(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2008/0144848 A1
`(43) Pub. Date:
`Jun. 19, 2008
`Buck et al.
`
`US 2008O144848A1
`
`Publication Classification
`
`(51) Int. Cl.
`(2006.01)
`H04B 3/20
`(52) U.S. Cl. .......................................................... 381A66
`(57)
`ABSTRACT
`An echo reduction system includes a signal analysis filter that
`converts an input into Sub-band signals. A signal down-sam
`pling circuit down-samples the Sub-band signals at a first
`down-sampling rate. An echo analysis filter converts a loud
`speaker signal into echo Sub-band signal that are further pro
`cessed by an echo down-sampling circuit. The circuit down
`samples the echo Sub-band signals at a second down
`sampling rate to generate down-sampled echo Sub-band
`signals. An echo compensation filter folds the down-sampled
`echo Sub-band signals with an estimated impulse response of
`a loudspeaker-room-input system. A second echo down-sam
`pling circuit down-samples the folded down-sampled echo
`Sub-band signals at a third down-sampling rate to generate
`estimated echo Sub-band signals. The first down-sampling
`rate is equal to the product of the second and third down
`sampling rates.
`
`(54) LOW COMPLEXITY ECHO COMPENSATION
`SYSTEM
`
`(76) Inventors:
`
`Markus Buck, Biberach (DE); Tim
`Haulick, Blaubeuren (DE); Martin
`Rossler, Ulm (DE); Gerhard Uwe
`Schmidt, Ulm (DE); Walter
`Schnug, Memmingen (DE)
`
`Correspondence Address:
`BRINKSHOFER GILSON & LONE
`P.O. BOX 10395
`CHICAGO, IL 60610
`
`(21) Appl. No.:
`
`11/955,862
`
`(22) Filed:
`
`Dec. 13, 2007
`
`(30)
`
`Foreign Application Priority Data
`
`Dec. 18, 2006 (EP) .................................. O6O26232.6
`
`
`
`Communication System 160
`
`Near-End/LRM 124
`
`120
`
`Echo
`Compensation
`System
`104
`
`Page 1 of 15
`
`GOOGLE EXHIBIT 1012
`
`

`

`Patent Application Publication
`
`Jun. 19, 2008 Sheet 1 of 8
`
`US 2008/O144848A1
`
`
`
`Communication System 160
`
`Near-End/LRM 124
`
`Echo
`Compensation
`System
`
`104
`
`Figure 1
`
`Page 2 of 15
`
`

`

`Patent Application Publication
`
`Jun. 19, 2008 Sheet 2 of 8
`
`US 2008/0144848A1
`
`
`
`Z 3 InãIH
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Page 3 of 15
`
`

`

`Patent Application Publication
`
`Jun. 19, 2008 Sheet 3 of 8
`
`US 2008/O144848A1
`
`
`
`Signal Analysis Filter Bank 206
`
`Signal
`Down-Sampling
`Circuit
`310
`
`Signal
`Down-Sampling
`Circuit
`310
`
`Signal
`Down-Sampling
`Circuit
`310
`
`Signal
`Sub-Band
`Filter 304
`guana
`
`Signal
`Sub-Band
`Filter 304
`guana
`
`Signal
`Sub-Band
`Filter 304
`guana
`
`Signal
`Down-Sampling
`Circuit
`310
`rr2 r
`
`Signal
`Sub-Band
`Filter 304
`guana
`
`y(n)
`
`y(n)
`
`Figure 3
`
`Page 4 of 15
`
`

`

`Patent Application Publication
`
`Jun. 19, 2008 Sheet 4 of 8
`
`US 2008/0144848A1
`
`
`
`Echo Analysis Filter Bank
`
`Echo Sub-Band
`Filter 220
`gu.and
`
`Echo Sub-Band
`Filter 220
`gu.and
`
`Echo Sub-Band
`Filter 220
`gu,and
`
`Echo Sub-Band
`Filter 220
`gu,and
`
`1. Echo
`Down-Sampling
`Circuit 404
`
`1. Echo
`Down-Sampling
`Circuit 404
`
`1. Echo
`Down-Sampling
`Circuit 404
`
`l'Echo
`Down-Sampling
`Circuit 404
`
`Echo Compensation Filter 416
`2nd Echo Down-Sampling
`Circuit
`420
`
`Figure 4
`
`Page 5 of 15
`
`

