Patent Publication Number: US-9852729-B2

Title: Low latency and memory efficient keyword spotting

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/903,814, entitled LOW LATENCY AND MEMORY EFFICIENT KEYWORD SPOTTING and filed May 28, 2013, now U.S. Pat. No. 9,390,708, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     Computing devices can be used to process a user&#39;s spoken commands, requests, and other utterances into written transcriptions. In a common application, a user can speak into a microphone of a computing device, and an automatic speech recognition module executing on the computing device can process the audio input and determine what the user said. Additional modules executing on the computing device can process the transcription of the utterance to determine what the user meant and/or perform some action based on the utterance. 
     Automatic speech recognition systems typically include an acoustic model and a language model. The acoustic model is used to generate hypotheses regarding which subword units (e.g., phonemes) correspond to an utterance based on the acoustic features of the utterance. The language model is used to determine which of the hypotheses generated using the acoustic model is the most likely transcription of the utterance based on lexical features of the language in which the utterance is spoken. 
     Some speech recognition systems are configured to spot particular keywords in a user utterance. Recognition of such keywords can trigger other actions. For example, an automatic speech recognition system may be used to process utterance audio and generate a transcript of the utterance. The system can then determine whether the transcript includes a particular keyword that the system is configured to spot. If the keyword has been positively spotted, another application or process may be initiated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of various inventive features will now be described with reference to the following drawings. Throughout the drawings, reference numbers may be re-used to indicate correspondence between referenced elements. The drawings are provided to illustrate example embodiments described herein and are not intended to limit the scope of the disclosure. 
         FIG. 1  is a flow diagram of an illustrative process for performing speech recognition processing on an utterance to spot one or more keywords according to one embodiment. 
         FIG. 2  is a block diagram of an illustrative user device that may implement the process for performing speech recognition processing on user utterances to spot one or more keywords. 
         FIG. 3  is a diagram of an illustrative finite state transducer (“FST”) for performing keyword recognition according to an embodiment. 
         FIG. 4  is a diagram of traceback data for states of a finite state transducer over the course of multiple frames of audio data. 
         FIG. 5  is a diagram of multiple memory blocks for storing traceback data, illustrating several pointers that may be used in the traceback storage process and the allocation of a new memory block. 
         FIG. 6  is a diagram of differences, over time, between the likelihood that particular frames of input data correspond to a keyword and the likelihood that the frames correspond to other speech or non-speech sounds. 
     
    
    
