Editing Continuously variable slope delta modulation (section)

{{Redirect|CVSD|other uses of this abbreviation|Central Valley School District (disambiguation)|and|Chino Valley School District (disambiguation)}}
'''Continuously variable slope delta modulation''' ('''CVSD''' or '''CVSDM''') is a [[Speech coding|voice coding]] method. It is a [[delta modulation]] with variable step size (i.e., special case of [[adaptive DPCM|adaptive delta modulation]]), first proposed by Greefkes and Riemens in 1970.

CVSD encodes at 1 bit per sample, so that audio sampled at 16&nbsp;kHz is encoded at 16&nbsp;kbit/s.

The encoder maintains a reference sample and a step size.  Each input sample is compared to the reference sample.  If the input sample is larger, the encoder emits a ''1'' bit and adds the step size to the reference sample.  If the input sample is smaller, the encoder emits a ''0'' bit and subtracts the step size from the reference sample.  The encoder also keeps the previous ''N'' bits of output (''N'' = 3 or ''N'' = 4 are very common) to determine adjustments to the step size; if the previous ''N'' bits are all 1s or 0s, the step size is increased.  Otherwise, the step size is decreased (usually in an exponential manner, with <math>\tau</math> being in the range of 5&nbsp;ms).  The step size is adjusted for every input sample processed.

To allow for bit errors to fade out and to allow (re)synchronization to an ongoing bitstream, the output register (which keeps the reference sample) is normally realized as a [[leaky integrator]] with a time constant (<math>\tau</math>) of about 1 ms.

The decoder reverses this process, starting with the reference sample, and adding or subtracting the step size according to the bit stream.  The sequence of adjusted reference samples are the reconstructed waveform, and the step size is adjusted according to the same all-1s-or-0s logic as in the encoder.

Adaptation of step size allows one to avoid slope overload (step of [[quantization (signal processing)|quantization]] increases when the signal rapidly changes) and decreases [[quantization error|granular noise]] when the signal is constant (decrease of step of quantisation).

CVSD is sometimes called a compromise between simplicity, low bitrate, and quality. Common bitrates are 9.6–128&nbsp;kbit/s.

Like other delta-modulation techniques, the output of the decoder [[lossy compression|does not exactly match]] the original input to the encoder.