`

`Patent Application Publication
`
`Jun. 19, 2008 Sheet 5 of 8
`
`US 2008/0144848A1
`
`
`
`Synthesis Filter Bank 240
`
`Synthesis
`Sub-Band
`Filter 520
`gusyn
`
`Synthesis
`Sub-Band
`Filter 520
`gusyn
`
`Synthesis
`Sub-Band
`Filter 520
`Susyn
`
`Synthesis
`Sub-Band
`Filter 520
`
`Up-Sampling
`Circuit 510
`
`r 1 oI2
`
`Up-Sampling
`Circuit 510
`flo2
`
`Up-Sampling
`Circuit 510
`f 12
`
`Up-Sampling
`Circuit 510
`
`Figure 5
`
`Page 6 of 15
`
`

`

`Patent Application Publication
`
`Jun. 19, 2008 Sheet 6 of 8
`
`US 2008/0144848A1
`
`600 N
`
`610
`
`Convert Microphone Signal to Down
`Sampled Microphone Sub-Band Signals
`
`620 Convert Reference Audio Signal to Down
`Sampled Reference Sub-Band Signals (1"
`Stage of Down-Sampling)
`
`630
`
`Store Down-Sampled Reference Sub-Bank
`Signals in Ring Buffer
`
`640
`
`Down-Sample Corresponding to Down
`Sampled Microphone Sub-Band Signals (2"
`Stage of Down-Sampling)
`
`650
`
`
`
`660
`
`670
`
`Estimate Sub-Band Echos
`
`Subtract Estimated Sub-Band Echo
`Signals From Microphone Sub-Band
`Signals to Generate Echo-Compensated
`Sub-Band Signals
`
`Synthesize Echo-Compensated
`Sub-Band Signal
`
`Figure 6
`
`Page 7 of 15
`
`

`

`Patent Application Publication
`
`Jun. 19, 2008 Sheet 7 of 8
`
`US 2008/0144848A1
`
`
`
`Near-End
`(Loudspeaker-Room
`Microphone)
`720
`
`Microphone
`
`Figure 7
`
`Page 8 of 15
`
`

`

`Patent Application Publication
`
`Jun. 19, 2008 Sheet 8 of 8
`
`US 2008/O144848A1
`
`
`
`8 9 InãIJ
`
`(u)"Na
`
`Z8
`
`Page 9 of 15
`
`

