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A-law

Description

ITU-T companding standard used in the conversion  between analog and  digital signals in PCM systems. A-law is used primarily in  European telephone  networks and is similar to the North American mu-law standard.

Complete  Definition

An A-law algorithm is a regular companding algorithm. For  optimizing or  modifying the dynamic range of an analog signal for digitizing,  it is utilized  in European digital communications systems. It is fairly akin to  the mu-law  algorithm used in Japan and North America. The wide-ranging active  range of  speech does not lend itself well to proficient linear digital  encoding.

A-law algorithm encoding efficiently lessens the dynamic range of the  signal.  Thus, the coding effectiveness gets increased and results in a   signal-to-distortion ratio which is advanced to that obtained by linear  encoding  for a particular number of bits. A-law algorithm is an audio  compression scheme  defined by Consultative Committee for International  Telephony and Telegraphy or  CCITT G.711 which constrict  16-bit linear PCM data down to  8  bits of logarithmic data. By restricting the linear sample values to 12   magnitude bits, the A-law compression is defined by its equation. In the   equation, A is the compression parameter and x is the normalized integer to be   compressed. It is a linear estimation of logarithmic input/output relationship.   It is put into practice by using 8-bit code words. 8-bit code words permit for  a  bit rate of 64 kilobits per second.

By multiplying the sampling rate by the size of the code word, this is   calculated.A-law algorithm breaks a vibrant range into a total of 16 segments.   There are 8 positive and 8 negative segments. Each segment is twice the length   of the preceding one. Within each segment, homogeneous quantization is used.  For  coding the 8-bit word, 1st (MSB) identifies polarity. Then, bits 2, 3 and  4  recognize segment. The final 4 bits quantize the segment.A-law algorithm has   certain edges over the mu-law algorithm. The A-law algorithm provides a  somewhat  superior dynamic range than the mu-law algorithm, at the cost of  substandard  relative alteration for small signals. For an unvarying PCM, A-law  algorithm calls for 13-bits. For using  A-law algorithm, a worldwide link is  mandatory.The application of A-law  algorithm has been observed extensively


Both a-law and u-law are  companders for the G.711 voice  codec

A-law Compander formula (from Cisco's  Waveform Coding Techniques )

A-law-formula.gif

A-Law Wikipedia   Definition

Also useful is the : ITU  website and  search on a-law

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Version history
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Last update:
‎06-17-2009 06:17 AM
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