Abstract:
A method of controlling a noise cancellation system, for use in an audio device, comprises: generating an ambient noise signal representative of ambient noise; filtering and applying gain to the ambient noise signal to generate a noise cancellation signal; passing the noise cancellation signal to a speaker; and generating an error signal from an error microphone, wherein the gain applied to the ambient noise signal is controlled based on the error signal, and the method further comprises: determining from the error signal whether the audio device is in an off-ear position, and controlling the noise cancellation system based on said determination as to whether the audio device is in the off-ear position.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/406,850, filed on Oct. 26, 2010. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a noise cancellation system, and in particular to a noise cancellation system for inclusion in a sound reproducing device, and to a method of operation of such a noise cancellation system that is able to detect when the sound reproducing device is in a primary operating position. 
     2. Description of the Related Art 
     Noise cancellation systems are known, in which ambient noise is detected by means of one or more microphone, and the resulting ambient noise signal is applied to signal processing circuitry to generate a corresponding ambient noise cancellation signal. The ambient noise cancellation signal is then applied to a speaker, which is typically also being used to play wanted sounds to the user. Systems of this type are typically used in sound reproducing devices that are intended to be used close to the ear of the user, such as headphones, earphones or handsets, and the wanted sounds might be music, or speech, for example. 
     Effective noise cancellation is achieved when the signal processing circuitry generates an ambient noise cancellation signal that, when played through the speaker, generates a sound that is equal in magnitude but opposite in phase to the ambient sounds, as they reach the ear of the user. Thus, the signal processing circuitry performs a signal processing operation that must take account amongst other things of the difference between the ambient noise that is detected by the noise microphone, or microphones, and the ambient noise that reaches the ear of the user. In the case of headphones or earphones, this difference might be relatively constant, because the headphones or earphones are usually worn in a fixed position. However, in the case of a handset, this difference can vary quite substantially, because the user can hold the handset against his head in different ways. 
     Although noise cancellation can be effective when the desired signal processing is provided, ineffective noise cancellation can appear as an additional noise source, and can therefore be distracting to the user of the device. In addition, noise cancellation requires a power source, such as a battery, and generating ineffective noise cancellation signals is wasteful of the battery. 
     GB-2441835A discloses a noise cancellation system, in which an error microphone is positioned in the sound reproducing device, in order to detect the sounds that reach the ear of the user. The signals from the error microphone are then used to adapt the signal processing circuitry, which can then be used to generate effective noise cancellation. However, when the sound reproducing device is positioned away, or completely removed, from the user&#39;s ear, the noise cancellation system is unable to provide effective noise cancellation. 
     It is known to provide a proximity detector in a mobile phone handset to detect when the handset is against the users&#39; head. For example, the proximity detector can be based on an infrared source and detector. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of controlling a noise cancellation system, for use in an audio device, the method comprising:
         generating an ambient noise signal representative of ambient noise;   filtering and applying gain to the ambient noise signal to generate a noise cancellation signal;   passing the noise cancellation signal to a speaker; and   generating an error signal from an error microphone,   wherein the gain applied to the ambient noise signal is controlled based on the error signal,   and the method further comprising:   determining from the error signal whether the audio device is in an off-ear position, and   controlling the noise cancellation system based on said determination as to whether the audio device is in the off-ear position.       

     According to a second aspect of the present invention, there is provided a noise cancellation system, for use with an audio device, the noise cancellation system comprising:
         a first input for receiving an ambient noise signal representative of ambient noise;   a filter circuit for filtering and applying gain to the ambient noise signal to generate a noise cancellation signal;   an output for the noise cancellation signal;   a second input for receiving an error signal from an error microphone; and   a controller for controlling the gain applied to the ambient noise signal based on the error signal,   and the controller being further adapted to:   determine from the error signal whether the audio device is in an off-ear position, and   control the noise cancellation system based on said determination as to whether the audio device is in the off-ear position.       

