Robust adaptive filter for echo cancellation
An adaptive filter is programmed with an algorithm based on a normalized Least Mean Squares (nLMS) algorithm that adapts each sample time. The algorithm is modified to be more efficient in a variety of DSPs by computing multiple errors, one per sample, before updating coefficients. The update equation utilizes the multiple errors to achieve adaptation at a similar performance to known nLMS algorithms that adapt each sample time but without the instability that is observed in low echo-to-near-end-noise ratio (ENR) input conditions. Varying the relaxation step size prevents divergence. The DSP utilizes either one or more MAC units.
1. In a telephone including an audio frequency circuit having a transmit channel, a receive channel, and at least one echo canceling circuit coupled between said channels, the improvement comprising:
an adaptive filter in said echo canceling circuit; and
a coefficient update circuit coupled to said adaptive filter for modifying the coefficients in said adaptive filter in steps in response to an error signal and in accordance with a multiple error per sample over plural samples, least mean squares algorithm for reducing said error signal.
2. The telephone as set forth in claim 1 wherein the step size of said samples decreases near convergence.
3. The telephone as set forth in claim 1 wherein said algorithm is a normalized least mean squares algorithm.
4. The telephone as set forth in claim 1 wherein said audio frequency circuit further includes a control circuit for interrupting adaptation of said filter during double talk conditions.
5. The telephone as set forth in claim 1 wherein said algorithm requires 7.1 to 10.2 MIPS to perform the adaptive filter and coefficient update for a single tap.
6. A method for reducing echo in a telephone, said method comprising the steps of:
filtering a first signal with a filter having adaptive coefficients;
detecting an error signal based on a difference between the filtered first signal and a second signal; and
modifying the adaptive coefficients in steps in response to the error signal and in accordance with a multiple error per sample over plural samples, least mean squares algorithm.
7. The method as set forth in claim 6 and further comprising the steps of:
monitoring signals within said telephone to detect double talk; and
interrupting said modification step in response to a detection of double talk.
8. The method as set forth in claim 6 and further comprising the steps of:
monitoring signals within said telephone to detect double talk; and
delaying said modification step in response to a detection of double talk.
9. The method as set forth claim 6 wherein the first signal is from a microphone and the second signal is output to a speaker, whereby said method reduces acoustic echo in said telephone.
10. The method as set forth claim 6 wherein the first signal is a line input signal and the second signal is a line output signal, whereby said method reduces line echo in said telephone.