Sustainability of Digital Formats
 Planning for Library of Congress Collections

Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact
Format Description Categories >> Browse Alphabetical List

AMR, Adaptive Multi-Rate Speech Codec

>> Back
Table of Contents
Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name AMR, Adaptive Multi-Rate Speech Codec
Description

Audio data compression scheme optimized for speech coding, adopted in October 1998 as the standard speech codec by 3GPP (3d Generation Partnership Project) and now widely used in GSM (Global System for Mobile Communications). In its evolved form, GSM is also known as UMTS (Universal Mobile Telecommunications System).

The AMR bit rates of 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kb/s are based on frames that contain 160 samples and are 20 milliseconds long. AMR uses different techniques, including Algebraic Code Excited Linear Prediction (ACELP) compression, Discontinuous Transmission (DTX), voice activity detection (VAD) and comfort noise generation (CNG). The usage of AMR requires optimized link adaptation that selects the best codec mode to meet the local radio channel and capacity requirements. AMR is also a file format for storing spoken audio; many modern mobile telephone handsets will store short recordings in the AMR format, and commercial programs exist to convert between this and other formats such as MP3. (Preceding adapted from the Wikipedia Adaptive_multi-rate_compression entry.)

At this Web site, AMR-WB+ is treated as a subtype of AMR and, in turn, AMR-WB is treated as a subtype of AMR-WB+ since AMR-WB is more constrained. Comments welcome.

Production phase Final-state for end-user delivery.
Relationship to other formats
    Has subtype AMR-WB+, AMR-WB+, Extended Adaptive Multi-Rate - Wideband Speech Codec
    Used by QuickTime File with AMR encoded audio, not documented at this Web site at this time.
    Used by Other audio file formats not documented at this Web site.

Local use Explanation of format description terms

LC experience or existing holdings The subtype AMR-WB+ has been selected as the audio codec for LC-produced Digital Talking Books; see also DTB_Ext.
LC preference None

Sustainability factors Explanation of format description terms

Disclosure Open standard. Developed by 3GPP, the 3d Generation Partnership Project, a collaboration of telecommunications industry and standards groups in Europe, Asia, and North America. The key European organization is ETSI (European Telecommunications Standards Institute).
    Documentation

3GPP TS 26.071, Digital cellular telecommunications system (Phase 2+); Universal Mobile Telecommunications System (UMTS); AMR Speech Codec; General description, and other specifications listed in Format specifications below. This Web page is intended to be somewhat generic regarding AMR; readers should note, however, that the December 2004 version of this specification is marked as Version 6.0.0 Release 6. The initial version was published in 1999.

According to the VoiceAge AMR Web page (consulted in April 2007), AMR was standardized by ETSI in 1999 and, in 2006, it was included in the CableLabs PacketCable 2.0 specification. These specifications have not been investigated for this Web site at this time.

Adoption Not investigated for this page.
    Licensing and patents Not investigated for this page.
Transparency Depends upon algorithms and tools to read; will require sophistication to build tools.
Self-documentation Not investigated for this page.
External dependencies Not investigated for this page.
Technical protection considerations Not investigated for this page.

Quality and functionality factors Explanation of format description terms

Sound
Normal rendering Not investigated for this page.
Fidelity (high audio resolution) Low; this is a format to carry voices to telephones using reduced bandwidth. Higher quality is provided by the wideband variants of AMR: AMR-WB and AMR-WB+.
Multiple channels None.
Functionality beyond normal rendering Not investigated for this page.

File type signifiers Explanation of format description terms

Tag Value Note
Filename extension amr
From The File Extension Source.
Internet Media Type audio/AMR
From The File Extension Source
Magic numbers Hex: 0x2321414d520a
ASCII: #!AMR\n
From RFC 4867
Tag Value Note
Filename extension 3gp
For files containing various forms of data associated with the 3GPP and 3GPP2 projects; from the File Extension Source. Comments welcome on the applicability of 3gp files to AMR data.
Internet Media Type audio/3gpp
For files containing various forms of audio data associated with the 3GPP and 3GPP2 projects; from the File Extension Source. Comments welcome on the applicability of this MIME type to AMR data.
Magic numbers Hex: 00 00 00
For files with the 3gp extension; from The File Extension Source. Comments welcome on the applicability of 3gp files to AMR data.

Notes Explanation of format description terms

General

From the VoiceAge Web site (consulted in April 2007): The proven, highly efficient and very robust AMR (Adaptive Multi-Rate) narrowband codec is the 3GPP mandatory standard codec for narrowband speech and multimedia messaging services over 2.5G/3G wireless systems based on evolved GSM core networks (WCDMA, EDGE, GPRS). . . . AMR operating at various bit rates is built into every GSM and WCDMA phone, ensuring that content generated by AMR can be played by virtually any wireless phone in the world . . . . AMR operates on narrowband (200-3400 Hz) signals at variable bit rates in the range of 4.75 to 12.2 kbps. It provides toll quality speech starting at 7.4 kbps, with near-toll quality and better robustness at lower rates and better reproduction of non-speech sounds at higher rates. . . . AMR is the only narrowband speech codec offering eight different bit rates that can be adapted according to network congestion.

Although there are file formats associated with AMR (see Signifiers above), the Library of Congress interest in AMR is related to the encoding aspects, since the subtype AMR-WB+ is used in DTB_Ext, DTB (Digital Talking Book), Extension for AMR-WB+ Speech Codec.

History The Wikipedia AMR entry states that 3GPP adoption occurred in 1998; the 3GPP Web site provides 1999 as the first publication date for the descriptive specification. The VoiceAge Web site (consulted in April 2007) included this information: "Initially developed for the GSM system, the single most deployed 2G mobile telecommunication system worldwide, AMR was also standardized by the European Telecommunications Standards Institute (ETSI) in 1999, and in 2006, it was included in the CableLabs PacketCable 2.0 specification."

Format specifications Explanation of format description terms


Useful references

URLs


Last Updated: 02/20/2014