Interlacing

21 03 2008

No, that isn’t an obscure sociological term about personal interactions. It’s an obscure technical term. In the video world it refers to the way in which an image is projected on a screen.

In a nutshell, the way it works is this. For a typical US television set, each frame of image is divided into 625 (whoops, my bad, I mean 525) lines. All of the odd numbered lines are scanned across the television screen in 1/60th of a second and then, while the image is still sitting in our brains, all of the even numbered frames are scanned across the screen. The brain combines each of these fields (as each of the groups of scan lines is called) into one full image. Voila. A frame.

Note that this is very different from film, in which the entire frame is displayed at the same time.

The various flavors of Hi Definition can be either interlaced or not (this last is called “progressive”). That is what the letters “p” and “i” mean when someone (who is usually try to sell you something) tells you that “This set is 1080i” or “You’ll like this better because it’s 720p.” Note that, in both cases, you should run for the hills — or, at least, the closest Internet station to help you survive the bullshit meter.

So, why am I giving you this long-winded lesson in tech terminology

Will Richardson who is a film director/editor for The Heliconia Press (a sporting publisher and DVD content creation company) publishes a great blog called The Video Animal. Recent postings include a series on HD on The Cheap, which are well worth reading. His most recent posting is the start of a new series entitled How To Post Video On The Web. This part is all about interlacing, and describes how to get rid of it. His approach to defining “interlacing” is a little different from mine, and mashes it up with a concept called “persistence of vision” which the theory that describes why the human mind can see a series of 24 or 30 still images in one second, and perceive them as one fluid moving image.

But his description of how to get rid of the interlacing when posting something to the web is clear and concise. He also recommends reading Adobe’s guide to compression, which I highly recommend reading if you have trouble sleeping at night. Seriously, though, you’ve got to love a technical guide when it contains the following line:

Compression technologies take advantage of the strengths and weaknesses of human senses by reducing data that isn’t likely to be perceived.

Now that’s riveting reading, isn’t it?

Cockiness aside, take a look at this series from Will, who has a fine and friendly writing style to help you through these difficult subjects. And you’ll also learn quite a bit about filmmaking on the cheap from him.


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2 responses to “Interlacing”

21 03 2008
John Walker (13:05:10) :

You mean 525.

21 03 2008
videoanimal (17:12:41) :

Thanks for the nod Mr.Hollyn! I think your description of how interlacing works is a little easier to understand than mine…damn. Ah well. Your observations are well received, thankyou.

I also agree that the adobe primer is about as boring to read as watching paint dry. I forgot to mention that and will add a note to that effect!

Will

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