Patent Publication Number: US-11037574-B2

Title: Speaker recognition and speaker change detection

Description:
TECHNICAL FIELD 
     This invention relates to speaker recognition. 
     In this document, the term speaker recognition is used to refer to a process in which information is obtained about the identity of a speaker. For example, this process may involve determining whether or not the speaker is a specific individual (speaker verification), or may involve identifying the speaker, for example from a group of enrolled speakers (speaker identification). 
     The term speech recognition is used to refer to a process in which information is obtained about the content of speech, for example in order to be able to determine what the speaker is saying. 
     BACKGROUND 
     Systems are known in which a verbal command from a speaker is recognised and processed, subject to the speaker&#39;s identity being verified. One such system is disclosed in GB-2515527A, for example, in which, if it is determined that a predetermined trigger phrase has been spoken, a speaker recognition process is performed in a first device. If the speaker recognition process determines that the predetermined trigger phrase was spoken by a specific enrolled user, the signal representing the speech is passed to a speech recognition engine, which may be provided in a separate second device. Typically, such a system may be used to allow the user to issue voice commands, causing the system to perform some action, or retrieve some requested information, for example. 
     One problem that could in theory arise in such a system is an attack, in which a third party attempts to gain unauthorised access to the system, by speaking after the enrolled user has spoken the predetermined trigger phrase. The speaker recognition process would recognise that the enrolled user was speaking initially, and the third party&#39;s speech may be of a sufficiently short duration that the speaker recognition process is unable to determine that the entirety of the speech was not spoken by the enrolled user. 
     SUMMARY 
     According to an aspect of the present invention, there is provided a method of speaker recognition, comprising: 
     receiving an audio signal comprising speech; 
     performing a biometric process on a first part of the audio signal, wherein the first part of the audio signal extends over a first time period; 
     obtaining a speaker recognition score from the biometric process for the first part of the audio signal; 
     performing a biometric process on a plurality of second parts of the audio signal, wherein the second parts of the audio signal are successive sections of the first part of the audio signal, and wherein each second part of the audio signal extends over a second time period and the second time period is shorter than the first time period; 
     obtaining a respective speaker recognition score from the biometric process for each second part of the audio signal; and 
     determining whether there has been a speaker change based on the respective speaker recognition scores for successive second parts of the audio signal. 
     In some embodiments, determining whether there has been a speaker change based on speaker recognition scores for successive second parts of the audio signal comprises determining that there has been a speaker change if speaker recognition scores for successive second parts of the audio signal show a variation that exceeds a threshold. 
     The method may comprise:
         storing the received audio signal;   performing a keyword detection process on the received audio signal; and   when a predetermined keyword is detected, identifying a start of the first time period at a start of the predetermined keyword.       

     In that case, the method may comprise:
         performing automatic speech recognition on the received audio signal from an end of the predetermined keyword.       

     In that case, the method may comprise:
         performing automatic speech recognition on the received audio signal from an end of the predetermined keyword only if the speaker recognition score obtained for the first part of the audio signal indicates that the speech is the speech of a predetermined speaker.       

     In some embodiments, the method comprises:
         performing a text-dependent biometric process on the received audio signal representing the predetermined keyword; and   performing a text-independent biometric process on the received audio signal following the predetermined keyword.       

     The second time period may be in the range 0.5 s-1.5 s. 
     The method may comprise performing the biometric process on the first part of the audio signal at a plurality of times during the first time period, and obtaining a respective speaker recognition score at each of the plurality of times, wherein the speaker recognition score obtained at each of the plurality of times is a cumulative score relating to the first part of the audio signal up until that time. 
     The method may comprise defining an end point of the first part of the audio signal in response to determining that there has been a speaker change, and then performing said biometric process on the first part of the audio signal. 
     The method may comprise using said speaker recognition score obtained for the first part of the audio signal, only in response to determining that there has not been a speaker change during the first part of the audio signal. 
     The step of obtaining the speaker recognition score from the biometric process for the first part of the audio signal may comprise combining the respective speaker recognition scores obtained from the biometric process for each second part of the audio signal. 
     This has the advantage that the speech that a speaker change detection process is able to determine when some of the detected speech was not spoken by the enrolled user, helping to ensure that the attack described above cannot be successful. 
     According to a second aspect of the present invention, there is provided a method of speaker change detection, comprising:
         receiving an audio signal representing speech;   performing at least one first speaker change detection process on the received audio signal to obtain information about times at which there may have been a speaker change;   performing a biometric process on a plurality of successive sections of the audio signal;   obtaining a speaker recognition score from the biometric process for each section of the audio signal;   obtaining information about times at which there may have been a speaker change, based on the speaker recognition scores obtained from the biometric process; and   determining whether there has been a speaker change based on information obtained from the first speaker change detection process and based on information obtained from the speaker recognition scores.       

