AMR-NB: Difference between revisions

From MultimediaWiki
Jump to navigation Jump to search
mNo edit summary
(Tabulate IF1 and IF2)
Line 10: Line 10:
=== IF1 format ===
=== IF1 format ===


  Frame type (4 bits)
{| border=1
  Frame quality indicator (1 bit)
! bits !! low level meaning || high level meaning
  Mode indication (3 bits)
|-
  Mode request (3 bits)
| 4
  CRC (8 bits)
| Frame type
  Class A bits
| rowspan=2 | AMR header
  Class B bits
|-
  Class C bits
| 1
 
| Frame quality indicator
|-
| 3
| Mode indication
| rowspan=3 | AMR auxiliary information
|-
| 3
| Mode request
|-
| 8
| CRC
|-
|  
| Class A bits
| rowspan=3 | AMR core frame
|-
|  
| Class B bits
|-
|  
| Class C bits
|}
 
=== IF2 format ===
=== IF2 format ===


  Frame type (4 bits)
{| border=1
  Class A bits
! bits !! low level meaning || high level meaning
  Class B bits
|-
  Class C bits
| 4
  Padding (called "Bit Stuffing" in spec)
| Frame type
| AMR header
|-
|  
| Class A bits
| rowspan=3 | AMR core frame
|-
|  
| Class B bits
|-
|  
| Class C bits
|-
|  
| Padding (called "Bit stuffing" in the specification)
|  
|}


=== Field meaning ===
=== Field meaning ===

Revision as of 21:55, 22 June 2007

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)
  • 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'