Convert MTS to M4A — Free Online Tool
Extract and convert the audio track from AVCHD camcorder footage (.mts) into M4A, an AAC-encoded MPEG-4 audio container optimized for iTunes, Apple devices, and podcast workflows. The conversion strips the H.264 video stream entirely and re-encodes the AC-3 or AAC audio from your camcorder recording into a clean, standalone AAC file.
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FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
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Settings
Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
MTS files from Sony and Panasonic camcorders wrap H.264 video alongside AC-3 (Dolby Digital) or AAC audio inside an MPEG-2 Transport Stream container. Since M4A is a pure audio format, the video stream is completely discarded using the -vn flag — no video encoding takes place at all. If the source audio is AC-3 (very common in AVCHD footage), FFmpeg decodes it and re-encodes it as AAC at 128k bitrate, which is the native codec for M4A. If the source is already AAC, FFmpeg still transcodes it to ensure clean compatibility with the MPEG-4 container structure. The result is a lightweight, widely compatible audio file containing only the audio from your original camcorder recording.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg program. In this browser-based tool, FFmpeg runs locally via WebAssembly (FFmpeg.wasm) — your MTS file never leaves your device. |
-i input.mts
|
Specifies the input file — your AVCHD camcorder recording in MTS format. FFmpeg will read the MPEG-2 Transport Stream container and detect the H.264 video and AC-3 or AAC audio streams inside. |
-c:a aac
|
Sets the audio codec to AAC (Advanced Audio Coding), which is the native and required codec for M4A files. FFmpeg decodes the source AC-3 or AAC audio from the MTS file and re-encodes it as AAC. |
-b:a 128k
|
Sets the AAC audio bitrate to 128 kilobits per second. This is a standard quality level suitable for speech, interviews, and general audio from camcorder recordings; increase to 192k or 256k if the source audio contains high-fidelity music or ambient sound. |
-vn
|
Disables video output entirely, instructing FFmpeg to ignore and discard the H.264 video stream from the MTS file. This flag is essential because M4A is an audio-only container and cannot hold a video stream. |
output.m4a
|
The output filename with the .m4a extension, telling FFmpeg to write the result into an MPEG-4 audio container. The .m4a extension signals to Apple devices, iTunes, and most media players that this is an audio-only AAC file. |
Common Use Cases
- Extract the spoken audio from a wedding or event recording to create a clean audio keepsake or transcript without the large video file
- Pull interview audio from AVCHD camcorder footage to import into GarageBand, Logic Pro, or iTunes for podcast production
- Extract the audio commentary or narration track from a camcorder recording to create a standalone voiceover file for video editing in Final Cut Pro
- Convert camcorder audio to M4A for direct sync with iPhones and iPads via iTunes or the Apple Music library, which natively supports M4A/AAC
- Reduce file size drastically when only the audio content of a lecture, speech, or performance recording is needed for archiving or distribution
- Extract ambient audio or field recordings captured on an AVCHD camcorder to use as sound design elements in audio production software
Frequently Asked Questions
It depends on the source audio codec in your MTS file. If your AVCHD camcorder recorded AC-3 (Dolby Digital) audio — which is very common — the conversion involves a full decode-and-reencode cycle to AAC, which introduces some generation loss. If your camcorder recorded AAC audio, you are still transcoding rather than stream-copying, so minor quality loss applies. At 128k AAC, the output is transparent for speech and generally acceptable for music, but if you need higher fidelity, you can increase the bitrate to 192k or 256k using the -b:a flag.
The dramatic file size reduction happens because M4A discards the H.264 video stream entirely, which typically accounts for 90–98% of an MTS file's size. A 4GB AVCHD recording might produce an M4A file of only 50–100MB, since you are retaining only the audio channel encoded at 128k AAC. This is expected behavior and not a sign that audio quality has been severely degraded.
No — M4A is a single-audio-track container and does not support multiple simultaneous audio tracks. FFmpeg will automatically select the default or first audio stream from the MTS file. If your AVCHD footage contains separate audio tracks (e.g., a main mic and a backup mic), only one will be included in the output. To extract a specific track, you would add a stream selector like -map 0:a:1 to the command to target the second audio stream.
Some basic metadata may transfer, but MTS Transport Stream files store metadata in a format that is not directly compatible with the iTunes-style metadata tags used by M4A (such as title, artist, and album fields). Recording timestamp data embedded in the AVCHD structure is generally lost during the conversion. If metadata preservation is important, you can add tags manually using FFmpeg's -metadata flag or an audio tagging tool like MusicBrainz Picard after conversion.
Replace the 128k value in the -b:a 128k flag with your preferred bitrate. For higher fidelity audio, use -b:a 192k or -b:a 256k. For smaller files where speech intelligibility is the only concern (such as voiceovers or interviews), -b:a 96k or -b:a 64k will reduce file size further with modest quality reduction. The full command with a higher bitrate would look like: ffmpeg -i input.mts -c:a aac -b:a 192k -vn output.m4a
Yes — on macOS or Linux, you can run a simple shell loop: for f in *.mts; do ffmpeg -i "$f" -c:a aac -b:a 128k -vn "${f%.mts}.m4a"; done. On Windows Command Prompt, use: for %f in (*.mts) do ffmpeg -i "%f" -c:a aac -b:a 128k -vn "%~nf.m4a". This is especially useful when digitizing a large collection of AVCHD recordings and you only need the audio content from each file.
Technical Notes
AVCHD MTS files use the MPEG-2 Transport Stream container, which was designed for broadcast and streaming robustness rather than archival or editing use. The audio codec inside varies by camcorder model and recording setting — Sony camcorders frequently use AC-3 (Dolby Digital) at 256kbps, while some models offer linear PCM or AAC. When converting to M4A, FFmpeg must fully decode whatever audio codec is present and re-encode to AAC, since M4A only supports AAC, FLAC, Opus, or Vorbis as audio codecs. The -vn flag is critical here because without it, FFmpeg would attempt to include the video stream and would likely produce an error or an incompatible file, since M4A has no video codec support. M4A uses the same underlying MPEG-4 container as MP4 with a different file extension to signal audio-only content; Apple devices and iTunes treat it as a first-class audio format with full support for gapless playback and chapter markers (though chapter data from the source MTS file is not preserved). One known limitation is that MTS files sometimes contain multiple program streams (common in broadcast-captured content), and FFmpeg may require the -analyzeduration and -probesize flags to be increased if the file fails to open correctly.