G.722 describes the characteristics of an audio (50 to 7 000 Hz) coding system which may be used for a variety of higher quality speech applications. The coding system uses sub-band adaptive differential pulse code modulation (SB-ADPCM) within a bit rate of 64 kbps. The system is referred as 64 kbps (7 kHz) audio coding.
In the SB-ADPCM technique used, the frequency band is split into two sub-bands (higher and lower) and the signals in each sub-band are encoded using ADPCM. The system 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.
The 64 kbps (7 kHz) audio encoder uses 64 kbps for audio coding at all times irrespective of the mode of operation. The audio coding algorithm has been chosen such that, without sending any indication to the encoder, the least significant bit or two least significant bits of the lower sub-band may be used downstream from the 64 kbps (7 kHz) audio encoder in order to substitute the auxiliary data channel bits. However, to maximize the audio performance for a given mode of operation, the 64 kbps (7 kHz) audio decoder must be optimized to the bit rate available for audio coding. G.722 describes three variants of the SB-ADPCM decoder and, for applications requiring an auxiliary data channel, an indication must be forwarded to select in the decoder the variant appropriate to the mode of operation.
The 64 kbps (7 kHz) audio encoder includes a transmit audio part which converts an audio signal to a uniform digital signal which is coded using 14 bits with 16 kHz sampling and a SB-ADPCM encoder which reduces the bit rate to 64 kbps.
The 64 kbps (7 kHz) audio decoder includes a SB-ADPCM decoder which performs the reverse operation to the encoder, noting that the effective audio coding bit rate at the input of the decoder can be 64, 56 or 48 kbps depending on the mode of operation and a receive audio part which reconstructs the audio signal from the uniform digital signal which is encoded using 14 bits with 16 kHz sampling.
The three basic possible modes of operation which correspond to the bit rates available for audio coding at the input of the decoder are: mode 1 - 64 kbps, mode 2 - 56 kbps, and mode 3 - 48 kbps.