Adaptive multi-rate compression

Adaptive Multi-Rate (AMR) is a patented 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. 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.^[1]

Usage

The frames contain 160 samples and are 20 milliseconds long.^[2] 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).

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)

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 embedding of possibly patented source code into many Open Source projects.

See also

• AMR-WB
• AMR-WB+
• Half Rate
• Full Rate
• Sampling rate
• IS-641
• 3GP

References

External links

• Open Source software to convert the .amr format: Amr2Wav, RetroCode, both are in an early developmental stage
• Mobile AMR Converter is freeware
• AMR Player is freeware to play AMR audio files, and can convert AMR with MP3/WAV audio format.
• 3GPP TS 26.090 - Mandatory Speech Codec speech processing functions; Adaptive Multi-Rate (AMR) speech codec; Transcoding functions
• 3GPP TS 26.071 - Mandatory Speech Codec speech processing functions; AMR Speech Codec; General Description
• 3GPP codecs specifications; 3G and beyond / GSM, 26 series
• RFC 4867 - RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs