Patent Publication Number: US-11023755-B2

Title: Detection of liveness

Description:
TECHNICAL FIELD 
     Embodiments described herein relate to methods and devices for detecting liveness of a speaker. As one example, the liveness detection can be used for detecting a replay attack on a voice biometrics system. 
     BACKGROUND 
     Biometrics systems are becoming widely used. In a voice biometrics system, a user trains the system by providing samples of their speech during an enrolment phase. In subsequent use, the system is able to discriminate between the enrolled user and non-registered speakers. Voice biometrics systems can in principle be used to control access to a wide range of services and systems. 
     One way for a malicious party to attempt to defeat a voice biometrics system is to obtain a recording of the enrolled user&#39;s speech, and to play back the recording in an attempt to impersonate the enrolled user and to gain access to services that are intended to be restricted to the enrolled user. 
     This is referred to as a replay attack, or as a spoofing attack. 
     In a facial recognition, or other type of biometrics system, the system recognises a characteristic of the user. Again, one way for a malicious party to attempt to defeat such a biometrics system is to present the system with a photograph or video recording of the enrolled user. 
     SUMMARY 
     According to an aspect of the present invention, there is provided a method, for use in a voice biometrics system, for detecting the liveness of a speaker and determining whether a received speech signal may be a product of a replay attack, which comprises: generating an ultrasound signal; receiving an audio signal comprising a reflection of the ultrasound signal; using the received audio signal comprising the reflection of the ultrasound signal to detect the liveness of a speaker; monitoring ambient ultrasound noise; and adjusting the operation of the voice biometrics system, based on a level of the reflected ultrasound and the monitored ambient ultrasound noise. Detecting the liveness of a speaker comprises determining whether a received speech signal may be a product of a replay attack. 
     According to another aspect of the present invention, there is provided a system for liveness detection, the system being configured for performing the method of the first aspect. 
     According to another aspect of the present invention, there is provided a device comprising such a system. The device may comprise a mobile telephone, an audio player, a video player, a mobile computing platform, a games device, a remote controller device, a toy, a machine, or a home automation controller or a domestic appliance. 
     According to another aspect of the present invention, there is provided a computer program product, comprising a computer-readable tangible medium, and instructions for performing a method according to the first aspect. 
     According to another aspect of the present invention, there is provided a non-transitory computer readable storage medium having computer-executable instructions stored thereon that, when executed by processor circuitry, cause the processor circuitry to perform a method according to the first aspect. According to another aspect of the present invention, there is provided a device comprising such a non-transitory computer readable storage medium. The device may comprise a mobile telephone, an audio player, a video player, a mobile computing platform, a games device, a remote controller device, a toy, a machine, or a home automation controller or a domestic appliance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a better understanding of the present invention, and to show how it may be put into effect, reference will now be made to the accompanying drawings, in which: 
         FIG. 1  illustrates a smartphone; 
         FIG. 2  is a schematic diagram, illustrating the form of the smartphone; 
         FIG. 3  illustrates a situation in which a replay attack is being performed; 
         FIG. 4  is a flow chart illustrating a method of detecting liveness; 
         FIG. 5  illustrates a speech processing system, including a system for detecting liveness; 
         FIG. 6  is a flow chart illustrating a part of the method of detecting liveness; 
         FIGS. 7 a , 7 b , and 7 c    illustrate various possible uses of smartphones; 
         FIG. 8  is a flow chart illustrating a part of the method of detecting liveness; 
         FIG. 9  is a flow chart illustrating a part of the method of detecting liveness; 
         FIG. 10  is a block diagram, illustrating a part of the system for detecting liveness; and 
         FIG. 11  illustrates results of the method of detecting liveness. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     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. 
     One example of the invention is illustrated with reference to its use in a smartphone, by way of example, though it will be appreciated that it may be implemented in any suitable device, as described in more detail below. 
       FIG. 1  illustrates a smartphone  10 , having a microphone  12  for detecting ambient sounds. In normal use, the microphone is of course used for detecting the speech of a user who is holding the smartphone  10 . 