     Obtaining information about times at which there may have been a speaker change, based on the speaker recognition scores obtained from the biometric process, may comprise:
         examining successive speaker recognition scores for the sections of the audio signal in order to determine a series of difference values between pairs of consecutive speaker recognition scores for the successive sections of the audio signal; and   determining that there may have been a speaker change when one of said difference values exceeds a threshold value.       

     The first speaker change detection process may comprise one or more of:
         tracking a fundamental frequency of the speech;   tracking an angle of arrival of the speech at a device; and   comparing feature vectors extracted from the audio signal in successive time windows of the audio signal.       

     The method may comprise obtaining a respective metric from the or each first speaker change detection process and from the speaker recognition scores obtained from the biometric process, wherein each respective metric indicates a likelihood that there has been a speaker change, and determining whether there has been a speaker change based on said metrics. 
     When the method comprises obtaining a respective metric from the or each first speaker change detection process and from the speaker recognition scores obtained from the biometric process, the method may comprise:
         combining the respective metrics to form a combined metric;   comparing the combined metric with a threshold value; and   determining whether there has been a speaker change based on a result of the comparison.       

     When the method comprises obtaining a respective metric from the or each first speaker change detection process and from the speaker recognition scores obtained from the biometric process, the method may comprise:
         comparing each metric with a respective threshold value; and   determining whether there has been a speaker change based on a result of the respective comparisons.       

     In that case, the method may comprise: determining that there has been a speaker change if any one of the metrics exceeds its respective threshold value. 
     Alternatively, the method may comprise:
         determining that there has been a speaker change if a majority of the metrics exceed their respective threshold values.       

     As a further alternative, the method may comprise:
         determining that there has been a speaker change if all of the metrics exceed their respective threshold values.       

     This has the advantage that a simple and reliable speaker change detection process is provided. 
     According to a further aspect, there is provided a system configured to perform any of the methods defined above. For example, the system may comprise: an input for receiving an audio signal representing speech; and a processor configured to perform a method in accordance with any of the methods defined above. 
     The system may be implemented in electronic device, for example a smartphone or other communications device, a smart speaker, a tablet or laptop computer, a games console, a home control system, a home entertainment system, an in-vehicle entertainment system, or a domestic appliance. 
     According to a further aspect, there is provided a non-transitory storage medium having stored thereon software code which, when run on a suitable processor, performs any of the methods defined above. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a system in accordance with an aspect of the invention. 
         FIG. 2  illustrates an example of a first device in the system of  FIG. 1 . 
         FIG. 3  illustrates a first attack on a system using speaker recognition. 
         FIG. 4  illustrates a second attack on a system using speaker recognition. 
         FIG. 5  is a block diagram, illustrating a system in accordance with an aspect of the invention. 
         FIG. 6  is a flow chart, illustrating a method in accordance with an aspect of the invention. 
         FIG. 7  is a block diagram, illustrating a part of the system of  FIG. 5 . 
         FIG. 8  is a flow chart, illustrating a method in accordance with an aspect of the invention. 
         FIG. 9  is a timing diagram, illustrating the operation of the system and method. 
     
