VoIP Voice and Fax Signal Processing

4.6 VAD/CNG IN LOW-BIT-RATE Codecs

In this section, an overview on low-bit-rate codecs VAD/CNG is given. Low-bit-rate codecs are given in Chapter 3. The codecs such as Internet low bit rate codec (iLBC) [URL (iLBC)], G.722 [ITU-T-G.722 (1988)], and G.728 [ITU-T-G.728 (1992)] do not have built-in VAD/CNG support. Some users considered G.711-Appendix-II as a VAD/CNG scheme with non-VAD/CNG codecs. Several low-bit-rate codecs do support VAD/CNG. Low-bit-rate codecs provide better compression. Because of the availability of higher network bandwidth from service providers, VAD/CNG is not used in deployments, even though VAD/CNG is supported in the codec and as a product feature for maintaining better interoperability. Some features of VAD/CNG operations in low-bit-rate codecs are given here.

G.729AB and G.729B [ITU-T-G.729B (1996)] support VAD/CNG, and the suffix B conveys VAD/CNG support. VAD packets are of 2 bytes with parameters, namely 1 bit for a switched predictor index of the LSF quantizer, 5 bits for the first-stage vector of the LSF quantizer, 4 bits of the second-stage vector of the LSF quantizer, and 5 bits of gain (energy). It has close relevance with G.711 VAD-II. In G.711 VAD-II, all 10 bytes are directly sent, and in G.729AB, these are coded with a table index. Hence, the payload during VAD/CNG reduces to 2 bytes in G.729AB.

G.723.1A supports VAD/CNG with 4 bytes of payload, and G.723.1 is without VAD/CNG support. At the encoder, for each SID frame, the algorithm computes ...