This speech codec uses sub-band adaptive differential pulse code modulation (ADPCM) and provides 7 kHz wideband audio at data rates from 48 to 64 kbps. This is useful in fixed network VoIP applications where the required bandwidth is typically not prohibitive. This speech codec offers a significant improvement in speech quality over older narrowband speech codecs such G.711 without an excessive increase in implementation complexity.

This speech codec splits the frequency band into two sub-bands, a higher and a lower sub-band, and then each sub-band is encoded using ADPCM. It has three basic modes of operation corresponding to the bit rates used for 7 kHz audio coding: 64, 56, and 48 kbps. The latter two modes allow an auxiliary data channel of 8 and 16 kbps respectively to be provided within the 64 kbps by making use of bits from the lower sub-band.