Patent Publication Number: US-7219062-B2

Title: Speech activity detection using acoustic and facial characteristics in an automatic speech recognition system

Description:
FIELD OF INVENTION 
   The present invention relates generally to automatic speech recognition systems and methods and more particularly to an automatic speech recognition system and method wherein an automatic speech recognizer only responsive to acoustic speech utterances is activated only in response to acoustic energy having a spectrum associated with the speech utterances and at least one facial characteristic associated with the speech utterances. 
   BACKGROUND OF THE INVENTION 
   Currently available speech recognition systems determine the beginning and end of utterances by responding to the presence and absence of only acoustic energy having a spectrum associated with the utterances. If a microphone associated with the speech recognition system is in an acoustically noisy environment including, for example, speakers other than the speaker whose voice is to be recognized or activated machinery, including telephones (particularly ringing telephones), the noise limits the system performance. Such speech recognition systems attempt to correlate the acoustic noise with words it has learned for a particular speaker, resulting in the speech recognition system producing an output that is unrelated to any utterance of the speaker whose voice is to be recognized. In addition, the speech recognition system may respond to the acoustic noise in a manner having an adverse effect on its speech learning capabilities. 
   We are aware that the prior art has considered the problems associated with an acoustically noisy environment by detecting acoustic energy and facial characteristics of a speaker whose voice is to be recognized. For example, Maekawa et al, U.S. Pat. No. 5,884,257, and Stork et al, U.S. Pat. No. 5,621,858, disclose voice recognition systems that respond to acoustic energy of a speaker, as well as facial characteristics associated with utterances by the speaker. In Maekawa et al., lip movement is detected by a visual system including a light source and light detector. The system includes a speech period detector which derives a speech period signal by detecting the strength and duration of the movement of the speaker&#39;s lips. The system also includes a voice recognition system and an overall judgment section which determines the content of an utterance based on the acoustic energy in the utterance and movement of the lips of the speaker. In Stork et al., lip, nose and chin movement are detected by a video camera. Output signals of a spectrum analyzer responsive to acoustic energy and a position vector generator responsive to the video camera supply signals to a speech classifier trained to recognize a limited set of speech utterances based on the output signals of the spectrum analyzer and position vector generator. 
   In both Maekawa et al. and Stork et al., complete speech recognition is performed in parallel to image recognition. Consequently, the speech recognition processes of these prior art devices would appear to be somewhat slow and complex, as well as require a significant amount of power, such that the devices do not appear to be particularly well-suited as remote control devices for controlling equipment. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, a speech recognition system comprises (1) an acoustic detector for detecting speech utterances of a speaker, (2) a visual detector for detecting at least one facial characteristic associated with speech utterances of the speaker, and (3) a processing arrangement connected to be responsive to the acoustic and visual detectors for deriving a signal. The signal has first and second values respectively indicative of the speaker making and not making speech utterances such that the first value is derived only in response to the acoustic detector detecting a finite, nonzero acoustic response while the visual detector detects at least one facial characteristic associated with speech utterances of the speaker. A speech recognizer for deriving an output indicative of the speech utterances as detected only by the acoustic detector is connected to be responsive to the acoustic detector only while the signal has the first value. 
   Another aspect of the invention relates to a method of recognizing speech utterances of a speaker with an automatic speech recognizer only responsive to acoustic speech utterances of the speaker. The method comprises: (1) detecting acoustic energy having a spectrum associated with speech utterances, (2) detecting at least one facial characteristic associated with speech utterances of the speaker, and (3) activating the automatic speech recognizer only in response to the detected acoustic energy having a spectrum associated with speech utterances while the at least one facial characteristic associated with speech utterances of the speaker is occurring. 
   Preferably, activation of the automatic speech recognizer is prevented in response to any of: (1) no acoustic energy having a spectrum associated with speech utterances being detected while no facial characteristic associated with speech utterances of the speaker is detected, (2) acoustic energy having a spectrum associated with speech utterances being detected while no facial characteristic associated with speech utterances of the speaker is detected, and (3) no acoustic energy having a spectrum associated with speech utterances being detected while at least one facial characteristic associated with speech utterances of the speaker is detected. 
