Stereophonic Acoustic Echo Cancellation - Lund University

175 Pages·2001·1.89 MB·English

Stereophonic Acoustic Echo Cancellation:
Theory and Implementation
Peter Eneroth
Lund 2001
Departmentof Electroscience
LundUniversity
Box 118,S-22100 LUND
SWEDEN
No.22
ISSN 1402-8662
ISBN 91-7874-110-6
(cid:13)c Peter Eneroth2001
Printed in Sweden byKFS AB, Lund.
January2001
Abstract
The thesis treats theory and implementation aspects for stereophonic acoustic echo
cancellation.
In Paper I a complete implementation of a stereophonic acoustic echo canceler
basedonthetwo-channelfastrecursiveleast-squaresalgorithmispresented. Ananal-
ysisofthesystemcalculationcomplexityisalsogiven,inadditiontosimulationresults
on recordedreal-lifedata.
Paper II presents a comparison between adaptive filters for usage in stereophonic
acoustic echo cancellation. The comparisonincludes, in addition to the standard nor-
malized least mean square algorithm, the two-channel fast recursive least-squares al-
gorithm and a two-channel frequency-domain adaptive algorithm. In the paper, the
convergence rate, the calculation complexity, the signal transmission delay, and the
memoryusagefor theevaluatedsystemsare shown.
Adaptivefiltersappliedinsubbandstructuresmayneedtomodelafewnon-causal
taps even if the fullband impulse response is causal. This phenomenon is analyzed
in Paper III. Formulas to calculate the number of non-causal taps needed are also
presentedin thepaper.
Thefundamentalproblemin stereophonicacousticecho cancellationis the signal
correlation between the two channels. In Paper IV it is shown how a perceptual au-
dio coder may decrease this correlation and thereby increase the performance of the
stereophonicacousticechocanceler.
Paper V investigatesthe possibilities of using joint subband filterbanks or time to
frequency-domaintransformsforecho cancellationandperceptualaudiocoding. The
usage of joint filterbanks/transforms not only decreases the computational complex-
ity of the system, it also reduces the total signal transmission delay of the system if
properlydesigned.
iii

