- 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 (i.e. special case of adaptive delta modulation), first proposed by Greefkes and Riemens in 1970.
CVSD encodes at 1 bit per sample, so that audio sampled at 16kHz is encoded at 16kbit/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 τ being in the range of 5 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 leaky integrator with a time constant (τ) 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 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, and quality. Common bitrates are 9.6 kbit/s to 128 kbit/s.
Like other delta-modulation techniques, the output of the decoder does not exactly match the original input to the encoder.
Applications
12 kbit/s CVSD is used by Motorola's SECURENET line of digitally encrypted two-way radio products.
16 kbit/s CVSD is used by military digital telephones [DNVT, DSVT] for use in deployed areas to provide voice recognition quality audio.
64 kbit/s CVSD is one of the options to encode voice signals in telephony-related Bluetooth service profiles, e.g. between mobile phones and wireless headsets. The other options are PCM with logarithmic a-law or μ-law quantization.
Numerous arcade games, such as Sinistar and Smash TV, play pre-recorded speech through an HC-55516 CVSD decoder.[1][2]
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
- Specification of the Bluetooth System 2.0 + EDR, Core System Package, Part B "Baseband Specification", Section 9 "Audio", November 2004
Categories:- Audio codecs
- Speech codecs
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