   In the preferred embodiment, the beginning of each speech utterance is assuredly coupled to the speech recognizer. The beginning of each speech utterance is assuredly coupled to the speech recognizer by: (a) delaying the speech utterance, (b) recognizing the beginning of each speech utterance, and (c) responding to the recognized beginning of each speech utterance to couple the delayed speech utterance associated with the beginning of each speech utterance to the speech recognizer and thereafter sequentially coupling the remaining delayed speech utterances to the speech recognizer. It is assured that no detected acoustic energy is coupled to the speech recognizer upon the completion of a speech utterance. Assurance that no detected acoustic energy is coupled to the speech recognizer upon the completion of a speech utterance is provided by: (a) delaying the acoustic energy associated with the speech utterance, (b) recognizing the completion of each speech utterance, and (c) responding to the recognized completion of each speech utterance to decouple delayed acoustic energy occurring after the completion of each speech utterance from the speech recognizer. 
   In the preferred apparatus embodiment, the delay is provided by a ring buffer that is effectively indexed so that segmented detected acoustic energy at the beginning of the utterance and segmented detected acoustic energy at the end of the utterance and segmented detected acoustic energy between the beginning and end of the utterance are coupled to the speech recognizer to the exclusion of acoustic energy prior to the beginning of the utterance and acoustic energy subsequent to the end of the utterance. 
   The processing arrangement in first and second embodiments respectively includes a lip motion and a face recognizer. The face recognizer is preferably arranged for enabling the signal to have the first value only in response to the face of the speaker being at a predetermined orientation relative to the visual detector. The face recognizer also preferably: (1) detects and distinguishes the faces of a plurality of speakers, and (2) enables the signal to have the first value only in response to the speaker having a recognized face. 
   In the second embodiment, the processing arrangement also includes a speaker identity recognizer for: (1) detecting and distinguishing speech patterns of a plurality of speakers, and (2) enabling the signal to have the first value only in response to the speaker having a recognized speech pattern. 
   The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, especially when taken in conjunction with the accompanying drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a block diagram of a preferred embodiment of the speech recognition system in accordance with one embodiment of the present invention; and 
       FIG. 2  is a block diagram of a modified portion of the speech recognition system of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE DRAWING 
   Reference is now made to the  FIG. 1  of the drawing wherein microphone  10  and video camera  12  are respectively responsive to acoustic energy in a spectrum including utterances of a speaker and optical energy associated with at least one facial characteristic, particularly lip motion, of utterances by the speaker. Microphone  10  and camera  12  respectively derive electrical signals that are replicas of the acoustic and optical energy incident on them in the spectra they are designed to handle. 
   The electrical output signal of microphone  10  drives analog to digital converter  14  which in turn drives acoustic energy detector circuit  16  and speech segmentor circuit  18  in parallel. Acoustic energy detector  16  derives a bi-level output signal having a true value in response to the digital output signal of converter  14  having a value indicating that acoustic energy above a predetermined threshold is incident on microphone  10 . Speech segmentor  18  derives a digital signal that is divided into sequential speech segments, such as phonemes, for utterances of the speaker speaking into microphone  10 . 
   Speech segmentor  18  supplies the sequential speech segments in parallel to random access memory (RAM)  22  and dynamic ring buffer  24 . RAM  22  includes an enable input terminal  23  connected to be responsive to the bi-level output signal of acoustic energy detector  16 . In response to energy detector  16  deriving a true value, as occurs when microphone  10  is responsive to a speaker making an utterance or ambient noise, RAM  22  is enabled to be responsive to the output of speech segmentor  18 . When enabled, sequential memory locations, i.e., addresses, in RAM  22  are loaded with the sequential segments that segmentor  18  derives by virtue of a data input of the RAM being connected to the segmentor output. This is true regardless of whether the sequential segments are speech utterances or noise. RAM  22  has sufficient capacity to store the sequential speech segments of a typical utterance by the speaker as segmentor  18  is deriving the segments so that the first and last segments of a particular utterance, or noise, are stored at predetermined addresses in the RAM. 
