The MP3 is known to be one of the most popular audio file formats known to most people around the civilized world today. However, many people may not realize it, but they also may be using audio files with the AMR file format for a good number of times too. If you find yourself buying a cell phone model which came out at least 2 years ago, chances are it will probably have a voice recorder integrated with it. When you copy these recordings onto your computer, you will find out that these files have .amr file extension.

AMR stands for Adaptive Multi-Rate, and it is named for the philosophy behind it. There are unique data constraints which are defined by the compression scheme which makes audio files. These constraints are integral to the principle that as interference from the audio source increases, codec rates are lowered to allow more effective application of error correction. It is for this matter that AMR audio file players and encoders have a good range of data rates for the overall scheme to adjust to.

AMR is used primarily in speech coding, and it also incorporates data compression to ensure that more .amr files can be saved in a single device. When an AMR file is played, one will not necessarily notice a considerable variance in audio quality. Of course, this is considering if the original source is of good quality, and there are no significant noise interferences in the background. It is also possible to record music to be saved in AMR file, however the chances of the audio quality being top quality are slim.

The AMR file format is best known for applications and other features integrated in modern cellular phones. These programs range from simple audio player/recorders to programs linked to VoIP (Voice over Internet Protocol) technology.

There have been other file formats used by cell phones prior to the introduction of AMR, such as the EFR or Enhanced Full-Rate codec. AMR’s flexibility to adjust its bit rate depending on the quality of the audio source makes it stand out among these other alternatives. Additionally, the features involved in the AMR format make the resultant file intricate in all considerable aspects.

First, there is VAD, or Voice Activity Detection. This AMR feature simply differentiates instances of speech from instances of silence coming from an audio source. There are times where the AMR file format utilizes VAD to improve the quality of the speech recording. Depending on the properties set on a particular cell phone or program, AMR can use VAD to stop recording during dead air to save space and to shave time off the length of the file.

There’s also CNG, short for Comfort Noise Generation. As the name implies, this feature introduces background static in a very low and tolerable level. Why would adding static while recording a good thing? It minimizes a considerable number of bad effects derived from absolute silence. For one, it keeps the codec rates in a decent level, in preparation for the possibility of disruption from sudden increases in volume. It also gives the person on both sides of the line the assurance that the phone connection is still active, keeping either one of them from hanging up during instances of dead air.

Discontinuous Transmission (DTX) is a feature used particularly in mobile devices with AMR audio file format capabilities. This feature saves power by controlling the device’s transmitter switch to make sure that the battery and amplifiers are not used during those instances of dead air during wireless conversations. It would mute a mobile phone when there is no voice input being made.

These three features of AMR are used together for the main purpose of reducing the usage of precious bandwidth during periods of silence.

Finally, there’s ACELP, which is short for Algebraic Code Excited Linear Prediction. AMR makes use of this sub-algorithm in integral parts of encoding, specifically during times when codec rates are adjusted to ensure continuous recording despite audio interference. As the name implies, it is a set of computation which determine the next best codec rate for the AMR encoder to adjust, where the audio source shows an abrupt change in volume or interference.

What does all of this mean? It simply implies that files of AMR format may be underestimated in terms of audio quality, but its features still make it the premier file format when it comes to recording conversations and other forms of speech from a cell phone. Almost all recordings which come from mobile devices may be converted to audio file formats of better quality, but they have to go through the basic process of AMR formatting first.