    
    
     DETAILED DESCRIPTION 
     The description below sets forth example embodiments according to this disclosure. Further example embodiments and implementations will be apparent to those having ordinary skill in the art. Further, those having ordinary skill in the art will recognize that various equivalent techniques may be applied in lieu of, or in conjunction with, the embodiments discussed below, and all such equivalents should be deemed as being encompassed by the present disclosure. 
       FIG. 1  shows an example of a system  10  in which the invention may be implemented. In this example, a speaker recognition process takes place in a first device  12 . As illustrated here, the first device  12  takes the form of a smartphone, but it may be any (portable) electronic device with some sort of voice operability, for example a smart speaker, a tablet or laptop computer, a games console, a home control system, a home entertainment system, an in-vehicle entertainment system, a domestic appliance, or the like. 
     The device  12  includes a speaker recognition block. Thus, the device  12  also includes at least one microphone and, when it is determined that a sound detected by the microphone(s) contains speech, the speaker recognition block may for example determine whether or not the speaker is a specific individual, or may identify the speaker from a group of enrolled speakers. 
     In this embodiment, if it is determined that the speaker is an enrolled speaker, the signal representing the speech may be passed to a separate second device  14  located remotely from the first device  12 . For example, the second device  14  may take the form of a server located in the cloud  16 , accessible by the first device over a wireless communications network. The second device  14  includes a speech recognition block. When a signal representing speech is supplied to the second device  14 , the speech recognition block may obtain information about the content of the speech, for example in order to be able to determine the content of a command. 
     Although an embodiment is shown here, in which the speaker recognition and the speech recognition take place in separate devices, in other examples, the speaker recognition and the speech recognition take place in the same device, for example a smartphone, a smart speaker, a tablet or laptop computer, a games console, a home control system, a home entertainment system, an in-vehicle entertainment system, a domestic appliance, or the like. 
       FIG. 2  shows one possible form of the first device  12 . In this example, the first device  12  includes an interface  30 , for connecting to other devices; a memory  32 , for storing data and program instructions; and a processor  34 , for performing operations in accordance with program instructions stored in the memory  32 . Thus, with reference to  FIG. 1  and the description thereof above, a speaker recognition block may be implemented by suitable program instructions stored in the memory  32 , causing the processor  34  to perform the speaker recognition functionality. 
       FIG. 3  illustrates a possible attack on a system using speech recognition, and specifically a voice assistant system that allows an enrolled user to speak commands that the system will act upon. The voice assistant system uses speaker recognition to ensure that a command was spoken by the enrolled user, before it acts upon the command. 
     The voice assistant system is woken from a low-power operating mode by a trigger phrase, in this case “Wake up phone”. Speech that follows the trigger phrase is sent to a speech recognition system, in order to identify the meaning of the speech. In addition, the signal representing the trigger phrase and the following speech is sent to a speaker recognition system, in order to determine whether the speaker is an enrolled user, for example the owner of the device or a person authorised by the owner. 
     One problem with such systems is that the reliability of the speaker recognition process depends on the duration of the speech that is considered. Longer utterances allow the speaker recognition system to operate with greater certainty. In addition, in a system that is woken using a predetermined trigger phrase, the speaker recognition system can use a text-dependent system to analyse the trigger phrase, but must use a text-independent system to analyse the speech following the trigger phrase. Since text-dependent speaker recognition is generally more reliable than text-independent speaker recognition, more weight might be given to the results of the text-dependent speaker recognition than to the results of the text-independent speaker recognition. This means that, provided that the trigger phrase is spoken by the enrolled user, then even if the speech following the trigger phrase is spoken by a different person, it might take some significant time before the overall result of the speaker recognition system recognises that there has been a change of speaker, and that it is no longer accurate to say that the speech was uttered by the enrolled user. 
     Thus,  FIG. 3  shows a situation where an enrolled user, Speaker  1 , utters the predetermined trigger phrase “Wake up phone”. Immediately thereafter, another person, Speaker  2 , utters a command “Order me a beer”. This is referred to as a competitive command. 
       FIG. 4  shows a situation where an enrolled user, Speaker  1 , utters the predetermined trigger phrase “Wake up phone”, and then also utters a command “Order me a pizza”. Immediately thereafter, another person, Speaker  2 , utters an additional command “And a beer”. This is referred to as a tailgating attack. 
     Methods and systems described herein involve speaker change detection, so that attacks such as those illustrated in  FIGS. 3 and 4  can be overcome. 
