# Man Mohan Sondhi
**Man Mohan Sondhi** (published as M. Mohan Sondhi; born 18 December 1933) is a signal-processing researcher who spent his career at Bell Laboratories in Murray Hill, New Jersey, and is known as the developer of the adaptive [echo canceller](https://www.edgepedia.org/echo-canceller), the device that removes echoes from long-distance telephone connections without interrupting the return path. He joined [Bell Labs](https://www.edgepedia.org/bell-labs)' Acoustics Research department in 1962 and retired in 2001 after 39 years, working throughout on problems in the processing of speech signals.[1](https://doi.org/10.1121/1.4782771) He was elected a Bell Labs Fellow in 1999 and received the IEEE Sumner Prize for his work on echo cancellation.[2](https://ieee-jp.org/section/kansai/activity/meeting_020.html) Man Mohan Sondhi was elected to the National Academy of Engineering.
| Fact | Detail |
|---|---|
| Born | 18 December 1933; US citizen[3](https://id.loc.gov/authorities/names/n91063913.html) |
| Education | B.Sc. (Honours) Physics, Delhi University, 1950; Diploma in Electrical Communication Engineering, Indian Institute of Science, Bangalore, 1953; M.S. 1955 and Ph.D. 1957, University of Wisconsin[4](https://doi.org/10.1002/j.1538-7305.1967.tb04231.x) • [2](https://ieee-jp.org/section/kansai/activity/meeting_020.html) |
| Career | Bell Laboratories Acoustics Research department, 1962–2001 (39 years)[1](https://doi.org/10.1121/1.4782771) |
| Signature work | "An Adaptive Echo Canceller", Bell System Technical Journal, 1967[4](https://doi.org/10.1002/j.1538-7305.1967.tb04231.x) |
| Measured performance | About 20 dB of echo cancellation with convergence in about 0.2 to 0.5 second for average speech levels[4](https://doi.org/10.1002/j.1538-7305.1967.tb04231.x) |
| Honors | IEEE Sumner Prize for echo cancellation; Bell Labs Fellow, 1999[2](https://ieee-jp.org/section/kansai/activity/meeting_020.html) |
| Output | About 120 papers and 10 patents[2](https://ieee-jp.org/section/kansai/activity/meeting_020.html) |
| Honor | Elected to the National Academy of Engineering |
## Early life and education
Sondhi was born on 18 December 1933 and later became a US citizen, according to the Library of Congress authority record drawn from the cataloging data of *Advances in Speech Signal Processing* (1991).[3](https://id.loc.gov/authorities/names/n91063913.html)
He studied physics at [Delhi University](https://www.edgepedia.org/delhi-university), taking a B.Sc. (Honours) degree in 1950, and then a Diploma in Electrical Communication Engineering at the [Indian Institute of Science](https://www.edgepedia.org/indian-institute-of-science) in [Bangalore](https://www.edgepedia.org/bangalore), awarded in 1953. He took M.S. (1955) and Ph.D. (1957) degrees in Electrical Engineering at the University of Wisconsin.[4](https://doi.org/10.1002/j.1538-7305.1967.tb04231.x) • [2](https://ieee-jp.org/section/kansai/activity/meeting_020.html) According to the *Bell System Technical Journal*, he joined Bell Telephone Laboratories in 1962.[4](https://doi.org/10.1002/j.1538-7305.1967.tb04231.x)
## Career at Bell Laboratories
Sondhi joined the Acoustics Research department at Bell Labs in 1962, eight days before AT&T launched Telstar, the first telecommunications satellite, on 10 July 1962. In his own account, the launch was destined to have a significant effect on his career, because satellite circuits created precisely the long transmission delays that made telephone echoes severe.[1](https://doi.org/10.1121/1.4782771) • [5](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.706.5784)
From 1962 until he retired in 2001, a span of 39 years, his work concerned problems connected with processing speech signals.[1](https://doi.org/10.1121/1.4782771) • [2](https://ieee-jp.org/section/kansai/activity/meeting_020.html) His research spanned speech signal processing, adaptive filtering, acoustical inverse problems, speech recognition and analysis, and the analysis and synthesis of speech by means of articulatory modeling. When AT&T Bell Labs split up in 1996, the laboratories became the research arm of [Lucent Technologies](https://www.edgepedia.org/lucent-technologies), and Sondhi's department passed with them.[5](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.706.5784) After retiring in 2001 he worked as a consultant at Avaya Labs Research and a visiting researcher at ATR.[2](https://ieee-jp.org/section/kansai/activity/meeting_020.html)
## Representative work
The two Bell System Technical Journal papers of 1966 and 1967 are the core of his contribution. The 1966 paper described a new device for canceling echo without interrupting the return path, named an echo canceller to distinguish it from conventional [echo suppressors](https://www.edgepedia.org/echo-suppressor); it was self-adapting, automatically tracking variations in the echo path that arise during a conversation, such as the connection or disconnection of extension phones ([doi:10.1002/j.1538-7305.1966.tb02442.x](https://doi.org/10.1002/j.1538-7305.1966.tb02442.x)).[6](https://doi.org/10.1002/j.1538-7305.1966.tb02442.x) The 1967 paper presented the full method: a replica of the echo is synthesized by a filter that, under the control of a feedback loop, adapts to the transmission characteristic of the echo path and tracks variations of the path during a conversation, and the replica is subtracted from the return signal. Under ideal conditions the echo converges to zero, and the paper derives estimates of the rate of convergence ([doi:10.1002/j.1538-7305.1967.tb04231.x](https://doi.org/10.1002/j.1538-7305.1967.tb04231.x)).[4](https://doi.org/10.1002/j.1538-7305.1967.tb04231.x) A follow-up study gave the theory and computer simulation of the self-adapting canceler, noting that eliminating echoes becomes increasingly difficult as roundtrip delay grows.[7](https://doi.org/10.1121/1.1943065)
