AMR-NB

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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.