How does the cpu/gpu tell each individual lcd in a monitor to do? And how does it understand without 2.1M+ wires?
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The computer prepares a signal that contains the intensity values for each pixel in the first row in a sequence, followed by the second row, and so on until it has scanned across all pixels and returns to the top again to draw the next frame. The time spent on each pixel is agrred upon by the standard and resolution that the computer and the monitor have agreed upon in advance. This happens in all commonly used analog and digital connections.
The trick is called “multiplexer”
It’s basically a junction of wires, a single wire splits into many and the multiplexer makes sure that only one of the channels is open at a time. So you send all the pixels in sequence and every time one is read the multiplexer switches to the next channel.
A multiplexer is basically a tree structure of left/right junctions, so a binary number can easily tell wich output should be active. 10111011 would basically mean signal goes left/right/left/left/left/right/left/left. With these 8 switch stages you can already reach 2^8 =256 different outputs. So for 2.1M outputs you need about 21 stages because 2^21 =2,097,152
Each pixel in the screen has a tiny memory cell. OLED displays have an analog memory cell with a simple current controller for the LED. In LCD screens, the LCD cell itself has some relapse time. The memory is short-term, and has to be renewed regularly – otherwise it looses data.
There is column wires, that run vertically – they are used to store data in the pixels. There is also row wires, that run horizontally – they seal and unseal electrical “valves”, that connect the cells to the column wires. Row wires select a row, that will be updated.
The video port sends pixel values one at a time – row by row, column by column. The monitor accumulates them into an internal row buffer memory. When the buffer is full – it sets the voltages on the column wires and then activates one of the row wires.
The GPU and CPU do not directly work with the screen: they work with its virtual copy – the framebuffer. The framebuffer is a RAM chip, and it organized exactly like a real screen – with rows and columns and a row buffer. The difference – GPU and CPU can activate rows in any order by sending a “row select” command with a row number, and then request or modify individual columns with a “column select”. The video port controller chip will periodically read the entire framebuffer and send the data to the monitor.
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