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I know they're both container formats and what matters is the actual codec used, but what I don't know is if video converting software will do anything to change the codec, or if it just repackages.

The reason I need to know is that I have several .avi files with subtitle files, and I'm wanting to turn them into .mkv so I can attach the subs and not need a second subtitle file anymore. Will my new .mkv files be identical in video and audio quality to the original .avi?

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potentially a duplicate of… -- although that question skips over the actual quality question posed here. – quack quixote May 29 '10 at 23:54
up vote 1 down vote accepted

There are some telling signs. Repackaging is very fast, like copying a file. Encoding is on the order of real-time; sometimes much slower, sometimes a bit faster.

Any given converter may change containers without re-encoding, but that may depend on the source codec, the target container, the target codec, and which options you choose.

If it repackages, the quality will be the same. If you re-encode to a lossless codec, the quality will be the same. In any other case, the quality will go down, mathematically, because is is re-encoding. Will you be able to subjectively tell the difference? Maybe.

AVI and MKV are fairly codec-agnostic, so if you don't change the codec, or change the frame size, it is unlikely to re-encode.

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this is correct. @Matt, you might be interested in MkvToolNix; i posted answers here and here that discuss doing exactly what you're talking about. MkvToolNix is apparently available in MacPorts. – quack quixote May 29 '10 at 23:52

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