G721

G.721 describes a method for compressing 64 kbit/s voice signals to 32 kbit/s.

Known as Adaptive Delta Pulse CodeModulation( ADPCM), the method is based on the difference between adjacent signals and the prediction of the next following signal.

The G.721 speech codec achieves half the data rate ofG.711 at a resolution of 4 bits at 32 kbit/s, with hardly any noticeable loss in quality. The speech quality has a mean opinion score( MOS) of 4.2 in contrast to 4.4 for G.711.