Compress an MTS or M2TS file

MTS and M2TS are the same thing in different places

Both are AVCHD, the format Sony and Panasonic agreed on for consumer HD camcorders, and both are an MPEG-2 Transport Stream underneath. The extension tells you where the file has been rather than what it is: .MTS is what sits on the camera's memory card, buried under PRIVATE/AVCHD/BDMV/STREAM/, and .m2ts is the same stream after import software or a Blu-ray authoring tool has copied it somewhere else.

A transport stream is a broadcasting format, not a file format. It is built out of fixed 188-byte packets carrying their own timing information, so that a receiver can join halfway through and still work out what it is looking at. That design is why a corrupted MTS usually still plays from the next clean packet, and it is also why the container is slightly wasteful — a few bytes of header on every 188 is a small tax you pay forever in exchange for robustness you do not need once the file is on a disk.

Why they are so large

The video inside is H.264, which sounds efficient and is, but it was written at a camcorder capture bitrate rather than a delivery one. Consumer AVCHD runs between roughly 17 and 24 Mbps, and the later progressive modes go to 28. A camera has milliseconds per frame to make encoding decisions, is running on a battery, and is recording something that will not happen again, so it spends bits freely on purpose. Ten minutes at that rate is close to two gigabytes.

Which brings up the other thing people notice: a long recording arrives as several files. AVCHD cards are formatted FAT32, and FAT32 cannot hold a file over 4 GB, so cameras cut a new clip at around 2 GB and keep going. A forty-minute recording is not one forty-minute file, it is a handful of consecutive ones. Joining them back together is a separate job from compressing them, and this tool does not do it — feed the pieces in one at a time, or join them in a desktop tool first.

The good news is that all of this slack is real. Re-encoding AVCHD to a delivery bitrate typically removes most of the file with very little visible cost, which is why even a generous ceiling is usually more than enough.

The interlacing problem, and what this tool does not do

Most AVCHD camcorders record 1080i: fifty or sixty fields a second, each holding every other line of the picture, woven together into frames. That was the right choice for a television and it is the wrong shape for anything with a progressive display, which is everything now. On motion you get comb teeth — thin horizontal stripes along the edge of anything that moved between the two fields.

Being straight about it: this tool does not deinterlace. The filter slot is used by the resolution setting, and no deinterlace pass is added, so an interlaced source re-encoded here comes out with the combing baked into the picture. Downscaling to 720p or 480p will blur it, because the scaler resamples vertically and the teeth get averaged away, but that is a side effect rather than a fix and it costs you sharpness everywhere else.

If your footage is 1080i and the combing matters, this is the case where a desktop tool is the right answer. HandBrake has a proper deinterlacer with sensible defaults, and ffmpeg on the command line is one yadif away from the same thing. If the camera was set to 24p or 60p, none of this applies and you can ignore the whole section.

Remux fixes playback and nothing else

The video inside an MTS is H.264, which does have a defined place in an MP4, so a stream copy is possible here in a way it is not for WMV. Choose Remux and the transport stream is unwrapped and the same H.264 written into an MP4 in seconds, at which point the file plays in a browser, on a phone, in anything.

It will be the same size. Remux changes the box, not the contents, and the contents are the problem. For AVCHD that makes remuxing a useful first move only when playback is the whole issue — you want to look at the footage before deciding what to keep, say. The moment size matters you need a real re-encode.

One caveat if you do remux. AVCHD audio is AC-3 or Blu-ray PCM, and the latter has no MP4 mapping, so a copy can fail on the sound while the video would have been fine. Remux mode only offers keeping the audio or dropping it, so if it fails, switch to Best compression with Audio set to Compress and the track is re-encoded to AAC.

Getting the file into the tool in the first place

This is the practical hurdle nobody warns you about. Operating systems often have no MIME type registered for .MTS, and the file picker here filters on video types, so an MTS can be greyed out in the dialog and the drop zone can ignore it silently when you drag it in. If that happens, switch the picker to show all files, or rename the file to .m2ts or .ts, which more systems recognise. The contents are unaffected either way.

Once it is loaded, target-size mode will not work on it. The tool reads clip length using the browser's own video element, and browsers do not open raw transport streams, so it will report that it cannot read the length and stop. Use Best compression instead.

For settings: output MP4, resolution 1080p if the source is 1080 and you want to keep it, 720p if the footage is going anywhere near a phone or a web page. Set audio to Compress rather than leaving it on Keep original, because AC-3 and PCM are both better off re-encoded to AAC here. And expect the encode to take a while — you are decoding a couple of gigabytes of high-bitrate H.264 and writing it back out inside a browser tab.

MTS / M2TS compression questions

Compressing to a size limit instead?

If what brought you here is a number rather than the fact that it is a MTS / M2TS file, start from the ceiling instead. Each of these opens the compressor with that target already set, ready for your MTS / M2TS:

• 5 MB
• 8 MB
• 10 MB
• 16 MB
• 20 MB
• 25 MB
• 50 MB
• 100 MB
• 200 MB
• 500 MB

Other formats

• Compress MP4 The universal container. Usually already H.264, so the gains come from bitrate and resolution.
• Compress MOV iPhone and QuickTime recordings. Frequently HEVC or ProRes, and frequently huge.
• Compress MKV Long recordings, rips and archives. Big enough that upload-based tools often refuse them.
• Compress WebM Already efficient — so the realistic gains are smaller, and worth understanding first.
• Compress AVI Legacy codecs, dramatic savings. Read in, MP4 out.
• Compress WMV Old Windows exports. Nothing plays them now, and remuxing is not an option.
• Compress FLV A dead container, often with a perfectly modern video inside. Try remux first.
• Compress M4V Apple's MP4 under another name. Renaming it often solves the whole problem.

The formats reference puts MTS / M2TS beside the other containers and shows which codec combinations play where.