AMR-NB: Difference between revisions
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For the specifics of the bit stream layout (i.e. for the bits to parameter mappings) see [http://www.3gpp.org/ftp/Specs/html-info/26101.htm 26.101 AMR speech codec frame structure]. | For the specifics of the bit stream layout (i.e. for the bits to parameter mappings) see [http://www.3gpp.org/ftp/Specs/html-info/26101.htm 26.101 AMR speech codec frame structure] and [http://www.3gpp.org/ftp/Specs/html-info/26090.htm 26.090 AMR speech codec transcoding functions]. | ||
[[Category:Vocoders]] | [[Category:Vocoders]] | ||
[[Category:Audio Codecs]] | [[Category:Audio Codecs]] | ||
[[Category: Formats missing in FFmpeg]] | [[Category: Formats missing in FFmpeg]] | ||
Revision as of 10:18, 8 August 2007
- 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. The following tables of data are taken from this specification unless otherwise stated.
IF1 format
| bits | low level meaning | high level meaning |
|---|---|---|
| 4 | Frame type | AMR header |
| 1 | Frame quality indicator (0 bad/1 good) | |
| 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) |
Classes
| Class | Importance explanation |
|---|---|
| A | Data that is most sensitive to error. Any error in these bits leads to a corrupted speech frame that should not be decoded without appropriate error concealment. This class of bits is protected by an 8-bit CRC. |
| B | Less sensitive data that are present in all speech frames. |
| C | Least sensitive data present only in higher bit rate frames. |
- Class A is protected by an 8-bit CRC with polynomial x^8+x^6+x^5+x^4+1 computed over the Class A bits.
- There is no significant step-wise change in subjective importance at class boundaries.
- The distribution of bits is ordered from most to least subjective importance at both the class level and within the classes.
| Frame type | Total bits | Class A bits | Class B bits | Class C bits |
|---|---|---|---|---|
| 0 | 95 | 42 | 53 | 0 |
| 1 | 103 | 49 | 54 | 0 |
| 2 | 118 | 55 | 63 | 0 |
| 3 | 134 | 58 | 76 | 0 |
| 4 | 148 | 61 | 87 | 0 |
| 5 | 159 | 75 | 84 | 0 |
| 6 | 204 | 65 | 99 | 40 |
| 7 | 244 | 81 | 103 | 60 |
| 8 | 39 | 39 | 0 | 0 |
| 9 | 43 | 43 | 0 | 0 |
| 10 | 38 | 38 | 0 | 0 |
| 11 | 37 | 37 | 0 | 0 |
For the specifics of the bit stream layout (i.e. for the bits to parameter mappings) see 26.101 AMR speech codec frame structure and 26.090 AMR speech codec transcoding functions.