       FIG. 5  is a block schematic diagram, illustrating a system configured for including speaker change detection as part of the speaker recognition system, and  FIG. 6  is a flow chart, illustrating an example of a method of performing speech recognition, in a system as illustrated in  FIG. 5 , for example. It should be noted that, although  FIG. 6  is presented as a flow chart, in which steps are performed successively, this represents just one embodiment. In other embodiments, the order of the steps may be different from that shown in  FIG. 6 , and/or steps may be performed in parallel, and/or one or more steps may be performed only when another step has been completed (and, in some cases, when the other step has produced one specific result). 
       FIG. 5  shows a system  60 , which includes a microphone  62  for detecting sounds in the vicinity. In embodiments in which the system is implemented in a smartphone or other device, the microphone  62  may be the microphone of that device. 
     The system  60  also includes a pre-processing block  64 , which performs initial processing on the audio signal generated by the microphone  62 , and stores the result in a buffer  66 . For example, speaker recognition and speech recognition processes typically operate on signals that have been divided into frames having a duration of 10-30 ms, and so the pre-processing block  64  may perform this division. Specifically, the pre-processing block  64  may divide the signal into frames, and may include a voice activity detection block, configured to determine which frames contain speech. 
     In the system illustrated in  FIG. 5 , the system also includes an optional voice keyword detection (VKD) block  68 . This may be configured to detect whether the audio signal represents a predetermined keyword, or trigger phrase. The VKD block  68  may act only on frames that are determined as containing speech. 
     Thus, the received audio signal is stored, a keyword detection process is performed on the received audio signal, and, when a predetermined keyword is detected, a start of the predetermined keyword is identified as a start of a first time period. 
     Thus, in some embodiments, the audio signal is only passed for subsequent processing if it is determined that the signal contains speech. In some embodiments, the audio signal is only passed for subsequent processing if it is determined that the signal contains the predetermined keyword. 
     Thus, provided that any condition as described above is met, the audio signal, or at least part of the audio signal, is passed to a first biometric process block  70 , which performs speaker recognition. For example, if the voice keyword detection block  68  determines that the signal contains the predetermined trigger phrase, the part of the signal beginning at the start of the predetermined trigger phrase (i.e. at the start of the first time period identified above) is sent from the buffer  66  to the first biometric process block  70 . 
     Therefore, referring to  FIG. 6 , at step  90 , an audio signal comprising speech is received. 
     At step  92 , a biometric process is performed on the received first part of the audio signal. The first part of the audio signal is considered to extend over the first time period mentioned above. The biometric process may be performed at a plurality of times during the first time period. 
     In one embodiment, the biometric process performed at step  92  includes both a text-dependent process and a text-independent process. In this case, the text-dependent process may be used to analyse the segment of the received signal that represents the trigger phrase, while the text-independent process is used to analyse the segment of the received signal that follows the trigger phrase. 
     As shown at step  94  of  FIG. 6 , the step of performing the biometric process at step  92  may therefore comprise obtaining a speaker recognition score from the biometric process for the first part of the audio signal. When the biometric process is performed at a plurality of times, the speaker recognition score obtained at each of the plurality of times may be a cumulative score relating to the entire first part of the audio signal up until that time. 
     The score that is obtained in step  94  may be an output that indicates whether or not the speaker is a specific individual (in the case of speaker verification), or identifies the speaker from a group of enrolled speakers (in the case of speaker identification). Alternatively, the score that is obtained in step  94  may be a numerical value that can for example be compared with an appropriate threshold in a subsequent process in order to allow a determination as to whether or not the speech should be considered to be that of the specific individual (in the case of speaker verification), or which of a group of enrolled speakers should be considered to be speaking (in the case of speaker identification). 
     In some embodiments, the biometric process is performed on the complete utterance that includes the trigger phrase. For example, the entire signal from the start of the trigger phrase until it is determined that the speech has stopped or the speaker has changed can be analysed. Calculating the cumulative score at the plurality of times means that, in this case, as soon as it is determined that the speech has stopped or the speaker has changed, there is an up-to-date score that has recently been calculated. In other embodiments, only a part of the signal is analysed. 
     As shown in  FIG. 5 , the first part of the audio signal, i.e. the part of the audio signal starting at the start of the predetermined trigger phrase, is also passed to a speaker change detection process  72 . In the illustrated embodiment, the speaker change detection process is a biometric speaker change detection process. 
     Thus, as shown in  FIG. 6 , at step  96 , a biometric process is performed on a plurality of second parts of the audio signal, wherein the second parts of the audio signal are successive sections of the first part of the audio signal, and wherein each second part of the audio signal extends over a second time period and the second time period is shorter than the first time period. For example, the second time period may be in the range 0.1 s-5 s. More specifically, the second time period may be in the range 0.5 s-1.5 s. Choosing a longer value for the second time period means that the scores obtained from the biometric process during the respective second time periods will likely be more consistent. However, choosing a shorter value means that there will be more such scores in a given utterance, making it easier to identify a speaker change from the series of scores, and so it is necessary to choose a suitable value based on these two factors. 
