AMR (Adaptive Multi-Rate) is a compressed audio file format used for storing audio data encoded with the patented Adaptive Multi-Rate audio codec. The format was originally introduced and popularized by the Ericsson corporation, but can now be found on every modern mobile device.
The AMR codec is based primarily on the Algebraic Code-Excited Linear Prediction algorithm (ACELP), developed by the VoiceAge Corporation. In October 1998, AMR was adopted by the 3GPP organization as the standard audio codec for the 3G mobile system specification, making Adaptive Multi-Rate encoding dominant in VoIP and mobile communications. With that decision, an increasing number of applications were being developed to natively work with and export AMR files, gradually setting the AMR file format as the standard for storing audio on mobile devices.
However, AMR compression is only useful on human speech recordings, as the ACELP algorithm is designed specifically with that purpose in mind. Therefore, AMR-encoding more complex audio data, such as music for example, usually produces unacceptable results in terms of accuracy and sound quality. Modern smartphones provide alternative applications designed primarily for recording music, which produce better results. These applications don't use AMR at all and let you store your recordings in a more convenient format like WAV instead.
Speaking of AMR files, there is a common misconception about the type of codecs AMR files use. There are indeed two variations of the AMR codec: AMR-NB (Narrow Band) and AMR-WB (Wide Band), but AMR files only use the AMR-NB codec. The AMR-NB codec provides encoding at low bitrates, while its Wide Band alternative is used mostly for transmitting high-quality voice over the mobile network and/or for storing speech recordings in AWB files.
Although widely popular on mobile devices, the AMR file format is still uncommon for desktop environments. Fortunately, AMR files can easily be converted to more convenient desktop audio formats such as WAV or OGG, using free applications like the MIKSOFT Mobile Media Converter.
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