     The smartphone  10  also has two loudspeakers  14 ,  16 . The first loudspeaker  14  is located at the top of the smartphone  10 , when it is held in its normal operating position for making a voice call, and is used for playing the sounds that are received from the remote party to the call. 
     The second loudspeaker  16  is located at the bottom of the smartphone  10 , and is used for playing back media content from local or remote sources. Thus, the second loudspeaker  16  is used for playing back music that is stored on the smartphone  10  or sounds associated with videos that are being accessed over the internet. 
     The illustrated smartphone  10  also has two additional microphones  12   a ,  12   b . The additional microphones, if present in the device, may be provided at any suitable location. In this illustrated device, one microphone  12   a  is located at the top end of the front of the device, while another microphone  12   b  is located at the top end of the side of the device. 
       FIG. 2  is a schematic diagram, illustrating the form of the smartphone  10 . 
     Specifically,  FIG. 2  shows various interconnected components of the smartphone  10 . It will be appreciated that the smartphone  10  will in practice contain many other components, but the following description is sufficient for an understanding of the present invention. 
     Thus,  FIG. 2  shows the microphone  12  mentioned above. In this particular illustrated embodiment, the smartphone  10  is provided with multiple microphones  12 ,  12   a ,  12   b , etc.  FIG. 2  also shows the loudspeakers  14 ,  16 . 
       FIG. 2  also shows a memory  18 , which may in practice be provided as a single component or as multiple components. The memory  18  is provided for storing data and program instructions. 
       FIG. 2  also shows a processor  20 , which again may in practice be provided as a single component or as multiple components. For example, one component of the processor  20  may be an applications processor of the smartphone  10 . 
       FIG. 2  also shows a transceiver  22 , which is provided for allowing the smartphone  10  to communicate with external networks. For example, the transceiver  22  may include circuitry for establishing an internet connection over a WiFi local area network and/or over a cellular network. 
       FIG. 2  also shows audio processing circuitry  24 , for performing operations on the audio signals detected by the microphone  12  as required. For example, the audio processing circuitry  24  may filter the audio signals or perform other signal processing operations. 
     The audio signal processing circuitry is also able to generate audio signals for playback through the loudspeakers  14 ,  16 , as discussed in more detail below. 
       FIG. 2  also shows that the smartphone  10  may include one or more sensors  26 . In certain embodiments, the sensor(s) may include any combination of the following: gyroscopes, accelerometers, proximity sensors, light level sensors, touch sensors, and a camera. 
     In this illustrated embodiment, the smartphone  10  is provided with voice biometric functionality, and with control functionality. Thus, the smartphone  10  is able to perform various functions in response to spoken commands from an enrolled user. The biometric functionality is able to distinguish between spoken commands from the enrolled user, and the same commands when spoken by a different person. Thus, certain embodiments of the invention relate to operation of a smartphone or another portable electronic device with some sort of voice operability, for example 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, in which the voice biometric functionality is performed in the device that is intended to carry out the spoken command. Certain other embodiments relate to systems in which the voice biometric functionality is performed on a smartphone or other device, which then transmits the commands to a separate device if the voice biometric functionality is able to confirm that the speaker was the enrolled user. 
     In some embodiments, while voice biometric functionality is performed on the smartphone  10  or other device that is located close to the user, the spoken commands are transmitted using the transceiver  22  to a remote speech recognition system, which determines the meaning of the spoken commands. For example, the speech recognition system may be located on one or more remote server in a cloud computing environment. Signals based on the meaning of the spoken commands are then returned to the smartphone  10  or other local device. In other embodiments, the speech recognition system is also located on the device  10 . 
     One attempt to deceive a voice biometric system is to play a recording of an enrolled user&#39;s voice in a so-called replay or spoof attack. 
       FIG. 3  shows an example of a situation in which a replay attack is being performed. Thus, in  FIG. 3 , the smartphone  10  is provided with voice biometric functionality. In this example, the smartphone  10  is in the possession, at least temporarily, of an attacker, who has another smartphone  30 . The smartphone  30  has been used to record the voice of the enrolled user of the smartphone  10 . The smartphone  30  is brought close to the microphone  12  inlet of the smartphone  10 , and the recording of the enrolled user&#39;s voice is played back. If the voice biometric system is unable to determine that the enrolled user&#39;s voice that it recognises is a recording, the attacker will gain access to one or more services that are intended to be accessible only by the enrolled user. 