     Even in embodiments in which the biometric process performed at step  92  includes both a text-dependent process and a text-independent process, the biometric process performed at step  96  may include only a text-independent process. This provides a way to obtain consistent scores across the whole audio signal. 
     The biometric process performed at step  96  may be a scheme that allows scoring per frame, such as a scheme using a Gaussian Mixture Model (GMM). The biometric process performed at step  92  may be a scheme that allows scoring per frame, such as a scheme using a Gaussian Mixture Model (GMM), in embodiments in which a cumulative score is continually updated during the speech. In embodiments in which the biometric process performed at step  92  is performed only on a complete utterance, a system that supports scoring per utterance, such as a scheme using Joint Factor Analysis (JFA) may be used. In such embodiments, the complete utterance may be the entire signal from the start of the trigger phrase until it is determined that the speech has stopped or the speaker has changed. Alternatively, the complete utterance may be the signal that follows the end of the trigger phrase until it is determined that the speech has stopped or the speaker has changed. 
     At step  98 , a speaker recognition score is obtained from the biometric process for each second part of the audio signal. For example, the biometric process may involve extracting features from the relevant part of the audio signal, and comparing these features with a corresponding set of features (also referred to as a model, or a voice print) from the speech of a speaker. For these purposes, the model that is used as the basis of the comparison may be a Universal background Model, or may be a speaker-specific model, for example for the expected speaker (such as the enrolled user of the device). 
     The speaker recognition score that is obtained may then be a measure of the distance of the extracted features from the model that is used as the basis for the comparison. The speaker recognition score may for example be expressed as a likelihood that the speech is from a specific previously-enrolled speaker. 
     At step  100 , it is determined whether there has been a speaker change based on speaker recognition scores for successive second parts of the audio signal. For example, when the speaker recognition score that is obtained is a measure of the distance of the extracted features from a speaker model, it would be expected that the score would be relatively consistent for each second part of the audio signal for the voice of a single speaker. Thus, if a speaker recognition score that is obtained for one second part of the audio signal is significantly different from the scores obtained for preceding second parts, this may be taken as an indication that the speaker has changed. 
     Thus, determining whether there has been a speaker change based on speaker recognition scores for successive second parts of the audio signal may comprise determining that there has been a speaker change if speaker recognition scores for successive second parts of the audio signal detection show a variation that exceeds a threshold. 
     For example, this determination may simply involve comparing each score to the immediately preceding score, and determining that there has been a speaker change if the difference between the two speaker recognition scores exceeds a threshold value. 
     Alternatively, a statistical analysis may be performed, comparing each score with a metric derived from some or all previously calculated scores, and determining that there has been a speaker change if the most recent score falls outside an expected distribution. In one scheme, a running mean of all previously calculated scores is formed, and any newly calculated score that is sufficiently far from that mean is considered as a possible indicator of a speaker change. More generally, the Kolmogorov-Smirnov test can be used to compare a newly calculated score with a cumulative distribution function (for example an empirical cumulative distribution function) calculated from previously calculated scores, in order to determine whether the newly calculated score is likely to indicate a speaker change. 
     In this example, the speaker change detection is based entirely on the biometric speaker change detection. However, an alternative method of speaker change detection combines the biometric method with at least one other method. 
       FIG. 7  is a block schematic diagram, illustrating a system for speaker change detection, and  FIG. 8  is a flow chart, illustrating an example of a method of performing speaker change detection, in a system as illustrated in  FIG. 7 , for example. 
       FIG. 7  shows a system  72 , for example for use in the system of  FIG. 5 , although there are other situations in which speaker change detection is useful, and the system and method of  FIGS. 7 and 8  can be used in any such situation. 
     The audio signal representing speech, for example retrieved from a buffer as shown in  FIG. 5 , is passed to a speaker change detection block  120 . Thus, in step  150  of the process shown in  FIG. 8 , the signal is received and, in step  152 , at least one first speaker change detection process is performed on the received audio signal to obtain information about times at which there may have been a speaker change. 
     For example, the at least one first speaker change detection process may be one or more process selected from the group consisting of:
         tracking a fundamental frequency of the speech;   tracking an angle of arrival of the speech at a device; and   comparing feature vectors extracted from the audio signal in successive time windows of the audio signal, for example as described in J. Ajmera, I. McCowan, and H. Bourlard, “Robust speaker change detection”, 2004 IEEE Signal Processing Letters, vol. 11, no. 8, pp. 649-651, August 2004.       

