What is an AMR file?
A file bearing the .amr extension is an audio format based on the Adaptive Multi-Rate (AMR) codec, a standard specifically engineered for efficient speech encoding. This technology utilizes a multi-rate narrowband speech codec that processes audio signals within a constrained frequency range optimal for the human voice. It operates across a spectrum of bit rates, from a low of 4.75 kbit/s up to 12.2 kbit/s, achieving what is recognized as toll quality speech—comparable to a traditional landline call—starting at just 7.4 kbit/s. A key to its efficiency is link adaptation, a smart technology that dynamically selects the most appropriate bit rate from eight possible options based on real-time network conditions. This ensures optimal clarity while conserving bandwidth.
Key Characteristics of AMR Files
| Characteristic | Description |
|---|---|
| Audio Quality | Speech-Optimized. Excellent for voice, but poor for music or complex sounds due to its narrowband frequency response. |
| File Extension | .amr |
| File Size | Extremely Small. The most efficient format for storing voice, with files often 10x smaller than equivalent MP3 recordings. |
| Compatibility | Excellent on Mobile/Modern Players. Native on billions of mobile devices. May require a specific codec on older desktop systems. |
| Primary Use Case | Voice Recording & Telephony. Ideal for mobile voice calls, voice memos, IVR systems, and any application where speech intelligibility is key. |
| Bitrate | Adaptive (4.75 - 12.2 kbit/s). Dynamically adjusts to audio complexity. |
AMR File Format
AMR file format uses many coding techniques, the ACELP (Algebraic Code Excited Linear Prediction) algorithm one of the best technique; designed for compressing human spoken audio in a more efficient way. The AMR was set as the standard voice or speech codec by 3GPP in 1999. The AMR file format is also used to store the spoken audio by using Adaptive Multi-Rate audio codec which is used by many smart phones to store recorded speeches.
File format structure
The AMR( Adaptive Multi-Rate ) is an audio format; widely used in various mobile applications and devices, typically in audio player/recorder or in VoIP kind of applications. AMR can be further classified as:
- AMR-NB( NarrowBand )
- AMR-WB( WideBand )
Usually, AMR refers to AMR-NB. The AMR file format has the following structure:
Each AMR file contains a 6-byte header that recognize the file as an AMR audio. This header is always set to:
- 0x23
- 0x21
- 0x41
- 0x4D
- 0x52
- 0x0A
This is usually simialr across all AMR-NB files. If the header following a standard, it is likely that the file is corrupted and should not be used. the AMR file consists of a whole number of packed frames of audio. These frames each compose 20ms of audio. Each frame can be encoded using any of the valid AMR-NB modes (0-7, 8 SID in DTX mode). Since, the frames can be encoded at different bit rates, this typical method is called Adaptive Multi-Rate(AMR).
AMR modes
Following are the different AMR modes and their corresponding bitrates:
| MODE | BIT RATES |
|---|---|
| 0 | AMR 4.75 - Encodes at 4.75kbit/s |
| 1 | AMR 5.15 - Encodes at 5.15kbit/s |
| 2 | AMR 5.9 - Encodes at 5.9kbit/s |
| 3 | AMR 6.7 - Encodes at 6.7kbit/s |
| 4 | AMR 7.4 - Encodes at 7.4kbit/s |
| 5 | AMR 7.95 - Encodes at 7.95kbit/s |
| 6 | AMR 10.2 - Encodes at 10.2kbit/s |
| 7 | AMR 12.2 - Encodes at 12.2kbit/s |
Frame size of AMR modes in bytes (including the header byte) are given below:
| CMR | MODE | FRAME SIZE( in bytes ) |
|---|---|---|
| 0 | AMR 4.75 | 13 |
| 1 | AMR 5.15 | 14 |
| 2 | AMR 5.9 | 16 |
| 3 | AMR 6.7 | 18 |
| 4 | AMR 7.4 | 20 |
| 5 | AMR 7.95 | 21 |
FAQ
Q1: Is AMR good for music?
A: No, AMR is highly optimized for speech and will produce poor-quality music recordings because it filters out the high and low frequencies essential for music.
Q2: How can I play an AMR file on my computer?
A: You can play AMR files using versatile media players like VLC Media Player or by installing an AMR codec pack for Windows Media Player.
Q3: Why is my voice memo saved as an AMR file?
A: Your phone’s voice recorder uses AMR by default to save storage space, as it is the most efficient format for capturing speech clearly.
Q4: Can I convert an AMR file to MP3?
A: Yes, you can convert AMR to MP3 using audio conversion software like Audacity or online converters, but the sound quality will not improve as MP3 cannot recreate the frequencies AMR discarded.
Q5: Is AMR still used today?
A: Yes, while newer codecs exist, AMR remains widely used for voice memos, some messaging apps, and as a fallback codec in mobile networks due to its proven efficiency and compatibility.
