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G.723 is an obsolete ITU-T Recommendation that is now superseded by standard G.726. It was written as an extension to G.721, which was titled “32 kbit/s adaptive differential pulse code modulation (ADPCM).” The aim of G.723 was to add the bit rates 24 kbit/s and 40 kbit/s. 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.
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.
The full title of the defunct specification explains what it does: “Extensions of Recommendation G.721 adaptive differential pulse code modulation to 24 and 40 kbit/s for digital circuit multiplication equipment application.” G.721 transmitted only at 32 kbit/s, but G.723 added two bit rates to the capability. G.723 and its neighboring recommendations were 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.
G.721 was established in 1984, and G.723 in 1988. The G.726 Recommendation superseded both G.721 and G.723 in 1990. G.726 also uses ADPCM, but it adds 16kbit/s to the other bit rates, as well as offering other advantages.