263:: It is fairly normal in conversation for both parties to speak at the same time, at least briefly. Because each echo suppressor will then detect voice energy coming from the far-end of the circuit, the effect would ordinarily be for loss to be inserted in both directions at once, effectively blocking both parties. To prevent this, echo suppressors can be set to detect voice activity from the near-end speaker and to fail to insert loss (or insert a smaller loss) when both the near-end speaker and far-end speaker are talking. This, of course, temporarily defeats the primary effect of having an echo suppressor at all.
184:). In some cases, these terms are more precise, as there are various types and causes of echo with unique characteristics, including acoustic echo (sounds from a loudspeaker being reflected and recorded by a microphone, which can vary substantially over time) and line echo (electrical impulses caused by, e.g., coupling between the sending and receiving wires, impedance mismatches, electrical reflections, etc., which varies much less than acoustic echo). In practice, however, the same techniques are used to treat all types of echo, so an acoustic echo canceller can cancel line echo as well as acoustic echo.
319:
400:
must be adaptive because the characteristics of the near-end's speaker and microphone are generally not known in advance. The acoustical attributes of the near-end's room are also not generally known in advance, and may change (e.g., if the microphone is moved relative to the speaker, or if individuals walk around the room causing changes in the acoustic reflections). By using the far-end signal as the stimulus, modern systems use an adaptive filter and can
36:
306:) rather than as standalone devices. The integration of echo cancellation directly into the switch meant that echo cancellers could be reliably turned on or off on a call-by-call basis, removing the need for separate trunk groups for voice and data calls. Today's telephony technology often employs echo cancellers in small or handheld communications devices via a software
247:
In the earlier days of telecommunications, echo suppression was used to reduce the objectionable nature of echos to human users. One person speaks while the other listens, and they speak back and forth. An echo suppressor attempts to determine which is the primary direction and allows that channel to
528:
for that same period. Once the connection is completed, they send their signals into the phone lines as normal, but also into the delay line. When their signal is reflected back, it is mixed with the inverted signal from the delay line, which cancels out the echo. This allowed both modems to use the
399:
The primary challenge for an echo canceller is determining the response characteristics of the filter to be applied to the far-end signal such that it resembles the resultant near-end echo. The filter is essentially a model of speaker, microphone and the room's acoustical attributes. Echo cancellers
276:
Dead-set: If the far-end party on a call is in a noisy environment, the near-end speaker will hear that background noise while the far-end speaker is talking, but the echo suppressor will suppress this background noise when the near-end speaker starts talking. The sudden absence of background noise
285:
developed echo canceler theory in the early 1960s, which then resulted in laboratory echo cancelers in the late 1960s and commercial echo cancelers in the 1980s. An echo canceller works by generating an estimate of the echo from the talker's signal, and subtracts that estimate from the return path.
560:
telephone wires also make use of automated echo cancellation to allow simultaneous bidirectional data communication. The computational complexity in implementing the adaptive filter is much reduced compared to voice echo cancelling because the transmit signal is a digital bit stream. Instead of a
511:
Standard telephone lines use the same pair of wires to both send and receive audio, which results in a small amount of the outgoing signal being reflected back. This is useful for people talking on the phone, as it provides a signal to the speaker that their voice is making it through the system.
