PATENT DOCUMENT

Publication Number: US-11380310-B2
Application Number: US-202016998786-A
Country: US
Kind Code: B2

Title: Low-latency intelligent automated assistant

Abstract:
Systems and processes for operating a digital assistant are provided. In an example process, low-latency operation of a digital assistant is provided. In this example, natural language processing, task flow processing, dialogue flow processing, speech synthesis, or any combination thereof can be at least partially performed while awaiting detection of a speech end-point condition. Upon detection of a speech end-point condition, results obtained from performing the operations can be presented to the user. In another example, robust operation of a digital assistant is provided. In this example, task flow processing by the digital assistant can include selecting a candidate task flow from a plurality of candidate task flows based on determined task flow scores. The task flow scores can be based on speech recognition confidence scores, intent confidence scores, flow parameter scores, or any combination thereof. The selected candidate task flow is executed and corresponding results presented to the user.

Claims:
What is claimed is: 
     
       1. An electronic device, comprising:
 one or more processors; and 
 memory storing one or more programs configured to be executed by the one or more processors, the one or more programs including instructions for:
 receiving a user utterance; 
 determining, based on a plurality of candidate text representations of the user utterance, a plurality of candidate user intents for the user utterance, wherein each candidate user intent of the plurality of candidate user intents corresponds to a respective candidate task flow of a plurality of candidate task flows; 
 determining a plurality of task flow scores for the plurality of candidate task flows, each task flow score of the plurality of task flow scores corresponding to a respective candidate task flow of the plurality of candidate task flows; 
 selecting, based on the plurality of task flow scores, a first candidate task flow of the plurality of candidate task flows; and 
 executing the first candidate task flow, including presenting, to the user, results from executing the first candidate task flow. 
 
 
     
     
       2. The electronic device of  claim 1 , wherein the one or more programs include further instructions for:
 for each candidate task flow of the plurality of candidate task flows:
 resolving one or more flow parameters of the respective candidate task flow, wherein a respective task flow score for the respective candidate task flow is based on resolving the one or more flow parameters of the respective candidate task flow. 
 
 
     
     
       3. The electronic device of  claim 2 , wherein resolving the one or more flow parameters of the respective candidate task flow comprises searching a data source for one or more values corresponding to the one or more flow parameters, the data source corresponding to one or more properties of a respective candidate user intent of the plurality of candidate user intents. 
     
     
       4. The electronic device of  claim 1 , wherein:
 each candidate text representation of the plurality of candidate text representations has an associated speech recognition confidence score; and 
 each task flow score of the plurality of task flow scores is based on a respective speech recognition confidence score of a respective candidate text representation of the plurality of candidate text representations. 
 
     
     
       5. The electronic device of  claim 1 , wherein:
 each candidate user intent of the plurality of candidate user intents has an associated intent confidence score; and 
 each task flow score of the plurality of task flow scores is based on a respective intent confidence score of a respective candidate user intent of the plurality of candidate user intents. 
 
     
     
       6. The electronic device of  claim 1 , wherein:
 a first candidate user intent of the plurality of candidate user intents is determined from a first candidate text representation of the plurality of candidate text representations; and 
 a second candidate user intent of the plurality of candidate user intents is determined from a second candidate text representation of the plurality of candidate text representations. 
 
     
     
       7. The electronic device of  claim 1 , wherein the one or more programs include further instructions for:
 ranking the plurality of candidate task flows in accordance with the plurality of task flow scores, wherein selecting the first candidate task flow is based on the ranking of the plurality of candidate task flows. 
 
     
     
       8. A method for operating a digital assistant, the method comprising:
 at an electronic device having one or more processors and memory:
 receiving a user utterance; 
 determining, based on a plurality of candidate text representations of the user utterance, a plurality of candidate user intents for the user utterance, wherein each candidate user intent of the plurality of candidate user intents corresponds to a respective candidate task flow of a plurality of candidate task flows; 
 determining a plurality of task flow scores for the plurality of candidate task flows, each task flow score of the plurality of task flow scores corresponding to a respective candidate task flow of the plurality of candidate task flows; 
 selecting, based on the plurality of task flow scores, a first candidate task flow of the plurality of candidate task flows; and 
 executing the first candidate task flow, including presenting, to the user, results from executing the first candidate task flow. 
 
 
     
     
       9. The method of  claim 8 , further comprising:
 for each candidate task flow of the plurality of candidate task flows:
 resolving one or more flow parameters of the respective candidate task flow, wherein a respective task flow score for the respective candidate task flow is based on resolving the one or more flow parameters of the respective candidate task flow. 
 
 
     
     
       10. The method of  claim 9 , wherein resolving the one or more flow parameters of the respective candidate task flow comprises searching a data source for one or more values corresponding to the one or more flow parameters, the data source corresponding to one or more properties of a respective candidate user intent of the plurality of candidate user intents. 
     
     
       11. The method of  claim 8 , wherein:
 each candidate text representation of the plurality of candidate text representations has an associated speech recognition confidence score; and 
 each task flow score of the plurality of task flow scores is based on a respective speech recognition confidence score of a respective candidate text representation of the plurality of candidate text representations. 
 
     
     
       12. The method of  claim 8 , wherein:
 each candidate user intent of the plurality of candidate user intents has an associated intent confidence score; and 
 each task flow score of the plurality of task flow scores is based on a respective intent confidence score of a respective candidate user intent of the plurality of candidate user intents. 
 
     
     
       13. The method of  claim 8 , wherein:
 a first candidate user intent of the plurality of candidate user intents is determined from a first candidate text representation of the plurality of candidate text representations; and 
 a second candidate user intent of the plurality of candidate user intents is determined from a second candidate text representation of the plurality of candidate text representations. 
 
     
     
       14. The method of  claim 8 , further comprising:
 ranking the plurality of candidate task flows in accordance with the plurality of task flow scores, wherein selecting the first candidate task flow is based on the ranking of the plurality of candidate task flows. 
 
     
     
       15. A non-transitory computer-readable storage medium storing one or more programs configured to be executed by one or more processors of an electronic device, the one or more programs including instructions for:
 receiving a user utterance; 
 determining, based on a plurality of candidate text representations of the user utterance, a plurality of candidate user intents for the user utterance, wherein each candidate user intent of the plurality of candidate user intents corresponds to a respective candidate task flow of a plurality of candidate task flows; 
 determining a plurality of task flow scores for the plurality of candidate task flows, each task flow score of the plurality of task flow scores corresponding to a respective candidate task flow of the plurality of candidate task flows; 
 selecting, based on the plurality of task flow scores, a first candidate task flow of the plurality of candidate task flows; and 
 executing the first candidate task flow, including presenting, to the user, results from executing the first candidate task flow. 
 
     
     
       16. The non-transitory computer-readable storage medium of  claim 15 , wherein the one or more programs further include instructions for:
 for each candidate task flow of the plurality of candidate task flows:
 resolving one or more flow parameters of the respective candidate task flow, wherein a respective task flow score for the respective candidate task flow is based on resolving the one or more flow parameters of the respective candidate task flow. 
 
 
     
     
       17. The non-transitory computer-readable storage medium of  claim 16 , wherein resolving the one or more flow parameters of the respective candidate task flow comprises searching a data source for one or more values corresponding to the one or more flow parameters, the data source corresponding to one or more properties of a respective candidate user intent of the plurality of candidate user intents. 
     
     
       18. The non-transitory computer-readable storage medium of  claim 15 , wherein:
 each candidate text representation of the plurality of candidate text representations has an associated speech recognition confidence score; and 
 each task flow score of the plurality of task flow scores is based on a respective speech recognition confidence score of a respective candidate text representation of the plurality of candidate text representations. 
 
     
     
       19. The non-transitory computer-readable storage medium of  claim 15 , wherein:
 each candidate user intent of the plurality of candidate user intents has an associated intent confidence score; and 
 each task flow score of the plurality of task flow scores is based on a respective intent confidence score of a respective candidate user intent of the plurality of candidate user intents. 
 
     
     
       20. The non-transitory computer-readable storage medium of  claim 15 , wherein:
 a first candidate user intent of the plurality of candidate user intents is determined from a first candidate text representation of the plurality of candidate text representations; and 
 a second candidate user intent of the plurality of candidate user intents is determined from a second candidate text representation of the plurality of candidate text representations. 
 
     
     
       21. The non-transitory computer-readable storage medium of  claim 15 , wherein the one or more programs further include instructions for:
 ranking the plurality of candidate task flows in accordance with the plurality of task flow scores, wherein selecting the first candidate task flow is based on the ranking of the plurality of candidate task flows. 
 
     
     
       22. An electronic device, comprising:
 one or more processors; and 
 memory storing one or more programs configured to be executed by the one or more processors, the one or more programs including instructions for:
 receiving a user utterance; 
 determining, based on a plurality of candidate text representations of the user utterance, a plurality of candidate user intents for the user utterance, wherein each candidate user intent of the plurality of candidate user intents corresponds to a respective candidate task flow of a plurality of candidate task flows, and wherein a first candidate task flow of the plurality of candidate task flows corresponds to a first candidate user intent of the plurality of candidate user intents; 
 determining a plurality of task flow scores for the plurality of candidate task flows, each task flow score of the plurality of task flow scores corresponding to a respective candidate task flow of the plurality of candidate task flows, wherein:
 a first task flow score of the plurality of task flow scores is for the first candidate task flow; 
 determining the plurality of task flow scores includes determining a flow parameter value corresponding to a property of the first candidate user intent, the flow parameter value not specified in the user utterance; and 
 the first task flow score is based on whether the flow parameter value can be determined by the electronic device; 
 
 selecting, based on the plurality of task flow scores, the first candidate task flow; and 
 executing the first candidate task flow, including presenting, to the user, results from executing the first candidate task flow. 
 
 
     
     
       23. The electronic device of  claim 22 , wherein the first task flow score is further based on a first intent confidence score of the first candidate user intent. 
     
     
       24. The electronic device of  claim 23 , wherein the first task flow score is a highest task flow score of the plurality of task flow scores, and wherein the first intent confidence score is not a highest intent confidence score of a plurality of intent confidence scores that correspond to the plurality of candidate user intents. 
     
     
       25. The electronic device of  claim 22 , wherein:
 the first candidate user intent is determined from a first candidate text representation of the plurality of candidate text representations; and 
 the first task flow score is further based on a first speech recognition confidence score of the first candidate text representation. 
 
     
     
       26. The electronic device of  claim 25 , wherein the first task flow score is a highest task flow score of the plurality of task flow scores, and wherein the first speech recognition confidence score is not a highest speech recognition confidence score of a plurality of speech recognition confidence scores that correspond to the plurality of candidate text representations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/679,595, filed on Aug. 17, 2017, entitled “LOW-LATENCY INTELLIGENT AUTOMATED ASSISTANT,” which claims priority from U.S. Provisional Ser. No. 62/505,546, filed on May 12, 2017, entitled “LOW-LATENCY INTELLIGENT AUTOMATED ASSISTANT,” which are hereby incorporated by reference in their entirety for all purposes. 
    
