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Old February 19th, 2004, 08:13 PM   #1
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What the heck?! DVDA layoff glitch...

OK, it's a small thing, BUT ("but" I'm a perfectionist; "but" this is for a friend; "but" I don't want to repeat this on my own show) when I laid off the DVD for a wedding I shot, and was then viewing the scene/chapter breakdowns, inspecting everything before I handed it over, I discovered that whereas in DVDA I had clearly marked/cut in on that scene/chapter at the father of the bride raising his glass and toasting "to X and Y," the DVD cuts in on that scene/chapter with "and Y," completely leaving out the groom's name. Incredibly, it's simply not there. I immediately trotted back to the computer, brought up DVDA, checked to see if I could've screwed something up somehow after my final, "one more time to be sure" check (I get totally anal with my editing hat on), cuz I couldn't believe I would've missed THAT, and it was NOT cut off in DVDA. It played exactly as I had intended it to play.

Now I've laid this off too many times at this point, gone back and done it all again (obsessively tweaking and polishing) considering this something I'm not even getting paid for. However, I don't want to repeat this kind of glitch when I lay stuff off professionally. So any clues as to what the heck happened, what I should look out for to avoid this in the future?!
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Old February 19th, 2004, 10:25 PM   #2
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How much of a timeframe difference are you talking about? DVDs can only have chapter points on I frames so, typically, you can be +/- a half second.
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Old February 20th, 2004, 12:01 AM   #3
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"I" frames? What's an "I" frame, Edward? Back in my assist. editor days I had to worry about "A" frames because of pulldown (film/telecine issues), but I'm not familiar with this. Insofar as timing, will have to check tomorrow to be precise (have to crash... am zonked and it's an early day at work). But what's missing is only as long as it took to cut off the father of the bride's two words. Was "To A and B". All that came through was the "...and B."

Where can I read up on this business of only being able to have chapter points on "I" frames stuff, and it being +/- half a second? Geesh, that's a 12 frame difference. That's huge. Definitely good to know, albiet a bit disheartening. I take it from what you said that this is for the DVD format in general, as opposed to a DVDA thing that's "off?"
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Old February 20th, 2004, 12:47 AM   #4
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yeah that crap about the I-Frames really stinks...Fortunately, there's an mpeg2 encoder out there called Cinema Craft SP,,, just a tad pricey, but it lets you put in your chapter points b4 the mepg is encoded. In doing so, it makes your chapter points I-Frames for the precision we all require =)

Mike
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Old February 20th, 2004, 05:09 AM   #5
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Fixing I frames

Another way to approach it is to:

a decide now exactly what GOP you will use whe you encode your video to MPEG2. (A GOP is a non-political group of pictures, the sequence of types of frames in the MPEG stream. A DVD will usually (in PAL) be something like IBBPBBPBBPBBPBBPBB)

b during your edit, make sure that all cuts (and anything else that will eventually become a chapter point on the DVD) occur just before what will become an I frame - in the example above this means before frame 0, 18, 36, etc.

c encode your carefully edited piece with exactly the GOP that you chose.

d marvel at the precise frame accurate placement of chapter points that results:)

I know that's a pain, but it adds little time to the edit once you get the hang of it.

Regards


Julian
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Old February 20th, 2004, 08:07 AM   #6
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I frames are frames that are fully compressed frames in MPEG2.
The B&P frames are only the changes from the previous picture
(P) or the changes from the previous and from the next (B).

As said by others, this can easily moved the chapter point about
half a second. One other thing to note is that DVD players some-
times can loose a bit of time when switching to a chapter point
as well. So they start playing and the signal only gets switched
on so to speak a fraction of a second later. This might result in
missing such pieces. In a lot of movie DVD's you see a lot of
chapter points on slow moving points in the movie so if it skips
a couple of frames you are not missing much.
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Old February 20th, 2004, 09:09 AM   #7
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(Rats, my post just vanished... will try again.)

Interesting stuff, guys. Good to know that at least there's a reason for what happened. So Julian (or anybody), what does that translate to in NTSC land? If PAL is 25 fps, there's not a direct connection I see to the 0, 18, 36 etc. (course, I'm rushing to get to work). What is the frame count to avoid over here?

