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How to deal with digital blanking bars on archive SD content

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Graham Stewart
How to deal with digital blanking bars on archive SD content
on Apr 29, 2020 at 6:32:50 pm

I’m having a headache dealing with digital video which has been captured from archive SD PAL sources (old TV shows on beta tapes etc).

I didn’t capture this video personally (it comes from the BBC digital archive where I work) so cannot vouch for the capture systems used.... but in almost all such footage I’ve come across there are, within my 768x576 pixel frame, two thin vertical black bars down each side (I assume horizontal for NTSC readers). I understand that this is often called the “blanking area” and, from what I understand, it’s to accommodate the growth and decay of the analogue waveform so that there is no abrupt clipping etc. As you can see from the attached screenshots, it's maybe only up to 10 pixels either side at the very most.

My question is — what is the correct way to handle this blanking area when I’m repurposing the video for online content, or even just capturing still screenshots? Specifically, should I be stretching the image horizontally only (ie. scaling only the x-axis and not both x and y) to ensure the correct 4:3 aspect ratio so that images on-screen (people, objects etc) don’t appear to “thin” or “squashed”?

My hunch is that the answer is ’yes’. My reasoning is as follows:— 768x576 is pretty much 4:3 aspect ratio. But with bars down the sides the “active picture area” is clearly less than 768 pixels wide and therefore cannot be 4:3.

What troubles me is that when I view DVDs on my computer (using the “DVD Player” on Mac OS) the frame size is 768x576 but I still see bars down each side. Are DVD manufacturers (or Apple’s software teams) getting it wrong too?

Thanks!





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Pat Horridge
Re: How to deal with digital blanking bars on archive SD content
on Apr 30, 2020 at 6:42:18 am

OK this is complex.

The move from analogue to digital required that none of the analogue image was cropped horizontally.
The actual pixel count is 720x576 for PAL
And of that 704 is active picture. Allowing 16 pixels blanking to be retained. This would vary by a few pixels left and right based on the timing of the original analogue signal.
But that's 4:3 aspect pixels (or later 26:9 aspect pixels)
To view them correctly in our square pixel world of computers and HD you have to scale horizontal.
So for 4:3 it becomes 768x576 and for 26:9 it becomes 1024x576.
But this includes the original blanking scaled up as well.
To retain aspect you must zoom expanding width and height.
But this will soften the image more.
Remember you have already expanded the width from 720 anyway.
One solution is to crop the black edges using a pip effect and use the active edge of the original zoomed image underneath. It basically repeats the edges slight.
Hope that helps.

Pat Horridge
Broadcast & Post Consultant, Trainer, Avid Certified Instructor
Free online Tutorials at VET digital media academy online http://vimeo.com/channels/752951
pat@vet.co.uk



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