What Is the Adaptive Multi-Rate (AMR) Format?

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Released in 1999, AMR, also known as Adaptive Multi-Rate, is an audio file format that's especially efficient at compressing and storing voice recordings compared to common formats like MP3, WMA, and AAC.

AMR is a lossy format with files commonly identified with the .amr extension—the exception to this rule is the 3GP container format can also be used to store AMR streams along with video.

AMR Narrowband and Wideband Versions

There are essentially two AMR format standards which are AMR-NB and AMR-WB. The first one (AMR-NB), is a narrowband version which is commonly used in situations where low bitrates are sufficient -- such as a basic voice recording facility you may have on your MP3 player. The frequency range used for AMR-NB is 300-3400 Hz which is capable of producing sound quality that is comparable to traditional telephone. This narrowband version uses the following bitrates:

  • 04.75 kbps
  • 05.15 kbps
  • 05.90 kbps
  • 06.70 kbps
  • 07.40 kbps
  • 07.95 kbps
  • 10.20 kbps
  • 12.20 kbps

The second version of AMR is the wideband type which is represented by the acronym, AMR-WB. As the name would suggest, this is an enhanced vocoder that uses a wider bandwidth than AMR-NB in order to store voice at a much higher quality -- the frequency range used is for this is 50 -7000 Hz. The bitrates used for the wideband version of AMR are:

  • 06.60 kbps
  • 08.85 kbps
  • 12.65 kbps
  • 14.25 kbps
  • 15.85 kbps
  • 18.25 kbps
  • 19.85 kbps
  • 23.05 kbps
  • 23.85 kbps

Due to its higher frequency range and therefore superior speech quality, AMR-WB is optimized for use in GSM (Global System for Mobile Communications) and UMTS (Universal Mobile Telecommunications System) technologies -- otherwise known as 2G and 3G mobile networks respectively. 

AMR vs. MP3 for Voice Recordings

Although the MP3 format is probably the most popular audio format in existence, it isn't particularly efficient (compared to AMR) when it comes to encoding speech. The AMR format, on the other hand, is excellent at such a task and is the preferred format even though it isn't as widely supported by hardware and software.

The most common application for AMR that you are likely to come across in digital music is using a portable device (such as an MP3 player or smartphone) to capture sound; many MP3 players these days can double as voice recorders using a built-in condenser microphone. In order to make efficient use of the MP3 player's limited storage -- especially if flash-based-- the manufacturer of the device may choose to use the AMR format. Files in the AMR format are significantly smaller on average than the more popular formats used to store music -- like MP3, AAC, WAV, and WMA for instance.​