`

`US 2008/0144848 A1
`
`Jun. 19, 2008
`
`LOW COMPLEXITY ECHO COMPENSATION
`SYSTEM
`
`PRIORITY CLAIM
`0001. This application claims the benefit of priority from
`European Patent Application No. EP 0602 6232.6, filed Dec.
`18, 2006, which is incorporated by reference.
`
`BACKGROUND OF THE INVENTION
`0002 1. Technical Field
`0003. This disclosure relates to echo compensation. In
`particular, this disclosure relates to an echo compensation
`system that reduces or eliminates echoes in a communication
`system.
`0004 2. Related Art
`0005 Echo reduction or suppression may be used in com
`munication systems, such as hands-free sets and speech rec
`ognition systems. Communication systems may include an
`input device that detects a signal. Such as a speech signal. The
`input device may also detect undesirable signals, such as
`echoes.
`0006 Echoes may occur when sound is reflected from a
`Surface. Such signals may be detected and re-transmitted
`back to a source. Echoes may be annoying to the user and may
`result in a communication failure.
`
`SUMMARY
`0007. An echo reduction system includes a signal analysis
`filter that converts an input into Sub-band signals. A signal
`down-sampling circuit down-samples the Sub-band signals at
`a first down-sampling rate. An echo analysis filter converts a
`loudspeaker signal into echo Sub-band signal that are further
`processed by an echo down-sampling circuit. The circuit
`down-samples the echo Sub-band signals at a second down
`sampling rate to generate down-sampled echo Sub-band sig
`nals. An echo compensation filter folds the down-sampled
`echo Sub-band signals with an estimated impulse response of
`a loudspeaker-room-input system. A second echo down-Sam
`pling circuit down-samples the folded down-sampled echo
`Sub-band signals at a third down-sampling rate to generate
`estimated echo Sub-band signals. The first down-sampling
`rate is equal to the product of the second and third down
`sampling rates.
`0008. Other systems, methods, features, and advantages
`will be, or will become, apparent to one with skill in the art
`upon examination of the following figures and detailed
`description. It is intended that all Such additional systems,
`methods, features and advantages be included within this
`description, be within the scope of the invention, and be
`protected by the following claims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`0009. The system may be better understood with reference
`to the following drawings and description. The components in
`the figures are not necessarily to Scale, emphasis instead
`being placed upon illustrating the principles of the invention.
`Moreover, in the figures, like-referenced numerals designate
`corresponding parts throughout the different views.
`0010 FIG. 1 is an echo compensation system.
`0011
`FIG. 2 is an alternative echo compensation system.
`0012 FIG. 3 is a signal analysis filter bank.
`0013 FIG. 4 is an echo analysis filter bank.
`0014 FIG. 5 is synthesis filter bank.
`
`0015 FIG. 6 is a echo compensation process.
`(0016 FIG. 7 is an input array.
`0017 FIG. 8 is an echo compensation system and an input
`array.
`
`DETAILED DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`0018 FIG. 1 is an echo compensation system 104. A near
`end environment may include a near-end input 116, or micro
`phone, and a near-end output or loudspeaker 120, which may
`form a “loudspeaker-room-input' (LRM) system 124 or
`“loudspeaker-room-microphone” (LRM) system. The input
`116 may convert a speech signal “so(n) of a local or near-end
`speaker 130, and an echo signal x(n) originating from the
`loudspeaker 120, into analog or digital signals (e.g. micro
`phone signals).
`0019. A far-end environment 136 may include a far-end
`input 138 and a far-end output or loudspeaker 140. The far
`end input 138 may detect a speech signal of a far-end speaker
`150. Speech from the far-end speaker 150 may be reproduced
`by the near-end loudspeaker 120, and may be received by the
`near-end input 116 as reflected sound oran echo signal “x(n).”
`The echo compensation system 104 may reduce or eliminate
`the echo signal “x(n). The echo compensation system 104
`may be part of the near-end environment 124, and may be
`included in a communication system 160 associated with the
`near-end environment. A second echo compensation system
`may be part of the far-end environment 136, and may reduce
`or eliminate echoes in the far-end environment 136. The
`communication system 160 may be a hands-free telephone or
`head-set, wireless telephone, or other communication device.
`The echo compensation system 104 may be part of a speech
`recognition system.
`0020 FIG. 2 is an alternative echo compensation system
`104. In this system a loudspeaker 120 may generate a refer
`ence audio signal "X(n) modeled by the impulse response
`“h(n) 204 of the near-end or LRM environment 124 to gen
`erate an echo signal “d(n). The input 116 may convert a
`speech signal “s(n) that corresponds to speech 'so(n), into
`analog or digital signals. The echo signal “d(n) may be
`combined with the speech signal's(n) to generate the input
`or microphone signaly(n). The output signaly(n) may be
`transmitted to the signal analysis filter bank 206 of the echo
`compensation system 104.
`0021 FIG.3 is a signal analysis filter bank 206. The signal
`analysis filter bank 206 may include a plurality of signal
`sub-band filters 304 that may divide the output signal “y(n)”
`into a plurality of Sub-bands signals. The number of signal
`sub-band filters 304 may range from u-0 to M-1. The signal
`sub-band filters 304 may have filter coefficients according to
`Equation 1 below:
`(Eqn. 1)
`gu.N-1, anal
`g, analguoanaguiana
`where the upper index T may indicate a transposition opera
`tion, and N may denote the filter length.
`0022. A signal down-sampling circuit 310 may communi
`cate with each signal sub-band filter 304, and may down
`sample each of the Sub-band signals, respectively, by a first
`down-sampling factor of “r.” The first down-sampling factor
`r” may be equal or Substantially equal to the product of a
`second down-sampling factor “r” and a third down-sampling
`factor “r” according to Equation 2 below:
`
`Page 10 of 15
`
`

