|
Codec |
G.711 |
G.723.1 |
G.726-32 |
G.729 |
|
Coding rate (kb/s) |
64 |
5.3-6.3 |
32 |
8 |
|
Frame size (ms) |
0.125 |
30 |
0.250 |
10 |
|
Algorithm |
PCM |
MP-MLQ |
ADPCM |
CS-ACELP |
|
Processing delay (ms) |
0.125 |
30 |
0.250 |
10 |
|
Look ahead delay (ms) |
0 |
7.5 |
0 |
5 |
|
Payload (Bytes) |
1 |
20-24 |
1 |
10 |
|
Quality |
Good |
Good/fair |
Good |
Good |
|
Complexity |
Lowest |
Highest |
Low |
High |
Table 1. Selected ITU voice coders and key characteristics.
참고 사이트
http://www.comsoc.org/livepubs/surveys/public/2004/apr/scheets.html
Voice Coder
POTS networks almost universally use the ITU G.711 64 kb/s PCM standard. As previously noted, this type of coder outputs 8 bits every 1/8000th of a second, that is, the frame of this coder is 1/8000th of a second, and the frame size is 8 bits.
Other coders exist that can reduce the generated bit rate, but usually with a slightly reduced perceived quality. Table 1 compares selected ITU coders, including their associated bit rates [2, 3].
Of these codes, G.729 has evoked considerable interest for VoIP providers as it has comparable quality to G.711, but at a greatly reduced bit rate. G.729 has a frame length of 10 msec, and its compression algorithm outputs 80 bits every 1/100th of a second, yielding an 8 kb/s bit rate.
POTS TDM backbones are based around fixed-rate coders. For example, a G.711 coder outputs 64 kb/s at all times, regardless of whether or not the voice source is talking or listening. The statistical as-needed allocation of backbone bandwidth on VoIP networks offers the opportunity to deploy variable-rate coders, which output traffic at one bit rate when the voice source is talking, and a lower bit rate (possibly zero) when the voice source is quiet. For example, G.729B with silence suppression examines each 10 msec voice frame and makes a voice/no voice decision. If the coder detects voice energy, it will output the standard 80 bits of compressed, digitized voice for that frame. If the coder decides this frame does not contain voice energy, the coder will output a reduced block of bits containing comfort noise, information that the receiver will use to generate background noise so that the user does not think the connection has been lost [3]. Alternatively, the coder could output nothing at all and the receiver could generate comfort noise.
Experiments have shown that in a typical two-way interactive voice conversation, voice sources are only active 40 percent of the time [4]. The 60 percent idle time includes pauses while listening to the other party talking, as well as pauses between sentences, and even pauses between some words. A G.729B coder with silence suppression will output 8 kb/s during talk spurts (40 percent of the time), and nothing or a reduced bit rate during intervening silence intervals (60 percent of the time). The use of silence suppression potentially allows a G.729 coder to reduce its average output from 8 kb/s to 3.2 kb/s (alternating 8 kb/s and 0 kb/s bursts).
