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Where does the 'extra' data come from when transcoding?
  • I wasn't sure what category this fit in exactly....

    This is something I've never quite understood, but have always wondered. Forgive my basic knowledge here, but I've tried to research this without a specific answer as to what is going on.

    When I convert from an .MTS file to ProRes422 HQ, the file get's huge. Why is that? Where is that extra data coming from if it's not there to begin with? i.e. higher bitrate. I understand the codec has a standard bitrate it will always maintain, but why do it if the original file bitrate is so much lower?

    For example, a stock GH2 mts for me streams at about 22 Mbits/s, I convert it to ProRes422 HQ, that same file is now playing at 216 Mbits/s. - With no discernible image quality difference.

    What is going on in the codec that makes the file so huge? Other than workflow issues, why would you want to transcode from MTS? Does converting it to ProRes (or another codec) 'Unpack' the data in a more efficient way?

    Why not use something like ProRes422 LT if you're starting from a much lower data rate like a native MTS file provides?

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  • It's less compressed. It's getting decompressed, then recompressed to a less-lossy or lossless codec.

  • Transcoding h.264 (.MTS as you put it) to a higher quality format like ProRes accomplishes two things:

    1. converts the highly compressed video into an edit-friendly intraframe format;
    2. "upgrades" the color space for better quality retention as the video is repeatedly processed through various post-production processes (i.e., adding effects and color-grading).

    An increased bitrate is necessary to accomplish these while maintaining the original picture quality (an insufficient bitrate would noticeably degrade the image during transcoding - not good). There are different grades of ProRes that suit the quality needs of different acquisition formats. Film acquisition for a special effects heavy sci-fi movie would probably be encoded to something like ProRes 4444. So, there is a purpose for the very high bitrate flavors of ProRes. But if you're originating from a GH2 (a hacked one even) the most you would reasonably need would be PRHQ - for most, LT is probably good enough. If LT is still too much for you to handle storage wise, there's PR Proxy (but it does start to visibly degrade image quality, use at least LT if at all possible).