Relaxed Look-Ahead Pipelined LMS Adaptive Filters and Their Application to ADPCM Coder

The relaxed look-ahead technique is presented as an attractive technique for pipelining adaptive filters. Unlike conventional look-ahead, the relaxed look-ahead does not attempt to maintain the input-output mapping between the serial and pipelined architectures but preserves the adaptation characteristics. The use of this technique results in a small hardware overhead which would not be possible with conventional look-ahead. The relaxed look-ahead is employed to develop finegrained pipelined architectures for least mean-squared (LMS) adaptive filtering. Convergence analysis results are presented for the pipelined architecture. The proposed architecture achieves the desired speed-up with marginal or no degradation in the convergence behavior. Past work in pipelined transversal LMS filtering are shown to be special cases of this architecture. Simulation results verifying the convergence analysis results for the pipelined LMS filter are presented. The pipelined LMS 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. Theoretical analysis indicates that the output signal-to-noise ratio (SNR) is degraded with increase in speed-up. Simulations with image data indicate that speed-ups of up to 44 can be achieved with less than 1 dB loss in SNR.