     In addition, the audio signal representing speech is passed to a biometric speaker change detection block  122 . This operates in a similar manner to that described above. Thus, the audio signal representing speech is passed to a second biometric process block  124  and, in step  154  of  FIG. 8 , a biometric process is performed on a plurality of successive sections of the audio signal. In step  156 , a speaker recognition score is obtained from the biometric process for each section of the audio signal. As described above, the speaker recognition score that is obtained for each section may be a measure of the distance of the extracted features of that section of the audio signal from the model that is used as the basis for the comparison. 
     The speaker recognition scores are passed to a difference calculation block  126 . Thus, in step  158 , successive speaker recognition scores for the sections of the audio signal are examined in order to determine a series of difference values between pairs of consecutive speaker recognition scores for the successive sections of the audio signal. These difference values are then passed to a speaker change determination block  128 , which uses the difference values for providing an indication of whether there has been a speaker change. 
     Output signals from the speaker change determination block  128  and the speaker change detection block  120  are passed to a combination block  130  and, in step  160 , it is determined whether there has been a speaker change based on information obtained from the first speaker change detection process and on said series of difference values. 
     Thus, multiple metrics may be combined to form a single output score. Each metric may for example be a measure of the likelihood that there has been a speaker change during a particular time period, which may be as long as the entire utterance. If there are n metrics, and each of the metrics (i.e. the metric obtained from the speaker change determination block  128  and the one or more metric obtained from the speaker change detection block  120 ) has a value α i , these values can be given respective weightings ω i , and a combined output score S can be calculated as: 
     
