Usually sigma-delta modulators (/spl Sigma//spl Delta/M) are modelled by replacing the quantizer with an additive white noise source. This linear modelling leads to decoder structures realizing approximately a linear rectangular filter. However, due to the low resolution quantizer the /spl Sigma//spl Delta/M is an inherently nonlinear system. This might mean that a nonlinear analysis could yield more effective decoder structures including better linear filters. In this paper a new linear filtering approach for decoder design is proposed. It achieves a decoding performance superior to rectangular filtering.