View source for Adaptive Multi-Rate audio codec

{{Short description|Audio compression format optimized for speech coding}}
{{Infobox file format
| name                   = Adaptive Multi-Rate (AMR)
| icon =
| logo =
| caption =
| extension              = .amr, .3ga<!-- Samsung Galaxy phones use .3ga -->
| mime                   = audio/amr, audio/3gpp, audio/3gpp2
| type code =
| uniform type =
| magic =
| owner =
| released               = {{Start date|1999|06|23|df=yes}}<!-- {{Start date|YYYY|mm|dd|df=yes}} --><ref name="3gpp-26090">{{cite web |url=http://www.3gpp.org/ftp/Specs/html-info/26090.htm | title=3GPP TS 26.090 - Mandatory Speech Codec speech processing functions; Adaptive Multi-Rate (AMR) speech codec; Transcoding functions |publisher=3GPP |access-date=2010-07-21}}</ref><ref name="3gpp-26071">{{cite web |url=http://www.3gpp.org/ftp/Specs/html-info/26071.htm | title=3GPP TS 26.071 - Mandatory speech CODEC speech processing functions; AMR speech Codec; General description |publisher=3GPP |access-date=2010-07-21}}</ref>
| latest release version = 14.0.0
| latest release date    = {{Start date and age|2017|03|17|df=yes}}<!-- {{Start date and age|YYYY|mm|dd|df=yes}} -->
| type                   = [[Lossy compression|Lossy]] [[Audio file format|audio]]
| container for =
| contained by =
| extended from =
| extended to =
| standard =
| open                   = Yes
| free                   = No
| url                    = 
}}

The '''Adaptive Multi-Rate''' ('''AMR''', '''AMR-NB''' or '''GSM-AMR''') '''audio codec''' is an [[audio compression format]] optimized for [[speech coding]]. AMR is a multi-rate [[narrowband]] speech [[codec]] that encodes narrowband (200–3400&nbsp;Hz) signals at variable bit rates ranging from 4.75 to 12.2&nbsp;kbit/s with toll quality<ref>{{cite web |title=What's toll-quality voice? |url=https://www.itworld.com/article/2783105/what-s-toll-quality-voice-.html |website=ITworld |date=13 December 2000 |access-date=26 July 2019}}</ref> speech starting at 7.4&nbsp;kbit/s.<ref>[http://tools.ietf.org/html/rfc4867#page-5 RFC 4867 - RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs] Page 35</ref>

AMR was adopted as the standard speech [[codec]] by [[3GPP]] in October 1999 and is now widely used in [[GSM]]<ref>{{cite web | url=http://www.eetimes.com/electronics-news/4139026/Sorting-Through-GSM-Codecs-A-Tutorial | title=Sorting Through GSM Codecs: A Tutorial | date=11 July 2003 }}</ref> 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 software|free]] and proprietary programs exist (see [[#Software support|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 <code>.amr</code>. 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 (digital)|container format]] based on ISO base media file format.<ref>[http://tools.ietf.org/html/rfc4867#page-35 RFC 4867 - RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs] Page 35</ref>

== Usage ==
The frames contain 160 samples and are 20 milliseconds long.<ref name="3gpp-26090" /> AMR uses various techniques, such as [[algebraic code-excited linear prediction|ACELP]], [[discontinuous transmission|DTX]], [[voice activity detection|VAD]] and [[comfort noise|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&nbsp;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, eight are available in a [[Full Rate|full rate channel (FR)]] and six on a [[Half Rate|half rate channel (HR)]].

