The Standard

ITU-T G.728 is low delay speech coder standard, for compressing toll quality speech (8000 samples/second). G.728 Annex G (G.728 G) is a fixed point specification of the coder working at a bit rate of 16000 bits/second. G.728 Annex I (G.728 I) is the packet loss concealment (PLC) technique used along with G.728 G. G.728 coders are widely used for applications that require very low algorithmic delay. The typical application of this speech coder is in telephony over packet networks, especially voice over cable and VoIP. This is a very robust speech coder, with very good speech quality, comparable to 32 kbit/s ADPCM.

G.728 coders are based on the principle of Low Delay-Code Excited Linear Prediction (LD-CELP). This coder has the conventional approach of analysis-by-synthesis for fixed codebook search, which is generally available in all CELP class coders. The low algorithmic delay of the coder is achieved using backward adaptation of predictors and gain. Though the algorithmic delay can be as low as 0.625 msec (5 samples), the frame size of the coder is 2.5 msec (20 samples).

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