AMR (Adaptive Multi-Rate Audio)
The codec that kept mobile calls intelligible when network conditions were rough.
| Full name | Adaptive Multi-Rate Audio |
| Extension | .amr |
| MIME type | audio/amr |
| Developer | ETSI (standardized by 3GPP; original codec developed by Ericsson, Nokia, and Siemens) |
| Released | 1999 |
| Type | Lossy audio codec, speech-optimized |
| Bit rates | 4.75, 5.15, 5.90, 6.70, 7.40, 7.95, 10.2, 12.2 kbit/s |
| Sample rate | 8000 Hz (narrowband) |
What is a AMR file?
AMR is a speech codec designed for mobile networks. It compresses voice audio into very small files while keeping speech clear enough for calls and voice messages. It became the standard codec for GSM and 3G networks worldwide.
AMR stands for Adaptive Multi-Rate Audio. It is a narrowband codec that encodes frequencies between 200 Hz and 3400 Hz, which covers most of the human voice range. The codec picks from eight different bit rates (4.75 to 12.2 kbit/s) depending on network conditions, dropping quality slightly when the connection is poor rather than dropping the call. It uses an algorithm called ACELP (Algebraic Code Excited Linear Prediction) to model speech efficiently.
History
ETSI launched a standardization program in October 1997 to find a better speech codec for GSM networks. After competitive evaluation, ETSI selected a codec jointly developed by Ericsson, Nokia, and Siemens, and formally approved it in June 1999. 3GPP adopted AMR as the mandatory speech codec for GSM and UMTS in 1999, which put it on hundreds of millions of handsets within a few years.
How it works
An AMR file begins with the ASCII magic string '#!AMR\n' that identifies the format and codec version. After that header, speech is stored as consecutive 20-millisecond frames. Each frame is prefixed by a one-octet header containing a 4-bit frame type field that indicates which of the eight bit rates was used for that frame. Frame data is octet-aligned, with zero-bit padding added at the end if the speech bits do not fill the last byte completely.
What it is used for
- Voice messages recorded on Android phones and older feature phones
- VoIP call recordings on mobile networks
- Archiving phone interview audio where file size matters more than music fidelity
- Embedded systems and IoT devices that record short voice prompts
How to open it
Most smartphones can play AMR files natively since the codec is built into Android and older Nokia and feature phones. On desktop, VLC Media Player and FFmpeg handle AMR without plugins.
Pros and cons
Strengths
- Extremely small file sizes: one minute of speech at 12.2 kbit/s uses under 100 KB
- Adaptive bit rate means quality adjusts gracefully to poor network conditions
- Built into virtually every GSM and 3G device manufactured since 1999
- Standardized by 3GPP, so playback is consistent across hardware vendors
Trade-offs
- Narrowband only: frequencies above 3400 Hz are cut, so music sounds thin and hollow
- Not suitable for anything other than speech; sound effects and music degrade noticeably
- Limited to 8000 Hz sample rate, far below CD quality or even FM radio quality
- Requires a decoder license for some commercial uses, which limited early desktop support
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AMR FAQ
Why do AMR files only sound good for voices?
AMR was built to encode the frequency range of human speech (200-3400 Hz) and nothing else. Music and other audio contains frequencies well above that range, which AMR simply discards.
Is AMR the same as AMR-WB?
No. AMR (also called AMR-NB or narrowband) samples at 8000 Hz and tops out at 12.2 kbit/s. AMR-WB (wideband) samples at 16000 Hz and reaches 23.85 kbit/s, giving noticeably better voice quality on HD Voice calls.
Can I convert AMR to MP3 without losing quality?
You will not lose any additional quality during conversion, but the quality is already limited by how AMR encoded the audio. Converting to MP3 cannot restore frequencies AMR discarded at recording time.
Why did I receive a .amr file from a WhatsApp or phone contact?
Older Android phones and many feature phones record voice messages and memos in AMR by default because it produces tiny files suitable for sending over mobile data connections.