How is information sent to and interpreted by computer monitors?

[Bluskyz]

I've been wandering about the protocol or method by which computer monitors receive and interpret information it is sent. Can the information be send down a single wire in a series of pulses? Say for example, you had a monitor that could interpret the following string of code: 1111 1010 1010 1000 1000 1010 0000. The first initial 1111 would tell the monitor that the following string of information would be the start of the information required to determine which pixels would be on or off. The 10 in the beginning of each subsequent group of 4 bits would indicate that a 1 or 0 immediately following would determine whether the current pixel is on or off respectively. Finally, 0 at the end of the group of 4 bits could either be a marker telling the monitor to proceed onto the next pixel or it could just be a place holder with no real value. I guess I'm asking if it would, in theory, be this easy to tell a monitor or some display device how to paint the screen. I know this example code was in no way optimized or perhaps practical but could it be done with a simple continuous string of binary code? Thanks for your replies.

 

[Simon Bridge]

It's usually more complicated than that - you don't just send a bunch of ones and zeros to a monitor ... something you should be able to tell by examining the plug.
Have you seen:
http://computer.howstuffworks.com/monitor3.htm

 

[meBigGuy]

Pixels are not just on or off. Each pixel might be made up of 3 phosphor dots, each of which can be any of 256 (8 bit) levels of brightness (or less, depending on the monitor - 24bit is "truecolor"). The monitor interface is either analog (like VGA) or digital (like DVI, HDMI, etc). The digital ones follow a serial/parallel protocol, but not quite like you described.

http://en.wikipedia.org/wiki/Color_depth for info on "bits per pixel"
http://en.wikipedia.org/wiki/Digital_Visual_Interface for info on the serial protocols.

 

[Okefenokee]

It's not easy. A video signal is not a single line of data. It's divided into data and clock signals. The data signals are divided into red, green, and blue for each pixel. The clock signals tell the receiver that a pixel is ready to be read, the next horizontal line is starting, the frame is done, etc. All the communication has timing requirements that have to be matched between the sender and receiver.

If you want to get into it there are projects that use FPGA's to create HDMI and VGA signals.

Here's a couple that you can check out.

http://www.fpga4fun.com/PongGame.html
http://www.fpga4fun.com/HDMI.html

You can learn about video signals just from reading those.

 

[meBigGuy]

To make it even more complex, some digital video links don't send a clock. The clock must be derived from the data transitions by a phase locked loop. Even DVI is complicated with respect to clock and data, in that the 1 to 10 clock to data relationship is not phase aligned and needs to be derived.

 

[donpacino]

A serial system such as the one you describes is similar to systems used in very small LCD and LED screens. They will use SPI or I2C for communication.

 

[Bluskyz]

Thanks for your reply. I looked a bit more into this and it turns out that there are television protocols such as NTSC and PAL, albeit these aren't far computers as far as I know. From what I can tell, early black and white TVs and later color ones pretty much did send pulses of current down a single wire. The voltage ranged between 0.0 volts to 0.7 volts and thus denoted the luminosity of the pixel. I'll have to do more research into this.

Interestingly, the fact that luminosity is determined by the voltage being applied is a life saver. What if the data had been sent with just discrete ones and zeros. You may have had to have like 6 or more bits dedicated to just defining the luminosity of just one pixel.

 

[meBigGuy]

It's a lot easier to do it digitally, trust me. Getting high resolution video bandwidth amplifiers and good sync detection is really tricky. Just try feeding an analog VGA signal into a modern LCD monitor (with VGA inputs) and start messing with the phasing to get it onto the right pixels (first thing it does is get A/D converted).

As for "pulses of current down a wire", I have no idea what you are referring to (none of them used pulses). NTSC has encoded linear chroma and luminosity information combined with vertical and horizontal sync. The chroma is 2 channels modulated onto two quardrature 3.58MHz subcarriers. S-Video has separate pins for chroma and luminance.

 

[Simon Bridge]

It is technologically possible to make a monitor that will respond as described.
But what would be the point?

TVs used to be used as monitors in the bad old days - the input was to a tuner circuit so the graphics adapter had to output RF.

Old-style monochrome monitors had just two wires for communication.
http://en.wikipedia.org/wiki/Monochrome_monitor

It is technically possible to used timed pulses to write to this sort of monitor since you know the scanning speed and the dimensions of the screen.