The Adaptive Multi-Rate (AMR or AMR-NB) audio codec is a patented audio data compression scheme optimized for speech coding. AMR was adopted as the standard speech codec by 3GPP in October 1999 and is now widely used in GSM[3] 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 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 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.[4]

Usage

The frames contain 160 samples and are 20 milliseconds long.[1] AMR uses various 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

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 samples 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)

Licensing and patent issues

AMR codecs incorporate several patents of Nokia Corporation, Telefonaktiebolaget L. M. Ericsson, VoiceAge Corporation and Nippon Telegraph and Telephone Corporation.[5][6] 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.[7][8]

The initial fee for professional content creation tools and "real-time channel" products is 6,500 USD. The minimum annual royalty is $10,000, which (in the first year) excludes the initial fee. Per-channel license fees fall from $0.99 to $0.50 with volume, up to a maximum of $2 million annually.[5][6]

In the category of personal computer products (e.g. media players), the AMR decoder is licensed for free. The license fee for a sold encoder falls from $0.40 to $0.30 with volume, up to a maximum of $300,000 annually. The minimum annual royalty is not applied to licensed products which fall under the category of personal computer products and use only the free decoder.[5][6]

More information:

Software support

  • 3GPP TS 26.073 - AMR speech Codec (C source code) - reference implementation[9]
  • Audacity (beta version 1.3) via the FFmpeg integration libraries[10]
  • FFmpeg with OpenCORE AMR libraries[11]
  • Android[12]
  • 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.
  • Nokia Multimedia Converter 2.0 can convert (create) samples, one can use Nokia's conversion tool to create both .amr and .awb files. It works in Windows 7 as well if the setup is run in XP compatibility mode.




from：http://en.wikipedia.org/wiki/Adaptive_Multi-Rate