His later work extended adaptive filtering and carried echo cancellation into acoustics. A 1976 *Proceedings of the IEEE* paper gave new results on the performance of a well-known class of adaptive filters. In 1995 an *IEEE Signal Processing Letters* paper set out the fundamental problem of stereophonic acoustic echo cancellation, and a 1998 *IEEE Transactions on Speech and Audio Processing* paper described a hybrid mono/stereo acoustic echo canceler.[8](https://doi.org/10.1002/047134608x.w6703) According to *Echo Cancellation for Speech Signals* (Wiley Encyclopedia of Telecommunications), he co-authored a chapter covering line echoes, adaptive cancellation, and single-channel and multichannel acoustic echo cancellation.[8](https://doi.org/10.1002/047134608x.w6703) He also wrote two retrospective accounts of the field and of his own career in *IEEE Signal Processing Magazine* in 2006, including "The History of Echo Cancellation", which opens the history on 10 July 1962 with the Telstar launch.[5](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.706.5784) • [10](https://doi.org/10.1109/msp.2006.1657810)
## How adaptive echo cancellation works and why it mattered
Before the canceller, telephone networks controlled echoes with suppressors, which interrupt the return path; that becomes harmful when conversation is simultaneous. Sondhi's device instead synthesizes a replica of the echo path response, convolves the incoming signal with the replica, and subtracts the resulting echo estimate from the echo path output, without interrupting transmission.[11](https://doi.org/10.1109/proc.1980.11774) According to its authors in the Bell System Technical Journal, the 1967 paper reported simulated cancellation of about 20 dB with a convergence time of roughly 0.2 to 0.5 second for average speech levels; convergence slows tenfold for a 20-dB decrease in signal level, and the system needs an initial echo-only interval, as short as 0.5 second for loud input, to converge.[4](https://doi.org/10.1002/j.1538-7305.1967.tb04231.x)
The motivation was delay. The introduction of transmission delays in excess of 300 ms over satellite circuits forced a general reexamination of suppression methods and led directly to the development of the echo canceller.[11](https://doi.org/10.1109/proc.1980.11774) By 1980 a VLSI canceller chip promised inexpensive implementation, and worldwide tests had shown the effectiveness of cancellation for satellite transmission;[11](https://doi.org/10.1109/proc.1980.11774) the 1984 review recorded that after two decades in the research lab the technique was finding widespread practical application in voice and full-duplex data transmission.[12](https://people.eecs.berkeley.edu/~messer/PAPERS/IEEE/Mar84-1.pdf) Sondhi wrote that one of his Bell Labs contributions yielded considerable profits to AT&T.[1](https://doi.org/10.1121/1.4782771)
## Honors and recognition
Sondhi received the IEEE Sumner Prize for his work on echo cancellation and was elected a Bell Labs Fellow in 1999.[2](https://ieee-jp.org/section/kansai/activity/meeting_020.html) He authored or co-authored about 120 papers and 10 patents, co-edited a book on speech signal processing, co-authored a book on echo cancellation, served as associate editor of *IEEE Transactions on Acoustics, Speech, and Signal Processing*, and served two years as a distinguished lecturer of the ASSP society.[2](https://ieee-jp.org/section/kansai/activity/meeting_020.html)
## References
1. [On my association with Bell Labs, Journal of the Acoustical Society of America](https://doi.org/10.1121/1.4782771)
2. [IEEE Kansai Section lecture announcement with biography of Dr Man Mohan Sondhi](https://ieee-jp.org/section/kansai/activity/meeting_020.html)
3. [Sondhi, M. Mohan, 1933-, Library of Congress Name Authority File](https://id.loc.gov/authorities/names/n91063913.html)
4. [An Adaptive Echo Canceller, Bell System Technical Journal, 1967](https://doi.org/10.1002/j.1538-7305.1967.tb04231.x)
5. [The History of Echo Cancellation, IEEE Signal Processing Magazine, September 2006](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.706.5784)
6. [A Self-Adaptive Echo Canceller, Bell System Technical Journal, 1966](https://doi.org/10.1002/j.1538-7305.1966.tb02442.x)
7. [Theory and Computer Simulation of a Self-Adapting Echo Canceler, Journal of the Acoustical Society of America](https://doi.org/10.1121/1.1943065)
8. [Echo Cancellation for Speech Signals, Wiley Encyclopedia of Telecommunications](https://doi.org/10.1002/047134608x.w6703)
9. [Adaptive Echo Cancelation for Voice Signals, Springer Handbook of Speech Processing, 2007](https://doi.org/10.1007/978-3-540-49127-9_45)
10. [Reflections on my career at Bell Labs, IEEE Signal Processing Magazine, July 2006](https://doi.org/10.1109/msp.2006.1657810)
11. [Silencing echoes on the telephone network, Proceedings of the IEEE, 1980](https://doi.org/10.1109/proc.1980.11774)
12. [Echo Cancellation, tutorial review, 1984](https://people.eecs.berkeley.edu/~messer/PAPERS/IEEE/Mar84-1.pdf)
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