I’m just used to it working but I don’t understand at all how it works. Like how do the electric signals coming from the computer lead to each individual pixel showing the rexactly right color at the exactly right time?
In: Technology
The graphics card in your computer does the calculations to figure out what color to display on each pixel and then sends that information over a cable to your monitor which simply outputs what its told.
The magic is called “multiplexing”. You have one wire with a series of signals arriving one after the other. The multiplexer is then like a junction with one input wire and many output wires. A control signal switches wich output is on to route the input in that direction.
In the case of a screen it simply has to sequencially send signals to each pixel one after the other. The pixels themselves are pretty much little LEDs with a small driver that takes the digital input signal and converts it to an analog power to make the pixel light up that much. Each pixel is made from 3 subpixels responsible for Red/green/blue so you can mix every colour from that.
Together these two systems turn a stream of digital signal that only switches between on/off quickly but encoding different numbers into a full picture on a screen.
not sure, if it works this way today, but imagine there are 3 layers of lcd displays over each other. each filters a prime color, normaly red, green and blue, that means that it only let those colors through. the computer sends a set of three values to the monitor, that tell, how much a color shall be filtered. if you got a green point, it would be red 50, green220, and blue 50. much more green light comes through the filters so the point is green.
The screen always expects the pixel data to come in the same order: starting from the top row, from left pixel to the right, then the next row, and so on.
OLED screens have a small capacitor inside each pixel that acts as an analog memory cell: the charge in that capacitor regulates power to the LED. This means that the screen can remember the picture for a brief moment: you can partially charge the capacitor, and then seal the pixel – it will remember the brightness until the next update.
The capacitors are charged or discharged through *column wires*, that run vertically along the screen. *Row wires* control the seal/unseal of the rows: when a row wire is activated, pixels are connected to their column wires. When the wire is deactivated – transistor switches disconnect them.
A controller chip receives the data from the computer and stores it in the internal RAM. When one full row is received, it sets the voltages on column wires and activates one row wire.The capacitors are recharged, changing the brightness of LEDs that are connected to them.
It doesn’t really “know” anything. It’s just a machine doing a job. I’m going to oversimplify, not all monitors work like this but it’s a good description of the important parts.
Part of that job is “listening” to the signal coming down the cable. The machine that does this job has a “clock” signal it uses. Signals computers send are often like music, they have a “rhythm” and the things that listen for signals only read the “note” in between beats. The “note” in this case will be a signal that is either high voltage (“on” or “1”) or low voltage (“off” or “0”). There is a complex code based on series of those signals sort of like Morse code that is how the computer communicates with the monitor.
So the monitor, after startup, starts reading the “notes” on the cable and waits until it sees a certain pattern. That pattern means “this is the start of a frame”.
So when the monitor gets that signal, another part of it starts to work. This is a tiny computer responsible for changing the colors of the pixels. It doesn’t really remember what color any of them are, it’s not that sophisticated.
Now the “signal reader” expects the next few signals to represent the color of a pixel. There’s a special code for how the high and low voltages correspond to a color. Once it reads enough signals, it tells the “pixel changer” to change the “first” pixel to the color it just read. Then it listens to the signal for more patterns and tells the “pixel changer” what color the “second” pixel should be. This happens once for every pixel, so at the end every pixel has been told what color to be. If it didn’t change, it just gets told to “change” to the same color.
How does the “pixel changer” actually pull this off? Well, it’s kind of the same. It has a special circuit that works kind of like a switch to connect itself to any pixel it wants. Once it’s connected to a pixel, it can control how much red, green, or blue that pixel is going to display. So it uses the signal it gets from the “signal reader”, coverts that to the appropriate amounts of red/green/blue, then outputs a signal to the pixel that means “change to these amounts of red/green/blue”.
(It’s kind of more complicated than that. There’s not really “a pixel” in modern displays. It’s really just a tiny area that happens to have a lot of little different-colored lights. Making “a pixel” be a certain color involves controlling how bright each light is in a specific area. That makes a the area on the screen that counts as “a pixel” look like the correct color.)
A lot of this is more complex in reality, but we can imagine it as those three little systems talking to each other using a special code. The “signal reader” listens for signals from your computer that tell it the color for each pixel. It tells the “pixel changer” what color each pixel should be. The “pixel changer” does the work to adjust the actual pixels so they display the right color.
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