The ITU-T Recommendation for adaptive differential pulse code modulation (ADPCM) voice coding and compression of high-fidelity 7 kHz voice at 64/56/48 kbps. See G series and ITU-T.
"G.721." YourDictionary, n.d. Web. 06 June 2018. <http://www.yourdictionary.com/g-721>.
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An ITU standard for speech codecs that uses the ADPCM method and provides toll quality audio at 16 Kbps. See G. standards, G.721, G.726 and ADPCM.