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Why does interlaced video have vertical black bars?

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Graham Stewart
Why does interlaced video have vertical black bars?
on Jun 27, 2011 at 11:57:35 pm

Despite looking all over the internet and asking video editors I know, I've never yet found a satisfactory answer to this question:

When I bring DV PAL footage into my Mac, why are there always very thin black vertical bars on each side and why is the image being squeezed slightly?

I think it may have something to do with interlacing.

If you look at my screenshot here: http://gallery.me.com/grahamstewart#100099/QT-207-20vs-20X-20interlace-20ha...

You can see that the top image (QuickTime 7 Player) exhibits the issue I'm talking about (the presence of combing on the man shows this is interlaced).

But the black bars have disappeared in the bottom image (QuickTIme Player X) - and you can see that it looks slightly horizontally stretched by comparison. I'm guessing this is something to do with the fact that QuickTime Player X DE-INTERLACES video automatically?

The question remains though... what is it about interlacing that introduces black bars and an incorrect aspect ratio??


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David Roth Weiss
Re: Why does interlaced video have vertical black bars?
on Jun 28, 2011 at 12:09:28 am

[Graham Stewart] "The question remains though... what is it about interlacing that introduces black bars and an incorrect aspect ratio??
"


It has nothing whatsoever to do with interlacing. QT Pro 7 (NOT X - NEVER USE X FOR ANYTHING) has a setting you need to change. Go to the Window>>Show Movie Properties - click on the menu bar in the properties window that opens - check "Conform aperture to:" and select "clean."

Now, toy with the other possible settings there and see what they do and remember for all eternity.


David Roth Weiss
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http://www.drwfilms.com

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Graham Stewart
Re: Why does interlaced video have vertical black bars?
on Jun 29, 2011 at 11:28:25 pm

Thanks David, but the bars also appear in Final Cut (where there isn't a "clean" option). The only alternative is to stretch the picture to get rid of the bars and hope the aspect ratio is reasonably correct.

Also, are you able to explain what causes it? Is it something to do with tape to DV conversion? I'm thinking maybe the DV convertor is sampling 768 square pixels, but because PAL broadcast is 720 non-square pixels, the black borders are accounting for the difference? However I still don't understand why DV VTRs don't account for this?


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