What is interlacing?

[Sonydjsnmix]

Is it those horizontal lines in the footages between movements?

[badmartialarts]

No, it isn't. Those are interlacing artifacts. It's a subtle distinction, but it's important to remember that interlacing in and of itself is not bad, in fact it's required to watch anything on a non-progressive TV. It comes from the fact that a TV uses a electron gun to shoot a screen with electrons, making patches of phosphorescent chemicals on the glass light up, red, green, or blue. The more energy in the electron, the brighter the red, green, or blue dot will turn. Now, the electron gun can fire pretty fast. But there is a problem, the phosphor dots don't stay lit up for very long. So to prevent the screen from flickering a bit as the dots turn off and then back on the next time the gun hits it, the TV signal is interlaced. Half of the lines are done in one pass, then the other half, like this: (imagine these line are pointed left to right)

Code: Select all

|||||||||| :Progressive signal


| | | | |   :Interlaced signal
 | | | | |

See? In the first progressive scan one, there has to be ten lines sent before the first line can be redrawn, meaning the phosphors have to stay lit for that long. But in an interlaced signal, you only have 5 lines sent, then the 5 lines around those lines. That way, if the phosphors start to fade out it'll be harder to tell.

Now, the interlacing artifacts start to come into play when to mess with the footage, like putting it onto a DVD. Some companies don't do this right, and you get those little lines everywhere because they mess up the field order and send two sets of the same interlacing pattern instead of alternating for a frame or two, or not setting up the fields right, or a whole bunch of other reasons. Sometimes you see the lines when you slow down footage, since your eyes now have time to pick out the fact that only half the lines are there at a time. The way to get rid of interlacing is to turn the footage back into the progressive signal, so you don't have to worry about keeping the fields and frames in the right alignment and whatnot. www.animemusicvideos.org/guides/avtech has a lot of good information about how to do this. I've typed enough.

[Sonydjsnmix]

thanks! I get it now

[Willen]

Detailed explanation of interlacing: http://www.inwards.com/~dbb/interlace_myths.html

Why do we use interlaced video at all?

When TV's were invented in the 1920s the type of phosphor used to produce the colours did not respond very fast. This meant it was impossible to get a picture in one shot; instead we would get a flickery strobing effect moving down the screen! To solve this they decided that instead of putting the lines on the screen one at a time (i.e. lines: 1, 2, 3, 4, 5) they would put them on every other line in one pass (i.e. 1, 3, 5, 7, 9) and then in-between the previous lines on the second pass (i.e. lines: 2, 4, 6, 8 etc.). This allowed a whole picture to be produced in two very fast scans and allowed enough time for the slow phosphor dots to recover. This, then, prevented any strobing effects from appearing - success! This process is called interlacing!

from here: http://nickyguides.digital-digest.com/interlace.htm