Relaxed look-ahead pipelined LMS adaptive filters and their application to ADPCM coder

Relaxed look-ahead is presented as an attractive technique for pipelining adaptive filters. Unlike conventional look-ahead, relaxed look-ahead does not attempt to maintain the input-output (I/O) mapping between the serial and pipelined architectures but preserves the adaptation characteristics, resulting in a small hardware overhead. Relaxed look-ahead is employed to develop fine-grained pipelined architectures for least-mean-square (LMS) adaptive filtering. The proposed architecture achieves the desired speed-up, with little or no degradation in the convergence behavior. Simulation results verifying the convergence analysis results for the pipelined LMS filter are presented. The filter is then employed to develop a high-speed adaptive differential pulse-code-modulation (ADPCM) codec. The new architecture has a negligible hardware overhead which is independent of the number of quantizer levels, the predictor order and the pipelining level. Additionally, the pipelined codec has a much lower output latency than the level of pipelining. Simulations with image data indicate that speed-ups of up to 43 can be achieved with less than 1-dB loss in SNR