Telephonic communications – Echo cancellation or suppression – Using digital signal processing
Reexamination Certificate
1999-10-14
2003-06-17
Isen, Forester W. (Department: 2644)
Telephonic communications
Echo cancellation or suppression
Using digital signal processing
C379S406090, C379S406080, C379S406060, C379S406010, C379S406120, C379S406130, C379S406020
Reexamination Certificate
active
06580795
ABSTRACT:
CROSS REFERENCE TO RELATED, COPENDING APPLICATIONS
Related subject matter is found in the following copending patent applications:
1. Application Ser. No. 09/129,076, entitled “Method and Device for Detecting Near-End Voice,” invented by Tom Hong Li and Michael McLaughlin, filed Aug. 4, 1998 and assigned to the assignee hereof.
2. Application Ser. No. 09/282,914, entitled “Method and Apparatus for Controlling a Full-Duplex Communication System,” invented by John Eugene Lane, Patrick K. Kelly, Robert V. Holland, and Garth D. Hillman, filed Mar. 31, 1999 and assigned to the assignee hereof.
FIELD OF THE INVENTION
This invention relates in general to signal processing systems, and more particularly to echo cancellers for full-duplex communication systems.
BACKGROUND OF THE INVENTION
Recent advances in signal processing technology have allowed the development of new products. One product is the full-duplex speakerphone. Prior technology only allowed half-duplex operation because the proximity between the loudspeaker and the microphone caused positive feedback and echo. Half-duplex speakerphones are annoying to users because the speakerphone output is muted while the speaker is talking. The party at the other end is unable to interrupt the conversation until the speaker is quiet for a given length of time.
However, signal processing technology is able to measure room acoustics and automatically cancel echo thereby generated. The signal processor typically uses an adaptive finite impulse response (AFIR) filter whose coefficients are weighted in accordance with the room acoustics. Each AFIR filter coefficient is multiplied by an audio input signal sample which is delayed by a predetermined number of samples from the current input signal sample. For example, if the room causes an echo 50 milliseconds (ms.) after an input signal, samples with time delay up to 50 milliseconds (ms.) are required for the filter and the AFIR filter coefficients are adapted iteratively to cancel this echo. Thus, the signal processor is able to cancel the echo.
Echo cancellers have become more sophisticated over time. For example a simple echo canceller may be replaced by a set of so-called sub-band echo cancellers. Sub-band echo cancellers separate the input signal into smaller frequency bands and perform echo cancellation separately on each of the bands. Associated with each frequency band is a separate FIR filter. While a sub-band echo canceller provides much better cancellation, it also requires more processing.
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Lane John E.
Li Zhao
Chiu Joanna G.
Isen Forester W.
Singh Ramnandan
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