Abstract:
A noise cancellation system is provided, for generating a noise cancellation signal to be added to a wanted signal to mitigate the effects of ambient noise. The system comprises: an input, for receiving an input signal representing ambient noise; a detector, for detecting a magnitude of said input signal; and a voice activity detector, for determining voiceless periods when said input signal does not contain a signal representing a voice. The detector is adapted to detect the magnitude of said input signal during said voiceless periods, and the system is adapted to operate in a first mode when said input signal is above a threshold value, and a second mode when said input signal is below the threshold value. The first mode comprises generating a noise cancellation signal with a first magnitude for at least partially cancelling the ambient noise. The second mode comprises generating a noise cancellation signal with a second magnitude that is less than the first magnitude.

Description:
[0001]    The present invention relates to noise cancellation systems, and in particular to a method for controlling the noise cancellation on the basis of the detected ambient noise. 
       BACKGROUND 
       [0002]    Noise cancellation systems are known, in which an electronic noise signal representing ambient noise is applied to a signal processing circuit, and the resulting processed noise signal is then applied to a speaker, in order to generate a sound signal. In order to achieve noise cancellation, the generated sound should approximate as closely as possible the inverse of the ambient noise, in terms of its amplitude and its phase. 
         [0003]    In particular, feedforward noise cancellation systems are known, for use with headphones or earphones, in which one or more microphones mounted on the headphones or earphones detect an ambient noise signal in the region of the wearer&#39;s ear. In order to achieve noise cancellation, the generated sound then needs to approximate as closely as possible the inverse of the ambient noise, after that ambient noise has itself been modified by the headphones or earphones. One example of modification by the headphones or earphones is caused by the different acoustic path the noise must take to reach the wearer&#39;s ear, travelling around the edge of the headphones or earphones. 
         [0004]    However, noise cancellation systems are generally employed in applications where it is highly desirable to reduce power consumption. For example, portable music players and mobile phones have limited battery resources, and therefore efforts should be made in order to reduce the drain on these resources. Noise cancellation is one such drain, and therefore it is desirable to design a noise cancellation system that is as efficient as possible. 
       SUMMARY OF INVENTION 
       [0005]    According to a first aspect of the present invention, there is provided a noise cancellation system, for generating a noise cancellation signal to be added to a wanted signal to mitigate the effects of ambient noise. The noise cancellation system comprises an input, for receiving an input signal representing ambient noise; a detector, for detecting a magnitude of said input signal; and a voice activity detector, for determining voiceless periods when said input signal does not contain a signal representing a voice. The detector is adapted to detect the magnitude of said input signal during said voiceless periods. The system is adapted to operate in a first mode when said input signal is above a threshold value, and a second mode when said input signal is below the threshold value. The first mode comprises generating a noise cancellation signal with a first magnitude for at least partially cancelling the ambient noise. The second mode comprises generating a noise cancellation signal with a second magnitude that is less than the first magnitude. 
         [0006]    According to a second aspect of the present invention, there is provided a noise cancellation system, for generating a noise cancellation signal to be added to a wanted signal to mitigate the effects of ambient noise. The system comprises an input, for receiving a signal representing ambient noise; a detector, for detecting a magnitude of said ambient noise signal; and a voice activity detector, for determining voiceless periods when said input signal does not contain a signal representing a voice. The detector is adapted to detect the magnitude of said input signal during said voiceless periods. The system is adapted to operate in a normal mode when said ambient noise signal is above a threshold value, and adapted to switch off when the ambient noise signal is below the threshold value. The normal mode comprises generating a noise cancellation signal for at least partially cancelling the ambient noise. 
         [0007]    The present invention also provides corresponding methods to each of the noise cancellation systems described above. 
         [0008]    According to alternative aspects of the present invention, there is provided a noise cancellation system, for generating a noise cancellation signal to be added to a wanted signal to mitigate the effects of ambient noise. The system comprises an input, for receiving an input signal representing ambient noise; and a detector, for detecting a magnitude of said input signal. The system is adapted to operate in a first mode when said input signal is above a threshold value, and a second mode when said input signal is below the threshold value. The first mode comprises generating a noise cancellation signal with a first magnitude for at least partially cancelling the ambient noise. The second mode comprises generating a noise cancellation signal with a second magnitude that is less than the first magnitude. The system is adapted to transition from the first mode to the second mode when the magnitude of the input signal falls below the first threshold value, and the system is adapted to transition from the second mode to the first mode when the magnitude of the input signal rises above a second threshold value, where the second threshold value is greater than the first threshold value. 