     DETAILED DESCRIPTION 
     Speech recognition systems may be used to spot particular keywords in a user utterance in addition to, or instead of, generating transcriptions of full utterances. Spotting keywords can be useful when certain actions are to be triggered in response to a keyword being uttered (e.g., navigating a voice-activated menu system). A standard HMM-based large vocabulary continuous speech recognition (“LVCSR”) system that processes utterances to generate transcriptions may be configured to trigger whenever a keyword is part of a recognized word sequence. However, LVCSR decoding is expensive computationally. A common approach is to simplify the LVCSR system by replacing the parts not related to the keyword by a universal background model (“UBM”). The UBM reduces computational load, but also reduces accuracy. A second drawback of the standard approach, independent of using an UBM or not, is latency: only after processing the last feature of an utterance the system can determine whether the keyword was uttered. An end-of-utterance is usually defined as a period of substantial length without speech. This behavior may not be useful in certain real-world applications, where an immediate response to the keyword is required even if it is followed by further speech. For example, assuming silence after the keyword may not be practical in systems that are to be addressed verbally, followed immediately or substantially immediately by a spoken command (e.g., “computer, play music by Frank Sinatra”). 
     Aspects of this disclosure relate to a keyword spotter that is triggered substantially immediately in response to a keyword being spoken, or shortly thereafter, thereby reducing the latency inherent in existing LVCSR systems. In addition, the keyword spotter of the present disclosure does not assume or require silence after the keyword, thereby improving latency over existing UBM-based keyword spotters. The keyword spotter can be configured to assume that each frame of audio input data may be the final frame of a keyword. In some embodiments, the system may determine a probability for each frame regarding whether the frame corresponds to a model of a keyword sound. The system may also determine a probability that the frame corresponds to general speech or non-speech sounds. A keyword likelihood may be continuously updated based on the probability that each frame corresponds to a model of a keyword sound. A background likelihood may also be continuously updated based on the probability that each frame corresponds to a model of general speech or non-speech sounds. When the difference between the keyword likelihood and the background likelihood exceeds a threshold, the system may determine that a keyword has been uttered. The system can then trigger some action, return some data, etc. 
     Additional aspects of the disclosure relate to the efficient management of traceback information, generated during speech processing, so that a second processing pass may be performed on the frames that the keyword spotter has determined are likely frames of a keyword. By more efficiently handling the memory used to store traceback data, a second speech recognition pass with a larger model (e.g., one that is not limited to only speech, non-speech, and keyword states) may be performed without adding a substantial delay to the overall process. As a result, the system can provide improved keyword recognition accuracy when compared to one-pass keyword spotters. In some embodiments, traceback information may be discarded for frames that are outside a window of time within which a keyword would be spotted. For example, a speech recognition hypothesis that is based on several seconds of data probably does not correspond to a single keyword or phrase, which would typically encompass less than a second of audio data. Discarding traceback information for such frames can reduce the amount of memory required to store traceback data. In addition, the process of pruning dangling back pointers from the traceback data can be eliminated due to the limited amount of data that is maintained. 
     Aspects of the embodiments described in the disclosure will focus, for the purpose of illustration, on a device configured to both receive spoken input and process the spoken input to spot keywords. However, the systems and techniques described herein may be implemented over a network, such as by a network-accessible speech processing system. In addition, certain features described in the disclosure may be applied to speech processing outside of the keyword spotting context. Various aspects of the disclosure will now be described with regard to certain examples and embodiments, which are intended to illustrate but not limit the disclosure. 
     Process for Keyword Spotting 
     With reference to an illustrative embodiment,  FIG. 1  shows a process  100  for spotting keywords in an utterance. Advantageously, the process  100  includes efficient memory management when storing traceback information that may be used for subsequent speech processing passes. Such efficient memory management can facilitate an efficient second speech processing pass, thereby improving keyword recognition accuracy. In addition, the keyword spotting used in the process  100  may provide rapid spotting of keywords within utterances (e.g., silence after the keyword is not required or assumed), thereby allowing other modules, applications, or systems to be triggered by the recognition of a keyword even though an utterance has not yet been fully processed. 
     In addition to spotting keywords within utterances (or by themselves), the process  100  may be used to spot special keywords, also known as “wake words.” For example, a device implementing the process  100  may be configured to identify when a user speaks a wake word that indicates that the user is addressing the device rather than, e.g., other users. The user may use the wake word to address the device and indicate that a spoken command will follow (e.g., in the utterance “phone, play music by Frank Sinatra,” the wake word is “phone” and the command is “play music by Frank Sinatra”). In this way, the wake word may be a verbal command indicator. The device need not be controlled via some physical input component (e.g., a keyboard, mouse, etc.), but rather may be activated and controlled through spoken commands without any non-verbal indication that a spoken command is coming. In some embodiments, the device may “listen” to all available audio, and perform some ASR processing and keyword spotting to determine when a keyword, wake word, or other command identifier is spoken. When a user speaks a wake word and issues a spoken command to the device, the device can recognize that the wake word was spoken, and initiate some application or other functionality that is to be triggered by the wake word. One example of a system for wake word identification and processing is disclosed in U.S. patent application Ser. No. 13/711,510, filed on Dec. 11, 2012 and incorporated herein by reference for all that it discloses. 
