Continuously variable slope delta modulation
Encyclopedia
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
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 (
) 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.
'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.
Speech coding
Speech coding is the application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic data compression algorithms to represent the resulting...
method. It is a delta modulation
Delta modulation
Delta modulation is an analog-to-digital and digital-to-analog signal conversion technique used for transmission of voice information where quality is not of primary importance. DM is the simplest form of differential pulse-code modulation where the difference between successive samples is encoded...
with variable step size (i.e. special case of adaptive delta modulation
Adaptive DPCM
Adaptive differential pulse-code modulation is a variant of differential pulse-code modulation that varies the size of the quantization step, to allow further reduction of the required bandwidth for a given signal-to-noise ratio....
), 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
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
Leaky integrator
In mathematics, a leaky integrator equation is a specific differential equation, used to describe a component or system that takes the integral of an input, but gradually leaks a small amount of input over time. It appears commonly in hydraulics, electronics, and neuroscience in single-neuron models...
with a time constant (
) 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
Quantization (signal processing)
Quantization, in mathematics and digital signal processing, is the process of mapping a large set of input values to a smaller set – such as rounding values to some unit of precision. A device or algorithmic function that performs quantization is called a quantizer. The error introduced by...
increases when the signal rapidly changes) and decreases granular noise
Quantization error
In analog-to-digital conversion, the difference between the actual analog value and quantized digital value is called quantization error or quantization distortion. This error is either due to rounding or truncation...
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 MotorolaMotorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
'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
Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...
service profiles, e.g. between mobile phones and wireless headsets. The other options are PCM with logarithmic a-law
A-law algorithm
An A-law algorithm is a standard companding algorithm, used in European digital communications systems to optimize, i.e., modify, the dynamic range of an analog signal for digitizing.It is similar to the μ-law algorithm used in North America and Japan....
or μ-law quantization.
Numerous arcade games, such as Sinistar
Sinistar
Sinistar is an arcade game released by Williams in 1982. It belongs to a class of video games from the 1980s called "twitch games". Other "twitch games" include Tempest, Defender, and Robotron: 2084. Sinistar was developed by Sam Dicker, Jack Haeger, Noah Falstein, RJ Mical and Richard Witt...
and Smash TV
Smash TV
Smash TV is a 1990 arcade game created by Eugene Jarvis and Mark Turmell for Williams. Home versions were developed for various platforms and most were published by Acclaim Entertainment.-Description:...
, play pre-recorded speech through an HC-55516 CVSD decoder.