Single bit sigma-delta modulator with nonlinear quantization for µ-law coding

This paper presents a 1st order sigma-delta-modulator with µ-law coding for dynamic compression of audio signals. The nonlinearity is located in the feedback loop of the modulator and does not require an internal multibit A/D converter. The modulator requires just one opamp, one comparator, and a minimum of digital logic to generate a four bit feedback and to ensure a stable operation. The µ-law-function for µ=255 is approximated by a standardised piecewise linear 16 segment function which is commonly used in telecommunication. The chip has been designed and fabricated in a standard 1.5µm n-well CMOS process, occupies a die area of 2 mm², and has been successfully tested.