     At the same time, or separately, when the smartphone  10  is provided with a camera-based biometric functionality, such as a facial recognition system, an attacker may use the display of the smartphone  30  to show a photo or video of the enrolled user, in an attempt to defeat the facial recognition system. 
     Embodiments described herein therefore attempt to perform liveness detection, for example detecting the presence of a person speaking any voice sounds that are detected. 
       FIG. 4  is a flow chart, illustrating a method of liveness detection, for example for use in a biometrics system, and in this illustrated example used for detecting a replay attack on a voice biometrics system, and  FIG. 5  is a block diagram illustrating functional blocks in one example of a speech processing system that includes the voice biometrics system. 
     Specifically, in step  50  in the method of  FIG. 4 , a signal is received on an input  70  of the system shown in  FIG. 5 . Thus, the input  70  may be connected to the microphone  12  shown in  FIG. 1  or the multiple microphones  12 ,  12   a ,  12   b , etc shown in  FIG. 2 . 
     The received signal is passed to a voice activity detector (VAD)  72 , which detects when the received signal contains speech. 
     The received signal is also passed to a keyword detection block  74 . If it is determined by the voice activity detector  72  that the received signal contains speech, the keyword detection block  74  is activated, and it acts to detect the presence of a predetermined keyword in the detected speech. For example, the speech processing system of a smartphone might as a default operate in a low power mode, reflecting the fact that speech processing will be required for only a small fraction of the operating life of the device. The speech processing system may then be taken out of the low-power mode by the user uttering the predetermined keyword or phrase, such as “Hello phone”. 
     The received signal is also passed to a speaker recognition block  76 . If it is determined by the keyword detection block  74  that the predetermined keyword is present in the detected speech, the speaker recognition block  76  then attempts to determine whether the person who uttered the predetermined keyword is the registered user of the device and/or of a particular application on the device. Suitable biometric techniques are known for determining whether the speaker of the speech that is present in the received signal is the registered user. 
     If it is determined by the speaker recognition block  76  that the person who uttered the predetermined keyword is the registered user of the device and/or of the particular application on the device, then the received signal is passed to a speech processing block  78 , which may be present on the device or may be located remotely, in the cloud. The speech processing block  78  then determines the content of the speech. If the speech contains a command, for example, then the speech processing block  78  generates a suitable signal for causing that command to be acted upon. 
     The system shown in  FIG. 5  includes a mechanism for performing liveness detection, and hence for detecting whether the received signal containing speech has originated from a replay attack, as illustrated in  FIG. 3 . 
     Thus, in step  52  of the method shown in  FIG. 4 , an ultrasound signal is generated and transmitted, by the ultrasound generate and transmit block  80  shown in  FIG. 5 . The ultrasound transmit block  80  may operate at all times. In other embodiments, the ultrasound transmit block  80  operates only when it receives an enable signal on its input  82 . The enable signal may be generated, for example, when the voice activity detector  72  determines that the received signal contains speech, or when the keyword detection block  74  detects the presence of the predetermined keyword, or when the speaker recognition block  76  starts to perform a biometric technique to determine whether the person who uttered the predetermined keyword is the registered user. 
     The ultrasound signal may be a single tone sine wave, or other configurations may be used, for example a chirp signal. The frequency of the ultrasound signal may be selected to be relatively close to 20 kHz for transmittability reasons, while being high enough to ensure that it is not audible. 
     In step  54  of the method shown in  FIG. 4 , a reflection of the generated ultrasound signal is detected. 
     In the system shown in  FIG. 5 , a signal is received on an input  84 , and passed to an ultrasound detection block  86 . For example, the input  84  may be connected to one or more of the multiple microphones  12 ,  12   a ,  12   b , etc shown in  FIG. 2 , to receive any signal detected thereby. 
