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January 12, 2005

How HDV is handled in new iMovie 05 and Final Cut Express HD
So I went by the Apple booth to get the skinny on the HDV support. I asked about how they were editing long GOP MPEG-2, and the first answer I got was that it was put in a QuickTime wrapper. I asked again how they were handling the 15 frame GOP (group of pictures), and was sent to somebody else, who it turns out was a Shake guy.

He told me that they were using some kind of an interstitial codec, and sent me over to find someone else.

So here's the scoop:

The HDV is read in from the tape in real time, but then it is transcoded to an all i-frame intermediate codec. The reason for this is that within the HDV data structure, one "clean and whole" frame is recorded, then the next 15 frames are recorded as changes from the previous frame. So if you want to know what a frame looks like that is 12 frames after an I-frame, you have to calculate what the prior 12 frames looked like going back to that I-frame. Time consuming and problematic.

So the usual fix for this is to transcode to a different codec.

Apple's codec for this is the Apple Intermediate (or is it Interstitial?) Codec, a 4:2:0 codec that matches the 4:2:0 color space of the HDV footage. It's a compressed codec that is all I-frames, so is easy to edit with.

(Continued at HD For Indies)
Posted by yatta at 02:11 AM