Subband adaptive filtering for acoustic echo control using allpass polyphase IIR filterbanks

Adaptive filtering in subbands is an attractive alternative to full-band schemes in many applications because of the potential for faster convergence and lower computational cost. However, the analysis of a signal into a subband representation and the synthesis back into its original full-band form carries three main penalties. These are that (1) the subsampling process often introduces aliasing, (2) the subband analysis and synthesis processes carry a computational overhead, thereby reducing the gain in efficiency, and (3) the subband analysis and synthesis processes introduce delay into the signal path. A subband scheme is presented that aims to minimize these penalties, thereby allowing the potential advantages of the subband approach to be more fully realized. The scheme is based on infinite impulse response (IIR) filterbanks, formed from allpass polyphase filters, which exhibit very high quality filtering compared to typical finite impulse response (FIR) implementations, have relatively low complexity, introduce a limited degree of phase distortion and have low delay. The scheme, in conjunction with normalized least mean squares (NLMS) adaptive filters, is tested in an acoustic echo control application and shown to give better convergence, lower delay, and lower computational cost than a comparable FIR subband scheme.