Adaptive Multi-Rate audio codec: Difference between revisions

Revision as of 11:18, 23 January 2018

Revision as of 11:33, 23 January 2018

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The '''Adaptive Multi-Rate (AMR or AMR-NB or GSM-AMR) audio codec''' is an [[audio compression format]] optimized for [[speech coding]]. AMR speech codec consists of 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 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>http://www.eetimes.com/electronics-news/4139026/Sorting-Through-GSM-Codecs-A-Tutorial</ref> and [[UMTS]]. It uses [[link adaptation]] to select from one of eight different bit rates based on link conditions.

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== 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.

The frames contain 160 samples and are 20 milliseconds long.<ref name="3gpp-26090" /> AMR uses various techniques,

There are a total of 14 modes of the AMR codec, eight are available in a full rate channel (FR) and six on a 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 [[Discontinuousdiscontinuous Transmissiontransmission]] (DTX), with [[Voicevoice Activityactivity Detectiondetection]] (VAD) and [[Comfort noise|Comfortcomfort Noisenoise Generationgeneration]] (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 [[Lineline spectral pairs]] (LSP).

** The residual waveform is coded using [[Algebraicalgebraic Codecode Excitedexcited Linearlinear Predictionprediction]] (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)

* [[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 ==