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Interlacing tech question

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Jared Gardner
Interlacing tech question
on Nov 10, 2013 at 1:29:21 pm

This may not be the appropriate section but I think this falls under 'compression' in some way. I'm confused about exactly how interlacing worked in the old days so let's try and clear this up:

1) Do the two fields eventually display simultaneously, like one field is drawn, then sticks on the screen, then the other field displays and completes the image, or is it that both fields are displayed at completely separate times, but it gives us the illusion that both are there?

2) I read that interlacing helped with flickering. Is this 'flickering' they're talking about the absence of the image on the screen for a split second due to taking so long for it to be transmitted? Like one of those lights that turn on and off really fast in a haunted house attraction or something.

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Rich Rubasch
Re: Interlacing tech question
on Nov 15, 2013 at 9:19:05 pm

It's more about horizontal (and to some degree vertical) motion. With interlacing you get the "effect" of recording at 60 frames per second, however only the first 1/60th of the second is recorded into a field (half the vertical resolution) and the second field is recorded to the second field, making up the frame.

With 30p interlaced recording, the recorder stops the motion and records the entire frame made up of the two fields. But the horizontal motion might have a more juddery look.

24p has even more pronounced judder since it only grabs frames at 24 frames per second.

Old CRTs operated at the 60 hertz rate so writing fields gave the smoothest image. (sports etc) Now with flat panels I think a progressive video looks better so we normally record 29.97 30p interlaced and interpret as frames (non interlaced) even though the video playback is actually interlaced (running at 60i)

True 60p (think Sony CineAlta and HDCAM) can record at a true 60 FRAMES per second. No small feat!


Rich Rubasch
Tilt Media Inc.
Video Production, Post, Studio Sound Stage

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