     According to a third aspect of the present invention, there is provided an audio system, comprising:
         an audio device, comprising a first microphone, for generating an ambient noise signal representative of ambient noise; a speaker; an error microphone located close to the speaker; and   a noise cancellation system according to the second aspect of the invention, wherein the first microphone is connected to the first input of the noise cancellation system, the error microphone is connected to the second input of the noise cancellation system, and the speaker is connected to the output of the noise cancellation system.       

     This may have the advantages that it can be determined without using additional devices that the audio device is in an off-ear position, and/or that the noise cancellation system can be controlled in order to avoid at least some unwanted effects when the device is placed back on or about the user&#39;s ear. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of a first sound reproduction device in accordance with an aspect of the present invention. 
         FIG. 2  shows the sound reproduction device of  FIG. 1  in use. 
         FIG. 3  is a schematic diagram of a noise cancellation system in the sound reproduction device of  FIG. 1 , in accordance with an aspect of the present invention. 
         FIG. 4  is a flow chart, illustrating the method of setting a gain value, in the noise cancellation system of  FIG. 3 . 
         FIG. 5  is a flow chart, illustrating in more detail a part of the method illustrated in  FIG. 4 . 
         FIG. 6  illustrates a possible result of the method of  FIGS. 4 and 5 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows the general form of a sound reproduction device  10 , having a wanted signal source  12 . The sound reproduction device  10  may for example be a mobile phone handset, in which case the wanted signal source  12  may be the circuitry that generates the signal representing the voice that has been transmitted to the mobile phone. As another example, the sound reproduction device  10  may be an earphone, in which case the wanted signal source  12  may be an input for receiving a signal from a playback device, or the like. 
     The wanted signal is applied through a first input of an adder  14  to a speaker driver amplifier  16 , and to a speaker  18 . 
     The sound reproduction device  10  also includes a first microphone  20 , for detecting ambient noise in the vicinity of the sound reproduction device. The sound reproduction device  10  also includes a second microphone  22 . As described in more detail below, the second microphone  22  detects sounds in the vicinity of the speaker  18 . 
     Signals from the first microphone  20  and the second microphone  22 , and wanted signals from the wanted signal source  12 , are applied to noise cancellation circuitry  24 . The noise cancellation circuitry  24  generates a noise cancellation signal, which is applied to a second input of the adder  14 , so that it is added to the wanted signal as the latter is applied to the speaker driver amplifier  16 , and to the speaker  18 . 
     If the signal processing performed in the noise cancellation circuitry  24  can be controlled appropriately, then the effect of applying the noise cancellation signal to the speaker  18  is to generate a sound that will cancel out the ambient noise to at least some extent, thereby making the wanted sounds more clearly audible. 
       FIG. 2  shows the sound reproduction device  10  in use. Specifically, in this illustrated example, the sound reproduction device  10  takes the form of a mobile phone handset, which is shown positioned against the outer ear  30  on the head  32  of the user. As is conventional, the handset includes a speaker  18  on its front surface, towards the upper end thereof, above a display  34  and a keypad  36 . 
     In this illustrated example, there are two first microphones  20 , positioned on opposite sides of the upper edge of the handset, such that they can detect ambient noise that will be heard by the user. 
     The second microphone  22  is positioned close to, for example in front of, the speaker  18 . 
       FIG. 3  shows in more detail the form of the noise cancellation circuitry  24 . In this case, the noise cancellation takes the form of adaptive feedforward noise cancellation. That is, the signal representing the detected ambient noise is filtered, and the resulting noise cancellation signal is applied to the speaker. At the same time, an error signal is generated by a microphone close to the speaker, and the error signal is used to adapt the form of the filtering that is applied. 
     Thus, the noise cancellation circuitry  24  shown in  FIG. 3  includes first inputs  40 ,  42 , for receiving signals from the two first microphones  20 , and an adder  44  for forming a combined signal that represents the ambient noise. The noise cancellation circuitry  24  also includes a second input  46 , for receiving signals from the second microphone  22 , and a third input  48 , for receiving signals from the wanted signal source. 