         [0009]    There is also provided a noise cancellation system, for generating a noise cancellation signal to be added to a wanted signal to mitigate the effects of ambient noise. The system comprises an input, for receiving a signal representing ambient noise; and a detector, for detecting a magnitude of said ambient noise signal. The system is adapted to operate in a normal mode when said ambient noise signal is above a threshold value, and adapted to switch off when the ambient noise signal is below the threshold value. The normal mode comprises generating a noise cancellation signal for at least partially cancelling the ambient noise. The system is adapted to transition from the normal mode to being switched off when the magnitude of the input signal falls below the first threshold value, and the system is adapted to transition from being switched off to the normal mode when the magnitude of the input signal rises above a second threshold value, where the second threshold value is greater than the first threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which: 
           [0011]      FIG. 1  illustrates a noise cancellation system in accordance with an aspect of the invention; 
           [0012]      FIG. 2  illustrates a signal processing circuit in accordance with an aspect of the invention in the noise cancellation system of  FIG. 1 ; 
           [0013]      FIG. 3  is a schematic graph showing one embodiment of the variation of applied gain with the detected noise envelope; and 
           [0014]      FIG. 4  is a schematic graph showing another embodiment of the variation of applied gain with the detected noise envelope. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates in general terms the form and use of a noise cancellation system in accordance with the present invention. 
         [0016]    Specifically,  FIG. 1  shows an earphone  10 , being worn on the outer ear  12  of a user  14 . Thus,  FIG. 1  shows a supra-aural earphone that is worn on the ear, although it will be appreciated that exactly the same principle applies to circumaural headphones worn around the ear and to earphones worn in the ear such as so-called ear-bud phones. The invention is equally applicable to other devices intended to be worn or held close to the user&#39;s ear, such as mobile phones and other communication devices. 
         [0017]    Ambient noise is detected by microphones  20 ,  22 , of which two are shown in  FIG. 1 , although any number more or less than two may be provided. Ambient noise signals generated by the microphones  20 ,  22  are combined, and applied to signal processing circuitry  24 , which will be described in more detail below. In one embodiment, where the microphones  20 ,  22  are analogue microphones, the ambient noise signals may be combined by adding them together. Where the microphones  20 ,  22  are digital microphones, i.e. where they generate a digital signal representative of the ambient noise, the ambient noise signals may be combined alternatively, as will be familiar to those skilled in the art. Further, the microphones could have different gains applied to them before they are combined, for example in order to compensate for sensitivity differences due to manufacturing tolerances. 
         [0018]    This illustrated embodiment of the invention also contains a source  26  of a wanted signal. For example, where the noise cancellation system is in use in an earphone, such as the earphone  10  that is intended to be able to reproduce music with a relatively high quality, the source  26  may be an inlet connection for a wanted signal from an external source such as a sound reproducing device. In other applications, for example where the noise cancellation system is in use in a mobile phone or other communication device, the source  26  may include wireless receiver circuitry for receiving and decoding radio frequency signals. 
         [0019]    The wanted signal from the source  26  is applied through the signal processing circuitry  24  to a loudspeaker  28 , which generates a sound signal in the vicinity of the user&#39;s ear  12 . In addition, the signal processing circuitry  24  generates a noise cancellation signal that is also applied to the loudspeaker  28 . 
         [0020]    One aim of the signal processing circuitry  24  is to generate a noise cancellation signal, which, when applied to the loudspeaker  28 , causes it to generate a sound signal in the ear  12  of the user that is the inverse of the ambient noise signal reaching the ear  12 . 
         [0021]    In order to achieve this, the signal processing circuitry  24  needs to generate from the ambient noise signals generated by the microphones  20 ,  22  a noise cancellation signal that takes into account the properties of the microphones  20 ,  22  and of the loudspeaker  28 , and also takes into account the modification of the ambient noise that occurs due to the presence of the earphone  10 . 