    
     FIELD 
     This relates generally to intelligent automated assistants and, more specifically, to low-latency intelligent automated assistants. 
     BACKGROUND 
     Intelligent automated assistants (or digital assistants) can provide a beneficial interface between human users and electronic devices. Such assistants can allow users to interact with devices or systems using natural language in spoken and/or text forms. For example, a user can provide a speech input containing a user request to a digital assistant operating on an electronic device. The digital assistant can interpret the user&#39;s intent from the speech input and operationalize the user&#39;s intent into tasks. The tasks can then be performed by executing one or more services of the electronic device, and a relevant output responsive to the user request can be returned to the user. 
     Digital assistants are frequently implemented on mobile computing platforms, such as smart phones and tablet computers. However, such mobile computing platforms can have limited computing resources (e.g., memory and processor power) and thus digital assistants implemented on such platforms can suffer from longer processing times and thus greater latency in responding to user requests. This can result in poor user experience, which can limit the widespread adoption of digital assistants on mobile platforms. 
     SUMMARY 
     Systems and processes for operating a digital assistant are provided. In one example process, low-latency operation of a digital assistant is provided. In this example, a stream of audio is received. In particular, a first portion of the stream of audio containing a user utterance is received from a first time to a second time and a second portion of the stream of audio is received from the second time to a third time. The process determines whether the first portion of the stream of audio satisfies a predetermined condition. In response to determining that the first portion of the stream of audio satisfies a predetermined condition, operations are at least partially performed between the second time and the third time. The operations include determining, based on one or more candidate text representations of the user utterance, a plurality of candidate user intents for the user utterance. Each candidate user intent of the plurality of candidate user intents corresponds to a respective candidate task flow of a plurality of candidate task flows. The operations also include selecting a first candidate task flow of the plurality of candidate task flows. In addition, the operations include executing the first candidate task flow without providing an output to a user of the device. The process determines whether a speech end-point condition is detected between the second time and the third time. In response to determining that a speech end-point condition is detected between the second time and the third time, results from executing the selected first candidate task flow are presented to the user. 
     Performing, at least partially between the second time and the third time, and in response to determining that the first portion of the stream of audio satisfies a predetermined condition, operations that include determining a plurality of candidate user intents, selecting a first candidate task flow, and executing the first candidate task flow can enable the electronic device to at least partially complete these operations while waiting for the speech end-point condition to be detected. This can enhance operability of the electronic device by reducing the amount of computation needed to be performed after detecting the speech end-point condition, which in turn can reduce the overall latency between receiving the user utterance and presenting the results to the user. 
     In another example process for low-latency operation of a digital assistant, a stream of audio is received. In particular, a first portion of the stream of audio containing a user utterance is received from a first time to a second time and a second portion of the stream of audio is received from the second time to a third time. The process determines whether the first portion of the stream of audio satisfies a predetermined condition. In response to determining that the first portion of the stream of audio satisfies a predetermined condition, operations are at least partially performed between the second time and the third time. The operations include causing generation of a text dialogue that is responsive to the user utterance. The operations also include determining whether the memory of the device stores an audio file having a spoken representation of the text dialogue. In response to determining that the memory of the device does not store an audio file having a spoken representation of the text dialogue, the operations include generating an audio file having a spoken representation of the text dialogue and storing the audio file in the memory. The process determines whether a speech end-point condition is detected between the second time and the third time. In response to determining that a speech end-point condition is detected between the second time and the third time, the spoken representation of the text dialogue is outputted to a user of the device by playing the stored audio file. 
     Performing, at least partially between the second time and the third time and in response to determining that the first portion of the stream of audio satisfies a predetermined condition, operations that include causing generation of a text dialogue and generating an audio file having a spoken representation of the text dialogue can enable the electronic device to at least partially complete these operations while waiting for the speech end-point condition to be detected. This can enhance operability of the electronic device by reducing the amount of computation needed to be performed after detecting the speech end-point condition, which in turn can reduce the overall latency between receiving the user utterance and outputting the spoken representation of the text dialogue to the user. 
     Systems and processes for robust operation of a digital assistant are also provided. In an example process, a user utterance is received. Based on a plurality of candidate text representations of the user utterance, a plurality of candidate user intents for the user utterance are determined. Each candidate user intent of the plurality of candidate user intents corresponds to a respective candidate task flow of a plurality of candidate task flows. A plurality of task flow scores for the plurality of candidate task flows are determined. Each task flow score of the plurality of task flow scores corresponds to a respective candidate task flow of the plurality of candidate task flows. Based on the plurality of task flow scores, a first candidate task flow of the plurality of candidate task flows is selected. The first candidate task flow is executed, including presenting to the user results from executing the first candidate task flow. 
     Determining a plurality of task flow scores for the plurality of candidate task flows and selecting the first candidate task flow based on the plurality of task flow scores can enable the electronic device to evaluate the reliability and feasibility of each candidate task flow prior to selecting and executing the first candidate task flow. This can enhance operability of the electronic device by improving the likelihood that the selected first candidate task flow coincides with the user&#39;s actual desired goal for providing the user utterance. In turn, this can allow the electronic device to operate with greater accuracy and reliability when identifying and performing tasks in response to a user utterance. 
     Executable instructions for performing the functions described herein are, optionally, included in a non-transitory computer-readable storage medium or other computer-program product configured for execution by one or more processors. Executable instructions for performing these functions are, optionally, included in a transitory computer-readable storage medium or other computer program product configured for execution by one or more processors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a system and environment for implementing a digital assistant, according to various examples. 
         FIG. 2A  is a block diagram illustrating a portable multifunction device implementing the client-side portion of a digital assistant, according to various examples. 
         FIG. 2B  is a block diagram illustrating exemplary components for event handling, according to various examples. 
         FIG. 3  illustrates a portable multifunction device implementing the client-side portion of a digital assistant, according to various examples. 
         FIG. 4  is a block diagram of an exemplary multifunction device with a display and a touch-sensitive surface, according to various examples. 
         FIG. 5A  illustrates an exemplary user interface for a menu of applications on a portable multifunction device, according to various examples. 
         FIG. 5B  illustrates an exemplary user interface for a multifunction device with a touch-sensitive surface that is separate from the display, according to various examples. 
         FIG. 6A  illustrates a personal electronic device, according to various examples. 
         FIG. 6B  is a block diagram illustrating a personal electronic device, according to various examples. 
         FIG. 7A  is a block diagram illustrating a digital assistant system or a server portion thereof, according to various examples. 
         FIG. 7B  illustrates the functions of the digital assistant shown in  FIG. 7A , according to various examples. 
         FIG. 7C  illustrates a portion of an ontology, according to various examples. 
         FIG. 8  is a block diagram illustrating a portion of a digital assistant module, according to various examples. 
         FIG. 9  is a timeline illustrating the timing of low-latency operation of a digital assistant, according to various examples. 
         FIG. 10  is a timeline illustrating the timing of low-latency operation of a digital assistant, according to various examples. 
         FIGS. 11A-11B  illustrate a process for operating a digital assistant, according to various examples. 
         FIG. 12  illustrates a process for operating a digital assistant to generate a spoken dialogue response, according to various examples. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of examples, reference is made to the accompanying drawings in which are shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the various examples. 
     As discussed above, digital assistants implemented on mobile computing platforms can suffer from longer processing times and thus greater latency in responding to user requests. In particular, certain processes performed by digital assistants, such as natural language processing, task flow processing, and/or speech synthesis, can be computationally intensive and contribute significantly to the response latency. In some digital assistant systems, the aforementioned processes are initiated only after a speech end-point condition is detected. Detecting a speech end-point condition establishes that the user has finished providing his/her spoken request. However, a speech end-point condition is frequently detected based on an absence of user speech for greater than a predetermined duration (e.g., 600 ms, 700 ms, or 800 ms). This means that the total latency experienced by the user after the user finishes providing his/her spoken request can include the predetermined duration needed to detect a speech end-point condition and the computational time required for the digital assistant system to process the spoken request (e.g., by performing natural language processing, task flow processing, and/or speech synthesis). Given the limited computational resources of mobile computing platforms, this total latency can be considerable enough to significantly impact user engagement. It can thus be desirable to reduce the total latency experienced by the user from the time the user finishes providing his/her spoken request to the time the digital assistant system presents a response to the user request. 
     Techniques for reducing response latency for digital assistant systems are described herein. In particular, in some exemplary processes, natural language processing, task flow processing, and/or speech synthesis can be initiated during the predetermined duration needed to detect a speech end-point condition. For instance, in a specific example, natural language processing, task flow processing, and/or speech synthesis can be initiated upon detecting a short pause (e.g., 50 ms, 75 ms, or 100 ms) in user speech. If the short pause develops into a long pause (e.g., 600 ms, 700 ms, or 800 ms) that corresponds to a speech end-point condition, natural language processing, task flow processing, and/or speech synthesis would be at least partially completed at the time the speech end-point condition is detected. This can result in a reduction in the total latency experienced by the user. 
     In one example process for reducing response latency in a digital assistant system, a stream of audio is received. In particular, a first portion of the stream of audio containing a user utterance is received from a first time to a second time and a second portion of the stream of audio is received from the second time to a third time. The process determines whether the first portion of the stream of audio satisfies a predetermined condition. In response to determining that the first portion of the stream of audio satisfies a predetermined condition, operations are at least partially performed between the second time and the third time. The operations include determining, based on one or more candidate text representations of the user utterance, a plurality of candidate user intents for the user utterance. Each candidate user intent of the plurality of candidate user intents corresponds to a respective candidate task flow of a plurality of candidate task flows. The operations also include selecting a first candidate task flow of the plurality of candidate task flows. In addition, the operations include executing the first candidate task flow without providing an output to a user of the device. In some examples, executing the first candidate task flow includes generating spoken dialogue that is responsive to the user utterance without outputting the generated spoken dialogue. The process determines whether a speech end-point condition is detected between the second time and the third time. In response to determining that a speech end-point condition is detected between the second time and the third time, results from executing the selected first candidate task flow are presented to the user. In some examples, presenting the results includes outputting the generated spoken dialogue. 
     Techniques for robust operation of a digital assistant are also described herein. In particular, due to the inherent ambiguity in human speech, there is inherent uncertainty during speech recognition and natural language processing of human speech. As a result, speech recognition and natural language processing errors can frequently occur when digital assistant systems process spoken user requests. Such errors, when propagated through task flow processing, can at times result in fatal errors (e.g., no response) or in the performance of tasks that do not correspond to the user&#39;s desired goal. 
     An illustrative example of a task flow processing error caused by a speech recognition error is provided. In this example, the user provides the spoken request “What are Mike Dunleavy&#39;s stats?” During speech recognition processing, the digital assistant system can erroneously transcribe the spoken request as “What are Mike Dunleavey&#39;s stats?” Subsequently, during natural language processing, the digital assistant system can recognize (e.g., based on the word “stats”) that the user is requesting for sports information and that “Mike Dunleavey” is a sports-related entity. Based on this interpretation, the digital assistant system can perform task flow processing and select a task flow that includes procedures for searching sports-related data sources for “Mike Dunleavey.” However, during execution of the selected task flow, the digital assistant system may be unable to locate any information related to “Mike Dunleavey” in the sports-related sources due to the speech recognition error. As a result, the digital assistant system can fail to provide any substantive response to the user&#39;s request. 
     In another illustrative example of a task flow processing error, the user can provide the spoken request “Directions to Fidelity Investments.” In this example, the digital assistant system can successfully transcribe the spoken request as “Directions to Fidelity Investments.” During subsequent natural language processing, the digital assistant system can recognize (e.g., based on the word “directions”) that the user is requesting for directions. However, rather than interpreting “Fidelity Investments” as a business, the digital assistant system can erroneously interpret “Fidelity Investments” as a person in the user&#39;s contact list (e.g., based on the existence of an entry corresponding to “Fidelity Investments” in the user&#39;s contact list). Based on this erroneous interpretation, the digital assistant system can perform task flow processing and select a task flow that includes procedures for searching the user&#39;s contact list for an address corresponding to “Fidelity Investments” and obtaining directions to that address. However, during execution of the selected task flow, the digital assistant system may be unable to find any address corresponding to “Fidelity Investments” in the user&#39;s contact list. Specifically, although the user has an entry corresponding to “Fidelity Investments” in his/her contact list, the entry may only include phone number information, but not address information. As a result, the digital assistant system can fail to provide any substantive response to the user&#39;s request. 
     Based on the illustrative examples described above, a digital assistant system that implements more robust task flow processing can be desirable. In accordance with some techniques described herein, multiple candidate task flows associated with multiple candidate user intents can be evaluated for reliability prior to selecting and executing a particular task flow. The evaluation process can be based on task flow scores determined for every candidate task flow. The task flow score for a respective candidate task flow can be based on, for example, a speech recognition confidence score of a respective speech recognition result, an intent confidence score of a respective natural language processing result, a flow parameter score of the respective candidate task flow, or any combination thereof. In some examples, the flow parameter score can be based on whether one or more missing flow parameters for the respective candidate task flow can be resolved. For example, referring to the above illustrative examples, the flow parameter score can be based on whether missing flow parameters (e.g., “sports entity” and “address” flow parameters) associated with “Mike Dunleavey” and “Fidelity Investments” can be resolved. In these examples, the flow parameter scores can be low because the missing flow parameters cannot be resolved. The digital assistant system can select a suitable candidate task flow based on the task flow scores of the candidate task flows. For example, a candidate task flow having a task flow score that is maximized based on the combined speech recognition confidence score, intent confidence score, and flow parameter score can be selected. By selecting a suitable candidate task flow based on determined task flow scores for every candidate task flow, the selected candidate task flow can be more likely to coincide with the user&#39;s intended goal. Moreover, fatal error may be less likely to occur during execution of a selected candidate task flow. 
     In an example process for robust operation of a digital assistant, a user utterance is received. Based on a plurality of candidate text representations of the user utterance, a plurality of candidate user intents for the user utterance are determined. Each candidate user intent of the plurality of candidate user intents corresponds to a respective candidate task flow of a plurality of candidate task flows. A plurality of task flow scores for the plurality of candidate task flows are determined. Each task flow score of the plurality of task flow scores corresponds to a respective candidate task flow of the plurality of candidate task flows. Based on the plurality of task flow scores, a first candidate task flow of the plurality of candidate task flows is selected. The first candidate task flow is executed, including presenting, to the user, results from executing the first candidate task flow. 
     Although the following description uses the terms “first,” “second,” etc. to describe various elements, these elements should not be limited by the terms. These terms are only used to distinguish one element from another. For example, a first input could be termed a second input, and, similarly, a second input could be termed a first input, without departing from the scope of the various described examples. The first input and the second input are both inputs and, in some cases, are separate and different inputs. 
     The terminology used in the description of the various described examples herein is for the purpose of describing particular examples only and is not intended to be limiting. As used in the description of the various described examples and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “includes,” “including,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The term “if” may be construed to mean “when” or “upon” or “in response to determining” or “in response to detecting,” depending on the context. Similarly, the phrase “if it is determined” or “if [a stated condition or event] is detected” may be construed to mean “upon determining” or “in response to determining” or “upon detecting [the stated condition or event]” or “in response to detecting [the stated condition or event],” depending on the context. 
     1. System and Environment 
       FIG. 1  illustrates a block diagram of system  100  according to various examples. In some examples, system  100  implements a digital assistant. The terms “digital assistant,” “virtual assistant,” “intelligent automated assistant,” or “automatic digital assistant” refer to any information processing system that interprets natural language input in spoken and/or textual form to infer user intent, and performs actions based on the inferred user intent. For example, to act on an inferred user intent, the system performs one or more of the following: identifying a task flow with steps and parameters designed to accomplish the inferred user intent, inputting specific requirements from the inferred user intent into the task flow; executing the task flow by invoking programs, methods, services, APIs, or the like; and generating output responses to the user in an audible (e.g., speech) and/or visual form. 
     Specifically, a digital assistant is capable of accepting a user request at least partially in the form of a natural language command, request, statement, narrative, and/or inquiry. Typically, the user request seeks either an informational answer or performance of a task by the digital assistant. A satisfactory response to the user request includes a provision of the requested informational answer, a performance of the requested task, or a combination of the two. For example, a user asks the digital assistant a question, such as “Where am I right now?” Based on the user&#39;s current location, the digital assistant answers, “You are in Central Park near the west gate.” The user also requests the performance of a task, for example, “Please invite my friends to my girlfriend&#39;s birthday party next week.” In response, the digital assistant can acknowledge the request by saying “Yes, right away,” and then send a suitable calendar invite on behalf of the user to each of the user&#39;s friends listed in the user&#39;s electronic address book. During performance of a requested task, the digital assistant sometimes interacts with the user in a continuous dialogue involving multiple exchanges of information over an extended period of time. There are numerous other ways of interacting with a digital assistant to request information or performance of various tasks. In addition to providing verbal responses and taking programmed actions, the digital assistant also provides responses in other visual or audio forms, e.g., as text, alerts, music, videos, animations, etc. 
     As shown in  FIG. 1 , in some examples, a digital assistant is implemented according to a client-server model. The digital assistant includes client-side portion  102  (hereafter “DA client  102 ”) executed on user device  104  and server-side portion  106  (hereafter “DA server  106 ”) executed on server system  108 . DA client  102  communicates with DA server  106  through one or more networks  110 . DA client  102  provides client-side functionalities such as user-facing input and output processing and communication with DA server  106 . DA server  106  provides server-side functionalities for any number of DA clients  102  each residing on a respective user device  104 . 
     In some examples, DA server  106  includes client-facing I/O interface  112 , one or more processing modules  114 , data and models  116 , and I/O interface to external services  118 . The client-facing I/O interface  112  facilitates the client-facing input and output processing for DA server  106 . One or more processing modules  114  utilize data and models  116  to process speech input and determine the user&#39;s intent based on natural language input. Further, one or more processing modules  114  perform task execution based on inferred user intent. In some examples, DA server  106  communicates with external services  120  through network(s)  110  for task completion or information acquisition. I/O interface to external services  118  facilitates such communications. 
     User device  104  can be any suitable electronic device. In some examples, user device is a portable multifunctional device (e.g., device  200 , described below with reference to  FIG. 2A ), a multifunctional device (e.g., device  400 , described below with reference to  FIG. 4 ), or a personal electronic device (e.g., device  600 , described below with reference to  FIG. 6A-B .) A portable multifunctional device is, for example, a mobile telephone that also contains other functions, such as PDA and/or music player functions. Specific examples of portable multifunction devices include the iPhone®, iPod Touch®, and iPad® devices from Apple Inc. of Cupertino, Calif. Other examples of portable multifunction devices include, without limitation, laptop or tablet computers. Further, in some examples, user device  104  is a non-portable multifunctional device. In particular, user device  104  is a desktop computer, a game console, a television, or a television set-top box. In some examples, user device  104  includes a touch-sensitive surface (e.g., touch screen displays and/or touchpads). Further, user device  104  optionally includes one or more other physical user-interface devices, such as a physical keyboard, a mouse, and/or a joystick. Various examples of electronic devices, such as multifunctional devices, are described below in greater detail. 
     Examples of communication network(s)  110  include local area networks (LAN) and wide area networks (WAN), e.g., the Internet. Communication network(s)  110  is implemented using any known network protocol, including various wired or wireless protocols, such as, for example, Ethernet, Universal Serial Bus (USB), FIREWIRE, Global System for Mobile Communications (GSM), Enhanced Data GSM Environment (EDGE), code division multiple access (CDMA), time division multiple access (TDMA), Bluetooth, Wi-Fi, voice over Internet Protocol (VoIP), Wi-MAX, or any other suitable communication protocol. 
     Server system  108  is implemented on one or more standalone data processing apparatus or a distributed network of computers. In some examples, server system  108  also employs various virtual devices and/or services of third-party service providers (e.g., third-party cloud service providers) to provide the underlying computing resources and/or infrastructure resources of server system  108 . 
     In some examples, user device  104  communicates with DA server  106  via second user device  122 . Second user device  122  is similar or identical to user device  104 . For example, second user device  122  is similar to devices  200 ,  400 , or  600  described below with reference to  FIGS. 2A, 4, and 6A -B. User device  104  is configured to communicatively couple to second user device  122  via a direct communication connection, such as Bluetooth, NFC, BTLE, or the like, or via a wired or wireless network, such as a local Wi-Fi network. In some examples, second user device  122  is configured to act as a proxy between user device  104  and DA server  106 . For example, DA client  102  of user device  104  is configured to transmit information (e.g., a user request received at user device  104 ) to DA server  106  via second user device  122 . DA server  106  processes the information and return relevant data (e.g., data content responsive to the user request) to user device  104  via second user device  122 . 
     In some examples, user device  104  is configured to communicate abbreviated requests for data to second user device  122  to reduce the amount of information transmitted from user device  104 . Second user device  122  is configured to determine supplemental information to add to the abbreviated request to generate a complete request to transmit to DA server  106 . This system architecture can advantageously allow user device  104  having limited communication capabilities and/or limited battery power (e.g., a watch or a similar compact electronic device) to access services provided by DA server  106  by using second user device  122 , having greater communication capabilities and/or battery power (e.g., a mobile phone, laptop computer, tablet computer, or the like), as a proxy to DA server  106 . While only two user devices  104  and  122  are shown in  FIG. 1 , it should be appreciated that system  100 , in some examples, includes any number and type of user devices configured in this proxy configuration to communicate with DA server system  106 . 
     Although the digital assistant shown in  FIG. 1  includes both a client-side portion (e.g., DA client  102 ) and a server-side portion (e.g., DA server  106 ), in some examples, the functions of a digital assistant are implemented as a standalone application installed on a user device. In addition, the divisions of functionalities between the client and server portions of the digital assistant can vary in different implementations. For instance, in some examples, the DA client is a thin-client that provides only user-facing input and output processing functions, and delegates all other functionalities of the digital assistant to a backend server. 
     2. Electronic Devices 
     Attention is now directed toward embodiments of electronic devices for implementing the client-side portion of a digital assistant.  FIG. 2A  is a block diagram illustrating portable multifunction device  200  with touch-sensitive display system  212  in accordance with some embodiments. Touch-sensitive display  212  is sometimes called a “touch screen” for convenience and is sometimes known as or called a “touch-sensitive display system.” Device  200  includes memory  202  (which optionally includes one or more computer-readable storage mediums), memory controller  222 , one or more processing units (CPUs)  220 , peripherals interface  218 , RF circuitry  208 , audio circuitry  210 , speaker  211 , microphone  213 , input/output (I/O) subsystem  206 , other input control devices  216 , and external port  224 . Device  200  optionally includes one or more optical sensors  264 . Device  200  optionally includes one or more contact intensity sensors  265  for detecting intensity of contacts on device  200  (e.g., a touch-sensitive surface such as touch-sensitive display system  212  of device  200 ). Device  200  optionally includes one or more tactile output generators  267  for generating tactile outputs on device  200  (e.g., generating tactile outputs on a touch-sensitive surface such as touch-sensitive display system  212  of device  200  or touchpad  455  of device  400 ). These components optionally communicate over one or more communication buses or signal lines  203 . 
     As used in the specification and claims, the term “intensity” of a contact on a touch-sensitive surface refers to the force or pressure (force per unit area) of a contact (e.g., a finger contact) on the touch-sensitive surface, or to a substitute (proxy) for the force or pressure of a contact on the touch-sensitive surface. The intensity of a contact has a range of values that includes at least four distinct values and more typically includes hundreds of distinct values (e.g., at least 256). Intensity of a contact is, optionally, determined (or measured) using various approaches and various sensors or combinations of sensors. For example, one or more force sensors underneath or adjacent to the touch-sensitive surface are, optionally, used to measure force at various points on the touch-sensitive surface. In some implementations, force measurements from multiple force sensors are combined (e.g., a weighted average) to determine an estimated force of a contact. Similarly, a pressure-sensitive tip of a stylus is, optionally, used to determine a pressure of the stylus on the touch-sensitive surface. Alternatively, the size of the contact area detected on the touch-sensitive surface and/or changes thereto, the capacitance of the touch-sensitive surface proximate to the contact and/or changes thereto, and/or the resistance of the touch-sensitive surface proximate to the contact and/or changes thereto are, optionally, used as a substitute for the force or pressure of the contact on the touch-sensitive surface. In some implementations, the substitute measurements for contact force or pressure are used directly to determine whether an intensity threshold has been exceeded (e.g., the intensity threshold is described in units corresponding to the substitute measurements). In some implementations, the substitute measurements for contact force or pressure are converted to an estimated force or pressure, and the estimated force or pressure is used to determine whether an intensity threshold has been exceeded (e.g., the intensity threshold is a pressure threshold measured in units of pressure). Using the intensity of a contact as an attribute of a user input allows for user access to additional device functionality that may otherwise not be accessible by the user on a reduced-size device with limited real estate for displaying affordances (e.g., on a touch-sensitive display) and/or receiving user input (e.g., via a touch-sensitive display, a touch-sensitive surface, or a physical/mechanical control such as a knob or a button). 
     As used in the specification and claims, the term “tactile output” refers to physical displacement of a device relative to a previous position of the device, physical displacement of a component (e.g., a touch-sensitive surface) of a device relative to another component (e.g., housing) of the device, or displacement of the component relative to a center of mass of the device that will be detected by a user with the user&#39;s sense of touch. For example, in situations where the device or the component of the device is in contact with a surface of a user that is sensitive to touch (e.g., a finger, palm, or other part of a user&#39;s hand), the tactile output generated by the physical displacement will be interpreted by the user as a tactile sensation corresponding to a perceived change in physical characteristics of the device or the component of the device. For example, movement of a touch-sensitive surface (e.g., a touch-sensitive display or trackpad) is, optionally, interpreted by the user as a “down click” or “up click” of a physical actuator button. In some cases, a user will feel a tactile sensation such as an “down click” or “up click” even when there is no movement of a physical actuator button associated with the touch-sensitive surface that is physically pressed (e.g., displaced) by the user&#39;s movements. As another example, movement of the touch-sensitive surface is, optionally, interpreted or sensed by the user as “roughness” of the touch-sensitive surface, even when there is no change in smoothness of the touch-sensitive surface. While such interpretations of touch by a user will be subject to the individualized sensory perceptions of the user, there are many sensory perceptions of touch that are common to a large majority of users. Thus, when a tactile output is described as corresponding to a particular sensory perception of a user (e.g., an “up click,” a “down click,” “roughness”), unless otherwise stated, the generated tactile output corresponds to physical displacement of the device or a component thereof that will generate the described sensory perception for a typical (or average) user. 
     It should be appreciated that device  200  is only one example of a portable multifunction device, and that device  200  optionally has more or fewer components than shown, optionally combines two or more components, or optionally has a different configuration or arrangement of the components. The various components shown in  FIG. 2A  are implemented in hardware, software, or a combination of both hardware and software, including one or more signal processing and/or application-specific integrated circuits. 
     Memory  202  includes one or more computer-readable storage mediums. The computer-readable storage mediums are, for example, tangible and non-transitory. Memory  202  includes high-speed random access memory and also includes non-volatile memory, such as one or more magnetic disk storage devices, flash memory devices, or other non-volatile solid-state memory devices. Memory controller  222  controls access to memory  202  by other components of device  200 . 
     In some examples, a non-transitory computer-readable storage medium of memory  202  is used to store instructions (e.g., for performing aspects of processes described below) for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In other examples, the instructions (e.g., for performing aspects of the processes described below) are stored on a non-transitory computer-readable storage medium (not shown) of the server system  108  or are divided between the non-transitory computer-readable storage medium of memory  202  and the non-transitory computer-readable storage medium of server system  108 . 
     Peripherals interface  218  is used to couple input and output peripherals of the device to CPU  220  and memory  202 . The one or more processors  220  run or execute various software programs and/or sets of instructions stored in memory  202  to perform various functions for device  200  and to process data. In some embodiments, peripherals interface  218 , CPU  220 , and memory controller  222  are implemented on a single chip, such as chip  204 . In some other embodiments, they are implemented on separate chips. 
     RF (radio frequency) circuitry  208  receives and sends RF signals, also called electromagnetic signals. RF circuitry  208  converts electrical signals to/from electromagnetic signals and communicates with communications networks and other communications devices via the electromagnetic signals. RF circuitry  208  optionally includes well-known circuitry for performing these functions, including but not limited to an antenna system, an RF transceiver, one or more amplifiers, a tuner, one or more oscillators, a digital signal processor, a CODEC chipset, a subscriber identity module (SIM) card, memory, and so forth. RF circuitry  208  optionally communicates with networks, such as the Internet, also referred to as the World Wide Web (WWW), an intranet and/or a wireless network, such as a cellular telephone network, a wireless local area network (LAN) and/or a metropolitan area network (MAN), and other devices by wireless communication. The RF circuitry  208  optionally includes well-known circuitry for detecting near field communication (NFC) fields, such as by a short-range communication radio. The wireless communication optionally uses any of a plurality of communications standards, protocols, and technologies, including but not limited to Global System for Mobile Communications (GSM), Enhanced Data GSM Environment (EDGE), high-speed downlink packet access (HSDPA), high-speed uplink packet access (HSUPA), Evolution, Data-Only (EV-DO), HSPA, HSPA+, Dual-Cell HSPA (DC-HSPDA), long term evolution (LTE), near field communication (NFC), wideband code division multiple access (W-CDMA), code division multiple access (CDMA), time division multiple access (TDMA), Bluetooth, Bluetooth Low Energy (BTLE), Wireless Fidelity (Wi-Fi) (e.g., IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n, and/or IEEE 802.11ac), voice over Internet Protocol (VoIP), Wi-MAX, a protocol for e mail (e.g., Internet message access protocol (IMAP) and/or post office protocol (POP)), instant messaging (e.g., extensible messaging and presence protocol (XMPP), Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions (SIMPLE), Instant Messaging and Presence Service (IMPS)), and/or Short Message Service (SMS), or any other suitable communication protocol, including communication protocols not yet developed as of the filing date of this document. 
     Audio circuitry  210 , speaker  211 , and microphone  213  provide an audio interface between a user and device  200 . Audio circuitry  210  receives audio data from peripherals interface  218 , converts the audio data to an electrical signal, and transmits the electrical signal to speaker  211 . Speaker  211  converts the electrical signal to human-audible sound waves. Audio circuitry  210  also receives electrical signals converted by microphone  213  from sound waves. Audio circuitry  210  converts the electrical signal to audio data and transmits the audio data to peripherals interface  218  for processing. Audio data are retrieved from and/or transmitted to memory  202  and/or RF circuitry  208  by peripherals interface  218 . In some embodiments, audio circuitry  210  also includes a headset jack (e.g.,  312 ,  FIG. 3 ). The headset jack provides an interface between audio circuitry  210  and removable audio input/output peripherals, such as output-only headphones or a headset with both output (e.g., a headphone for one or both ears) and input (e.g., a microphone). 
     I/O subsystem  206  couples input/output peripherals on device  200 , such as touch screen  212  and other input control devices  216 , to peripherals interface  218 . I/O subsystem  206  optionally includes display controller  256 , optical sensor controller  258 , intensity sensor controller  259 , haptic feedback controller  261 , and one or more input controllers  260  for other input or control devices. The one or more input controllers  260  receive/send electrical signals from/to other input control devices  216 . The other input control devices  216  optionally include physical buttons (e.g., push buttons, rocker buttons, etc.), dials, slider switches, joysticks, click wheels, and so forth. In some alternate embodiments, input controller(s)  260  are, optionally, coupled to any (or none) of the following: a keyboard, an infrared port, a USB port, and a pointer device such as a mouse. The one or more buttons (e.g.,  308 ,  FIG. 3 ) optionally include an up/down button for volume control of speaker  211  and/or microphone  213 . The one or more buttons optionally include a push button (e.g.,  306 ,  FIG. 3 ). 
     A quick press of the push button disengages a lock of touch screen  212  or begin a process that uses gestures on the touch screen to unlock the device, as described in U.S. patent application Ser. No. 11/322,549, “Unlocking a Device by Performing Gestures on an Unlock Image,” filed Dec. 23, 2005, U.S. Pat. No. 7,657,849, which is hereby incorporated by reference in its entirety. A longer press of the push button (e.g.,  306 ) turns power to device  200  on or off. The user is able to customize a functionality of one or more of the buttons. Touch screen  212  is used to implement virtual or soft buttons and one or more soft keyboards. 
     Touch-sensitive display  212  provides an input interface and an output interface between the device and a user. Display controller  256  receives and/or sends electrical signals from/to touch screen  212 . Touch screen  212  displays visual output to the user. The visual output includes graphics, text, icons, video, and any combination thereof (collectively termed “graphics”). In some embodiments, some or all of the visual output correspond to user-interface objects. 
     Touch screen  212  has a touch-sensitive surface, sensor, or set of sensors that accepts input from the user based on haptic and/or tactile contact. Touch screen  212  and display controller  256  (along with any associated modules and/or sets of instructions in memory  202 ) detect contact (and any movement or breaking of the contact) on touch screen  212  and convert the detected contact into interaction with user-interface objects (e.g., one or more soft keys, icons, web pages, or images) that are displayed on touch screen  212 . In an exemplary embodiment, a point of contact between touch screen  212  and the user corresponds to a finger of the user. 
     Touch screen  212  uses LCD (liquid crystal display) technology, LPD (light emitting polymer display) technology, or LED (light emitting diode) technology, although other display technologies may be used in other embodiments. Touch screen  212  and display controller  256  detect contact and any movement or breaking thereof using any of a plurality of touch sensing technologies now known or later developed, including but not limited to capacitive, resistive, infrared, and surface acoustic wave technologies, as well as other proximity sensor arrays or other elements for determining one or more points of contact with touch screen  212 . In an exemplary embodiment, projected mutual capacitance sensing technology is used, such as that found in the iPhone® and iPod Touch® from Apple Inc. of Cupertino, Calif. 
     A touch-sensitive display in some embodiments of touch screen  212  is analogous to the multi-touch sensitive touchpads described in the following U.S. Pat. No. 6,323,846 (Westerman et al.), U.S. Pat. No. 6,570,557 (Westerman et al.), and/or U.S. Pat. No. 6,677,932 (Westerman), and/or U.S. Patent Publication 2002/0015024A1, each of which is hereby incorporated by reference in its entirety. However, touch screen  212  displays visual output from device  200 , whereas touch-sensitive touchpads do not provide visual output. 
     A touch-sensitive display in some embodiments of touch screen  212  is as described in the following applications: (1) U.S. patent application Ser. No. 11/381,313, “Multipoint Touch Surface Controller,” filed May 2, 2006; (2) U.S. patent application Ser. No. 10/840,862, “Multipoint Touchscreen,” filed May 6, 2004; (3) U.S. patent application Ser. No. 10/903,964, “Gestures For Touch Sensitive Input Devices,” filed Jul. 30, 2004; (4) U.S. patent application Ser. No. 11/048,264, “Gestures For Touch Sensitive Input Devices,” filed Jan. 31, 2005; (5) U.S. patent application Ser. No. 11/038,590, “Mode-Based Graphical User Interfaces For Touch Sensitive Input Devices,” filed Jan. 18, 2005; (6) U.S. patent application Ser. No. 11/228,758, “Virtual Input Device Placement On A Touch Screen User Interface,” filed Sep. 16, 2005; (7) U.S. patent application Ser. No. 11/228,700, “Operation Of A Computer With A Touch Screen Interface,” filed Sep. 16, 2005; (8) U.S. patent application Ser. No. 11/228,737, “Activating Virtual Keys Of A Touch-Screen Virtual Keyboard,” filed Sep. 16, 2005; and (9) U.S. patent application Ser. No. 11/367,749, “Multi-Functional Hand-Held Device,” filed Mar. 3, 2006. All of these applications are incorporated by reference herein in their entirety. 
     Touch screen  212  has, for example, a video resolution in excess of 100 dpi. In some embodiments, the touch screen has a video resolution of approximately 160 dpi. The user makes contact with touch screen  212  using any suitable object or appendage, such as a stylus, a finger, and so forth. In some embodiments, the user interface is designed to work primarily with finger-based contacts and gestures, which can be less precise than stylus-based input due to the larger area of contact of a finger on the touch screen. In some embodiments, the device translates the rough finger-based input into a precise pointer/cursor position or command for performing the actions desired by the user. 
     In some embodiments, in addition to the touch screen, device  200  includes a touchpad (not shown) for activating or deactivating particular functions. In some embodiments, the touchpad is a touch-sensitive area of the device that, unlike the touch screen, does not display visual output. The touchpad is a touch-sensitive surface that is separate from touch screen  212  or an extension of the touch-sensitive surface formed by the touch screen. 
     Device  200  also includes power system  262  for powering the various components. Power system  262  includes a power management system, one or more power sources (e.g., battery, alternating current (AC)), a recharging system, a power failure detection circuit, a power converter or inverter, a power status indicator (e.g., a light-emitting diode (LED)) and any other components associated with the generation, management and distribution of power in portable devices. 
     Device  200  also includes one or more optical sensors  264 .  FIG. 2A  shows an optical sensor coupled to optical sensor controller  258  in I/O subsystem  206 . Optical sensor  264  includes charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) phototransistors. Optical sensor  264  receives light from the environment, projected through one or more lenses, and converts the light to data representing an image. In conjunction with imaging module  243  (also called a camera module), optical sensor  264  captures still images or video. In some embodiments, an optical sensor is located on the back of device  200 , opposite touch screen display  212  on the front of the device so that the touch screen display is used as a viewfinder for still and/or video image acquisition. In some embodiments, an optical sensor is located on the front of the device so that the user&#39;s image is obtained for video conferencing while the user views the other video conference participants on the touch screen display. In some embodiments, the position of optical sensor  264  can be changed by the user (e.g., by rotating the lens and the sensor in the device housing) so that a single optical sensor  264  is used along with the touch screen display for both video conferencing and still and/or video image acquisition. 
     Device  200  optionally also includes one or more contact intensity sensors  265 .  FIG. 2A  shows a contact intensity sensor coupled to intensity sensor controller  259  in I/O subsystem  206 . Contact intensity sensor  265  optionally includes one or more piezoresistive strain gauges, capacitive force sensors, electric force sensors, piezoelectric force sensors, optical force sensors, capacitive touch-sensitive surfaces, or other intensity sensors (e.g., sensors used to measure the force (or pressure) of a contact on a touch-sensitive surface). Contact intensity sensor  265  receives contact intensity information (e.g., pressure information or a proxy for pressure information) from the environment. In some embodiments, at least one contact intensity sensor is collocated with, or proximate to, a touch-sensitive surface (e.g., touch-sensitive display system  212 ). In some embodiments, at least one contact intensity sensor is located on the back of device  200 , opposite touch screen display  212 , which is located on the front of device  200 . 
     Device  200  also includes one or more proximity sensors  266 .  FIG. 2A  shows proximity sensor  266  coupled to peripherals interface  218 . Alternately, proximity sensor  266  is coupled to input controller  260  in I/O subsystem  206 . Proximity sensor  266  is performed as described in U.S. patent application Ser. No. 11/241,839, “Proximity Detector In Handheld Device”; Ser. No. 11/240,788, “Proximity Detector In Handheld Device”; Ser. No. 11/620,702, “Using Ambient Light Sensor To Augment Proximity Sensor Output”; Ser. No. 11/586,862, “Automated Response To And Sensing Of User Activity In Portable Devices”; and Ser. No. 11/638,251, “Methods And Systems For Automatic Configuration Of Peripherals,” which are hereby incorporated by reference in their entirety. In some embodiments, the proximity sensor turns off and disables touch screen  212  when the multifunction device is placed near the user&#39;s ear (e.g., when the user is making a phone call). 
     Device  200  optionally also includes one or more tactile output generators  267 .  FIG. 2A  shows a tactile output generator coupled to haptic feedback controller  261  in I/O subsystem  206 . Tactile output generator  267  optionally includes one or more electroacoustic devices such as speakers or other audio components and/or electromechanical devices that convert energy into linear motion such as a motor, solenoid, electroactive polymer, piezoelectric actuator, electrostatic actuator, or other tactile output generating component (e.g., a component that converts electrical signals into tactile outputs on the device). Contact intensity sensor  265  receives tactile feedback generation instructions from haptic feedback module  233  and generates tactile outputs on device  200  that are capable of being sensed by a user of device  200 . In some embodiments, at least one tactile output generator is collocated with, or proximate to, a touch-sensitive surface (e.g., touch-sensitive display system  212 ) and, optionally, generates a tactile output by moving the touch-sensitive surface vertically (e.g., in/out of a surface of device  200 ) or laterally (e.g., back and forth in the same plane as a surface of device  200 ). In some embodiments, at least one tactile output generator sensor is located on the back of device  200 , opposite touch screen display  212 , which is located on the front of device  200 . 
     Device  200  also includes one or more accelerometers  268 .  FIG. 2A  shows accelerometer  268  coupled to peripherals interface  218 . Alternately, accelerometer  268  is coupled to an input controller  260  in I/O subsystem  206 . Accelerometer  268  performs, for example, as described in U.S. Patent Publication No. 20050190059, “Acceleration-based Theft Detection System for Portable Electronic Devices,” and U.S. Patent Publication No. 20060017692, “Methods And Apparatuses For Operating A Portable Device Based On An Accelerometer,” both of which are incorporated by reference herein in their entirety. In some embodiments, information is displayed on the touch screen display in a portrait view or a landscape view based on an analysis of data received from the one or more accelerometers. Device  200  optionally includes, in addition to accelerometer(s)  268 , a magnetometer (not shown) and a GPS (or GLONASS or other global navigation system) receiver (not shown) for obtaining information concerning the location and orientation (e.g., portrait or landscape) of device  200 . 
     In some embodiments, the software components stored in memory  202  include operating system  226 , communication module (or set of instructions)  228 , contact/motion module (or set of instructions)  230 , graphics module (or set of instructions)  232 , text input module (or set of instructions)  234 , Global Positioning System (GPS) module (or set of instructions)  235 , Digital Assistant Client Module  229 , and applications (or sets of instructions)  236 . Further, memory  202  stores data and models, such as user data and models  231 . Furthermore, in some embodiments, memory  202  ( FIG. 2A ) or  470  ( FIG. 4 ) stores device/global internal state  257 , as shown in  FIGS. 2A and 4 . Device/global internal state  257  includes one or more of: active application state, indicating which applications, if any, are currently active; display state, indicating what applications, views or other information occupy various regions of touch screen display  212 ; sensor state, including information obtained from the device&#39;s various sensors and input control devices  216 ; and location information concerning the device&#39;s location and/or attitude. 
     Operating system  226  (e.g., Darwin, RTXC, LINUX, UNIX, OS X, iOS, WINDOWS, or an embedded operating system such as VxWorks) includes various software components and/or drivers for controlling and managing general system tasks (e.g., memory management, storage device control, power management, etc.) and facilitates communication between various hardware and software components. 
     Communication module  228  facilitates communication with other devices over one or more external ports  224  and also includes various software components for handling data received by RF circuitry  208  and/or external port  224 . External port  224  (e.g., Universal Serial Bus (USB), FIREWIRE, etc.) is adapted for coupling directly to other devices or indirectly over a network (e.g., the Internet, wireless LAN, etc.). In some embodiments, the external port is a multi-pin (e.g., 30-pin) connector that is the same as, or similar to and/or compatible with, the 30-pin connector used on iPod® (trademark of Apple Inc.) devices. 
     Contact/motion module  230  optionally detects contact with touch screen  212  (in conjunction with display controller  256 ) and other touch-sensitive devices (e.g., a touchpad or physical click wheel). Contact/motion module  230  includes various software components for performing various operations related to detection of contact, such as determining if contact has occurred (e.g., detecting a finger-down event), determining an intensity of the contact (e.g., the force or pressure of the contact or a substitute for the force or pressure of the contact), determining if there is movement of the contact and tracking the movement across the touch-sensitive surface (e.g., detecting one or more finger-dragging events), and determining if the contact has ceased (e.g., detecting a finger-up event or a break in contact). Contact/motion module  230  receives contact data from the touch-sensitive surface. Determining movement of the point of contact, which is represented by a series of contact data, optionally includes determining speed (magnitude), velocity (magnitude and direction), and/or an acceleration (a change in magnitude and/or direction) of the point of contact. These operations are, optionally, applied to single contacts (e.g., one finger contacts) or to multiple simultaneous contacts (e.g., “multitouch”/multiple finger contacts). In some embodiments, contact/motion module  230  and display controller  256  detect contact on a touchpad. 
     In some embodiments, contact/motion module  230  uses a set of one or more intensity thresholds to determine whether an operation has been performed by a user (e.g., to determine whether a user has “clicked” on an icon). In some embodiments, at least a subset of the intensity thresholds are determined in accordance with software parameters (e.g., the intensity thresholds are not determined by the activation thresholds of particular physical actuators and can be adjusted without changing the physical hardware of device  200 ). For example, a mouse “click” threshold of a trackpad or touch screen display can be set to any of a large range of predefined threshold values without changing the trackpad or touch screen display hardware. Additionally, in some implementations, a user of the device is provided with software settings for adjusting one or more of the set of intensity thresholds (e.g., by adjusting individual intensity thresholds and/or by adjusting a plurality of intensity thresholds at once with a system-level click “intensity” parameter). 
     Contact/motion module  230  optionally detects a gesture input by a user. Different gestures on the touch-sensitive surface have different contact patterns (e.g., different motions, timings, and/or intensities of detected contacts). Thus, a gesture is, optionally, detected by detecting a particular contact pattern. For example, detecting a finger tap gesture includes detecting a finger-down event followed by detecting a finger-up (liftoff) event at the same position (or substantially the same position) as the finger-down event (e.g., at the position of an icon). As another example, detecting a finger swipe gesture on the touch-sensitive surface includes detecting a finger-down event followed by detecting one or more finger-dragging events, and subsequently followed by detecting a finger-up (liftoff) event. 
     Graphics module  232  includes various known software components for rendering and displaying graphics on touch screen  212  or other display, including components for changing the visual impact (e.g., brightness, transparency, saturation, contrast, or other visual property) of graphics that are displayed. As used herein, the term “graphics” includes any object that can be displayed to a user, including, without limitation, text, web pages, icons (such as user-interface objects including soft keys), digital images, videos, animations, and the like. 
     In some embodiments, graphics module  232  stores data representing graphics to be used. Each graphic is, optionally, assigned a corresponding code. Graphics module  232  receives, from applications etc., one or more codes specifying graphics to be displayed along with, if necessary, coordinate data and other graphic property data, and then generates screen image data to output to display controller  256 . 
     Haptic feedback module  233  includes various software components for generating instructions used by tactile output generator(s)  267  to produce tactile outputs at one or more locations on device  200  in response to user interactions with device  200 . 
     Text input module  234 , which is, in some examples, a component of graphics module  232 , provides soft keyboards for entering text in various applications (e.g., contacts  237 , email  240 , IM  241 , browser  247 , and any other application that needs text input). 
     GPS module  235  determines the location of the device and provides this information for use in various applications (e.g., to telephone  238  for use in location-based dialing; to camera  243  as picture/video metadata; and to applications that provide location-based services such as weather widgets, local yellow page widgets, and map/navigation widgets). 
     Digital assistant client module  229  includes various client-side digital assistant instructions to provide the client-side functionalities of the digital assistant. For example, digital assistant client module  229  is capable of accepting voice input (e.g., speech input), text input, touch input, and/or gestural input through various user interfaces (e.g., microphone  213 , accelerometer(s)  268 , touch-sensitive display system  212 , optical sensor(s)  264 , other input control devices  216 , etc.) of portable multifunction device  200 . Digital assistant client module  229  is also capable of providing output in audio (e.g., speech output), visual, and/or tactile forms through various output interfaces (e.g., speaker  211 , touch-sensitive display system  212 , tactile output generator(s)  267 , etc.) of portable multifunction device  200 . For example, output is provided as voice, sound, alerts, text messages, menus, graphics, videos, animations, vibrations, and/or combinations of two or more of the above. During operation, digital assistant client module  229  communicates with DA server  106  using RF circuitry  208 . 
     User data and models  231  include various data associated with the user (e.g., user-specific vocabulary data, user preference data, user-specified name pronunciations, data from the user&#39;s electronic address book, to-do lists, shopping lists, etc.) to provide the client-side functionalities of the digital assistant. Further, user data and models  231  include various models (e.g., speech recognition models, statistical language models, natural language processing models, ontology, task flow models, service models, etc.) for processing user input and determining user intent. 
     In some examples, digital assistant client module  229  utilizes the various sensors, subsystems, and peripheral devices of portable multifunction device  200  to gather additional information from the surrounding environment of the portable multifunction device  200  to establish a context associated with a user, the current user interaction, and/or the current user input. In some examples, digital assistant client module  229  provides the contextual information or a subset thereof with the user input to DA server  106  to help infer the user&#39;s intent. In some examples, the digital assistant also uses the contextual information to determine how to prepare and deliver outputs to the user. Contextual information is referred to as context data. 
     In some examples, the contextual information that accompanies the user input includes sensor information, e.g., lighting, ambient noise, ambient temperature, images or videos of the surrounding environment, etc. In some examples, the contextual information can also include the physical state of the device, e.g., device orientation, device location, device temperature, power level, speed, acceleration, motion patterns, cellular signals strength, etc. In some examples, information related to the software state of DA server  106 , e.g., running processes, installed programs, past and present network activities, background services, error logs, resources usage, etc., and of portable multifunction device  200  is provided to DA server  106  as contextual information associated with a user input. 
     In some examples, the digital assistant client module  229  selectively provides information (e.g., user data  231 ) stored on the portable multifunction device  200  in response to requests from DA server  106 . In some examples, digital assistant client module  229  also elicits additional input from the user via a natural language dialogue or other user interfaces upon request by DA server  106 . Digital assistant client module  229  passes the additional input to DA server  106  to help DA server  106  in intent deduction and/or fulfillment of the user&#39;s intent expressed in the user request. 
     A more detailed description of a digital assistant is described below with reference to  FIGS. 7A-7C . It should be recognized that digital assistant client module  229  can include any number of the sub-modules of digital assistant module  726  described below. 
     Applications  236  include the following modules (or sets of instructions), or a subset or superset thereof:
         Contacts module  237  (sometimes called an address book or contact list);   Telephone module  238 ;   Video conference module  239 ;   E-mail client module  240 ;   Instant messaging (IM) module  241 ;   Workout support module  242 ;   Camera module  243  for still and/or video images;   Image management module  244 ;   Video player module;   Music player module;   Browser module  247 ;   Calendar module  248 ;   Widget modules  249 , which includes, in some examples, one or more of: weather widget  249 - 1 , stocks widget  249 - 2 , calculator widget  249 - 3 , alarm clock widget  249 - 4 , dictionary widget  249 - 5 , and other widgets obtained by the user, as well as user-created widgets  249 - 6 ;   Widget creator module  250  for making user-created widgets  249 - 6 ;   Search module  251 ;   Video and music player module  252 , which merges video player module and music player module;   Notes module  253 ;   Map module  254 ; and/or   Online video module  255 .       