       FIG. 2  illustrates an example user device  200  that may perform the process  100 . The user device  200  may correspond to a wide variety of electronic devices. Illustratively, the user device  200  may be a mobile device, such as a mobile phone, personal digital assistant (“PDA”), mobile gaming device, media player, electronic book reader, tablet computer, laptop computer, or the like. In some embodiments, the user device  200  may be a substantially stationary device, such as a television with program execution capabilities and network connectivity (a “smart TV”), set-top box, gaming console, home entertainment system, desktop computer, server computer, or the like. 
     In some embodiments, as shown, the user device  200  may be a computing device that includes one or more processing units  202 , a computer readable medium drive  204 , an input/output device interface  206 , and a memory  210 . Various data and software modules may be stored in the memory  210  at any time, such as an operating system  212 , an ASR module  216 , traceback data  216 , and the like. The user device  200  shown in  FIG. 2  and described herein is illustrative only, and not intended to be limiting. In some embodiments, the user device  200  may include additional, fewer, or alternative modules or components. 
     With reference to  FIG. 1 , the process  100  may begin at block  102 . The process  100  may be embodied in a set of executable program instructions stored on, e.g., the computer readable medium drive  204  of the user device  200 . When the process  100  is initiated, the executable program instructions (e.g., the ASR module  214 ) can be loaded into memory  210  of the user device  200  and executed by the one or more processing units  202  that are in communication with the memory  210 . 
     The process  100  may be automatically initiated upon receipt of an audio stream, and the user device  200  may begin processing the stream as it is received. The audio stream may be a substantially real-time stream of user utterance audio. In some embodiments, as described above, the client device  200  may “listen” for audio for extended periods of time, or indefinitely until powered off or terminated, etc. Input/output device interface  206  may facilitate the receipt of an utterance from a user  250 . For example a microphone may be coupled to, or in communication with, the input/output device interface  206  to accept speech input on which to perform speech recognition and keyword spotting. When a user  250  speaks, audio data of the user&#39;s utterance may be passed by the microphone to the input/output device interface  206 . The input/output device interface  206  may communicate the audio data to the processing unit  202 , or it may have direct access to the memory  210  and can therefore store the audio data in the memory  210  for processing. 
     The ASR module  214  or some other module or component may extract features from the input audio. Audio input may be partitioned into a sequence of small units of audio, also known as frames. Illustratively, each frame may include 10 ms of audio. A feature vector may be computed or generated based on each frame of audio data, such that a sequence of feature vectors corresponding to the sequence of frames is generated. The feature vectors can include data regarding the acoustic features of the frame and other data associated with the frame. 
     At block  104 , the ASR module  214  or some other module or component can compute speech recognition scores (e.g., likelihoods or probabilities) for each frame based on a hidden Markov model (“HMM”) for the keyword being recognized. In some embodiments, the HMM may be modeled by an FST.  FIG. 3  shows an example FST  300  that may be used in the process  100 . The FST  300  is a graphical representation of the HMM used in the HMM-based recognition system. Speech recognition decoding with the FST  300  involves aligning feature vectors with the states of the FST  300  (e.g., the numbered circles), and determining or computing a score, such as a likelihood that the frame represented by the feature vector correctly corresponds to a portion of the keyword (e.g., phoneme or other subword unit) associated with each state. 
     The FST  300  represents an HMM for a particular keyword, in this case “Geo.” The process  100  can therefore use this FST  300  to spot instances of the keyword “Geo” in user utterances. The FST  300  is illustrative only, and is not intended to be limiting. The number of individual states in the FST can vary depending upon the particular keyword to be decoded. In some embodiments, the FST may represent an HMM for a phrase of multiple words, rather than a specific keyword. In some embodiments, the FST may represent HMMs for several stand-alone keywords, and may therefore be used to spot multiple different keywords at a time, rather than only one keyword. In some embodiments, the process  100  may use multiple, separate FSTs in parallel to spot different keywords or perform different functions. 
     The keyword “Geo” has three phonemes, and therefore three states in the FST  300 : states 4, 5, and 6. Each of the states has an arc to a subsequent state and also a self looping arc. Each arc is associated with a number that indicates the probability density function (“PDF”) for the phoneme that corresponds to the current state. Illustratively, state 4 is the originator of a self-looping arc and an arc pointing to state 5. The number 17 that appears near both of the arcs represents the particular PDF for state 4 (e.g., the Gaussian, of a Gaussian mixture model, for the first phoneme of “Geo”). In a similar manner, state 5 is the originator of arcs associated with the Gaussian mixture model (“GMM”) index for the second phoneme of the keyword, and the state 6 is the originator of arcs associated with the GMM index for the third phoneme of the keyword. Note that some arcs in the FST  300  are not associated with a PDF (e.g., they do not have numbers next to them), and therefore do not consume or absorb a frame during processing. The frames may also be known as epsilon arcs. For example, the arc between states 0 and 3 is not associated with a PDF, and therefore a frame of audio data is not consumed in the transition from state 0 to 3. Rather, a label has been added to the arc from state 0 to 3 (e.g., “&lt;kw”), indicating that this is the beginning of the keyword. Such labels are illustrative only, and are not used to calculate keyword or background likelihoods. The FST  300  illustrated in  FIG. 3  requires at least 4 frames of input data, starting at state 0, to reach state 9 where the overall likelihood for the keyword is calculated (e.g., the arcs betweens states 3-4, 4-5, 5-6, and 6-7 each consume a frame because they are associated with a PDF, while the arcs between states 0-3, 7-8 and 8-9 do not consume any frames because they are not associated with any PDF). 
