I'd like to convert some AVCHD footage (.m2ts) to mp4 files. Can anybody here recommend a good converter (preferably free) that'll do a good job?
FFmpeg should be able to do this - assuming you don't mind getting down with the command line (there may be some GUI options out there too possibly).
My Olympus E-P3 produces .MTS files - I assume they're similar (AVC/AC3 in a Blu-Ray Video wrapper).
I ran the following command: ffmpeg -i 00001.MTS -vcodec copy -acodec copy -f mp4 00001.mp4
That just tells FFmpeg to copy the video and audio streams but put them in a new mp4 format (the -f option). The resultant mp4 still has the same AVC/AC3 combination but now in a MP4 wrapper.
I played both source and output in VLC and they looked the same - obviously you should satisfy yourself that quality has been maintained.
I'll wager that most GUI tools to do this task use FFmpeg under the hood.
For Mac, there is a very simple and efficient solution that results in no loss of quality whatsoever, however it does require you to install some software.
Install the clipgrap app. This contains the necessary codecs required for step 2. You don't have to use the app again, and I just left it in the 'Utilities' folder in 'Applications.'
Download the 'Rewrap2M4V' file from this page. Put it on your desktop.
Now drag and drop your M2TS file onto the Automator icon of the file you just downloaded. An MP4 file will be created in the directory of the M2TS file.
Basically, the automator script changes the wrapper on the file from the awkward Panasonic one to a more friendly MP4. You can now edit or do whatever you wish with it.
The advantage of this solution is that it does not hinder quality through conversion as many converters do. It does not affect the actual file content itself. It is also very quick.