       
         
           
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     The weighting given to each metric may be arranged to give increased weight to more accurate metrics. There are multiple ways to calculate the weighting. For example, one way uses the Equal Error Rate calculated for the different metrics. Thus, if a group of metrics have respective Equal Error Rates EER i , the weighting given to each metric value can be calculated as: 
     
       
         
           
             
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     Similarly, the weighting may accordingly be calculated using the False Acceptance Rate, FAR, or False Rejection Rate, FRR, for each metric. 
     Thus, in step  160 , it may be determined whether there has been a speaker change based on a determination as to whether the single combined output score exceeds a threshold value. 
     Alternatively, each metric may be compared with its own respective threshold value, and the determination as to whether there has been a speaker change may be based on the set of comparison results. For example, it may be determined that there has been a speaker change if any one of the metrics exceeds its respective threshold value. Alternatively, it may be determined that there has been a speaker change if a majority of the metrics exceed their respective threshold values. Alternatively, it may be determined that there has been a speaker change if all of the metrics exceed their respective threshold values. 
     There are therefore various ways in which it might be determined if there has been a speaker change. 
     In addition, there are various steps that might be taken in response to determining that there has been a speaker change. Returning to  FIG. 5 , the signal stored in the buffer  66  may be passed to an automatic speech recognition block  74 . In a system having a voice keyword detection block  68 , automatic speech recognition may be performed only on the received audio signal starting from the end of the predetermined keyword. 
     The speech recognition block  74  is able to determine the meaning of the spoken words contained in the signal that it receives, and, for example, interpret these as a command, and take suitable action in response to the command. In order to prevent the attacks described with reference to  FIGS. 3 and 4 , for example, the action taken in response to a speaker change detection might for example be to send to the automatic speech recognition block  74  only the speech that occurs before a speaker change. Then, if there is a speaker change, for example in a competitive command or a tailgating attack, the speech of the second speaker will not be send to the automatic speech recognition block  74  for processing and interpretation. 
     In other examples, the detection of a speaker change might be used as a signal to the first biometric process  70  to stop performing the speaker recognition. 
     In other examples, the stored speech signal is retrieved first by the speaker change detection process  72 , and steps  96  and  98  of  FIG. 6  are performed. When it is determined in step  100  that there has been a speaker change, the stored speech signal is retrieved by the first biometric process  70 , and step  92  is performed. In this case, it is only necessary for the first biometric process  70  to retrieve the stored speech signal up until the time at which the speaker change was detected. Thus, the time at which the speaker change is detected defines an end point of the first part of the audio signal. If it is determined in step  100  that there has been no speaker change, the stored speech signal is retrieved by the first biometric process  70  only at the end of the whole utterance, and step  92  is performed on the whole utterance. In these examples, the first biometric process  70  may determine whether the speaker was the expected enrolled speaker and, if so, a signal may be sent to the buffer  66 , so that the audio signal stored in the buffer  66  is passed to an automatic speech recognition block  74  for interpretation of the enrolled speaker&#39;s utterance. 
     In other examples, the stored speech signal is retrieved by the first biometric process  70 , for example at the end of a whole utterance, and step  92  is performed on the whole utterance or other retrieved signal. As usual, the first biometric process  70  may determine whether the speaker was the expected enrolled speaker. Either in parallel with this, or only after determining that the speaker was the expected enrolled speaker, the stored speech signal is retrieved first by the speaker change detection process  72 , and steps  96  and  98  of  FIG. 6  are performed. In this case, when it is determined in step  100  that there has been a speaker change, the stored speech signal may be considered to have been tampered with, and may be discarded. Only in response to determining that there has not been a speaker change during the first part of the audio signal, the speaker recognition score obtained for the first part of the audio may be used. Thus, if the score indicates that the speaker was the expected enrolled speaker, a signal may be sent to the buffer  66 , so that the audio signal stored in the buffer  66  is passed to an automatic speech recognition block  74  for interpretation of the enrolled speaker&#39;s utterance. 
     In other examples, the stored speech signal is retrieved first by the speaker change detection process  72 , and steps  96  and  98  of  FIG. 6  are performed. When it is determined in step  100  that there has been a speaker change, the stored speech signal is retrieved by the first biometric process  70 , and step  92  is performed. In this case, it is only necessary for the first biometric process  70  to retrieve the stored speech signal up until the time at which the speaker change was detected. Thus, the time at which the speaker change is detected defines an end point of the first part of the audio signal. If it is determined in step  100  that there has been no speaker change, step  92  is performed on the whole utterance. In this case, the step of obtaining the speaker recognition score from the biometric process for the first part of the audio signal may comprise combining the respective speaker recognition scores obtained from the biometric process for each second part of the audio signal. Thus, as part of the speaker detection process, a separate speaker recognition score is obtained for each second part of the audio signal. When it has been determined that there is no speaker change during the whole utterance, a speaker recognition score for the whole utterance may be obtained by combining (for example averaging) these separate speaker recognition scores. Similarly, when it is determined that there has been a speaker change, a speaker recognition score for the first part of the signal up until the speaker change may be obtained by combining (for example averaging) the separate speaker recognition scores for the same part of the signal. Again, the first biometric process  70  may determine from this speaker recognition score for the first part of the signal whether the speaker was the expected enrolled speaker and, if so, a signal may be sent to the buffer  66 , so that the audio signal stored in the buffer  66  is passed to an automatic speech recognition block  74  for interpretation of the enrolled speaker&#39;s utterance. 
       FIG. 9  is a timing diagram, illustrating operation of the system and method described herein, in one example situation. 
     