         [0022]    According to the present invention, the signal processor  24  includes means for measuring the level of ambient noise and for controlling the addition of the noise cancellation signal to the source signal based on the level of ambient noise. For example, in environments where ambient noise is low or negligible, noise cancellation may not improve the sound quality heard by the user. That is, the noise cancellation may even add artefacts to the sound stream to correct for ambient noise that is not present. Further, the activity of the noise cancellation system during such periods consumes power that is wasted. Therefore, when the noise signal is low, the noise cancellation signal may be reduced, or even turned off altogether. This saves power and prevents the noise signal from adding unwanted noise to the voice signal. 
         [0023]    However, when the noise cancellation system is present in a mobile phone or headset, for example, the ambient noise may be detected in isolation from the user&#39;s own voice. That is, a user may be speaking on a mobile phone or headset in an otherwise empty room, but the noise cancellation system may still not detect that noise is low due to the user&#39;s voice. 
         [0024]      FIG. 2  shows in more detail one embodiment of the signal processing circuitry  24 . An input  40  is connected to receive a noise signal, for example directly from the microphones  20 ,  22 , representative of the ambient noise. The noise signal is input to an analogue-to-digital converter (ADC)  42 , and is converted to a digital noise signal. The digital noise signal is input to a noise cancellation block  44 , which outputs a noise cancellation signal. The noise cancellation block  44  may for example comprise a filter for generating a noise cancellation signal from a detected ambient noise signal, i.e. the noise cancellation block  44  substantially generates the inverse signal of the detected ambient noise. The filter may be adaptive or non-adaptive, as will be apparent to those skilled in the art. 
         [0025]    The noise cancellation signal is output to a variable gain block  46 . The control of the variable gain block  46  will be explained later. Conventionally, a gain block may apply gain to the noise cancellation signal in order to generate a noise cancellation signal that more accurately cancels the detected ambient noise. Thus, the noise cancellation block  44  will typically comprise a gain block (not shown) designed to operate in this manner. However, according to one embodiment of the present invention the applied gain is varied according to the detected amplitude, or envelope, of ambient noise. The variable gain block  46  may therefore be in addition to a conventional gain block present in the noise cancellation block  44 , or may represent the gain block in the noise cancellation block  44  itself, adapted to implement the present invention. 
         [0026]    The signal processor  24  further comprises an input  48  for receiving a voice or other wanted signal, as described above. Thus, in the case of a mobile phone, the wanted signal is the signal that has been transmitted to the phone, and is to be converted to an audible sound by means of the speaker  28 . In general, the wanted signal will be digital (e.g. music, a received voice, etc), in which case the wanted signal is added to the noise cancellation signal output from the variable gain block  46  in an adding element  52 . However, in the case that the wanted signal is analogue, the wanted signal is input to an ADC (not shown), where it is converted to a digital signal, and then added in the adding element  52 . The combined signal is then output from the signal processor  24  to the loudspeaker  28 . 
         [0027]    Further, according to the present invention, the digital noise signal is input to an envelope detector  54 , which detects the envelope of the ambient noise and outputs a control signal to the variable gain block  46 .  FIG. 3  shows one embodiment, where the envelope detector  54  compares the envelope of the noise signal to a threshold value N 1 , and outputs the control signal based on the comparison. For example, if the envelope of the noise signal is below the threshold value N 1 , the envelope detector  54  may output a control signal such that zero gain is applied, effectively turning off the noise cancellation function of the system  10 . Similarly, the envelope detector  54  may output a control signal to actually turn off the noise cancellation function of the system  10 . In the illustrated embodiment, if the envelope of the noise signal is below the first threshold value N 1 , the envelope detector  54  outputs a control signal such that the gain is gradually reduced with decreasing noise such that, when a second, lower, threshold value N 2  is reached, zero gain is applied. In between the threshold values N 1  and N 2 , the gain is varied linearly; however, a person skilled in the art will appreciate that the gain may be varied in a stepwise manner, or exponentially, for example. 
         [0028]      FIG. 4  shows a schematic graph of a further embodiment, in which the envelope detector  54  employs a first threshold value N 1  and a second threshold value N 2  in such a way that a hysteresis is built into the system. The solid line of the graph represents the applied gain when the system is transitioning from a “full” noise cancellation signal to a zero noise cancellation signal; and the chain line represents the applied gain when the system is transitioning from a zero noise cancellation signal to a full noise cancellation signal. In the illustrated embodiment, when the system is initially generating a full noise cancellation signal, but the ambient noise then falls below the first threshold N 1 , the applied gain is reduced until zero gain is applied at a value N 1 ′ of ambient noise. When the system is initially switched off, or generating a “zero” noise cancellation signal, and the envelope of the ambient noise rises above the second threshold value N 2 , the applied gain is increased until a full noise cancellation signal is generated at a value N 2 ′ of ambient noise. The second threshold value may be set higher than the value N 1 ′, at which value the noise cancellation was previously switched off, such that a hysteresis is built into the system. The hysteresis prevents rapid fluctuations between noise cancellation “on” and “off” states when the envelope of the noise signal is close to the first threshold value. 
