Definition - What does G.721 mean?
G.721 is an obsolete ITU-T Recommendation that is now superseded by standard G.726. The title of the defunct specification is “32 kbit/s adaptive differential pulse code modulation (ADPCM).” The standard was one of the ITU-T G series, which is titled "Transmission systems and media, digital systems and networks." ADPCM is a technology used to encode digital audio.
Techopedia explains G.721
The International Telecommunication Union (ITU) is an arm of the United Nations that deals with information and communication technologies. ITU-T is a subset of that agency called the Telecommunication Standardization Sector. Among the many standards specified by the organization are the G Series Recommendations. As with any vibrant standards organization, these specifications are under a constant state of development and improvement.
G.721 and its neighboring recommendations are in the family of standards pertaining to the processing of digital audio signals. ADPCM converts analog signals to digital through sound sampling. ADPCM is considered a codec (coder-decoder) because of its transformation of the audio signal. The G.721 Recommendation was defined with a transmission rate of 32 kbit/s.
G.721 was established in 1984. The G.726 Recommendation superseded both G.721 and G.723 in 1990. G.726 also uses ADPCM, but it has advantages over G.721, such as the ability to transmit at multiple bit rates.