Abstract:
A system includes a loudspeaker, a microphone, an echo canceling circuit, and an accelerometer coupled to the loudspeaker for providing a reference signal, a(n), to the echo canceling circuit. Preferably, the accelerometer is attached to the loudspeaker. A speaker signal, x(n), is used to drive the loudspeaker and the system also includes a switch for coupling either a(n) or x(n) to the echo canceling circuit, depending upon the volume or loudness of the sounds involved.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to acoustic echo cancellation and, in particular, to using an accelerometer to remove non-linearities caused by acoustic transducers. 
         [0002]    The sound from a loudspeaker can be reflected or coupled back to a microphone after some finite delay, producing an echo. In its simplest form, the echo (sound) corresponds to the electrical signal in the apparatus producing the sound and the system is considered linear. Unfortunately, this is an ideal situation. Transducers, such as microphones and loudspeakers, are nonlinear, even when operated in their optimum range. This invention is described in the context of the acoustic echo in telephones but is more broadly useful. 
         [0003]    Nonlinear effects significantly reduce the performance of acoustic echo cancelers, particularly in compact, hands-free kits for cellphones. Many of the problems associated with hands-free kits have been attributed to inexpensive, low-quality loudspeakers that are poorly isolated from the enclosure of a kit. When such a loudspeaker is overdriven, saturation effects associated with the loudspeaker and its amplifier distort sound in a nonlinear manner. An acoustic echo of such sound contains a mixture of linear and nonlinear components. A typical acoustic echo canceler estimates only the linear acoustic impulse response of the loudspeaker-enclosure-microphone system. The remaining nonlinear components can be large and audible, particularly at high volume. 
         [0004]    An overdriven amplifier causes nonlinear distortion by creating harmonics and inter-modulation distortion from the clipping of large amplitude signals; see U.S. Pat. No. 4,809,336 (Pritchard). Enclosure vibration due to mechanical coupling between a loudspeaker and an enclosure, especially at lower voice frequencies, causes significant nonlinear distortion that is picked up by the microphone. The loudspeaker itself is a major source of nonlinear distortion. The nonlinearities can be acoustic, electromagnetic, or mechanical, such as distortion of the cone or diaphragm or the voice coil hitting an end of travel. 
         [0005]    One can place a microphone next to a loudspeaker. This method is not very useful because new sounds and background noise are also picked up by the microphone. An optical sensor, such as a laser and photodetector, can be used to measure the displacement of the cone of a loudspeaker. This method requires sophisticated hardware when a much less expensive solution is desired. 
         [0006]    Several other approaches have been used to cancel the nonlinear echo. One approach is nonlinear preprocessing of the signal to the loudspeaker. This method attempts to compensate for known nonlinear characteristics. Predistortion can degrade the quality of speech or loudness from a loudspeaker. Another approach is to use a nonlinear adaptive filter. Most or all known techniques have high computational cost and converge slowly, which makes them unsuitable for most real-time applications. Existing techniques for controlling non-linear echo use mathematical models of the nonlinearity. Echo canceler performance depends on the accuracy of the model. An accurate model can be very difficult to obtain because the non-linearities may change with time and may not be the same for two specimens of the same product. 
         [0007]    Bulk delay is the time it takes an echo of the digital signal to a loudspeaker to be detected at a microphone. Synchronizing the microphone analog-to-digital conversion and the loudspeaker digital-to-analog conversion is challenging in real-time environments, especially if a non-real-time operating system is used. 
         [0008]    It is known in the art to minimize the mechanical vibration picked up by a microphone using an accelerometer; U.S. Published Application Nos. 2006/0155346 (Miller), 2007/0167671 (Miller); and 2008/0132750 (Miller). U.S. Pat. No. 5,117,401 (Feintuch) discloses coupling a microphone or an accelerometer to a noise source for noise cancellation. U.S. Pat. No. 6,470,275 (Dubinsky) discloses using an accelerometer for noise cancellation in a device for drilling bore holes. U.S. Published Application No. 2005/0201549 (Dedieu et al.) discloses detecting the tilt angle and high frequency vibration of a loudspeaker using an accelerometer. 