{| class="wikitable"
! Mode !! Bitrate (kbit/s) !! Channel !! Compatible with
|-
| AMR_12.20 || 12.20 || FR || [[Enhanced full rate|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 || [[IS-641|TIA/EIA IS-641 TDMA enhanced full rate]]
|-
| AMR_6.70 || 6.70 || FR/HR || [[PDC-EFR|ARIB 6.7&nbsp;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&nbsp;kHz/13-bit (160 samples for 20&nbsp;ms frames), filtered to 200–3400&nbsp;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&nbsp;kbit/s.
* Generates frame length of 95, 103, 118, 134, 148, 159, 204, or 244 bits for AMR FR bit rates 4.75, 5.15, 5.90, 6.70, 7.40, 7.95, 10.2, or 12.2&nbsp;kbit/s, respectively. AMR&nbsp;HR frame lengths are different.
* AMR utilizes [[discontinuous transmission]] (DTX), with [[voice activity detection]] (VAD) and [[Comfort noise|comfort noise generation]] (CNG) to reduce bandwidth usage during silence periods
* Algorithmic delay is 20&nbsp;ms per frame. For bit-rates of 12.2, there is no "algorithm" look-ahead delay. For other rates, look-ahead delay is 5&nbsp;ms. Note that there is 5&nbsp;ms "dummy" look-ahead delay, to allow seamless frame-wise mode switching with the rest of rates.
* AMR is a hybrid speech coder, and as such transmits both speech parameters and a waveform signal
** [[Linear predictive coding]] (LPC) is used to synthesize the speech from a residual waveform. The LPC parameters are encoded as [[line spectral pairs]] (LSP).
** The residual waveform is coded using [[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 score]]s of 4.14 for AMR (12.2&nbsp;kbit/s), compared to 4.45 for [[G.711]] (μ-law){{citation needed|date=October 2019}}
* PSQM testing under network stress yields [[mean opinion score]]s of 3.79 for AMR (12.2&nbsp;kbit/s), compared to 4.13 for [[G.711]] (μ-law)

== Licensing and patent issues ==
AMR codecs incorporate several [[patent]]s of [[Nokia]], [[Ericsson]], [[Nippon Telegraph and Telephone|NTT]] and VoiceAge,<ref name="voiceage-amr_licterms">{{cite web |author=VoiceAge Corporation |publisher=VoiceAge Corporation |url=http://www.voiceage.com/amr_licterms.php |title=AMR Licensing Terms |date=2007-10-14 |access-date=2009-09-12 |archive-url=https://web.archive.org/web/20071014162046/http://www.voiceage.com/amr_licterms.php |archive-date=2007-10-14}}</ref><ref name="voiceage-amr_licterms2">{{cite web|author=VoiceAge Corporation |publisher=VoiceAge Corporation |url=http://www.voiceage.com/amr_licterms.php |title=AMR Licensing Terms |date=June 2007 |access-date=2009-09-12 |url-status=dead |archive-url=https://web.archive.org/web/20071014162046/http://www.voiceage.com/amr_licterms.php |archive-date=2007-10-14 }}</ref> the last one being the ''License Administrator'' for the AMR [[patent pool]]s. VoiceAge also accepts submission of patents for determination of their possible essentiality to these standards.<ref name="voiceage-licpatentcalls">{{cite web|author=VoiceAge Corporation |publisher=VoiceAge Corporation |url=http://www.voiceage.com/licpatentcalls.php |title=Licensing - Patent Calls |access-date=2009-09-12 |url-status=dead |archive-url=https://web.archive.org/web/20071014161420/http://www.voiceage.com/licpatentcalls.php |archive-date=2007-10-14 }}</ref><ref name="archive-licpatentcalls">{{cite web |author=VoiceAge Corporation |url=http://www.voiceage.com/licpatentcalls.php |title=Licensing - Patent Calls |date=2007-10-14 |access-date=2009-09-12 |archive-url = https://web.archive.org/web/20071014161420/http://www.voiceage.com/licpatentcalls.php |archive-date = 2007-10-14}}</ref>

The initial fee for professional content creation tools and "real-time channel" products is US$6,500.{{When|date=November 2015}} 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.<ref name="voiceage-amr_licterms"/><ref name="voiceage-amr_licterms2"/>

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 that fall under the category of personal computer products and use only the free decoder.<ref name="voiceage-amr_licterms"/><ref name="voiceage-amr_licterms2"/>

More information:
* [http://www.voiceage.com/licensing.php VoiceAge licensing information], including pricing to license the AMR codecs
* [https://web.archive.org/web/20070630180010/http://www.3gpp.org/legal/legal.htm 3GPP legal issues]
* [https://web.archive.org/web/20070630180057/http://www.3gpp.org/faq/faq_2005_2.htm#A3.1 The 3G Patent Platform and its licensing policy]
* [http://www.penguin.cz/~utx/amr AMR Codecs as Shared Libraries] {{Webarchive|url=https://web.archive.org/web/20210411005805/http://www.penguin.cz/~utx/amr |date=2021-04-11 }} — legal notices for usage of amrnb and amrwb libraries based on the reference implementation

