Patent Publication Number: US-6707912-B2

Title: Method and apparatus for setting a step size for an adaptive filter coefficient of an echo canceller

Description:
FIELD OF THE INVENTION 
     The present invention pertains to echo cancellers, and more particularly to an adaptive step-size thereof. 
     BACKGROUND OF THE INVENTION 
     Operation of bi-directional hands-free communication devices, such as two-way radios, speaker phones which are commonly referred to as hands-free telephones, teleconferencing devices and car-kits for cellular telephones, requires management of those signals emitted by an audio speaker that are coupled to a microphone. Such devices are utilized in systems having a communication channel, such as a cable, twisted-wire, optical fiber, a frequency bandwidth for signals transmitted by air, or the like, which connects a local, or “near-end,” device to a remote, or “far-end,” device. Signals output by the speaker of a device that are detected by the microphone of the device are commonly called echoes. Echoes which occur at one end of the communication link are generally a nuisance to users at the other end. In severe cases, echoes can result in a phenomena known as “howling” which is very unpleasant at both ends of a communication link. 
     Echo cancellers have been developed to cancel echo signals. Echo cancellers employ a filter to estimate the echo signal in a communication device. The echo canceller subtracts the echo estimate from the signal output by the microphone to produce an echo suppressed signal. 
     Although echo cancellers work well in some environments, the effective cancellation of echo signals in a hands-free vehicle environment is particularly challenging. Least means squares (LMS) error minimization recursive filters are often used for echo control. Although these filters are very stable, nonlinear system effects, as well as limitations of algorithmic and arithmetic precision, limit the effectiveness of echo cancellers incorporating such filters. Post processing stages are thus employed to suppress residual echoes. Post processing can include attenuation of the output signal using a variable gain control or a filter, or other known post processing techniques. 
     Unfortunately, post processing can result in significant degradation and attenuation of desired transmission signals that are present when both users are speaking simultaneously (double talk condition). For example, post processing attenuation can result in the echo canceled taking on half-duplex characteristics, such that only one user can speak at a time. Additionally, post processing typically introduces perceptible changes, or attenuation, of the background noise, which is present in noisy environments such as vehicle interiors. When this noise variation correlates with speech activity in the signal received at the far end, it is objectionable to far-end users. 
     A variety of methods have been developed to improve echo canceller performance. Such methods include storing initial tap values for the echo canceller. Storing such initial tap values may speed up echo canceller operation upon initialization. However, if the present initial conditions are dramatically different from the previous initial conditions, starting from the previous conditions will not be beneficial. 
     Another known method of improving echo canceller operation employs an adaptive step size, such that the error signal gain fed back to the adaptive filter is varied as a function of the magnitude of the error signal. If the error signal is large, the step size will be large. If the error signal is small, the step size is small. Although echo cancellers operating in this manner provide improved performance by adapting more quickly, the echo canceller adaptation must still occur without having initial state unique to the environment of the echo canceller. 
     Accordingly, there is a need or an improved echo canceller to provide more rapid convergence based upon the environment of the echo canceller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit schematic in block diagram form illustrating a communication device including an echo canceller. 
     FIG. 2 is a circuit schematic in block diagram form illustrating an echo canceller. 