         [0009]    In view of the foregoing, it is therefore an object of the invention to generate a reference signal for echo cancellation from an accelerometer. 
         [0010]    Another object of the invention is to provide an echo canceler that uses one adaptive filter for both the linear part of an echo and the nonlinear part of the echo. 
         [0011]    A further object of the invention is to provide an echo canceler that measures, rather than estimates, the linear and nonlinear components of the response of a loudspeaker. 
         [0012]    Another object of the invention is to provide an echo canceler that has a reference signal that is substantially insensitive to near-side speech and noise. 
         [0013]    A further object of the invention is to provide an echo canceler that tracks time-varying changes in the response of a loudspeaker. 
         [0014]    Another object of the invention is to provide an echo canceler that tolerates distortion by a loudspeaker, the amplifier driving the loudspeaker, or the enclosure for the loudspeaker. 
         [0015]    A further object of the invention is to provide an echo canceler that measures all nonlinearities in the signal to a loudspeaker, including amplifier distortion, loudspeaker distortion, and case rattle. 
         [0016]    Another object of the invention is to provide an echo canceler that uses the signal from an accelerometer rather than the signal to a loudspeaker as the reference signal for echo cancellation. 
         [0017]    A further object of the invention is to provide an echo canceler that uses linear adaptive filtering to solve a nonlinear control problem, thereby reducing the number of processing cycles compared to using a nonlinear adaptive filter. 
         [0018]    Another object of the invention is to provide an echo canceler that significantly reduces problems with bulk delay by generating a reference signal using an accelerometer coupled to a loudspeaker. 
       SUMMARY OF THE INVENTION 
       [0019]    The foregoing objects are achieved in this invention in which a system includes a loudspeaker, a microphone, an echo canceling circuit, and an accelerometer coupled to the loudspeaker for providing a reference signal, a(n), to the echo canceling circuit. Preferably, the accelerometer is attached to the loudspeaker. A speaker signal, x(n), is used to drive the loudspeaker and the system also includes a switch for coupling either a(n) or x(n) to the echo canceling circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0021]      FIG. 1  is a perspective view of a speakerphone; 
           [0022]      FIG. 2  is a perspective view of a hands-free kit; 
           [0023]      FIG. 3  is a perspective view of a cellphone; 
           [0024]      FIG. 4  is a block diagram of a cellphone; 
           [0025]      FIG. 5  is a block diagram of an audio processing circuit; 
           [0026]      FIG. 6  is a block diagram of an echo canceler constructed in accordance with a preferred embodiment of the invention; 
           [0027]      FIG. 7  is a series of charts illustrating the operation of the invention; and 
           [0028]      FIG. 8  is a chart illustrating echo return loss enhancement. 
       
    
    
       [0029]    Those of skill in the art recognize that, once an analog signal is converted to digital form, all subsequent operations can take place in one or more suitably programmed microprocessors. Reference to “signal,” for example, does not necessarily mean a hardware implementation or an analog signal. Data in memory, even a single bit, can be a signal. In other words, a block diagram can be interpreted as hardware, software, e.g. a flow chart or an algorithm, or a mixture of hardware and software. Programming a microprocessor is well within the ability of those of ordinary skill in the art, either individually or in groups. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0030]      FIG. 1  illustrates a conference phone or speaker phone such as found in business offices. Telephone  10  includes microphone  11  and loudspeaker  12  in a sculptured case.  FIG. 2  illustrates what is known as a hands-free kit for providing audio coupling to a cellphone, illustrated in  FIG. 3 . Hands-free kits come in a variety of implementations but generally include powered loudspeaker  16  attached to plug  17 , which fits an accessory outlet or a cigarette lighter socket in a vehicle. A hands-free kit also includes cable  18  terminating in plug  19 . Plug  19  fits the headset socket on a cellphone, such as socket  21  ( FIG. 3 ) in cellphone  22 . Some kits use RF signals, like a cordless phone, to couple to a telephone. A hands-free kit also typically includes a volume control and some control switches, e.g. for going “off hook” to answer a call. A hands-free kit also typically includes a visor microphone (not shown) that plugs into the kit. Audio processing circuitry constructed in accordance with the invention can be included in a hands-free kit or in a cellphone. 
