Drawing lines

Fields Of Dreams

There are two main parameters we are concerned with when drawing an image: How long does it take to complete a line (horizontal sync), and how long does it take to draw an entire frame (vertical sync, or refresh rate). Your video card will supply the monitor with reference horizontal and vertical sync signals, and the monitor uses these signals to drive its internal circuitry to ensure that the monitor stays in sync with the video card's current display settings.

Within the monitor, there are some simple oscillator circuits tuned according to the reference signal. With standard television, the horizontal sync is 525 lines X 30 fps = 15.75kHz. Computer monitors are capable of varying horizontal frequencies based on your current settings, but for the most part, they start at 15.75kHz and go up to an average of 85-95kHz maximum. In the following example, lets use a television's 15.75kHz horizontal frequency to examine this is a bit closer.

Since time period is the inverse of frequency (frequency divided by 1000), 15.75kHz works out to be 1/15.75kHz=63.5us (microseconds). What this means is that it takes 63.5ms for a single line to be drawn on the screen.

Draw them lines!

At this point, the vertical retrace pulse causes the electron gun to turn off. The flyback discharges, forcing the electron gun to "flyback" (hence the name) to the starting position, drop one line down, and start all over again. This process is repeated 525 times for each frame. Once it reaches the bottom of the screen, the CRT turns off, and the electron gun returns to the topmost position to begin drawing the next frame.

The actual number of lines drawn on the screen is somewhat less than 525 due to blanking time, which is the time in which the CRT is off while it returns from the right to left to draw the next line, and while it retraces back to the top of the screen for refresh. This leaves us with a usual 428 to 339(depending on the quality of your set) lines displayed in broadcast television.

Interlaced and non-interlaced scanning

Computer monitors rely on the exact same technology as TVs; they just do it with greater precision. With television, the entire scene is an image of reality, converted into electronic format. A typical scene is made up of all of those random curves commonly found in nature, and many of the objects within the scene are all in some sort of motion. Because of this, the image itself, even though displayed at a much lower frame rate, appears to move very smoothly especially when television frames often include motion blur. When you're staring at a computer screen, you are essentially looking at a slide show of the same static, or nearly static image being redrawn over and over again. Couple this with the primarily "square" nature of screen objects, and it makes the "flicker" effect much more noticeable.