Echo Cancellation (Vaseghi - Advanced Digital Signal Processing and Noise Reduction)

Advanced Digital Signal Processing and Noise Reduction, Second Edition.Saeed V. VaseghiCopyright © 2000 John Wiley & Sons LtdISBNs: 0-471-62692-9 (Hardback): 0-470-84162-1 (Electronic)14ECHO CANCELLATION14.114.214.314.414.514.614.7Introduction: Acoustic and Hybrid EchoesTelephone Line Hybrid EchoHybrid Echo SuppressionAdaptive Echo CancellationAcoustic EchoSub-band Acoustic Echo CancellationSummaryEcho is the repetition of a waveform due to reflection from pointswhere the characteristics of the medium through which the wavepropagates changes.

Echo is usefully employed in sonar and radar fordetection and exploration purposes. In telecommunication, echo can degradethe quality of service, and echo cancellation is an important part ofcommunication systems. The development of echo reduction began in thelate 1950s, and continues today as new integrated landline and wirelesscellular networks put additional requirement on the performance of echocancellers. There are two types of echo in communication systems: acousticecho and telephone line hybrid echo.

Acoustic echo results from a feedbackpath set up between the speaker and the microphone in a mobile phone,hands-free phone, teleconference or hearing aid system. Acoustic echo maybe reflected from a multitude of different surfaces, such as walls, ceilingsand floors, and travels through different paths. Telephone line echoes resultfrom an impedance mismatch at telephone exchange hybrids where thesubscriber's 2-wire line is connected to a 4-wire line. The perceptual effectsof an echo depend on the time delay between the incident and reflectedwaves, the strength of the reflected waves, and the number of paths throughwhich the waves are reflected. Telephone line echoes, and acoustic feedbackechoes in teleconference and hearing aid systems, are undesirable andannoying and can be disruptive.

In this chapter we study some methods forremoving line echo from telephone and data telecommunication systems,and acoustic feedback echoes from microphone–loudspeaker systems.397Acoustic and Hybrid Echoes14.1 Introduction: Acoustic and Hybrid EchoesEcho can severely affect the quality and intelligibility of voice conversationin a telephone system. The perceived effect of an echo depends on itsamplitude and time delay.

In general, echoes with an appreciable amplitudeand a delay of more than 1 ms are noticeable. Provided the round-trip delayis on the order of a few milliseconds, echo gives a telephone call a sense of“liveliness”. However, echoes become increasingly annoying andobjectionable with the increasing round-trip delay and amplitude inparticular for delays of more than 20 ms. Hence echo cancellation is animportant aspect of the design of modern telecommunication systems suchas conventional wireline telephones, hands-free phones, cellular mobile(wireless) phones, or teleconference systems.

There are two types of echo ina telephone system (Figure 14.1):(a) acoustic echo due to acoustic coupling between the speaker andthe microphone in hands-free phones, mobile phones andteleconference systems;(b) electrical line echo due to mismatch at the hybrid circuitconnecting a 2-wire subscriber line to a 4-wire truck line in thepublic switched telephone network.In the early days of expansion of telephone networks, the cost ofrunning a 4-wire line from the local exchange to subscribers’ premises wasconsidered uneconomical. Hence, at the exchange the 4-wire truck lines areconverted to 2-wire subscribers local lines using a 2/4-wire hybrid bridgecircuit. At the receiver due to any imbalance between the 4/2-wire bridgecircuit, some of the signal energy of the 4-wire circuit is bounced backEcho cancellersHybrid echoMobile switchingcentreAcoustic echoFigure 14.1 Illustration of echo in a mobile to land line system.398Echo Cancellationtowards the transmitter, constituting an echo signal.

If the echo is more thana few milliseconds long then it becomes noticeable, and can be annoyingand disruptive.In digital mobile phone systems, the voice signals are processed at twopoints in the network: first voice signals are digitised, compressed andcoded within the mobile handset, and then processed at the radio frequencyinterface of the network. The total delay introduced by the various stages ofdigital signal processing range from 80 ms to 100 ms, resulting in a totalround-trip delay of 160–200 ms for any echo.

