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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 format 1 (IF1) and 2. 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 , mode indication and mode request - for frame types 0-7 they all should be the same and indicate one of eight possible AMR bitrates, frame types 8-11 represent silence, 12-14 are reserved and frame type 15 means there is no data at all
  • 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'