Patent Publication Number: US-10325613-B1

Title: Acoustic delay estimation

Description:
TECHNICAL FIELD 
     The present invention relates to the field of acoustics, and in particular to a method and apparatus for determining the inherent acoustic delay or audio latency in an audio system. 
     BACKGROUND 
     It is often desirable to measure the time delay of a signal through an audio system. For example, it may be necessary to measure the difference in time delay as an audio signal passes through each of a plurality of speakers compared to a reference audio signal when trying to ensure synchronization between the speakers. Traditionally, time delay measurement techniques in an audio system involve using a known impulse signal. One method involves performing a cross-correlation between a transmitted impulse signal and the recorded audio signal. This method involves a training period while the adaptive algorithm adapts to the audio characteristics of the room, and requires calibration tones or known reference tones. Other methods include use of time-domain reflectometry where a pulse or a short sine wave burst is transmitted from the audio system. Measurements are then made of the timing of the return echo. These methods are susceptible to ambient noise and multi-modal reverberation and/or echo in a room. As a result, the recorded audio signal or return echo signal is not an exact replica of the original transmitted signal. 
     Also, adaptive filters are used for echo cancellation. In certain applications, such as in the case of TV set top boxes, the echo is delayed by an amount of time that exceeds the capacity of the adaptive filter. Increasing the filter size is not practical for digital signal processing reasons. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a principal object of the present invention to overcome at least some of the disadvantages of prior art acoustic signal delay measurement techniques. This is provided in one embodiment by an acoustic signal delay measurement apparatus comprising: an acoustic signal input terminal; an acoustic signal output terminal; at least one echo input terminal; an adjustable tapped delay line exhibiting a plurality of taps, a first end of the tapped delay line coupled to the acoustic signal input terminal, each of the taps exhibiting a respective predetermined delay; a processor, an output of the processor coupled to a control input of the adjustable tapped delay line; and a plurality of adaptive filters, a first input of each of the plurality of adaptive filters coupled to a respective one of the at least one echo input terminal, a second input of each of the plurality of adaptive filters coupled to a respective one of the plurality of taps and an output of each of the plurality of adaptive filters coupled to a respective input of the processor, wherein the processor is arranged to determine a system delay responsive to: the amount of time it takes for one of the plurality of adaptive filters to converge; and the delay of the tap associated with the converged adaptive filter. 
     Additional features and advantages of the invention will become apparent from the following drawings and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding sections or elements throughout. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings: 
         FIG. 1A  illustrates a high level schematic diagram of a first embodiment of an acoustic signal delay measurement apparatus; 
         FIG. 1B  illustrates a high level flow chart of a method of operation of the apparatus of  FIG. 1A ; 
         FIGS. 1C-1F  illustrate various high level graphs showing an example of the apparatus of  FIG. 1A ; 
         FIG. 2  illustrates a high level schematic diagram of a second embodiment of an acoustic signal delay measurement apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
       FIG. 1A  illustrates a high level schematic diagram of an acoustic signal delay measurement apparatus  10 . Acoustic signal delay measurement apparatus  10  comprises: an acoustic signal input terminal  20 ; an acoustic signal output terminal  30 ; a plurality of echo input terminals  40 ; an adjustable tapped delay line  50  comprising a plurality of taps; a plurality of adaptive filters  70 ; a processor  80 ; and a system module  90 . Each adaptive filter  70  comprises: a digital filter  100 ; and an adder  110 . In one embodiment, digital filter  100  comprises a least mean squares (LMS) filter. Acoustic signal delay measurement apparatus  10  is illustrated in an embodiment where the number of echo input terminals  40  is the same as the number of adaptive filters  70 , however this is not meant to be limiting in any way. In another embodiment, the number of echo input terminals  40  is less than the number of adaptive filters  70 . System module  90  comprises various circuitry and software functionalities and represents the system delay. 
     Acoustic signal input terminal  20  is coupled to an input of system module  90  and a first end of adjustable tapped delay line  50 . Each tap of adjustable tapped delay line  50  is coupled to a first input of a respective adaptive filter  70 , the first input of each adaptive filter  70  representing a first input of the respective digital filter  100 . A second input of each adaptive filter  70  is coupled to a respective echo input terminal  40 , the second input of each adaptive filter  70  representing a first input of the respective adder  110 . In the embodiment, as illustrated in relation to acoustic signal delay measurement apparatus  200  of  FIG. 2 , where the number of echo input terminals  40  is less than the number of adaptive filters  70 , a plurality of adaptive filters  70  are coupled to a single echo input terminal  40 , as will be described further below. Each echo input terminal  40  is arranged to receive a digitized acoustic signal received at a respective microphone  120 . An output of each digital filter  100  is coupled to a second input of the respective adder  110 . An output of each adder  110  is coupled to a control input of the respective digital filter  100  and to a respective input of processor  80 . An output of processor  80  is coupled to a control input of adjustable tapped delay line  50 . An output of system module  90  is coupled to acoustic signal output terminal  30  and acoustic signal output terminal  30  is further coupled to a speaker  130 . 
     The operation of acoustic signal delay measurement apparatus  10  is described in relation to the high level flow chart of  FIG. 1B . In stage  1000 , a digitized acoustic signal is received at acoustic signal input terminal  20 . The received signal enters both system module  90  and adjustable tapped delay line  50 . In stage  1010 , processor  80  sets the delay at each tap of adjustable tapped delay line  50 , i.e. the delay of the received signal experienced by each adaptive filter  70 . In one embodiment, the delays are initially set to be integer multiples of a predetermined value. In one further embodiment, the delays between adjacent taps are generally equal. In another further embodiment, the delays are given as:
 