`

`US 2008/0144848 A1
`
`Jun. 19, 2008
`
`0023 The second and third down-sampling factors “r”
`and “r” may be integer values. The second down-sampling
`factor “r” may be equal to about 64, and the third down
`sampling factor “r” may be equal to about 2. Thus, the first
`down-sampling factor “r” may be equal to about 128, where
`the number of sub-bands, “M” may be equal to about 256.
`Other values of the second down-sampling factor “r” may be
`used, such as values between about 32 and about 512. Other
`values for the third down-sampling factor “r” may be used,
`such as integer values between about 2 and about 4. When
`using the higher values for the second and third down-Sam
`pling factors, the first down-sampling factor “r” may be equal
`to about 2048 (r-rr=4x512). The sampling rate of the input
`signal may be equal to about 11025 Hz. The signal down
`sampling circuits 310 may generate down-sampled Sub-band
`signals “y,(n).”
`0024. In some systems, only part of the microphone signal
`“y(n) may be divided into sub-bands by the signal analysis
`filter bank 206. For example, a predetermined frequency
`range of the output signal “y(n) may be divided into sub
`bands, while a remaining frequency band or frequency region
`may be analyzed without sub-band division.
`0025. The echo compensation system 104 of FIG. 2 may
`include an echo analysis filter 216 or filter bank. The echo
`analysis filter 216 may include a plurality of echo sub-band
`filters 220, which may divide the reference audio signal or
`echo signal “X(n)' into a plurality of echo Sub-bands signals.
`0026 FIG. 4 is the echoanalysis filter 216. The number of
`echo Sub-band filters 220 may range from L-0 to M-1. The
`echo sub-band filters 220 may have filter coefficients “g,
`which may be the same or similar to the filter coefficients (g,
`ana) of the signal Sub-band filters 304 of the signal analysis
`filter bank 206.
`0027. The echo analysis filter 216 may include a plurality
`of first echo down-sampling circuits 404. Each first echo
`down-sampling circuit 404 may communicate with respec
`tive echo sub-band filters 220, and may down-sample each of
`the echo Sub-band signals by the second down-sampling fac
`tor “r.” The sub-band signals down-sampled by the first echo
`down-sampling circuit 404 may be stored in a ring buffer 410
`or other memory storage.
`0028. An echo compensation filter 416 may receive the
`output of each of the first echo down-sampling circuits 404
`(or ring buffer output). The echo compensation filter 416 may
`have a filter response “h.(n)” 418. In some systems, the
`coefficients of the echo compensation filter 416 may be
`adapted to perform echo compensation after initial down
`sampling by the first echo down-sampling circuits 404 using
`the first down-sampling factor “r” where the value of “r
`may be equal to about 64.
`0029. Filter coefficients of the echo compensation filter
`416 may be adapted for each sub-band using a Normalized
`Least Mean Square process according to Equation 3 below:
`
`in + 1) = fin)+c(n)".
`|x(n)|
`
`(Eqn. 3)
`
`where the signal vector X(n)-X(n), X(n-1),..., X(n-N+1) T.
`where N is the length of the filter h(n), and denotes a norm,
`and
`where the error signal e(n)-y(n)-d(n)-y(n)-h'(n)x(n). The
`quantity c(n) may be the adaptation step size.
`
`0030 The echo compensation filter 416 may include a
`second echo down-sampling circuit 420, which may further
`down-sample the output of the first echo down-sampling cir
`cuits 404 (or ring buffer 410). In some systems, the coeffi
`cients of the echo compensation filter 416 may be adapted to
`perform echo compensation after down-sampling by the sec
`ond echo down-sampling circuits 420 using the third down
`sampling factor “r. To reduce computational loads, the filter
`coefficients may be adapted after the signals may be down
`sampled using the third down-sampling factor “r”.
`0031. The echo analysis filter bank 216 of FIG. 2 may
`output Sub-band signals (short-time spectra) in the frequency
`(S2) domain according to Equation 4 below:
`
`r1-1
`
`xen)-X-("i"). ('i)
`
`27
`
`27
`
`(Eqn. 4)
`
`where the echo compensation filter 416 may fold or combine
`the short-time spectra to generate echo compensated spectra
`according to Equation 5 below:
`
`f(e) X(ei)ii (e')
`r1-1
`
`(Eqn. 5)
`
`where H(e') may represent the frequency domain filter
`coefficients of the echo compensation filter 416.
`0032. The coefficients H(e') may represent temporally
`adapted estimates for the corresponding impulse response of
`the LRM environment H(e''), in accordance with the coef
`ficients of h(n) in the time domain.
`0033 Aliasing terms of the signal analysis filter bank 206
`for L=0 may be eliminated based on Equations 6 and 7 below:
`
`Goo?e
`
`x
`
`O
`
`(E-im)
`
`)=
`
`1, for n = 0
`0, for n e {1, ... , r1 - 1
`
`(Eqn. 6)
`
`27t
`1, if Os
`
`Edn. 7
`(Eqn. 7)
`
`27t
`Go (e') = arbitrary, if M <2< r
`27t
`0, if Oa -
`r
`
`0034. In Equations 6 and 7, the sub-bands “M” may have
`the same sub-band width. The signal sub-band filters 304
`G(e') for L-1. . . . . M-1, may be derived using Equa
`tions and 6 for the sub-band L-0 by a frequency shift opera
`tion.
`0035. The echo compensation filter 416 of FIG. 2 may
`generate estimated sub-band signals"d(n)” corresponding to
`the echo signal “d(n) captured by the input 116 in the LRM
`environment 124. A summing circuit 230 may subtract the
`estimated sub-band signals “d(n)" from the down-sampled
`sub-band signals "y, (n)" to generate sub-band error signals
`
`Page 11 of 15
`
`