     The ambient noise signal output by the adder  44  is applied to an adaptive filter  50  to generate an ambient noise cancellation signal, which is supplied to an output terminal  52  for eventual application to the speaker  18  as described above. 
     As is well known, effective noise cancellation requires that the filter characteristics of the filter  50  should be well matched to the other characteristics (for example, acoustical, mechanical and hardware characteristics) of the system. Thus, the filter  50  can have a frequency response characteristic that compensates for any frequency dependent variations in the responses of the noise microphones  20  or the loudspeaker  18 . Also, the filter  50  can have a frequency response characteristic that compensates for any frequency dependent variations in the ambient noise that reaches the user&#39;s ear around the handset as it is held close to the user&#39;s head. These characteristics of the filter  50  can be preset, based on knowledge of the handset in which the noise cancellation system  24  is to be used. 
     In addition, in this illustrated example, the filter characteristics of the filter  50  are adapted in use, based on the signals received from the second microphone  22 , which represent the error, i.e. the extent to which the signals reaching the ear of the user contain uncompensated ambient sounds, or contain sounds generated by the noise cancellation system that overcompensate for the actual ambient noise. In certain embodiments, the frequency dependent aspects of the filter characteristic can be adapted. 
     In this illustrated example, the gain of the adaptive filter  50  is adapted based on the signals received from the second microphone  22 , as described in more detail below. The signals from the second microphone  22  are received on the second input  46 , and passed to a first input of a subtractor  56 . The wanted input signals applied to the third input  48  are passed through an adaptive compensation filter  58  before being passed to a second input of the subtractor  56 , so that the effect of the wanted signal is removed from the detected error signal. 
     Specifically, the noise cancellation system  24  includes a controller  54 , for example in the form a digital signal processor, and  FIG. 4  is a flow chart, illustrating the process performed in the controller  54 . Although the discussion herein refers to the gain of the adaptive filter  50  being adapted, the gain of the adaptive compensation filter  58  must be adapted in a similar way, and the output from the controller  54  also controls this gain value. 
     As discussed above, the error microphone  22  captures the residual of the ambient noise and the anti-noise signal, produced by the noise cancellation system  24 , that is played out of the speaker  18 . If no anti-noise signal is generated, or the anti-noise signal is less than the ambient noise signal, then the residual signal, or error signal, will be in phase with the ambient noise detected by the microphone(s)  20 . If the anti-noise signal is higher than the ambient noise signal entering into the ear, then the residual signal, or error signal, will be out of phase with the ambient noise signal. Therefore, in this example, an algorithm that monitors the phase relation of the signals from the ambient noise microphone and the error microphone is used to adjust the gain of the noise cancellation system to converge to its optimum gain value. 
     Referring to  FIG. 4 , in step  60 , the process takes the signal received from the ambient noise microphones  20 , and in step  62  it takes the signal received from the error microphone  22 . 
     In each case, these signals are passed to a Fast Fourier Transform (FFT) block, in which data is sampled at 7.8 kHz and buffered to a block size of 128 samples. The process then examines the phases of the signal components at a number of frequencies in the frequency range over which noise cancellation is intended to be performed. In this example, the processor examines the phases in a Cartesian co-ordinate space of components of the signals received from the ambient noise microphones and the error microphone at 500 Hz, 560 Hz, 620 Hz, 680 Hz, 740 Hz and 800 Hz. 
     In step  64 , the phase difference between the signals received from the ambient noise microphones and the error microphone is calculated at each of these frequencies. Each of these calculated phase differences is then compared with 90°, with a decision of +1 being returned when the phase difference is less than 90°, and a decision of −1 being returned when the phase difference is greater than 90°. Then, the sum of these decisions is calculated. That is, if the calculated phase difference at all six frequencies is less than 90°, a sum of +6 is formed, while if the calculated phase difference at all six frequencies is greater than 90°, a sum of −6 is formed, and intermediate sums can be formed if the six phase differences at the six frequencies do not all give the same result when compared with 90°. 
