3:2 pulldown and GOP-size (for Scenarist)

[dust12]

Hi all,

I got a problem trying to import a CCE-encoded file into Scenarist because Scenarist complains about >36 GOP-size. In the Scenarist forum somebody gave a reasonable explanation, but it doesn't solve the problem :-) Actually it is about DVD-encoding, but the problem is CCE-specific, so I'm posting it here.

Detailed problem explanation:

My source is a 23.97fps movie (film) which I'd like to encode for NTSC (29.97fps). Unfortunately, CCE doesn't have a 3:2 pulldown of its own, so I used the external "pulldown.exe" tool (v0.99), a commandline-tool that is applied to the 23.97fps MPG2-file CCE produces. As far as I understood, the following hapens: CCE thinks it will produce 24fps (I leave out the "...97" stuff away for simplicity :-). In CCE you can set GOP-sizes, and NTSC only allows a size of 18 pictures per GOP. So, if CCE produces a GOP "IBBPBB" (short example for simplicity :-) and I apply "pulldown.exe" to it, "pulldown.exe" obviously only takes the first frame, doubles it as 2 fields, then the next frame as 3 field-copies, and so on. This will produce a GOP that now loos like "II,BBB,BB,PPP,BB,BBB" (commas are added to explain the structure). Not only is this very ugly (too much redundant information), it also increases the GOP-size to 45 fields, and Scenarist doesn't like that.

It works perfectly if I use TMPGEnc for compression, because it has a 3:2 pulldown of its own ("3:2 pulldown when playback"), and this obviously pulldowns the stuff (or "pulls teh stuff down"? :-) before compression. Is there a way to do the same using CCE? I'm still using CCE2.50 (could switch to 2.62/64 if needed though)

Thanks in advance, Andy

PS: I just noticed now... obviouslky teh standard allows 2*18 = 36 fields if interlacing is donw, but still you would have to reduce the GOP-size to 14 in order to stay below 18 after pulldown. In any way, the pulldown procedure seems unreasonable to me, because it repeats even I-frames, if it's done AFTER MPG2-comrpession. So I'm wondering if it can be done BEFORE MPG2-comrpession using CCE? Maybe (didn't try that out yet, will do so though!) it can be done in an AVISynth script brefore CCE....

[auenf]

i started writing a new reply, but couldnt think straight, so i found this:

http://forum.doom9.org/showthread.php?s=&threadid=22069&highlight=GOP+size

it is better to encode the footage in 24fps and run pulldown on it than do pulldown on the avi and encode (interlaced), mainly cause in 24f progressive will compress a lot more than any interlaced content. underneath 4mbit would be almost entirely out of the question for interlaced content, whereas clean 24fps progressive footage can be fine down to 2.5mbit sometimes.

Enf...

[dust12]

Hmm yes, sounds logical. Do you happen to know how pulldown works? For example, if it has to repeat an I frame (2 or 3 times), will it insert the whole I frame again (making II or III out of it), which of course would make comrpession worse and actually increase the bitrate, maybe eben more than it should, or is it celvere enough to produce something like "IP" or "IPP" with P-frames telling thhere's no change to the former I-frame (B-frames are not useful I think)? Makes me wonder how TMPGEnc is dealing this... a GOP-analyzer could help there, if I had one :-)

Andy

[auenf]

pulldown.exe only adds flags for the player to do the repeat first field and top field first stuff, which doesnt affect encoding.

running the source thru telicine before encoding will affect it as you guess.

Enf...