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Cutting from black to video - getting clean frames? (CROSS POST)

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Ben Wharton
Cutting from black to video - getting clean frames? (CROSS POST)
on Apr 9, 2005 at 1:20:10 pm

(NOTE: Also placed on Encore forum - apologies, just not sure where the right experitise will be.)

This is probably an encoding/editing issue rather than an authoring issue, but I thought someone might know!


A compilation of sequences either fading up from black /cutting from black then fading to black / cutting to black

Encoded with Procoder CBR 6500 Video rate, Highest Quality (NOT Mastering) setting. Source material composite output dumps from client DVDs converted to DV codecs.

Fade ups/fade downs are perfect. But cuts FROM black to picure tear/flash across 2 or 3 frames. The material is letterboxed 4:3 (black borders top and bottom)

Question: Is this to do with the position of the first frame of video within the GOP structure OR is this to do with the fact that when the sequences were edited (on Avid Xpress Pro) there was nothing on the timeline before the first frame of video?

And whatever the reason is there a way to fix it? Any help / pointers massively appreciated.

(I get the feeling this is an old subject so apologies if this has been asked long long ago)

Ben


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arky
Re: Cutting from black to video - getting clean frames? (CROSS POST)
by
on Apr 9, 2005 at 6:59:21 pm

Ben, are these artifacts evident in the

DVD->DV footage

or only in the final

DVD->DV->ProcoderMPEG2

footage?

If it's the latter, then my guess is that you will achieve significant improvements by forcing I-frames at the scene-change points (although I must admit, I would expect ProCoder to have automatically done a decent job of identifying the need for I-frames at these specific points in the 'DV source' footage). You may find the best way to force such I-frames is to cut your DV file into segments at the problem-boundaries, then encode these segments as a batch, in ProCoder, to equivalent MPEG 2 segments (do NOT rely on the ProCoder's 'stitch' function, in this particular instance), and then concatenate these afterwards, preferably with MPEG Append, if you have a Mac to hand, or, alternatively, in your DVD authoring program's timeline, if it caters for this function. In this way, you will produce robust I-frames at precisely the points you require them in your MPEG stream.


John.


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Ben Wharton
Re: Cutting from black to video - getting clean frames? (CROSS POST)
on Apr 9, 2005 at 11:39:37 pm

Hi John

Thanks so much for the reply.

As I'd been naughty and cross posted this query, I had a discussion with Roadkill over at the Encore forum where I explined the pipeline more precisely:

DVD output (composite) -> OpenDL DV codec AVI

OpenDL AVI -> Avid Xpress Pro -> Edited Sequences -> QuickTime Reference File

QT Ref -> Procoder 2 -> MPEG2 files


And we discoveref it's something to do with QuickTime Reference files.



Either Avid Xpress Pro is producing QT Refs that Procoder 2 doesn't like
OR
Procoder just doesn't handle QT Refs properly in the first place
OR
QT Refs aren't an accurate way to pass information to any kind of encoding engine...

You can't spot anything amiss on the QT refs. Whether that's because you're seeing a deinterlaced image of an interlaced file I'm not sure.

There's also another thing in the mix - field dominance. As far as I understand, PAL DV is lower field first, but MPEG2 PAL should be upper field first. The video looks fine with this swapping of dominance (playback from burnt DVD on TV) but I wonder if it's what causes a problem with Procoder's encoding engine when going from black to video?

I've rendered out the sequences to DV AVIs, then put those throught Procoder instead and the cuts work perfectly in the resulting MPEG2 files. More rendering time... on this project it was just about OK, but on something larger? It's all such a bloody hassle to be honest!

Your suggestion about the I-frames is kinda what I assumed at the start but, like, Why Should I Have To Do It?!

I guess I should go ask Canopus if they know what's going on.

Thanks again.

Ben






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