That doesn't "fix" the potential DVD player lag Rob mentions, but at least now I'm aware of what to be on the lookout for.
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Old February 20th, 2004, 10:26 AM   #8
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Marcia,

it all depends on _your_ MPEG encoder. When _you_ encode video for DVD, are you offered a choice of MPEG2 settings, or are you shown what these settings are?

The magic number _you_ should use is governed by the GOP _you_ are using.

Regards,

Julian
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Old February 20th, 2004, 10:39 AM   #9
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It totally depends on the GOP structure but I was thinking it was every 15 frames. This means that if you place a chapter point exactly on the second mark or exactly at the 1/2 second mark, you should hit it dead on.
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Old February 20th, 2004, 06:09 PM   #10
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Okay, clear as mud. ;-)

Julian, you said it's the sequence of types of frames in the MPEG stream, but how do I find out what my DVD program is using/what that GOP structure (types of frames in the MPEG stream) is? As I said earlier, I understand "types of frames" in the film/telecine world (they went ABCD), but I don't get what you mean by "types of frames" in the DV world. Does this have anything to do with what it was rendered out as in Vegas when I prepped it to go to DVD Architect, cuz DVDA never gives me any options that make sense with regard to what we're talking about here. OTOH, Vegas does ask, and I rendered it as "MainConcept MPEG-2/DVD Architect 24p NTSC Video Stream" which makes it 23.976 fps. The footage was all shot on a DVX100 in 24pA mode.

In Vegas the timeline is 24 fps, and in DVDA it's at 30fps. So which half mark do I go by, 12 or 15? I'm guessing 15, which Edward suggests, since that matches the half mark for what DVDA is outputting. It hiccuped over a marker/chapter point that was TC 00:40:23:16. Does that mean then that, on my system, I avoid like the plague the 01/16 frames? And if so, how does that work when I want to set markers in Vegas to be imported into DVDA insofar as TC/the frame(s) I avoid?

Obviously I'm still a tad bit confused...
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Old February 20th, 2004, 09:25 PM   #11
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When you go to file - render as, click on the Custom button. On that screen, click on the video tab. There's an entry there that talks about I frames. What number is there? That's the number of frames between I frames.
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Old February 21st, 2004, 10:39 AM   #12
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Marcia,

sorry for being obscure - here goes an explanation:

I frames, B frames, P frames (and D frames actually) are not DV things, but are MPEG things. MPEG is the compression applied to video before putting it on a DVD.
An I frame is a complete frame, compressed a bit, but standing on its own. Given a single I frame you can view it - this may not sound like much, but it means a lot when compared to B and P frames.
A P frame only contains the difference between it and the previous I or P frame. So given a single P frame, you CANNOT view it, since you also need to have the related I or P frame before you can make sense of it.
A B frame depends on both the previous and following I and P frames, so also cannot be viewed on its own.

So you can see that jumping to any frame other than an I frame will cause a hiccup of sorts (the exact nature of the hiccup will depend on how the DVD player manufacturer has designed the player).


These I frames don't _have_ to appear every half second. However it is a convention that default values correspond to roughly every half second - so if an I frame gets corrupted it will be at most half a second before the video will correct itself.

Regards,

Julian
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Old February 21st, 2004, 02:31 PM   #13
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Thanks for the explanation, and your patience. I get it now! :-)

So much to learn...
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Old February 21st, 2004, 06:42 PM   #14
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Okay, as I'm working along I'm begining to think MAYBE I get it now, and maybe not. Before I print it to DVD (again!) I think it's best that I confirm something.

My mpeg settings (when rendered out from Vegas) tell me the I frame is on the 12th. And since it was rendered out from Vegas that way (24p), even though I'm setting the chapter markers up in DVD Architect, and it's timeline is 30fps, I should still place the markers on the 00 and 12 frames, correct? Then again... is that only if the chapter markers were imported from Vegas via the 24p mpeg layoff, but that since I'm putting in the chapter markers in DVDA (and it's reading at 30 fps), it should be at 00 and 15 owing to its 30fps timeline? (This second option doesn't make sense though given what you said earlier.)

00 frame and 12 to match Vegas' mpeg output and 24p timeline or 00 frame and 15 to match DVDA's? And I thought film was tricky.
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Old February 21st, 2004, 09:11 PM   #15
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If your video is 24p coming from Vegas, why aren't you making a 24p DVD? DVD Architect can handle that just fine.
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