     Examples of other applications  236  that are stored in memory  202  include other word processing applications, other image editing applications, drawing applications, presentation applications, JAVA-enabled applications, encryption, digital rights management, voice recognition, and voice replication. 
     In conjunction with touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , and text input module  234 , contacts module  237  are used to manage an address book or contact list (e.g., stored in application internal state  292  of contacts module  237  in memory  202  or memory  470 ), including: adding name(s) to the address book; deleting name(s) from the address book; associating telephone number(s), e-mail address(es), physical address(es) or other information with a name; associating an image with a name; categorizing and sorting names; providing telephone numbers or e-mail addresses to initiate and/or facilitate communications by telephone  238 , video conference module  239 , e-mail  240 , or IM  241 ; and so forth. 
     In conjunction with RF circuitry  208 , audio circuitry  210 , speaker  211 , microphone  213 , touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , and text input module  234 , telephone module  238  are used to enter a sequence of characters corresponding to a telephone number, access one or more telephone numbers in contacts module  237 , modify a telephone number that has been entered, dial a respective telephone number, conduct a conversation, and disconnect or hang up when the conversation is completed. As noted above, the wireless communication uses any of a plurality of communications standards, protocols, and technologies. 
     In conjunction with RF circuitry  208 , audio circuitry  210 , speaker  211 , microphone  213 , touch screen  212 , display controller  256 , optical sensor  264 , optical sensor controller  258 , contact/motion module  230 , graphics module  232 , text input module  234 , contacts module  237 , and telephone module  238 , video conference module  239  includes executable instructions to initiate, conduct, and terminate a video conference between a user and one or more other participants in accordance with user instructions. 
     In conjunction with RF circuitry  208 , touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , and text input module  234 , e-mail client module  240  includes executable instructions to create, send, receive, and manage e-mail in response to user instructions. In conjunction with image management module  244 , e-mail client module  240  makes it very easy to create and send e-mails with still or video images taken with camera module  243 . 
     In conjunction with RF circuitry  208 , touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , and text input module  234 , the instant messaging module  241  includes executable instructions to enter a sequence of characters corresponding to an instant message, to modify previously entered characters, to transmit a respective instant message (for example, using a Short Message Service (SMS) or Multimedia Message Service (MMS) protocol for telephony-based instant messages or using XMPP, SIMPLE, or IMPS for Internet-based instant messages), to receive instant messages, and to view received instant messages. In some embodiments, transmitted and/or received instant messages include graphics, photos, audio files, video files and/or other attachments as are supported in an MMS and/or an Enhanced Messaging Service (EMS). As used herein, “instant messaging” refers to both telephony-based messages (e.g., messages sent using SMS or MMS) and Internet-based messages (e.g., messages sent using XMPP, SIMPLE, or IMPS). 
     In conjunction with RF circuitry  208 , touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , text input module  234 , GPS module  235 , map module  254 , and music player module, workout support module  242  includes executable instructions to create workouts (e.g., with time, distance, and/or calorie burning goals); communicate with workout sensors (sports devices); receive workout sensor data; calibrate sensors used to monitor a workout; select and play music for a workout; and display, store, and transmit workout data. 
     In conjunction with touch screen  212 , display controller  256 , optical sensor(s)  264 , optical sensor controller  258 , contact/motion module  230 , graphics module  232 , and image management module  244 , camera module  243  includes executable instructions to capture still images or video (including a video stream) and store them into memory  202 , modify characteristics of a still image or video, or delete a still image or video from memory  202 . 
     In conjunction with touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , text input module  234 , and camera module  243 , image management module  244  includes executable instructions to arrange, modify (e.g., edit), or otherwise manipulate, label, delete, present (e.g., in a digital slide show or album), and store still and/or video images. 
     In conjunction with RF circuitry  208 , touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , and text input module  234 , browser module  247  includes executable instructions to browse the Internet in accordance with user instructions, including searching, linking to, receiving, and displaying web pages or portions thereof, as well as attachments and other files linked to web pages. 
     In conjunction with RF circuitry  208 , touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , text input module  234 , e-mail client module  240 , and browser module  247 , calendar module  248  includes executable instructions to create, display, modify, and store calendars and data associated with calendars (e.g., calendar entries, to-do lists, etc.) in accordance with user instructions. 
     In conjunction with RF circuitry  208 , touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , text input module  234 , and browser module  247 , widget modules  249  are mini-applications that can be downloaded and used by a user (e.g., weather widget  249 - 1 , stocks widget  249 - 2 , calculator widget  249 - 3 , alarm clock widget  249 - 4 , and dictionary widget  249 - 5 ) or created by the user (e.g., user-created widget  249 - 6 ). In some embodiments, a widget includes an HTML (Hypertext Markup Language) file, a CSS (Cascading Style Sheets) file, and a JavaScript file. In some embodiments, a widget includes an XML (Extensible Markup Language) file and a JavaScript file (e.g., Yahoo! Widgets). 
     In conjunction with RF circuitry  208 , touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , text input module  234 , and browser module  247 , the widget creator module  250  are used by a user to create widgets (e.g., turning a user-specified portion of a web page into a widget). 
     In conjunction with touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , and text input module  234 , search module  251  includes executable instructions to search for text, music, sound, image, video, and/or other files in memory  202  that match one or more search criteria (e.g., one or more user-specified search terms) in accordance with user instructions. 
     In conjunction with touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , audio circuitry  210 , speaker  211 , RF circuitry  208 , and browser module  247 , video and music player module  252  includes executable instructions that allow the user to download and play back recorded music and other sound files stored in one or more file formats, such as MP3 or AAC files, and executable instructions to display, present, or otherwise play back videos (e.g., on touch screen  212  or on an external, connected display via external port  224 ). In some embodiments, device  200  optionally includes the functionality of an MP3 player, such as an iPod (trademark of Apple Inc.). 
     In conjunction with touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , and text input module  234 , notes module  253  includes executable instructions to create and manage notes, to-do lists, and the like in accordance with user instructions. 
     In conjunction with RF circuitry  208 , touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , text input module  234 , GPS module  235 , and browser module  247 , map module  254  are used to receive, display, modify, and store maps and data associated with maps (e.g., driving directions, data on stores and other points of interest at or near a particular location, and other location-based data) in accordance with user instructions. 
     In conjunction with touch screen  212 , display controller  256 , contact/motion module  230 , graphics module  232 , audio circuitry  210 , speaker  211 , RF circuitry  208 , text input module  234 , e-mail client module  240 , and browser module  247 , online video module  255  includes instructions that allow the user to access, browse, receive (e.g., by streaming and/or download), play back (e.g., on the touch screen or on an external, connected display via external port  224 ), send an e-mail with a link to a particular online video, and otherwise manage online videos in one or more file formats, such as H.264. In some embodiments, instant messaging module  241 , rather than e-mail client module  240 , is used to send a link to a particular online video. Additional description of the online video application can be found in U.S. Provisional Patent Application No. 60/936,562, “Portable Multifunction Device, Method, and Graphical User Interface for Playing Online Videos,” filed Jun. 20, 2007, and U.S. patent application Ser. No. 11/968,067, “Portable Multifunction Device, Method, and Graphical User Interface for Playing Online Videos,” filed Dec. 31, 2007, the contents of which are hereby incorporated by reference in their entirety. 
     Each of the above-identified modules and applications corresponds to a set of executable instructions for performing one or more functions described above and the methods described in this application (e.g., the computer-implemented methods and other information processing methods described herein). These modules (e.g., sets of instructions) need not be implemented as separate software programs, procedures, or modules, and thus various subsets of these modules can be combined or otherwise rearranged in various embodiments. For example, video player module can be combined with music player module into a single module (e.g., video and music player module  252 ,  FIG. 2A ). In some embodiments, memory  202  stores a subset of the modules and data structures identified above. Furthermore, memory  202  stores additional modules and data structures not described above. 
     In some embodiments, device  200  is a device where operation of a predefined set of functions on the device is performed exclusively through a touch screen and/or a touchpad. By using a touch screen and/or a touchpad as the primary input control device for operation of device  200 , the number of physical input control devices (such as push buttons, dials, and the like) on device  200  is reduced. 
     The predefined set of functions that are performed exclusively through a touch screen and/or a touchpad optionally include navigation between user interfaces. In some embodiments, the touchpad, when touched by the user, navigates device  200  to a main, home, or root menu from any user interface that is displayed on device  200 . In such embodiments, a “menu button” is implemented using a touchpad. In some other embodiments, the menu button is a physical push button or other physical input control device instead of a touchpad. 
       FIG. 2B  is a block diagram illustrating exemplary components for event handling in accordance with some embodiments. In some embodiments, memory  202  ( FIG. 2A ) or  470  ( FIG. 4 ) includes event sorter  270  (e.g., in operating system  226 ) and a respective application  236 - 1  (e.g., any of the aforementioned applications  237 - 251 ,  255 ,  480 - 490 ). 
     Event sorter  270  receives event information and determines the application  236 - 1  and application view  291  of application  236 - 1  to which to deliver the event information. Event sorter  270  includes event monitor  271  and event dispatcher module  274 . In some embodiments, application  236 - 1  includes application internal state  292 , which indicates the current application view(s) displayed on touch-sensitive display  212  when the application is active or executing. In some embodiments, device/global internal state  257  is used by event sorter  270  to determine which application(s) is (are) currently active, and application internal state  292  is used by event sorter  270  to determine application views  291  to which to deliver event information. 
     In some embodiments, application internal state  292  includes additional information, such as one or more of: resume information to be used when application  236 - 1  resumes execution, user interface state information that indicates information being displayed or that is ready for display by application  236 - 1 , a state queue for enabling the user to go back to a prior state or view of application  236 - 1 , and a redo/undo queue of previous actions taken by the user. 
     Event monitor  271  receives event information from peripherals interface  218 . Event information includes information about a sub-event (e.g., a user touch on touch-sensitive display  212 , as part of a multi-touch gesture). Peripherals interface  218  transmits information it receives from I/O subsystem  206  or a sensor, such as proximity sensor  266 , accelerometer(s)  268 , and/or microphone  213  (through audio circuitry  210 ). Information that peripherals interface  218  receives from I/O subsystem  206  includes information from touch-sensitive display  212  or a touch-sensitive surface. 
     In some embodiments, event monitor  271  sends requests to the peripherals interface  218  at predetermined intervals. In response, peripherals interface  218  transmits event information. In other embodiments, peripherals interface  218  transmits event information only when there is a significant event (e.g., receiving an input above a predetermined noise threshold and/or for more than a predetermined duration). 
     In some embodiments, event sorter  270  also includes a hit view determination module  272  and/or an active event recognizer determination module  273 . 
     Hit view determination module  272  provides software procedures for determining where a sub-event has taken place within one or more views when touch-sensitive display  212  displays more than one view. Views are made up of controls and other elements that a user can see on the display. 
     Another aspect of the user interface associated with an application is a set of views, sometimes herein called application views or user interface windows, in which information is displayed and touch-based gestures occur. The application views (of a respective application) in which a touch is detected correspond to programmatic levels within a programmatic or view hierarchy of the application. For example, the lowest level view in which a touch is detected is called the hit view, and the set of events that are recognized as proper inputs is determined based, at least in part, on the hit view of the initial touch that begins a touch-based gesture. 
     Hit view determination module  272  receives information related to sub events of a touch-based gesture. When an application has multiple views organized in a hierarchy, hit view determination module  272  identifies a hit view as the lowest view in the hierarchy which should handle the sub-event. In most circumstances, the hit view is the lowest level view in which an initiating sub-event occurs (e.g., the first sub-event in the sequence of sub-events that form an event or potential event). Once the hit view is identified by the hit view determination module  272 , the hit view typically receives all sub-events related to the same touch or input source for which it was identified as the hit view. 
     Active event recognizer determination module  273  determines which view or views within a view hierarchy should receive a particular sequence of sub-events. In some embodiments, active event recognizer determination module  273  determines that only the hit view should receive a particular sequence of sub-events. In other embodiments, active event recognizer determination module  273  determines that all views that include the physical location of a sub-event are actively involved views, and therefore determines that all actively involved views should receive a particular sequence of sub-events. In other embodiments, even if touch sub-events were entirely confined to the area associated with one particular view, views higher in the hierarchy would still remain as actively involved views. 
     Event dispatcher module  274  dispatches the event information to an event recognizer (e.g., event recognizer  280 ). In embodiments including active event recognizer determination module  273 , event dispatcher module  274  delivers the event information to an event recognizer determined by active event recognizer determination module  273 . In some embodiments, event dispatcher module  274  stores in an event queue the event information, which is retrieved by a respective event receiver  282 . 
     In some embodiments, operating system  226  includes event sorter  270 . Alternatively, application  236 - 1  includes event sorter  270 . In yet other embodiments, event sorter  270  is a stand-alone module, or a part of another module stored in memory  202 , such as contact/motion module  230 . 
     In some embodiments, application  236 - 1  includes a plurality of event handlers  290  and one or more application views  291 , each of which includes instructions for handling touch events that occur within a respective view of the application&#39;s user interface. Each application view  291  of the application  236 - 1  includes one or more event recognizers  280 . Typically, a respective application view  291  includes a plurality of event recognizers  280 . In other embodiments, one or more of event recognizers  280  are part of a separate module, such as a user interface kit (not shown) or a higher level object from which application  236 - 1  inherits methods and other properties. In some embodiments, a respective event handler  290  includes one or more of: data updater  276 , object updater  277 , GUI updater  278 , and/or event data  279  received from event sorter  270 . Event handler  290  utilizes or calls data updater  276 , object updater  277 , or GUI updater  278  to update the application internal state  292 . Alternatively, one or more of the application views  291  include one or more respective event handlers  290 . Also, in some embodiments, one or more of data updater  276 , object updater  277 , and GUI updater  278  are included in a respective application view  291 . 
     A respective event recognizer  280  receives event information (e.g., event data  279 ) from event sorter  270  and identifies an event from the event information. Event recognizer  280  includes event receiver  282  and event comparator  284 . In some embodiments, event recognizer  280  also includes at least a subset of: metadata  283 , and event delivery instructions  288  (which include sub-event delivery instructions). 
     Event receiver  282  receives event information from event sorter  270 . The event information includes information about a sub-event, for example, a touch or a touch movement. Depending on the sub-event, the event information also includes additional information, such as location of the sub-event. When the sub-event concerns motion of a touch, the event information also includes speed and direction of the sub-event. In some embodiments, events include rotation of the device from one orientation to another (e.g., from a portrait orientation to a landscape orientation, or vice versa), and the event information includes corresponding information about the current orientation (also called device attitude) of the device. 
     Event comparator  284  compares the event information to predefined event or sub-event definitions and, based on the comparison, determines an event or sub event, or determines or updates the state of an event or sub-event. In some embodiments, event comparator  284  includes event definitions  286 . Event definitions  286  contain definitions of events (e.g., predefined sequences of sub-events), for example, event  1  ( 287 - 1 ), event  2  ( 287 - 2 ), and others. In some embodiments, sub-events in an event ( 287 ) include, for example, touch begin, touch end, touch movement, touch cancellation, and multiple touching. In one example, the definition for event  1  ( 287 - 1 ) is a double tap on a displayed object. The double tap, for example, comprises a first touch (touch begin) on the displayed object for a predetermined phase, a first liftoff (touch end) for a predetermined phase, a second touch (touch begin) on the displayed object for a predetermined phase, and a second liftoff (touch end) for a predetermined phase. In another example, the definition for event  2  ( 287 - 2 ) is a dragging on a displayed object. The dragging, for example, comprises a touch (or contact) on the displayed object for a predetermined phase, a movement of the touch across touch-sensitive display  212 , and liftoff of the touch (touch end). In some embodiments, the event also includes information for one or more associated event handlers  290 . 
     In some embodiments, event definition  287  includes a definition of an event for a respective user-interface object. In some embodiments, event comparator  284  performs a hit test to determine which user-interface object is associated with a sub-event. For example, in an application view in which three user-interface objects are displayed on touch-sensitive display  212 , when a touch is detected on touch-sensitive display  212 , event comparator  284  performs a hit test to determine which of the three user-interface objects is associated with the touch (sub-event). If each displayed object is associated with a respective event handler  290 , the event comparator uses the result of the hit test to determine which event handler  290  should be activated. For example, event comparator  284  selects an event handler associated with the sub-event and the object triggering the hit test. 
     In some embodiments, the definition for a respective event ( 287 ) also includes delayed actions that delay delivery of the event information until after it has been determined whether the sequence of sub-events does or does not correspond to the event recognizer&#39;s event type. 
     When a respective event recognizer  280  determines that the series of sub-events do not match any of the events in event definitions  286 , the respective event recognizer  280  enters an event impossible, event failed, or event ended state, after which it disregards subsequent sub-events of the touch-based gesture. In this situation, other event recognizers, if any, that remain active for the hit view continue to track and process sub-events of an ongoing touch-based gesture. 
     In some embodiments, a respective event recognizer  280  includes metadata  283  with configurable properties, flags, and/or lists that indicate how the event delivery system should perform sub-event delivery to actively involved event recognizers. In some embodiments, metadata  283  includes configurable properties, flags, and/or lists that indicate how event recognizers interact, or are enabled to interact, with one another. In some embodiments, metadata  283  includes configurable properties, flags, and/or lists that indicate whether sub-events are delivered to varying levels in the view or programmatic hierarchy. 
     In some embodiments, a respective event recognizer  280  activates event handler  290  associated with an event when one or more particular sub-events of an event are recognized. In some embodiments, a respective event recognizer  280  delivers event information associated with the event to event handler  290 . Activating an event handler  290  is distinct from sending (and deferred sending) sub-events to a respective hit view. In some embodiments, event recognizer  280  throws a flag associated with the recognized event, and event handler  290  associated with the flag catches the flag and performs a predefined process. 
     In some embodiments, event delivery instructions  288  include sub-event delivery instructions that deliver event information about a sub-event without activating an event handler. Instead, the sub-event delivery instructions deliver event information to event handlers associated with the series of sub-events or to actively involved views. Event handlers associated with the series of sub-events or with actively involved views receive the event information and perform a predetermined process. 
     In some embodiments, data updater  276  creates and updates data used in application  236 - 1 . For example, data updater  276  updates the telephone number used in contacts module  237 , or stores a video file used in video player module. In some embodiments, object updater  277  creates and updates objects used in application  236 - 1 . For example, object updater  277  creates a new user-interface object or updates the position of a user-interface object. GUI updater  278  updates the GUI. For example, GUI updater  278  prepares display information and sends it to graphics module  232  for display on a touch-sensitive display. 
     In some embodiments, event handler(s)  290  includes or has access to data updater  276 , object updater  277 , and GUI updater  278 . In some embodiments, data updater  276 , object updater  277 , and GUI updater  278  are included in a single module of a respective application  236 - 1  or application view  291 . In other embodiments, they are included in two or more software modules. 
     It shall be understood that the foregoing discussion regarding event handling of user touches on touch-sensitive displays also applies to other forms of user inputs to operate multifunction devices  200  with input devices, not all of which are initiated on touch screens. For example, mouse movement and mouse button presses, optionally coordinated with single or multiple keyboard presses or holds; contact movements such as taps, drags, scrolls, etc. on touchpads; pen stylus inputs; movement of the device; oral instructions; detected eye movements; biometric inputs; and/or any combination thereof are optionally utilized as inputs corresponding to sub-events which define an event to be recognized. 
       FIG. 3  illustrates a portable multifunction device  200  having a touch screen  212  in accordance with some embodiments. The touch screen optionally displays one or more graphics within user interface (UI)  300 . In this embodiment, as well as others described below, a user is enabled to select one or more of the graphics by making a gesture on the graphics, for example, with one or more fingers  302  (not drawn to scale in the figure) or one or more styluses  303  (not drawn to scale in the figure). In some embodiments, selection of one or more graphics occurs when the user breaks contact with the one or more graphics. In some embodiments, the gesture optionally includes one or more taps, one or more swipes (from left to right, right to left, upward and/or downward), and/or a rolling of a finger (from right to left, left to right, upward and/or downward) that has made contact with device  200 . In some implementations or circumstances, inadvertent contact with a graphic does not select the graphic. For example, a swipe gesture that sweeps over an application icon optionally does not select the corresponding application when the gesture corresponding to selection is a tap. 
     Device  200  also includes one or more physical buttons, such as “home” or menu button  304 . As described previously, menu button  304  is used to navigate to any application  236  in a set of applications that is executed on device  200 . Alternatively, in some embodiments, the menu button is implemented as a soft key in a GUI displayed on touch screen  212 . 
     In one embodiment, device  200  includes touch screen  212 , menu button  304 , push button  306  for powering the device on/off and locking the device, volume adjustment button(s)  308 , subscriber identity module (SIM) card slot  310 , headset jack  312 , and docking/charging external port  224 . Push button  306  is, optionally, used to turn the power on/off on the device by depressing the button and holding the button in the depressed state for a predefined time interval; to lock the device by depressing the button and releasing the button before the predefined time interval has elapsed; and/or to unlock the device or initiate an unlock process. In an alternative embodiment, device  200  also accepts verbal input for activation or deactivation of some functions through microphone  213 . Device  200  also, optionally, includes one or more contact intensity sensors  265  for detecting intensity of contacts on touch screen  212  and/or one or more tactile output generators  267  for generating tactile outputs for a user of device  200 . 
       FIG. 4  is a block diagram of an exemplary multifunction device with a display and a touch-sensitive surface in accordance with some embodiments. Device  400  need not be portable. In some embodiments, device  400  is a laptop computer, a desktop computer, a tablet computer, a multimedia player device, a navigation device, an educational device (such as a child&#39;s learning toy), a gaming system, or a control device (e.g., a home or industrial controller). Device  400  typically includes one or more processing units (CPUs)  410 , one or more network or other communications interfaces  460 , memory  470 , and one or more communication buses  420  for interconnecting these components. Communication buses  420  optionally include circuitry (sometimes called a chipset) that interconnects and controls communications between system components. Device  400  includes input/output (I/O) interface  430  comprising display  440 , which is typically a touch screen display. I/O interface  430  also optionally includes a keyboard and/or mouse (or other pointing device)  450  and touchpad  455 , tactile output generator  457  for generating tactile outputs on device  400  (e.g., similar to tactile output generator(s)  267  described above with reference to  FIG. 2A ), sensors  459  (e.g., optical, acceleration, proximity, touch-sensitive, and/or contact intensity sensors similar to contact intensity sensor(s)  265  described above with reference to  FIG. 2A ). Memory  470  includes high-speed random access memory, such as DRAM, SRAM, DDR RAM, or other random access solid state memory devices; and optionally includes non-volatile memory, such as one or more magnetic disk storage devices, optical disk storage devices, flash memory devices, or other non-volatile solid state storage devices. Memory  470  optionally includes one or more storage devices remotely located from CPU(s)  410 . In some embodiments, memory  470  stores programs, modules, and data structures analogous to the programs, modules, and data structures stored in memory  202  of portable multifunction device  200  ( FIG. 2A ), or a subset thereof. Furthermore, memory  470  optionally stores additional programs, modules, and data structures not present in memory  202  of portable multifunction device  200 . For example, memory  470  of device  400  optionally stores drawing module  480 , presentation module  482 , word processing module  484 , website creation module  486 , disk authoring module  488 , and/or spreadsheet module  490 , while memory  202  of portable multifunction device  200  ( FIG. 2A ) optionally does not store these modules. 
     Each of the above-identified elements in  FIG. 4  is, in some examples, stored in one or more of the previously mentioned memory devices. Each of the above-identified modules corresponds to a set of instructions for performing a function described above. The above-identified modules or programs (e.g., sets of instructions) need not be implemented as separate software programs, procedures, or modules, and thus various subsets of these modules are combined or otherwise rearranged in various embodiments. In some embodiments, memory  470  stores a subset of the modules and data structures identified above. Furthermore, memory  470  stores additional modules and data structures not described above. 
     Attention is now directed towards embodiments of user interfaces that can be implemented on, for example, portable multifunction device  200 . 
       FIG. 5A  illustrates an exemplary user interface for a menu of applications on portable multifunction device  200  in accordance with some embodiments. Similar user interfaces are implemented on device  400 . In some embodiments, user interface  500  includes the following elements, or a subset or superset thereof: 
     Signal strength indicator(s)  502  for wireless communication(s), such as cellular and Wi-Fi signals;
         Time  504 ;   Bluetooth indicator  505 ;   Battery status indicator  506 ;   Tray  508  with icons for frequently used applications, such as:
           Icon  516  for telephone module  238 , labeled “Phone,” which optionally includes an indicator  514  of the number of missed calls or voicemail messages;   Icon  518  for e-mail client module  240 , labeled “Mail,” which optionally includes an indicator  510  of the number of unread e-mails;   Icon  520  for browser module  247 , labeled “Browser;” and   Icon  522  for video and music player module  252 , also referred to as iPod (trademark of Apple Inc.) module  252 , labeled “iPod;” and   
           Icons for other applications, such as:
           Icon  524  for IM module  241 , labeled “Messages;”   Icon  526  for calendar module  248 , labeled “Calendar;”   Icon  528  for image management module  244 , labeled “Photos;”   Icon  530  for camera module  243 , labeled “Camera;”   Icon  532  for online video module  255 , labeled “Online Video;”   Icon  534  for stocks widget  249 - 2 , labeled “Stocks;”   Icon  536  for map module  254 , labeled “Maps;”   Icon  538  for weather widget  249 - 1 , labeled “Weather;”   Icon  540  for alarm clock widget  249 - 4 , labeled “Clock;”   Icon  542  for workout support module  242 , labeled “Workout Support;”   Icon  544  for notes module  253 , labeled “Notes;” and   Icon  546  for a settings application or module, labeled “Settings,” which provides access to settings for device  200  and its various applications  236 .   
               

