AUDIO

            The H.323 standard specifies that all terminals must support audio communications. At the very least, all terminals must support the G.711 protocol.

 G.711 is the international standard for encoding audio on a 64 kbps channel. It is a pulse code modulation scheme operating at 8 kHz sample rate, with 8 bits per sample. There are two variants of the G.711 protocol: A-law and mu-law. The difference between the two is that A-law represents smaller signals with greater fidelity. A-law is the standard for international circuits.

Terminal suppliers may provide other audio compression algorithms aside from G.711. Other protocols such as G.722, G.728, G.729, MPEG1 audio, and G.723.1 may also be used.

H.323 has also provided a new audio algorithm: G.723. G.723 is a dual-rate speech-coding standard, which operates on lower bit rates (5.3 – 6.4 kbps) while maintaining high perceptual quality. Both bit rates are mandatory parts of the encoder and decoder. The G.723.1 encoder provides one frame of audio every 30 milliseconds. The audio frame size is 20 bytes of pulse-coded modulation (PCM) samples for the low rate (5.3 kb/s) and 24 bytes for the high rate (6.4 kb/s).

Below is a summary of audio codecs.

Table 1. Audio Codecs