 T   N   =N*k*M   EQ. 1
 
where N is an integer number, T N  is the delay at tap N, k is a predetermined number below 1, and M is the time it takes to apply all the taps of a digital filter  100  to a received signal. Preferably, k is between 0.5-0.75.
 
     In stage  1020 , the coefficients of digital filter  100  of each adaptive filter  70  are set to initial predetermined values. In one embodiment, all the coefficients are set to zero. In another embodiment, the coefficients are set to previously stored values from a previous delay estimation. 
     In stage  1030 , each adaptive filter  70  begins processing the received signals. Particularly, the acoustic signal received at acoustic signal input terminal  20 , delayed by the delay of system module  90 , is output by speaker  130  and the echo thereof is picked up by microphones  120 . The echo is then sampled by an analog to digital (A/D) converter and the digitized signal is received at the first input of adder  110  of the respective adaptive filter  70 . Additionally, the acoustic signal received at acoustic signal input terminal  20  is applied, before the delay of system module  90 , to adjustable tapped delay line  50 . Thus, digital filter  100  of each adaptive filter  70  receives the original acoustic signal after a respective predetermined delay, as described above. The difference between the output of each digital filter  100  and the received echo signal is output by the respective adder  110  to the control input of the respective digital filter  100 . The coefficients of digital filter  100  are then adjusted until adaptive filter  70  converges, i.e. the difference at the output of adder  110  is below a predetermined threshold, as known to those skilled in the art at the time of the invention. Processor  80  analyzes the outputs of adders  110  to determine for each adaptive filter  70  whether it has converged or not. 
     In stage  1040 , once at least one adaptive filter  70  converges, processor  80  controls each adaptive filter  70  to cease adaptation, i.e. to stop adjusting the coefficients of the respective digital filter  100 . In stage  1050 , processor  80  reads the filter coefficients of the converged adaptive filter  70 , or plurality of adaptive filters if they converged at the same time. 
     In stage  1060 , processor  80  determines the delay within system module  90  responsive to the filter coefficients of the converged adaptive filter  70 . Particularly, processor  80  determines at which coefficient the respective digital filter  100  peaked, i.e. which coefficient has the highest value. In one embodiment, the filter coefficients are first smoothed by a predefined smoothing filter to remove any spikes in the coefficient values in order to correctly identify the point at which the filter peaked, as known to those skilled in the art at the time of the invention. The time interval from when the first coefficient of digital filter  100  is applied until the peak coefficient is applied is denoted TF. The acoustic signal delay is thus calculated as:
 
 T   D   =TF+T   N   EQ. 2
 
where T N  is the delay of the acoustic signal within adjustable tapped delay line  50  at the tap coupled to the converged adaptive filter  70 .
 
