Continuous Variable Slope Delta Modulation (CVSD) is a type of delta modulation in which the step size of the approximated signal is progressively increased or decreased.
Increasing or decreasing the step size serves to better adapt the approximated signal to the analog input signal. During the conversion, only the positive or negative changes compared to the previous value are indicated by a bit.
CVSD modulation is used for speech coding, for example in Bluetooth, and works with sampling rates of 32 kHz. There are also implementations that work with a lower sampling rate.