     The received signal is passed to the ultrasound detection block  86 , which may for example comprise one or more filter for selecting signals having a frequency that is close to the frequency of the ultrasound signal transmitted by the ultrasound transmit block  80 . Reflected ultrasound signals may be Doppler shifted in their frequency, but the Doppler shifts are unlikely to be much more than 100 Hz, and so the ultrasound detection block  86  may comprise a filter for selecting signals having a frequency that is within 100 Hz of the frequency of the ultrasound signal transmitted by the ultrasound transmit block  80 . 
     In step  56  of the method shown in  FIG. 4 , the received ultrasound signal detected by the ultrasound detection block  86  is passed to a Doppler detect block  88 , to detect Doppler shifts in the reflection of the generated ultrasound signal. Thus, the received reflected ultrasound signal is compared with the generated ultrasound signal to identify frequency shifts in the reflected signal that are caused by reflections off a moving surface, such as the face, and in particular the lips, of a person who is speaking to generate the detected speech signal. 
     In step  58  of the method shown in  FIG. 4 , it is determined based on the detected Doppler shifts whether these Doppler shifts provide good evidence for the liveness of a person generating the detected speech. 
     In the illustrated embodiment shown in  FIG. 5 , the output of the Doppler detect block  88  is applied to one input of a correlation block  90 . The received audio signal on the input  70  is applied to another input of the correlation block  90 . In an alternative embodiment, a signal generated by the voice activity detect block  72  is applied to the other input of the correlation block  90 . The output of the correlation block  90  is applied to a determination block  92  shown in  FIG. 5 . 
     If it is found by the correlation block  90  that there is a correlation between time periods in which Doppler shifts are detected in the reflection of the generated ultrasound signal, and time periods in which speech content is identified in the received speech signal, this indicates that the detected speech is generated by a live person moving their lips to generate the sound. If the degree of correlation is low, one possible reason for this may be that the detected speech is not generated by a live person moving their lips to generate the sound. One possible cause of this is that the detected speech is in fact generated by a replay attack. 
     Therefore, the determination block  92  produces an output signal that contains information about the liveness of the speaker, and hence about the likelihood that the detected speech was generated by a replay attack. This output signal is applied, in this illustrated embodiment, to the speaker recognition block  76 , which is performing one or more voice biometrics process to determine whether the speaker is the registered user of the device. The speaker recognition block  76  can then use the output signal as one of several factors that it uses to determine whether the speaker is in fact the registered user of the device. For example, there may be one or more factors which indicate whether the detected speech is the speech of the registered user, and one or more factors which indicate whether the detected speech may have resulted from a replay attack. 
     In other examples, the liveness detection can be used for other purposes, for example for detecting an attempt to defeat a facial recognition system by presenting a still or moving image of an enrolled user. 
     As discussed in more detail below, the purpose of generating the ultrasound signal is to detect the movement of a speaker&#39;s face, and in particular the lips, while speaking. For this to operate successfully, it is advantageous that the ultrasound signal may be varied depending on information about the use of the device. 
     Thus, as described above, step  52  of the process shown in  FIG. 4  involves generating and transmitting the ultrasound signal. 
       FIG. 6  is a flow chart, giving more detail about this step, in some embodiments. Specifically, in step  110  of the method, the system obtains information about a position of the device  10 . 
     For example, obtaining information about a position of the device may comprise obtaining information about an orientation of the device. Information about the orientation of the device may for example be obtained from gyroscopes and/or accelerometers provided as sensors  26  in the device  10 . 
     As one alternative, obtaining information about a position of the device may comprise obtaining information about a distance of the device from the voice source. Information about a distance of the device from the voice source may for example be obtained by detecting the levels of signals generated by the microphones  12 ,  12   a ,  12   b . For example, a higher signal level from one microphone may indicate that the voice source is closer to that microphone than to one or more other microphone. 
     As another alternative, obtaining information about a position of the device may comprise obtaining information about a position of the device relative to a supposed speaker. Information about the position of the device relative to a supposed speaker may for example be obtained from one or more proximity sensor provided as a sensor  26  in the device  10 . Information about the position of the device relative to a supposed speaker may also be obtained from one or more light level sensor provided as a sensor  26  in the device  10 . Information about the position of the device relative to a supposed speaker may also be obtained from one or more touch sensor provided as a sensor  26  in the device  10 , indicating how the device  10  is being held by a user. Information about the position of the device relative to a supposed speaker may also be obtained from a camera provided as a sensor  26  in the device  10 , which can track the position of a user&#39;s face relative to the device  10 . 