         [0031]      FIG. 4  is a block diagram of the major components of a cellphone. Typically, the blocks correspond to integrated circuits implementing the indicated function. Microphone  31 , loudspeaker  32 , and keypad  33  are coupled to signal processing circuit  34 . 
         [0032]    A cellphone includes both audio frequency and radio frequency circuits. Duplexer  35  couples antenna  36  to receive processor  37 . Duplexer  35  couples antenna  36  to power amplifier  38  and isolates receive processor  37  from the power amplifier during transmission. Transmit processor  39  modulates a radio frequency signal with an audio signal from circuit  34 . In non-cellular applications, such as speakerphones, there are no radio frequency circuits and signal processor  34  may be simplified somewhat. Echo cancellation takes place in audio processor  40 . It is audio processor  40  that is modified to include the invention. 
         [0033]      FIG. 5  is a more detailed block diagram of audio processor  40 . The following describes signal flow through the transmit channel, from microphone input  42  to line out  44 . The receive channel, from line in  46  to loudspeaker output  48 , operates similarly. 
         [0034]    The signal from input  42  is digitized in A/D converter  51  and coupled to summation circuit  52 . There is, as yet, no signal from echo canceling circuit  53  and the data proceeds intact to non-linear processor  54 . The output from non-linear processor  54  is coupled to D/A converter  57 , amplified in amplifier  58 , and coupled to line output  44 . Circuit  53  reduces acoustic echo by providing a modified signal from the transmit channel to summation circuit  52 . 
         [0035]      FIG. 6  illustrates a preferred embodiment of the invention. In  FIG. 6 , microphone  61  is coupled through amplifier  62  to analog to digital converter  64 . The output signal from converter  64  is coupled through summation circuit  65  to output  66 . The signal on output  66  is ultimately coupled to a “line out” terminal. Similarly, a signal on input  71 , from a “line in” terminal, is processed and coupled to digital to analog converter  72 . The signal in analog form is coupled through amplifier  73  to loudspeaker  74 . As indicated by dashed lines  77  and  78 , some of the sound from loudspeaker  77  is reflected back to microphone  61  as an echo. 
         [0036]    A digital representation, y(n), of the signal from microphone  61  is coupled to a first input of summation circuit  65 . The signal, y(n), contains two components, original sound and echo. The echo has portions that are linear and nonlinear. Original sound can include, for example, near-end speech and background noise. “Near-end” refers to one end of a two channel communication link between two parties to a telephone call. “Far end” refers to conditions on the telephone lines, including “line out” and “line in,” and signals from the telephone of the other party. 
         [0037]    For echo cancellation, an estimate, d(n), of the echo is coupled to a second input of summation circuit  65 . If the estimate is accurate, the echo component is canceled and signal e(n) represents only original sound. As described thus far, the circuit is a substantially conventional, near-end, acoustic echo canceler as described in  FIG. 5  when switch  85  is connected to input  71 . 
         [0038]    In accordance with one aspect of the invention, the vibrations of loudspeaker  74  are converted into an electrical signal by accelerometer  81 , such as a piezoelectric crystal. This signal is amplified in amplifier  86  and digitized in converter  87 , and used as reference signal a(n) for adaptive filter  83 . Adaptive filter  83  is preferably what is known in the art as a finite impulse response (FIR) filter. 
         [0039]    Signal a(n) is a reference signal to the adaptive filter  83  when switch  85  is in the position shown and includes any nonlinearities generated by loudspeaker  74  or amplifier  73 . Adaptive filter  83  estimates the combined linear response of loudspeaker-enclosure-microphone system. The adaptive filter is required to model only the linear acoustic impulse response because the adaptive filter reference signal is the nonlinearly distorted signal. In a conventional acoustic echo canceler, an adaptive filter has to model both the nonlinear response and the linear response of the system and, typically, does not model the nonlinear responses. 