203:
Echo cancellation involves first recognizing the originally transmitted signal that re-appears, with some delay, in the transmitted or received signal. Once the echo is recognized, it can be removed by subtracting it from the transmitted or received signal. This technique is generally implemented
199:
going in one direction on a circuit, and then muting or attenuating the signal in the other direction. Usually, the echo suppressor at the far end of the circuit does this muting when it detects voice coming from the near-end of the circuit. This muting prevents the speaker from hearing their own
290:
to generate a signal accurate enough to effectively cancel the echo, where the echo can differ from the original due to various kinds of degradation along the way. Since invention at AT&T Bell Labs echo cancellation algorithms have been improved and honed. Like all echo cancelling processes,
794:
485:
In some of these cases, sound from the loudspeaker enters the microphone almost unaltered. The difficulties in canceling echo stem from the alteration of the original sound by the ambient space. These changes can include certain frequencies being absorbed by soft furnishings and reflection of
532:
Echo cancellation is also applied by many telcos to the line itself and can cause data corruption rather than improving the signal. Some telephone switches or converters (such as analog terminal adapters) disable echo suppression or echo cancellation when they detect the 2100 or 2225 Hz
1038:
573:
Some modems use separate incoming and outgoing frequencies or allocate separate time slots for transmitting and receiving to eliminate the need for echo cancellation. Higher frequencies beyond the original design limits of telephone cables suffer significant
519:
split the signal frequencies, so that the devices on either end used different tones, allowing each one to ignore any signals in the frequency range it was using for transmission. However, this diminished the amount of bandwidth available to both sides.
236:, echo is the reflected copy of one's voice heard some time later. If the delay is fairly significant (more than a few hundred milliseconds), it is considered annoying. If the delay is very small (tens of milliseconds or less), the phenomenon is called
523:
Echo cancellation mitigated this problem. During the call setup and negotiation period, both modems send a series of unique tones and then listen for them to return through the phone system. They measure the total delay time, then configure a
565:
lookup table based echo cancelling scheme eliminates even the addition operation by simply addressing a memory with a truncated transmit bit stream to obtain the echo estimate. Echo cancellation is now commonly implemented with
427:, of the original signal and its echo. High values mean the echo is very weak, while low values mean the echo is very strong. Negative indicate the echo is stronger than the original signal, which if left unchecked would cause
266:
Clipping: Since the echo suppressor is alternately inserting and removing loss, there is frequently a small delay when a new speaker begins talking that results in clipping the first syllable from that speaker's speech.
310:, which provides cancellation of either acoustic echo or the residual echo introduced by a far-end PSTN gateway system; such systems typically cancel echo reflections with up to 64 milliseconds delay.
256:
any signal on the assumption that the signal is echo. Although the suppressor effectively deals with echo, this approach leads to several problems which may be frustrating for both parties to a call.
503:
systems that accept speech for input use AEC while speech prompts are played to prevent the system's own speech recognition from falsely recognizing the echoed prompts and other output.
191:
Although echo suppressors and echo cancellers have similar goals—preventing a speaking individual from hearing an echo of their own voice—the methods they use are different:
149:
from being created or removing it after it is already present. In addition to improving subjective audio quality, echo suppression increases the capacity achieved through
866:
260:
1056:
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However, this reflected signal causes problems for a modem, which is unable to distinguish between a signal from the remote modem and the echo of its own signal.
407:
Echo cancellation alone may be insufficient in many applications. Echo cancellation and suppression can work in conjunction to achieve acceptable performance.
635:
586:. Deep, narrow frequency gaps which cannot be remedied by echo cancellation often result. These are detected and mapped out during connection negotiation.
375:
of the far end speech that leaks through the hybrid. Once the adaption is complete, the error signal consists mostly of speech from the local telephone.
53:
917:
735:
188:
in particular is commonly used to refer to echo cancelers in general, regardless of whether they were intended for acoustic echo, line echo, or both.
438:(ERLE), which is the amount of additional signal loss applied by the echo canceller. Most echo cancellers are able to apply 18 to 35 dB ERLE.
100:
72:
348:
contains both the desired speech from the local telephone plus filtered speech from the far end. The echo canceller is the adaptive filter
1068:
79:
291:
these first algorithms were designed to anticipate the signal which would inevitably re-enter the transmission path, and cancel it out.
86:
542:
493:(DSP), this cost in processing capability may come at a premium, however, many embedded systems do have a fully functional AEC.
