Why do computer screens flicker in videos/TV shots

Discussion in 'General Discussion' started by Deepak, Sep 21, 2005.

  1. Deepak

    Deepak B3D Yoddha
    Veteran

    Joined:
    Jan 10, 2003
    Messages:
    2,687
    Likes Received:
    23
    Location:
    Nai Dilli (New Delhi), Bharat (India)
    I always have wondered why does it happen, any answers?
     
  2. Bolloxoid

    Newcomer

    Joined:
    May 15, 2003
    Messages:
    191
    Likes Received:
    0
    It's the combination of the monitor refresh rate and the sample rate of the video camera.
     
  3. London-boy

    London-boy Shifty's daddy
    Legend Subscriber

    Joined:
    Apr 13, 2002
    Messages:
    21,508
    Likes Received:
    5,153
    Different refresh rates. The video you're looking at runs at 30fps, and the monitor runs at 75 or more (the problem is that it's not a multiple of 30 or whatever framerate your video runs at). Therefore each frame on your video will see "one frame and a bit" from the computer monitor it's filming. I think that's the "descending bar" you see in videos, it's the overlapping.

    Same thing for pictures, it all depends on the shutter speed and how many frames (or how many fractions of a frame) are displayed in that tiny short time the shutter is open.
     
  4. Simon F

    Simon F Tea maker
    Moderator Veteran

    Joined:
    Feb 8, 2002
    Messages:
    4,560
    Likes Received:
    157
    Location:
    In the Island of Sodor, where the steam trains lie
    It's due to an old "friend" - aliasing. Temporal in this case.

    A CRT is producing "flashes", at X Hz, that fade out over a certain time period. If you try sampling those with a video camera (which is runing at a different rate, i.e 100Hz (PAL) or 120Hz(NTSC) FIELD RATE)), you sometimes hit the bright times and other times you hit the darker periods.

    OTOH if you sample with exactly the same rate, (eg a video camera filming a TV) you get a static result with part of the image bright and part dark.
     
    #4 Simon F, Sep 21, 2005
    Last edited by a moderator: Sep 21, 2005
  5. Geo

    Geo Mostly Harmless
    Legend

    Joined:
    Apr 22, 2002
    Messages:
    9,116
    Likes Received:
    213
    Location:
    Uffda-land
    My wife the TV engineer says the same thing. Tho I have noticed it tends to not be as god-awful looking in recent years. I suspect the increase in refresh rates must have helped.

    Did I ever tell you guys the story of the quack chiropractor I went to some years ago before my doctor gave up on trying to save the HMO money and we did back surgery? (side note: happily he also gave up before I was irretrievably hooked on vicodin!)

    This is NOT a general "chiropractors are quacks" story. There are very good ones, and I subsequently had one of that type. This first one tho was a new age fruit-cake.

    One of his therapies was to clear me of interference from electronic sources, particularly when he learned I work with computers all day. It seems part of my problem was that the pulsing of the monitor was screwing with my back. So he had me walk up to a monitor, put my hand on it, and then did some "Heal!" kind of thing (no palm to the forehead, thank ghu!).

    This must have been around 1998 or so, but the computer system he was using in his office was old even for the time. DOS, 12", 60hz. Like a clueless jackass, I then tried to explain to him about refresh rates, and his ancient system and monitor vs our much newer ones where I work. :roll: He confidently assured me I was cured of intereference from all monitors everywhere.

    I never went back there again. :lol:
     
  6. London-boy

    London-boy Shifty's daddy
    Legend Subscriber

    Joined:
    Apr 13, 2002
    Messages:
    21,508
    Likes Received:
    5,153
    LOL!
    I'm beginning to understand why you have 3 green squares. :lol:
     
    Geo likes this.
  7. _xxx_

    Banned

    Joined:
    Aug 3, 2004
    Messages:
    5,008
    Likes Received:
    86
    Location:
    Stuttgart, Germany
    Bah, cheats and robbery ;)
     
    London-boy likes this.
  8. Geo

    Geo Mostly Harmless
    Legend

    Joined:
    Apr 22, 2002
    Messages:
    9,116
    Likes Received:
    213
    Location:
    Uffda-land
    So that's the guideline now? "Don't trust anyone with more than two squares!" :lol:

    Ghu forbid I should sneak ahead of Reynolds and end up with the "Hmph! What's so special about him?" bullseye on my back. I may have to tell Digi to deal me red if it happens. :wink:

    It doesn't help that the member list doesn't include "Regulars", as some of those are three-squarers too. . .
     
  9. Geo

    Geo Mostly Harmless
    Legend

    Joined:
    Apr 22, 2002
    Messages:
    9,116
    Likes Received:
    213
    Location:
    Uffda-land
    Well, I could tell the story of the second chiropractor, the storefront discount one, who would have made a fine concentration camp guard. But that one doesn't have a monitor in it. :wink:

    It was the third chiropractor that was "just right". Much like MS software --useless, painful, bingo!
     
Loading...

Share This Page

  • About Us

    Beyond3D has been around for over a decade and prides itself on being the best place on the web for in-depth, technically-driven discussion and analysis of 3D graphics hardware. If you love pixels and transistors, you've come to the right place!

    Beyond3D is proudly published by GPU Tools Ltd.
Loading...