     If it is found that the sum of these decisions is greater than a predetermined threshold, typically zero, a positive value is output. If it is found that the sum of these decisions is less than the predetermined threshold, a negative value is output. If the sum of these decisions is equal to the threshold, a zero is output. 
     These output values are generated once per frame, or block of 128 samples. 
     If the output value is negative, it is determined that the anti-noise is higher in magnitude than the ambient noise entering the ear, and so a gain decrement value is output. If the output value is positive, it is determined that the anti-noise is less in magnitude than the ambient noise entering the ear, and so a gain increment value is output. 
     For example, a gain decrement value of fixed size is output whenever the average decision indicates that the phase difference is greater than 90°, and a gain increment value of the same fixed size is output whenever the average decision indicates that the phase difference is less than 90°. 
     As discussed above, the noise cancellation system  24  can cope well with the situation where the handset  10  is on, or positioned about, the user&#39;s ear, but is moved a small distance such that the amount of ambient noise reaching the user&#39;s ear changes slightly. 
     However, problems can arise when the handset  10  is removed from, or positioned away from, the user&#39;s ear. Specifically, when the handset is off the ear and exposed to the free air, the error signal will be overwhelmed by the ambient noise signal, and the small size of the speaker  18  will mean that it is unable to produce enough energy to cancel the ambient noise. In that case, step  64  will continue to output gain increment values, even when the gain value has reached its maximum value. 
     Thus, in the process of  FIG. 4 , it is determined in step  66  whether an off-ear condition is detected. If it is determined that the handset is on, or about, the user&#39;s ear, the process passes to step  70 , in which it is determined whether the gain value supplied from the controller to the adaptive filter  50  to be applied to the detected ambient noise signal, is at the relevant limiting value. Thus, if step  64  has output a gain increment value, it is determined in step  70  whether the current gain value is at a preset upper limiting gain value, and if step  64  has output a gain decrement value, it is determined in step  70  whether the current gain value is at a preset lower limiting gain value. 
     If it is determined in step  70  that the current gain value is at the relevant limiting gain value, then in step  72  the existing gain value continues to be output, but if it is determined in step  70  that the current gain value is not at the relevant limiting gain value, then in step  74  a new gain value is formed by applying the calculated increment or decrement to the existing gain value and the new gain value is output. 
     If it is determined in step  66  that an off-ear condition has been detected, the process passes to step  76 , in which it is determined whether the gain value supplied from the controller to the adaptive filter  50  to be applied to the detected ambient noise signal, is at a predetermined off-ear value, which lies between the upper and lower limiting gain values mentioned above. 
     If it is determined in step  76  that the current gain value is at the predetermined off-ear gain value, then in step  80  the existing gain value continues to be output, but if it is determined in step  76  that the current gain value is not at the predetermined off-ear gain value, then in step  78  a new gain value is formed by applying a negative step value to the existing gain value and the new gain value is output. 
       FIG. 5  is a further flow chart, showing in more detail how the process determines in step  66  whether an off-ear condition is to be detected, and  FIG. 6  shows the time history of various values in an illustrative example of the operation of the system. 
     In step  90  of the process shown in  FIG. 5 , it is determined whether the current gain value output by the controller  54  is at the predetermined upper limiting value. 
     In the time period Ta-Tb shown in  FIG. 6 , the handset  10  is being held against the user&#39;s ear. During this operation, the position of the handset might be changing slightly, and so the process of  FIG. 4  will typically be calculating an appropriate stream of increment and decrement values, as described with reference to step  64  in  FIG. 4 , with the result that the calculated gain value, indicated by the line  120  in  FIG. 6 , stays close to the middle of the range between the upper and lower limit values. Typically, the gain value  120  should be expected to fluctuate around the nominal value, which causes the adaptive filter  50  to apply a gain in the middle of its range of possible values. 