     It should be noted that the icon labels illustrated in  FIG. 5A  are merely exemplary. For example, icon  522  for video and music player module  252  is optionally labeled “Music” or “Music Player.” Other labels are, optionally, used for various application icons. In some embodiments, a label for a respective application icon includes a name of an application corresponding to the respective application icon. In some embodiments, a label for a particular application icon is distinct from a name of an application corresponding to the particular application icon. 
       FIG. 5B  illustrates an exemplary user interface on a device (e.g., device  400 ,  FIG. 4 ) with a touch-sensitive surface  551  (e.g., a tablet or touchpad  455 ,  FIG. 4 ) that is separate from the display  550  (e.g., touch screen display  212 ). Device  400  also, optionally, includes one or more contact intensity sensors (e.g., one or more of sensors  457 ) for detecting intensity of contacts on touch-sensitive surface  551  and/or one or more tactile output generators  459  for generating tactile outputs for a user of device  400 . 
     Although some of the examples which follow will be given with reference to inputs on touch screen display  212  (where the touch-sensitive surface and the display are combined), in some embodiments, the device detects inputs on a touch-sensitive surface that is separate from the display, as shown in  FIG. 5B . In some embodiments, the touch-sensitive surface (e.g.,  551  in  FIG. 5B ) has a primary axis (e.g.,  552  in  FIG. 5B ) that corresponds to a primary axis (e.g.,  553  in  FIG. 5B ) on the display (e.g.,  550 ). In accordance with these embodiments, the device detects contacts (e.g.,  560  and  562  in  FIG. 5B ) with the touch-sensitive surface  551  at locations that correspond to respective locations on the display (e.g., in  FIG. 5B, 560  corresponds to  568  and  562  corresponds to  570 ). In this way, user inputs (e.g., contacts  560  and  562 , and movements thereof) detected by the device on the touch-sensitive surface (e.g.,  551  in  FIG. 5B ) are used by the device to manipulate the user interface on the display (e.g.,  550  in  FIG. 5B ) of the multifunction device when the touch-sensitive surface is separate from the display. It should be understood that similar methods are, optionally, used for other user interfaces described herein. 
     Additionally, while the following examples are given primarily with reference to finger inputs (e.g., finger contacts, finger tap gestures, finger swipe gestures), it should be understood that, in some embodiments, one or more of the finger inputs are replaced with input from another input device (e.g., a mouse-based input or stylus input). For example, a swipe gesture is, optionally, replaced with a mouse click (e.g., instead of a contact) followed by movement of the cursor along the path of the swipe (e.g., instead of movement of the contact). As another example, a tap gesture is, optionally, replaced with a mouse click while the cursor is located over the location of the tap gesture (e.g., instead of detection of the contact followed by ceasing to detect the contact). Similarly, when multiple user inputs are simultaneously detected, it should be understood that multiple computer mice are, optionally, used simultaneously, or a mouse and finger contacts are, optionally, used simultaneously. 
       FIG. 6A  illustrates exemplary personal electronic device  600 . Device  600  includes body  602 . In some embodiments, device  600  includes some or all of the features described with respect to devices  200  and  400  (e.g.,  FIGS. 2A-4 ). In some embodiments, device  600  has touch-sensitive display screen  604 , hereafter touch screen  604 . Alternatively, or in addition to touch screen  604 , device  600  has a display and a touch-sensitive surface. As with devices  200  and  400 , in some embodiments, touch screen  604  (or the touch-sensitive surface) has one or more intensity sensors for detecting intensity of contacts (e.g., touches) being applied. The one or more intensity sensors of touch screen  604  (or the touch-sensitive surface) provide output data that represents the intensity of touches. The user interface of device  600  responds to touches based on their intensity, meaning that touches of different intensities can invoke different user interface operations on device  600 . 
     Techniques for detecting and processing touch intensity are found, for example, in related applications: International Patent Application Serial No. PCT/US2013/040061, titled “Device, Method, and Graphical User Interface for Displaying User Interface Objects Corresponding to an Application,” filed May 8, 2013, and International Patent Application Serial No. PCT/US2013/069483, titled “Device, Method, and Graphical User Interface for Transitioning Between Touch Input to Display Output Relationships,” filed Nov. 11, 2013, each of which is hereby incorporated by reference in their entirety. 
     In some embodiments, device  600  has one or more input mechanisms  606  and  608 . Input mechanisms  606  and  608 , if included, are physical. Examples of physical input mechanisms include push buttons and rotatable mechanisms. In some embodiments, device  600  has one or more attachment mechanisms. Such attachment mechanisms, if included, can permit attachment of device  600  with, for example, hats, eyewear, earrings, necklaces, shirts, jackets, bracelets, watch straps, chains, trousers, belts, shoes, purses, backpacks, and so forth. These attachment mechanisms permit device  600  to be worn by a user. 
       FIG. 6B  depicts exemplary personal electronic device  600 . In some embodiments, device  600  includes some or all of the components described with respect to  FIGS. 2A, 2B , and 4. Device  600  has bus  612  that operatively couples I/O section  614  with one or more computer processors  616  and memory  618 . I/O section  614  is connected to display  604 , which can have touch-sensitive component  622  and, optionally, touch-intensity sensitive component  624 . In addition, I/O section  614  is connected with communication unit  630  for receiving application and operating system data, using Wi-Fi, Bluetooth, near field communication (NFC), cellular, and/or other wireless communication techniques. Device  600  includes input mechanisms  606  and/or  608 . Input mechanism  606  is a rotatable input device or a depressible and rotatable input device, for example. Input mechanism  608  is a button, in some examples. 
     Input mechanism  608  is a microphone, in some examples. Personal electronic device  600  includes, for example, various sensors, such as GPS sensor  632 , accelerometer  634 , directional sensor  640  (e.g., compass), gyroscope  636 , motion sensor  638 , and/or a combination thereof, all of which are operatively connected to I/O section  614 . 
     Memory  618  of personal electronic device  600  is a non-transitory computer-readable storage medium, for storing computer-executable instructions, which, when executed by one or more computer processors  616 , for example, cause the computer processors to perform the techniques and processes described below. The computer-executable instructions, for example, are also stored and/or transported within any non-transitory computer-readable storage medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. Personal electronic device  600  is not limited to the components and configuration of  FIG. 6B , but can include other or additional components in multiple configurations. 
     As used here, the term “affordance” refers to a user-interactive graphical user interface object that is, for example, displayed on the display screen of devices  104 ,  200 ,  400 , and/or  600  ( FIGS. 1, 2, 4, and 6 ). For example, an image (e.g., icon), a button, and text (e.g., hyperlink) each constitutes an affordance. 
     As used herein, the term “focus selector” refers to an input element that indicates a current part of a user interface with which a user is interacting. In some implementations that include a cursor or other location marker, the cursor acts as a “focus selector” so that when an input (e.g., a press input) is detected on a touch-sensitive surface (e.g., touchpad  455  in  FIG. 4  or touch-sensitive surface  551  in  FIG. 5B ) while the cursor is over a particular user interface element (e.g., a button, window, slider or other user interface element), the particular user interface element is adjusted in accordance with the detected input. In some implementations that include a touch screen display (e.g., touch-sensitive display system  212  in  FIG. 2A  or touch screen  212  in  FIG. 5A ) that enables direct interaction with user interface elements on the touch screen display, a detected contact on the touch screen acts as a “focus selector” so that when an input (e.g., a press input by the contact) is detected on the touch screen display at a location of a particular user interface element (e.g., a button, window, slider, or other user interface element), the particular user interface element is adjusted in accordance with the detected input. In some implementations, focus is moved from one region of a user interface to another region of the user interface without corresponding movement of a cursor or movement of a contact on a touch screen display (e.g., by using a tab key or arrow keys to move focus from one button to another button); in these implementations, the focus selector moves in accordance with movement of focus between different regions of the user interface. Without regard to the specific form taken by the focus selector, the focus selector is generally the user interface element (or contact on a touch screen display) that is controlled by the user so as to communicate the user&#39;s intended interaction with the user interface (e.g., by indicating, to the device, the element of the user interface with which the user is intending to interact). For example, the location of a focus selector (e.g., a cursor, a contact, or a selection box) over a respective button while a press input is detected on the touch-sensitive surface (e.g., a touchpad or touch screen) will indicate that the user is intending to activate the respective button (as opposed to other user interface elements shown on a display of the device). 
     As used in the specification and claims, the term “characteristic intensity” of a contact refers to a characteristic of the contact based on one or more intensities of the contact. In some embodiments, the characteristic intensity is based on multiple intensity samples. The characteristic intensity is, optionally, based on a predefined number of intensity samples, or a set of intensity samples collected during a predetermined time period (e.g., 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10 seconds) relative to a predefined event (e.g., after detecting the contact, prior to detecting liftoff of the contact, before or after detecting a start of movement of the contact, prior to detecting an end of the contact, before or after detecting an increase in intensity of the contact, and/or before or after detecting a decrease in intensity of the contact). A characteristic intensity of a contact is, optionally based on one or more of: a maximum value of the intensities of the contact, a mean value of the intensities of the contact, an average value of the intensities of the contact, a top 10 percentile value of the intensities of the contact, a value at the half maximum of the intensities of the contact, a value at the 90 percent maximum of the intensities of the contact, or the like. In some embodiments, the duration of the contact is used in determining the characteristic intensity (e.g., when the characteristic intensity is an average of the intensity of the contact over time). In some embodiments, the characteristic intensity is compared to a set of one or more intensity thresholds to determine whether an operation has been performed by a user. For example, the set of one or more intensity thresholds includes a first intensity threshold and a second intensity threshold. In this example, a contact with a characteristic intensity that does not exceed the first threshold results in a first operation, a contact with a characteristic intensity that exceeds the first intensity threshold and does not exceed the second intensity threshold results in a second operation, and a contact with a characteristic intensity that exceeds the second threshold results in a third operation. In some embodiments, a comparison between the characteristic intensity and one or more thresholds is used to determine whether or not to perform one or more operations (e.g., whether to perform a respective operation or forgo performing the respective operation) rather than being used to determine whether to perform a first operation or a second operation. 
     In some embodiments, a portion of a gesture is identified for purposes of determining a characteristic intensity. For example, a touch-sensitive surface receives a continuous swipe contact transitioning from a start location and reaching an end location, at which point the intensity of the contact increases. In this example, the characteristic intensity of the contact at the end location is based on only a portion of the continuous swipe contact, and not the entire swipe contact (e.g., only the portion of the swipe contact at the end location). In some embodiments, a smoothing algorithm is applied to the intensities of the swipe contact prior to determining the characteristic intensity of the contact. For example, the smoothing algorithm optionally includes one or more of: an unweighted sliding-average smoothing algorithm, a triangular smoothing algorithm, a median filter smoothing algorithm, and/or an exponential smoothing algorithm. In some circumstances, these smoothing algorithms eliminate narrow spikes or dips in the intensities of the swipe contact for purposes of determining a characteristic intensity. 
     The intensity of a contact on the touch-sensitive surface is characterized relative to one or more intensity thresholds, such as a contact-detection intensity threshold, a light press intensity threshold, a deep press intensity threshold, and/or one or more other intensity thresholds. In some embodiments, the light press intensity threshold corresponds to an intensity at which the device will perform operations typically associated with clicking a button of a physical mouse or a trackpad. In some embodiments, the deep press intensity threshold corresponds to an intensity at which the device will perform operations that are different from operations typically associated with clicking a button of a physical mouse or a trackpad. In some embodiments, when a contact is detected with a characteristic intensity below the light press intensity threshold (e.g., and above a nominal contact-detection intensity threshold below which the contact is no longer detected), the device will move a focus selector in accordance with movement of the contact on the touch-sensitive surface without performing an operation associated with the light press intensity threshold or the deep press intensity threshold. Generally, unless otherwise stated, these intensity thresholds are consistent between different sets of user interface figures. 
     An increase of characteristic intensity of the contact from an intensity below the light press intensity threshold to an intensity between the light press intensity threshold and the deep press intensity threshold is sometimes referred to as a “light press” input. An increase of characteristic intensity of the contact from an intensity below the deep press intensity threshold to an intensity above the deep press intensity threshold is sometimes referred to as a “deep press” input. An increase of characteristic intensity of the contact from an intensity below the contact-detection intensity threshold to an intensity between the contact-detection intensity threshold and the light press intensity threshold is sometimes referred to as detecting the contact on the touch-surface. A decrease of characteristic intensity of the contact from an intensity above the contact-detection intensity threshold to an intensity below the contact-detection intensity threshold is sometimes referred to as detecting liftoff of the contact from the touch-surface. In some embodiments, the contact-detection intensity threshold is zero. In some embodiments, the contact-detection intensity threshold is greater than zero. 
     In some embodiments described herein, one or more operations are performed in response to detecting a gesture that includes a respective press input or in response to detecting the respective press input performed with a respective contact (or a plurality of contacts), where the respective press input is detected based at least in part on detecting an increase in intensity of the contact (or plurality of contacts) above a press-input intensity threshold. In some embodiments, the respective operation is performed in response to detecting the increase in intensity of the respective contact above the press-input intensity threshold (e.g., a “down stroke” of the respective press input). In some embodiments, the press input includes an increase in intensity of the respective contact above the press-input intensity threshold and a subsequent decrease in intensity of the contact below the press-input intensity threshold, and the respective operation is performed in response to detecting the subsequent decrease in intensity of the respective contact below the press-input threshold (e.g., an “up stroke” of the respective press input). 
     In some embodiments, the device employs intensity hysteresis to avoid accidental inputs sometimes termed “jitter,” where the device defines or selects a hysteresis intensity threshold with a predefined relationship to the press-input intensity threshold (e.g., the hysteresis intensity threshold is X intensity units lower than the press-input intensity threshold or the hysteresis intensity threshold is 75%, 90%, or some reasonable proportion of the press-input intensity threshold). Thus, in some embodiments, the press input includes an increase in intensity of the respective contact above the press-input intensity threshold and a subsequent decrease in intensity of the contact below the hysteresis intensity threshold that corresponds to the press-input intensity threshold, and the respective operation is performed in response to detecting the subsequent decrease in intensity of the respective contact below the hysteresis intensity threshold (e.g., an “up stroke” of the respective press input). Similarly, in some embodiments, the press input is detected only when the device detects an increase in intensity of the contact from an intensity at or below the hysteresis intensity threshold to an intensity at or above the press-input intensity threshold and, optionally, a subsequent decrease in intensity of the contact to an intensity at or below the hysteresis intensity, and the respective operation is performed in response to detecting the press input (e.g., the increase in intensity of the contact or the decrease in intensity of the contact, depending on the circumstances). 
     For ease of explanation, the descriptions of operations performed in response to a press input associated with a press-input intensity threshold or in response to a gesture including the press input are, optionally, triggered in response to detecting either: an increase in intensity of a contact above the press-input intensity threshold, an increase in intensity of a contact from an intensity below the hysteresis intensity threshold to an intensity above the press-input intensity threshold, a decrease in intensity of the contact below the press-input intensity threshold, and/or a decrease in intensity of the contact below the hysteresis intensity threshold corresponding to the press-input intensity threshold. Additionally, in examples where an operation is described as being performed in response to detecting a decrease in intensity of a contact below the press-input intensity threshold, the operation is, optionally, performed in response to detecting a decrease in intensity of the contact below a hysteresis intensity threshold corresponding to, and lower than, the press-input intensity threshold. 
     3. Digital Assistant System 
       FIG. 7A  illustrates a block diagram of digital assistant system  700  in accordance with various examples. In some examples, digital assistant system  700  is implemented on a standalone computer system. In some examples, digital assistant system  700  is distributed across multiple computers. In some examples, some of the modules and functions of the digital assistant are divided into a server portion and a client portion, where the client portion resides on one or more user devices (e.g., devices  104 ,  122 ,  200 ,  400 , or  600 ) and communicates with the server portion (e.g., server system  108 ) through one or more networks, e.g., as shown in  FIG. 1 . In some examples, digital assistant system  700  is an implementation of server system  108  (and/or DA server  106 ) shown in  FIG. 1 . It should be noted that digital assistant system  700  is only one example of a digital assistant system, and that digital assistant system  700  can have more or fewer components than shown, can combine two or more components, or can have a different configuration or arrangement of the components. The various components shown in  FIG. 7A  are implemented in hardware, software instructions for execution by one or more processors, firmware, including one or more signal processing and/or application specific integrated circuits, or a combination thereof. 
     Digital assistant system  700  includes memory  702 , one or more processors  704 , input/output (I/O) interface  706 , and network communications interface  708 . These components can communicate with one another over one or more communication buses or signal lines  710 . 
     In some examples, memory  702  includes a non-transitory computer-readable medium, such as high-speed random access memory and/or a non-volatile computer-readable storage medium (e.g., one or more magnetic disk storage devices, flash memory devices, or other non-volatile solid-state memory devices). 
     In some examples, I/O interface  706  couples input/output devices  716  of digital assistant system  700 , such as displays, keyboards, touch screens, and microphones, to user interface module  722 . I/O interface  706 , in conjunction with user interface module  722 , receives user inputs (e.g., voice input, keyboard inputs, touch inputs, etc.) and processes them accordingly. In some examples, e.g., when the digital assistant is implemented on a standalone user device, digital assistant system  700  includes any of the components and I/O communication interfaces described with respect to devices  200 ,  400 , or  600  in  FIGS. 2A, 4, 6A -B, respectively. In some examples, digital assistant system  700  represents the server portion of a digital assistant implementation, and can interact with the user through a client-side portion residing on a user device (e.g., devices  104 ,  200 ,  400 , or  600 ). 
     In some examples, the network communications interface  708  includes wired communication port(s)  712  and/or wireless transmission and reception circuitry  714 . The wired communication port(s) receives and send communication signals via one or more wired interfaces, e.g., Ethernet, Universal Serial Bus (USB), FIREWIRE, etc. The wireless circuitry  714  receives and sends RF signals and/or optical signals from/to communications networks and other communications devices. The wireless communications use any of a plurality of communications standards, protocols, and technologies, such as GSM, EDGE, CDMA, TDMA, Bluetooth, Wi-Fi, VoIP, Wi-MAX, or any other suitable communication protocol. Network communications interface  708  enables communication between digital assistant system  700  with networks, such as the Internet, an intranet, and/or a wireless network, such as a cellular telephone network, a wireless local area network (LAN), and/or a metropolitan area network (MAN), and other devices. 
     In some examples, memory  702 , or the computer-readable storage media of memory  702 , stores programs, modules, instructions, and data structures including all or a subset of: operating system  718 , communications module  720 , user interface module  722 , one or more applications  724 , and digital assistant module  726 . In particular, memory  702 , or the computer-readable storage media of memory  702 , stores instructions for performing the processes described below. One or more processors  704  execute these programs, modules, and instructions, and reads/writes from/to the data structures. 
     Operating system  718  (e.g., Darwin, RTXC, LINUX, UNIX, iOS, OS X, WINDOWS, or an embedded operating system such as VxWorks) includes various software components and/or drivers for controlling and managing general system tasks (e.g., memory management, storage device control, power management, etc.) and facilitates communications between various hardware, firmware, and software components. 
     Communications module  720  facilitates communications between digital assistant system  700  with other devices over network communications interface  708 . For example, communications module  720  communicates with RF circuitry  208  of electronic devices such as devices  200 ,  400 , and  600  shown in  FIG. 2A, 4, 6A -B, respectively. Communications module  720  also includes various components for handling data received by wireless circuitry  714  and/or wired communications port  712 . 
     User interface module  722  receives commands and/or inputs from a user via I/O interface  706  (e.g., from a keyboard, touch screen, pointing device, controller, and/or microphone), and generate user interface objects on a display. User interface module  722  also prepares and delivers outputs (e.g., speech, sound, animation, text, icons, vibrations, haptic feedback, light, etc.) to the user via the I/O interface  706  (e.g., through displays, audio channels, speakers, touch-pads, etc.). 
     Applications  724  include programs and/or modules that are configured to be executed by one or more processors  704 . For example, if the digital assistant system is implemented on a standalone user device, applications  724  include user applications, such as games, a calendar application, a navigation application, or an email application. If digital assistant system  700  is implemented on a server, applications  724  include resource management applications, diagnostic applications, or scheduling applications, for example. 
     Memory  702  also stores digital assistant module  726  (or the server portion of a digital assistant). In some examples, digital assistant module  726  includes the following sub-modules, or a subset or superset thereof: input/output processing module  728 , speech-to-text (STT) processing module  730 , natural language processing module  732 , dialogue flow processing module  734 , task flow processing module  736 , service processing module  738 , and speech synthesis processing module  740 . Each of these modules has access to one or more of the following systems or data and models of the digital assistant module  726 , or a subset or superset thereof: ontology  760 , vocabulary index  744 , user data  748 , task flow models  754 , service models  756 , and ASR systems  758 . 
     In some examples, using the processing modules, data, and models implemented in digital assistant module  726 , the digital assistant can perform at least some of the following: converting speech input into text; identifying a user&#39;s intent expressed in a natural language input received from the user; actively eliciting and obtaining information needed to fully infer the user&#39;s intent (e.g., by disambiguating words, games, intentions, etc.); determining the task flow for fulfilling the inferred intent; and executing the task flow to fulfill the inferred intent. 
     In some examples, as shown in  FIG. 7B , I/O processing module  728  interacts with the user through I/O devices  716  in  FIG. 7A  or with a user device (e.g., devices  104 ,  200 ,  400 , or  600 ) through network communications interface  708  in  FIG. 7A  to obtain user input (e.g., a speech input) and to provide responses (e.g., as speech outputs) to the user input. I/O processing module  728  optionally obtains contextual information associated with the user input from the user device, along with or shortly after the receipt of the user input. The contextual information includes user-specific data, vocabulary, and/or preferences relevant to the user input. In some examples, the contextual information also includes software and hardware states of the user device at the time the user request is received, and/or information related to the surrounding environment of the user at the time that the user request was received. In some examples, I/O processing module  728  also sends follow-up questions to, and receive answers from, the user regarding the user request. When a user request is received by I/O processing module  728  and the user request includes speech input, I/O processing module  728  forwards the speech input to STT processing module  730  (or speech recognizer) for speech-to-text conversions. 
     STT processing module  730  includes one or more ASR systems  758 . The one or more ASR systems  758  can process the speech input that is received through I/O processing module  728  to produce a recognition result. Each ASR system  758  includes a front-end speech pre-processor. The front-end speech pre-processor extracts representative features from the speech input. For example, the front-end speech pre-processor performs a Fourier transform on the speech input to extract spectral features that characterize the speech input as a sequence of representative multi-dimensional vectors. Further, each ASR system  758  includes one or more speech recognition models (e.g., acoustic models and/or language models) and implements one or more speech recognition engines. Examples of speech recognition models include Hidden Markov Models, Gaussian-Mixture Models, Deep Neural Network Models, n-gram language models, and other statistical models. Examples of speech recognition engines include the dynamic time warping based engines and weighted finite-state transducers (WFST) based engines. The one or more speech recognition models and the one or more speech recognition engines are used to process the extracted representative features of the front-end speech pre-processor to produce intermediate recognitions results (e.g., phonemes, phonemic strings, and sub-words), and ultimately, text recognition results (e.g., words, word strings, or sequence of tokens). In some examples, the speech input is processed at least partially by a third-party service or on the user&#39;s device (e.g., device  104 ,  200 ,  400 , or  600 ) to produce the recognition result. Once STT processing module  730  produces recognition results containing a text string (e.g., words, or sequence of words, or sequence of tokens), the recognition result is passed to natural language processing module  732  for intent deduction. In some examples, STT processing module  730  produces multiple candidate text representations of the speech input. Each candidate text representation is a sequence of words or tokens corresponding to the speech input. In some examples, each candidate text representation is associated with a speech recognition confidence score. Based on the speech recognition confidence scores, STT processing module  730  ranks the candidate text representations and provides the n-best (e.g., n highest ranked) candidate text representation(s) to natural language processing module  732  for intent deduction, where n is a predetermined integer greater than zero. For example, in one example, only the highest ranked (n=1) candidate text representation is passed to natural language processing module  732  for intent deduction. In another example, the five highest ranked (n=5) candidate text representations are passed to natural language processing module  732  for intent deduction. 
     More details on the speech-to-text processing are described in U.S. Utility application Ser. No. 13/236,942 for “Consolidating Speech Recognition Results,” filed on Sep. 20, 2011, the entire disclosure of which is incorporated herein by reference. 
     In some examples, STT processing module  730  includes and/or accesses a vocabulary of recognizable words via phonetic alphabet conversion module  731 . Each vocabulary word is associated with one or more candidate pronunciations of the word represented in a speech recognition phonetic alphabet. In particular, the vocabulary of recognizable words includes a word that is associated with a plurality of candidate pronunciations. For example, the vocabulary includes the word “tomato” that is associated with the candidate pronunciations of / / and / /. Further, vocabulary words are associated with custom candidate pronunciations that are based on previous speech inputs from the user. Such custom candidate pronunciations are stored in STT processing module  730  and are associated with a particular user via the user&#39;s profile on the device. In some examples, the candidate pronunciations for words are determined based on the spelling of the word and one or more linguistic and/or phonetic rules. In some examples, the candidate pronunciations are manually generated, e.g., based on known canonical pronunciations. 
     In some examples, the candidate pronunciations are ranked based on the commonness of the candidate pronunciation. For example, the candidate pronunciation / / is ranked higher than / /, because the former is a more commonly used pronunciation (e.g., among all users, for users in a particular geographical region, or for any other appropriate subset of users). In some examples, candidate pronunciations are ranked based on whether the candidate pronunciation is a custom candidate pronunciation associated with the user. For example, custom candidate pronunciations are ranked higher than canonical candidate pronunciations. This can be useful for recognizing proper nouns having a unique pronunciation that deviates from canonical pronunciation. In some examples, candidate pronunciations are associated with one or more speech characteristics, such as geographic origin, nationality, or ethnicity. For example, the candidate pronunciation / / is associated with the United States, whereas the candidate pronunciation / / is associated with Great Britain. Further, the rank of the candidate pronunciation is based on one or more characteristics (e.g., geographic origin, nationality, ethnicity, etc.) of the user stored in the user&#39;s profile on the device. For example, it can be determined from the user&#39;s profile that the user is associated with the United States. Based on the user being associated with the United States, the candidate pronunciation / / (associated with the United States) is ranked higher than the candidate pronunciation / / (associated with Great Britain). In some examples, one of the ranked candidate pronunciations is selected as a predicted pronunciation (e.g., the most likely pronunciation). 
     When a speech input is received, STT processing module  730  is used to determine the phonemes corresponding to the speech input (e.g., using an acoustic model), and then attempt to determine words that match the phonemes (e.g., using a language model). For example, if STT processing module  730  first identifies the sequence of phonemes / / corresponding to a portion of the speech input, it can then determine, based on vocabulary index  744 , that this sequence corresponds to the word “tomato.” 