     In addition to states for the keyword, the FST  300  has background states for all other speech sounds (state 1) and for all non-speech sounds (state 2). The GMMs that are used in conjunction with the FST  300  may include a subset of the Gaussians of the full acoustic model used the speech processing system. As a result, decoding with the FST  300  may be substantially faster than a full speech recognizer because the size of the FST  300  and the number of computations per feature vector is much smaller. In some embodiments, the speech and non-speech GMMs are derived by merging all speech-related and non-speech-related GMMs into a single speech-related GMM and non-speech-related GMM, followed by a clustering step which reduces the number of Gaussians in the new GMMs. In some embodiments, additional or alternative background models may be used. For example, background models for music, mechanical sounds, and the like may be used instead of, or in addition to, general speech and non-speech models. 
     As frames of audio data are processed, they may be matched against the PDF for each state (e.g., the first frame may be matched against the PDFs for states 1, 2, and 4; the second frame against states 1, 2, 4, and 5; the third frame against 1, 2, 4, 5, and 6, and so on). Scores (e.g., numbers representing likelihoods or probabilities) can be computed or otherwise determined for each state. Once a sufficient number of frames has been processed to reach the final state of the keyword (e.g., at least 4 frames to reach state 9 from state 0 in the FST  300  illustrated in  FIG. 3 ), a keyword score, such as a score representing the likelihood that the current frame represents the final frame of the keyword, can be determined at state 9. In addition, after each frame, a likelihood score for the speech model (at state 1) and non-speech model (at state 2) can be updated. The higher of the two likelihood scores may be used as the overall likelihood for the background models (at state 0). 
     The self-looping arcs represent frames that both begin and end in a particular state (e.g., each phoneme of a keyword may encompass multiple frames of audio data, depending upon the phoneme itself, how fast a user is speaking, etc.). The arcs that connect two states represent transition frames (e.g., the last frame associated with a particular phoneme, or the frame that encompasses the transition from one phoneme to the next). The arrows for each arc point in only one direction, and therefore each state may only be arrived at directly from one previous state (the straight arcs) or from itself (the self-looping arcs). 
     In one example, a user may speak an utterance that includes the word “Jeff.” The feature vectors for the first few frames (e.g., 1 or 2 frames), corresponding to the first phoneme of “Jeff,” are processed against states 1, 2, and 4 in the FST. Illustratively, a high probability may be calculated for those frames with respect to state 4, which represents the first phoneme of the keyword “Geo,” due to the similar or identical initial sounds of the two words. In addition, a moderately high probability may be calculated for those frames with respect to state 1, which represents all speech sounds, while a low probability may be calculated for state 2, which represents non-speech sounds. 
     For each feature vector that is processed against the FST, a background likelihood (e.g., for state 0 in the FST  300 ) can be computed or updated to indicate the likelihood that the feature vector corresponds to a background sound or no sound. Once a sufficient number of frames has been processed, a keyword likelihood score (e.g., for state 9 in the FST  300 ) can be computed or updated to indicate the likelihood that the feature vector corresponds to the last frame of a keyword. A difference between the keyword likelihood at state 9 and the better of the background likelihoods at state 0 (e.g., the better of the likelihoods at states 1 and 2) can be computed, and that difference may form the basis of determining that a keyword has been spotted, as described in detail below with respect to decision block  116 . 
     The feature vectors for the frames that correspond to the second phoneme in the word “Jeff” are also processed against the states in the FST. Illustratively, a relatively low probability may be calculated for those frames with respect to state 5, which represents the second phoneme of the keyword “Geo,” due to the different sounds of the second phonemes of each word. A moderately high probability may again be calculated for those frames with respect to state 1, which represents all speech sounds, and a low probability may again be calculated for state 2, which represents non-speech sounds. As the feature vectors for the word “Jeff” continue to be processed, a keyword likelihood at state 9 may be computed (e.g., after 4 or more feature vectors have been processed against the FST  300 ). As those frames that correspond to the third phoneme of “Jeff” are processed, the keyword likelihood may be low, compared to the background likelihood, due to the low likelihood that feature vectors for frames of the third phoneme in “Jeff” correspond to the third phoneme of “Geo.” 
     While processing the feature vectors against the FST (or afterwards), data regarding likely paths through the FST may be recorded. This data may be referred to as traceback data. Traceback data can indicate a relationship between feature vectors (e.g., between a current feature vector and a prior feature vector in a sequence of feature vectors). Traceback data may include a back pointer indicating a state prior to the current state for the current feature vector, a likelihood score indicating a likelihood that the frame associated with the feature vector corresponds to the current state, and/or other information that may be used to determine alignments. A second processing pass may use the traceback data in order to align frames with their likely corresponding states in the FST. For example, the traceback data can indicate sequences of multiple consecutive frames that most likely correspond to a single phoneme (e.g., how many times a state was arrived at via a self looping arc) and which frames most likely indicate the beginning or ending of a phoneme or a transition between phonemes (e.g., when a state was arrived at from an arc connecting to a prior state). 
       FIG. 4  shows a visual representation of traceback data for several frames of audio data (X1-X5 on the horizontal “time” axis). The nodes represent each state of the FST  300  for each frame of audio data. The back pointer arrows indicate the previous state for a current state of a particular frame of data. Illustratively, node  406  may represent state 5 from the FST  300  for a particular frame X3. As shown in  FIG. 2 , state 5 may only be arrived at via an arc originating at state 4 or originating at itself, state 5. The back pointer  412  in  FIG. 4  indicates that the prior state, at frame X2, was state 5 (e.g., the probability that the prior state was state 5 is higher than the probability that the prior state was 4, and therefore the most likely path to state 5 in frame X3 is a self looping arc rather than an arc from state 4). Node  404  represents state 5 for frame X2, and the back pointer  410  to node  408  indicates that most likely path to state 5 at frame X2 was a transition arc from state 4. 
