Why is quicktime forcing my videos to convert to Apple ProRes in order to play? This is new since upgrading to Mavericks

Why is quicktime forcing my videos to convert to Apple ProRes in order to play? This is new since upgrading to Mavericks. This is doubling the size of the files in most cases. Is this going to be necessary to view all my videos in QT?

The basic answer is that this is the way the QT X Player v10.3 is now programmed by Apple to operate under Mavericks. As to the codec specics, thaey depend on the source codecs and/or file container. For instance, AIC/PCM MOV and Animation/PCM MOV files are now converted to ProRes 422/PCM MOV files but DV/DV AVI, DV/DV DV, and PNG/PCM MOV files are now converted to H.264/AAC MOV files while DV/PCM MOV, HDV, H.264/AAC MOV/MP4/M4V and Motion JPEG/PCM files are not converted at all. In addition, I found files where MP3 audio is converted if in a AVI video file but "passed through" if the same data is in an MOV file container. Also noticed that AC3 audio fails to convert properly in the QT X Player v10.3 app but converts properly using the Finder level conversion routine—even to downconverting DD5.1 surround audio to Stereo audio. Basically, am beginning to thing there are tombs of "handwriting on the walls" for those who bother to read it.

As to viewing without conversion, you can still use the QT 7 Player, MPEG Streamclip, MPlayer, the Flip Player, or any other media play that can acces your system's codec component configuration for playback rather than a conversion asset as QT X v10.3 now does.