Multi-channel echo canceler and method using convolution of two training signals

DETAILED DESCRIPTION OF THE DRAWINGS In the preferred embodiments, the present invention provides, among other things, an echo canceler and method that uses specialized training signals to determine the reflective characteristics of an interface once a connection has been established.
The present invention also provides an apparatus and method that suppresses the side tone or echo in channels caused by reflections at the interface between two-wire and four-wire communication systems.
Echoes caused by long delays, (for example, delays greater than 64 milliseconds) are suppressed at a lower cost compared to current methods.
In one preferred embodiment of the present invention, the apparatus and method employ training signals prior to voice communication to estimate the reflected signals.
An inverse of the reflected training signal is generated using an adaptive filter and combined with the reflected training signal to cancel the reflected training signal.
Least-means-square (LMS) calculations are performed on the error signal to determine filter coefficients for the adaptive filter.
Once the filter coefficients are determined, the signal path is switched to the normal path and voice communication may begin.
During the subsequent voice communication, the filter coefficients remain the same.
In one of the preferred embodiments of the present invention, the training signals are a convolution of an impulse function with sin(x)/x function.
FIG.
1 is a simplified block diagram illustrating an echo canceler 10 in accordance with a preferred embodiment of the present invention.
FIG.
1 also illustrates an interface with a telephony system whereby a four-wire loop is converted to a two-wire line through hybrid 12.
Hybrid 12 provides the interface between the two types of communication systems and is not part of echo canceler 10.
Typical two-wire systems include the public switched telephone network (PSTN) while typical four-wire systems include mobile and cellular telephone systems.
In general, two-wire systems refer to communication systems that use one communication channel for both transmitting and receiving, while four-wire systems use separate channels for the transmit and receive paths