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.

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

 Frame type (4 bits)
 Frame quality indicator (1 bit)
 Mode indication (3 bits)
 Mode request (3 bits)
 CRC (8 bits)
 Class A bits
 Class B bits
 Class C bits
 

IF2 format

 Frame type (4 bits)
 Class A bits
 Class B bits
 Class C bits
 Padding (called "Bit Stuffing" in spec)

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)

• Frame quality indicator (1 bit) - flag that shows if frame is 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 so-called 'importance function'