     As shown, node  402  is not connected to any node at frame X2 by a back pointer. This means that no state at frame X2 was most likely arrived at via any arc originating from state 5 (e.g., each state for frame X2 was most likely arrived at via an arc originating from other states, rather than from state 5). Node  418  is not connected to any node at frame X3, even though it is connected to node  414  via back pointer  418 . In some speech processing systems, dead end nodes and dangling back pointers, such as nodes  418 ,  414 ,  402 , and back pointer  420 , are pruned from the traceback data in order to reduce the total amount of memory that is required to store the traceback data. However, such pruning can be computationally expensive. 
     Additionally, in some speech processing systems all back pointers are stored for all currently active states. This means that, e.g., the back pointer  410  to node  408  must be maintained because the most recently processed frame (X5) includes states that are connected, via back pointers, to node  408 . However, for a keyword spotting system that may run for extended periods of time, this means that some back pointers would need to be maintained indefinitely, even though a keyword is typically encompassed by less than one second of audio data. 
     Process  100  includes operations for more efficient management of memory than the existing speech processing systems described above. For example, rather than maintaining back pointers indefinitely, the system may use an assumption about the maximum length L of the keyword (e.g., in number of frames or length of time). In some embodiments, the expected maximum length L of the keyword may be based on one or more observations of prior utterances of the keyword. The system may then discard all back pointers older than t−L, where t is the current time frame. As a result, the maximum number of back pointers in the traceback data is S*L, where S is the total number of states in the FST. Typically, S may be a relatively small number (e.g., S&lt;100), such that the total maximum number of back pointers in the traceback data is also relatively small when compared to the traceback data maintained by systems that keep all back pointers for currently active states. 
     As shown in  FIG. 4 , the nodes and back pointers drawn with broken lines all relate to data from frame X1. In a system which ignores or discards back pointers that are older than a time t−L, and where L corresponds to 3 frames, then the system may ignore or discard all traceback data from before frame X2. As a result, all nodes and back pointers drawn in broken lines may be ignored, discarded, or overwritten. 
     In addition, the system may be configured to skip the expensive process of pruning dangling back pointers, because the total amount of traceback data that is maintained is relatively small, even with the dangling back pointers. This has the added benefit of keeping the traceback data in sequential memory spaces, which facilitates efficient processing. 
       FIG. 5  illustrates the traceback data  216  in the memory  210  of the user device  200 . In some embodiments, as shown, traceback data  216  may be stored in one or more memory blocks  502 ,  504 , and  506 . Each block may be sized to hold a particular number of traceback items (e.g., back pointers or other traceback information). The keyword spotting system may maintain several memory pointers to facilitate use of the memory blocks. For example, the system may use a next pointer  510  which stores an address of the first memory location in the next memory block (block  502  in this example), a memory block pointer  512  which stores an address of the first memory location in the current memory block (block  506  in this example), a current pointer  514  which stores an address of the memory location within the current memory block into which the most recent traceback item was stored, and an end pointer  516  which stores an address of the last memory location within the current memory block. As can be seen in  FIG. 5 , by not pruning the dangling back pointers from the traceback data  216 , the memory locations can remain sequential within a given memory block, which facilitates fast access and processing of traceback data  216  in subsequent speech processing passes (e.g., pointers can be simply incremented and decremented to access the next or previous traceback item because they are stored at the next or previous adjacent memory location, respectively). 
     With reference to  FIG. 1 , blocks  106 - 114  show an example implementation of storing traceback information for the current frame of audio data, as generated in block  104  and described above, in the traceback data  216  portion of the memory  210 . At decision block  106 , the ASR module  214  or some other module or component can determine whether the current traceback data memory block is full. In some embodiments, the ASR module  214  may do so by incrementing the address in the current pointer  514  and comparing it to the address in the end pointer  516 . If they are equivalent, then the current memory block is full, and the process  100  may proceed to block  110 . Otherwise, the process may proceed to block  108 , where the traceback item for the current frame is stored in the next available memory location (e.g., the memory location at the address in the current pointer  514 ). 
     At decision block  110 , the ASR module  214  or some other module or component can determine whether the next memory block contains traceback information associated with frames from a time before a threshold time. For example, the ASR module  214  can inspect a field or property associated with the next memory block in the ASR data  216  (e.g., the memory block that begins with the memory location at the address in the next pointer  510 ). If the data is associated with frames from before the threshold period of time described above (e.g., from before t−L), then the process  100  can proceed to block  114 . Otherwise, the process may proceed to block  112 . 
     At block  112 , the ASR module  214  or some other module or component may allocate a new memory block (e.g., memory block  508  in  FIG. 5 ) in which to store traceback items. The traceback item for the current frame may be stored at the first memory location of the new memory block, and all pointers may be updated accordingly. 
     At block  114 , the ASR module  214  or some other module or component may overwrite data in an existing memory block because the data is associated with old frames of data (e.g., frames of data that are older than a threshold, as determined in decision block  110 ). In some embodiments, the ASR module  214  may first delete data in the existing memory block, and then store the new traceback information in the memory block. 
     Table 1 presents pseudo code for managing the traceback data  216  memory blocks of  FIG. 5 , as described above. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 // memory block; holds SIZE traceback items 
               