     Then, in step  112 , the method involves adapting the generating and transmitting of the ultrasound signal based on the information about the position of the device. 
     Adapting the generating and transmitting of the ultrasound signal may for example comprise adjusting a transmit power of the ultrasound signal. As another example, when the device has multiple transducers  14 ,  16 , adapting the generating and transmitting of the ultrasound signal may comprise selecting the one or more transducer in which the ultrasound signal is generated, with the intention that the ultrasound signal should be generated from a transducer that is close to the user&#39;s mouth in order to be able to detect movement of the user&#39;s lips. 
     For example, obtaining information about a position of the device may comprise obtaining information about a distance of the device from the voice source, and adapting the generating and transmitting of the ultrasound signal may comprise adjusting a transmit power of the ultrasound signal, with a higher power being used when the device is further from the voice source, at least for distances below a certain limit. This allows the device to generate ultrasound signals that produce clearly detectable reflections, without risking transmitting ultrasound energy when the device is close to the user&#39;s ear. 
     As another example, obtaining information about a position of the device may comprise obtaining information as to which of multiple loudspeaker transducers is closest to the voice source (for example based on signal levels at microphones placed located close to those transducers), and adapting the generating and transmitting of the ultrasound signal may comprise transmitting the ultrasound signal mainly or entirely from that transducer. This allows the device to generate ultrasound signals from the transducer that is closest to the sound source, and thereby increase the chance of detecting usable reflection signals. 
     Other possibilities relate to specific ways in which speakers may use the device. 
     Thus, for example, when the device  10  is a mobile phone comprising at least a first transducer  16  at a lower end of the device and a second transducer  14  at an upper end of the device, adapting the generating and transmitting of the ultrasound signal based on the information about the position of the device may comprise transmitting the ultrasound signal at a relatively low power from the first transducer  16  if the information about the position of the device indicates that the device  10  is being used in a close talk mode. Close talk will be understood as a use of a phone where the phone is positioned adjacent the side of a user&#39;s face, and where communication is using the close-range earpiece speaker, e.g. as with a “traditional” phone handset positioning. 
     For example, the ultrasound signal may be transmitted at a level of 70-90 dB SPL at 1 cm in this mode. 
     The information about the position of the device may be considered to indicate that the device is being used in a close talk mode if, for example, accelerometers indicate that the device  10  is in an upright position, and proximity sensors detect that the device  10  is being held close to a surface that might be a user&#39;s face  120 , as shown in  FIG. 7   a.    
     More generally, adapting the generating and transmitting of the ultrasound signal based on the information about the position of the device may comprise transmitting the ultrasound signal from the second transducer if the information about the position of the device indicates that the device is being used in a generally vertical orientation. 
     As another example, when the device  10  is a mobile phone comprising at least a first transducer  16  at a lower end of the device and a second transducer  14  at an upper end of the device, adapting the generating and transmitting of the ultrasound signal based on the information about the position of the device may comprise transmitting the ultrasound signal at a relatively high power from the transducer  16  at the lower end of the device, if the information about the position of the device indicates that the device  10  may be being held by the user in front of their face  130 , with the lower microphone  12  pointing towards them, i.e. in a “pizza slice” version of a near talk mode, as shown in  FIG. 7   b.    
     Near-talk mode will be understood as where a phone is positioned in front of the user&#39;s face, and where use may be made of near-field loudspeakers and microphones. This position may be suitable for the purposes of a video call, e.g. using software products such as Skype™ from Microsoft or FaceTime™ from Apple. “Pizza slice” mode will be understood as a variation of near-talk mode, but where the phone is held in a relatively horizontal position (such that a microphone positioned at the lower end of the phone faces the user directly). 
     For example, the ultrasound signal may be transmitted at a level of 90-110 dB SPL at 1 cm in this mode. 
     The information about the position of the device may be considered to indicate that the device is being used in a “pizza slice” mode if, for example, accelerometers indicate that the device is in a horizontal position, and the signal level detected by the microphone  12  is higher than the signal level detected by the microphones  12   a ,  12   b.    
