An amplitude-controlled adaptive delta sigma modulator

An adaptive delta sigma modulator (ADSM) is proposed wherein the companding is done in a novel fashion. The output of a simple DEM is differentiated, and the resulting waveform is filtered in a syllabic filter after d.c. restoration. This yields a control signal that is proportional to the average number of transitions from one to zero and zero to one that occur at the coder output. The signal so obtained is inversely proportional to the input amplitude and can then be used to control the pulse height in the feedback path. At 40 kHz, the s.n.r. is greater than 21 dB over an input range of 30 dB with a peak s.n.r. of 24 dB (the input being a 800 Hz sinusoid). The s.n.r. is almost constant over the message band and improves at a rate of about 8 dB/octave increase in the clock rate.