Clipping detector for echo cancellation
An adaptive filter in an echo canceling circuit includes a clipping detector that affects the coefficients of the adaptive filter when clipping is detected. Clipping is defined as at least two successive maxima or minima in a predetermined period. The coefficients are either locked or the echo estimate is reduced to prevent overcorrection. In accordance with another aspect of the invention, lower order bits are ignored to avoid problems of bit resolution when comparing signals.
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 circuit coupled to said adaptive filter for changing coefficients in said adaptive filter, wherein said circuit includes a clipping detector.
2 . The telephone as set forth in claim 1 wherein said clipping detector includes a counter for counting consecutive maxima or minima of a signal, wherein the count in said counter affects the coefficients of said adaptive filter.
3 . The telephone as set forth in claim 2 where said count affects the coefficients by temporarily locking the coefficients.
4 . The telephone as set forth in claim 2 where said count affects the coefficients by setting the coefficients to a predetermined value.
5 . The telephone as set forth in claim 4 where said count affects the coefficients by temporarily locking the coefficients.
6 . The telephone as set forth in claim 2 wherein said signal is in digital form, including a number of binary bits, wherein at least the lowest order bit is ignored when evaluating said signal.
7 . The telephone as set forth in claim 2 wherein a count equal to or greater than two indicates clipping.
8 . The telephone as set forth in claim 1 wherein said clipping detector includes:
a first circuit for detecting the maximum amplitude of a signal;
a second circuit for detecting the minimum amplitude of a signal;
logic for combining data representing the minimum amplitude and the maximum amplitude to provide range data;
wherein said range data affects the coefficients of said adaptive filter.
9 . 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:
a clipping detector coupled to said echo canceling circuit, said clipping detector enabling the echo canceling circuit to operate on a signal outside the dynamic range of a channel.
10 . The telephone as set forth in claim 9 wherein said echo canceling circuit is implemented in software and the clipping detector enables the echo canceling circuit to operate on a signal clipped below the dynamic range allowed by the echo canceling algorithm.