         [0040]    The performance of echo canceler  60  has been measured in terms of echo return loss enhancement (ERLE), defined as 
         [0000]    
       
         
           
             
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         [0000]    where E is the statistical expectation. Experimental data was collected in a relatively quiet car. An eleven second speech signal was fed into a 50 mm×32 mm loudspeaker having a capacity of two watts. The echo signal was sensed by a 6 mm omnidirectional microphone. The speech file used consisted of 200 ms white noise followed by an artificial male voice at the far end (ITU-T Recommendation P.50). Sets of data were obtained at different sound pressure levels. The signals were recorded at 48 kHz sampling frequency with a 16-bit A/D converter and subsequently down-sampled to 16 kHz for analysis. A piezoelectric accelerometer weighing eight grams with dynamic range of ±150 g was mounted on the magnet of the loudspeaker to detect vibration as the loudspeaker reproduced the speech file. In one embodiment of the invention, the adaptive filter was implemented using a normalized least mean squares (NLMS) algorithm with 512 taps. The step size, μ, was set to 0.5 and regularization factor r was equal to 10 −6 . 
         [0041]      FIG. 7  includes three charts representative of the data collected. Chart  91  represents signal y(n). Chart  92  represents the output from summation circuit  65  when switch  85  is in the lower position; i.e. when x(n) is used for the estimate, as in the prior art. Chart  93  represents signal e(n) when a(n) provides the estimate. 
         [0042]      FIG. 8  is a chart of ERLE versus total non-coherent distortion. Curve  95  represents a(n) and remains almost flat as total non-coherent distortion increases. Curve  96  decreases very sharply with increasing total non-coherent distortion when the accelerometer is not used. At maximum distortion, representing a 40 dB increase in volume, using an accelerometer improves ERLE more than 15 dB. Although additional hardware is used to accommodate the accelerometer, the overall computational complexity is the same as for an acoustic echo canceler of the prior art that also uses an adaptive filter. 
         [0043]    An accelerometer is insensitive to near-end speech and background noise. If a second microphone had been used instead of an accelerometer for detecting nonlinear distortion, near-end speech and noise would inevitably be included in reference signal a(n) and the acoustic echo canceler would attempt to cancel it. An accelerometer can be 50 dB less sensitive to near-end speech and noise than a microphone, yet just as sensitive to echo. 
         [0044]    The invention thus provides an acoustic echo canceler using an accelerometer to generate a reference signal. An acoustic echo canceler constructed in accordance with the invention requires only one adaptive filter rather than a first filter for the linear part of the echo and a second filter for the nonlinear part of the echo. The canceler measures, rather than estimates, linear modifications to x(n), thereby reducing the estimation demand on the adaptive filter in an echo canceling circuit. The canceler uses linear adaptive filtering to solve a nonlinear control problem and does not require as many processing cycles as a nonlinear adaptive filter would require. 
         [0045]    An acoustic echo canceler constructed in accordance with the invention is largely insensitive to near side speech and noise but is sensitive to time-varying nonlinearities of the loudspeaker and significantly reduces problems with bulk delay. An acoustic echo canceler constructed in accordance with the invention also enables less expensive loudspeakers to be used, loudspeaker volumes to be increased, and smaller loudspeakers to be used. 
         [0046]    An acoustic echo canceler constructed in accordance with the invention measures all nonlinearities emanating from the loudspeaker, including amplifier distortion, loudspeaker distortion, and case rattle. The canceler does not predistort speaker signal, x(n), to minimize nonlinearities. 
         [0047]    Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, one can use speaker signal, x(n), at low volumes, when the signal from the accelerometer may be too small, and the accelerometer signal, a(n), at high volumes. This allows good performance at any volume. Near-end speech and background noise cause slight mechanical vibration of the loudspeaker and its enclosure. At normal to high volume, this vibration is insignificant. At low volume, the vibration could become a noticeable fraction of the signal from accelerometer  81 . When to switch is determined empirically and depends upon many factors, such as the loudspeaker, the enclosure, and the particular environment in which the system is located at the time. The accelerometer can be mounted on the enclosure of a hands-free system; for example, instead of on the loudspeaker itself. Whether circuit  65  is called a summation circuit, a subtraction circuit, or “combining means” does not matter. In an electronic computer, signals are subtracted using two&#39;s complement addition.