       FIGS. 1C-1F  illustrate an example where 3 adaptive filters  70  are provided. Digital filter  100  of each adaptive filter  70  comprises 2048 taps and k of EQ. 1 is equal to 0.5. The delay of system module  90  is 193 ms. Particularly,  FIG. 1C  illustrates a high level graph of the coefficients of digital filter  100  of the first adaptive filter  70 , where no delay is provided from adjustable tapped delay line  50 .  FIG. 1D  illustrates a high level graph of the coefficients of digital filter  100  of the second adaptive filter  70 , where a delay of 1024 sampling times is provided by adjustable tapped delay line  50 .  FIG. 1E  illustrates a high level graph of the coefficients of digital filter  100  of the third adaptive filter  70 , where a delay of 2048 sampling times is provided by adjustable tapped delay line  50 .  FIG. 1F  illustrates a high level graph comparing the operation times of each adaptive filter  70  with the operation of a theoretical 4096 tap filter for the 193 ms delayed acoustic signal. As illustrated, the third adaptive filter  70  converges quickly due to the short length of the filter and provides better convergence depth than the theoretical 4096 tap filter since there are fewer taps associated with the precursor, i.e. the period before the main impulse in the filter coefficients. Thus, an improved result is received while using a smaller filter. 
     In stage  1070 , the filter coefficients of converged adaptive filter  70  are copied into the other adaptive filters  70 . Additionally, the delay of each tap of adjustable tapped delay line  50  is set to the calculated delay of stage  1060 , i.e. the delays for all of the adaptive filters  70  are now substantially identical. Thus, the delay of system module  90  is compensated for and adaptive filters  70  will cancel acoustic echo received at microphones  120  even with an internal delay greater than the size of adaptive filters  70 . 
     Advantageously, the above described system and method allows for shorter adaptive filter lengths while still allowing for estimation and compensation for system delays longer than the length of the individual filters. After compensation, each adaptive filter will converge more quickly than if reset following system delay estimation. The above described method allows for an overall improvement in acoustic echo cancelling depth of convergence, while also allowing for greater tolerance to delay variation of acoustic echo path change before complete retraining, including overall system delay estimation, is required. 
     Additionally, the above described method can support multiple input channels, e.g. stereo, and multiple acoustic signal output terminals  30  since the delay is determined in relation to an internal tapped delay line. 
       FIG. 2  illustrates a high level schematic diagram of an acoustic signal delay measurement apparatus  200 . Acoustic signal delay measurement apparatus  200  is in all respects similar to acoustic signal delay measurement apparatus  10 , with the exception that a least one of the echo input terminals  40  is coupled to a plurality of adaptive filters  70 , i.e. a plurality of adaptive filters  70  is provided for at least one of the plurality of microphones  120 . Alternatively (not shown), only a single microphone  120  is provided. The operation of acoustic signal delay measurement apparatus  200  is in all respects similar to acoustic signal delay measurement apparatus  10  and in the interest of brevity will not be repeated. Although several adaptive filters  70  receive the same echo input from the respective microphone  120 , the delay applied by tapped delay line  50  for each adaptive filter  70  is different. Therefore, different delay possibilities can be checked using only a single echo input. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. In particular, the invention has been described with an identification of each powered device by a class, however this is not meant to be limiting in any way. In an alternative embodiment, all powered device are treated equally, and thus the identification of class with its associated power requirements is not required. 
     Unless otherwise defined, all technical and scientific terms used herein have the same meanings as are commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods are described herein. 
     All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the patent specification, including definitions, will prevail. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the appended claims and includes both combinations and subcombinations of the various features described hereinabove as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description.