Question about display technology

What you see is what you get, including photography, displays, and video equipment.

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Question about display technology

Postposted on Fri Jul 11, 2014 3:26 am

Greetings,

this question is just pure curiosity.

When filming a screen through camera it has been usual to see strange things that can't be seen through the naked eye. For example, with old CRTs when filmed you could see the screen refresh flickering as hell.

With the advent of LCDs you could see a very stable image due to the change in technology. But recently I've seen something very strange pointed to by my sister.

She has an HP laptop and by chance saw the screen in a recording through a CMOS camera. She was amazed by what she saw. The screen has a cycle of fading to black and recovering which is very slow. I mean, she just couldn't understand how she doesn't notice anything in everyday use. The screen seems completely faded during a second and then recovers. At the side it could be seen a desktop LCD monitor and the image was pretty constant with a slight flickering.

I don't know if this is some artifact of recording with CMOS cameras or modern laptops have some crazy stuff regarding how they refresh or if this is anything related to energy conservation in a laptop screen. The laptop screen is amazing, although it is a run of the mill 1366x768. She just bought the laptop for the display because it is the one that gives her less eye strain.

Anyone knows anything about this stuff or can point me to sources for research?

Thanks.
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Re: Question about display technology

Postposted on Fri Jul 11, 2014 3:48 am

With a cathode ray tube, the electron beam illuminated one pixel at a time, scanning from left to right, row-by-row. Since it was drawing the whole screen 60 (or even better 75+) times per second, it wasn't visible to the naked eye. To make it invisible to the camera as well, set your exposure time to be a multiple of the screen's refresh rate (e.g.: 1/60th of a second or 1/30th of a second for a 60 Hz display).

With an LCD display with a fluorescent backlight, the lamp output fluctuates at the frequency of the inverter. That's probably what you're seeing through the video camera. With LED backlighting, you should have a steady image.
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Re: Question about display technology

Postposted on Fri Jul 11, 2014 4:52 am

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Re: Question about display technology

Postposted on Fri Jul 11, 2014 5:55 am

Aye. TFTCentral is an awesome site, especially in the war against stupid new marketing terms designed to re-spin low-quality old tech.
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Re: Question about display technology

Postposted on Fri Jul 11, 2014 10:24 am

Thanks to all of you guys for all the replies! You have given me the clues I needed.

I think I understand what is happening. The frequency at which the monitor switches on and off to regulate brightness is nearly in phase with the camera, but not exactly. If it was perfectly in phase, I would get an steady image. But, as it is nearly there, it shows smooth transitions which are not real but perceived. I think Nyquist's theorem was at work here.

Thanks again.
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Re: Question about display technology

Postposted on Fri Jul 11, 2014 10:46 am

madtronik wrote: I think Nyquist's theorem was at work here.
Musically-minded folks might call it a beat frequency.
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Re: Question about display technology

Postposted on Fri Jul 11, 2014 10:52 am

madtronik wrote:I think I understand what is happening. The frequency at which the monitor switches on and off to regulate brightness is nearly in phase with the camera, but not exactly. If it was perfectly in phase, I would get an steady image. But, as it is nearly there, it shows smooth transitions which are not real but perceived. I think Nyquist's theorem was at work here.
Yes. You'd get a steady image, or black, depending on if they were exactly in or out of phase (assuming the duration of the backlight's "off" pulse matched the camera's "shutter speed" exactly). But because they're close but different, they come into and out of phase slowly and you get a "signal" that is a product of the two frequencies. Superheterodyne receivers take advantage of this effect to effectively bring a high frequency input down to a much lower frequency where it is easier to manipulate, using a second input that is close to but not exactly at the frequency of the target signal (this is, among other things, what made cheap radar detectors possible).

You actually used to see this in videos of CRTs if the camera and CRT were very close in frequency, but instead of a luminance variation you'd see a slow roll of the blanking interval going up (or down) the CRT. This is also related to the old wagon wheel effect.

And as JAE points out, anybody who has tuned a guitar using harmonics has taken advantage of this effect whether they understood what was happening or not.
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