A delay of this magnitude willmake any appreciable echo disruptive to the communication process. Owingto the inherent processing delay in digital mobile communication systems, itis essential and mandatory to employ echo cancellers in mobile phoneswitching centres.14.2 Telephone Line Hybrid EchoHybrid echo is the main source of echo generated from the public-switchedtelephone network (PSTN).

Echoes on a telephone line are due to thereflection of signals at the points of impedance mismatch on the connectingcircuits. Conventionally, telephones in a given geographical area areconnected to an exchange by a 2-wire twisted line, called the subscriber'slineline, which serves to receive and transmit signals. In a conventionalsystem a local call is set up by establishing a direct connection, at thetelephone exchange, between two subscribers’ loops. For a local call, thereis usually no noticeable echo either because there is not a significantimpedance mismatch on the connecting 2-wire local lines or because theHybrid AHybrid BEcho of BEcho of ASpeaker ASpeaker BFigure 14.2 Illustration of a telephone call set up by connection of 2-wiresubscriber's via hybrids to 4-wire lines at the exchange.399Telephone Line Hybrid EchoEcho of speaker BSignal from speaker ASpeaker AEcho from speaker ASignal from speaker BFigure 14.3 A 2-wire to 4-wire hybrid circuit.distances are relatively small and the resulting low-delay echoes areperceived as a slight amplification and “livening” effect.

For long-distancecommunication between two exchanges, it is necessary to use repeaters toamplify the speech signals; therefore a separate 2-wire telephone line isrequired for each direction of transmission.To establish a long-distance call, at each end, a 2-wire subscriber's linemust be connected to a 4-wire line at the exchange, as illustrated in Figure14.2. The device that connects the 2-wire subscriber's loop to the 4-wire lineis called a hybrid, and is shown in Figure 14.3.

As shown the hybrid isbasically a three-port bridge circuit. If the hybrid bridge were perfectlybalanced then there would be no reflection or echo. However, each hybridcircuit serves a number of subscribers’ lines. The subscribers' lines do notall have the same length and impedance characteristics; therefore it is notpossible to achieve perfect balance for all subscribers at the hybrids.

Whenthe bridge is not perfectly balanced, some of the signal energy on thereceiving 4-wire lines becomes coupled back onto itself and produces anecho. Echo is often measured in terms of the echo return loss (ERL); thehigher the echo return loss the lower will be the echo. Telephone line echoesare undesirable, and become annoying when the echo amplitude is relativelyhigh and the echo delay is long. For example when a long-distance call is400Echo Cancellationmade via a satellite the round-trip echo delay can be as long as 600 ms, andechoes can become disruptive. Also, as already mentioned, there areappreciable delays of up to 200 ms inherent in digital mobile phones, whichmake any echo quite noticeable.

For this reason the employment of echocancellers in mobile switching centres is mandatory.14.3 Hybrid Echo SuppressionThe development of echo reduction began in the late 1950s with the adventof echo suppression systems. Echo suppressors were first employed tomanage the echo generated primarily in satellite circuits. An echo suppresser(Figure 14.4) is primarily a switch that lets the speech signal through duringthe speech-active periods and attenuates the line echo during the speechinactive periods. A line echo suppresser is controlled by a speech/echodetection device.

The echo detector monitors the signal levels on theincoming and outgoing lines, and decides if the signal on a line from, say,speaker B to speaker A is the speech from the speaker B to the speaker A, orthe echo of speaker A. If the echo detector decides that the signal is an echothen the signal is heavily attenuated. There is a similar echo suppression unitfrom speaker A to speaker B. The performance of an echo suppresserdepends on the accuracy of the echo/speech classification subsystem.

Echoof speech often has a smaller amplitude level than the speech signal, butEcho/speechclassifierSpeaker BEcho suppressorFigure 14.4 Block diagram illustration of an echo suppression system.Adaptive Echo Cancellation401otherwise it has mainly the same spectral characteristics and statistics asthose of the speech.