Computer Dictionary of Information Technology

A-law

The ITU-T standard for nonuniform quantising logarithmic compression.

The equation for A-law is

      |    A
      | ------- (m/mp)                   |m/mp| =< 1/A
      | 1+ln A
  y = |
      | sgn(m)
      | ------ (1 + ln A|m/mp|)   1/A =< |m/mp| =< 1
      | 1+ln A

Values of u=100 and 255, A=87.6, mp is the Peak message value, m is the current quantised message value. (The formulae get simpler if you substitute x for m/mp and sgn(x) for sgn(m); then -1 <= x <= 1.)

Converting from u-LAW to A-LAW introduces quantising errors. u-law is used in North America and Japan, and A-law is used in Europe and the rest of the world and international routes.

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