Audio compression is a form of data compression designed to reduce the size of audio files. Audio compression algorithms are implemented in computer software as audio codecs. An audio codec is a computer program that compresses/decompresses digital audio data according to a given audio file format or streaming audio format. Most codecs are implemented as libraries which interface to one or more multimedia players.
There are many different technologies and standards by different standard organizations and companies for audio compression available. In the following table, the main ones are listed.
| Codec Type | Standard by | Modulation method | Bit rate (kb/s) | Sampling rate (kHz) |
| G.711 | ITU-T | PCM | 64 | 8 |
| G.721 | ITU-T | ADPCM | 32 | 8 |
| G.722 | ITU-T | ADPCM | 64 | 16 |
| G.722.1 | ITU-T | Transform-based | 24/32 | 16 |
| G.722.2 | ITU-T | AMR-WB | 6.60, 8.85, 12.65, 14.25, 15.85, 18.25, 19.85, 23.05 and 23.85 | 16 |
| G.723 | ITU-T | DPCM | 24/40 | 8 |
| G.723.1 | ITU-T | MPC-MLQ or ACELP | 5.6/6.3 | 8 |
| G.726 | ITU-T | ADPCM | 16/24/32/40 | 8 |
| G.727 | ITU-T | ADPCM | 5-, 4-, 3- and 2 | |
| G.728 | ITU-T | LDCELP | 16 | 8 |
| G.729 | ITU-T | CS-ACELP | 8 | 8 |
| GSM 06.10 | ETSI | RPE-LTP | 13 | 8 |
| LPC10 | USA Government DOD / NATO | LPC | 2.4 | 8 |
| Speex | Freeware | CELP | 8, 16, 32 | 2.15-24.6 (NB) 4-44.2 (WB) |
| iLBC (Internet Low Bitrate Vocoder) | IETF RFC 3951 Freeware |
LPC | 8, 16 | 13.3 |
| LPC | 8, 16 | 15.2 | ||
| DoD CELP | Department of Defense (DoD) USA | CELP | 4.8 | |
| EVRC | 3GPP2 | Enhanced Variable Rate CODEC | 9.6/4.8/1.2 | 8 |
| L16 | IETF RFC 3551 | Uncompressed audio data samples | 128 | Variable |
| Codec Type | Frame size (ms) | Compression delay | Mean Opinion Score (MOS) | Notes |
| G.711 | 0.125 | 0.75 | 4.1 | U-law (US, Japan) and A-law (Europe) companding. |
| G.721 | Sampling | Replaced by G.726. | ||
| G.722 | Sampling | Subband-codec that divides 16 kHz band into two subbands, each coded using ADPCM | ||
| G.722.1 | 20 | 40 | ||
| G.722.2 | Sampling | AMR-WB is standardized for usage in networks such as UMTS. | ||
| G.723 | Sampling | 30 | Superceded by G.726; This is a completely different codec than G.723.1 | |
| G.723.1 | 30 | 30 | 3.7 -3.9 | Part of H.324 video conferencing. |
| G.726 | 0.125 | 1 | 3.9 | Replaces G.721 and G.723. |
| G.727 | Sampling | Related to G.726. | ||
| G.728 | 0.625 | 0.625 | 3.6 | |
| G.729 | 10 | 15 | 3.9 | VOIP Applications |
| GSM 06.10 | 22.5 | Used for GSM cellular telephony. | ||
| LPC10 | 22.5 | Also known as FS-1015 or STANAG 4198. | ||
| Speex | 30(NB) 34(WB) |
30 – 34 | VOIP | |
| iLBC (Internet Low Bitrate Vocoder) | 30 | 30 | 4.0 | VOIP |
| 20 | 20 | 4.0 | ||
| DoD CELP | 30 | |||
| EVRC | 20 | Used in CDMA network. | ||
| L16 | Sampling |
Voice Modulation Methods
- PCM = Pulse Code Modulation
- ADPCM = Adaptive Differential Pulse Code Modulation
- AMR-WB = Adaptive Multi Rate – WideBand
- DPCM = Differential Pulse Code Modulation
- LDCELP = Low-Delay Code Excited Linear Prediction
- LPC = Linear-Predictive Codec
- CS-ACLEP = Conjugate-Structure Algebraic-Code-Excited Linear-Prediction
- MP-MLQ = Multi-Pulse, Multi-Level Quantization
- ACELP = Algebraic Code Excited Linear Prediction
Related Terms: CODEC, Pulse Code Modulation, G.7xx, Audio compression
Reference Links: http://www.javvin.com/protocolG7xx.html: Audio (Voice) Compression Protocols (CODEC) (G.711, G.721, G.722, G.726, G.727, G.728, G.729)