     More generally, adapting the generating and transmitting of the ultrasound signal based on the information about the position of the device may comprise transmitting the ultrasound signal from the first transducer if the information about the position of the device indicates that the device is being used in a generally horizontal orientation. 
     In the variant of the near talk mode, in which the device is held by the user in front of their face, for example so that they can see the screen on the device while speaking, adapting the generating and transmitting of the ultrasound signal based on the information about the position of the device may comprise transmitting the ultrasound signal at a relatively high power from the transducer  14  at the upper end of the device, or from transducers at both ends of the device. 
     As another example, adapting the generating and transmitting of the ultrasound signal based on the information about the position of the device may comprise preventing transmission of the ultrasound signal if the information about the position of the device indicates that the device is being used in a far field mode, for example with the device  10  being placed on a surface  140  some distance from the user  142 , as shown in  FIG. 7 c   . In this example, the information about the position of the device may indicate that the device is located more than a threshold distance (for example 50 cm) from the source of the sound. 
     This is because it may be determined that detecting the movement of a speaker&#39;s lips is only reliable enough for use when the indications are that the device may be being held close to the user&#39;s face. 
     As shown in  FIG. 5 , and as described above, the output of the Doppler detect block  88  is applied to one input of a correlation block  90 . The received audio signal on the input  70  is applied to another input of the correlation block  90 . The correlation block  90  determines whether there is a correlation between time periods in which Doppler shifts are detected in the reflection of the generated ultrasound signal, and periods in which there is speech. 
     The aim is to confirm that any Doppler shifts that are detected in the received reflection of the generated ultrasound signal do result from facial movements of a speaker, and are not the result of spurious reflections from other moving objects. 
       FIG. 8  is a flow chart, illustrating a method performed in the correlation block  90 . 
     First, it is determined in step  150 , whether the detected Doppler shifts correspond to a general speech articulation rate. The articulation rate is the rate at which syllables are produced during speech, and it has been found that, for most speech, a typical articulation rate is in the range of 4-10 Hz. The facial movements of the speaker (for example movements of the speaker&#39;s lips, cheeks, and nostrils) will typically occur at the same rate. Thus, in step  150 , it is determined whether the detected Doppler shifts correspond to facial movements at a frequency in the range of 4-10 Hz. 
     In step  152 , it is determined whether the detected Doppler shifts correspond to an articulation rate of the current speech. 
     Thus, the articulation rate of the speech contained in the received audio signal is extracted in the correlation block  90 . It is then determined whether the detected Doppler shifts correspond to facial movements at a frequency that corresponds to that extracted articulation rate. 
     If it is determined that the detected Doppler shifts correspond to facial movements at a frequency that corresponds to that extracted articulation rate, this can be taken as good evidence of liveness. 
     In a further possible step, in step  154  of the method shown in  FIG. 8 , it is determined whether there is a correlation between detected Doppler shifts in the reflection of the generated ultrasound signal, and speech content of the received speech signal. 
     It is recognised that one issue with using ultrasound as described herein, is that there may be interfering sources of ambient ultrasound noise. 
     Therefore,  FIG. 9  is a flow chart, illustrating one method performed in the Doppler detect block  88  and correlation block  90 . 
     Specifically, in step  170 , a level of ambient ultrasound noise is monitored. Then, in step  172 , the operation of the voice biometrics system is adjusted based on the levels of the reflected ultrasound and monitored ambient ultrasound noise. 
       FIG. 10  is a block diagram, illustrating schematically the operation of the Doppler detect block  88  and correlation block  90 .  FIG. 11  illustrates signals obtained at different stages of the operation. 
     Specifically, the signal from one or more microphones  12  is passed to a low pass filter  180 , for isolating the audio frequency components (for example, below 20 kHz) of the detected signal. The resulting audio signal, in one example, is shown in  FIG. 11 , signal a). 
     The signal level of the audio signal is found in a block  182  that finds the absolute value of the signal. The resulting envelope signal, in the same example, is shown in  FIG. 11 , signal b). 