     In some examples, STT processing module  730  uses approximate matching techniques to determine words in an utterance. Thus, for example, the STT processing module  730  determines that the sequence of phonemes / / corresponds to the word “tomato,” even if that particular sequence of phonemes is not one of the candidate sequence of phonemes for that word. 
     Natural language processing module  732  (“natural language processor”) of the digital assistant takes the n-best candidate text representation(s) (“word sequence(s)” or “token sequence(s)”) generated by STT processing module  730 , and attempts to associate each of the candidate text representations with one or more “actionable intents” recognized by the digital assistant. An “actionable intent” (or “user intent”) represents a task that can be performed by the digital assistant, and can have an associated task flow implemented in task flow models  754 . The associated task flow is a series of programmed actions and steps that the digital assistant takes in order to perform the task. The scope of a digital assistant&#39;s capabilities is dependent on the number and variety of task flows that have been implemented and stored in task flow models  754 , or in other words, on the number and variety of “actionable intents” that the digital assistant recognizes. The effectiveness of the digital assistant, however, also dependents on the assistant&#39;s ability to infer the correct “actionable intent(s)” from the user request expressed in natural language. 
     In some examples, in addition to the sequence of words or tokens obtained from STT processing module  730 , natural language processing module  732  also receives contextual information associated with the user request, e.g., from I/O processing module  728 . The natural language processing module  732  optionally uses the contextual information to clarify, supplement, and/or further define the information contained in the candidate text representations received from STT processing module  730 . The contextual information includes, for example, user preferences, hardware, and/or software states of the user device, sensor information collected before, during, or shortly after the user request, prior interactions (e.g., dialogue) between the digital assistant and the user, and the like. As described herein, contextual information is, in some examples, dynamic, and changes with time, location, content of the dialogue, and other factors. 
     In some examples, the natural language processing is based on, e.g., ontology  760 . Ontology  760  is a hierarchical structure containing many nodes, each node representing either an “actionable intent” or a “property” relevant to one or more of the “actionable intents” or other “properties.” As noted above, an “actionable intent” represents a task that the digital assistant is capable of performing, i.e., it is “actionable” or can be acted on. A “property” represents a parameter associated with an actionable intent or a sub-aspect of another property. A linkage between an actionable intent node and a property node in ontology  760  defines how a parameter represented by the property node pertains to the task represented by the actionable intent node. 
     In some examples, ontology  760  is made up of actionable intent nodes and property nodes. Within ontology  760 , each actionable intent node is linked to one or more property nodes either directly or through one or more intermediate property nodes. Similarly, each property node is linked to one or more actionable intent nodes either directly or through one or more intermediate property nodes. For example, as shown in  FIG. 7C , ontology  760  includes a “restaurant reservation” node (i.e., an actionable intent node). Property nodes “restaurant,” “date/time” (for the reservation), and “party size” are each directly linked to the actionable intent node (i.e., the “restaurant reservation” node). 
     In addition, property nodes “cuisine,” “price range,” “phone number,” and “location” are sub-nodes of the property node “restaurant,” and are each linked to the “restaurant reservation” node (i.e., the actionable intent node) through the intermediate property node “restaurant.” For another example, as shown in  FIG. 7C , ontology  760  also includes a “set reminder” node (i.e., another actionable intent node). Property nodes “date/time” (for setting the reminder) and “subject” (for the reminder) are each linked to the “set reminder” node. Since the property “date/time” is relevant to both the task of making a restaurant reservation and the task of setting a reminder, the property node “date/time” is linked to both the “restaurant reservation” node and the “set reminder” node in ontology  760 . 
     An actionable intent node, along with its linked concept nodes, is described as a “domain.” In the present discussion, each domain is associated with a respective actionable intent, and refers to the group of nodes (and the relationships there between) associated with the particular actionable intent. For example, ontology  760  shown in  FIG. 7C  includes an example of restaurant reservation domain  762  and an example of reminder domain  764  within ontology  760 . The restaurant reservation domain includes the actionable intent node “restaurant reservation,” property nodes “restaurant,” “date/time,” and “party size,” and sub-property nodes “cuisine,” “price range,” “phone number,” and “location.” Reminder domain  764  includes the actionable intent node “set reminder,” and property nodes “subject” and “date/time.” In some examples, ontology  760  is made up of many domains. Each domain shares one or more property nodes with one or more other domains. For example, the “date/time” property node is associated with many different domains (e.g., a scheduling domain, a travel reservation domain, a movie ticket domain, etc.), in addition to restaurant reservation domain  762  and reminder domain  764 . 
     While  FIG. 7C  illustrates two example domains within ontology  760 , other domains include, for example, “find a movie,” “initiate a phone call,” “find directions,” “schedule a meeting,” “send a message,” and “provide an answer to a question,” “read a list,” “providing navigation instructions,” “provide instructions for a task” and so on. A “send a message” domain is associated with a “send a message” actionable intent node, and further includes property nodes such as “recipient(s),” “message type,” and “message body.” The property node “recipient” is further defined, for example, by the sub-property nodes such as “recipient name” and “message address.” 
     In some examples, ontology  760  includes all the domains (and hence actionable intents) that the digital assistant is capable of understanding and acting upon. In some examples, ontology  760  is modified, such as by adding or removing entire domains or nodes, or by modifying relationships between the nodes within the ontology  760 . 
     In some examples, nodes associated with multiple related actionable intents are clustered under a “super domain” in ontology  760 . For example, a “travel” super-domain includes a cluster of property nodes and actionable intent nodes related to travel. The actionable intent nodes related to travel includes “airline reservation,” “hotel reservation,” “car rental,” “get directions,” “find points of interest,” and so on. The actionable intent nodes under the same super domain (e.g., the “travel” super domain) have many property nodes in common. For example, the actionable intent nodes for “airline reservation,” “hotel reservation,” “car rental,” “get directions,” and “find points of interest” share one or more of the property nodes “start location,” “destination,” “departure date/time,” “arrival date/time,” and “party size.” 
     In some examples, each node in ontology  760  is associated with a set of words and/or phrases that are relevant to the property or actionable intent represented by the node. The respective set of words and/or phrases associated with each node are the so-called “vocabulary” associated with the node. The respective set of words and/or phrases associated with each node are stored in vocabulary index  744  in association with the property or actionable intent represented by the node. For example, returning to  FIG. 7B , the vocabulary associated with the node for the property of “restaurant” includes words such as “food,” “drinks,” “cuisine,” “hungry,” “eat,” “pizza,” “fast food,” “meal,” and so on. For another example, the vocabulary associated with the node for the actionable intent of “initiate a phone call” includes words and phrases such as “call,” “phone,” “dial,” “ring,” “call this number,” “make a call to,” and so on. The vocabulary index  744  optionally includes words and phrases in different languages. 
     Natural language processing module  732  receives the candidate text representations (e.g., text string(s) or token sequence(s)) from STT processing module  730 , and for each candidate representation, determines what nodes are implicated by the words in the candidate text representation. In some examples, if a word or phrase in the candidate text representation is found to be associated with one or more nodes in ontology  760  (via vocabulary index  744 ), the word or phrase “triggers” or “activates” those nodes. Based on the quantity and/or relative importance of the activated nodes, natural language processing module  732  selects one of the actionable intents as the task that the user intended the digital assistant to perform. In some examples, the domain that has the most “triggered” nodes is selected. In some examples, the domain having the highest confidence value (e.g., based on the relative importance of its various triggered nodes) is selected. In some examples, the domain is selected based on a combination of the number and the importance of the triggered nodes. In some examples, additional factors are considered in selecting the node as well, such as whether the digital assistant has previously correctly interpreted a similar request from a user. 
     User data  748  includes user-specific information, such as user-specific vocabulary, user preferences, user address, user&#39;s default and secondary languages, user&#39;s contact list, and other short-term or long-term information for each user. In some examples, natural language processing module  732  uses the user-specific information to supplement the information contained in the user input to further define the user intent. For example, for a user request “invite my friends to my birthday party,” natural language processing module  732  is able to access user data  748  to determine who the “friends” are and when and where the “birthday party” would be held, rather than requiring the user to provide such information explicitly in his/her request. 
     It should be recognized that in some examples, natural language processing module  732  is implemented using one or more machine learning mechanisms (e.g., neural networks). In particular, the one or more machine learning mechanisms are configured to receive a candidate text representation and contextual information associated with the candidate text representation. Based on the candidate text representation and the associated contextual information, the one or more machine learning mechanism are configured to determine intent confidence scores over a set of candidate actionable intents. Natural language processing module  732  can select one or more candidate actionable intents from the set of candidate actionable intents based on the determined intent confidence scores. In some examples, an ontology (e.g., ontology  760 ) is also used to select the one or more candidate actionable intents from the set of candidate actionable intents. 
     Other details of searching an ontology based on a token string are described in U.S. Utility application Ser. No. 12/341,743 for “Method and Apparatus for Searching Using An Active Ontology,” filed Dec. 22, 2008, the entire disclosure of which is incorporated herein by reference. 
     In some examples, once natural language processing module  732  identifies an actionable intent (or domain) based on the user request, natural language processing module  732  generates a structured query to represent the identified actionable intent. In some examples, the structured query includes parameters for one or more nodes within the domain for the actionable intent, and at least some of the parameters are populated with the specific information and requirements specified in the user request. For example, the user says “Make me a dinner reservation at a sushi place at 7.” In this case, natural language processing module  732  is able to correctly identify the actionable intent to be “restaurant reservation” based on the user input. According to the ontology, a structured query for a “restaurant reservation” domain includes parameters such as {Cuisine}, {Time}, {Date}, {Party Size}, and the like. In some examples, based on the speech input and the text derived from the speech input using STT processing module  730 , natural language processing module  732  generates a partial structured query for the restaurant reservation domain, where the partial structured query includes the parameters {Cuisine=“Sushi”} and {Time=“7 μm”}. However, in this example, the user&#39;s utterance contains insufficient information to complete the structured query associated with the domain. Therefore, other necessary parameters such as {Party Size} and {Date} is not specified in the structured query based on the information currently available. In some examples, natural language processing module  732  populates some parameters of the structured query with received contextual information. For example, in some examples, if the user requested a sushi restaurant “near me,” natural language processing module  732  populates a {location} parameter in the structured query with GPS coordinates from the user device. 
     In some examples, natural language processing module  732  identifies multiple candidate actionable intents for each candidate text representation received from STT processing module  730 . Further, in some examples, a respective structured query (partial or complete) is generated for each identified candidate actionable intent. Natural language processing module  732  determines an intent confidence score for each candidate actionable intent and ranks the candidate actionable intents based on the intent confidence scores. In some examples, natural language processing module  732  passes the generated structured query (or queries), including any completed parameters, to task flow processing module  736  (“task flow processor”). In some examples, the structured query (or queries) for the m-best (e.g., m highest ranked) candidate actionable intents are provided to task flow processing module  736 , where m is a predetermined integer greater than zero. In some examples, the structured query (or queries) for the m-best candidate actionable intents are provided to task flow processing module  736  with the corresponding candidate text representation(s). 
     Other details of inferring a user intent based on multiple candidate actionable intents determined from multiple candidate text representations of a speech input are described in U.S. Utility application Ser. No. 14/298,725 for “System and Method for Inferring User Intent From Speech Inputs,” filed Jun. 6, 2014, the entire disclosure of which is incorporated herein by reference. 
     Task flow processing module  736  is configured to receive the structured query (or queries) from natural language processing module  732 , complete the structured query, if necessary, and perform the actions required to “complete” the user&#39;s ultimate request. In some examples, the various procedures necessary to complete these tasks are provided in task flow models  754 . In some examples, task flow models  754  include procedures for obtaining additional information from the user and task flows for performing actions associated with the actionable intent. 
     As described above, in order to complete a structured query, task flow processing module  736  needs to initiate additional dialogue with the user in order to obtain additional information, and/or disambiguate potentially ambiguous utterances. When such interactions are necessary, task flow processing module  736  invokes dialogue flow processing module  734  to engage in a dialogue with the user. In some examples, dialogue flow processing module  734  determines how (and/or when) to ask the user for the additional information and receives and processes the user responses. The questions are provided to and answers are received from the users through I/O processing module  728 . In some examples, dialogue flow processing module  734  presents dialogue output to the user via audio and/or visual output, and receives input from the user via spoken or physical (e.g., clicking) responses. Continuing with the example above, when task flow processing module  736  invokes dialogue flow processing module  734  to determine the “party size” and “date” information for the structured query associated with the domain “restaurant reservation,” dialogue flow processing module  734  generates questions such as “For how many people?” and “On which day?” to pass to the user. Once answers are received from the user, dialogue flow processing module  734  then populates the structured query with the missing information, or pass the information to task flow processing module  736  to complete the missing information from the structured query. 
     Once task flow processing module  736  has completed the structured query for an actionable intent, task flow processing module  736  proceeds to perform the ultimate task associated with the actionable intent. Accordingly, task flow processing module  736  executes the steps and instructions in the task flow model according to the specific parameters contained in the structured query. For example, the task flow model for the actionable intent of “restaurant reservation” includes steps and instructions for contacting a restaurant and actually requesting a reservation for a particular party size at a particular time. For example, using a structured query such as: {restaurant reservation, restaurant=ABC Café, date=3/12/2012, time=7 pm, party size=5}, task flow processing module  736  performs the steps of: (1) logging onto a server of the ABC Caf{tilde over (e)} or a restaurant reservation system such as OPENTABLE®, (2) entering the date, time, and party size information in a form on the website, (3) submitting the form, and (4) making a calendar entry for the reservation in the user&#39;s calendar. 
     In some examples, task flow processing module  736  employs the assistance of service processing module  738  (“service processing module”) to complete a task requested in the user input or to provide an informational answer requested in the user input. For example, service processing module  738  acts on behalf of task flow processing module  736  to make a phone call, set a calendar entry, invoke a map search, invoke or interact with other user applications installed on the user device, and invoke or interact with third-party services (e.g., a restaurant reservation portal, a social networking website, a banking portal, etc.). In some examples, the protocols and application programming interfaces (API) required by each service are specified by a respective service model among service models  756 . Service processing module  738  accesses the appropriate service model for a service and generate requests for the service in accordance with the protocols and APIs required by the service according to the service model. 
     For example, if a restaurant has enabled an online reservation service, the restaurant submits a service model specifying the necessary parameters for making a reservation and the APIs for communicating the values of the necessary parameter to the online reservation service. When requested by task flow processing module  736 , service processing module  738  establishes a network connection with the online reservation service using the web address stored in the service model, and send the necessary parameters of the reservation (e.g., time, date, party size) to the online reservation interface in a format according to the API of the online reservation service. 
     In some examples, natural language processing module  732 , dialogue flow processing module  734 , and task flow processing module  736  are used collectively and iteratively to infer and define the user&#39;s intent, obtain information to further clarify and refine the user intent, and finally generate a response (i.e., an output to the user, or the completion of a task) to fulfill the user&#39;s intent. The generated response is a dialogue response to the speech input that at least partially fulfills the user&#39;s intent. Further, in some examples, the generated response is output as a speech output. In these examples, the generated response is sent to speech synthesis processing module  740  (e.g., speech synthesizer) where it can be processed to synthesize the dialogue response in speech form. In yet other examples, the generated response is data content relevant to satisfying a user request in the speech input. 
     In examples where task flow processing module  736  receives multiple structured queries from natural language processing module  732 , task flow processing module  736  initially processes the first structured query of the received structured queries to attempt to complete the first structured query and/or execute one or more tasks or actions represented by the first structured query. In some examples, the first structured query corresponds to the highest ranked actionable intent. In other examples, the first structured query is selected from the received structured queries based on a combination of the corresponding speech recognition confidence scores and the corresponding intent confidence scores. In some examples, if task flow processing module  736  encounters an error during processing of the first structured query (e.g., due to an inability to determine a necessary parameter), the task flow processing module  736  can proceed to select and process a second structured query of the received structured queries that corresponds to a lower ranked actionable intent. The second structured query is selected, for example, based on the speech recognition confidence score of the corresponding candidate text representation, the intent confidence score of the corresponding candidate actionable intent, a missing necessary parameter in the first structured query, or any combination thereof. 
     Speech synthesis processing module  740  is configured to synthesize speech outputs for presentation to the user. Speech synthesis processing module  740  synthesizes speech outputs based on text provided by the digital assistant. For example, the generated dialogue response is in the form of a text string. Speech synthesis processing module  740  converts the text string to an audible speech output. Speech synthesis processing module  740  uses any appropriate speech synthesis technique in order to generate speech outputs from text, including, but not limited, to concatenative synthesis, unit selection synthesis, diphone synthesis, domain-specific synthesis, formant synthesis, articulatory synthesis, hidden Markov model (HMM) based synthesis, and sinewave synthesis. In some examples, speech synthesis processing module  740  is configured to synthesize individual words based on phonemic strings corresponding to the words. For example, a phonemic string is associated with a word in the generated dialogue response. The phonemic string is stored in metadata associated with the word. Speech synthesis processing module  740  is configured to directly process the phonemic string in the metadata to synthesize the word in speech form. 
     In some examples, instead of (or in addition to) using speech synthesis processing module  740 , speech synthesis is performed on a remote device (e.g., the server system  108 ), and the synthesized speech is sent to the user device for output to the user. For example, this can occur in some implementations where outputs for a digital assistant are generated at a server system. And because server systems generally have more processing power or resources than a user device, it is possible to obtain higher quality speech outputs than would be practical with client-side synthesis. 
     Additional details on digital assistants can be found in the U.S. Utility application Ser. No. 12/987,982, entitled “Intelligent Automated Assistant,” filed Jan. 10, 2011, and U.S. Utility application Ser. No. 13/251,088, entitled “Generating and Processing Task Items That Represent Tasks to Perform,” filed Sep. 30, 2011, the entire disclosures of which are incorporated herein by reference. 
     With reference back to  FIG. 7A , digital assistant module  726  further includes audio processing module  770  and latency management module  780 . Audio processing module  770  is configured to analyze a stream of audio received by digital assistant system  700  (e.g., at I/O processing module  728  and via microphone  213 ). In some examples, audio processing module  770  is configured to analyze the stream of audio to identify which portions contain user speech and which portions do not contain user speech. For example, audio processing module  770  divides the stream of audio into a sequence of overlapping audio frames. Each audio frame has a predetermined duration (e.g., 10 ms). Audio processing module  770  analyzes the audio features of each audio frame (e.g., using audio and/or speech models) to determine whether or not each audio frame contains user speech. The analyzed audio features can include time domain and/or frequency domain features. Time domain features include, for example, zero-crossing rates, short-time energy, spectral energy, spectral flatness, autocorrelation, or the like. Frequency domain features include, for example, mel-frequency cepstral coefficients, linear predictive cepstral coefficients, mel-frequency discrete wavelet coefficients, or the like. In some examples, audio processing module  770  provides audio frame information indicating which audio frames of the stream of audio contain user speech and which audio frames of the stream of audio do not contain user speech to other components of digital assistant module  726 . 
     In some examples, latency management module  780  receives the audio frame information from audio processing module  770 . Latency management module  780  uses this information to control the timing of various digital assistant processes to reduce latency. For example, latency management module  780  uses the audio frame information to detect pauses or interruptions in user speech in the stream of audio. In addition, the duration of each pause or interruption can be determined. In some examples, latency management module  780  applies one or more predetermined rules to determine whether a first portion of the stream of audio satisfies a predetermined condition. In some examples, the predetermined condition includes the condition of detecting, in the first portion of the stream of audio, an absence of user speech (e.g., a pause) for longer than a first predetermined duration (e.g., 50 ms, 75 ms, or 100 ms). In response to determining that the first portion of the stream of audio satisfies a predetermined condition, latency management module  780  initiates performance of natural language processing (e.g., at natural language processing module  732 ), task flow processing (e.g., at task flow processing module  736  or  836 ), and/or speech synthesis (e.g., at speech synthesis processing module  740 ) based on the user utterance contained in the first portion of the stream of audio. In some examples, latency management module  780  initiates performance of these processes while causing the digital assistant system to continue receiving a second portion of the stream of audio. 
     In some examples, latency management module  780  is configured to detect a speech end-point condition. Specifically, after determining that the first portion of the stream of audio satisfies a predetermined condition, latency management module  780  determines whether a speech end-point condition is detected. In some examples, latency management module  780  uses the audio frame information to determine whether a speech end-point condition is detected. For example, detecting the speech end-point condition can include detecting, in the second portion of the stream of audio, an absence of user speech for greater than a second predetermined duration (e.g., 600 ms, 700 ms, or 800 ms). The second predetermined duration is longer than the first predetermined duration. In some examples, detecting the speech end-point condition includes detecting a predetermined type of non-speech input from the user. For example, the predetermined type of non-speech input can be a user selection of a button (e.g., “home” or menu button  304 ) of the electronic device or an affordance displayed on the touch screen (e.g., touch screen  212 ) of the electronic device. In response to determining that a speech end-point condition is detected, latency management module  780  causes results generated by task flow processing module  736  or  836 , dialogue flow processing module  734 , and/or speech synthesis processing module  740  to be presented to the user. In some examples, the results include spoken dialogue. In some examples, latency management module  780  prevents the generated results from being presented to the user prior to determining that a speech end-point condition is detected. 
     In some examples, latency management module  780  determines that a speech end-point condition is not detected. Instead, latency management module  780  detects additional speech in the second portion of the stream of audio. Specifically, for example, the additional speech is a continuation of the utterance in the first portion of the stream of audio. In these examples, latency management module  780  re-initiates performance of natural language processing, task flow processing, and/or speech synthesis based on the user utterance across the first and second portions of the stream of audio. The latency reducing functions of latency management module  780  are described in greater detail below with reference to  FIGS. 9 and 10 . 
       FIG. 8  is a block diagram illustrating a portion of digital assistant module  800 , according to various examples. In particular,  FIG. 8  depicts certain components of digital assistant module  800  that can enable robust operation of a digital assistant, according to various examples. More specifically, the components depicted in digital assistant module  800  can function to evaluate multiple candidate task flows corresponding to a user utterance and improve the robustness and reliability of task flow processing. For simplicity, only a portion of digital assistant module  800  is depicted. It should be recognized that digital assistant module  800  can include additional components. For example, digital assistant module  800  can be similar or substantially identical to digital assistant module  726  and can reside in memory  702  of digital assistant system  700 . 
     As shown in  FIG. 8 , STT processing module  830  receives a user utterance (e.g., via I/O processing module  728 ). STT processing module  830  is similar or substantially identical to STT processing module  730 . In an illustrative example, the received user utterance is “Directions to Fidelity Investments.” STT processing module  830  performs speech recognition on the user utterance to determine a plurality of candidate text representations. Each candidate text representation of the plurality of candidate text representations corresponds to the user utterance. STT processing module  830  further determines an associated speech recognition confidence score for each candidate text representation. In some examples, the determined plurality of candidate text representations are the n-best candidate text representations having the n-highest speech recognition confidence scores. In the present example, STT processing module  830  determines three candidate text representations for the user utterance, which include “Directions to Fidelity Investments,” “Directions to deli restaurants,” and “Directions to Italian restaurants.” The candidate text representation “Directions to Fidelity Investments” can have the highest speech recognition confidence score and the candidate text representations “Directions to deli restaurants” and “Directions to Italian restaurants” can have lower speech recognition confidence scores. 
     STT processing module  830  provides the three candidate text representations and the associated speech recognition confidence scores to natural language processing module  832 . Natural language processing module  832  can be similar or substantially identical to natural language processing module  732 . Based on the three candidate text representations, natural language processing module  832  determines corresponding candidate user intents. Each candidate user intent is determined from a respective candidate text representation. Natural language processing module  832  further determines an associated intent confidence score for each candidate user intent. 
     Determining a candidate user intent from a respective candidate text representation includes, for example, parsing the respective candidate text representation to determine a candidate domain and candidate parse interpretations for the candidate text representation. The determined candidate user intent can be represented in the form of a structured query based on the determined candidate domain and parse interpretations. For instance, in the present example, natural language processing module  832  parses the candidate text interpretation “Directions to Fidelity Investments” to determine that a candidate domain is “get directions.” In addition, natural language processing module  832  recognizes that “Fidelity Investments” is an entry in the user&#39;s contact list and thus interprets it as a person/entity of the contact list (contacts=“Fidelity Investment”). Thus, a first candidate user intent determined for the candidate text interpretation “Directions to Fidelity Investments” can be represented by the structured query {Get directions, location=search(contacts=“Fidelity Investments”)}. In some examples, natural language processing module  832  can also interpret “Fidelity Investments” as a business. Thus, a second candidate user intent determined for the candidate text interpretation “Directions to Fidelity Investments” can be represented by the structured query {Get directions, location=search(contacts=“Fidelity Investments”)}. 
     The candidate text representations “Directions to deli restaurants” and “Directions to Italian restaurants” can similarly be parsed by natural language processing module  832  to determine respective candidate user intents. Specifically, natural language processing module  832  can interpret “deli” and “Italian” as types of cuisine associated with restaurants. Thus, candidate user intents determined for these candidate text interpretations can be represented by the structured queries {Get directions, location=search(restaurant, cuisine=“deli”)} and {Get directions, location=search(restaurant, cuisine=“Italian”)}, respectively. 
     Therefore, in the present example, natural language processing module  832  determines four candidate user intents from the three candidate text representations. The four candidate user intents are represented by the following respective structured queries:
         1. {Get directions, location=search(contacts=“Fidelity Investments”)}   2. {Get directions, location=search(business=“Fidelity Investments”)}   3. {Get directions, location=search(restaurant, cuisine=“deli”)}   4. {Get directions, location=search(restaurant, cuisine=“Italian”)}
 