     FIG. 3 is a flow chart illustrating operation of the echo canceller. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The step size for tap positions of an adaptive filter in an echo canceller for canceling an echo signal are individually selected based upon a profile of the environment. A training signal is injected into the echo canceller and to the echo path during a training interval, or period. The adaptive filter profiles the echo path during the injection of the training signal. Respective step sizes for the coefficients are generated as a function of the echo profile so generated. The step sizes for individual taps are thus generated as a function of the coefficients, such that tap positions which are expected to have large magnitudes will have a large step size permitting rapid adaptation whereas tap positions which are expected to have small coefficients will have small step sizes permitting accurate adaptation. 
     A communication device  100  is shown in FIG.  1 . The communication device  100  can be a two-way radio, a speaker phone which are commonly referred to as hands-free telephones, a teleconferencing device, a portable radiotelephone, a satellite telephone, a car-kit for a cellular telephone, or the like. 
     The communication device includes a loudspeaker  102  and a microphone  104 . The loudspeaker  102  is connected to the receive path of device  100 , which receive path includes a digital-to-analog (D/A) converter  106  connected to a switch  108 . The switch  108  selectively connects the D/A converter  106  to receive signals from either a noise source  112  or the receiver (not shown) of a transceiver  114 . The transceiver  114  communicates signals received from a remote communication device via antenna  116  for output to the speaker  102 . The noise source  112  generates digitized training signals, and may for example generate a random signal, white noise or any other suitable training signal. 
     The microphone  104  is coupled to the transmit path of the communication device  100 . The communication device  100  transmit path includes an analog-to-digital (A/D) converter  120 , which outputs digitized representations of the analog signal from microphone  104 . The signal output from the A/D converter  120  is combined with the output of an echo canceller  125  adaptive filter  124  in subtractor  126 . The subtractor subtracts the echo estimate output of the adaptive filter  124  from the output of the A/D converter  120  to generate a substantially echo-free signal at the output of the subtractor. 
     The subtractor is connected to a switch  127  controlled by a controller  128 . The controller  128  connects the output of the subtractor to the transceiver at all times except when the noise source generator  112  is connected to the D/A converter  106  during a training mode. This permits the adaptive filter to be trained without feeding noise to the transceiver  114 , which communicates with a far-end, or remote, party. 
     The controller  128  is also connected to receive coefficients from the adaptive filter  124  and provide gain control signals to a gain control  130 . The gain control  130  adjusts the adaptation step size individually for each tap, or coefficient, of the echo canceller as described hereinbelow. 
     Training may be initiated upon installation of the device  100 , by the user actuating a manual switch  140  (FIG.  2 ), or automatically on a periodic basis. It is envisioned that the user could initiate training by actuating switch  140  if they move the location of the microphone  104  and/or speaker  102  in a vehicle installation. Alternatively, if the hands-free unit is in a portable telephone the user could initialize the device each time the phone is cradled in a vehicle, or the switch  140  could be actuated only the first time that a phone is installed in a permanent location. Alternatively, the controller  128  can automatically enter training mode, for example on a periodic basis. 
     With reference to FIG. 2, during the training mode, switch  108  is connected to a noise signal generator  112 , which generates a signal used to train the echo canceller, such as white noise as mentioned above. The second switch  127  is open. The standard least mean square (LMS) algorithm used for updating coefficients of an adaptive filter is given in equation 1, wherein h n [k] represents the coefficient at the kth tap position of the adaptive filter at sample time n (h[1] for the first tap position, h[2] for the second tap position, and h[M] for the Mth tap position where the adaptive filter has M taps at the output of shift register  132 ), fixed parameter α is the step size constant (much less than 1), x n  is the received signal, e is the error signal output from the subtractor  126 , and c is a constant having a value significantly smaller than the expected value of |x n |: 
     
