Continuously variable slope delta modulation
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Continuously variable slope delta modulation is a voice coding method. It is delta modulation with variable step size.
Adaptation of step size allows one to avoid slope slope overload (step of quantisation increases when the signal rapidly changes) and decreases granular noise when the signal is constant (decrease of step of quantisation). Change of step of quantisation in the coder depends on the previous N output bits of the decoder (N = 3 or N = 4 are very common).
CVSD is sometimes called a compromise between simplicity/low bitrate/quality. Bitrates are 9.6 to 128 kbit/s.