Inside the Tube: A Look at Video Monitor Technology

When was the last time you looked at a video monitor? As a videomaker, you probably use monitors quite often, whether out in the field, in the editing suite, or both. And like many videomakers, you probably take them for granted. But take another look and ask a few questions. How do these things work? Are they really showing you what your video looks like? What makes one monitor better than another? How can you tell if a monitor matches your equipment and your needs?

In this article, we'll demystify the technology of video monitors to help you understand how they work and how they differ from one another. What is resolution? What does black screen mean? Is Trinitron real technology or just marketing hype? These are some of the mysteries we'll unravel.

We'll also discuss some features (composite and Y/C connectors, etc.) found on most monitors, as well as the higher-end capabilities (underscan, pulse-cross, blue-gun, autodegaussing, etc.) that make some monitors a joy to use.

So if you've ever been curious about that glowing tube you watch so often, read on. And if you're setting up a new video editing system, the information presented here will help you make the right choices.

To understand monitor technology, let's take a look inside the heart of the monitor: the picture tube or cathode ray tube (CRT).

The CRT in a black-and-white (monochrome) monitor is similar to that of a color monitor, but simpler in design, so we'll explore it first and the color CRT later.

Figure 1 shows a simplified cross section of a monochrome CRT. At the thin end of the tube (the neck), there's a group of elements that make up the electron gun. These elements create, accelerate and focus a thin beam of electrons (cathode rays) onto a phosphor coating at the opposite end of the tube. Wherever the electrons strike the phosphor coating, it glows. If the electron beam were to remain stationary, it would merely create a tiny glowing dot on the face of the monitor.

But between the electron gun and the phosphor coating, on the outside of the CRT neck, there are two sets of electromagnetic coils of wire, called deflection yokes. The vertical deflection yokes are at the top and bottom of the neck and horizontal deflection yokes (not shown in Figure 1) are on the left and right sides of the neck. These yokes deflect the electron beam rapidly, causing it to scan the entire face of the tube; this makes the screen glow. Now we're getting somewhere.

Let's imagine that we put a black-and-white monitor inside a transparent time-slowing machine that slows everything down by about 15,750 times. Without attaching any video signal, we turn on the monitor, turn up the brightness control and watch what happens. The electron beam creates a dim gray dot at the top left corner of the screen. The deflection yokes force the beam to trace a straight, almost horizontal gray line that slopes downward slightly as it travels to the right side of the screen. This takes about 4/5 of a second.

But suddenly the dot disappears from the end of the line at the right side of the screen, and then reappears a fifth of a second later on the left side of the screen. What happened? The monitor's blanking circuit momentarily diminished the electron beam while the deflection yokes forced the beam back to the left side of the screen.

The dot repeats this line-drawing performance every second (in our hyper-slow example, remember), leaving a space between each line. This continues for about four minutes until the dot gets down to the bottom center of the screen, at which time it has drawn a field of 241-1/2 gray lines. Then, with the help of the blanking circuit and the yokes, the dot disappears for 21 seconds and reappears at the top center of the screen.

Now the dot begins tracing another field of lines in between the existing lines, filling in the spaces it left before. But by the time the dot finishes tracing this second field, the glow of the first field is fading fast and needs retracing. So the dot continually retraces the lines, keeping the face of the CRT glowing dull gray.

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