       
           h   n   [k]=h   n−{overscore (1)}   [k ]+(α/(|| x   n   ||+c )) e   n   x   n-k .  (1) 
       
     
     During the training mode, the same predetermined step size α is used for all of the coefficients. 
     In the echo cancelling mode, switch  108  is connected to the receiver of transceiver  114  and switch  127  is connected to output a signal to the transmitter of transceiver  114 . In the echo profiling system, the step size constant α[k] is a function of the tap position as determined in the training mode and shown in the following equation: 
     
       
           h   n   [k]=h   n−1   [k ]{overscore (+)}(α k   [k ]/(|| x   n   ||+c )) e   n   x   n-k .  (2) 
       
     
     The step size for a particular tap position is selected as a function of the value of the coefficients near that tap position generated during training. Thus, the step size for the nth tap will be the derived from a set of coefficients centered about the nth tap at the end of the training interval. By using the coefficients derived during the training mode, taps near large coefficients will have a large step size and taps not near large coefficients will have a small step size. This increases the efficiency of the echo canceller and improves the accuracy of the echo estimate using information derived from the environment in which the echo canceller is employed. 
     As mentioned briefly above, the step size α[k] is generated independently for each tap of the adaptive filters using the following method. First, during a training period, a training signal is injected as indicated in block  302  of FIG. 3. A standard least means square (LMS) algorithm is used in filter  124  during a training period in which the coefficients h[n] of filter  124  are adapted using a single common step size α., as indicated in block  304 . Adaptation will continue until the echo residual signal drops below a predetermined threshold level as indicated in step  306 . Alternatively, the adaptation can take place for a predetermined time period, such as an expected training period. The value of the coefficients h[n] at the end of the training period are input to the controller  128 . These coefficients are stored, as indicated in step  308 , and the stored impulse response estimates are referred to as h′[n]. 
     The impulse response h′[n] is used to generate the echo profile vector as indicated in step  310 . Although the respective step sizes for the tap positions can be set based on many estimation techniques, the step sizes are preferably generated using a sum of coefficients, and most preferably from a sum of adjacent coefficients. By using a sum of adjacent coefficients, the step size is generated based on an average level rather than a single step size. 
     A particularly advantageous method of generating the respective step sizes for the tap positions of the echo canceller is described herein below. First a sum is generated from adjacent coefficients, wherein α′[k] is the:                        α   ′          [   k   ]       =       ∑     j   =     K   -   3         j   =     K   +   3              |       h   ′          [   j   ]       |                   h   ′          [   j   ]       =       0                 n     &gt;     M                 or                 n     &lt;   0.                   (   3   )                         
     Coefficient values at the ends of the adaptive filter are thus given a value of zero for purposes of calculating the coefficient step sizes whereas coefficients within the adaptive filter have the values stored at the conclusion of training. Each summation is then divided by the maximum summation max(α′(k), such that: 
     
       
         α[ k ]=(2 α′[k ])/(max(α′( k )).  (4) 
       
     
     By dividing the summation by the maximum summation, the step sizes are scaled to a maximum value of 2 to provide stability. 
     In the echo canceling mode, the finite impulse response (FIR) filter  124  coefficients h 1  to h N  are adapted using the coefficients generated during training, and thus are related to the profile information from the training mode. In this manner, the adaptation of the echo canceller coefficients is expedited using information unique to the environment where the echo canceller is located. Those coefficients in a range of coefficients producing a summation with a large value during profiling will produce a proportionally large step size. Those coefficients in a range of coefficients producing a summation with a proportionally small value, will have a proportionally small step size. 
     Echo profiling of the acoustic environment of echo canceller  124  during a training mode is thus used to initialize step size in gain control  130  on a tap-by-tap basis. This system operates in two modes, a training mode and a canceling mode. The training mode may take only a few seconds during installation of device  100  in an environment, such as a vehicle or a room, or upon user actuation of switch  140 . Other than during such brief training mode, the canceling mode is used. 
     A hands-free telephone can comprise a microphone, a speaker and the echo canceller. The echo canceller attenuates echoes of signals output by the speaker and detected by the microphone. It includes adaptive coefficients having respective step sizes, each step size being a function of an associated coefficient value of the echo canceller generated during training. The hands-free telephone can advantageously include at least one switch for enabling training of the echo canceller to generate the associated coefficients representative of an environment in which the hands-free telephone will operate. 
     Thus it can be seen that an improved system is disclosed that provides better performance than prior systems, while maintaining the low complexity and stability of the LMS adaptive filter. When employed in a hands free telephone, and particularly a portable telephone such as a wireless hands-free telephone, an effective and efficient method of adaptation is provided for the environment where the telephone is used.