665:
339:
to the local telephone and route speech from the telephone to the far end. However, the hybrid is never perfect, so its output
240:. If the delay is slightly longer, around 50 milliseconds, humans cannot hear the echo as a distinct sound, but instead hear a
68:
298:
allowed echo cancellers to be made smaller and more cost-effective. In the 1990s, echo cancellers were implemented within
270:
1033:, Bjor, Håkon E. & Raad, Bjørn H., "Directional coupler", published 1980-12-02, assigned to
1080:
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938:
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119:
17:
157:. Echo suppressors were developed in the 1950s in response to the first use of satellites for telecommunications.
1110:
639:
809:
93:
57:
921:
758:
789:, Kelly Jr., John L., "Self-adaptive echo canceller", published 1970-03-10, assigned to
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multiplication and an addition operation for every tap in the filter, only the addition is required. A
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154:
1030:
786:
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The resultant signal represents sounds present in the room excluding any direct or reverberated sound.
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886:
867:"Acoustic Echo Cancellation and Doubletalk Detection Using Estimated Loudspeaker Impulse Responses"
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205:
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46:
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Implementing AEC requires engineering expertise and a fast processor, usually in the form of a
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from providing no cancellation to 55 dB of cancellation in around 200 ms.
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The far-end signal is filtered and delayed to resemble the near-end signal.
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The total signal loss of the echo (ACOM) is the sum of the ERL and ERLE.
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or software, although it can be implemented in analog circuits as well.
918:"What is Echo Return Loss (ERL) and how does it affect voice quality?"
965:"Echo Cancellation Part 1: The Basics and Acoustic Echo Cancellation"
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282:
233:
142:
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An adaptive echo canceler for a telephone circuit. The function of
35:
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424:
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The filtered far-end signal is subtracted from the near-end signal.
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gives the near-end user the impression that the line has gone dead.
237:
220:
describe requirements and tests for echo cancellers in digital and
667:
Stereophonic
Acoustic Echo Cancellation: Theory and Implementation
478:
303:
808:
Murano, Kazuo; Unagami, Shigeyuki; Amano, Fumio (January 1990).
470:
systems which use ceiling speakers and microphones on the table
160:
Echo suppression and cancellation methods are commonly called
546:
538:
516:
636:"Octasic: Voice Quality Enhancement & Echo Cancellation"
556:
operating at frequencies above the voice band over standard
449:
Sources of echo are found in everyday surroundings such as:
993:"A New Digital Echo Canceler for Two-Wire Subscriber Lines"
221:
146:
562:
366:
by filtering the incoming far end speech into a replica
1059:. International Engineering Consortium. Archived from
529:
full spectrum available, doubling the possible speed.
271:
Voice activity detection § Performance evaluation
434:
The performance of an echo canceller is measured in
60:. Unsourced material may be challenged and removed.
807:
1102:
990:
874:IEEE Transactions on Speech and Audio Processing
379:The echo cancellation process works as follows:
537:associated with such calls, in accordance with
330:, is to route incoming speech from the far end
357:, which attempts to minimize the error signal
248:go forward. In the reverse channel, it places
781:
779:
383:A far-end signal is delivered to the system.
302:for the first time (in the Northern Telecom
486:different frequencies at varying strength.
153:by preventing echo from traveling across a
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473:Physical coupling where vibrations of the
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120:Learn how and when to remove this message
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27:Preventing or removing echo in telephony
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423:(ERL). This is the ratio, expressed in
145:to improve voice quality by preventing
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1103:
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801:
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456:A standard telephone or cellphone in
928:
810:"Echo Cancellation and Applications"
58:adding citations to reliable sources
29:
997:IEEE Transactions on Communications
991:Holte, N.; Stueflotten, S. (1981).
477:transfer to the microphone via the
410:
69:"Echo suppression and cancellation"
24:
1081:"Q-Sys Acoustic Echo Cancellation"
755:10.1002/j.1538-7305.1967.tb04231.x
463:Dedicated standalone speakerphones
25:
1127:
1071:. Ditech Networks. Archived from
1049:
939:"Echo Analysis for Voice over IP"
910:
791:Bell Telephone Laboratories, Inc.