               
                 struct MemoryBlock 
               
               
                 { 
               
            
           
           
               
               
            
               
                   
                 static const int SIZE; 
               
               
                   
                 MemoryBlock *next; 
               
               
                   
                 int maxTime; 
               
               
                   
                 Backpointer *begin, *end; 
               
               
                   
                 Backpointer *current; 
               
               
                   
                 MemoryBlock(MemoryBlock *next=0) 
               
               
                   
                 { 
               
            
           
           
               
               
            
               
                   
                 if (next == 0) { this−&gt;next = next; } else { this−&gt;next = 
               
            
           
           
               
            
               
                 this; } 
               
            
           
           
               
               
            
               
                   
                 maxTime = 0; 
               
               
                   
                 begin = new Backpointer[SIZE]; 
               
               
                   
                 end = begin + SIZE; 
               
               
                   
                 current = begin; 
               
            
           
           
               
               
            
               
                   
                 } 
               
            
           
           
               
            
               
                 }; 
               
               
                 // memory location of current memory block 
               
               
                 MemoryBlock *memoryBlock = new MemoryBlock; 
               
               
                 Backpointer * storeTracebackItem(int time, Backpointer &amp;bptr) 
               
               
                 { 
               
            
           
           
               
               
            
               
                   
                 memoryBlock−&gt;current += 1; 
               
               
                   
                 // current memory block is full 
               
               
                   
                 if (memoryBlock−&gt;current == memoryBlock−&gt;end) 
               
               
                   
