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ADPCM

Description

Adaptive Differential Pulse Code Modulation (ADPCM)

Complete  Definition

Adaptive DPCM (ADPCM) is a  variant of DPCM which in  term is a  form of PCM, DPCM varies the size of  the quantization  step, to allow further reduction of the required bandwidth for  a given  signal-to-noise ratio.

Delta modulation (differential PCM), another variant, uses one bit per   sample.

In telephony, a standard audio signal for a single phone call is encoded as   8000 analog samples per second, of 8 bits each, giving a 64 kbit/s digital   signal known as DS0. The default signal compression encoding on a DS0 is either   μ-law (mu-law) PCM (North America and Japan) or a-law PCM (Europe and  most of the rest of the world).  These are logarithmic compression systems where  a 12 or 13 bit linear PCM sample number is mapped into an 8  bit value.  This system is described by international standard G.711. An  alternative proposal for a floating  point representation, with 5 bit mantissa  and 3 bit radix, was abandoned.

Where circuit costs are high and loss of voice quality is acceptable, it   sometimes makes sense to compress the voice signal even further. An ADPCM   algorithm is used to map a series of 8 bit u-law (or a-law) PCM samples into a series of 4 bit ADPCM  samples.  In this way, the capacity of the line is doubled. The technique is  detailed in  the G.726 standard.

Later it was found that even further compression was possible and additional   standards were published. Some of these international standards describe  systems  and ideas which are covered by privately owned patents and thus use of  these  standards requires payments to the patent holders.

Some ADPCM techniques are used in Voice over IP communications.

ADPCM  Wikipedia Definition

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‎06-17-2009 06:28 AM
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