Why do computer monitors on TV flicker?

[grrl]

Many times on the news when there is some story filmed in an office, any computer monitors in the background show a horizontal line that moves from the bottom to the top of the monitors. What is this caused by?

 

[minendo]

Originally posted by: grrl
Many times on the news when there is some story filmed in an office, any computer monitors in the background show a horizontal line that moves from the bottom to the top of the monitors. What is this caused by?

You are seeing the refreshing of the screen.

 

[TerryMathews]

Most computer video cards (or at least their drivers) do not support interlacing 640x480. This would be why a Windows PC flickers on a TV and an Xbox doesn't, even though they may have very similar graphics controllers.

 

[Shalmanese]

Its due to the fact that the Computer monitors and Video Cameras are almost perfectly in sync with respect to refresh rate/frame rate. You get a similar effect if you look at propellor baldes spinnig up where they appear to stand still since they are syncing with your eyes. You can eliminate the flicker by simply changing the refesh rate and it seems that you see the charecteristic flicker less and less nowadays.

 

[grrl]

Its due to the fact that the Computer monitors and Video Cameras are almost perfectly in sync with respect to refresh rate/frame rate.

I figured it had to do with synching. So that means the refresh rate on the TV is greater than the monitor?

 

[CTho9305]

Originally posted by: grrl

Its due to the fact that the Computer monitors and Video Cameras are almost perfectly in sync with respect to refresh rate/frame rate.

I figured it had to do with synching. So that means the refresh rate on the TV is greater than the monitor?

Not necessarily.... it just means they are different. I would think a TV would usually be the slowest (60), and monitors generally higher than 60.

 

[drag]

electricy we all get goes at 60mhz. SO thats the standard speed for every little flickering dodads like TV's, cameras and floresent tubes run at. But even if they are not in sync you will still get a wave or a flicker. All TV's and monitors and cameras operate on a similar principle. You probably know of course that monitors on flash one line of pixels at any one time, it's just that we are incapable of seeing anything move faster than 50-60 fps, so everything smears together in our minds in one big blur. It's all just one big trick on our nervous system. A fly's nervous system is much smaller and theirfore much quicker it has no trouble seeing the invidual pixels light up/dim and the next row light up again! But then again in floresent lighted rooms it will only be light half the time and pitch black the other half and will seem like some sort of satanic disco to a fly.

You can see this effect if you move your hand up and down rapidly between your eyes and the monitor. Or better yet a some blades of a fan. You will see "shadows" of your hand/fan if it is moving rapidly enough as it covers up that one line of activated pixels for a instant.

So unless the camera is happening to scan in a picture the same moment that a tv or monitor is scanning in one you will see it flicker.

Movie/TV people have a device in which they can plug into the powersupple (ie eletrical power cord) that will sync up the electrical supplies to electronic tv-type devices so that they can video tape them with no flicker.

thats how I understand the flickering effect for all that it is worth (axprom. $0.02 of time and bandwidth)

 

[Peter]

You see the interference frequency. E.g. when the computer screen refreshes at 75 Hz and the TV camera is recording 60 images/second, you'll see an artifact effect that appears to be moving at 15 Hz. Very simply said.
Same effect that makes car wheels look standing still or even turning backwards when filmed or driving past a fence.

regards, Peter

 

[Xyrate]

i thought basically the cameras are not fast enought to capture monitors because they refresh so quickly?

 

[dejitaru]

electricy we all get goes at 60mhz. SO thats the standard speed for every little flickering dodads like TV's, cameras and floresent tubes run at.

No, it's at 60Hz, and that has nothing to do with the capture speed of the camera. Also, the video camera is probably battery-powered.

So unless the camera is happening to scan in a picture the same moment that a tv or monitor is scanning in one you will see it flicker.

Actually, if they are at exactly the same speed, the flicker will be much worse, you'll see a horizontal gradient.

i thought basically the cameras are not fast enought to capture monitors because they refresh so quickly?

You don't want the capture speed of the camera to be as fast as the monitor's refresh rate, otherwise there will be noticeable flicker.

A CRT only draws one pixel at any time, so it has to move very quickly. The effect is that you will see images and not pixels. A video camera captures about as fast as this one pixel moves, so there may be flicker.
If the refresh is too fast for the camera, it will exploit the blurring effect on which all CRT's rely.
To reduce flicker, increase the refresh rate of your monitor.

 

[Mingon]

it's just that we are incapable of seeing anything move faster than 50-60 fps

Has it really been 6 months since the last time this stupid comment was said, oh well

 

[sxr7171]

Originally posted by: Peter
You see the interference frequency. E.g. when the computer screen refreshes at 75 Hz and the TV camera is recording 60 images/second, you'll see an artifact effect that appears to be moving at 15 Hz. Very simply said. Same effect that makes car wheels look standing still or even turning backwards when filmed or driving past a fence. regards, Peter

I was going to mention the car/wagon wheel phenomenon but you beat me to it.

