The Free On-line Dictionary of Computing (30 December 2018):

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.

   [The Audio File Formats FAQ]

   (1995-02-21)