   Dynamic ring buffer  24  includes a sufficiently large number of stages to store the sequential speech segments segmentor  18  derives for a typical utterance. Thus, buffer  24  effectively continuously records and maintains the last few seconds of acoustic energy supplied to microphone  10 . RAM  22  and circuitry associated with it form a processing arrangement that effectively indexes dynamic ring buffer  24  to indicate when the first and last segments of utterances by the speaker who is talking into microphone  10  occur. If the acoustic energy incident on microphone  10  is not associated with an utterance, dynamic ring buffer  24  is not effectively indexed. Buffer  24  is part of a delay arrangement for assuring that (1) the beginning of each speech utterance is coupled to a speech recognizer and (2) upon completion of each utterance the speech recognizer is no longer responsive to a signal representing acoustical energy. 
   To perform indexing of buffer  24  only in response to utterances by the speaker who is talking into microphone  10 , the system illustrated in  FIG. 1  detects at least one facial characteristic associated with speech utterances of the speaker while acoustic energy is incident on microphone  10 . The facial characteristic of the embodiment of  FIG. 1  is detection of lip motion. To this end, video camera  12  derives a signal indicative of lip motion of the speaker speaking into microphone  10 . The lip motion signal that camera  12  derives drives lip motion detector  26  which derives a bi-level signal having a true value while lip motion detector  26  senses that the lips of the speaker are moving and a zero value while lip motion detector  26  senses that the lips of the speaker are not moving. 
   The bi-level output signals of acoustic energy detector  16  and motion detector  26  drive AND gate  28  which derives a bi-level signal having a true value only while the bi-level output signals of detector  16  and  26  both have true values. Thus, AND gate  28  derives a true value only while microphone  10  and camera  12  are responsive to speech utterances by the speaker; at all other times, the output of AND gate  28  has a zero, i.e., not true, value. 
   The output signal of AND gate  28  drives one shot circuits  30  and  32  in parallel. One shot  30  derives a short duration pulse in response to the leading edge of the output signal of AND gate  28 , i.e., in response to the output of the gate having a transition from the zero value to the true value. One shot  32  derives a short duration pulse in response to the trailing edge of the output signal of AND gate  28 , i.e., in response to the output of the gate having a transition from the true value to the zero value. Hence, one shot circuits  30  and  32  respectively derive short duration pulses only at the beginning and end of a speech utterance. One shot circuits  30  and  32  do not derive any pulses if (1) acoustic energy detector  16  derives a true value while lip motion detector  26  derives a zero value, (2) lip motion detector  26  derives a true value while acoustic energy detector  16  derives a zero value, or (3) neither of detectors  16  nor  26  derives a true value. 
   The output pulses of one shot circuits  30  and  32  are supplied as write enable signals to first and second predetermined addresses of RAM  22 . The first and second addresses are respectively for the first and last speech segments that segmentor  18  derives for a particular utterance. Hence, the first address stores the first speech segment that segmentor  18  derives for a particular utterance, while the second address stores the last speech segment that segmentor derives for that same utterance. RAM  22  is enabled to be responsive to the sequential segments that segmentor  18  derives and the output signals of one shot circuits  30  and  32  by virtue of acoustic energy detector  16  supplying the RAM enable input terminal  23  with a true value during the speech utterance. RAM  22  responds to a transition of the output of acoustic energy detector  16  from a true value to a zero value to read out the contents of the first and second addresses to input terminals of comparison circuits  34  and  36 , respectively. 
   Comparison circuits  34  and  36  are respectively connected to be responsive to the contents of the speech segments stored in the first and second addresses of RAM  22  and the output of dynamic ring buffer  24  to detect the location in the ring buffer of the first and last speech segments of the particular utterance. In particular, upon the completion of a particular speech utterance, RAM  22  supplies (1) one input terminal of comparison circuit  34  with a signal indicative of the speech content of the first speech segment of that utterance and (2) one input terminal of comparison circuit  36  with a signal indicative of the speech content of the last speech segment of that utterance. 