Contents
Abstract iii
Contents v
Acknowledgment vii
General Introduction 1
1 CommunicationSystem . . . . . . . . . . . . . . . . . . . . . . . 1
2 Echo Cancellation . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3 AdaptiveFiltering . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4 Audio Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5 Outlineof theThesis . . . . . . . . . . . . . . . . . . . . . . . . . 18
I A Real-time Implementation of a Stereophonic Acoustic
Echo Canceler 33
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2 Problemsand Solutionsof StereophonicAcousticEchoCancellation 36
3 ProposedStructurefor theStereophonicAcousticEchoCanceler . 39
4 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
A Analysis Filterbank . . . . . . . . . . . . . . . . . . . . . . . . . 52
B Synthesis Filterbank . . . . . . . . . . . . . . . . . . . . . . . . . 54
C PrototypeFilter forNon-criticalDownsampling . . . . . . . . . . 56
D PrototypeFilter Construction . . . . . . . . . . . . . . . . . . . . 58
E Real-time Implementation . . . . . . . . . . . . . . . . . . . . . . 63
II Comparison of Different Adaptive Algorithms for Stereo-
phonic Acoustic Echo Cancellation 75
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
2 AdaptiveAlgorithms . . . . . . . . . . . . . . . . . . . . . . . . . 78
3 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
v
vi Contents
III Analysisof Subband Impulse Responses in Subband Echo
Cancelers 91
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
2 System Description . . . . . . . . . . . . . . . . . . . . . . . . . 95
3 Analysis of theSubbandImpulseResponse . . . . . . . . . . . . . 96
4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
A CorrelationCalculations . . . . . . . . . . . . . . . . . . . . . . . 112
IV Influence of Audio Coding on Stereophonic Acoustic Echo
Cancellation 121
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
2 Problem Formulation. . . . . . . . . . . . . . . . . . . . . . . . . 123
3 Audio Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
4 MeasurementStudies . . . . . . . . . . . . . . . . . . . . . . . . 127
5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
V Joint Filterbanks for Echo Cancellation and Audio Cod-
ing 135
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
2 Problem Formulation. . . . . . . . . . . . . . . . . . . . . . . . . 139
3 Modified AudioCoder FilterbankStructuresfor EchoCancelers . . 144
4 FDAF and MDCT Based AudioCoders . . . . . . . . . . . . . . . 151
5 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
Acknowledgment
Without all the help and encouragement I have received during the last five years,
the contents of this thesis would be significantly different, and I am truly grateful to
everyonewho has made this work possible. Not only have I been given a lot of help,
I have also been given the opportunity to become friends with so many generous,
interestingand intelligentpeople.
I am without competition most grateful towards my supervisor Tomas Ga¨nsler. I
have always felt totally confident with his knowledge and his support to me. He has
guidedmetowardsveryinterestingtechnicalproblems,andintimeswhenIhavebeen
toocommittedin solvingtheseproblems,hehasforcedmetowrite. Afterleavingthe
department,Tomascontinuedbeing fullycommittedas my supervisor.
I would also like to thank Go¨ran Salomonsson, now retired from his position at
thedepartment. Itwas Go¨ran whoaccepted meas a graduatestudentand who guided
mein myearly work.
Inspirationis a fundamentalcorner-stonebehindresearch, and much of my inspi-
rationhascomefromBell Laboratories. JacobBenesty taughtmeplentyaboutmulti-
channel adaptive filtering, and has generously shared everything from knowledge to
deep friendship. During the fall of 1998, Steven Gay and I worked on the project
presentedin Paper I ofthisthesis. We spentmanyeveningsat thelabs,togetherfight-
ing againsttime and software bugs. Also Steve generouslyshared his knowledgeand
friendship. Dennis Morganwas always happy to discuss everythingfrom mathemati-
cal and technicaldetails to internationalpolitics. Being a perfectionist,he also taught
me plenty about technical writing. Finally, I would like to mention the head of the
group,GaryElko,whoenthusiasticallypushedtheworkintherightdirection,andev-
erybodywhohelpedcreatingsuch a friendlyatmosphereat Bell Laboratories,Lucent
Technologies.
WhenTomasGa¨nslerandIstartedourworkonstereophonicacousticechocancel-
lation,itwasajointprojecttogetherwithTeliaResearchABinHaninge,nowsituated
inFarsta. Ourcooperationwasextremelyfruitful,andIwillalwaysbegratefultoOve
Till, NiklasJohanssonand BirgitteWikman.
Most of the time was of course spent at the Department of Applied Electronics,
now the Department of Electroscience. Academic and educational inspiration was
given by Leif So¨rnmo and Per Ola Bo¨rjesson, and administrative issues were easily
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viii Acknowledgment
resolved with help from Birgitta Holmgren. Clas Angvall has been supportive in di-
verse matters and Erik Johnsson has provided me a bullet proof computer system.
I’m also grateful to all colleagues at the department, making almost every day at the
departmentjoyful.
Finally, I would like to thank my wife, Elisabeth Anderberg, my family and my
friendsfor encouragement,andfor givingmequalityof life.
General Introduction
Echo: “the repetitionof a soundcausedbyreflection ofsoundwaves.”
Merriam-Webster’sDictionary
Historically,echowasprobablyaninterestingandamusingphenomenonthatcould
be experienced in rare places, e.g. between mountain walls in the Alps. However, to
most people today, echo is also something annoying which can occur, e.g., during a
longdistancetelephonecall. Thedefinitionof echois still valid though,it is allabout
(delayed)reflectionsof soundwaves.
Inthisthesis,methodsforremovingechoincommunicationsystemsareproposed.
Whilemostofthetheoriespresentedherecanbeappliedtoreduceechoesintelephone
calls, the target application is hands-free communications systems, with dual audio
channels. Examples include stereophonic video conferencing systems and desktop
conferencingoncomputerswithdualloud-speakers. Thethesiswillbothconsiderthe
theoreticalproblemsand implementationaspects.
Thispartincludesgeneralbackgroundinformationneededinordertogettheother
parts of the thesis in a proper context. Here the underlying application is discussed,
together with notes about the historical background. The theoretical problem in echo
cancellationisshownandbasicechocancellationmethodsarediscussed. Finally,Sec-
tion 5 summarizesthefivepartsof thethesis.
1 Communication System
For the purposeof this thesis, a communicationsystem is a system that allows two or
more persons, or perhaps one person and one machine, to interact with speech over
anartificialchannel,forexampleatelephoneline. Thissystem mayalso deliverother
components,suchaspictures,video,documents,oranyothertypeofmedia,butinthis
thesis we will only discuss the audio component. The most commonly used system
1
2 GeneralIntroduction
in this category is of course the standard telephone system, also called the public
switchedtelephonenetwork(PSTN),butinrecentyearsmanyadditionalsystemshave
emerged,e.g.,cellular telephones,videoconferencing,anddesktopconferencing.
Natural speech signals are analogue signals, and they can be modeled as contin-
uous functions, with infinitely high resolution. Such functions are very ill-suited for
today’s digital communication and computer systems. Therefore, in the systems un-
derconsideration,thespeechsignalsaresampledandquantized. Thisprocessinvolves
measuringtheamplitudeofthesignalatgiventimeintervals(sampling)andpartition-
ingthesignalamplitudespaninsub-intervals,andrepresentingeachsub-intervalwith
a number (quantization). The digitized signal can advantageously be processed with
a digital system, e.g., a computer, and be transmitted via a digital communication
system,such as a modernPSTN.
The quality of the digital signal can be made arbitrarily good, by decreasing the
time between successivesamples and by increasing the numberof quantizationinter-
vals. Since the bandwidthof the original speech signal is limited, as is the sub-group
of audio signals that are audible to humans, it is not controversial to state that high
quality sampled audio signals are extremely redundant. That is, it is possible to re-
duce the amountof data used to representthe signal without, or with very small, loss
of quality. The sampling and quantization of the analog signal can be viewed as one
formof datacompression,and later in thethesiswe willsee othercompressionmeth-
ods,wheremoremathematicalmethodsareused to reducethe redundancyfurther.
Almost everyone who has made an intercontinental telephone call, or a call to a
cellular telephone in a poor network, has experienced echoes. This thesis presents
methods for how to reduce echoes in communication systems, with the emphasis on
echoes due to an acoustic coupling between the loud-speaker and the microphone.
Threepapersinthethesisfocusonsystemswithtwoaudiochannels,stereophonicau-
dio,describingwhythissituationismorecomplicatedandproposespossiblesolutions
for stereophonicacousticecho cancellation.
2 Echo Cancellation
Echoes in the PSTN have been a problem since the early days of telephone technol-
ogy,when the first transcontinentaltelephonenetworkswere built. The long distance
betweenthetwopartiesintroducesatimedelaytotheechosignal. Thisdelayincom-
bination with low echo attenuation reducesthe perceptualquality of the system. For-
tunately, the development of high speed transmission systems in the 1920’s reduced
theproblem[1].
In the 1960’s, satellite communication was introduced, and again the round-trip
delay became long enough to cause noticeable echo problems. A device called echo