Convert MTS (and M2TS) to MP4
MTS and M2TS files are what you find on the SD cards from Sony Handycams, Panasonic camcorders, and AVCHD Blu-ray discs. Same underlying format, just two different extensions - MTS on the card itself, M2TS on discs and after transfer with Sony's software. Modern devices refuse to play them: iOS says nope, Windows Media Player is inconsistent, most upload sites reject the extension. But the picture inside is already MP4-compatible H.264. Wave rewrites the wrapper to MP4 without touching the video and converts the camcorder's Dolby Digital (AC-3) sound to AAC - the audio format every phone and browser plays - so your footage becomes a file that plays everywhere, usually in seconds.
What actually changes
MTS and M2TS are the same container - MPEG-2 Transport Stream wrapped in BDAV, the AVCHD variant Sony and Panasonic standardised on around 2006 for consumer camcorders. The extension differs by tool: cameras record .mts on the SD card, Blu-ray-authored discs and PC-side tools use .m2ts. Wave treats them identically.
The payload is H.264 video plus AC-3 or LPCM audio. The video copies through bit-for-bit (a "remux" - see Remux vs re-encode), and the file's index (the moov atom) is placed at the front for fast-start playback. The audio is converted to AAC either way: AC-3 is legal inside MP4, but Chrome and Firefox can't decode it and play the clip silent, and MP4 doesn't carry LPCM (some older Panasonic camcorders, some AVCHD Blu-ray tracks) in a broadly-playable way. Stereo lands at 192 kbps; 5.1 stays 5.1 at 384 kbps. Converting a few minutes of audio takes seconds, so the whole job stays quick.
AVCHD launched in 2006 as a joint Sony / Panasonic specification and became the dominant consumer camcorder format for the next decade. Sony Handycams, Panasonic HDC-series, Canon Vixia, and JVC Everio all recorded AVCHD to SD cards and internal storage. The 2011 AVCHD 2.0 update added 1080p60 and 3D support; AVCHD Progressive shot at 28 Mbps for 1080p50/60 in the highest-quality mode. Modern 4K camcorders migrated to XAVC (which is H.264 in a plain MP4 container) but the AVCHD archive across families and small businesses is enormous.
The .mts extension appears when the file lives on the SD card in the camera's BDMV folder structure; the .m2ts extension is what Blu-ray authoring tools and Windows-side transfer utilities use for the same content. AVCHD-format Blu-ray discs use .m2ts inside their STREAM folder. Wave reads both without configuration - the byte-level structure is identical, only the filename convention differs.
Three steps.
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Step 1
Drop your .mts or .m2ts file onto the card.
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Step 2
Wave inspects the streams and shows Fast copy for the video track. Audio may need a quick re-encode if it's LPCM.
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Step 3
Convert and save the MP4.
Answers, specifically.
Why not just rename .mts to .mp4?
Because MTS is a transport stream inside - different packet structure, no MP4-style index. Players that trust the extension will crash or refuse the file. A real remux rewrites the container and builds an MP4 index, without touching the video pixels or audio samples.
My camcorder splits long recordings into 4 GB chunks. Can Wave stitch them?
Not automatically in the current build. Concatenate the chunks yourself first (any AVCHD-aware tool, or ffmpeg concat), then drop the joined file onto Wave for the remux to MP4.
Do subtitle or chapter tracks come across?
AVCHD MTS files rarely carry text subtitles; when they do, Wave preserves them. Chapter markers from AVCHD discs pass through as MP4 chapter atoms.
What about 4K MTS from newer camcorders?
4K H.264 (60p, high bitrate) remuxes cleanly. Very large files (over 4–5 GB per clip) push against browser memory even for a pure remux - Wave warns above 2 GB and the ceiling is 8 GB per file.
Will the file get smaller?
Typically slightly smaller. AVCHD transport-stream packaging adds a few percent of overhead that MP4's tighter container structure sheds. The picture and sound are identical.
My camcorder writes 4 GB chunks. How do I get the whole clip back?
AVCHD splits at 4 GB because the FAT32 filesystem used on SD cards cannot store larger files. To reassemble, use PMB (Sony), HD Writer AE (Panasonic), or ffmpeg concat before dropping into Wave. Newer camcorders on SDXC / exFAT cards do not split.
What happens to the AVCHD folder structure (BDMV, CLIPINF, PLAYLIST)?
Wave converts individual .mts files. The BDMV folder structure carries playlists and thumbnails that the camera used for its own preview interface; MP4 has no equivalent, so those are dropped. If you have valuable playlist information, note it before conversion.
Can I convert AVCHD Progressive (1080p60) footage?
Yes. AVCHD 2.0 Progressive shoots H.264 at up to 28 Mbps in 1080p60, which is standard MP4-compatible H.264 Main profile. Wave remuxes it in seconds with no quality change.