Each of the four candidate user intents has an associated intent confidence score. In this example, the four candidate user intents are arranged in decreasing order of intent confidence scores, with the first candidate user intent having the highest intent confidence score and the fourth candidate user intent having the lowest intent confidence score. Although in the present example, each of the determined candidate user intents has the same inferred domain (“get directions”), it should be recognized that, in other examples, the determined candidate user intents can include a plurality of different inferred domains.
       

     Natural language processing module  832  provides the four candidate user intents to task flow processing module  836 . For example, the structured queries for the four candidate user intents are provided to task flow processing module  836 . In addition, the associated speech recognition confidence scores and intent confidence scores are provided to task flow processing module  836 . In some examples, task flow processing module  836  is similar or substantially identical to task flow processing module  736 . 
     In some examples, task flow manager  838  of task flow processing module  836  initially only selects one of the four candidate user intents for processing. If task flow manager  838  determines that the initially selected candidate user intent cannot be successfully processed through task flow processing module  836 , task flow manager  838  selects another candidate user intent for processing. For example, the first candidate user intent having the highest intent confidence score is initially selected. Specifically, in the present example, the first candidate user intent represented by the structured query {Get directions, location=search(contacts=“Fidelity Investments”)} is selected by task flow manager  838 . Task flow manager  838  maps the first candidate user intent to a corresponding first candidate task flow (e.g., first candidate task flow  842 ). Notably, the structured query for the first candidate user intent is incomplete because it does not contain any value for the “location” property. As a result, the “location” task parameter in the corresponding first candidate task flow can be missing a required value for performing the task of getting directions to a location corresponding to “Fidelity Investments.” In this example, the first candidate task flow includes procedures for resolving the “location” task parameter. Specifically, the first candidate task flow includes procedures for searching the “Fidelity Investments” entry of the user&#39;s contact list to obtain a corresponding value (e.g., address value) for the “location” task parameter. 
     In some examples, task flow manager  838  determines a corresponding first task flow score for the first candidate task flow. The first task flow score can represent the likelihood that the corresponding candidate task flow is the correct candidate task flow to perform given the user utterance. In some examples, the first task flow score is based on a first flow parameter score. The first flow parameter score can represent a confidence of resolving one or more flow parameters for the first candidate task flow. In some examples, the first task flow score is based on any combination of a speech recognition confidence score for the corresponding candidate text representation “Directions to Fidelity Investments,” an intent confidence score for the first candidate user intent, and a first task parameter score. 
     Task flow resolver  840  determines the first flow parameter score by attempting to resolve the missing “location” flow parameter for the first candidate task flow. In the present example, task flow resolver  840  attempts to search the “Fidelity Investments” entry of the user&#39;s contact list to obtain a value (e.g., address value) for the “location” flow parameter. If task flow resolver  840  successfully resolves the “location” flow parameter by obtaining a value from the “Fidelity Investments” entry of the user&#39;s contact list, task flow resolver  840  can determine a higher value for the first flow parameter score. However, if task flow resolver  840  is unable to successfully resolve the “location” flow parameter (e.g., no address value is found in the “Fidelity Investments” entry of the user&#39;s contact list), task flow resolver  840  can determine a lower value for the first flow parameter score. Thus, the first flow parameter score can be indicative of whether one or more flow parameters of the first candidate task flow (e.g., the “location” parameter) can be successfully resolved. In some examples, task flow resolver  840  can utilize context data to attempt to resolve missing flow parameters. 
     In some examples, task flow manager  838  determines whether the first task flow score satisfies a predetermined criterion (e.g., greater than a predetermined threshold level). In the example where task flow resolver  840  successfully resolves the “location” flow parameter, the first flow parameter score can be sufficiently high to enable the first task flow score to satisfy the predetermined criterion. In this example, task flow manager  838  determines that the first task flow score satisfies the predetermined criterion and in response, task flow manager  838  executes the corresponding first candidate task flow without processing the remaining three candidate user intents. In the present example, executing the first candidate task flow can include searching for directions to the address obtained from the “Fidelity Investments” entry of the user&#39;s contact list and displaying the directions to the user on the electronic device. In some examples, executing the first candidate task flow further includes generating a dialogue text that is responsive to the user utterance and outputting a spoken representation of the dialogue text. For example, the outputted spoken representation of the dialogue text can be “OK, here are directions to Fidelity Investments.” 
     In the alternative example where task flow resolver  840  is unable to successfully resolve the “location” flow parameter, the first flow parameter score can be sufficiently low to result in the first task flow score not satisfying the predetermined criterion. In this example, task flow manager  838  forgoes executing the corresponding first candidate task flow and proceeds to select a second candidate user intent for processing. For example, the second candidate user intent having the second highest intent confidence score can be selected. In the present example, the selected second candidate user intent is represented by the second structured query {Get directions, location=search(business=“Fidelity Investments”)}. 
     Task flow manager  838  maps the second candidate user intent to a corresponding second candidate task flow (e.g., second candidate task flow  844 ). The second candidate task flow is processed in a similar manner as the first candidate task flow. Specifically, task flow resolver  840  determines a second flow parameter score for the second candidate task flow by attempting to resolve one or more missing task parameters for the second candidate task flow. For example, task flow resolver  840  attempts to search one or more “business” data sources (e.g., a business directory) to obtain an address value corresponding to the business “Fidelity Investments.” The determined second flow parameter score is based on whether task flow resolver  840  can successfully resolve the “location” flow parameter by searching the “business” data source. Based on the second flow parameter score, task flow manager  838  determines a second task flow score for the second candidate task flow. In some examples, the second task flow score is further based on the speech recognition confidence score for the corresponding candidate text representation “Directions to Fidelity Investments” and/or the intent confidence score for the second candidate user intent. If task flow manager  838  determines that the second task flow score satisfies the predetermined criterion, then the second candidate task flow is executed. However, if task flow manager  838  determines that the second task flow score does not satisfy the predetermined criterion, then task flow manager  838  forgoes executing the second candidate task flow and continues to evaluate the third candidate user intent (and if necessary, the fourth candidate user intent) until a corresponding candidate task flow is determined to have an associated task flow score that satisfies the predetermined criterion. 
     Although in the examples described above, task flow processing module  836  evaluates the candidate user intents serially to determine a candidate task flow having an associated task flow score that satisfies the predetermined criterion, it should be recognized that, in other examples, task flow processing module  836  can evaluate the candidate user intents in parallel. In these examples, task flow manager  838  maps each of the four candidate user intents to four respective candidate task flows. Task flow manager  838  then determines a respective task flow score for each candidate task flow. As discussed above, task flow processing module  836  determines each task flow score based on the speech recognition confidence score for the respective candidate text representation, the intent confidence score for the respective candidate user intent, the respective task parameter score, or any combination thereof. Task flow processing module  836  determines each respective task parameter score by attempting to resolve one or more missing task parameters for the respective candidate task flow. Based on the determined task flow scores for the four candidate task flows, task flow manager  838  ranks the four candidate task flows and selects the highest ranking candidate task flow. Task flow manager  838  then executes the selected candidate task flow. 
     In some examples, the highest ranking candidate task flow is the candidate task flow with the highest task flow score. In the present example, the highest ranking candidate task flow can correspond to the second candidate user intent represented by the structured query {Get directions, location=search(business=“Fidelity Investments”)}. Thus, in the present example, Task flow manager  838  can execute the second candidate user intent, which can include obtaining directions to a “Fidelity Investments” address obtained by searching one or more business data sources and presenting the directions to the user (e.g., by displaying a map with the directions). 
     It should be appreciated that, in some examples, the selected highest ranking candidate task flow need not correspond to the candidate user intent with the highest intent confidence score. For instance, in the present example, the selected second candidate task flow corresponds to the second candidate user intent (e.g., represented by the structured query {Get directions, location=search(business=“Fidelity Investments”)}), which does not have the highest intent confidence score. Further, in some examples, the selected highest ranking candidate task flow need not correspond to the candidate text representations with the highest speech recognition confidence score. Because the task flow score can be based on a combination of speech recognition confidence scores, intent confidence scores, and flow parameter scores, using the task flow scores to select a suitable candidate task flow can enable a candidate task flow that represents an optimization of speech recognition, natural language processing, and task flow processing to be selected. As a result, the selected candidate task flow can be more likely to coincide with the user&#39;s actual desired goal for providing the user utterance and less likely to fail (e.g., causes a fatal error) during execution. 
       FIGS. 9 and 10  are timelines  900  and  1000  illustrating the timing for low-latency operation of a digital assistant, according to various examples. In some examples, the timing for low-latency operation of a digital assistant is controlled using a latency management module (e.g., latency management module  780 ) of a digital assistant module (e.g., digital assistant module  726 ).  FIGS. 9 and 10  are described with references to digital assistant system  700  of  FIGS. 7A and 7B . 
     As shown in  FIG. 9 , digital assistant system  700  begins receiving stream of audio  902  at first time  904 . For example, digital assistant system  700  begins receiving stream of audio  902  at first time  904  in response to receiving user input that invokes digital assistant system  700 . In this example, stream of audio  902  is continuously received from first time  904  to third time  910 . Specifically, a first portion of stream of audio  902  is received from first time  904  to second time  908  and a second portion of stream of audio  902  is received from second time  908  to third time  910 . As shown, the first portion of stream of audio  902  includes user utterance  903 . 
     In some examples, digital assistant system  700  performs speech recognition as stream of audio  902  is being received. For example, latency management module  780  causes STT processing module  730  to beginning performing speech recognition in real-time as stream of audio  902  is being received. STT processing module  730  determines one or more first candidate text representations for user utterance  903 . 
     Latency management module  780  determines whether the first portion of stream of audio  902  satisfies a predetermined condition. For example, the predetermined condition can include the condition of detecting an absence of user speech in the first portion of stream of audio  902  for longer than a first predetermined duration (e.g., 50 ms, 75 ms, or 100 ms). It should be appreciated that, in other examples, the predetermined condition can include other conditions associated with the first portion of stream of audio  902 . In the present example, as shown in  FIG. 9 , the first portion of stream of audio  902  contains an absence of user speech between first intermediate time  906  and second time  908 . If latency management module  780  determines that this absence of user speech between first intermediate time  906  and second time  908  satisfies the predetermined condition (e.g., duration  912  is longer than the first predetermined duration), latency management module  780  causes the relevant components of digital assistant system  700  to initiate a sequence of processes that include natural language processing, task flow processing, dialogue flow processing, speech synthesis, or any combination thereof. Specifically, in the present example, in response to determining that the first portion of stream of audio  902  satisfies the predetermined condition, latency management module  780  causes natural language processing module  732  to begin performing, at second time  908 , natural language processing on the one or more first candidate text representations. This can be advantageous because natural language processing, task flow processing, dialogue flow processing, or speech synthesis can be at least partially completed between second time  908  and third time  910  while digital assistant system  700  is awaiting the detection of a speech end-point condition. As a result, less processing can be required after the speech end-point condition is detected, which can reduce the response latency of digital assistant system  700 . 
     As discussed above, latency management module  780  causes one or more of natural language processing, task flow processing, dialogue flow processing, and speech synthesis to be performed while the second portion of stream of audio  902  is being received between second time  908  and third time  910 . Specifically, between second time  908  and third time  910 , latency management module  780  causes natural language processing module  732  to determine one or more candidate user intents for user utterance  903  based on the one or more first candidate text representations. In some examples, latency management module  780  also causes task flow processing module  736  (or  836 ) to determine (e.g., at least partially between second time  908  and third time  910 ) one or more respective candidate task flows for the one or more candidate user intents and to select (e.g., at least partially between second time  908  and third time  910 ) a first candidate task flow from the one or more candidate task flows. In some examples, latency management module  780  further causes task flow processing module  736  (or  836 ) to execute (e.g., at least partially between second time  908  and third time  910 ) the selected first candidate task flow without providing an output to a user of digital assistant system  700  (e.g., without displaying any result or outputting any speech/audio on the user device). 
     In some examples, executing the first candidate task flow includes generating a text dialogue that is responsive to user utterance  903  and generating a spoken representation of the text dialogue. In these examples, latency management module  780  further causes dialogue flow processing module  734  to generate (e.g., at least partially between second time  908  and third time  910 ) the text dialogue and causes speech synthesis processing module  740  to generate (e.g., at least partially between second time  908  and third time  910 ) the spoken representation of the text dialogue. 
     In some examples, speech synthesis processing module  740  receives a request (e.g., from task flow processing module  736  or dialogue flow processing module  734 ) to generate the spoken representation of the text dialogue. In response to receiving the request, speech synthesis processing module  740  can determine (e.g., at least partially between second time  908  and third time  910 ) whether the memory (e.g., memory  202 ,  470 , or  702 ) of the electronic device (e.g., server  106 , device  104 , device  200 , or system  700 ) stores an audio file having a spoken representation of the text dialogue. In response to determining that the memory of the electronic device does store an audio file having a spoken representation of the text dialogue, speech synthesis processing module  740  awaits detection of an end-point condition before playing the stored audio file. In response to determining that the memory of the electronic device does not store an audio file having a spoken representation of the text dialogue, speech synthesis processing module  740  generates an audio file having a spoken representation of the text dialogue and stores the audio file in the memory. In some examples, generating and storing the audio file are at least partially performed between second time  908  and third time  910 . After storing the audio file, speech synthesis processing module  740  awaits detection of a speech end-point condition before playing the stored audio file. 
     Latency management module  780  determines whether a speech end-point condition is detected between second time  908  and third time  910 . For example, detecting the speech end-point condition can include detecting, in the second portion of stream of audio  902 , an absence of user speech for longer than a second predetermined duration (e.g., 600 ms, 700 ms, or 800 ms). It should be recognized that, in other examples, other speech end-point conditions can be implemented. In the present example, the second portion of stream of audio  902  between second time  908  and third time  910  does not contain any user speech. In addition, duration  914  between second time  908  and third time  910  is longer than the second predetermined duration. Thus, in this example, a speech end-point condition is detected between second time  908  and third time  910 . In response to determining that a speech end-point condition is detected between the second time and the third time, latency management module  780  causes digital assistant system  700  to present (e.g., at fourth time  1014 ) the results obtained from executing the first candidate task flow. For example, the results can be displayed on a display of the electronic device. In some examples, latency management module  780  causes output of the spoken representation of the text dialogue to the user by causing a respective stored audio file to be played. 
     Because at least a portion of natural language processing, task flow processing, dialogue flow processing, and speech synthesis is performed prior to detecting the speech end-point condition, less processing can be required after the speech end-point condition is detected, which can reduce the response latency of digital assistant system  700 . Specifically, the results obtained from executing the first candidate task flow can be presented more quickly after the speech end-point condition is detected. 
     In other examples, a speech end-point condition is not detected between second time  908  and third time  910 . For example, with reference to timeline  1000  in  FIG. 10 , stream of audio  1002  contains user speech between second time  1008  and third time  1012 . Thus, in this example, a speech end-point condition is not detected between second time  1008  and third time  1012 . Timeline  1000  of  FIG. 10  from first time  1004  to second time  1008  can be similar or substantially identical to timeline  900  of  FIG. 9  from first time  904  to second time  908 . In particular, the first portion of stream of audio  1002  containing user utterance  1003  is received from first time  1004  to second time  1008 . Latency management module  780  determines that the absence of user speech between first intermediate time  1006  and second time  1008  satisfies the predetermined condition (e.g., duration  1018  is longer than the first predetermined duration) and in response, latency management module  780  causes the relevant components of digital assistant system  700  to initiate, for the first time, a sequence of processes that include natural language processing, task flow processing, dialogue flow processing, speech synthesis, or any combination thereof. Specifically, in response to determining that the first portion of stream of audio  1002  satisfies the predetermined condition, latency management module  780  causes natural language processing module  732  to begin performing, at second time  1008 , natural language processing on one or more first candidate text representations of user utterance  1003  in the first portion of stream of audio  1002 . 
     Timeline  1000  of  FIG. 10  differs from timeline  900  of  FIG. 9  in that user utterance  1003  continues from the first portion of stream of audio  1002  (between first time  1004  and second time  1008 ) to the second portion of stream of audio  1002  (between second time  1008  and third time  1012 ) and stream of audio  1002  further extends from third time  1012  to fourth time  1014 . In this example, latency management module  780  determines that a speech end-point condition is not detected between second time  1008  and third time  1012  (e.g., due to detecting user speech) and in response, latency management module  780  causes digital assistant system  700  to forgo presentation of any results obtained from performing task flow processing between second time  1008  and third time  1012 . In other words, the natural language processing, task flow processing, dialogue flow processing, or speech synthesis performed between second time  1008  and third time  1012  can be discarded upon detecting user speech in the second portion of stream of audio  1002 . In addition, upon detecting user speech in the second portion of stream of audio  1002 , latency management module  780  causes digital assistant system  700  to process the user speech (continuation of user utterance  1003 ) in the second portion of stream of audio  1002 . Specifically, latency management module  780  causes STT processing module  730  to perform speech recognition on the second portion of stream of audio  1002  and determine one or more second candidate text representations. Each candidate text representation of the one or more second candidate text representations is a candidate text representation of user utterance  1003  across the first and second portions of stream of audio  1002  (e.g., from first time  1004  to third time  1012 ). Further, upon detecting user speech in the second portion of stream of audio  1002 , latency management module  780  causes digital assistant system  700  to continue receiving stream of audio  1002  from third time  1012  to fourth time  1014 . Specifically, a third portion of stream of audio  1002  is received from third time  1012  and fourth time  1014 . 
     The second and third portions of stream of audio  1002  are processed in a similar manner as the first and second portions of stream of audio  902 , described above with reference to  FIG. 9 . In particular, latency management module  780  determines whether the second portion of stream of audio  1002  satisfies the predetermined condition (e.g., absences of user speech for longer than a first predetermined duration). In the present example, as shown in  FIG. 10 , the second portion of stream of audio  1002  contains an absence of user speech between second intermediate time  1010  and third time  1012 . If latency management module  780  determines that this absence of user speech between second intermediate time  1010  and third time  1012  satisfies the predetermined condition (e.g., duration  1020  is longer than the first predetermined duration), latency management module  780  causes the relevant components of digital assistant system  700  to initiate, for a second time, a sequence of processes that include natural language processing, task flow processing, dialogue flow processing, speech synthesis, or any combination thereof. Specifically, in the present example, in response to determining that the second portion of stream of audio  1002  satisfies the predetermined condition, latency management module  780  causes natural language processing module  732  to begin performing, at third time  1012 , natural language processing on the one or more second candidate text representations. 
     As discussed above, latency management module  780  causes one or more of natural language processing, task flow processing, dialogue flow processing, and speech synthesis to be performed between third time  1012  and fourth time  1014 . In particular, between third time  1012  and fourth time  1014 , latency management module  780  causes natural language processing module  732  to determine, based on the one or more second candidate text representations, one or more second candidate user intents for user utterance  1003  in the first and second portions of stream of audio  1002 . In some examples, latency management module  780  causes task flow processing module  736  (or  836 ) to determine (e.g., at least partially between third time  1012  and fourth time  1014 ) one or more respective second candidate task flows for the one or more second candidate user intents and to select (e.g., at least partially between third time  1012  and fourth time  1014 ) a second candidate task flow from the one or more second candidate task flows. In some examples, latency management module  780  further causes task flow processing module  736  (or  836 ) to execute (e.g., at least partially between third time  1012  and fourth time  1014 ) the selected second candidate task flow without providing an output to a user of the digital assistant system (e.g., without displaying any result or outputting any speech/audio on the user device). 
     Latency management module  780  determines whether a speech end-point condition is detected between third time  1012  and fourth time  1014 . In the present example, the third portion of stream of audio  1002  between third time  1012  and fourth time  1014  does not contain any user speech. In addition, duration  1022  between third time  1012  and fourth time  1014  is longer than the second predetermined duration. Thus, in this example, a speech end-point condition is detected between third time  1012  and fourth time  1014 . In response to determining that a speech end-point condition is detected between third time  1012  and fourth time  1014 , latency management module  780  causes digital assistant system  700  to present (e.g., at fourth time  1014 ) the results obtained from executing the first candidate task flow. For example, the results can be displayed on a display of the electronic device. In other examples, presenting the results includes outputting spoken dialogue that is responsive to user utterance  1003 . In these examples, the spoken dialogue can be at least partially generated between third time  1012  and fourth time  1014 . 
     4. Process for Operating a Digital Assistant 
       FIGS. 11A-11B  illustrate process  1100  for operating a digital assistant, according to various examples. Some aspects of process  1100  relate to low-latency operation of a digital assistant. In addition, some aspects of process  1100  relate to more reliable and robust operation of a digital assistant. Process  1100  is performed, for example, using one or more electronic devices implementing a digital assistant. In some examples, process  1100  is performed using a client-server system (e.g., system  100 ), and the blocks of process  1100  are divided up in any manner between the server (e.g., DA server  106 ) and a client device (user device  104 ). In other examples, the blocks of process  1100  are divided up between the server and multiple client devices (e.g., a mobile phone and a smart watch). Thus, while portions of process  1100  are described herein as being performed by particular devices of a client-server system, it will be appreciated that process  1100  is not so limited. In other examples, process  1100  is performed using only a client device (e.g., user device  104 ) or only multiple client devices. In process  1100 , some blocks are, optionally, combined, the order of some blocks is, optionally, changed, and some blocks are, optionally, omitted. In some examples, additional steps may be performed in combination with process  1100 . 
     At block  1102 , a stream of audio (e.g., stream of audio  902  or  1002 ) is received (e.g., at I/O processing module  728  via microphone  213 ). The stream of audio is received, for example, by activating a microphone (e.g., microphone  213 ) of the electronic device (e.g., user device  104 ) and initiating the collection of audio data via the microphone. Activation of the microphone and initiating collection of audio data can be performed in response to detecting a predetermined user input. For example, detecting the activation of a “home” affordance of the electronic device (e.g., by the user pressing and holding the affordance) can invoke the digital assistant and initiate the receiving of the stream of audio. In some examples, the stream of audio is a continuous stream of audio data. The stream of audio data can be continuously collected and stored in a buffer (e.g., buffer of audio circuitry  210 ). 
     In some examples, block  1102  is performed in accordance with blocks  1104  and  1106 . Specifically, in these examples, the stream of audio is received across two time intervals. At block  1104 , a first portion of the stream of audio is received from a first time (e.g., first time  904  or  1004 ) to a second time (e.g., second time  908  or  1008 ). The first portion of the stream of audio contains, for example, a user utterance (user utterance  903  or  1003 ). At block  1106 , a second portion of the stream of audio is received from the second time (e.g., second time  908  or  1008 ) to a third time (e.g., third time  910  or  1012 ). The first time, second time, and third time are each specific points of time. The second time is after the first time and the third time is after the second time. In some examples, the stream of audio is continuously received from the first time through to the third time, wherein the first portion of the stream of audio is continuously received from the first time to the second time and the second portion of the stream of audio is continuously received from the second time to the third time. 
     At block  1108 , a plurality of candidate text representations of the user utterance are determined (e.g., using STT processing module  730 ). The plurality of candidate text representations are determined by performing speech recognition on the stream of audio. Each candidate text representation is associated with a respective speech recognition confidence score. The speech recognition confidence scores can indicate the confidence that a particular candidate text representation is the correct text representation of the user utterance. In addition, the speech recognition confidence scores can indicate the confidence of any determined word in a candidate text representation of the plurality of candidate text representations. In some examples, the plurality of candidate text representations are the n-best candidate text representations having the n-highest speech recognition confidence scores. 
     In some examples, block  1108  is performed in real-time as the user utterance is being received at block  1104 . In some examples, speech recognition is performed automatically upon receiving the stream of audio. In particular, words of the user utterance are decoded and transcribed as each portion of the user utterance is received. In these examples, block  1108  is performed prior to block  1110 . In other examples, block  1108  is performed after block  1110  (e.g., performed in response to determining that the first portion of the stream of audio satisfies a predetermined condition). 
     At block  1110 , a determination is made (e.g., using latency management module  780 ) as to whether the first portion of the stream of audio satisfies a predetermined condition. In some examples, the predetermined condition is a condition based on one or more audio characteristics of the stream of audio. The one or more audio characteristics include, for example, one or more time domain and/or frequency domain features of the stream of audio. Time domain features include, for example, zero-crossing rates, short-time energy, spectral energy, spectral flatness, autocorrelation, or the like. Frequency domain features include, for example, mel-frequency cepstral coefficients, linear predictive cepstral coefficients, mel-frequency discrete wavelet coefficients, or the like. 
     In some examples, the predetermined condition includes the condition of detecting, in the first portion of the stream of audio, an absence of user speech for longer than a first predetermined duration after the user utterance. Specifically, process  1100  can continuously monitor the first portion of the stream of audio (e.g., from the first time to the second time) and determine the start time and end time of the user utterances (e.g., using conventional speech detection techniques). If an absence of user speech is detected in the first portion of the stream of audio for longer than a first predetermined duration (e.g., 50 ms, 75 ms, or 100 ms) after the end time of the user utterance, it can be determined that the first portion of the stream of audio satisfies the predetermined condition. 
     In some examples, the presence or absence of user speech is detected based on audio energy level (e.g., energy level of the stream of audio within a frequency range corresponding to human speech, such as 50-500 Hz). In these examples, the predetermined condition includes the condition of detecting, in the first portion of the stream of audio, an audio energy level that is less than a predetermined threshold energy level for longer than a first predetermined duration after the end time of the user utterance. 
     In some examples, the predetermined condition includes a condition that relates to a linguistic characteristic of the user utterance. For example, the plurality of candidate text representations of block  1108  can be analyzed to determine whether an end-of-sentence condition is detected in the one or more candidate text representations. In some examples, the end-of-sentence condition is detected if the ending portions of the one or more candidate text representations match a predetermined sequence of words. In some examples, a language model is used to detect an end-of-sentence condition in the one or more candidate text representations. 
     In response to determining that the first portion of the stream of audio satisfies a predetermined condition, one or more of the operations of blocks  1112 - 1126  are performed. In particular, one or more of the operations of blocks  1112 - 1126  are performed automatically (e.g., without further input from the user) in response to determining that the first portion of the stream of audio satisfies a predetermined condition. Further, in response to determining that the first portion of the stream of audio satisfies a predetermined condition, one or more of the operations of blocks  1112 - 1126  are at least partially performed between the second time (e.g., second time  908  or  1008 ) and the third time (e.g., third time  910  or  1012 ) (e.g., while the second portion of the stream of audio is received at block  1106 ). 