`

`US 2008/0144848 A1
`
`Jun. 19, 2008
`
`10036) The estimated sub-band signals “d(n) may be gen
`erated based on the echo Sub-band signals previously stored
`in the ring buffer 410, which may represent echo sub-band
`signals sampled at the second down-sampling rater. How
`ever, such signals may be processed using the first down
`sampling rate “r” which may correspond to the down-Sam
`pling rate of the input Sub-band signals. Processing the echo
`signals using the second down-sampling rate “r” may result
`in low aliasing terms, while processing the microphone sig
`nals using the first, higher down-sampling rate “r” may
`reduce the computational load. Thus, adapting the filter coef
`ficients of the echo compensation filter 416, may be per
`formed at the first or highest down-sampling rate “r” which
`may be equal to about 128 when using 256 sub-bands. This
`may reduce the computational load and may reduce memory
`requirements. In some systems, the Sub-band error signals
`“e(n) may be filtered to reduce background noise that may
`be present in the microphone signal y(n).
`0037. The echo compensation system 104 of FIG. 2 may
`include a synthesis filter bank 240. FIG. 5 is the synthesis
`filter bank 240. The synthesis filter bank or synthesis filter
`240 may include a plurality of up-sampling circuits 510 and
`corresponding synthesis sub-band filters 520. Each up-sam
`pling circuit 510 may receive the corresponding sub-band
`error signals "e(n).” The up-sampling circuits 510 may up
`sample the Sub-band signals using the factor “r” as an up
`sampling factor. The up-sampling factor “r” may be equal to
`the first down-sampling factor “r” where r-(r)(r).
`0038. Each synthesis sub-band filter 520 may include
`high-pass filters, band-pass filters, and/or low-pass filters to
`reduce or eliminate artifacts. A summing circuit 530 may sum
`the output of each of the synthesis sub-band filters 520 to
`generate a synthesized speech signal “S(n). The synthesized
`speech signal may have reduced acoustic echo.
`0039. The analysis sub-band filters 304, the echo sub-band
`filters 220, and the synthesis sub-band filters 520 may include
`square root Hann window filters. Square-root Hann window
`filters may be efficient and robust in terms of stability. The
`filter length of the analysis sub-band filters 304, the echo
`sub-band filters 220, and/or the synthesis sub-band filters 520
`may be equal in length. The filter length may be about equal
`to the number of sub-bands into which the microphone signal
`and the reference audio signal are divided. Filter banks of
`“M” parallel filters may include a prototype low-pass filter
`ho(n)and modulated band-pass filtersh,(n)-ho(n) wa?" where
`we'. Such filters may be based on discrete Fourier
`transforms in the form of a polyphase process, which may
`provide a Substantially flat frequency response.
`0040. The analysis sub-band filters 304 and the echo sub
`band filters 220 may also be Hann window filters (rather than
`square-root Hann window filters), which may be raised to an
`exponent corresponding to a first rational number. The first
`rational number may be, for example, about 0.75. Other val
`ues for the first rational number may be used, such as values
`between about 0.50 to about 0.95. The synthesis sub-band
`filters 520 may also be Hann window filters (rather than
`square-root Hann window filters) that may be raised to an
`exponent corresponding to a second rational number. The
`second rational number may be, for example, about 0.25. Such
`that the sum of the first and second rational numbers may be
`about equal to 1. Other values for the second rational number
`may be used, such as values between about 0.05 to about 0.50.
`Because the signal analysis filter bank 206 may affect the
