Video Scan Lines

[sunnah]

I have been curious what is it that produces the video scan lines when seeing a tv screen through a video camera? For those who can remember the days before HDTV that almost always when one would see a computer screen or a TV screen in a movie or through any video recordings the screens would always have a horizontal scan happening. My question is what is it that produces it and why is it not seen anymore? Also is it possible to show those older screens in video with our scan lines?

[lowellriggsiam]

Refresh Rate?

[davexnet]

IS the OP referring to the old CRT-type screens (cathode ray tube) ? The picture is build from a scanning electron beam,
scanning horizontal lines from top to bottom. These must be the lines you can see.

[jagabo]

See the Raster Scan section:
http://www.squidoo.com/how-a-crt-tv-works
http://www.youtube.com/watch?v=TE_agdPYYWQ

[sunnah]

Thank you everyone for the insight. I am not exactly sure what I was thinking in my head was answered or not. Here is another shot:

Footage taken from a digital video camera of a television or computer monitor. It can be a family watching TV or a person working behind a desk. When that video footage is played back you can see significant scan lines running horizontally and the same with the person working on the computer.

Even if the TV was getting a satellite feed will it not do the same thing? I assume if both the TV and the feed are digital then the effect can possibly be avoided.

Make more sense?

[edDV]

Thank you everyone for the insight. I am not exactly sure what I was thinking in my head was answered or not. Here is another shot:

Footage taken from a digital video camera of a television or computer monitor. It can be a family watching TV or a person working behind a desk. When that video footage is played back you can see significant scan lines running horizontally and the same with the person working on the computer.

Even if the TV was getting a satellite feed will it not do the same thing? I assume if both the TV and the feed are digital then the effect can possibly be avoided.

Make more sense?

Do you mean flicker rather than scan lines?

Movies and most TV series series are shot at 24fps (23.976 actually). "NTSC" TV screens update at a 59.94 Hz rate (50Hz for PAL). This field/frame update mismatch causes the flicker you see in movies.

There are effects specialists that convert and run CRT/LCD screens at 48 or 72 fps (multiples of 23.976) so that the image looks natural when shot at film rate.

If you shoot a TV with a camcorder there is a slight vertical rate mismatch which often results in a horizontal bar running up or down the screen. This is corrected by genlocking the camcorder and monitor to the same sync generator.

[jagabo]

Are you talking about something like this:



or like this:



(The dark or light band usually scrolls up or down the screen.)

These are both defects seen when filming a CRT display (in movies they are often simulated, ie, they aren't really made by shooting a TV, they shot on film normally and added the effects later). Both are caused by the nature of the way the CRT is scanned by the electron beam -- each phosphor dot glows when it is scanned but quickly fades. LCDs don't show this type of problem because they aren't scanned, the backlight is on continuously and the picture stays on the screen until it's changed.

[sunnah]

Thank you for clearing that up for me And I will not continue this archaic topic any longer