     In response to determining that the first portion of the stream of audio does not satisfy a predetermined condition, block  1110  continues to monitor the first portion of the stream of audio (e.g., without performing blocks  1112 - 1126 ) until it is determined that the predetermined condition is satisfied by the first portion of the stream of audio. 
     Determining whether the first portion of the stream of audio satisfies a predetermined condition and performing, at least partially between the second time and the third time, one or more of the operations of blocks  1112 - 1126  in response to determining that the first portion of the stream of audio satisfies a predetermined condition can reduce the response latency of the digital assistant on the electronic device. In particular, the electronic device can at least partially complete these operations while waiting for the speech end-point condition to be detected. This can enhance operability of the electronic device by reducing the operations needed to be performed after detecting the speech end-point condition. In turn, this can reduce the overall latency between receiving the user utterance (block  1104 ) and presenting the results to the user (block  1130 ). 
     At block  1112 , a plurality of candidate user intents for the user utterance are determined (e.g., using natural language processing module  732 ). In particular, natural language processing is performed on the one or more candidate text representations of block  1108  to determine the plurality of candidate user intents. Each candidate user intent of the plurality of candidate user intents is an actionable intent that represents one or more tasks, which when performed, would satisfy a predicted goal corresponding to the user utterance. In some examples, each candidate user intent is determined in the form of a structured query. 
     In some examples, each candidate text representation of the one or more candidate text representations of block  1108  is parsed to determine one or more respective candidate user intents. In some examples, the plurality of candidate user intents determined at block  1112  include candidate user intents corresponding to different candidate text representations. For example, at block  1112 , a first candidate user intent of the plurality of candidate user intents can be determined from a first candidate text representation of block  1108  and a second candidate user intent of the plurality of candidate user intents can be determined from a second candidate text representation of block  1108 . 
     In some examples, each candidate user intent is associated with a respective intent confidence score. The intent confidence scores can indicate the confidence that a particular candidate user intent is the correct user intent for the respective candidate text representation. In addition, the intent confidence scores can indicate the confidence of corresponding domains, actionable intents, concepts, or properties determined for the candidate user intents. In some examples, the plurality of candidate user intents are the m-best candidate user intents having the m-highest intent confidence scores. 
     At block  1114 , a plurality of candidate task flows are determined (e.g., using task flow processing module  736  or  836 ) from the plurality of candidate user intents of block  1112 . Specifically, each candidate user intent of the plurality of candidate user intents is mapped to a corresponding candidate task flow of the plurality of candidate task flows. Each candidate task flow includes procedures for performing one or more actions that fulfill the respective candidate user intent. For candidate user intents having incomplete structured queries (e.g., partial structured queries with one or more missing property values), the corresponding candidate task flows can include procedures for resolving the incomplete structured queries. For example, the candidate task flows can include procedures for determining one or more flow parameters (e.g., corresponding to the one or more missing property values) by searching one or more data sources or querying the user for additional information. Each candidate task flow further includes procedures for performing one or more actions represented by the corresponding candidate user intent (e.g., represented by the complete structured query of the candidate user intent). 
     At block  1116 , a plurality of task flow scores are determined (e.g., using task flow processing module  736  or  836 ) for the plurality of candidate task flows. Each task flow score of the plurality of task flow scores corresponds to a respective candidate task flow of the plurality of candidate task flows. The task flow score for a respective candidate task flow can represent the likelihood that the respective candidate task flow is the correct candidate task flow to perform given the user utterance. For example, the task flow score can represent the likelihood that the user&#39;s actual desired goal for providing the user utterance is fulfilled by performing the respective candidate task flow. 
     In some examples, each task flow score is based on a flow parameter score for the respective candidate task flow. In these examples, block  1116  includes determining (e.g., using task flow manager  838 ) a respective flow parameter score for each candidate task flow. The flow parameter score for a respective candidate task flow can represent a confidence of resolving one or more flow parameters for the respective candidate task flow. In some examples, determining a flow parameter score for a respective candidate task flow includes resolving one or more flow parameters for the respective candidate task flow. Specifically, for each candidate task flow, process  1100  determines, at block  1118 , whether the respective candidate task flow includes procedures for resolving one or more flow parameters. The one or more flow parameters can correspond, for example, to one or more missing property values of a corresponding incomplete structured query. In some examples, the one or more flow parameters are parameters that are not expressly specified in the user utterance. If process  1100  determines that the respective candidate task flow includes procedures for resolving one or more flow parameters, the procedures can be executed (e.g., using task flow resolver  840 ) to resolve the one or more flow parameters. In some examples, executing the procedures causes one or more data sources to be searched. In particular, the one or more data sources are searched to obtain one or more values for the one or more flow parameters. In some examples, the one or more data sources correspond to one or more properties of the respective candidate user intent. 
     If the one or more flow parameters for the respective candidate task flow can be resolved (e.g., by successfully obtaining one or more values for the one or more flow parameters from the one or more data sources), then the flow parameter score determined for the respective candidate task flow can be high. Conversely, if the one or more flow parameters for the respective candidate task flow cannot be resolved (e.g., due to a failure to obtain one or more values for the one or more flow parameters from the one or more data sources), then the flow parameter score determined for the respective candidate task flow can be low. 
     Determining task flow scores and/or task parameter scores for the plurality of candidate task flows can be advantageous for evaluating the reliability of each candidate task flow prior to selecting and executing any candidate task flow. In particular, the task flow scores and/or task parameter scores can be used to identify candidate task flows that cannot be resolved. This allows process  1100  to only select (e.g., at block  1122 ) and execute (e.g., at block  1124 ) candidate task flows that can be resolved, which improves the reliability and robustness of task flow processing by the digital assistant. 
     In some examples, each task flow score of the plurality of task flow scores is based on the intent confidence score of a respective candidate user intent corresponding to the respective candidate task flow. Further, in some examples, each task flow score of the plurality of task flow scores is based on the speech recognition confidence score of the respective candidate text representation corresponding to the respective candidate task flow. In some examples, each task flow score is based on a combination of the flow parameter score for the respective candidate task flow, the intent confidence score of the respective candidate user intent, and the speech recognition confidence score of the respective candidate text representation. 
     At block  1120 , the plurality of candidate task flows are ranked (e.g., using task flow manager  838 ) according to the plurality of task flow scores of block  1116 . For example, the plurality of candidate task flows are ranked from the highest task flow score to the lowest task flow score. 
     At block  1122 , a first candidate task flow of the plurality of candidate task flows is selected (e.g., using task flow manager  838 ). In particular, the first candidate task flow of the plurality of candidate task flows is selected based on the plurality of task flow scores and the ranking of block  1120 . For example, the selected first candidate task flow is the highest ranked candidate task flow of the plurality of candidate task flows (e.g., having the highest task flow score). 
     In some examples, the selected first candidate task flow has the highest task flow score, but corresponds to a candidate user intent having an intent confidence score that is not the highest intent confidence score among the plurality of candidate user intents. In some examples, the selected first candidate task flow corresponds to a text representation having a speech recognition score that is not the highest speech recognition score among the plurality of candidate text representations. 
     In the examples described above, process  1100  evaluates each of the plurality of candidate task flows in parallel to select the first candidate task flow having the highest task flow score. It should be appreciated, however, that in other examples, process  1100  can instead evaluate the plurality of candidate task flows serially. For instance, in some examples, a first task flow score is initially determined only for a candidate task flow corresponding to a candidate user intent having the highest intent confidence score. If the first task flow score satisfies a predetermined criterion (e.g., greater than a predetermined threshold level), then the corresponding candidate task flow is selected at block  1122 . If, however, the first task flow score does not satisfy the predetermined criterion (e.g., less than the predetermined threshold level), then a second task flow score is determined for another candidate task flow corresponding to a candidate user intent having the next highest intent confidence score. Depending on whether or not the second task flow score satisfies the predetermined criterion, the another candidate task flow corresponding to the second task flow score can be selected at block  1122 , or additional task flow scores can be subsequently determined for additional candidate task flows based on the associated intent confidence scores. 
     Selecting the first candidate task flow based on the plurality of task flow scores can enhance the accuracy and reliability of the digital assistant on the electronic device. In particular, using the plurality of task flow scores, process  1100  can avoid selecting candidate task flows that cannot be resolved. This can reduce the likelihood of task flow processing errors during execution of the selected first candidate task flow. Moreover, because candidate task flows that cannot be resolved are less likely to coincide with the user&#39;s actual goals, selecting the first candidate task flow based on the plurality of task flow scores can increase the likelihood that the selected first candidate task flow coincides with the user&#39;s actual desired goal. As a result, the accuracy and reliability of the digital assistant on the electronic device can be improved by selecting the first candidate task flow based on the plurality of task flow scores. 
     At block  1124 , the first candidate task flow selected at block  1122  is executed (e.g., using task flow manager  838 ). Specifically, one or more actions represented by the first candidate task flow are performed. In some examples, results are obtained by executing the first candidate task flow. The results can include, for example, information requested by the user in the user utterance. In some examples, not all actions represented by the first candidate task flow are performed at block  1124 . Specifically, actions that provide an output to the user of the device are not performed at block  1124 . For example, block  1124  does not include displaying, on a display of the electronic device, the results obtained by executing the first candidate task flow. Nor does block  1124  include providing audio output (e.g., speech dialogue or music) on the electronic device. Thus, in some examples, the first candidate task flow is executed without providing any output to the user prior to detecting a speech end-point condition at block  1128 . 
     In some examples, executing the first candidate task flow at block  1124  can include performing the operations of block  1126 . At block  1126 , a text dialogue that is responsive to the user utterance is generated (e.g., using task flow manager  838  in conjunction with dialogue flow processing module  734 ). In some examples, the generated text dialogue includes results obtained from executing the first candidate task flow. In some examples, the text dialogue is generated at block  1126  without outputting the text dialogue or a spoken representation of the text dialogue to the user. In some examples, block  1126  further includes additional operations for generating a spoken representation of the text dialogue for output (e.g., operations of blocks  1202 - 1208  in process  1200 , described below with reference to  FIG. 12 ). In these examples, block  1126  can include generating a plurality of speech attribute values for the text dialogue. The plurality of speech attribute values provide information that can be used to generate the spoken representation of the text dialogue. In some examples, the plurality of speech attribute values can include a first speech attribute value that specifies the text dialogue (e.g., a representation of the text dialogue that can be used by a speech synthesis processing module to convert the text dialogue into corresponding speech). In some examples, the plurality of speech attribute values can specify one or more speech characteristics for generating the spoken representation of the text dialogue, such as language, gender, audio quality, type (e.g., accent/localization), speech rate, volume, pitch, or the like. 
     At block  1128 , a determination is made as to whether a speech end-point condition is detected between the second time (e.g., second time  908  or  1008 ) and the third time (e.g., third time  910  or  1012 ). A speech end-point refers to a point in the stream of audio where the user has finished speaking (e.g., end of the user utterance). The determination of block  1128  is made, for example, while the stream of audio is being received from the second time to the third time at block  1102 . In some examples, the determination of block  1128  is performed by monitoring one or more audio characteristics in the second portion of the stream of audio. For instance, in some examples, detecting the speech end-point condition can include detecting, in the second portion of the stream of audio, an absence of user speech for greater than a second predetermined duration (e.g., 600 ms, 700 ms, or 800 ms). In these examples, block  1128  includes determining whether the second portion of the stream of audio contains a continuation of the user utterance in the first portion of the stream of audio. If a continuation of the user utterance in the first portion of the stream of audio is detected in the second portion of the stream of audio, then process  1100  can determine that a speech end-point condition is not detected between the second time and the third time. If a continuation of the user utterance in the first portion of the stream of audio is not detected in the second portion of the stream of audio for greater than the second predetermined duration, process  1100  can determine that a speech end-point condition is detected between the second time and the third time. The absence of user speech can be detected using similar speech detection techniques described above with respect to block  1110 . In some examples, the second predetermined duration is longer than the first predetermined duration of block  1110 . 
     In some examples, detecting the speech end-point condition includes detecting a predetermined type of non-speech input from the user between the second time and the third time. For example, a user may invoke the digital assistant at the first time by pressing and holding a button (e.g., “home” or menu button  304 ) of the electronic device. In this example, the predetermined type of non-speech input can be the user releasing the button (e.g., at the third time). In other examples, the predetermined type of non-speech input is a user input of an affordance displayed on the touch screen (e.g., touch screen  212 ) of the electronic device. 
     In response to determining that a speech end-point condition is detected between the second time and the third time, block  1130  is performed. Specifically, at block  1130 , results from executing the selected first candidate task flow at block  1124  are presented to the user. In some examples, block  1130  includes outputting the results on the electronic device to the user. For example, the results are displayed on a display of the electronic device. The results can include, for example, the text dialogue generated at block  1126 . In some examples, the results are presented to the user in the form of audio output. For example, the results can include music or speech dialogue. 
     In some examples, presenting the results at block  1130  includes performing the operations of block  1132 . Specifically, at block  1132 , a spoken representation of the text dialogue generated at block  1126  is outputted. Outputting the spoken representation of the text dialogue includes, for example, playing an audio file having the spoken representation of the text dialogue. In some examples, outputting the spoken representation of the text dialogue includes performing one or more of the blocks of process  1200 , described below with reference to  FIG. 12 . For example, outputting the spoken representation of the text dialogue includes determining whether the memory of the electronic device stores an audio file having the spoken representation of the text dialogue (block  1204 ). In response to determining that the memory of the electronic device stores an audio file having the spoken representation of the text dialogue, the spoken representation of the text dialogue is outputted by playing the stored audio file (block  1212 ). In response to determining that the memory of the electronic device does not store an audio file having the spoken representation of the text dialogue, an audio file having the spoken representation of the text dialogue is generated (block  1206 ) and stored (block  1208 ) in the memory of the electronic device. In response to determining that the speech end-point condition is detected (block  1128  or  1210 ), the stored audio file is played to output the spoken representation of the text dialogue (block  1212 ). 
     As discussed above, at least partially performing the operations of blocks  1112 - 1126  and/or  1202 - 1208  between the second time and the third time (prior to detecting the speech end-point condition at block  1128  or  1210 ) can reduce the number of operations required to be performed upon detecting the speech end-point condition. Thus, less computation can be required upon detecting the speech end-point condition, which can enable the digital assistant to provide a quicker response (e.g., by presenting the results at block  1130  or outputting spoken dialogue at block  1132  or  1212 ) upon detecting the speech end-point condition. 
     With reference back to block  1128 , in response to determining that a speech end-point condition is not detected between the second time and the third time, process  1100  forgoes performance of block  1130  (and block  1132 ). For example, if process  1100  determines that the second portion of the stream of audio contains a continuation of the user utterance, then no speech end-point condition is detected between the second time and the third time and process  1100  forgoes performance of blocks  1130  and  1132 . Specifically, process  1100  forgoes presenting results from executing the selected first candidate task flow of block  1122 . In examples where text dialogue is generated, process  1100  further forgoes output of a spoken representation of the text dialogue. Furthermore, if process  1100  is still performing any of the operations of blocks  1112 - 1126  or blocks  1202 - 1208  with respect to the utterance in the first portion of the stream of audio, process  1100  ceases to perform these operations upon determining that a speech end-point condition is not detected between the second time and the third time. 
     In some examples, in response to determining that a speech end-point condition is not detected between the second time and the third time, process  1100  can return to one or more of blocks  1102 - 1126  to process the speech in the second portion of the stream of audio. Specifically, upon detecting a continuation of the user utterance in the second portion of the stream of audio, speech recognition is performed (block  1108 ) on the continuation of the user utterance in the second portion of the stream of audio. Additionally, in some examples, process  1100  continues to receive the stream of audio (block  1102 ) after the third time. Specifically, a third portion of the stream of audio can be received (block  1102 ) from the third time (third time  1012 ) to a fourth time (fourth time  1014 ). 
     In some examples, the speech recognition results of the continuation of the user utterance in the second portion of the stream of audio is combined with the speech recognition results of the user utterance in the first portion of the stream of audio to obtain a second plurality of candidate text representations. Each candidate text representation of the second plurality of candidate text representations is a text representation of the user utterance across the first and second portions of the stream of audio. 
     A determination is made (block  1110 ) as to whether the second portion of the stream of audio satisfies a predetermined condition. In response to determining that the second portion of the stream of audio satisfies a predetermined condition, one or more of the operations of blocks  1112 - 1126  are performed with respect to the second plurality of candidate text representations. In particular, in response to determining that the second portion of the stream of audio satisfies a predetermined condition, one or more of the operations of blocks  1112 - 1130  are at least partially performed between the third time (e.g., third time  1012 ) and the fourth time (e.g., fourth time  1014 ) (e.g., while receiving the third portion of the stream of audio at block  1102 ). 
     Based on the second plurality of candidate text representations, a second plurality of candidate user intents for the user utterance in the first and second portions of the stream of audio are determined (block  1112 ). A second plurality of candidate task flows are determined (block  1114 ) from the second plurality of candidate user intents. Specifically, each candidate user intent of the second plurality of candidate user intents is mapped to a corresponding candidate task flow of the second plurality of candidate task flows. A second candidate task flow is selected from the second plurality of candidate task flows (block  1122 ). The selection can be based on a second plurality of task flow scores determined for the second plurality of candidate task flows (block  1116 ). The selected second candidate task flow is executed (block  1124 ) without providing any output to the user prior to detecting a speech end-point condition. In some examples, second results are obtained from executing the second candidate task flow. In some examples, executing the second candidate task flow includes generating a second text dialogue (block  1126 ) that is responsive to the user utterance in the first and second portions of the stream of audio. In some examples, the second text dialogue is generated without outputting the second text dialogue or a spoken representation of the second text dialogue to the user prior to detecting a speech end-point condition. In some examples, additional operations for generating a spoken representation of the second text dialogue for output are performed (e.g., operations in blocks  1202 - 1208  of process  1200 , described below with reference to  FIG. 12 ). 
     In some examples, a determination is made (block  1128 ) as to whether a speech end-point condition is detected between the third time and the fourth time. In response to determining that a speech end-point condition is detected between the third time and the fourth time, second results from executing the selected second candidate task flow are presented to the user (block  1130 ). In some examples, presenting the second results includes outputting, to the user of the device, the spoken representation of the second text dialogue by playing a stored second audio file (e.g., a stored second audio file generated at block  1206 ). 
       FIG. 12  illustrates process  1200  for operating a digital assistant to generate a spoken dialogue response, according to various examples. In some examples, process  1200  is implemented as part of process  1100  for operating a digital assistant. Process  1100  is performed, for example, using one or more electronic devices implementing a digital assistant. Implementing process  1200  in a digital assistant system can reduce the latency associated with text-to-speech processing. In some examples, process  1200  is performed using a client-server system (e.g., system  100 ), and the blocks of process  1200  are divided up in any manner between the server (e.g., DA server  106 ) and a client device (user device  104 ). In some examples, process  1200  is performed using only a client device (e.g., user device  104 ) or only multiple client devices. In process  1100 , some blocks are, optionally, combined, the order of some blocks is, optionally, changed, and some blocks are, optionally, omitted. In some examples, additional operations may be performed in combination with process  1200 . 
     At block  1202 , a text dialogue is received (e.g., at speech synthesis processing module  740 ). In some examples, the text dialogue is generated by the digital assistant system (e.g., at block  1126 ) in response to a received user utterance (e.g., at block  1102 ). In some examples, the text dialogue is received with a plurality of associated speech attribute values (e.g., speech attribute values, described above with reference to block  1126 ). In some examples, the plurality of speech attribute values specify one or more speech characteristics for generating the spoken representation of the text dialogue. The one or more speech characteristics include, for example, language, gender, audio quality, type (e.g., accent/localization), speech rate, volume, pitch, or the like. The combination of the text dialogue and the plurality of speech attribute values can represent a request to generate a spoken representation of the text dialogue in accordance with the one or more speech characteristics defined in the plurality of speech attribute values. 
     In response to receiving the text dialogue at block  1202 , block  1204  is performed. At block  1204 , a determination is made (e.g., using speech synthesis processing module  740 ) as to whether the memory (e.g., memory  202 ,  470 , or  702 ) of the electronic device (e.g., device  104 ,  200 ,  600 , or  700 ) stores an audio file having the spoken representation of the text dialogue. For example, block  1204  includes searching the memory of the electronic device for an audio file having the spoken representation of the text dialogue. In some examples, the memory of the electronic device contains one or more audio files. In these examples, block  1204  includes analyzing each audio file of the one or more audio files to determine whether one of the one or more audio files includes a plurality of speech attribute values that match the plurality of speech attribute values for the text dialogue received at block  1202 . If an audio file of the one or more audio files has a first plurality of speech attribute values that match the plurality of speech attribute values for the text dialogue, then it would be determined that the memory stores an audio file having the spoken representation of the text dialogue. 
     In some examples, block  1204  includes searching the file names of the one or more audio files stored in the memory of the electronic device. In these examples, the file name of each audio file is analyzed to determine whether the file name represents a plurality of speech attribute values that match the plurality of speech attribute values for the text dialogue. Specifically, each file name can encode (e.g., using md5 hash) a plurality of speech attribute values. Thus, analyzing the file names of the one or more audio files stored in the memory can determine whether the memory stores an audio file having the spoken representation of the text dialogue. 
     In response to determining that the memory of the electronic device stores an audio file having the spoken representation of the text dialogue, process  1200  forgoes performance of blocks  1206  and  1208  and proceeds to block  1210 . In response to determining that the memory of the electronic device does not store an audio file having the spoken representation of the text dialogue, block  1206  is performed. 
     At block  1206 , an audio file having the spoken representation of the text dialogue is generated (e.g., using speech synthesis processing module  740 ). In particular, speech synthesis is performed using the text dialogue and the associated plurality of speech attribute values to generate the audio file of the spoken representation of the text dialogue. The spoken representation of the text dialogue is generated according to the one or more speech characteristics specified in the plurality of speech attribute values. 
     At block  1208 , the audio file generated at block  1206  is stored in the memory of the electronic device. In some examples, the audio file having the spoken representation of the text dialogue can indicate the plurality of speech attribute values for the text dialogue. Specifically, in some examples, the audio file having the spoken representation of the text dialogue is stored with a file name that encodes the plurality of speech attribute values for the text dialogue (e.g., using md5 hash). 
     In some examples, blocks  1202 - 1208  are performed without providing any output (e.g., audio or visual) to the user. Specifically, neither the text dialogue nor the spoken representation of the text dialogue is outputted to the user prior to determining that the speech end-point condition is detected at block  1210 . Blocks  1202 - 1208  of process  1200  are performed at least partially prior to a speech end-point condition being detected at block  1210 . This can be advantageous for reducing the response latency of the digital assistant on the electronic device. 
     At block  1210 , a determination is made (e.g., using latency management module  780 ) as to whether a speech end-point condition is detected. Block  1208  is similar or substantially identical to block  1128 , described above. For example, the determination can be made between the second time (e.g., second time  908 ) and the third time (e.g., third time  910 ) while the second portion of the stream of audio is received at block  1102 . In response to determining that a speech end-point condition is detected, block  1212  is performed. Specifically, at block  1212 , the spoken representation of the text dialogue is outputted to the user by playing the stored audio file. Block  1212  is similar or substantially identical to block  1132 . In response to determining that a speech end-point condition is not detected, process  1200  forgoes output of the spoken representation of the text dialogue (block  1214 ). For example, process  1100  can remain at block  1210  to await detection of the speech end-point condition. 
     In the examples described above, a suitable candidate task flow is first selected (block  1122 ) and the selected candidate task flow is then executed (block  1124 ). Moreover, an audio file of spoken dialogue is generated (block  1206 ) only for the selected candidate task flow. However, it should be recognized that, in other examples, a suitable candidate task flow can be selected at block  1122  while executing a plurality of candidate task flows at block  1124 . In certain implementations, executing the plurality of candidate task flows prior to selecting a suitable candidate task flow can be advantageous for reducing latency. Specifically, determining the task flow scores (block  1116 ) and selecting a suitable candidate task flow (block  1122 ) based on the determined task flow scores can be computationally intensive and thus to reduce latency, the plurality of candidate task flows can be executed in parallel while determining the task flow scores and selecting a suitable candidate task flow. In addition, a plurality of respective audio files containing spoken dialogues for the plurality of candidate task flows can be generated at block  1206  while determining the task flow scores and selecting a suitable candidate task flow. By performing these operations in parallel, when a suitable candidate task flow is selected, the selected candidate task flow would have been, for example, at least partially executed and the respective audio file containing spoken dialogue for the selected candidate task flow would have been, for example, at least partially generated. As a result, response latency can be further reduced. Upon detecting a speech end-point condition at block  1128  or  1210 , the result corresponding to the selected candidate task flow can be retrieved from the plurality of results and presented to the user. In addition, the audio file corresponding to the selected candidate task flow can be retrieved from the plurality of audio files and played to output the corresponding spoken dialogue for the selected candidate task flow. 
     The operations described above with reference to  FIGS. 11A-11B and 12  are optionally implemented by components depicted in  FIGS. 1-4, 6A -B,  7 A- 7 C, and  8 . For example, the operations of processes  1100  and  1200  may be implemented by I/O processing module  728 , STT processing module  730 , natural language processing module  732 , dialogue flow processing module  734 , task flow processing module  736 , speech synthesis processing module  740 , audio processing module  770 , latency management module  780 , task flow manager  838 , and task flow resolver  840 . It would be clear to a person having ordinary skill in the art how other processes are implemented based on the components depicted in  FIGS. 1-4, 6A -B,  7 A- 7 C, and  8 . 
     In accordance with some implementations, a computer-readable storage medium (e.g., a non-transitory computer-readable storage medium) is provided, the computer-readable storage medium storing one or more programs for execution by one or more processors of an electronic device, the one or more programs including instructions for performing any of the methods or processes described herein. 
     In accordance with some implementations, an electronic device (e.g., a portable electronic device) is provided that comprises means for performing any of the methods or processes described herein. 
     In accordance with some implementations, an electronic device (e.g., a portable electronic device) is provided that comprises a processing unit configured to perform any of the methods or processes described herein. 
     In accordance with some implementations, an electronic device (e.g., a portable electronic device) is provided that comprises one or more processors and memory storing one or more programs for execution by the one or more processors, the one or more programs including instructions for performing any of the methods or processes described herein. 
     The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the techniques and their practical applications. Others skilled in the art are thereby enabled to best utilize the techniques and various embodiments with various modifications as are suited to the particular use contemplated. 
     Although the disclosure and examples have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the disclosure and examples as defined by the claims.

