Telecom ABC - A
Adaptive Multi-Rate (AMR) is the new speech codec for GSM and UMTS. There are two different versions of AMR. The basic version, also called AMR-Narrowband (AMR-NB), which is mainly intended for use by GSM and AMR-Wideband (AMR-WB), which is mainly intended for use by UMTS. Wideband AMR uses a speech bandwidth of 50 - 7000 Hz, whereas the bandwidth of narrowband AMR is 300 - 3400 Hz. This gives wideband AMR a more natural speech quality.
The speech codec modes perform also error correction and bad-frame detection. This is done by a combination of convolutional coding for the error correction and a cyclic redundancy check (CRC) for the detection of bad frames.
Choosing the right mode
The codec mode chosen in the uplink may be different from the one used in the downlink direction, but the channel mode (HR or FR) must be the same.
AMR also contains voice activity detection and discontinuous transmission (VAD/DTX). These are used to switch off the encoding and transmission during periods of silence, thereby reducing radio interference and extending the battery lifetime.
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