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.