AMR » History » Version 1
laforge, 02/19/2016 10:47 PM
| 1 | 1 | laforge | = AMR (Adaptive Multi Rate) = |
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| 3 | The AMR codec is the latest and most advanced member of the GSM voice codec family. |
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| 4 | |||
| 5 | == Specification == |
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| 6 | |||
| 7 | AMR is specified in the following specs: |
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| 8 | |||
| 9 | * TS 26.090: AMR Speech Codec; Speech Transcoding Functions |
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| 10 | * mathematical description of the encoding functions and adaptive/algebraic codebook |
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| 11 | * TS 26.091: AMR Speech Codec; Error concealment of lost frames |
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| 12 | * TS 26.092; AMR Speech Codec; Comfort Noise Aspects |
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| 13 | * how to detect background noise, encode its parameters and re-create it at the decoder |
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| 14 | * TS 26.093: AMR Speech Codec; Source Controlled Rate Operation (old 06.93) |
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| 15 | * Controlled Rate Operation is what happens during DTX (discontinuous transmission) |
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| 16 | * |
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| 17 | * TS 26.094: AMR Speech Codec; Voice Activity Detector |
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| 18 | * TS 26.101: AMR Speech Codec; Frame Structure |
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| 19 | * defines FR1 and FR2 frame formats used in the back-haul/core network |
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| 20 | * TS 26.073: AMR Speech Codec; ANSI C source code |
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| 21 | * reference implementation of the codec |
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| 22 | * TS 26.074: AMR Speech Codec; Test sequences |
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| 23 | * test sequences for testing a codce implementation |
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| 24 | * TS 45.009: Link Adaption |
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| 25 | * describe the mechanism + inband signalling for rate/link adaption |
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| 26 | * SID, codec mode indication/request/command, RATSCCH |
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| 27 | |||
| 28 | Related specs |
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| 29 | * TS 28.062: Tandem Free Operation |
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| 30 | * a method by which the double-transcoding in a MO-to-MT call can be avoided |
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| 31 | * TS 26.071: Overall description of speech processing |
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| 32 | |||
| 33 | == Details == |
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| 34 | |||
| 35 | === Codec Negotiaiton / Adaption === |
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| 36 | |||
| 37 | At any given time, there is an active set of four speech codecs. Which four is defined by a number of different procedures, e.g. |
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| 38 | * Layer 3 signalling (CHANNEL MODE MODIFY, ASSIGNMENT COMMAND, HANDOVER COMMAND) |
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| 39 | * RATSCCH based signalling, using stealing frames from the speech channel (like FACCH) |
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| 40 | |||
| 41 | During call set-up and/or handover, there is a ''MultiRate configuration IE'' (TS 44.018 10.5.2.21aa) that gives |
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| 42 | a description of the multi-rate function to the MS. |
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| 43 | There can also be CHANNEL MODE MODIFY and HANDOVER COMMAND messages, containing the same IE. |
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| 44 | |||
| 45 | If at channel activation time there is no codec mode specified by Layer3 or RATSCCH, there are some |
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| 46 | default rules (TS 45.009 Chapter 3.4.3) that define which of the codecs will be used for the first speech frames. |
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| 47 | The content of this configuration is a set of codec rates and C/I (carrier/inteference) thresholds, |
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| 48 | which are to be applied. Also, the initial codec mode can be specified. |
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| 49 | |||
| 50 | When one of the network-specified thresholds is reached, the codec mode command/command procedure is |
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| 51 | invoked. |
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| 52 | |||
| 53 | * Downlink |
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| 54 | * if the MS detects a C/I threshold on incoming downlink bursts was hit, it requests a new codec mode using the ''CODEC MODE REQUEST (CMR)'' |
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| 55 | * Uplink |
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| 56 | * if the BTS detects a C/i threshold on incoming uplink bursts was hit, it will instruct the MS to use a different codec in the uplink direction using a ''CODEC MODE COMMAND (CMC)'' |
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| 57 | |||
| 58 | Furthermore, the sender frequently uses the ''CODEC MODE INDICATION (CMI)'' to let the recipient know which codec was used to encode the speech frames. |
