Abstract:
System speakers are switched to function as sound input transducers to improve recognizer performance and to support recognizer features. A crossbar switch is selectively activated, either manually or under software control, to allow system loudspeakers to function as sound input transducers that supplement the recognition system microphone or microphone array. Using loudspeakers as “microphones” improves speech recognition in noisy environments, thus attaining better recognition performance with little added system cost. The loudspeakers, positioned in physically separate locations also provide spatial information that can be used to determine the location of the person speaking and thereby offer different functionality for different persons. Acoustic models are selected based on environmental and vehicle operating conditions and may be adapted dynamically using ambient information obtained using the loudspeakers as sound input transducers.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to speech recognition systems. More particularly, the invention relates to an improved recognizer system, useful in a variety of applications and with a variety of electronic systems that use loudspeakers to provide sound to the user. The invention advantageously switches the loudspeakers from their normal sound reproduction mode to a voice or sound input mode and the voice or sound signal so input is then processed to enhance recognizer performance and to support additional recognizer features. 
   BACKGROUND OF THE INVENTION 
   To deploy an automatic speech recognizer in an automobile, or at another location, one or more microphones may need to be installed. Using multiple microphones can improve recognition results in noisy environments, but the installation costs can be prohibitive, particularly where the recognition system is installed in a system that was not originally designed for that purpose. In automotive applications, speech recognition features are typically integrated into the audio system of the car, using a single microphone, or a microphone array, that has a single cable for connecting it to the audio system. In such case, the audio system includes an input port to which the microphone cable is connected. Thus, even when the audio system includes such a port, it can be cost prohibitive to retrofit such an audio system with a recognizer that takes advantages of additional microphones (i.e., microphones in addition to the microphone or microphone array that was engineered for the system). 
   Using multiple microphones helps with removing noise. It also helps when more than one person is speaking, as the recognizer may be able to select the desired speaker by utilizing spatial information. In a multiple microphone system, this would be done by properly combining the signals received from the multiple microphones to acquire the spatial information. In an automotive application, it could be useful to have a recognition system that responds to certain voice commands only when uttered by the vehicle driver. With a single microphone, it can be very difficult to determine whether the person uttering the command is the driver, as opposed to another vehicle passenger. With multiple microphones it is much easier to discriminate among the speakers, particularly if the microphones are scattered throughout the vehicle. However, with current technology there is no economical way to accomplish this. 
   Using multiple microphones can also be beneficial in other applications. A second exemplary application involves deployment of automatic speech recognition for control of home entertainment systems. As in the car application, multiple microphones can help to remove noise and to select the desired speaker. Additionally, in home applications multiple microphones can be further applied to help reduce the adverse effects upon speech recognition of room reverberations. 
   SUMMARY OF THE INVENTION 
   The present invention provides an improved speech recognition system that may be coupled to an audio system or audio/video system to add speech recognition features to those systems and improve recognition performance. The system employs a multi-channel signal processor and a signal switch. The switch is adapted for placement between the audio system or audio/video system and the associated loudspeakers. In one state, the switch connects the loudspeakers to the audio system, so that the audio signal content may be supplied to the speakers for playback in the usual fashion. When switched to a second state, the switch decouples the loudspeakers from the audio system and instead couples them to input channels (one channel per loudspeaker) of the multi-channel signal processor. A microphone is coupled to another input channel of the multi-channel signal processor. The signal processor may be configured to provide a number of different processing operations, such as noise removal operations and spatial speaker localization operations. The output of the multi-channel processor may be fed to a speech recognizer which in turn controls system functions within the audio system or audio/video system. 
