G.7xx, including G.711, G.721, G.722, G.726, G.727, G.728, G.729, is an suite of ITU-T standards for audio compression and de-commpression. It is primarily used in telephony. In telephony, there are 2 main algorithms defined in the standard, mu-law algorithm (used in America) and a-law algorithm (used in Europe and the rest of the world). Both are logarithmic, but the later a-law was specifically designed to be simpler for a computer to process. The G.7xx protocol suite compose the following protocols:
- G.711 - Pulse code modulation (PCM) of voice frequencies on an 64 kbps channel.
- G.721 - 32 kbit/s adaptive differential pulse code modulation (ADPCM)
- G.722 - 7 kHz audio-coding within 64 kbit/s
- G.722.1 - Coding at 24 and 32 kbit/s for hands-free operation in systems with low frame loss
- G.722.2 - Wideband coding of speech at around 16 kbit/s using adaptive multi-rate wideband (AMR-WB)
- G.726 - 40, 32, 24, 16 kbit/s adaptive differential pulse code modulation (ADPCM)
- G.727 - 5-, 4-, 3- and 2-bit/sample embedded adaptive differential pulse code modulation (ADPCM)
- G.728 - Coding of speech at 16 kbit/s using low-delay code excited linear prediction
- G.729 - Coding of speech at 8 kbit/s using conjugate-structure algebraic-code-excited linear-prediction (CS-ACELP)
Protocol Structure
Comparison of major Audio Codec:
| Name | standardized by | description | bit rate (kb/s) | sampling rate (kHz) | frame size (ms) | remarks |
| (ADPCM) DVI | Intel, IMA | ADPCM | 32 | 8 | sample | |
| G.711 | ITU-T | Pulse code modulation (PCM) | 64 | 8 | sample | mu-law (US, Japan) and A-law (Europe) companding |
| G.721 | ITU-T | Adaptive differential pulse code modulation (ADPCM) | 32 | 8 | sample | Now described in G.726; obsolete. |
| G.722 | ITU-T | 7 kHz audio-coding within 64 kbit/s | 64 | 16 | sample | 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 | See also |
| 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code modulation to 24 and 40 kbit/s for digital circuit multiplication equipment application | 24/40 | 8 | sample | Superceded by G.726; obsolete.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. DSP Group. 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 | ITU-T | 40, 32, 24, 16 kbit/s adaptive differential pulse code modulation (ADPCM) | 16/24/32/40 | 8 | sample | ADPCM; replaces G.721 and G.723. |
| G.727 | ITU-T | 5-, 4-, 3- and 2-bit/sample embedded adaptive differential pulse code modulation (ADPCM) | var. | ? | sample | 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 | CELP. Annex J offers variable-bit rate operation for DCME. | |
| 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) |
| GSM 06.10 | ETSI | Regular Pulse Excitation Long Term Predictor (RPE-LTP) | 13 | 8 | 22.5 | Used for GSM cellular telephony. |
Related protocols: RTSP, RTP, RTCP, H.323, H.245, H.263
Sponsor Source: G.7xx is a suite of ITU-T (http://www.itu.int/ITU-T/ ) standard.