AMR-NB

• 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.