                 { 
               
            
           
           
               
               
            
               
                   
                 // next memory block cannot be overwritten, allocate new 
               
            
           
           
               
            
               
                 one 
               
            
           
           
               
               
            
               
                   
                 if memoryBlock−&gt;next−&gt;maxTime + maxKeywordLength &gt; 
               
               
                   
                 time 
               
               
                   
                 { 
               
            
           
           
               
               
            
               
                   
                 memoryBlock−&gt;next = new MemoryBlock(memoryBlock− 
               
            
           
           
               
            
               
                 &gt;next); 
               
            
           
           
               
               
            
               
                   
                 } 
               
               
                   
                 // initialize next memory block 
               
               
                   
                 memoryBlock = memoryBlock−&gt;next; 
               
               
                   
                 memoryBlock−&gt;current = memoryBlock−&gt;begin; 
               
            
           
           
               
               
            
               
                   
                 } 
               
               
                   
                 // store traceback item 
               
               
                   
                 memoryBlock−&gt;maxTime = time; 
               
               
                   
                 *memoryBlock−&gt;current = bptr; 
               
               
                   
                 // return new memory location of traceback item 
               
               
                   
                 return memoryBlock−&gt;current; 
               
            
           
           
               
            
               
                 } 
               
               
                   
               
            
           
         
       
     
     After storing the traceback item for the current frame according to the features described above with respect to blocks  108 ,  112 , or  114 , the process  100  may proceed to decision block  116  to determine whether a keyword has been spotted. In some embodiments, the keyword spotting determination of block  116  may be made prior to, or in parallel with, the traceback data storage and memory management process of blocks  106 - 114 . 
     At decision block  116 , the system can determine whether a keyword has been spotted. As described above, the system may determine that the keyword has been spotted if the difference between the keyword likelihood at state 9 of the FST  300  and the background likelihood at state 0 exceeds some threshold, with the keyword likelihood being larger than the background likelihood. If so, the process  100  can proceed to block  120 . Otherwise, the process  100  can return to block  104 , where the next frame of audio data is processed. 
     In some embodiments, the system may not immediately trigger for a keyword upon determining that the difference in likelihoods described above exceeds a threshold. Such an approach may not find the precise ending of a keyword, because the difference in likelihoods may exceed the threshold a few frames before the keyword ends. Finding the exact end time is important for post-processing, such as the second speech recognition pass described below for confirming the recognition of the keyword. To ensure that each frame of the keyword has been processed, a timeout may be implemented before the system triggers for the keyword. 
     In some embodiments, the difference between the keyword likelihood and background likelihood is computed continuously (e.g., for each frame), as described above. If the keyword likelihood is greater than the background likelihood, and if the difference exceeds a threshold and is the largest difference in a time window [t−d, t+d] (where d is some number of frames based on, e.g., a compromise between the amount of latency that is acceptable to users and the amount of time needed to ensure that the end of the keyword has been reached), then the system can trigger for the keyword at time t (e.g., the frame at time t is the last frame of the keyword). By using this approach, the system can trace an increase in the likelihood difference and also tolerate an unsteady, non-smooth increase. 
     In addition, after triggering for the keyword, the system can impose a timeout before the keyword is subsequently spotted again. For example, the timeout may be set to a period of time that is shorter than the length of the keyword, such as D frames, where T&gt;D&gt;=d, and where T is the approximate length of the keyword. Using these parameters, the keyword cannot end again at time t+D, and therefore the system need not use resources determining whether the keyword is ending during the timeout. 
       FIG. 6  shows an illustrative graph of a likelihood difference over time, as observed during the process  100 . As shown, a local maximum  602  may occur while processing an utterance, and the local maximum  602  may exceed the threshold for triggering a keyword. However, the frame corresponding to the local maximum may not be the last frame of the keyword. By waiting to observe a maximum  606  at time t within the window [t−d, t+d], the system can reduce or eliminate the chance of triggering for a keyword before the last frame has been identified. The keyword may be triggered at the end  606  of the window [t−d, t+d]. Table 2 presents pseudo code for tracking the likelihood difference, as described above. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 const float scoreDiffThreshold; 
               
               
                   
                 const int windowLength; 
               
               
                   
                 const int timeoutLength; 
               
               
                   
                 assert(timeoutLength &gt; windowLength) 
               
