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In computer screens, is every pixel turned on during one frame?

How can you address millions of pixels at the same time?

Or is it like in CRT where it is scanned left to right, top to bottom?

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    \$\begingroup\$ What's a "computer screen"? There's at least 5 different mainstream technologies, numerous ways to drive each one, plenty of different chips designed for that purpose and no on. And then different drivers and software. So this is way too broad to be answered. \$\endgroup\$ – Lundin Sep 30 '20 at 11:30
  • \$\begingroup\$ of course im talking about normal computer monitors \$\endgroup\$ – Noob_Guy Sep 30 '20 at 11:34
  • \$\begingroup\$ LED/LCD computer monitors \$\endgroup\$ – Noob_Guy Sep 30 '20 at 11:34
  • \$\begingroup\$ @Noob_Guy yes, Lundins answer is correct as he talked about normal computer monitors too. \$\endgroup\$ – schnedan Sep 30 '20 at 11:35
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There are different technologies used for "computer screens".

Today's most common technology is TFT-LCD which mostly uses the Active Matrix adressing scheme to build up an image.

... is every pixel turned on during one frame?

In a TFT-LCD the pixels are on continuously, if the display has a refresh rate of 60 Hz then the pixel's brightness is updated 60 times per second.

The pixels have to be on continuously, if that was not the case (the pixel is off and dark most of the time) then they would have to be extremely bright when it is on to display an image. On a CRT this sort of happens (the dot is very bright) but the phosphors make the dot glow for some time.

How can you address millions of pixels at the same time?

It is possible but:

  • very expensive
  • not needed

Your eyes aren't that quick to respond so if all pixels are addressed in such a short time that your eyes won't notice it then there is no issue in addressing the pixels in a certain sequence.

Or is it like in CRT where it is scanned left to right, top to bottom?

Something like that indeed, but not pixel-for-pixel, instead one row (or column) is refreshed in one go. Then the next row (or colum) etc. How this precisely done depends on the actual display.

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  • \$\begingroup\$ But if we multiplex by row, say a 1920x1080 monitor, then you would need a microchip with 1920 x 3 (RGB) = 5760 pins to address all pixels in row simultaneously. Do we have such a microchip? \$\endgroup\$ – Noob_Guy Sep 30 '20 at 14:00
  • \$\begingroup\$ @Noob_Guy Are you assuming that these pins are directly connected to a microcontroller? They're not, they're connected to display driver chips which have many pins. In a 1920x1080 display there will be several of these chips. These drivers are usually controlled through an LVDS type connection which uses much less pins. Chips with 6000 pins can be made but will need to be large (to fit all the pins) and therefore expensive. The solution is to use many smaller chips with a few hundred pins each. \$\endgroup\$ – Bimpelrekkie Sep 30 '20 at 14:32
  • \$\begingroup\$ are those "display driver chips" also microcontrollers? or are they just buffers? and what is LVDS, is it an IC communication protocol similar to SPI, I2C, etc.? \$\endgroup\$ – Noob_Guy Sep 30 '20 at 18:01
  • \$\begingroup\$ In TFT you update only the pixels that changed. This is why writing one character is fast but when you change the whole picture you can see the screen blinks. \$\endgroup\$ – Moty Sep 30 '20 at 18:33
  • \$\begingroup\$ In TFT you update only the pixels that changed It depends on the type of LCD if this is true. For a high resolution PC display I do not believe this is done as it is "too much work" to figure out which pixels changed and which did not. On a small, low power monochrome LCD indeed this might be done though. \$\endgroup\$ – Bimpelrekkie Sep 30 '20 at 18:59

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