Multiplex communications – Duplex – Transmit/receive interaction control
Patent
1995-05-31
1997-05-20
Hsu, Alpus H.
Multiplex communications
Duplex
Transmit/receive interaction control
379411, H04B 323
Patent
active
056318995
ABSTRACT:
Acoustic echos are canceled by employing a first echo canceler having a comparatively short first impulse response synthesis capability which is connected between a transmit path and receive path for generating a first error signal and for canceling echo signals in the transmit path, and at least a second echo canceler having a comparatively long second impulse response synthesis capability connected in parallel with the first echo canceler between the transmit and the receive path. The second echo canceler is supplied with the first error signal from the first echo canceler and is adaptively operating simultaneously with but independent of the first echo canceler to further cancel echos in the transmit path. More specifically, the first echo canceler is intended to capture the direct path acoustic echo and any early arriving echos that are stable which are not time yawing. Since the direct path acoustic echo and the stable early arriving echos are not time varying, the adaptive adaptation rate, i.e. loop gain, of the first echo canceler adaptation circuitry can be chosen to be a very small value so that it can adapt accurately in the presence of undetected near-end speech. Then, with the direct path acoustic echo and any stable early arriving echos having been substantially removed by the first echo canceler, the first error signal output from the first echo canceler has a comparatively weaker echo component to corrupt any near-end speech that might be present and, consequently, the near-end speech can be more reliably detected by monitoring the first error signal from the first echo canceler. Since the near-end speech can be more reliably detected by employing the first error signal from the first echo canceler, the adaptation rate of the second echo canceler can be chosen to be at a significantly faster rate without undo impairment caused by undetected near-end speech. In a specific embodiment of the invention, the first echo canceler employs a transversal filter having predetermined fixed coefficients in order to cancel the direct path acoustic echo component and any stable early arriving echos.
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Hsu Alpus H.
Lucent Technologies - Inc.
Rao Seema S.
Stafford Thomas
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