== Software support ==
* 3GPP TS 26.073{{snd}}AMR speech Codec (C source code){{snd}}reference implementation<ref>3GPP (2008-12-11) [http://www.3gpp.org/ftp/Specs/archive/26_series/26.073/26073-800.zip 3GPP TS 26.073 - AMR speech Codec], Retrieved 2009-09-08</ref>
* [[Audacity (audio editor)|Audacity]] (beta version 1.3) via the FFmpeg integration libraries<ref name="audacity-amr">Retrieved on 2010-02-28</ref>  (both input and output format)
* [[FFmpeg]] with OpenCORE AMR libraries<ref name="ffmpeg-amr">[http://www.ffmpeg.org/general.html FFmpeg General Documentation - AMR external library], Retrieved on 2009-07-08</ref>
* [[Android (operating system)|Android]]<ref name="android-amr">[http://android.git.kernel.org/?p=platform/external/opencore.git;a=tree;f=codecs_v2/audio/gsm_amr;hb=HEAD Android AMR codecs], Retrieved on 2009-07-08  {{webarchive|url=https://web.archive.org/web/20090218232855/http://android.git.kernel.org/?p=platform%2Fexternal%2Fopencore.git%3Ba%3Dtree%3Bf%3Dcodecs_v2%2Faudio%2Fgsm_amr%3Bhb%3DHEAD |date=February 18, 2009 }}</ref> Used for voice recorder.
* [http://www.penguin.cz/~utx/amr AMR Codecs as Shared Libraries] {{Webarchive|url=https://web.archive.org/web/20210411005805/http://www.penguin.cz/~utx/amr |date=2021-04-11 }}{{snd}}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: [https://sourceforge.net/projects/retrocode/ RetroCode], [https://sourceforge.net/projects/amr2wav/ Amr2Wav], both are in an early developmental stage
* [http://www.amrplayer.com/ AMR Player] is freeware to play AMR audio files, and can convert AMR from/to MP3/WAV audio format.
* [https://web.archive.org/web/20120422173730/http://www.developer.nokia.com/info/sw.nokia.com/id/d1c17a7f-1231-4385-8c17-04f28f4f2d8e/Nokia_Multimedia_Converter_2.0.html 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.
* [[MPlayer]] ([[SMPlayer]], <abbr title="Konqueror Media Player">KMPlayer</abbr><ref>[http://www.softadept.com/read/kmplayer-internal-audio-decoder-preferences-overview/ KMPlayer Internal Audio Decoder Preferences] {{webarchive|url=https://web.archive.org/web/20141022231354/http://www.softadept.com/read/kmplayer-internal-audio-decoder-preferences-overview/ |date=2014-10-22 }}, Retrieved 2014-10-22</ref>)
* Parole Media Player 0.8.1 (in Ubuntu 16.04)
* [[QuickTime]] Player and multimedia framework
* [[RealPlayer]] version 11 and later
* [[VLC media player]] version 1.1.0 and later (input format only, not output format)
* [[ffdshow]]
* Apple [[iPhone]] (can play back AMR files)
* [[iOS]] & [[macOS]] (iMessage)
* [[BlackBerry]] smartphones (used for voice recorder file format, while [[BlackBerry 10]] cannot play AMR format)
* [[K-Lite Codec Pack]]
* [[Media Player Classic Home Cinema#Media Player Classic - Home Cinema|Media Player Classic Home Cinema, around 1.7.1]]
* [[foobar2000]] with the component [https://github.com/unjello/foo_input_amr/ foo_input_amr]

== See also ==
* [[Adaptive Multi-Rate Wideband]] (AMR-WB)
* [[Extended Adaptive Multi-Rate – Wideband]] (AMR-WB+)
* [[Half Rate]]
* [[Full Rate]]
* [[Enhanced Full Rate]] (EFR)
* [[Sampling rate]]
* [[IS-641]]
* [[3GP]]
* [[Comparison of audio coding formats]]
* [[RTP audio video profile]]

== References ==
{{reflist|30em}}

== External links ==
* [http://www.3gpp.org/ftp/Specs/html-info/26090.htm 3GPP TS 26.090{{snd}}Mandatory Speech Codec speech processing functions; Adaptive Multi-Rate (AMR) speech codec; Transcoding functions]
* [http://www.3gpp.org/ftp/Specs/html-info/26071.htm 3GPP TS 26.071{{snd}}Mandatory Speech Codec speech processing functions; AMR Speech Codec; General Description]
* [http://www.3gpp.org/ftp/Specs/html-info/26-series.htm 3GPP codecs specifications; 3G and beyond / GSM, 26 series]
* {{IETF RFC|4867|link=no}}{{snd}}RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs
* {{IETF RFC|4281|link=no}}{{snd}}The Codecs Parameter for "Bucket" Media Types

{{Compression formats}}

[[Category:Speech codecs]]
[[Category:1999 software]]