[Audio][Copy]Adaptive multi-rate compression

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Adaptive multi-rate compression
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Adaptive Multi-Rate Narrow Band (AMR-NB) Filename extension .amr
Internet media type audio/amr
Type of format Audio
Adaptive Multi-Rate (AMR) is an audio data compression scheme optimized for speech coding. AMR was adopted as the standard speech codec by 3GPP in October 1998 and is now widely used in GSM and UMTS. It uses link adaptation to select from one of eight different bit rates based on link conditions.

AMR is also a file format for storing spoken audio using the AMR codec. Many modern mobile telephone handsets will allow you to store short recordings in the AMR format, both Open Source (see the external links) and commercial programs exist to convert between this and other formats such as MP3, although it should be remembered that AMR is a speech format and is unlikely to give ideal results for other audio. The common filename extension is .amr.

Contents [hide]
1 Usage
2 Features
3 Licensing and patent issues
4 See also
5 External links
6 External references



[edit] Usage
The frames contain 160 samples and are 20 milliseconds long. AMR uses different techniques, such as ACELP, DTX, VAD and CNG. The usage of AMR requires optimized link adaptation that selects the best codec mode to meet the local radio channel and capacity requirements. If the radio conditions are bad, source coding is reduced and channel coding is increased. This improves the quality and robustness of the network connection while sacrificing some voice clarity. In the particular case of AMR this improvement is somewhere around S/N = 4-6 dB for usable communication. The new intelligent system allows the network operator to prioritize capacity or quality per base station.

There are a total of 14 modes of the AMR codec, 8 are available in a full rate channel (FR) and 6 on a half rate channel (HR).

Mode Bitrate (kbit/s) Channel Compatible with
AMR_12.20 12.20 FR ETSI GSM enhanced full rate
AMR_10.20 10.20 FR
AMR_7.95 7.95 FR/HR
AMR_7.40 7.40 FR/HR TIA/EIA IS-641 TDMA enhanced full rate
AMR_6.70 6.70 FR/HR ARIB 6.7 kbit/s enhanced full rate
AMR_5.90 5.90 FR/HR
AMR_5.15 5.15 FR/HR
AMR_4.75 4.75 FR/HR

AMR_SID 1.80 FR/HR


[edit] Features
Sampling frequency 8 kHz/13-bit (160 samples for 20 ms frames), filtered to 200-3400 Hz.
The AMR codec uses eight source codecs with bit-rates of 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kbit/s.
Generates frame length of 95, 103, 118, 134, 148, 159, 204, or 244 bits for bit rates 4.75, 5.15, 5.90, 6.70, 7.40, 7.95, 10.2, or 12.2 kbit/s, respectively
AMR utilizes Discontinuous Transmission (DTX), with Voice Activity Detection (VAD) and Comfort Noise Generation (CNG) to reduce bandwidth usage during silence periods
Algorithmic delay is 20 ms per frame. For bit-rates of 12.2, there is no 'algorithm' look-ahead delay. For other rates, look-ahead delay is 5 ms. Note that there is 5 ms 'dummy' look-ahead delay, to allow seamless frame-wise mode switching with the rest of rates.
AMR is a hybrid speech coder which uses Algebraic Code Excited Linear Prediction (ACELP)
The complexity of the algorithm is rated at 5, using a relative scale where G.711 is 1 and G.729a is 15.
PSQM testing under ideal conditions yields Mean Opinion Scores of 4.14 for AMR (12.2 kbit/s), compared to 4.45 for G.711 (u-law)
PSQM testing under network stress yields Mean Opinion Scores of 3.79 for AMR (12.2 kbit/s), compared to 4.13 for G.711 (u-law)

[edit] Licensing and patent issues
AMR codecs incorporate several patents. For more information about this, please refer to:

VoiceAge licensing information, including pricing to license the AMR codecs
3GPP legal issues
The 3G Patent Platform and its licensing policy
AMR Codecs as Shared Libraries - amrnb and amrwb libraries development site. These libraries are based on the reference implementation and were created to prevent ugly embedding of possibly patented source code into many Open Source projects.

[edit] See also
AMR-WB
AMR-WB+
Half Rate
Full Rate
Sampling rate
IS-641

[edit] External links
Open Source software to convert the .amr format: Amr2Wav, RetroCode, both are in an early developmental stage
Mobile AMR Converter is freeware
Total Video Player will play .amr files.

[edit] External references
3G TS 26.071 - Mandatory Speech Codec speech processing functions; AMR Speech Codec; General Description
[hide]v • d • eMultimedia compression formats

Video compression ISO/IEC MJPEG · Motion JPEG 2000 · MPEG-1 · MPEG-2 · MPEG-4 ASP · MPEG-4/AVC

ITU-T H.120 · H.261 · H.262 · H.263 · H.264

Others AMV · AVS · Bink · Dirac · Indeo · Pixlet · RealVideo · RTVideo · SheerVideo · Smacker · Snow · Theora · VC-1 · VP6 · VP7 · VP8 · WMV


Audio compression ISO/IEC MPEG-1 Layer III (MP3) · MPEG-1 Layer II · MPEG-1 Layer I · AAC · HE-AAC

ITU-T G.711 · G.718 · G.719 · G.722 · G.722.1 · G.722.2 · G.723 · G.723.1 · G.726 · G.728 · G.729 · G.729.1 · G.729a

Others AC3 · AMR · Apple Lossless · ATRAC · FLAC · iLBC · Monkey's Audio · μ-law · Musepack · Nellymoser · OptimFROG · RealAudio · RTAudio · SHN · Siren · Speex · Vorbis · WavPack · WMA · TAK


Image compression ISO/IEC/ITU-T JPEG · JPEG 2000 · lossless JPEG · JBIG · JBIG2 · PNG · WBMP

Others BMP · GIF · ICER · ILBM · PCX · PGF · TGA · TIFF · JPEG XR / HD Photo


Media containers General 3GP · ASF · AVI · Bink · DMF · DPX · EVO · FLV · Matroska · MPEG-PS · MPEG-TS · MP4 · MXF · NUT · Ogg · Ogg Media · QuickTime · RealMedia · Smacker · RIFF · VOB

Audio only AIFF · AU · WAV


See Compression Methods for methods and Compression Software Implementations for codecs


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Categories: Sound technology | Speech codecs | Audio engineering