The 4-bit FT field of each frame
header names the frame’s mode. Because each mode has a fixed bit count, FT also gives
the frame’s length. These tables come from 3GPP TS 26.101 (AMR), TS 26.201 (AMR-WB) and
RFC 4867. Class A bits are the ones most sensitive to errors, and class C the least.
AMR (narrowband)
| FT | Mode | Rate | Bits | Class A | Class B | Class C | Bytes in a file, with header |
|---|---|---|---|---|---|---|---|
| 0 | AMR 4.75 | 4.75 kbit/s | 95 | 42 | 53 | 0 | 13 |
| 1 | AMR 5.15 | 5.15 kbit/s | 103 | 49 | 54 | 0 | 14 |
| 2 | AMR 5.90 | 5.90 kbit/s | 118 | 55 | 63 | 0 | 16 |
| 3 | AMR 6.70 | 6.70 kbit/s | 134 | 58 | 76 | 0 | 18 |
| 4 | AMR 7.40 | 7.40 kbit/s | 148 | 61 | 87 | 0 | 20 |
| 5 | AMR 7.95 | 7.95 kbit/s | 159 | 75 | 84 | 0 | 21 |
| 6 | AMR 10.2 | 10.2 kbit/s | 204 | 65 | 99 | 40 | 27 |
| 7 | AMR 12.2 | 12.2 kbit/s | 244 | 81 | 103 | 60 | 32 |
| 8 | AMR SID | comfort noise | 39 | 39 | 0 | 0 | 6 |
| 9 | GSM-EFR SID | not in files | 43 | not used in the file format | – | ||
| 10 | TDMA-EFR SID | not in files | 38 | not used in the file format | – | ||
| 11 | PDC-EFR SID | not in files | 37 | not used in the file format | – | ||
| 12–14 | reserved | 1 | |||||
| 15 | NO_DATA | no transmission | 0 | 0 | 0 | 0 | 1 |
The frame length is the bits rounded up to bytes, plus the header byte. The AMR 12.2 mode is the same codec as GSM Enhanced Full Rate, and 7.40 matches IS-641 and 6.70 PDC-EFR. An AMR SID frame holds 35 bits of comfort-noise data, one STI bit (0 for a first SID, 1 for an update) and a 3-bit mode indication.
AMR-WB (wideband)
| FT | Mode | Rate | Bits | Bytes in a file, with header |
|---|---|---|---|---|
| 0 | AMR-WB 6.60 | 6.60 kbit/s | 132 | 18 |
| 1 | AMR-WB 8.85 | 8.85 kbit/s | 177 | 24 |
| 2 | AMR-WB 12.65 | 12.65 kbit/s | 253 | 33 |
| 3 | AMR-WB 14.25 | 14.25 kbit/s | 285 | 37 |
| 4 | AMR-WB 15.85 | 15.85 kbit/s | 317 | 41 |
| 5 | AMR-WB 18.25 | 18.25 kbit/s | 365 | 47 |
| 6 | AMR-WB 19.85 | 19.85 kbit/s | 397 | 51 |
| 7 | AMR-WB 23.05 | 23.05 kbit/s | 461 | 59 |
| 8 | AMR-WB 23.85 | 23.85 kbit/s | 477 | 61 |
| 9 | AMR-WB SID | comfort noise | 40 | 6 |
| 10–13 | reserved | 1 | ||
| 14 | SPEECH_LOST | lost frame | 0 | 1 |
| 15 | NO_DATA | no transmission | 0 | 1 |
An AMR-WB SID frame’s 40 bits are all class A: 35 of comfort-noise data, one STI bit and a 4-bit mode indication.
Modes and sample rates
| Codec | Sample rate | Frame | Samples | Audio band |
|---|---|---|---|---|
| AMR | 8,000 Hz | 20 ms | 160 | about 200 to 3,400 Hz |
| AMR-WB | 16,000 Hz | 20 ms | 320 | about 50 to 7,000 Hz |