   Another aspect of the invention involves the automatic inclusion of environmental conditions to achieve more accurate speech recognition in noisy environments, such as within automotive vehicles. Speech recognition from a moving vehicle can be severely degraded by the ambient noise. The ambient noise in a vehicle is typically a time-varying phenomenon and may emanate from a variety of different sources such as:
         noise from the engine and the revolving mechanical parts of the vehicle, vibration noise from the surface contact of the wheels and roadway, the noise from air drawn into the vehicle through ducts or open windows, noise from passing/overtaking vehicles, clicks from turn indicators, etc. Each type of vehicle generates different noise frequency characteristics (e.g., BMW series generates wide-band noises and Volvo series generates narrow-band noises).       

   The improved recognizer system of the invention will automatically extract the environmental information through the available in-vehicle sensors, including the in-vehicle loudspeakers used as sound transducers as explained herein. The system processes this information to determine the type(s) of noise present in the ambient background and uses the processed information to select the optimal acoustic models for speech recognition. In addition, the ambient background information so obtained may be used to train different noise models for different noise conditions, as they are experienced during vehicle operation. The trained noise models may then be selected, based on current noise conditions, when recognition is performed. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a block diagram illustrating a presently preferred embodiment of the improved recognition system; 
       FIG. 2  is a signal processing diagram illustrating the spectrum magnitude method applied by the signal processing system; 
       FIG. 3  is a detailed block diagram of an embodiment of the speech recognition system, illustrating how system control functions can be implemented through voiced commands; 
       FIG. 4  is a perspective view of an automobile cockpit, illustrating how the invention may be integrated into the audio system of the vehicle; 
       FIG. 5  is a diagrammatic view of an audio/video system, illustrating an embodiment of the recognition system suitable for control of home electronic components by voiced command; and 
       FIG. 6  is a block diagram of the inventive system for training and using noise models adapted to different noise conditions within the vehicle. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     FIG. 1  shows an exemplary embodiment of the improved speech recognizer system. In the illustrated embodiment the recognizer system is coupled to an audio system  10 , which has a plurality of audio output ports, as at  12 , through which audio output signals are provided. In the illustrated embodiment, the audio system  10  supplies audio signals to a set of front speakers  14   a  and  14   b  and a set of rear speakers  16   a  and  16   b . The front and rear speakers each provide left channel and right channel information, respectively. In  FIG. 1 , a single line has been shown to represent the left-right stereo pair. This has been done to simplify the drawing. Those skilled in the art will appreciate that separate sets of conductors are normally used to supply the left and right channels, respectively. 
   The improved recognition system is incorporated into the audio system by provision of the crossbar switch  18 . As illustrated, switch  18  has a plurality of input ports  20  to which the audio system  10  is coupled and a plurality of ports  22  to which the loudspeakers are coupled. The crossbar switch is further coupled through a signal processor input bus  24 , which may include plurality of signal lines that communicate with the multi-channel signal processor  26 . 
   Crossbar switch  18  has two switching states. In a first state the ports  20  are coupled to the ports  22 . In this first state the audio system  10  is thus coupled to the loudspeakers, thereby allowing audio signals to be routed to the loudspeakers for playback in the usual fashion. 
   Crossbar switch  18  has a second switching state that decouples the loudspeakers from the audio system and instead couples the loudspeakers to the signal processor  26 . In this second switching state the loudspeakers function as sound input transducers (i.e., as microphone devices). 
   In one embodiment the crossbar switch couples and decouples all loudspeakers simultaneously. In that embodiment, all loudspeakers are switched between audio playback state and sound input transducer state simultaneously. In an alternate embodiment, crossbar switch  18  is capable of independent speaker channel switching. In this embodiment a selected speaker can be switched from audio playback to sound input mode while the remaining loudspeakers remain in playback mode. If desired, the crossbar switch can be also provided with signal attenuators to reduce the sound output volume of loudspeakers in the playback mode when one or more loudspeakers have been switched to the sound input mode. 
   The signal processor  26  also includes an audio input to which a microphone  28  is coupled. Microphone  28  serves as the primary input for receiving voiced commands that are then processed by the speech recognizer  30 . Signal processor  26  digitizes the audio input signals from microphone  28  and from the input channels  24  and then processes the resulting digital data to optimize it for use by the recognizer  30 . Such optimization can include performing noise cancellation algorithms (discussed below) and speaker localization or source separation algorithms (also discussed below). 
