Adaptive Multi-Rate Audio
Convert AMR voice recordings to MP3 or WAV directly in your browser — no upload, no server, full privacy. FileDex uses FFmpeg WebAssembly to decode the mobile speech codec and re-encode it locally on your device.
Your files never leave your device
Common questions
How do I convert AMR to MP3 in my browser?
Drop your AMR file into FileDex above. Select MP3 as the output format and click Convert. FileDex decodes the AMR speech codec and re-encodes to MP3 entirely in your browser using FFmpeg WebAssembly — no file is uploaded to any server.
Why does my AMR file sound so low quality?
AMR-NB samples audio at only 8 kHz with a maximum bitrate of 12.2 kbps — designed for phone-quality speech, not music. Converting to MP3 will not improve the audio quality since the high-frequency data was never captured. AMR-WB (HD Voice) at 16 kHz sounds noticeably better.
What programs can open AMR files?
VLC Media Player plays AMR natively on all platforms. Android devices have built-in AMR support. On Windows, AMR Player is a lightweight free option. For conversion, FFmpeg handles both AMR-NB and AMR-WB via the opencore-amr library.
Is AMR still used in modern phones?
AMR-WB remains active for VoLTE (HD Voice) calls on 4G networks. For voice recordings, most modern phones now default to AAC or Opus instead of AMR. The EVS codec is replacing AMR-WB in 5G networks.
What makes .AMR special
What is an AMR file?
AMR (Adaptive Multi-Rate) is an audio compression format optimized specifically for speech coding on mobile networks. Developed by the European Telecommunications Standards Institute (ETSI) and standardized by 3GPP, AMR became the mandatory speech codec for GSM networks and is widely used in 3G (UMTS) networks. The format earned its "Adaptive Multi-Rate" name because it dynamically switches between eight different bitrates (4.75 to 12.2 kbps for AMR-NB) based on network conditions and available bandwidth.
Continue reading — full technical deep dive
AMR prioritizes small file sizes and intelligibility of human speech — qualities that make it ideal for voice calls and recordings on older mobile hardware, but unsuitable for music where audio fidelity matters.
How to open AMR files
- VLC Media Player (Windows, macOS, Linux) — Free, plays AMR natively
- QuickTime Player (macOS) — Built-in support on Mac
- AMR Player (Windows) — Lightweight free player dedicated to AMR
- FFmpeg — Command-line tool for playback and conversion
- Android — Native support in the built-in voice recorder and media player
Technical specifications
| Property | Value |
|---|---|
| Variants | AMR-NB (Narrowband, 8 kHz), AMR-WB (Wideband, 16 kHz) |
| Sample rate | 8,000 Hz (NB) / 16,000 Hz (WB) |
| Bitrates | AMR-NB: 4.75–12.2 kbps; AMR-WB: 6.6–23.85 kbps |
| Channels | Mono only |
| Frame size | 20 ms per frame |
| Standard | 3GPP TS 26.071 (NB), TS 26.171 (WB) |
| Magic bytes | 23 21 41 4D 52 (#!AMR in ASCII) |
AMR-NB vs AMR-WB
| Variant | Sample rate | Quality | Use |
|---|---|---|---|
| AMR-NB (Narrowband) | 8 kHz | Phone-quality speech | 2G/3G calls, basic voice memos |
| AMR-WB (Wideband) | 16 kHz | HD Voice quality | VoLTE, modern VoIP, better intelligibility |
AMR-WB (also called G.722.2) is significantly better quality — voices sound more natural and are easier to understand at the same bitrate. It is the codec behind "HD Voice" calls on modern carriers.
