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VoIP Codec
VoIP sends voice information in digital form over the internet. The process to sample analogical waves into digital information is made by an encoder-decoder (CODEC).
Codec converts analog signals to a digital packet, and another identical codec at the far end of the communication converts the digital packet back into an analog signal.
In addition, Codec compress the sequence of data to save network bandwidth. Some codec also support silence suppression, where silence package is not encoded or transmitted.
Types of Codec:
| Number |
Bandwidt (kb/s) |
Sampling rate (kHz) |
Frame size (ms) |
Comments |
| G.711 |
64 |
8 |
Sampling |
G.711u/a often refered to as u-law/a-law: where a-law is the European version and u-law the US/Japanese version |
| G.721 |
32 |
8 |
Sampling |
Now described in G.726; obsolete. |
| G.722 |
64 |
16 |
Sampling |
Subband-codec that divides 16 kHz band into two subbands, each coded using ADPCM |
| G.722.1 |
24/32 |
16 |
20 |
|
| G.723 |
24/40 |
8 |
Sampling |
Superceded by G.726; obsolete. This is a completely different codec than G.723.1 |
| G.723.1 |
5.6/6.3 |
8 |
30 |
Part of H.324 video conferencing. It encodes speech or other audio signals in frames using linear predictive analysis-by-synthesis coding. The excitation signal for the high rate coder is Multipulse Maximum Likelihood Quantization (MP-MLQ) and for the low rate coder is Algebraic-Code-Excited Linear-Prediction (ACELP). |
| G.726 |
16/24/32/40 |
8 |
Sampling |
ADPCM; replaces G.721 and G.723. |
| G.727 |
var. |
|
Sampling |
ADPCM. Related to G.726 |
| G.728 |
16 |
8 |
2.5 |
CELP. |
| G.729** |
8 |
8 |
10 |
Low delay (15 ms) |
| GSM 06.10 |
13 |
8 |
22.5 |
Used for GSM cellular telephony. |
| LPC10 |
2.4 |
8 |
22.5 |
10 coefficients. |
| Speex |
8, 16, 32 |
2.15-24.6 (NB)
4-44.2 (WB) |
30 ( NB )
34 ( WB ) |
|
| iLBC |
8 |
13.3 |
30 |
|
| DoD CELP |
4.8 |
|
30 |
|
| EVRC |
9.6/4.8/1.2 |
8 |
20 |
Se usa en redes CDMA |
| DVI |
32 |
Variable |
Sampling |
|
| L16 |
128 |
Variable |
Sampling |
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