`quality of the enhanced microphone signal more than the
`
`synthesis filter bank 240, the first rational number may be
`larger than the second rational number.
`0041
`FIG. 6 is an echo compensation process. A micro
`phone signal may be divided into a plurality of Sub-band
`microphone signals and may be down-sampled by a prede
`termined first down-sampling factor “r” (Act 610). A refer
`ence audio signal or echo signal may be divided into a plu
`rality of Sub-band signals and may be down-sampled by a
`predetermined second down-sampling factor “r” (Act 620)
`to generate first down-sampled Sub-band echo signals. The
`down-sampled Sub-band echo signals may be stored in a ring
`buffer (Act 630). The second down-sampling rate “r” may be
`selected to Suppress aliasing. The reference audio signal may
`represent an audio signal received from a remote communi
`cation party, which may be output by a loudspeaker at the
`“near-end.”
`0042. The first down-sampled sub-band echo signals may
`be down-sampled a second time using a third down-sampling
`factor “r” (Act 640) so that the total amount of down-sam
`pling may be about equal to r(r)(r), which may be the same
`as the down-sampling factor applied to the microphone sig
`nal. Filter coefficients of the echo compensation filter 416
`may be adapted to generate estimated Sub-band signals (Act
`650). The estimated sub-band signals may correspond to the
`echo signal. Filter adaptation may be performed using the first
`down-sample factor of r-(r)(r). The estimated sub-band
`echoes may be subtracted from the input Sub-signals (Act
`660) to generate echo compensated microphone Sub-band
`signals. The echo compensated microphone sub-band signals
`may be synthesized and combined (Act 670) to generate an
`enhanced audio signal for transmission to a remote commu
`nication party.
`0043 FIG. 7 is an input array (e.g. a microphone) array
`710. The input array 710 may include a plurality of devices
`that convert Sound into analog signals or digital data, and may
`include directional microphones, which may represent “k”
`number of microphone “channels.” The input array 710 may
`be arranged in a "loudspeaker-room-microphone (LRM)
`environment 720, which may represent a near-end system.
`Each input 116 may generate an operational signal “s (n).”
`The loudspeaker 120 may generate the reference audio signal
`“x(n), which may be modeled by the impulse response
`“hon) 730 of a near-endor LRMenvironment 720 to generate
`(or cause) an echo signal "d(n). The echo signal “d(n) may
`be combined with the microphone signal “s, (n) in the LRM
`environment 720 to generate the output signal “y (n).”
`0044) The output signals “y (n) may be transmitted to an
`echo compensation system 804 of FIG.8. The echo compen
`sation system 804 may include the plurality of signal analysis
`filter banks 206, which may receive the corresponding micro
`phonesignaly(n). Each signal analysis filterbank 206 may
`be the same or similar to the signal analysis filter bank of
`FIGS. 2 and 3. Each output signal “y (n) may correspond to
`a specific microphone channel of the input array 710. Each
`signal analysis filter bank or filter 206 may generate a down
`sampled microphone sub-band signal "y, (n) correspond
`ing to the specific microphone channel “k.”
`0045. The echo compensation system 804 may include a
`plurality of the echo compensation filters 416. The number of
`echo compensation filters 416 may be equal to the number of
`input channels “k. Each echo compensation filter 416 may be
`the same or similar to the echo compensation filter of FIG. 2.
`Each echo compensation filter 416 may receive the output of
`the echoanalysis filter bank 216. The echo analysis filter bank
`
`Page 12 of 15
`
`

