Continuously variable slope delta modulation

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Continuously variable slope delta modulation (CVSD or CVSDM) is a voice coding method. It is a delta modulation with variable step size, first proposed by Greefkes and Riemens in 1970.

CVSD encodes at 1 bit per sample, so that audio sampled at 16kHz is compressed to 16kbit/s. It is a lossy codec; the output of the decoder does not match the input to the encoder.

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 1's or 0's, the step size is doubled. Otherwise, the step size is halved. The step size is adjusted for every input sample processed.

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 doubled or halved according to the same all-1's-or-0's logic as in the encoder.

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

CVSD is sometimes called a compromise between simplicity/low bitrate/quality. Bitrates are 9.6 to 128 kbit/s.

[edit] Applications

12kbit/s CVSD is used by Motorola's SECURENET line of digitally encrypted two-way radio products.

[edit] References

  • J. A. Greefkes and K. Riemens, "Code Modulation with Digitally Controlled Companding for Speech Transmission," Philips Tech. Rev., pp. 335-353, 1970.
  • N.S. Jayant, "Digital coding of speech waveforms: PCM, DPCM, and DM quantizers," Proc. IEEE, vol. 62, no. 5, pp. 61 1-632, May 1974.
  • R. Steele, Delta Modulation Systems, Pentech Press, London, England, 1975.
  • N. S. Jayant and P. Noll, Digital Coding of Waveforms: Principles and Applications to Speech and Video, Prentice-Hall, Englewood Cliffs, N. J., 1984.
  • A description of the algorithm, plus speech samples
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