An International Telecommunications Union (ITU-T) standard for for audio (speech) compression and decompression that is used in digital transmission systems, and in particular, used for the coding of analog signals into digital signals.
G.721 is an ITU-T standard codec that uses Adaptive Differential Pulse Code Modulation (ADPCM); a form of Pulse Code Modulation (PCM), to produce a digital signal with a lower bit rate than standard PCM. This ITU standard for speech codecs uses ADPCM on a 32 kbit/s channel. NOTE: G.721 was first introduced in 1984. In 1990 this standard was folded into G.726 along with G.723. (See also ADPCM).
See G.7xx for more information on how these standards are used in telephony networks.