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 AValues 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]