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In response to the limitations of analog transmission, the telephony network migrated to digital
transmission using pulse code modulation (PCM) or adaptive differential pulse code modulation
(ADPCM). In both cases, analog sound is converted into digital form by sampling the analog sound
8000 times per second and converting each sample into a numeric code.
3. 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:
♦ 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 public switched telephone network (PSTN) or
through PBXes.
♦ 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.
♦ G.726—Describes ADPCM coding at 40, 32, 24, and 16 kbps. ADPCM-encoded voice can be
interchanged between packet voice, PSTN, and PBX networks if the PBX networks are configured
to support ADPCM.
♦ G.728—Describes a 16-kbps low-delay variation of CELP voice compression. CELP voice coding
must be translated into a public telephony format for delivery to or through the PSTN.
♦ 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.
Each CODEC provides a certain quality of speech. The quality of transmitted speech is a subjective
response of the listener. A common benchmark used to determine the quality of sound produced by
specific CODECs is the mean opinion score (MOS). With MOS, a wide range of listeners judge the
quality of a voice sample (corresponding to a particular CODEC) on a scale of 1 (bad) to 5 (excellent).
The scores are averaged to provide the mean opinion score for that sample. Table 1shows the
relationship between CODECs and MOS scores.
Compression Method
G.711 PCM
G.726 ADPCM
G.728 LD-CELP
G.729 CS-ACELP
G.729 x 2 Encodings
G.729 x 3 Encodings
G.729a CS-ACELP
G.723.1 MP-MLQ
G.723.1 ACELP
Although it might seem logical from a financial standpoint to convert all calls to low-bit rate CODECs to
save on infrastructure costs, you should exercise additional care when designing voice networks with
low-bit rate compression. There are drawbacks to compressing voice. One of the main drawbacks is
Compression Methods and MOS Scores
Table 12-1
Bit Rate (kbps)
64
32
16
8
8
8
8
6.3
5.3
Model Name
Framing Size
0.125
4.1
0.125
3.85
0.625
3.61
10
3.92
10
3.27
10
2.68
10
3.7
30
3.9
30
3.65
- 457 -
MOS Result
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