A Beginner's Guide on AMR Files

Written by John Harris, Updated on October 06, 2023

AMR stands for Adaptive Multi Rate. AMR is a common audio file format used for various mobile devices such as cell phones and other mobile devices for applications ranging from normal audio players/recorders to VoIP applications. AMR files are mainly optimized for recording speech, i.e., creating voice messages and voice memos on cell phones. It was developed by Ericson and became the standard speech codec in 1998. Most modern mobile telephone handsets will support AMR.

AMR files can be encoded and decoded using the AMR codec. The AMR codec is royalty-free, meaning it can be used without paying any licensing fees. This makes AMR a popular choice for storing and transmitting audio data.

AMR files can be played using a variety of media players, including Windows Media Player, VLC Media Player, and QuickTime Player.

These are the compressed audio files that are extensively used by many 3G cell phones for voice recordings such as MMS messages. It incorporates the Algebraic Code Excited Linear Prediction (ACELP) algorithm to compress human speech audio securely. However, AMR file does not provide good audio quality for audio playback, i.e., non-voice content.

AMR is the only narrowband speech codec that provides eight different bit rates, which can be adapted according to the network congestion. This enables a significant enhancement in the Quality of the audio. AMR files use a link adaptation technique to select from one of the eight-bit rates based on link conditions.

File Extension: .amr

Mime type: audio/amr

Types of AMR files:

There are two AMR file formats: AMR- Narrow Band and AMR-Broad band. Below is a brief description of the two:

AMR-NB (Narrowband): This version is commonly used in situations like basic voice recording on your MP3 player, where low bitrates are sufficient. The frequency range used for AMR-NB is 300-3400 Hz, which can produce sound quality equivalent to the traditional telephone. This narrowband version uses the bitrates of 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 (Wideband): As the name suggests, this enhanced vocoder uses a wider bandwidth than AMR-NB. It stores voice at a much higher quality, i.e., the frequency range used in this ranges between 50 -7000 Hz. Because of its higher frequency range and superior speech quality, AMR-WB is improved for use in GSM (Global System for Mobile Communications), i.e., 2G technology, and UMTS (Universal Mobile Telecommunications System), i.e., 3G technologies.

The bitrates used for the AMR Wideband version include 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.

Main features of AMR:

  • AMR codec uses eight basic bit rates: 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kbps.
  • AMR utilizes Discontinuous Transmission (DTX) in combination with Voice Activity Detection (VAD) and Comfort Noise Generation (CNG); this reduces the use of bandwidth during a quiet sound.
  • Coded AMR media can be encapsulated in the 3GP or MPEG-4 file formats and used along the multimedia applications' delivery chain.

Applications of AMR

  • Multimedia services for 3G mobile communication systems
  • Unified Messaging
  • Content creation tools and content downloads
  • Internet applications
  • Streaming
  • Voice over IP
  • Portable audio devices
  • Conferencing
  • Wi-Fi telephony
  • Digital radio broadcasting
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