               
                   
                 float bestScoreDiff = −infinity; 
               
               
                   
                 int bestScoreDiffDurtation = 0; 
               
               
                   
                 int timeout = 0; 
               
               
                   
                 def detectKeyword(scoreDiff): 
               
            
           
           
               
               
            
               
                   
                 if timeout &gt; 0: 
               
            
           
           
               
               
            
               
                   
                 timeout −= 1; 
               
               
                   
                 return false; 
               
            
           
           
               
               
            
               
                   
                 if scoreDiff &gt; bestScoreDiff: 
               
            
           
           
               
               
            
               
                   
                 bestScoreDiff = scoreDiff; 
               
               
                   
                 bestScoreDiffDuration = 0; 
               
            
           
           
               
               
            
               
                   
                 if bestScoreDiff &gt; scoreDiffThreshold: 
               
            
           
           
               
               
            
               
                   
                 bestScoreDiffDuration += 1 
               
               
                   
                 if bestScoreDiffDuration == windowLength: 
               
            
           
           
               
               
            
               
                   
                 bestScoreDiff = −infinity; 
               
               
                   
                 bestScoreDiffDuration = 0; 
               
               
                   
                 timeout = timeoutLength − windowLength 
               
               
                   
                 return true 
               
            
           
           
               
               
            
               
                   
                 return false 
               
               
                   
                   
               
            
           
         
       
     
     At block  118 , the ASR module  214  or some other module or component may extract alignments from the traceback data stored as described above. In some embodiments, the alignments can be used to optionally rescore the feature vectors that correspond to the spotted keyword, as described below. 
     At block  120 , the ASR module  214  or some other module or component may optionally rescore the feature vectors that correspond to the spotted keyword. The ASR module  214  system may use a full LVCSR model and speech recognition pass to do so, thereby improving the accuracy of the keyword recognition over the fast keyword spotting process described above. In order to reduce the computational overhead and latency introduced by performing a second speech recognition pass with a full LVCSR model, the ASR module  214  may use information generated during the first keyword spotting pass and use a tighter beam for pruning the decoding graph. For example, the ASR module  214  may use the traceback information and keyword likelihoods generated during the first keyword spotting pass. The ASR module  214  may maintain a narrow beam of alternative likely states because it is only confirming that the frames most likely correspond to the keyword. In some embodiments, the ASR module  214  may keep active, for each frame, only those states for which likelihood of the state is within some threshold of the likelihood for the state of the keyword at the same frame. 
     The system may not determine what word or words the frames most likely correspond to if they do not correspond to the keyword; once it is determined that the keyword is not the most likely transcription, the process  100  can terminate (or return to block  102  or  104  to continue process an utterance or process a subsequent utterance). In addition, because the system may not determine what the correct transcription is, the system may not maintain back pointers during the re-scoring process, thereby further improving efficiency. 
     Terminology 
     Depending on the embodiment, certain acts, events, or functions of any of the processes or algorithms described herein can be performed in a different sequence, can be added, merged, or left out altogether (e.g., not all described operations or events are necessary for the practice of the algorithm). Moreover, in certain embodiments, operations or events can be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors or processor cores or on other parallel architectures, rather than sequentially. 
     The various illustrative logical blocks, modules, routines and algorithm steps described in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. The described functionality can be implemented in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the disclosure. 
     The steps of a method, process, routine, or algorithm described in connection with the embodiments disclosed herein can be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module can reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of a non-transitory computer-readable storage medium. An exemplary storage medium can be coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium can be integral to the processor. The processor and the storage medium can reside in an ASIC. The ASIC can reside in a user terminal. In the alternative, the processor and the storage medium can reside as discrete components in a user terminal. 
     Conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. 
     Conjunctive language such as the phrase “at least one of X, Y and Z,” unless specifically stated otherwise, is to be understood with the context as used in general to convey that an item, term, etc. may be either X, Y or Z, or a combination thereof. Thus, such conjunctive language is not generally intended to imply that certain embodiments require at least one of X, at least one of Y and at least one of Z to each be present. 
     While the above detailed description has shown, described and pointed out novel features as applied to various embodiments, it can be understood that various omissions, substitutions and changes in the form and details of the devices or algorithms illustrated can be made without departing from the spirit of the disclosure. As can be recognized, certain embodiments of the inventions described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others. The scope of certain inventions disclosed herein is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.