`

`US 2008/0144848 A1
`
`Jun. 19, 2008
`
`216 may be the same or similar to the echoanalysis filter bank
`of FIGS. 2 and 4. Each echo compensation filter 416 may
`generate estimated sub-band signals "d(n)" based on the
`response “h.(n)” 806 of the corresponding echo compensa
`tion filter 416.
`0046. A plurality of signal combining circuits 810 may
`Subtract the corresponding estimated Sub-band signals “
`d(n) from the down-sampled sub-band signal "y, (n)" to
`generate sub-band error signals "e, (n),” which may corre
`spond to the specific input channel “k.” The estimated sub
`band signals “d(n) may be generated by folding or com
`bining the echo compensation filter response “h.(n)” 806
`with the corresponding Sub-band signals based on the refer
`ence audio signal “X(n). Adaptation of the echo compensa
`tion filter coefficients and the folding of the coefficients with
`the Sub-band reference signals may be performed at a down
`sampling rate that may be about equal to the down-sampling
`rate of the signal analysis filter bank 206.
`0047. A beam-forming circuit 820 may receive and pro
`cess the sub-band error signals "e, (n).” The beam-forming
`circuit 820 may be adaptive or may be non-adaptive. The
`beam-forming circuit 820 may be a “delay and sum' beam
`forming circuit. The beam-forming circuit 820 may process
`the signals using a process described by "Optimum Array
`Processing, Part IV of Detection, Estimation, and Modulation
`Theory,” by H. L. van Trees, Wiley & Sons, New York 2002,
`which is incorporated by reference.
`0048. The beam-forming circuit 820 may be a “delay-and
`Sum' beam-former or it may be a Generalized Sidelobe Can
`celler. The Generalized Sidelobe Canceller may include a first
`or lower adaptive signal processing path with a blocking
`matrix and an adaptive noise-canceling circuit. The General
`ized Sidelobe Canceller may include a second or upper non
`adaptive signal processing path with a fixed beam-former.
`The beam-forming circuit 820 may process the signals using
`a process described by “An alternative approach to linearly
`constrained adaptive beam forming.” by Griffiths, IEEE
`Transactions on Antennas and Propagation, Vol. 30, p. 27.
`1982, which is incorporated by reference.
`0049. The beam-forming circuit 820 may combine the
`sub-band error signals “e, (n) for each of the “k” input
`channels to generate beam-formed Sub-band signals. A beam
`formed filter circuit 830 may receive the beam-formed sub
`band signals and may suppress a residual echo to enhance the
`quality of the beam-formed Sub-band signals. The beam
`formed filter circuit 830 may include a Wiener filter. The
`Weiner filter may reduce background noise in the frequency
`domain according to Equation 8 below:
`(Eqn. 8)
`W(en)=1-S (en)/S (en)
`where S(e', n) may denote an estimated short-time power
`density of the background noise, and
`where S(e', n) may denote a short-time power density of
`the full-band error signal.
`0050. The beam-formed filter circuit 830 may generate
`enhanced sub-band signals “s,(n).” A synthesis filter bank
`840 may receive the enhanced sub-band signals “s,(n).” The
`synthesis filter bank 840 may include a plurality of up-sam
`pling circuits 850 and corresponding synthesis sub-band fil
`ters 860. Each up-sampling circuit 850 may receive the cor
`responding enhanced sub-band signals “s,(n).” The
`up-sampling circuits 850 may up-sample the Sub-band sig
`nals using the factor “r” as the up-sampling factor. The up
`sampling factor “r” may be equal to the first down-sampling
`
`factor “r” where r-(r)(r). Each synthesis sub-band filter 860
`may include high-pass filters, band-pass filters, and/or low
`pass filters to reduce or eliminate artifacts. A Summing circuit
`870 may sum the output of each of the synthesis sub-band
`filters 860 to generate a synthesized speech signal “s(n).”
`0051. The logic, circuitry, and processing described above
`may be encoded in a computer-readable medium Such as a
`CDROM, disk, flash memory, RAM or ROM, an electromag
`netic signal, or other machine-readable medium as instruc
`tions for execution by a processor. Alternatively or addition
`ally, the logic may be implemented as analog or digital logic
`using hardware. Such as one or more integrated circuits (in
`cluding amplifiers, adders, delays, and filters), or one or more
`processors executing amplification, adding, delaying, and fil
`tering instructions; or in Software in an application program
`ming interface (API) or in a Dynamic Link Library (DLL),
`functions available in a shared memory or defined as local or
`remote procedure calls; or as a combination of hardware and
`software.
`0.052 The logic may be represented in (e.g., stored on or
`in) a computer-readable medium, machine-readable medium,
`propagated-signal medium, and/or signal-bearing medium.
`The media may comprise any device that contains, stores,
`communicates, propagates, or transports executable instruc
`tions for use by or in connection with an instruction execut
`able system, apparatus, or device. The machine-readable
`medium may selectively be, but is not limited to, an elec
`tronic, magnetic, optical, electromagnetic, or infrared signal
`or a semiconductor system, apparatus, device, or propagation
`medium. A non-exhaustive list of examples of a machine
`readable medium includes: a magnetic or optical disk, a Vola
`tile memory such as a Random Access Memory “RAM, a
`Read-Only Memory “ROM, an Erasable Programmable
`Read-Only Memory (i.e., EPROM) or Flash memory, or an
`optical fiber. A machine-readable medium may also include a
`tangible medium upon which executable instructions are
`printed, as the logic may be electronically stored as an image
`or in another format (e.g., through an optical scan), then
`compiled, and/or interpreted or otherwise processed. The
`processed medium may then be stored in a computer and/or
`machine memory.
`0053. The systems may include additional or different
`logic and may be implemented in many different ways. A
`controller may be implemented as a microprocessor, micro
`controller, application specific integrated circuit (ASIC), dis
`crete logic, or a combination of other types of circuits or logic.
`Similarly, memories may be DRAM, SRAM, Flash, or other
`types of memory. Parameters (e.g., conditions and thresh
`olds) and other data structures may be separately stored and
`managed, may be incorporated into a single memory or data
`base, or may be logically and physically organized in many
`different ways. Programs and instruction sets may be parts of
`a single program, separate programs, or distributed across
`several memories and processors. The systems may be
`included in a wide variety of electronic devices, including a
`cellular phone, a headset, a hands-free set, a speakerphone,
`communication interface, or an infotainment system.
`0054 While various embodiments of the invention have
`been described, it will be apparent to those of ordinary skill in
`the art that many more embodiments and implementations are
`possible within the scope of the invention. Accordingly, the
`invention is not to be restricted except in light of the attached
`claims and their equivalents.
`
`Page 13 of 15
`
`

