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Convertessa feeds AVCHD .MTS clips to FFmpeg and writes ordinary MP4 files that macOS, Photos, iMovie and every browser open without argument. The H.264 picture is re-encoded at the quality you choose, the camcorder's Dolby Digital track becomes AAC, and the transport-stream packets give way to an MP4 index so seeking is instant. Point it at the card's STREAM folder and the whole shoot converts in one queue.
AVCHD camcorders do not write video files so much as broadcast streams. An .MTS is an MPEG-2 transport stream — the same 188-byte packet structure used for digital television — which the format chose because a stream of small packets survives having the card pulled mid-record. Inside it sits H.264 video and, on most Sony, Panasonic and Canon bodies, Dolby Digital (AC-3) audio. QuickTime has no demuxer for it, so the file that plays perfectly on the camera's flip-out screen does nothing on your desktop.
Two habits of the camera cause most of the confusion:
00000.MTS, 00001.MTS, and onward. They are genuinely separate files, and converting them gives you separate MP4s. Rejoining a take is an editing job, not a conversion one.BDMV folder wrapped around the stream holds playlists, clip information and thumbnails. Copy only STREAM off the card and you lose the camera's index but keep everything that matters, because each MTS is self-contained.FFmpeg does the work. The packet stream is replaced by MP4's sample index, which is why the converted file scrubs instantly while the original stuttered. AC-3 becomes AAC, the audio codec every phone, browser and editor accepts. The loss that surprises people is the shooting date: AVCHD writes date, time and camera settings into the stream itself, and MP4 has no equivalent field, so the converted file carries the date you converted it. Note the original down first if it matters for an archive.
Then check interlacing. A camcorder set to 1080i records fields rather than whole frames. Convertessa does not deinterlace, so that field structure passes into the MP4 — fine in most players, visible as comb teeth on fast motion when you scrub frame by frame or pull a still.
Mount the camera or its SD card and drag PRIVATE/AVCHD/BDMV/STREAM into Convertessa. Every numbered chunk queues in card order, each converting to its own MP4 with the original filename kept.
Select MP4 and set quality. AVCHD records generously for a consumer format, so a middle setting typically produces a much smaller file that looks the same on anything short of a frame-by-frame comparison.
Run the queue and open a clip with movement in it. Panning shots and hands are where an over-aggressive quality setting shows first, and where 1080i field structure becomes visible if the camera shot interlaced.
Images, audio, video, documents, archives, eBooks and fonts — all converted natively, right on your Mac.
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Convert hundreds of files at once with per-type group defaults and individual overrides.
First-class support for HEIC, AVIF and WebP — encode and decode, both ways. Read JPEG XL and a dozen more modern formats too.
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Right-click any file in Finder and choose Convert. A Quick Action handles it on the spot — no need to open the app first.
Shrink Convertessa to a tidy mini window that tucks into a corner — just a drop zone and a format picker for quick, one-off conversions.
No cloud. No account. No upload. Every conversion happens entirely on-device, using the power already in your Mac. What you convert is nobody’s business but yours.
Script conversions, wire them into your build, or batch a folder from the terminal. The same engine, no GUI required.
Read the full CLI documentation →Shrink Convertessa down to a small, focused window — drop, convert, done, with your recent conversions one click away.
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No. The camera split a single take at a file-size boundary, but the pieces are independent files and Convertessa converts each one as it finds it. You get 00000.mp4 and 00001.mp4, in order, cut at exactly the frame the camera cut. Join them afterwards in an editor, where you can also cross the seam cleanly. Converting first is the right order anyway — most editors handle MP4 far more comfortably than raw transport streams.
AVCHD is designed for recording to a card in real time, so it writes a steady, generous stream of data whether the frame is a static wall or a football match. A modern re-encode spends data where the picture needs it and almost nothing where it does not. A half-size output from a mid quality setting is normal, and on locked-off tripod footage the gap is wider still.
AC-3 is re-encoded to AAC, which is what MP4 players expect. Stereo camcorder audio comes across as stereo. If your body recorded a 5.1 track, remember that MP4 will carry the channels but most of what you will play it on — laptop speakers, a browser, a phone — folds it down anyway. For surround work destined for an edit, keep the original MTS files alongside the MP4s.
No. AVCHD stores date, time and shooting parameters inside the stream in a structure MP4 has no slot for, so it does not travel. The converted file shows the date you converted it. If provenance matters, read the dates off the clips first, or keep the original card image — an archived copy of BDMV preserves both the streams and the camera's own index.
Not in any way that affects you. Both are the same transport stream; camcorders writing to SD cards tend to use .MTS, while AVCHD discs and hard-drive models use .M2TS. Convertessa treats them the same way and produces the same MP4. Some camera software also renames files on import, which is why the same footage can arrive under either extension.
Yes, and it is the fastest way to deal with a shoot:
convertessa /Volumes/UNTITLED/PRIVATE/AVCHD/BDMV/STREAM/*.MTS --to mp4 --quality 80 -o ~/Movies/Shoot
Every clip lands in the destination folder with its number intact, so card order is preserved. Leave the card mounted until the run finishes; reading directly off SD is slower than copying the folder to your drive first.
Usually yes, and for a structural reason. A transport stream carries no index — a player has to scan packets to find a point in time, which is why scrubbing an MTS feels like wading. MP4 stores a sample table up front, so the player jumps straight to the frame. The decoding load is similar; it is the seeking that transforms.