Usage
The frames contain 160 samples and are 20 milliseconds long. AMR uses various techniques, such as ACELP, DTX, VAD and CNG. The usage of AMR requires optimized link adaptation that selects the best codec mode to meet the local radio channel and capacity requirements. If the radio conditions are bad, source coding is reduced and channel coding is increased. This improves the quality and robustness of the network connection while sacrificing some voice clarity. In the particular case of AMR this improvement is somewhere around S/N = 4-6 dB for usable communication. The new intelligent system allows the network operator to prioritize capacity or quality per base station.
There are a total of 14 modes of the AMR codec, 8 are available in a full rate channel (FR) and 6 on a half rate channel (HR).
| Mode | Bitrate (kbit/s) | Channel | Compatible with |
|---|---|---|---|
| AMR_12.20 | 12.20 | FR | ETSI GSM enhanced full rate |
| AMR_10.20 | 10.20 | FR | |
| AMR_7.95 | 7.95 | FR/HR | |
| AMR_7.40 | 7.40 | FR/HR | TIA/EIA IS-641 TDMA enhanced full rate |
| AMR_6.70 | 6.70 | FR/HR | ARIB 6.7 kbit/s enhanced full rate |
| AMR_5.90 | 5.90 | FR/HR | |
| AMR_5.15 | 5.15 | FR/HR | |
| AMR_4.75 | 4.75 | FR/HR | |
| AMR_SID | 1.80 | FR/HR |
Read more about this topic: Adaptive Multi-Rate Audio Codec
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