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CODEC
Continuous Variable Slope Delta modulation
- this page is a stub
Continuously Variable Slope Delta Modulation, commonly abbreviated
CVSD or CVSDM, is an improved form of Delta Modulation,
invented in 1970 by J.A. Greefkes and K. Riemens at
Philips Research Laboratories in The Netherlands.
Like Delta Modulation it encodes at 1 bit per sample, but has a variable
step size. It is a special case of adaptive delta modulation [1].
It is used in many professional and military products, with sample rates
in the range of 9.6 to 128 kb/s
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Delta Modulation (DM) is a single-bit modulation technique which produces a
bit-stream (i.e. a stream of '0' and '1' values). The encoder maintains
a reference value and a step size. The analog input signal (2) is
sampled at regular intervals, e.g. 10,000 times per second, under
control of a clock (1). Each new sample is compared to the reference value.
If it is higher, the output bit will be '1' and the step will be added to the
referency value. If it is lower, the output bit will be '0' and the step size
will be subtracted from the reference value. This results in a stream of 1s
and 0s (4).
The bit-rate is equal to the sampling rate, typically between 9,600 and
16,000 bps.
The output can be used to reconstruct (an approximation of) the original
signal (3). It will never match the original signal (2) exactly,
but the higher the sample rate, the better the approximation and the lower
the distortion of the signal.
Delta Modulation (DM) requires no synchronization bits at all and is less
sensitive to transmission errors than PCM. Error rates of up to 5% still
produce a reasonable level of intelligibility, whilst a 1% error rate is
enough to put PCM out of business.
In practice, Delta Modulation has been superseded by improved versions
like CVSD and ΔΣ.
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Delta Modulation on this website
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With Delta Modulation (DM), the encoder maintains a reference value and
a step size. Each input sample is compared to the reference sample. If
it is larger, the encoder produces a '1' bit and adds the step size to the
reference sample. If the input sample is smaller than the reference sample,
the encoder produces a '0' bit and subtracts the step size from the
reference sample.
Continuously Variable Slope Delta modulation (CVSD) is an improved
version of Delta Modulation in which the step size is
continuously variable rather than fixed. It is based on the values of the
previous N output bits, in which N is usually 3 or 4. If the previous
N bits are all 1s or 0s, the step size is increased. Otherwise it is decreased.
In addition, the output register — which keps the reference value — is
implemented as a leaky integrator with a time constant (τ) of approx.
1 ms, to allow for bit errors to fade out and to allow (re)synchronisation
to an ongoing bit stream [1].
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© Crypto Museum. Created: Wednesday 08 May 2024. Last changed: Friday, 30 May 2025 - 13:02 CET.
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