AMR » History » Version 2
laforge, 02/19/2016 10:48 PM
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| 2 | 2 | laforge | h1. AMR (Adaptive Multi Rate) |
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| 5 | 1 | laforge | The AMR codec is the latest and most advanced member of the GSM voice codec family. |
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| 8 | 2 | laforge | h2. Specification |
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| 11 | 1 | laforge | AMR is specified in the following specs: |
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| 13 | 2 | laforge | * TS 26.090: AMR Speech Codec; Speech Transcoding Functions |
| 14 | ** mathematical description of the encoding functions and adaptive/algebraic codebook |
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| 15 | * TS 26.091: AMR Speech Codec; Error concealment of lost frames |
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| 16 | * TS 26.092; AMR Speech Codec; Comfort Noise Aspects |
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| 17 | ** how to detect background noise, encode its parameters and re-create it at the decoder |
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| 18 | * TS 26.093: AMR Speech Codec; Source Controlled Rate Operation (old 06.93) |
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| 19 | ** Controlled Rate Operation is what happens during DTX (discontinuous transmission) |
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| 20 | ** |
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| 21 | * TS 26.094: AMR Speech Codec; Voice Activity Detector |
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| 22 | * TS 26.101: AMR Speech Codec; Frame Structure |
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| 23 | ** defines FR1 and FR2 frame formats used in the back-haul/core network |
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| 24 | * TS 26.073: AMR Speech Codec; ANSI C source code |
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| 25 | ** reference implementation of the codec |
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| 26 | * TS 26.074: AMR Speech Codec; Test sequences |
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| 27 | ** test sequences for testing a codce implementation |
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| 28 | * TS 45.009: Link Adaption |
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| 29 | ** describe the mechanism + inband signalling for rate/link adaption |
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| 30 | ** SID, codec mode indication/request/command, RATSCCH |
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| 31 | 1 | laforge | |
| 32 | Related specs |
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| 33 | 2 | laforge | * TS 28.062: Tandem Free Operation |
| 34 | ** a method by which the double-transcoding in a MO-to-MT call can be avoided |
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| 35 | * TS 26.071: Overall description of speech processing |
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| 38 | 2 | laforge | h2. Details |
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| 42 | h3. Codec Negotiaiton / Adaption |
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| 45 | 1 | laforge | 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. |
| 46 | 2 | laforge | * Layer 3 signalling (CHANNEL MODE MODIFY, ASSIGNMENT COMMAND, HANDOVER COMMAND) |
| 47 | * RATSCCH based signalling, using stealing frames from the speech channel (like FACCH) |
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| 48 | 1 | laforge | |
| 49 | 2 | laforge | During call set-up and/or handover, there is a _MultiRate configuration IE_ (TS 44.018 10.5.2.21aa) that gives |
| 50 | 1 | laforge | a description of the multi-rate function to the MS. |
| 51 | There can also be CHANNEL MODE MODIFY and HANDOVER COMMAND messages, containing the same IE. |
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| 53 | If at channel activation time there is no codec mode specified by Layer3 or RATSCCH, there are some |
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| 54 | 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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| 55 | The content of this configuration is a set of codec rates and C/I (carrier/inteference) thresholds, |
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| 56 | which are to be applied. Also, the initial codec mode can be specified. |
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| 58 | When one of the network-specified thresholds is reached, the codec mode command/command procedure is |
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| 59 | invoked. |
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| 61 | 2 | laforge | * Downlink |
| 62 | ** 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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| 63 | * Uplink |
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| 64 | ** 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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| 65 | 1 | laforge | |
| 66 | 2 | laforge | Furthermore, the sender frequently uses the _CODEC MODE INDICATION (CMI)_ to let the recipient know which codec was used to encode the speech frames. |
