PassFinder AP2520 VoIP Router/Gateway Operation Manual
Version 1.10 / Mar. 2002
AddPac Technology Co., Ltd.
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166
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5.1.2.
Codecs and MOS(Mean Opinion Score)
5.1.2.1.
Codecs
PCM and ADPCM are examples of "waveform" CODEC techniques. Waveform
CODECs are compression techniques that exploit the redundant
characteristics of the waveform itself.
In addition to waveform CODECs, there are source CODECs that compress
speech by sending only simplified parametric information about voice
transmission; these CODECs require less bandwidth. Source CODECs include
linear predicative coding (LPC), code-excited linear prediction (CELP), and
multi-pulse, multi-level quantization (MP-MLQ).
Coding techniques are standardized by the ITU-T in its G-series
recommendations. The most popular coding standards for telephony and
voice packet are:
z
G.711---Describes the 64-kbps PCM voice coding technique. In G.711,
encoded voice is already in the correct format for digital voice delivery in
the PSap or through PBXs.
z
G.723.1---Describes a compression technique that can be used for
compressing speech or audio signal components at a very low bit rate as
part of the H.324 family of standards. This CODEC has two bit rates
associated with it: 5.3 and 6.3 kbps. The higher bit rate is based on ML-MLQ
technology and provides a somewhat higher quality of sound. The lower bit
rate is based on CELP and provides system designers with additional
flexibility.
z
G.729---Describes CELP compression where voice is coded into 8-kbps
streams. There are two variations of this standard (G.729 and G.729 Annex
A) that differ mainly in computational complexity; both provide speech
quality similar to 32-kbps ADPCM.
