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I'm having trouble imagining what actually happens inside an LCD screen.

I picture digital RGB data for each pixel getting sent to the screen through a connector. Lets say we have 24-bit color for each pixel, so 8 bits each for red, green, and blue subpixel. Inside the LCD, for each subpixel a voltage must be applied to control its intensity. This is where a DAC must come in to convert each 8 bit intensity value (digital) to a proportional voltage (analog). But, surely there is no DAC for every subpixel? My laptop screen is 1920x1080 so it has over 6 million subpixels.

So, how are these DACs integrated into the architecture of this LCD? How many DACs are in a single LCD screen?

I read that a transistor is used to select a particular liquid crystal cell and charge it/light it up, and a capacitor holds the charge briefly until the next refresh cycle. So that would eliminate the need for 6 million DACs, although it would mean you need 6 million transistors and capacitors. But I'm still wondering about these DACs - they never seem to get mentioned because people consider LCD screens "digital".

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  • \$\begingroup\$ "although it would mean you need 6 million transistors" yep. Have a look at any search result of "tft structure" or similar. \$\endgroup\$ – PlasmaHH May 13 '15 at 8:29
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From this patent about TFTs:

The pixels are organized in a matrix like this: enter image description here

and each pixel circuit looks like this:

enter image description here

During operation, voltage levels according to desired brightness levels for each pixel of a single row are put on the vertical lines Cn. After this, a signal on RAm and RBm activates the transistors of all pixels in that row, and the voltage levels Ci are transferred to a capacitor inside the pixel. The transistors are deactivated, and the whole process continues with the next pixel row.

That capacitor not only stores the voltage applied, it also is part of the liquid crystal cell and its electric field causes the crystals to move into the orientation needed for the desired brightness.

Here is also a drawing of the layout of a pixel:

enter image description here

Finally, I have to say I have no idea why there are two transistors, but I also didn't read the full document. The stuff is not as easy as described here, but it's OK to say that both transistors are used to put the voltage level of Cn onto the capacitor.

Now, you only need as many DACs as there are columns, but DACs can be quite simple and small.

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    \$\begingroup\$ The two transistors are a special feature of this patent for reducing power consumtion. Conventional TFTs have only one there. \$\endgroup\$ – PlasmaHH May 13 '15 at 9:33
  • \$\begingroup\$ You may not even need one DAC per column, given that the pixels are scanned, and that there is thus only ever one of the pixel transistors on at any given time, you only actually need one DAC whose output feeds every pixel (through analogue buffers probably). You just need an analogue switch for each column to allow only the currently selected column to charge (like the row switches). \$\endgroup\$ – Tom Carpenter May 13 '15 at 17:17

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