   In the embodiment illustrated in  FIG. 1 , signal processor  26  also effects software control over the crossbar switch  18  via the control line  32  shown in dotted lines in FIG.  1 . When a user utters a voiced command, the command is initially picked up by microphone  28 . Signal processor  26 , upon detecting speech input from microphone  2 B, sends a control signal to the crossbar switch, causing it to switch to the sound input mode for one or more of the loudspeakers  14   a ,  14   b ,  16   a , and  16   b . In this embodiment the signal processor thus automatically switches the system for improved recognizer performance based on receipt of a voice command through microphone  28 . 
   This automatic operation can be accomplished in a variety of ways. One way uses the signal processor  26  to continually monitor the sound input level and other spectral characteristics of the input from microphone  28 . The signal processor acquires information about the ambient background noise by averaging the input signal from microphone  28  over a predetermined time interval that is substantially longer than the voiced commands for which the system is designed to recognize. The ambient background level is then subtracted out from the signal input from microphone  28 , so that voiced command utterances are readily discriminated from the background ambient noise level. 
   If desired, the signal processor can also receive an audio signal through the input bus  24 . This input signal can supply the signal processor with the audio signal being sent to the loudspeakers. By subtracting out this signal (which microphone  28  is picking up) the microphone can be made further sensitive to voiced commands. 
   An alternate processing technique relies upon recognizer  30  to recognize the voiced commands received through microphone  28  and initially processed by signal processor  26  without having information from the loudspeakers. In this alternate embodiment the recognizer can detect particular utterances, such as particular command words or phrases, and then send a control signal to signal processor  26 , informing it that the crossbar switch  18  needs to be switched to the sound input mode. Thus, a particular voiced command by a user can be used to signal the system that it needs to switch to the sound input mode whereby one or more of the loudspeakers serve as auxiliary sound input transducers. 
   Another more sophisticated embodiment uses the confidence level achieved by the recognizer to determine when noise cancellation or other signal processing operations are needed. Upon detecting such conditions, the signal processor is notified via the control line  34  and it, in turn, signals the crossbar switch via line  32  to switch to the sound input state. This functionality may be implemented by monitoring the recognition score or probability of match score generated by the recognizer as it operates upon the input data. When recognition confidence drops below a predetermined level, the recognizer detects this and sends a control message to the signal processor  26 . 
   Because the crossbar switch is under software control, by the signal processor  26  and also by the recognizer in some applications, the loudspeakers can be used to acquire useful information about the recognition environment that would not otherwise be available through the single microphone  28 . In the environment learning mode, the loudspeakers are individually switched, one at a time, while a predetermined time segment of input sound is sampled and stored for further analysis. By cycling through all of the loudspeakers in this fashion, the system acquires spatial information about the sound field within which the microphone  28  is placed. Acquiring information of the sound field can be quite beneficial in fine tuning the signal processing algorithms used to enhance recognition. For example, if the system needs to recognize a particular person who is speaking among a group of persons, the sound field information can tell where that person is located relative to the others. Once the location has been determined, the utterances of the other persons can be rejected based on spatial cues. 
   The learning mode described above may be performed at very high speed by utilizing a solid-state crossbar switching circuit. Thus the system can cycle through successive loudspeakers, to acquire sound field information, without the audio content of the playback material being noticeably degraded. 
     FIG. 2  shows a signal processing algorithm that may be used to enhance recognizer performance. As illustrated, signals from the loudspeakers and microphone, respectively, are converted into the spectrum magnitude domain by processing blocks  50  and  52 , respectively. An alignment operation is then performed at  54  and the resulting aligned signal is then subtracted from the spectrum magnitude signal originating from microphone  28 . After subtraction as at block  56 , the processed signal is then passed to the recognizer  30 . 
