Voice transmission is analogical, whereas the
data network is digital. The process to sample analogical
waves into digital information is made by an encoder-decoder
(CODEC). There are many standards to sample an analogical
voice signal into a digital one. The process is often quite
complex. Most of the conversions use pulse code modulation
(PCM) or variations
In addition, the CODEC zip the sequence of data, and sometimes
provides echo cancellation. The compression of the
waveform can save bandwidth. This is especially interesting
in low speed connections so you can have more VoIP connections
at the same time. Another way to save bandwidth is
using the silence suppression. The goal is not to send packages
when there is no voice in the conversations.
| Number |
Standard by |
Description |
Bit rate (kb/s) |
Sampling rate (kHz) |
Frame size (ms) |
Remarks
| MOS (Mean Opinion Score)
|
| G.711 * |
ITU-T |
Pulse code modulation (PCM) |
64 |
8 |
Sampling |
U-law (US, Japan) and A-law (Europe) companding |
4.1
|
| G.721 |
ITU-T |
Adaptive differential pulse code modulation (ADPCM) |
32 |
8 |
Sampling |
Now described in G.726; obsolete. |
|
| G.722 |
ITU-T |
7 kHz audio-coding within 64 kbit/s |
64 |
16 |
Sampling |
Subband-codec that divides 16 kHz band into two subbands, each coded using ADPCM |
|
| G.722.1 |
ITU-T |
Coding at 24 and 32 kbit/s for hands-free operation in systems with low frame loss |
24/32 |
16 |
20 |
|
|
| G.723 |
ITU-T |
Extensions of Recommendation G.721 adaptive differential pulse code modulation to 24 and 40 kbit/s for digital circuit multiplication equipment application |
24/40 |
8 |
Sampling |
Superceded by G.726; obsolete. This is a completely different codec than G.723.1 |
|
| G.723.1 |
ITU-T |
Dual rate speech coder for multimedia communications transmitting at
5.3 and 6.3 kbit/s |
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). |
3.8-3.9 |
| G.726 |
ITU-T |
40, 32, 24, 16 kbit/s adaptive differential pulse code modulation
(ADPCM) |
16/24/32/40 |
8 |
Sampling |
ADPCM; replaces G.721 and G.723. |
3.85 |
| G.727 |
ITU-T |
5-, 4-, 3- and 2-bit/sample embedded adaptive differential pulse
code modulation
(ADPCM) |
var. |
|
Sampling |
ADPCM. Related to G.726 |
|
| G.728 |
ITU-T |
Coding of speech at 16 kbit/s using low-delay code excited linear prediction |
16 |
8 |
2.5 |
CELP. |
3.61 |
| G.729 ** |
ITU-T |
Coding of speech at 8 kbit/s using conjugate-structure
algebraic-code-excited linear-prediction (CS-ACELP) |
8 |
8 |
10 |
Low delay (15 ms) |
3.92 |
| GSM 06.10 |
ETSI |
RegularPulse Excitation LongTerm Predictor (RPE-LTP) |
13 |
8 |
22.5
| Used for GSM cellular telephony. |
|
| LPC10 |
USA Government |
Linear-predictive codec |
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 |
American Department of Defense (DoD) USA Government |
|
4.8 |
|
30 |
|
|
EVRC |
3GPP2 |
Enhanced Variable Rate CODEC |
9.6/4.8/1.2 |
8 |
20 |
Se usa en redes CDMA |
|
| DVI |
Interactive Multimedia Association (IMA) |
DVI4 uses an adaptive delta pulse code modulation (ADPCM) |
32 |
Variable |
Sampling |
|
|
| L16 |
|
Uncompressed audio data samples |
128 |
Variable |
Sampling |
|
|
