Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-04-06
2001-01-30
Pham, Chi H. (Department: 2731)
Pulse or digital communications
Receivers
Interference or noise reduction
C370S286000, C379S413020, C381S071100
Reexamination Certificate
active
06181753
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an echo
oise canceler for removing echo and noise components from an input signal.
Echo cancelers and noise cancelers are used in voice communication terminals such as videoconferencing terminals, automobile-mounted hands-free telephone sets, and portable telephone sets. Unexamined Japanese Patent Application No. 8789/1996 describes a voice communication system in which a noise canceler removes noise components from the output of an echo canceler, and the output of the noise canceler is used in adjusting adaptive filter coefficients in the echo canceler. The reason for this arrangement is that use of the output of the noise canceler in adjusting the adaptive filter coefficients enables the residual echo to be reduced to a lower level. In a variation of this arrangement, the output of the echo canceler, which is the input of the noise canceler, is used to adjust the adaptive filter coefficients until the residual echo has been reduced to the local background noise level, then the output of the noise canceler is used to achieve a further reduction.
A problem in this arrangement is the processing delay of the noise canceler. Most recent noise cancelers divide the input signal into frames and process one frame at a time. Noise cancelers employing spectral subtraction, for example, operate in this way. Spectral subtraction and other frame-based noise cancellation methods have the advantage of high accuracy, but they generate an unavoidable processing delay equal to or greater than the frame length.
In the arrangement described above, a long processing delay in the noise canceler can prevent the adaptive filter coefficients in the echo canceler from converging, or can cause the coefficient values to oscillate. Further details will be given below.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an echo
oise canceler that compensates for the processing delay in the noise canceler.
Another object of the invention is to achieve a high level of echo reduction.
Yet another object is to save power.
The invented echo
oise canceler comprises an echo canceler and a noise canceler. The echo canceler has an adaptive filter that uses a plurality of coefficients to generate an echo replica from a plurality of recent sample values of a received signal. The echo replica is subtracted from a local input signal to cancel an echo of the received signal, generating a first residual signal. The noise canceler cancels local background noise in the first residual signal, generating a second residual signal having a certain processing delay with respect to the first residual signal.
The adaptive filter has a sample register that stores the above-mentioned recent sample values of the received signal, a delayed sample register that stores a plurality of older sample values of the received signal, and a coefficient adjuster that adjusts the above-mentioned coefficients on the basis of the older sample values and the second residual signal.
The echo canceler preferably has a detector that, when the level of local background noise is low, switches off the noise canceler and causes the coefficient adjuster to use the first residual signal and the recent sample values in adjusting the coefficients.
The delay between the sample values stored in the sample register and the sample values stored in the delayed sample register compensates for the processing delay of the noise canceler, so that the coefficient adjustment is carried out consistently. Use of the second residual signal then leads to a high level of echo reduction.
Switching off the noise canceler when the local background noise level is low saves power.
REFERENCES:
patent: 5315585 (1994-05-01), Iizuka et al.
patent: 5631899 (1997-05-01), Duttweiler
patent: 5638311 (1997-06-01), Fujii et al.
patent: 5663955 (1997-09-01), Iyengar
patent: 5796819 (1998-08-01), Romesburg
patent: 5859907 (1999-01-01), Kawahara et al.
patent: 5937060 (1999-08-01), Oh
patent: 8-8789 (1996-01-01), None
Ariyama Yoshihiro
Takada Masashi
Frank Robert J.
OKI Electric Industry Co., Ltd.
Pham Chi H.
Sartori Michael A.
Tran Khai
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