Adaptive Multi-Rate audio codec

뮤리찌 2011. 2. 18. 11:50

2011. 2. 18. 11:50

From Wikipedia, the free encyclopedia

Adaptive Multi-Rate (AMR)

Filename extension
.amr

Internet media type
audio/amr, audio/3gpp, audio/3gpp2

Initial release
23 June 1999^[1]^[2]

Latest release
9.0.0 / 18 December 2009; 13 months ago

Type of format
Audio compression format, audio file format

The Adaptive Multi-Rate (AMR or AMR-NB) audio codec is a patented audio data compression scheme optimized forspeech coding. AMR was adopted as the standard speech codec by 3GPP in October 1998 and is now widely used in GSMand 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 can store short audio recordings in the AMR format, and both free and proprietary programs exist (see Software support) to convert between this and other formats, 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. There also exists another storage format for AMR that is suitable for applications with more advanced demands on the storage format, like random access or synchronization with video. This format is the 3GPP-specified 3GP container format based on ISO base media file format.^[3]

[hide]

[edit]Usage

The frames contain 160 samples and are 20 milliseconds long.^[1] 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 of Nokia Corporation, Telefonaktiebolaget L. M. Ericsson, VoiceAge Corporation and Nippon Telegraph and TelephoneCorporation.^[4]^[5] VoiceAge Corporation is the License Administrator for the AMR and AMR-WB+ patent pools. VoiceAge also accepts submission of patents for determination of their possible essentiality to these standards.^[6]^[7]

The initial fee for professional content creation tools and "real-time channel" products is $6,500. The minimum annual royalty shall be $10,000, excluding the initial fee in year 1 of the license agreement.^[4]^[5]

AMR decoder in a category of personal computer products (e.g. media players) is licensed for free. The license fee for a sold encoder is $0.40. The minimum annual royalty will not apply to licensed products which fall under category of personal computer products and which contain only the free decoder.^[4]^[5]

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 - legal notices for usage of amrnb and amrwb libraries based on the reference implementation

[edit]Software support

3GPP TS 26.073 - AMR speech Codec (C-source code) - reference implementation^[8]
Audacity via the FFmpeg integration libraries ^[9]
FFmpeg with OpenCORE AMR libraries ^[10]
Android ^[11]
AMR Codecs as Shared Libraries - amrnb and amrwb libraries development site. These libraries are based on the reference implementation and were created to prevent embedding of possibly patented source code into many Open Source projects.
Open Source software to convert the .amr format: RetroCode, Amr2Wav, both are in an early developmental stage
AMR Player is freeware to play AMR audio files, and can convert AMR with MP3/WAV audio format.
MPlayer (SMPlayer, KMPlayer)
QuickTime player and multimedia framework
RealPlayer version 11 and later
VLC media player version 1.1.0 and later
ffdshow

Others

AVS · Bink · CineForm · Cinepak · Dirac · DV · Indeo · Microsoft Video 1 · OMS Video · Pixlet · RealVideo · RTVideo · SheerVideo · Smacker ·Sorenson Video & Sorenson Spark · Theora · VC-1 · VC-2 · VC-3 · VP3 · VP6 · VP7 · VP8 · WMV

Audio

ISO/IEC

MPEG-1 Layer III (MP3) · MPEG-1 Layer II (Multichannel) · MPEG-1 Layer I · AAC · HE-AAC · MPEG Surround · MPEG-4 ALS · MPEG-4 SLS ·MPEG-4 DST · MPEG-4 HVXC · MPEG-4 CELP

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

Others

AC-3 · AMR · AMR-WB · AMR-WB+ · Apple Lossless · ATRAC · CELT · DRA · DTS · EVRC · EVRC-B · FLAC · GSM-HR · GSM-FR · GSM-EFR · iLBC ·iSAC · Monkey's Audio · TTA (True Audio) · MT9 · A-law · μ-law · Musepack · Nellymoser · OptimFROG · OSQ · QCELP · RealAudio · RTAudio ·SD2 · SHN · SILK · Siren · SMV · Speex · SVOPC · TwinVQ · VMR-WB · Vorbis · WavPack · WMA

Image

ISO/IEC/ITU-T

JPEG · JPEG 2000 · JPEG XR · lossless JPEG · JBIG · JBIG2 · PNG

Others

APNG · BMP · DjVu · EXR · GIF · ICER · ILBM · MNG · PCX · PGF · TGA · QTVR · TIFF · WBMP · WebP

Containers

ISO/IEC

MPEG-PS · MPEG-TS · ISO base media file format · MPEG-4 Part 14 · Motion JPEG 2000 · MPEG-21 Part 9

ITU-T

H.222.0 · T.802

Others

3GP and 3G2 · AMV · ASF · AIFF · AVI · AU · Bink · DivX Media Format · DPX · EVO · Flash Video · GXF · M2TS · Matroska · MXF · Ogg ·QuickTime File Format · RealMedia · REDCODE RAW · RIFF · Smacker · MOD and TOD · VOB · WAV · WebM

See Compression methods for methods and Compression software implementations for codecs

Categories: Speech codecs

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This page was last modified on 6 February 2011 at 19:38.

http://en.wikipedia.org/wiki/Adaptive_Multi-Rate_audio_codec

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