ZDO Corporation

Differential Pulse Code Modulation (DPCM) encodes PCM values as differences between the current and previous values. In other words, it reduces the number of bits required.

Adaptive Differential Pulse Code Modulation (ADPCM) varies the size of quantization step to allow further reduction.

M-law Algorithm

Mostly used in North America, Japan.

F(x) = sgn (ln(1 - M|x|) / ln(1 + M)), -1 <= x <= 1

M = 255 (8 bits)

Used because speech has a wide dynamic range that does not lend itself well to efficient linear digital encoding.

A-law Algorithm

Used in Europe and most part of the world.

x <= A            F(x) =  (Ax) / (1 + ln A)

1/A <= x <= 1  F(x) = (1 + ln Ax) / (1 + ln A)

A-law takes a 12 or 16 bit audio and convert it to a 8 bit value.