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View Full Version : War of words on Adobe forums re GH1 pulldown removal



Ozpeter
08-11-2009, 03:52 AM
http://forums.adobe.com/thread/463780


I wrote an article a long time ago about the problems of working with AVCHD footage. You'll find it here. (http://hottek.net/2007/10/avhd-ready-for-prime-time.html)
Since that time I've had several occasions to work with the footage. None of them have been by choice. The problems pop up more often than not. They don't seem to be related directly to the different cameras. They seem to be more related to the motion in the image. I'm sure that they are caused by the MPEG GOP compression scheme that squeezes HD frame sizes into a data stream smaller than DV. That's 6 times the number of pixels at a full megabit per second less data than DV (24Mbit/sec vs 25Mbit/sec). If the hardware that's compressing this video on the fly has any problems at all you'll end up with fouled up data. There's no way around it. There's no easy fix.


I wonder which of the protagonists was actually right, and perhaps more usefully, is there anything useful for us here in this discussion, most of which goes over my head?

j
08-11-2009, 06:04 AM
That's funny, I just pictured them on stage in togas fighting over codec theories.

Up too late prepping for a shoot...

Barry_Green
08-11-2009, 09:10 AM
From my experience, Rick was trying to help, but he's wrong and Gnurpi was right.

The footage is recorded right. But After Effects is inconsistent in the way it displays the footage. There's a bug there somewhere. That's the part Rick wasn't understanding.

I've gone through exactly what Gnurpi was going through -- you step by step through the frames, and it's all perfectly fine, and then it isn't -- the cadence gets "off". I even started writing down exactly what the timecode frame numbers were, where duplicate frames would show up, to see if I could figure out the pattern -- and when I'd go back to verify, the duplicate frames wouldn't be there anymore! It seems random. But sometimes I'd go through the sequence and get WWSSW WWSSW WWSSW WWWSS and I'd be all "huh?" And I'd start over and get WWSSW WWSSW WWSSW WWSSW and so I'd think I was seeing things, so I'd try again and get WWSSW WSSWW WWWSS and by that time I knew there was something wigged out. It's the same raw footage, how can it possibly be displayed with three different pulldown patterns?

It's a bug in the way After Effects (and Premiere Pro) are decoding frame-by-frame long-GoP images, at least from AVCHD. I don't know if it affects other formats.

But the footage is absolutely recorded properly. Shedworx VoltaicHD, and CineForm NeoScene, decode it properly. So I think it's probably due to the way AFX ram-caches frames out of a group, because it really only seems to show up when you're single-stepping through the frames. Playing at speed, it looks right -- because it is right.

Ozpeter
08-12-2009, 03:56 AM
In a previous post I related how stepping though footage in different NLE's gives very different results - goes to reinforce that how you see GH1 footage depends partly on how you're looking at it (if you see what I mean).