from
The Free On-line Dictionary of Computing (8 July 2008)
A-law
<standard> 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)