Common use cases
- Mobile voice recordings: Android's built-in voice recorder often saves in AMR format by default on older devices
- MMS messages: Audio clips sent in multimedia messages use AMR for compact file size
- VoIP and cellular calls: The AMR codec runs in real-time on baseband processors during actual phone calls
- Voicemail storage: Mobile carriers and voicemail systems store messages as AMR files
Converting AMR to MP3 or WAV
AMR files are not widely supported outside mobile devices. Convert to MP3 or WAV for compatibility with all players:
Using FFmpeg:
# AMR to MP3
ffmpeg -i recording.amr recording.mp3
# AMR to WAV
ffmpeg -i recording.amr recording.wav
# AMR-WB (higher quality) to MP3 at 128kbps
ffmpeg -i recording.amr -b:a 128k output.mp3
Online tools: CloudConvert, Zamzar, and similar services convert AMR files in the browser without software installation.
Audio quality limitations
AMR was optimized for speech, not music. The 8 kHz sample rate of AMR-NB limits audio to frequencies below 4 kHz — music sounds "tinny" and lacks bass. If you receive an AMR file containing music, converting it to MP3 will not recover lost quality; the high-frequency information was never captured. AMR recordings are perfectly clear for speech but will never approach music quality regardless of conversion format.
.AMR compared to alternatives
| Formats | Criteria | Winner |
|---|---|---|
| .AMR vs .MP3 | Audio quality AMR-NB samples at 8 kHz (4 kHz bandwidth), producing phone-quality speech. MP3 at 128 kbps with 44.1 kHz sampling captures the full audible spectrum. AMR is optimized for speech intelligibility at minimal bitrate, not audio fidelity. | MP3 wins |
| .AMR vs .OPUS | Speech compression Opus outperforms AMR at all bitrates for both speech and music, with lower latency and no licensing restrictions. Opus at 12 kbps exceeds AMR-NB quality at 12.2 kbps while supporting wideband and fullband modes. | OPUS wins |
| .AMR-NB vs .AMR-WB | Voice clarity AMR-WB (HD Voice) doubles the sampling rate to 16 kHz, capturing frequencies up to 7 kHz versus 4 kHz for AMR-NB. Voices sound more natural with better consonant discrimination. | AMR-WB wins |
Convert .AMR to...
Technical reference
- MIME Type
audio/amr- Magic Bytes
23 21 41 4D 52#!AMR text signature.- Developer
- European Telecommunications Standards Institute
- Year Introduced
- 1999
- Open Standard
- Yes
#!AMR text signature.
Binary Structure
AMR files have a minimal structure. The file begins with a 6-byte magic number: 23 21 41 4D 52 0A (#!AMR followed by newline) for AMR-NB, or 23 21 41 4D 52 2D 57 42 0A (#!AMR-WB followed by newline) for AMR-WB. After the magic header, the file contains a sequence of consecutive codec frames with no index, no seeking table, and no metadata. Each AMR-NB frame starts with a 1-byte header containing a 4-bit frame type index (FT) that determines the bitrate mode (0-7 for speech, 8 for comfort noise, 15 for no data) and the frame payload size. AMR-NB frames are 20 ms each at 8 kHz sampling. There is no container structure, no timestamp index, and no random-access capability — playback requires sequential decoding from the start.
| Offset | Length | Field | Example | Description |
|---|---|---|---|---|
0x00 | 6 bytes | Magic Number | 23 21 41 4D 52 0A (#!AMR\n) | File signature identifying AMR-NB format. AMR-WB uses 9-byte signature: #!AMR-WB\n. |
0x06 | 1 byte | Frame Header | 3C | Bits 7-4: frame type index (FT), determines codec mode and frame size. Bits 3-0: quality indicator. |
0x07 | variable | Frame Payload | varies | Encoded speech data. Size depends on FT: mode 0 = 12 bytes, mode 7 = 31 bytes (AMR-NB). No padding. |
Attack Vectors
- Malformed frame header with invalid frame type index can cause out-of-bounds read in decoders that use the FT field as a direct array index for frame size lookup
- AMR files embedded in 3GP containers inherit the 3GP container's attack surface, including malformed atom parsing vulnerabilities
Mitigation: FileDex processes AMR files locally using FFmpeg WASM in a sandboxed browser environment. No data is sent to external servers.