     As the gain value is not equal to the upper limiting value, the answer to the question in step  90  is negative, and the process passes to step  92 , with the result that the process of  FIG. 4  passes from step  66  to step  70 . 
     At time Tb, the handset  10  is moved away from the user&#39;s ear, and the result is that the calculation in step  64  starts to output a substantially continuous stream of increment values. Thus, during the time period Tb-Tc shown in  FIG. 6  the gain value  120  is continually increasing, until it reaches the upper limiting value at the time Tc. As the gain value is not equal to the upper limiting value in the time period Tb-Tc, the answer to the question in step  90  remains negative, and the process passes to step  92 , with the result that the process of  FIG. 4  passes from step  66  to step  70 . 
     When the gain value  120  reaches the upper limiting value at the time Tc, the answer to the question in step  90  becomes positive, and the process passes to step  94 . In step  94 , the gain increment or decrement values most recently calculated in step  64  are examined. 
     In step  96 , it is determined whether a threshold condition is met. For example, the gain increment or decrement values calculated over a sliding window, which may for example again be of length 10 frames, are examined. The threshold condition might then for example be satisfied if some threshold number of increment values, such as 8, 9 or 10, is found to have been generated during that 10 frame window. 
     If the threshold condition described in step  96  has not been met, as during the period Tc-Td in  FIG. 6 , it is determined that the off-ear condition has not yet been met, and the process passes to step  92 , with the result that the process of  FIG. 4  passes from step  66  to step  70 . 
     At time Td in  FIG. 6 , it is determined that the off-ear condition has been met, and the process passes to step  98 , with the result that the process of  FIG. 4  passes from step  66  to step  76 . 
     In step  76 , it is determined whether the current gain value is equal to the predetermined off-ear value. Initially, it will be equal to the upper limiting value, which is higher than the off-ear value, and so the process will pass to step  78 , and the negative step will be applied to the current gain value. 
     When the negative step has been applied to the current gain value often enough for the current gain value to become equal to the predetermined off-ear value, at time Te, the determination at step  76  becomes positive, and thereafter the process of  FIG. 4  passes to step  80 , in which an unchanged gain value is output. Thus, while the handset  10  is held away from the user&#39;s ear for a protracted period, the gain is ramped to the off-ear value. This prevents the situation where, if the gain value were instead maintained at the upper limiting value, the user would notice an unnecessarily (and perhaps unacceptably) large amount of anti-noise energy when placing the handset back on his ear. 
     This gain value unchanged at the off-ear value persists until time Tf, at which it is determined that the off-ear condition no longer applies, and thereafter the process of  FIG. 4  passes from step  66  to step  70 , as described previously. For example, when it is found that the most recently calculated increment or decrement values are not all, or are not predominantly, increment values, as discussed above with reference to steps  66  and  96 , it can be determined that the off-ear condition no longer applies. Thereafter, the process of  FIG. 4  can pass from step  66  to step  70 , rather than to step  76 . 
     Thus, there is provided a mechanism for ensuring that the gain value applied to the adaptive filter  50  takes account of the situation when the device is held off the ear of the user. 
     Although the invention has been described here with reference to its use in a handset, the same principle can be used in other devices that include noise cancellation, such as headphones, earphones or the like. Where the method described herein is used in headphones or earphones having a pair of speakers, the method can be applied separately to the signals applied to those two speakers. Thus, when a pair of earbuds includes an ambient noise microphone and an error microphone on each earbud, it is possible to determine separately from the signals received from these microphones whether each earbud is in an off-ear condition. 
     When it is determined that one earbud of a pair of earbuds is in an off-ear condition, the gain applied to the noise cancellation signal in that one earbud can be controlled appropriately, as described above. Alternatively, when it is determined that one earbud of a pair of earbuds is in an off-ear condition, it is possible to stop sending the wanted signal and the noise cancellation signal to that one earbud to save power. Alternatively, or additionally, the wanted signals and the noise cancellation signals to a pair of earbuds can be switched off completely when it is determined that both earbuds of a pair are in an off-ear condition.