Convert MPEG-4 Audio (.m4a) to Adaptive Multi-Rate Audio (.amr) online for free. Fast, secure audio conversion with no watermarks or registration.
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AMR (Adaptive Multi-Rate) is the speech codec that powered GSM voice calls worldwide. Converting M4A to AMR compresses audio down to telephone-grade quality — 8 kHz mono at 4.75-12.2 kbps. This extreme compression makes sense only for telephony applications: IVR prompts, voicemail greetings, automated call systems, and legacy feature phone content. Music in AMR is essentially unintelligible, but human speech remains clear enough for communication.
Enterprise telephony infrastructure frequently requires AMR format. Automated calling systems, IVR menu prompts, and voicemail systems on GSM networks accept AMR as their primary audio format. iPhone Voice Memos (M4A) being converted to AMR enables their use in these telephony systems. Some IoT devices with limited processing power also use AMR for voice playback.
FFmpeg decodes the M4A's AAC or ALAC audio, resamples to 8 kHz mono (AMR-NB requirement), and encodes via libopencore-amrnb. The encoder applies ACELP (Algebraic Code-Excited Linear Prediction) speech modeling, which assumes the input is human voice. Eight bitrate modes exist: 4.75, 5.15, 5.9, 6.7, 7.4, 7.95, 10.2, and 12.2 kbps. The output .amr file begins with the standard 6-byte magic header '#!AMR\n'.
At 12.2 kbps, AMR-NB produces telephone-quality speech — clear enough for communication but with noticeable compression artifacts. Music is severely degraded: only the fundamental pitch of a melody may survive, with all texture and harmonics stripped. This conversion is not suitable for music content.
| Device | M4A | AMR |
|---|---|---|
| Windows PC | Partial | Partial |
| macOS | Native | Partial |
| iPhone/iPad | Native | Partial |
| Android | Partial | Partial |
| Linux | Partial | Partial |
| Web Browser | No |
Resolution: N/A
Bitrate: 320 kbps
OGG Vorbis preferred
Resolution: N/A
Bitrate: 256 kbps
AAC format required
Resolution: N/A
Bitrate: 128 kbps
Lossless FLAC/WAV for best quality
Resolution: N/A
Bitrate: 128 kbps
MP3 mono for spoken word
M4A to AMR serves telephony and embedded device use cases exclusively. Always use the highest available bitrate (12.2 kbps) and restrict this conversion to speech-only content.
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| No |
| 特徴 | M4A | AMR |
|---|---|---|
| 正式名称 | MPEG-4 Audio | Adaptive Multi-Rate Audio |
| 拡張子 | .m4a | .amr |
| 最適な用途 | Better than MP3 | Speech optimized |