   While RAM  22  is driving comparison circuits  34  and  36  with the signals indicative of the speech content of the first and last speech segments of the utterance, dynamic ring buffer  24  is enabled by the transition at the trailing edge of the bi-level output of acoustic energy detector  16  to sequentially derive, at a high frequency (i.e., a frequency considerably higher than the frequency at which the segments are transduced by microphone  10 ) the speech segments it stores. To this end, buffer  24  includes a read out enable input terminal  37  connected to be responsive to the trailing edge transition that detector  16  derives. While enabled for read out, dynamic ring buffer  24  supplies the sequential speech segments it derives in parallel to second input terminals of comparison circuits  34  and  36 . 
   Comparison circuit  34  derives a pulse only in response to the speech segment that buffer  24  derives being the same as the first segment that RAM  22  supplies to comparison circuit  34 . Comparison circuit  36  derives a pulse only in response to the speech segment that buffer  24  derives being the same as the last segment that RAM  22  supplies to comparison circuit  36 . Gate  38  has first and second control input terminals respectively connected to be responsive to the output pulses of comparison circuits  34  and  36  and a data input terminal connected to be responsive to the sequential speech segments dynamic ring buffer  24  derives. Gate  38  is constructed so that in response to comparison circuit  34  supplying the first control input terminal of the gate with a pulse, the gate is opened and remains open until it is closed by comparison circuit  36  supplying the second control input terminal of the gate with a pulse. 
   While gate  38  is open, it passes to automatic speech recognizer  40  the first through the last speech segments dynamic ring buffer  24  supplies to its data input terminal. Automatic speech recognizer  40  can be of any known type that responds only to signals representing acoustic energy and produces an output signal indicative of the speech utterances of the speaker talking into microphone  10  while the speaker is being observed by video camera  12 . The output signal of speech recognizer  40  drives output device  42 . Examples of output device  42  are a computer character generator for driving a computer display with alphanumeric characters commensurate with the utterances or a machine for performing tasks commensurate with the utterances. 
   The speech recognition system of  FIG. 1  can be modified by the arrangement illustrated in  FIG. 2  so that the speech recognition system will not respond to speech utterances when the speaker is not looking at camera  12  and so that it can respond to speech utterances and the faces of a plurality of speakers. The apparatus illustrated in  FIG. 2  is connected to respond to the output signal of acoustic energy detector  16 ,  FIG. 1 , and replaces lip motion detector  26  and AND gate  28 . 
   The apparatus of  FIG. 2  includes face recognizer  50 , connected to be responsive to the output signal of video camera  12 , and speaker identity recognizer  52 , connected to be responsive to the output signal of acoustic energy detector  16 . Face recognizer  50  and speech identity recognizer  52  are connected to other circuit elements and to speech recognizer  40  so that the speech recognizer is activated only when the speaker is facing video camera  12 , that is, has a predetermined orientation relative to the video camera. Hence, if the speaker turns away from and is not looking directly into video camera  12  because the speaker is talking to someone and does not desire to have his/her voice recognized by recognizer  40 , recognizer  40  is not activated. Speech recognizer  40  is only activated if the face recognizer  50  and speech recognizer  52  identify the same person. Face recognizer  50  and speech recognizer  52  are trained during at least one training period to recognize the face and speech of more than one person and speech recognizer  40  is activated only if the face and speech are recognized as being for the same person. 
   To these ends, speaker identity recognizer  52  includes memory  54  having one input connected to be responsive to the speech signal output of analog to digital converter  14  and a second input connected to be responsive to the output of acoustic energy detector  16  so that memory  54  stores short-term utterances of the speaker while detector  16  derives a true value. Upon the completion of the utterance, memory  54  supplies a digital signal indicative of the utterance to one input of comparator  56 , having a second input responsive to memory  58  which stores digital signals indicative of the speech patterns of a plurality of speakers who have trained speech recognizer  40 . 