   Processing in this fashion effectively subtracts the background noise from the speech, so that the speech can be processed more effectively by the recognizer  30 . The processing operation is typically calibrated prior to use by allowing the reference microphone to sample only background noise. If the reference microphone receives both speech and noise, then a source separation technique may be used. The source separation technique uses independent component analysis (ICA) to separate the speech and noise. The microphone will have speech and noise, and the loudspeakers being used as sound input transducers will also have speech and noise, but with a different transfer function. In the frequency domain these two input signals can be written according to the matrix equation below: 
         [           m   1               m   2           ]     =         [           a   11           a   12               a   21           a   22           ]     ⁡     [         s           n         ]       =     M   ⁡     [         s           n         ]             
 
   In the above matrix equation M 1  and M 2  are the two input signals, while a 11 , a 12 , a 21  and a 22  are transfer functions. The s and n terms are speech and noise, respectively. If the matrix M is not singular, the signal and noise signals can be recovered by: 
         [         s           n         ]     =       M     -   1       ⁡     [           m   1               m   2           ]           
 
   The independent component analysis will find the inverse of M, using a gradient descent algorithm. The recovered speech is then fed to the speech recognizer  30 . If applied directly to the sound signal, ICA can take a considerable amount of computational power. This power can be substantially reduced if the signal is split into frequency bands and ICA is applied, band by band. The frequency band representation may be sent directly to the recognizer. 
   Referring now to  FIG. 3 , an exemplary application of the improved recognizer is illustrated. Many of the component parts illustrated have been described above and will not be repeated here. As illustrated in  FIG. 3 , the recognizer is coupled to a control system  60 , which may, in turn, be coupled to the audio system  10  (or audio/video system). The control system can also be connected to other devices for which operational control is desired. The control system may be provided with a memory for storing a feature set database illustrated diagrammatically at  62 . The feature set database stores the identity of various devices and operational features and functions of those devices in association with the identity of different persons who will be using the system. The feature set database is used to dictate which of the various control functions certain individual persons have authority to operate. In an automotive application, for example, certain vehicular functions may be designated for the vehicle driver only. Using information about the spatial location of the vehicle occupants, the system is able to ascertain whether the driver or one of the passengers has uttered a particular voice command. It the command is one that is reserved for the driver only, it will be performed only if the driver utters it. The command will be ignored if other vehicle occupants utter it. 
   While it should be apparent that the recognition system of the invention can be used in a variety of different applications, two examples of such systems will be provided next in order to illustrate some of the ways that the invention may be deployed. 
   Referring to  FIG. 4 , a vehicle cockpit is shown at  80 . The vehicle audio system is shown at  82 , with two of the audio system loudspeakers illustrated at  84  and  86 . Other loudspeakers would be also provided in other locations of the vehicle (not shown). A microphone  28  is provided in a suitable location, such as within the rearview mirror assembly. If desired, the rearview mirror assembly may also have a system activation button  88  that the user presses to turn on the recognition system. Such button is optional, as the recognition system can be configured to work automatically, as described previously. The recognizer of the invention can be housed in a suitable package having connectors for plugging between the audio system  82  and the loudspeakers. The package is designed to accept the standard wiring harness plugs and jacks found within the vehicle. This has an advantage in that the wiring harness and loudspeaker installation may be the same for vehicles with speech recognition and without it. This saves on manufacturing and inventory costs. 
     FIG. 5  illustrates a home entertainment system with the recognition system employed. In the home entertainment system, the microphone  28  may be placed in a suitable location, such as at a fixed location within the viewing room, or within the remote control of one of the components. When placed in the remote control, wireless or infrared communication may be used to communicate the spoken utterance back to the signal processing unit. 
