What MTS and M2TS actually are
MTS and M2TS are the file extensions used by AVCHD, the HD recording format Sony, Panasonic, Canon, and JVC ship on their camcorders — the .m2ts variant is also what sits on Blu-ray discs. Inside, the video is H.264, but it is wrapped in an MPEG-2 transport stream, and the audio is usually AC3 (Dolby Digital) or, on some bodies, uncompressed PCM. That mix is fine for the camcorder, but it is a recording structure, not an editing or sharing one.
What this tool does, exactly
Native ffmpeg re-encodes the video to H.264 at CRF 23, re-encodes the audio to AAC at 160 kbps, forces the pixel format to yuv420p for universal hardware-decoder support, and writes the result as an MP4 with the +faststart flag so it begins playing the moment it streams. The AVCHD video is already H.264, but the transport-stream packaging and the AC3 / PCM audio are exactly what break in editors, so a clean re-encode of both streams is far more reliable than a remux.
Why your AVCHD footage won't edit cleanly
AVCHD is a broadcast-style recording structure, with the real media buried in a BDMV / STREAM folder hierarchy. Many players choke on the MPEG-2 transport-stream layout, and a lot of editors either refuse the Dolby / AC3 audio or import the clips with no sound at all. Re-encoding the streams into a plain H.264 + AAC MP4 produces a file that drops straight into Premiere, DaVinci Resolve, Final Cut, iMovie, and CapCut and plays everywhere the original .mts or .m2ts would not.
Sister tools
For the related broadcast container, TS to MP4 converter handles raw MPEG-2 transport streams, and MPG to MP4 converter takes the older MPEG-1 / MPEG-2 program streams. For camcorder cousins, MOV to MP4 converter handles QuickTime and iPhone sources, and AVI to MP4 converter covers older camcorders and capture cards. Shrinking the finished MP4? Video compressor.
Skipping the conversion entirely
AVCHD exists because camcorders write it to record HD footage to disc and tape-style media. If what you actually need is to record and share — a demo, a walkthrough, a quick screen capture — Clipy records your screen straight to a shareable link in standard H.264 MP4, with no local file, no AVCHD folder to dig through, no format conversion, no watermark, and no install.