   Comparator  56  derives a true output signal in response to the output signal of speaker memory  54  matching one of the speech patterns that memory  58  stores. Comparator  56  derives a separate true signal for each of the speakers having a speech pattern stored in memory  58 . In  FIG. 2 , it is assumed that memory  58  stores speech patterns for first and second different speakers, whereby comparator  56  includes output leads  57  and  59 , respectively provided for the first and second speakers. In response to comparator  56  recognizing the speaker as having speech characteristics the same as the speech pattern that memory  58  stores for the first and second speakers, comparator  57  respectively supplies true values to output leads  57  and  59 . 
   Face recognizer  50  includes memory  60  having an input connected to be responsive to the output of video camera  12  so that memory  60  stores one frame of an image being viewed by video camera  12 . Upon completion of the frame, memory  60  supplies a digital signal indicative of the frame contents to one input of comparator  62 , having a second input responsive to memory  64  which stores digital signals indicative of the facial patterns of each of the plurality of speakers; the facial patterns memory  64  stores are derived while the speakers are looking directly into camera  12 , that is, while the faces of the speakers have a predetermined orientation relative to the camera. Comparator  62  derives a true output signal in response to the output signal of memory  60  matching one of the facial patterns that memory  64  stores. Comparator  62  derives a separate true signal for each of the speakers with facial images stored in memory  64 . In the example of  FIG. 2 , memory  64  stores facial images for the first and second speakers, whereby comparator  64  includes output leads  66  and  68 , respectively provided for the first and second speakers. In response to comparator  64  recognizing the speaker as having a facial image the same as one of the facial images that memory  60  stores for the first and second speakers, comparator  62  respectively supplies true values to output leads  66  and  68 . 
   During a training period for each of the speakers, each of the speakers recites a predetermined sequence of words, and the speaker is looking directly into video camera  12 . At this time, speaker memory  54  is connected to an input of memory  58  to cause the memory  58  to store speech patterns for each of the plurality of speakers who train speech recognizer  40 . At the same time, image memory  60  is connected to an input of memory  64 , to cause memory  64  to store a facial image for each of the plurality of speakers who train speech recognizer  40 . During the training period for each of the speakers, the output of speech segmentor  16  is supplied to the input of speech recognizer  40  to enable the speech recognizer to learn the speech patterns of each of the speakers, in a manner known to those skilled in the art. 
   The output signals of comparators  56  and  62  on leads  57  and  66  are supplied to inputs of AND gate  70 , while the output signals of the comparators on leads  59  and  68  are supplied to inputs of AND gate  72 . Hence, AND gate  70  derives a true value only in response to face recognizer  50  and speech identity recognizer  52  both recognizing that a speaker is the first speaker who is looking directly into camera  12 . Similarly, AND gate  72  derives a true value only in response to face recognizer  50  and speech identity recognizer  52  both recognizing that a speaker is the second speaker who is looking directly into camera  12 . AND gates  70  and  72  derive bi-level signals that are supplied to OR gate  74  which derives a true value in response to either the first or second speakers being identified from the voice and facial characteristics thereof. 
   The output signal of OR gate  74  drives one shots in the same manner that the output of AND gate  28  drives the one shots. Consequently, the speech signal of the first or second speaker is supplied to speech recognizer  40  in the same manner that the speech signal is supplied to speech recognizer  40  in the embodiment of  FIG. 1 . 
   To enable speech recognizer  40  of  FIG. 2  to recognize both speakers, the outputs of AND gates  70  and  72  are supplied to speech recognizer  40 . Speech recognizer  40  responds to the outputs of AND gates  70  and  72  to analyze the speech of the correct speaker, in a manner known to those skilled in the art. 
   While there has been described and illustrated a specific embodiment of the invention, it will be clear that variations in the details of the embodiment specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example, the discrete circuit elements can be replaced by a programmed computer.