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CVSD
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


Delta modulation
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 superceeded by improved versions like CVSD and ΔΣ.

Delta Modulation on this website
Spendex-10 tactical speech encryptor with Delta Modulation
CVSD
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].

CVSD on this website
KY-57 (VINSON) Wide-band Voice and Data Encryption Unit
Spendex 50 (DBT), military secure crypto phone
PFX-PM portable radio with digital encryption
KY-99 (MINTERM) Narrow-band Voice and Data Terminal
Digital Subscriber Voice Terminal
Secure Terminal Equipment
STE
Motorola Saber II secure portable radio
BID/250 (Lamberton) SAVILLE-based voice encryption unit for Clansman DMU
Racal PRM-4515 Cougar handheld radio
Racal Cougar PRM-4735 body-wearable covert radio with voice encryption
Thales PRM-5120 Cougar covert radio with voice encryption
Digital voice encryptor
Gretacoder 103, telephone scrambler
Gretacoder 104, voice scrambler
Telsy TDS-2004 telephone voice encryptor
Telsy TDS-2004M mobile voice encryptor
RT-1439 right angle view
Terma ET-10, Digital Tactical Terminal with secure voice and data capability
VINSON family of digital wideband voice encryptors
ANDVT family of digita; narrowband/wideband voice encryptors
Indictor embeddable encryption module
Examples
System/family Example Modulation Rate (kb/s) Manufacturer/user
RT-3600 Spendex 10 DM 9.6 Philips
SECURENET Saber CVSD 12 Motorola
VINSON KY-57 CVSD 16 US/ NATO
ANDVT KY-99 CVSD, LPC 16 US Military
DSVT KY-68 CVSD 16/32 US Military
Bluetooth - CVSD, PCM 64 -
Literature
  1. Code modulation with digitally controlled companding for speech transmission
    J.A. Greefkes and K. Riemens. Philips Technical Review, Volume 31, 1970, No. 11/12.
References
  1. Wikipedia, Continuously variable slope delta modulation
    Visited 8 May 2024.

  2. J.A. Greefkes and K. Riemens
    Code modulation with digitally controlled companding for speech transmission

    Philips Technical Review, Volume 31, 1970, No. 11/12.
Further information
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Crypto Museum. Created: Wednesday 08 May 2024. Last changed: Thursday, 09 May 2024 - 06:52 CET.
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