         [0029]    A person skilled in the art will appreciate that rather than gradually reducing or increasing the applied gain, the noise cancellation may be switched off or on when the ambient noise crosses the first and second thresholds, respectively. However, in this embodiment the envelope detector  54  of the signal processor  24  may comprise a ramping filter to smooth transitions between different levels of gain. Harsh transitions may sound strange to the user, and by choosing an appropriate time constant for the ramping filter, they can be avoided. 
         [0030]    Although in the description above an envelope detector is used to determine the level of ambient noise, alternatively the amplitude of the noise signal may be used instead. The term “noise level”, also used in the description, may apply to the amplitude or envelope, or some other magnitude of the noise signal. 
         [0031]    Of course, there are many possible alternative methods, not explicitly mentioned here, of altering the addition of the noise cancellation signal to the wanted signal in accordance with the detected ambient noise that would be apparent to those skilled in the art. The present invention is not limited to any one of the described methods, except as defined in the claims appended hereto. 
         [0032]    According to a further embodiment of the invention, the digital noise signal output from the ADC  42  is input to the envelope detector  52  via a gate  56 . The gate  56  is controlled by a voice activity detector (VAD)  58 , which also receives the digital noise signal output from the ADC  42 . The VAD  58  then operates the gate  56  such that the noise signal is allowed through to the envelope detector  52  only during voiceless periods. The operation of the gate  56  and the VAD  58  will be described in greater detail below. The VAD  58  and gate  56  are especially beneficial when the noise cancellation system  10  is realized in a mobile phone, or a headset, i.e. any system where the user is liable to be speaking whilst using the system. 
         [0033]    The use of a voice activity detector is advantageous because the system includes one or more microphones  20 ,  22  which detect ambient noise, but which are also close enough to detect the user&#39;s own speech. When it is determined that the gain of the noise cancellation system should be controlled on the basis of the ambient noise, it is advantageous to be able to detect the ambient noise level during periods when the user is not speaking. 
         [0034]    In the illustrated embodiment of the invention, the ambient noise level is taken to be the noise level during the quietest period within a longer period. Thus, in one embodiment, where the signal from the microphones  20 ,  22  is converted to a digital signal at a sample rate of 8 kHz, the digital samples are divided into frames, each comprising 256 samples, and the average signal magnitude is determined for each frame. Then, the ambient noise level at any time is determined to be the frame, from amongst the most recent 32 frames, having the lowest average signal magnitude. 
         [0035]    Thus, it is assumed that, in each period of 32×256 samples (=approximately 1 second), there will be one frame where the user will not be making any sound, and the detected signal level during this frame will accurately represent the ambient noise. 
         [0036]    The gain applied to the noise cancellation signal is then controlled based on ambient noise level determined in this manner. Of course, however, many methods are known for detecting voice activity, and the invention is not limited to any particular method, except as defined in the claims as appended hereto. 
         [0037]    Various modifications may be made to the embodiments described above without departing from the scope of the claims appended hereto. For example, a digital noise signal may be input directly to the signal processor  28 , and in this case the signal processor  28  would not comprise ADC  42 . Further, the VAD  58  may receive an analogue version of the noise signal, rather than the digital signal. 
         [0038]    The present invention may be employed in feedforward noise cancellation systems, as described above, or in so-called feedback noise cancellation systems. The general principle of adapting the addition of the noise cancellation signal to the wanted signal in accordance with the detected ambient noise level is applicable to both systems. 
         [0039]    Noise cancellation systems according to the present invention may be employed in many devices, as would be appreciated by those skilled in the art. For example, they may be employed in mobile phones, headphones, earphones, headsets, etc. 
         [0040]    The skilled person will recognise that the above-described apparatus and methods may be embodied as processor control code, for example on a 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/digital hardware. 
         [0041]    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 processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.