7 kHz audio-coding within 64 kbit/s
|Publication Date:||1 September 2012|
Scope and outline description
This Recommendation 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 kbit/s. The system is henceforth referred to as 64 kbit/s (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 kbit/s. The latter two modes allow an auxiliary data channel of 8 and 16 kbit/s respectively to be provided within the 64 kbit/s by making use of bits from the lower sub-band.
Figure 1 identifies the main functional parts of the 64 kbit/s (7 kHz) audio codec as follows:
i) 64 kbit/s (7 kHz) audio encoder comprising:
- a transmit audio part which converts an audio signal to a uniform digital signal which is coded using 14 bits with 16 kHz sampling;
- a SB-ADPCM encoder which reduces the bit rate to 64 kbit/s.
ii) 64 kbit/s (7 kHz) audio decoder comprising:
- 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 kbit/s depending on the mode of operation;
- 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 following two parts, identified in Figure 1 for clarification, will be needed for applications requiring an auxiliary data channel within the 64 kbit/s:
- a data insertion device at the transmit end which makes use of, when needed, 1 or 2 audio bits per octet depending on the mode of operation and substitutes data bits to provide an auxiliary data channel of 8 or 16 kbit/s respectively;
- a data extraction device at the receive end which determines the mode of operation according to a mode control strategy and extracts the data bits as appropriate.
Clause 1.2 contains a functional description of the transmit and receive audio parts, clause 1.3 describes the modes of operation and the implication of inserting data bits on the algorithms, whilst clauses 1.4 and 1.5 provide the functional descriptions of the SB-ADPCM encoding and decoding algorithms respectively. Clause 1.6 deals with the timing requirements. Clause 2 specifies the transmission characteristics of the 64 kbit/s (7 kHz) audio codec and of the transmit and receive audio parts, clauses 3 and 4 give the principles of the SB-ADPCM encoder respectively whilst clauses 5 and 6 specify the computational details of the quadrature mirror filters (QMF) and of the ADPCM encoders and decoders respectively.
Networking aspects and test sequences are addressed in Appendices I and II respectively to this Recommendation.
Recommendation ITU-T G.725 contains specifications for in-channel handshaking procedures for terminal identification and for mode control strategy, including interworking with existing 64 kbit/s PCM terminals.