 

[zephyrprime]

You're seeing the point in time/space where the sync on the camera and the electron beam on the monitor are at the same point. The rest of the computer screen is dimmer because it's not being immediately lit by the electron beam, it's only phoshorescing(sp bad).

 

[AnthraX101]

Originally posted by: zephyrprime
You're seeing the point in time/space where the sync on the camera and the electron beam on the monitor are at the same point. The rest of the computer screen is dimmer because it's not being immediately lit by the electron beam, it's only phoshorescing(sp bad).

Oh dang it. I had a good post all typed up and got an error and lost it. *sigh*

Quick overview:

Yeah, and you see a band because the camera doesn't capture a single moment in time, it has an exposure length. Therefore, the film was being exposed (or the digital detectors were being sampled, etc.) for the length of time that it took the monitor to refresh from the top left pixel in the band to the lower right pixel in the bright band. Then, while the camera was advancing the film (or just sitting around waiting for its next cue) the monitor refreshed from the first dark pixel in one frame, to the first bright pixel in the next. (Or, if the refresh rate is further off of the sampleing rate, then it did that plus a full screen, or 2 full screens, or 3, etc.) That is why the band can scroll. It can actualy stop scrolling and reverse direction too, if the clocks drift over one another.

AnthraX101

 

[drag]

it's just that we are incapable of seeing anything move faster than 50-60 fps

Has it really been 6 months since the last time this stupid comment was said, oh well

Why is it stupid to say that as humans we are incapable of discerning a object vibrating faster than 60hz?
And were do you get off saying something like that and not backing it up? you could at least of put a link up or something, or are you afraid that might infringe upon your intellectual superiority to me? Or is it that I didn't phrase it correctly enough for you?

If you can I would like you to educate me. Because If you could see motion beyond 60 that's anything beyond a blur I think that we would have a hardtime watching TV, because it would be just one line moving top to bottom on a black screen. Especially since it is only realy running interlaced images at 30fps. hmm.. maybe I am just stupid, eh.

Sure we can make out individual objects that may flash(lets say a single frame of a picture of a popcorn bag) by at up to about 200fps on a screen, but I would enjoy seeing you trying to tell me what that object was.

 

[MrDudeMan]

Originally posted by: drag

it's just that we are incapable of seeing anything move faster than 50-60 fps

Has it really been 6 months since the last time this stupid comment was said, oh well

Why is it stupid to say that as humans we are incapable of discerning a object vibrating faster than 60hz?
And were do you get off saying something like that and not backing it up? you could at least of put a link up or something, or are you afraid that might infringe upon your intellectual superiority to me? Or is it that I didn't phrase it correctly enough for you?

If you can I would like you to educate me. Because If you could see motion beyond 60 that's anything beyond a blur I think that we would have a hardtime watching TV, because it would be just one line moving top to bottom on a black screen. Especially since it is only realy running interlaced images at 30fps. hmm.. maybe I am just stupid, eh.

Sure we can make out individual objects that may flash(lets say a single frame of a picture of a popcorn bag) by at up to about 200fps on a screen, but I would enjoy seeing you trying to tell me what that object was.

actually if you do a search for the old thread, you CAN see something moving at unbelievable FPS...the human mind doesnt capture things in frames per second, it captures a stream of "video" and you see it as it is

determining how many FPS the eye is capable of discerning is not easy, if even possible, because you cant relate the two

i dont have a link because it doesnt really matter and this is off topic of the original question, but do a search and you will find the old thread which was a much better explaination



edit: CRTs also dont draw a line at a time, its an electron gun that shoots a pixel by itself to the surface, it just does it millions of times per second, well maybe not millions, but a lot

 

[drag]

cool all i wanted was a explaination

 

[dejitaru]

actually if you do a search for the old thread, you CAN see something moving at unbelievable FPS...the human mind doesnt capture things in frames per second, it captures a stream of "video" and you see it as it is

You don't capture streaming images. You have visual memory. That, with the high capture rate of your brain, creates a nice blurred video. If the appropriate part of the brain is damaged, you see things at a lower framerate, moving objects appear as "snapshots".

 

[spidey07]

Most have already hit on it - refresh rate.

Set the monitor to 60 Hz to sync with the 60 fields/sec of video and there is no flicker.

 

[Shalmanese]

the fps you see depends on enviroment as well. TV movies are filmed so they have an inherent motion blur. If you freeze frame a TV shot, it will look blurry. Since this is what it would sort of look like if you "freeze framed" your eye, you can get away with lower fps.

Saying you cant tell what a thing is at 200fps is irrellevant. If your playing Quake or UT, you dont need to conciously recognise anything, it all gets processed sub-conciously.