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Correct tracking between the transmitter and receiver is difficult to maintain when a long integrator time constant is used. We describe a differential quantizer which has a digital integrator; this integrator enables perfect tracking to be achieved at the output of the integrator without any adjustments. The differential quantizer gives high quality pictures when seven, eight, and nine quantizer output levels are used. We present a scheme for transmitting the nine-level signal at the rate of three bits per picture element. Picture quality is improved significantly by adding low-amplitude dither patterns to the input signal to mask contours. The coder is more susceptible to transmission errors than coders having an analogue integrator with a short time constant; we discuss two methods for reducing the susceptibility.
Limb et al. (Mon,) studied this question.