Issues in the design of low oversampling ratio single bit sigma-delta modulators

The linear delta modulator is shown to be a special instance of the more general sigma-delta modulator, in which the signal and noise transfer functions are equal. High order sigma-delta systems may be designed which retain the single pole -6 dB/octave overloading characteristics, but with the advantages of higher order noise shaping. Techniques for adaptive step size, both instantaneous and syllabic may be adopted to give companding operation. With slight damping of the loop integrators, the cutoff phenomenon may be used to give a noise gating effect. These techniques have been used for a design study for the integration of a switched capacitor speech coder/decoder with a bit rate of 32 to 64 kbits/s. The resulting system gives a significant performance improvement with similar complexity compared with a linear pulse code (PCM) approach using the same bit rate. In this application, there is no requirement for digital decimators and interpolators to process the digital data