AMR-NB
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- samples: http://samples.mplayerhq.hu/amr/
- specification: http://www.3gpp.org/ftp/Specs/html-info/26-series.htm
- Wikipedia article: http://en.wikipedia.org/wiki/Adaptive_Multi-Rate
AMR-NB (Adaptive Multi-Rate Narrowband) is a vocoder employed in low-bitrate applications like mobile phones.
Bit stream frame format
Specification (26.101) describes two possible frame types - interface formats 1 and 2 (often abbreviated IF1 and IF2). IF2 is byte-aligned.
IF1 format
| bits | low level meaning | high level meaning |
|---|---|---|
| 4 | Frame type | AMR header |
| 1 | Frame quality indicator | |
| 3 | Mode indication | AMR auxiliary information |
| 3 | Mode request | |
| 8 | CRC | |
| Class A bits | AMR core frame | |
| Class B bits | ||
| Class C bits |
IF2 format
| bits | low level meaning | high level meaning |
|---|---|---|
| 4 | Frame type | AMR header |
| Class A bits | AMR core frame | |
| Class B bits | ||
| Class C bits | ||
| Padding (called "Bit stuffing" in the specification) |
Field meaning
| Frame type | Frame content |
|---|---|
| 0 | AMR 4.75kbps |
| 1 | AMR 5.15kbps |
| 2 | AMR 5.90kbps |
| 3 | AMR 6.70kbps (PDC-EFR) |
| 4 | AMR 7.40kbps (TDMA-EFR) |
| 5 | AMR 7.95kbps |
| 6 | AMR 10.2kbps |
| 7 | AMR 12.2kbps (GSM-EFR) |
| 8 | AMR SID |
| 9 | GSM-EFR SID |
| 10 | TDMA-EFR SID |
| 11 | PDC-EFR SID |
| 12-14 | Reserved for future use |
| 15 | No data (no transmission/no reception) |
Notes
- Frame quality indicator bit: 0 bad/corrupted (can use error concealment), 1 good
- CRC (8 bits) - CRC with polynomial x^8+x^6+x^5+x^4+1 computed over Class A bits
- Class A bits - the most important data
- Class B bits - less important data
- Class C bits - additional not very important data that may be present only on higher bitrates (frame types 6 and 7)
How to smear bits between all these classes is defined by a so-called 'importance function'