`

`US 2008/0144848 A1
`
`Jun. 19, 2008
`
`We claim:
`1. A method for reducing echoes in an input signal con
`taining an audio signal and an echo signal, the echo signal
`generated by a loudspeaker signal in a loudspeaker-room
`microphone system, the method comprising:
`converting a portion of the input signal to microphone
`Sub-band signals;
`down-sampling the microphone Sub-band signals by a first
`down-sampling rate;
`converting the loudspeaker signal to echo Sub-band Sig
`nals;
`down-sampling the echo Sub-band signals by a second
`down-sampling rate to generate down-sampled echo
`Sub-band signals, the second down-sampling rate less
`than the first down-sampling rate;
`storing the down-sampled echo Sub-band signals;
`folding the down-sampled echo Sub-band signals with an
`estimated impulse response of the loudspeaker-room
`microphone system;
`down-sampling the folded down-sampled echo Sub-band
`signals by a third down-sampling rate to generate esti
`mated echo Sub-band signals, where the first down-Sam
`pling rate is equal to the product of the second and third
`down-sampling rates;
`Subtracting for each Sub-band, the estimated echo Sub
`band signal from the corresponding microphone Sub
`band signal to obtain error Sub-band signals; and
`up-sampling and synthesizing the error Sub-band signals to
`generate an echo compensated audio signal.
`2. The method according to claim 1, where generating the
`estimated echo Sub-band signal comprises adapting filter
`coefficients of an echo compensation filter based on the stored
`down-sampled echo Sub-band signals at a rate equal to the
`first down-sampling rate.
`3. The method according to claim 2, where the filter coef
`ficients are adapted usin