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

Compress and store voice recordings

Released in 1999, Adaptive Multi-Rate (AMR) 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 that the 3GP container format can also store AMR streams and video.

AMR Narrowband and Wideband Versions

There are essentially two AMR format standards: AMR-NB and AMR-WB.

AMR-NB is a narrowband version that's 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 to 3400 Hz, which can produce sound quality comparable to a 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

AMR-WB is the wideband AMR. As the name suggests, this is an enhanced vocoder that uses a wider bandwidth than AMR-NB to store voice at a higher quality. The frequency range used for this is 50 to 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 superior speech quality, AMR-WB is optimized for Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) 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 encoding speech. However, the AMR format is excellent for encoding speech and is preferred even though it isn't widely supported by hardware and software.

The most common application for AMR that you are likely to come across in digital music is when using a portable device such as an MP3 player or smartphone to capture sound. Many MP3 players can double as voice recorders by using a built-in condenser microphone.

To efficiently use the MP3 player's limited storage (especially if flash-based), the device's manufacturer may choose to use the AMR format. Files in the AMR format are smaller on average than the popular formats used to store music, like MP3, AAC, WAV, and WMA.​

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