386:The far-end signal is reproduced.
200:voice returning from the far end.
963:Kosanovic, Bogdan (2002-04-11).
734:Sondhi, Man Mohan (March 1967).
34:
1023:
984:
714:"Echo in Voice over IP Systems"
444:
45:needs additional citations for
13:
1:
865:Åhgren, Per (November 2005).
743:Bell System Technical Journal
621:
817:IEEE Communications Magazine
453:Hands-free car phone systems
436:echo return loss enhancement
313:
224:applications, respectively.
7:
736:"An adaptive echo canceler"
673:(Thesis). Lund University.
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286:This technique requires an
10:
1132:
501:interactive voice response
268:
227:
170:acoustic echo cancellation
155:telecommunications network
1009:10.1109/TCOM.1981.1094923
601:Least mean squares filter
515:For this reason, earlier
506:
296:digital signal processing
195:Echo suppressors work by
162:acoustic echo suppression
568:Digital Signal Processor
491:digital signal processor
206:digital signal processor
197:detecting a voice signal
896:10.1109/TSA.2005.851995
664:Eneroth, Peter (2001).
1111:Communication circuits
1069:"Echo basics tutorial"
576:attenuation distortion
376:
178:line echo cancellation
321:
281:In response to this,
269:Further information:
1035:Elektrisk Bureau A/S
415:Echo is measured as
141:are methods used in
54:improve this article
1057:"Echo cancellation"
176:), and more rarely
151:silence suppression
1088:QSC Audio Products
584:impedance matching
570:(DSP) techniques.
377:
328:hybrid transformer
294:Rapid advances in
204:digitally using a
1003:(11): 1573–1581.
611:Signal reflection
139:echo cancellation
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16:(Redirected from
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880:(6): 1231–1237.
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419:echo return loss
411:Quantifying echo
135:Echo suppression
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582:and incomplete
541:recommendation
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468:conference room
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1075:on 2011-07-10.
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1063:on 2007-03-08.
1051:
1050:External links
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1091:. Retrieved
1073:the original
1061:the original
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974:. Retrieved
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946:. Retrieved
922:the original
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848:. Retrieved
823:(1): 49–55.
820:
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766:. Retrieved
759:the original
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717:. Retrieved
692:. Retrieved
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644:. Retrieved
640:the original
630:
616:Voice engine
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558:twisted-pair
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445:Current uses
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110:January 2011
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52:Please help
47:verification
44:
580:bridge taps
475:loudspeaker
261:Double-talk
250:attenuation
1105:Categories
1093:2016-07-28
1031:US 4237463
787:US 3500000
694:2015-06-25
622:References
554:DSL modems
526:delay line
466:Installed
80:newspapers
1116:Telephony
1017:1558-0857
882:CiteSeerX
837:0163-6804
689:1402-8662
606:Mix-minus
552:ISDN and
314:Operation
283:Bell Labs
234:telephony
143:telephony
970:EE Times
850:14 April
768:14 April
646:14 April
590:See also
425:decibels
402:converge
254:suppress
238:sidetone
904:2575877
578:due to
479:handset
304:DMS-250
228:History
94:scholar
1040:
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976:7 July
948:2 July
902:
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845:897792
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719:2 July
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507:Modems
481:casing
326:, the
168:) and
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1084:(PDF)
900:S2CID
870:(PDF)
841:S2CID
813:(PDF)
762:(PDF)
739:(PDF)
671:(PDF)
547:G.165
543:G.164
539:ITU-T
218:P.340
214:G.168
101:JSTOR
87:books
1013:ISSN
978:2014
950:2014
852:2014
833:ISSN
770:2014
721:2014
685:ISSN
675:ISBN
648:2014
499:and
460:mode
222:PSTN
216:and
147:echo
137:and
73:news
1005:doi
892:doi
825:doi
751:doi
563:RAM
545:or
232:In
186:AEC
182:LEC
174:AEC
166:AES
56:by
1107::
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