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     Line  180  shows the speech that is detected by the pre-processing block  64  in  FIG. 5 . In this example, this speech begins at time t 0  with a trigger phrase  182 , and continues thereafter with subsequent speech  184 . 
     Line  186  shows the operation of the voice keyword detection (VKD) block  68  in  FIG. 5 . In this example, at time ti, the VKD block identifies that the trigger phrase has been spoken, and generates a signal  188 . 
     In response, the signal from the start of the trigger phrase is retrieved from the buffer  66  and passed to the first biometric process  70  and the speaker change detection process  72 . 
     The first biometric process Vbio 1  therefore starts operating at time t 2 , after the shortest possible delay from the time when the VKD block identifies that the trigger phrase has been spoken, and operates on the audio signal. During the time that the retrieved signal contains the trigger phrase  190 , the first biometric block performs a text-dependent (TD) speaker recognition process  192  on the received signal. During the following period when the retrieved signal contains the subsequent speech  194 , the first biometric block performs a text-independent (TI) speaker recognition process  196  on the received signal. 
     The first biometric block performs a cumulative process, and so, as shown generally at  198 , it generates a series of score values at successive times  200 ,  202  etc. At any desired time, these can be compared with a threshold value T. If the score exceeds the threshold value T, then this can be considered sufficient to determine that the speaker is a specific enrolled user. 
     The speaker change detection process uses a second biometric process Vbio 2 , and this similarly starts operating at time t 2 , and operates on the audio signal. In this case, both during the time that the retrieved signal contains the trigger phrase  204 , and during the following period when the retrieved signal contains the subsequent speech  206 , the second biometric block performs a text-independent (TI) speaker recognition process  208  on the received signal. 
     The second biometric block performs a process that is not cumulative, and so, as shown generally at  210 , it generates a series of score values at successive times  212 ,  214 , etc., that are somewhat independent of each other. However, when the same person is speaking, these scores are somewhat similar, reflecting the fact that the speech of that person is generally consistent. Thus, every score until the score at time  216  falls within the range from a to b. 
     However, the next score, at time  218 , has a value of c, significantly outside the range from a to b. This sudden difference may be taken as an indication that a different person is speaking. 
     Line  220  shows the speaker change detection output and, in response to the score at time  218 , this produces a speaker change detection signal  222 . 
     This signal  222  can therefore be used to mark the end of the speech of the first speaker. This can therefore be used to ensure that only the speech of the first speaker is sent for speech recognition. 
     It will be noted that, although the change of speaker also causes a reduction in the cumulative score output by the first biometric process at time  224 , this is not sufficient to take the score below the threshold value T, and it can be seen that the score may not fall below the threshold value T for some significant time. Thus, it can be seen that simply relying on the cumulative scoring biometric process would be inadequate to detect the speaker change. 
     The skilled person will recognise that some aspects of the above-described apparatus and methods, for example the discovery and configuration methods may be embodied as processor control code, for example on a non-volatile carrier medium such as a disk, CD- or DVD-ROM, programmed memory such as read only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. For many applications, embodiments will be implemented on a DSP (Digital Signal Processor), 
     ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array). Thus the code may comprise conventional program code or microcode or, for example code for setting up or controlling an ASIC or FPGA. The code may also comprise code for dynamically configuring re-configurable apparatus such as re-programmable logic gate arrays. Similarly the code may comprise code for a hardware description language such as Verilog™ or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate, the code may be distributed between a plurality of coupled components in communication with one another. Where appropriate, the embodiments may also be implemented using code running on a field-(re)programmable analogue array or similar device in order to configure analogue hardware. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference numerals or labels in the claims shall not be construed so as to limit their scope.