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Geek Culture / When a pc is video taped why does it flicker?

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Archaikz
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Posted: 2nd Sep 2003 01:31
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Just an ot thread but i've always wandered this.
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Yian
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Posted: 2nd Sep 2003 02:05
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I think it's because the pc screen is updated faster than the camera captures each frame...

-john D.
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the_winch
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Posted: 2nd Sep 2003 02:25
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It could be becuase they are out of sink with each other the camera reads a frame when the monitor is halfway through displaying one.
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Yian
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Posted: 2nd Sep 2003 02:54
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cool i never thought of it that way

-john D.
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randi
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Posted: 2nd Sep 2003 05:43
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Those are the reasons.

You know if you have a strobe light and you turn it on in a room with a ceiling fan...
You can adjust the strobe speed until it matches the fan speed and it will look as if the fan has stopped.
Set the strobe speed faster and it looks as if the fan goes in the wrong direction.

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Van B
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Posted: 2nd Sep 2003 13:51
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Yeah, monitors are usually about 75Hz - TV's are 50Hz or 60Hz, so the extra 15Hz on the monitor doubles over itself causing a brighter band at varying places on the monitor. There are some monitors (like for 3D rendering or 2D artwork) that don't have this problem, because they hold the image longer to eleviate flicker (means you get a nice trail from the mouse pointer though).


Van-B

My cats breath smells of cat food.
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Shock
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Posted: 2nd Sep 2003 22:56
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yeah, vanb is right.

but tv's run at different speeds, i.e. you can get those pure thinggy's that run at 100hz, or you could have a tv like the old 14" shoved in my cupboard that runs at about 40hz.

it depends how fast your camera is at catching stuff. video tapes have a standard rate (have a search on google), im not sure if dvd's are any different (any better?).

if you want to get rid of the annoying lines, just turn your monitor's refresh rate as low as it can go. you wont get rid of everything unless your monitor could go down to 35-50hz, but still, 65hz would help your problem


Sticking feathers up your butt doesn't make you a chicken.
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Shock
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Posted: 2nd Sep 2003 22:57
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add: the security camera's in my house run @ 35hz.


Sticking feathers up your butt doesn't make you a chicken.
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Magpie
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Posted: 3rd Sep 2003 01:46
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If that's the reason (i'm not saying it isn't) why does the same thing (sort of) happen when you take a photo of the screen?

Damn, just worked it out. (because you take the photo while it's refreshing or someting)

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Mentor
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Posted: 4th Sep 2003 00:11 Edited at: 4th Sep 2003 00:14
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correct, the image is partialy refreshed, but you can avoid this with a tft display since they stay constant until the data in the matrix is altered, they seem totaly flicker free (unless the Backlight is flourescent and flickers), I just took a photo with a tft display in the frame and when I checked it the image was clear on the monitor, that never happens with crt setups, you always get strobe effects, you get the same thing in old movies when you see a prop driven plane on the runway, the propeler blades seem to run at the wrong speed or backwards in some shots.

Mentor.
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Arrow
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Posted: 4th Sep 2003 00:27
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Quote: "Yeah, monitors are usually about 75Hz - TV's are 50Hz or 60Hz, so the extra 15Hz on the monitor doubles over itself causing a brighter band at varying places on the monitor"
Not quite, the same effect will happen whether you film a stander TV or a moniter.


Teenage Male Geek + Female Remotly Interested in Common Geek Activities = Teenage Male Jackass
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Pricey
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Posted: 4th Sep 2003 17:43
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randi's idea was cool! i will try when i have a strobe light and ceiling fan!

My programs don't crash... your computer does

http://johnsgamesuk.topcities.com
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Van B
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Posted: 4th Sep 2003 18:35
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TV's run at about 24 frames per second, so really that's what the update rate of the image is, the update rate of the screen is 50/60hz, although some fancy ones are 100hz. All this is unimportant, the important thing is that a vertical sync cathode ray screen is distorted when filmed or photographed whereas a slow refresh screen like a TFT monitor or perhaps an LCD projector don't have the problem.


Van-B

My cats breath smells of cat food.
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Shadow Robert
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Posted: 4th Sep 2003 18:47
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cinematic 24fps
pal 25fps (50hz)
ntsc 30fps (60hz)

not really much more to add here than that...

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