A bit-rate/bandwidth scalable speech coder based on ITU-T G.723.1 standard

The paper presents a new scalable coder based on the ITU-T G.723.1 standard which is one of the most famous speech coders for VoIP applications. In order to support both bit-rate scalability and bandwidth scalability, the proposed coder adopts a split-band approach, where the input signal, sampled at 16 kHz, is decomposed into two equal frequency bands. The lower-band speech is coded with a standard coder, such as the G.723.1 standard. In addition, the low-band enhancement layer for lower-band speech improves the perceptual quality of decoded speech by employing additional coding units based on a cascaded codebook approach. The higher-band signal is encoded using an MDCT-based transform coding scheme. The proposed coder, at a bit-rate of 19.4 kbit/s, provides speech quality comparable to the ITU-T 24 kbit/s G.722.1 coder, while it also has interoperability with G.723.1.