DPCM - Differential Pulse Code Modulation

By: Preethi Ramkumar

Meaning of DPCM – “Differential Pulse Code Modulation”, is a modulation technique invented by the British Alec Reeves in 1937. It is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals. Every sample is quantized to a series of symbols in a digital code, which is usually a binary code. PCM is used in digital telephone systems. It is also the standard form for digital audio in computers and various compact disc formats. Several PCM streams may be multiplexed into a larger aggregate data stream. This technique is called Time-Division Multiplexing, or (TDM). TDM was invented by the telephone industry, but today the technique is an integral part of many digital audio workstations such as Pro Tools.In conventional PCM, the analog signal may be processed (e.g. by amplitude compression) before being digitized. Once the signal is digitized, the PCM signal is not subjected to further processing (e.g. digital data compression). Some forms of PCM combine signal processing with coding. Older versions of these systems applied the processing in the analog domain as part of the A/D process, newer implementations do so in the digital domain. These simple techniques have been largely rendered obsolete by modern transform-based signal compression techniques.

Differential (or Delta) pulse-code modulation encodes the PCM values as differences between the current and the previous value. For audio this type of encoding reduces the number of bits required per sample by about 25% compared to PCM. Adaptive DPCM is a variant of DPCM that varies the size of the quantization step, to allow further reduction of the required bandwidth for a given signal-to-noise ratio.

Other Related Definitions:

“…Differential pulse-code modulation (DPCM): Pulse-code modulation in which an analog signal is sampled and the difference between the actual value of each sample and its predicted value, derived from the previous sample or samples, is quantized and converted, by encoding, to a digital signal. ” [Alliance for Telecommunications Industry Solutions (ATIS)]

“…At the time of the PCM process, the differences between input sample signals are minimal. Differential PCM (DPCM) is designed to calculate this difference and then transmit this small difference signal instead of the entire input sample signal. Since the difference between input samples is less than an entire input sample, the number of bits required for transmission is reduced. This allows for a reduction in the throughput required to transmit voice signals. Using DPCM can reduce the bit rate of voice transmission down to 48 kbps.” [Cisco]

“…PCM (Pulse Code Modulation) permits adjacent samples to be described from extreme opposite ends of the sample range. As this situation rarely occurs it should be able to convey the baseband data using fewer bits. DPCM allows this to be achieved by describing only the changes between the samples.” [Mpirical]

“…A relatively simple solution is to encode the differences between successive samples rather than the samples themselves. The resulting technique is called differential pulse code modulation (DPCM). Since differences between samples are expected to be smaller than the actual sampled amplitudes, fewer bits are requrired to represent the differences.” [Electrical & Computer Engineering (ECE) Department]

“…Differential pulse code modulation (DPCM) is a procedure of converting an analog into a digital signal in which an analog signal is sampled and then the difference between the actual sample value and its predicted value (predicted value is based on previous sample or samples) is quantized and then encoded forming a digital value.” [Informatik]

Related Links:

Mathworks - DPCM Terminology
SpeechAnalysis - Differential Pulse Code Modulation (DPCM)
Cisco - Differential Pulse Code Modulation
GTAV - Differential Pulse Code Modulation (DPCM)
Drexel - How to use Differential Pulse Code Modulation files?
Everything2 - Differential Pulse Code Modulation
Informatik - What is DPCM?
Technologyforall - How do we convert analog voice into digital form?
Sethares - Stochastic Analysis of the Modulator and Differential Pulse Code Modulator
Telecom - Speech Coder Types
Cornell - ECE 476 Cornell University
Ict - VcDemo Compression Modules
Temple - General Overview

Technical Resources:

Cmlab - Differential Pulse Code Modulation (DPCM)

Products and Solutions:

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Innovative Technologies in Education
Mpirical
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Blogs, News, feeds…

http://www.garfield.library.upenn.edu/classics1982/A1982NC82300001.pdf
http://www.apl.jhu.edu/Notes/Geckle/525759/lecture4.pdf

Books:

Differential pulse code modulation of broadcast-rate television signals by Theodore Michael Lyszczarz
Adaptive differential pulse code modulation (ADPCM) with the DSP56001 digital signal processor by Jinfeng Wang
Pulse Code Modulation Systems Design (Artech House Telecommunications Library) by William N. Waggener
An analysis of leaky integrator differential PCM systems with stationary first order Gauss Markov input by Ethirajan Janardhanan

See Also:

DPCM Resources

 

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