Adaptive Multi-Rate is a codec based on an audio compression format which is used for speech coding. It is considered a hybrid of speech codes and is capable of transmitting both waveform signals and speech parameters. For most third-generation (3G) cellular systems it is a mandatory speech codec and is a preferred codec for the Global System for Mobile Communications (GSM) standard. Adaptive Multi-Rate provides superior audio performance, better coverage and quality, and is easier to implement than previous formats.
The Adaptive Multi-Rate codec provides speech rates ranging from 4.75 to 12.2 kbits/s. The associate sampling frequency is 8 kHz with speech encoding happening on 20 ms frames. It also utilizes technology like comfort noise generation, discontinuous transmission and voice activity detection in order to decrease the usage of bandwidth during periods of silence. One of the distinct features of Adaptive Multi-Rate is its adaptability to radio channel environments and selecting optimal speech, unlike other speech codecs which function at a fixed level of error protection and at a fixed rate. Under bad radio conditions, channel coding is increased and sourced coding is decreased in the case of Adaptive Multi-Rate. Adaptive Multi-Rate makes use of link adaptation in order to choose one of the eight possible bit rates.
Adaptive Multi-Rate has many advantages over other codecs. It can be tailored to meet the different needs of the operators. It also can bring in improved speech quality with the help of codec mode adaptation, even in half-rate mode. It has better power control and improved handover compared to other codecs. It also has increased resistance to interference and errors.
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