     The signal from the one or more microphones  12  is also passed to a high pass filter  184 , for isolating the ultrasound components (for example, above 20 kHz) of the detected signal. This may contain the wanted reflection of the generated ultrasound signal, but may also contain interfering ambient ultrasound noise. 
     The level of the ultrasound signal is determined by a level detector  186 . 
     The ultrasound signal is then passed to a demodulation block  188 , where it is downconverted to the audio band, and any Doppler shifted reflections are found. This is achieved by mixing the received ultrasound signal with the ultrasound signal that was generated and transmitted. The received ultrasound signal can be passed through a band pass filter before downconversion if required, in order to remove other ultrasound signals not originating from the transmitted signal. In addition, the output of the mixing step can be low-pass filtered. 
     The resulting signal, in one example, is shown in  FIG. 11 , signal c). 
     The signal level of the Doppler shifted reflected signal is found in a block  190  that finds the absolute value of the signal. 
     It can thus be seen from  FIG. 11  that there is a correlation between the detected Doppler shifts in the reflection of the generated ultrasound signal, and speech content of the received speech signal. 
     In order to obtain a robust result, a correlation operation is performed, as shown at block  192  of  FIG. 10 . 
     However, before performing the correlation, it is noted that, while the audio signal is effectively the result of the facial movements of the speaker, the Doppler shifts in the reflected ultrasound signal will result from the velocity of the facial movements. Therefore, in some embodiments, either the audio signal is differentiated (for example by passing through a block  194  in the form of a band pass filter with a pass-band of, say, 10-200 Hz, an envelope block, or a differentiator), or the ultrasound signal is integrated (for example by passing through a block  196  in the form of a leaky integrator or a band pass filter with a pass-band of, say, 10-200 Hz). 
     The correlator  192  then performs a frame-by-frame cross correlation on the signals. If the correlation result Rxy is above a threshold then it is determined that there is enough of a correlation, between the detected Doppler shifts and the speech content of the received speech signal, to conclude that there is evidence of a live speaker, and hence that the speech may not result from a replay attack. If there is not good evidence of liveness of a speaker, this may be an indication that the received speech signal may be a product of a replay attack. 
     The operation of the system may be adjusted, based on a level of the reflected ultrasound and the monitored ambient ultrasound noise, as detected by the level detector  186 . 
     For example, the reliance that is placed on the determination as to whether the received speech signal may be the result of a replay attack may be adjusted, based on the level of the monitored ambient ultrasound noise. The determination, as to whether the received speech signal may be the result of a replay attack, will typically be made based on more than one factor. It is recognised that the presence of large ambient ultrasound signals will impact on the reliability of this system, and so the reliance that is placed on the determination may be reduced, as the level of the monitored ambient ultrasound noise increases. More specifically, if the level of the monitored ambient ultrasound noise exceeds a first threshold level, the result of the correlation may be ignored completely, or the correlation may not be performed. 
     For lower levels of interference, the adjustment of the operation of the system may involve adapting the threshold correlation value that is used in determining whether there is enough of a correlation, between the detected Doppler shifts and the speech content of the received speech signal, to conclude that there is evidence of a live speaker. Specifically, for low levels of ultrasound interference, a high threshold correlation value can be used. For somewhat higher levels of ultrasound interference (still below the first threshold mentioned above), lower threshold correlation values can be used, to take account of the fact that the presence of interference will automatically reduce the correlation values obtained from the correlator  192 . 
     The skilled person will recognise that some aspects of the above-described apparatus and 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 of the invention 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. 
     Note that as used herein the term module shall be used to refer to a functional unit or block which may be implemented at least partly by dedicated hardware components such as custom defined circuitry and/or at least partly be implemented by one or more software processors or appropriate code running on a suitable general purpose processor or the like. A module may itself comprise other modules or functional units. A module may be provided by multiple components or sub-modules which need not be co-located and could be provided on different integrated circuits and/or running on different processors. 
     Embodiments may be implemented in a host device, especially a portable and/or battery powered host device such as a mobile computing device for example a laptop or tablet computer, a games console, a remote control device, a home automation controller or a domestic appliance including a domestic temperature or lighting control system, a toy, a machine such as a robot, an audio player, a video player, or a mobile telephone for example a smartphone. 
     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.