The colors! The colors!

Producing a Color Image

Simple CRTs only contain a single electron gun, and can only produce black and white images (called "monochrome"), but the more complex CRTs found in modern TVs and monitors have three guns, one each for red, blue and green. The additive mixing of the different electron gun outputs, and varied beam strengths are used to create different colors as each beam strikes the correct screen phosphor.

You can create different colors by applying a different intensity to each color phosphor. As you might guess, creating a color image is three times as complex as a B/W image because now, instead of only one electron beam to manipulate, there are three, but it still works amazingly well. Each of the three electron guns has its own yolk. A television can focus different combinations of the three beams at each screen pixel to make each glow a specific color. Using those three RGB beams, the CRT can produce any combination of up to 16 million colors.

The Flyback Transformer

So now we have this energy cannon that shoots an electron beam at a screen, we have to find a way to make that beam move, otherwise, it will just sit there and strike a single spot over and over, making a single lit spot on the center of your screen. Enter the flyback transformer. For the automotive buffs, this device is very similar to an engine ignition coil. A low power signal is applied to the flyback's magnetic coil for a specific period of time, which generates a magnetic field. When the low power source is turned off, all of that built up magnetic energy is discharged into a high power output. This output is applied to the electron gun's yolk and provides it the power it needs to generate the magnetic field used to manipulate the electron beam.

Depending on the size of the CRT, the flyback can generate between around 10kVs to 50kV of energy. When the gun completes drawing a line on screen, the flyback discharges. This shuts off the electron gun, and collapses the yolk's magnetic field, forcing the beams focus to return to the other side of the screen. The gun then drops down a line and starts the process all over again.

Vertical and Horizontal Synchronization

So far we have explained the electron gun, the yolk, the flyback transformer, and quite a few other things inside the monitor all going about their business. Now we have to make them all dance together. All the parts need to be synchronized. Two basic sync signals need to be applied to the CRT. The horizontal sync signal determines the amount of time it takes for the CRT to draw a single line across the screen. The vertical sync signal determines how long it takes the CRT to get from the top of the screen, to the bottom and then back to the starting position.