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AMR Definition: What is the AMR Format?

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Introduction to AMR
In digital audio, the letters AMR are short for Adaptive Multi-Rate and relate to the AMR audio format. This audio file format, which was first released in 1999, is especially efficient at compressing and storing voice recordings compared to common formats like MP3, WMA, and AAC for example. It 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 be used to store AMR streams along with video. Incidentally, this type of voice coding technique is sometimes referred to as vocoding.

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 bandwith 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.

Why Does my Portable Use the AMR format Rather Than MP3 for Voice Recordings?
Although the 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 with 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 condensed microphone. In order to make efficient use of the MP3 player's limited storage -- especially if flash based-- the manufacturer of th 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.

Also Known As: AMR-NB, AMR-WB, 3GP, 3GGP
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