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.

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'