Metadata:
Filing Date: 20200820
Publication Date: 20220705
Grant Date: 20220705
Priority Date: 20170512
Inventors: ACERO, ALEJANDRO
ZHANG, HEPENG
Assignee: APPLE INC
CPC Classifications: [{"code": "G10L15/183", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L15/22", "inventive": true, "first": true, "tree": "[]"}, {"code": "G10L15/30", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L13/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L2015/223", "inventive": false, "first": false, "tree": "[]"}, {"code": "G10L25/78", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/16", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L2015/223", "inventive": false, "first": false, "tree": "[]"}, {"code": "G10L25/78", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/16", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L15/22", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L25/87", "inventive": false, "first": false, "tree": "[]"}, {"code": "G10L25/87", "inventive": false, "first": false, "tree": "[]"}, {"code": "G10L15/22", "inventive": true, "first": true, "tree": "[]"}, {"code": "G10L15/1822", "inventive": true, "first": true, "tree": "[]"}, {"code": "G10L13/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L15/183", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L25/87", "inventive": false, "first": false, "tree": "[]"}, {"code": "G10L15/22", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L2015/223", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/16", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L25/78", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L15/30", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L15/1822", "inventive": true, "first": true, "tree": "[]"}, {"code": "G10L15/183", "inventive": true, "first": false, "tree": "[]"}, {"code": "G10L13/04", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 64097963