   In some implementations, it may be beneficial to provide the recognizer with different acoustic models for different noise conditions. The recognizer system of the invention makes provision for this using ambient noise measuring and acoustic model selection system illustrated in FIG.  6 . The system maintains a pool of acoustic models, stored in acoustic model memory  100 . An intelligent decision logic unit  102  predicts or determines the current noise conditions based on a variety of factors that are supplied as inputs to the logic unit  102 , as illustrated. The logic unit supplies an ambient noise identification signal at  104  to an acoustic model selection module  106 . The selection module  106  selects the appropriate acoustic model from memory  100 , based on the signal at  104  and supplies this model to the model adaptation module  108 . Model selections can be made prior to and/or during the recognition session. Module  108 , in turn generates or supplies the adapted model to the pattern matching engine  110  of the recognizer. The intelligent decision logic unit  102  may also be configured to supply a control signal at  112  to provide background noise information to the model adaptation module  108 . 
   In addition to providing adapted acoustic models for recognition, the system may also be configured to perform noise compensation upon the input speech signal prior to recognition and/or to change compensation parameters during a recognition session. As illustrated in  FIG. 6 , an ambient noise identification signal is supplied at  114  to a noise compensation module  116 . Signal  114  provides the noise compensation module with information about the type of noise in the current ambient background. The noise compensation module performs processing of the input speech signal to remove or reduce the effects of the noise. In a presently preferred embodiment, noise compensation is performed in a parametric domain, after the input speech signal has been processed by the feature extraction module  118 , as illustrated. 
   The front-end (noise compensation) processing operations can be selected according to current noise conditions. If the noise is minimal, then perceptual linear prediction features can be selected for recognition. If the noise is greater then a sub-band feature can be selected for recognition. If the noise is null, Mel frequency cepstral coefficient features may be selected. 
   While there can be a wide assortment of different factors that affect what noise is present in the ambient background, the following are offered as some examples. Suitable sensors would be provided to capture the following operating parameters:
         Engine is on or off.   Speed of the vehicle, e.g., 30 mph (residential), 40 mph (city), 65 mph (highway).   Accelerator position (the speed will be lower if the vehicle is climbing a mountain, but the accelerator will be more fully depressed and engine noise will be greater).   Engine rpm.   Age of the vehicle.   Model of the vehicle (sports car, family sedan, minivan, SUV, motor home, school bus, etc.). This information can also serve to inform the logic unit of the number of speakers that can be estimated a-priori.   Window open or closed.   Sensors under vehicle seat(s) or at the entrance of each door, so the system can precisely estimate the number of persons inside the vehicle. (Pets, like cats and dogs, can be recognized similarly, and the system will detect that these occupants will not be providing speech input to be recognized.)   Windshield wipers on and off.   Convertible top up or down/sunroof open or closed.   Radio, music, dvd on or off.   Global Positioning Satellite (GPS)—vehicle location. This information is used to learn street location. The type of roadway surface can be stored for each location and this information used to predict noise level. In this regard, a concrete roadway provides a different background noise than a blacktop, gravel or dirt surface. In addition, the background noise associated with each type of surface changes differently when wet. GPS information may also be used to determine whether the vehicle is approaching a train track (railway crossing) or moving near the ocean (surf noise) or climbing up a mountain.   Real-time weather and traffic conditions.   Air conditioning system is on or off.       

   The acoustic models stored in memory  100  can be preconfigured and downloaded through suitable data connection, such at a vehicle service center or via vehicle internet connection. Alternatively, the system can measure background noise, using the sound transducers as described herein, and then generate its own acoustic models. Generating the models itself, allows the models to become adapted as the vehicle age changes. As the vehicle ages, its noise characteristics change (more rattles, louder muffler, etc.). 
   The acoustic models may be trained according to most of the noisy conditions and the best fitting model is selected according to the deterministic information from all of the sensors (described above). Model adaptation can also be done on the selected model to enhance the inter-speaker and intra-speaker variabilities.  FIG. 6  thus illustrates a model training module  120  that provides new models, or re-trains existing models within memory  100 . The model training module receives information about ambient noise conditions from the loudspeaker system  122 , and/or from microphone  124 . 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.