An Echo Canceller with Reduced Arithmetic Precision

Abstract

It is well known that implementation of adaptive digital filters for echo cancellation in full-duplex transmission over telephone lines requires a large number of bits for the tap representation. This is due to the large dynamic ranges of the echo and far-end signal. In this paper, we introduce a new echo canceller: the "controlled gain echo canceller" (CGEC) which uses an adaptive gain control at the output of the classical echo canceller (CEC). A feedback loop permits approximate regulation of the front-edge CEC output power at a nominal level, independently of the echo and far-end signal levels. By this means, the precision required for adaptation is reduced to a minimum value. The analysis of adaptation, convergence, residual echo power, and computational complexity is given for the CGEC and compared to the similar quantities in a CEC; computer simulation results are presented. As an example, a 64 taps CGEC with only 16 bits instead of 20 can achieve secure binary data transmission (with bit error rate less than10 ^{-6}) for a far-end signal-to-noise ratio of 16 dB and for an echo to far-end signal ratio of 20 dB, independently of the echo and far-end signal powers.