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.
The Monochrome Monitor
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.
Follow the Bouncing Dot
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.
Tracing an Image
Now let’s imagine that we put a video camera into the transparent time-slowing machine with the monitor. Let’s also connect video cables from the camcorder to the monitor so that we can watch the image output from the camcorder on the monitor’s screen. We aim the video camera at a still photograph. Let’s throw the switch and power up the entire system.
Just as before, a dim gray dot appears at the top left corner of the screen and begins to trace a line. But inside the monitor, something interesting happens. Part of the video signal entering the monitor finds its way to the electron gun and causes the electron beam’s intensity to vary wildly. On the monitor’s screen, the dot flickers and flashes, leaving what looks like a string of bright and dark dots and dashes across the screen. The dot jumps back to the left side of the screen and draws another line with a similar but slightly different pattern. After several lines appear, we recognize that the monitor is drawing an image of the photograph that the video camera "sees."
Figure 3 shows the photograph’s image as it appears on the monitor. One of the 483 visible lines traced by the electron beam is highlighted in white for clarity. The waveform below the photograph corresponds to the variations in the electron beam’s intensity as it traces this specific line.
Now that we’ve seen how the monochrome CRT works in slow motion, let’s remove the monitor and video camera from our imaginary time-slowing machine. In real time, the monitor’s electron beam traces out each line from left to right and returns to the left in 1/15,750 of a second. It traces a field of 241-1/2 lines from top to bottom and returns up top in 1/60 of a second. And it traces a full frame of 483 lines and repositions to start again in 1/30 of a second.
The CRT in a color monitor is similar to, but more complicated than, a monochrome CRT. Instead of only one electron beam, the color CRT uses three beams, one for each primary color (red, green, and blue). And instead of having a continuous coating of phosphor on the inside of the tube’s face, the color CRT has a large number of tiny phosphor dots or stripes. The color CRT also uses a metal screen or grille between the electron beam and the phosphors.
Currently, there are three types of color CRTs. Figure 4 show a simplified representation of the arrangement of electron beams, screen and phosphors in the three types of color CRTs.
The Delta Configuration
The first type of color CRT, which RCA developed and introduced in 1950, is the Delta or Triad CRT. It uses three electron guns in a triangular, or delta, configuration, and phosphor dots in the same triad configuration. The three electron guns emit beams that travel toward the faceplate, converge, and pass through one of hundreds of thousands of round holes in a metal screen called a shadow mask or aperture mask. The beams emerge from the hole and diverge, and each beam strikes an individual phosphor on the screen. The electron guns, shadow mask and phosphor dots are all precisely aligned so the "red" beam hits the red dot, the "green" beam hits the green dot, and the "blue" beam hits the blue dot. (Of course, the electron beams themselves have no color. And each phosphor dot is dark until electrons strike it, whereupon it glows its particular primary color.)
The triad of red, green and blue phosphor dots are so small and so close to each other that, when seen from a normal viewing distance, they appear to be one glowing dot. So, depending on the intensity of each electron beam, the triad can display any color at any brightness. Multiply this by a few hundred thousand triads, and you’ve got a screen full of living color.
In 1969, Sony introduced the first in-line configuration CRT called Trinitron. It uses a single electron gun with three cathodes to produce three electron beams in line with each other on the same horizontal plane. It also uses vertical slits stripes instead of phosphor dots, and a screen of vertical slits called an aperture grille. The in-line electron beams and vertical phosphor stripes improve horizontal resolution. The vertical aperture grille allows the Trinitron CRT faceplate to have a vertically flat surface instead of spherical like a Delta CRT of other in-line CRTs. The "flatscreen" reduces image distortion at the corners of the screen, and reflects ambient light away from the viewer. But to scan the vertically flat faceplate properly, the electron gun must be further from the faceplate. so Trinitron CRTs are generally longer and heavier than Delta CRTs.
In 1972, RCA introduced an in-line CRT similar to Sony’s. It uses three separate in-line electron guns and an aperture mask with vertical slits. As in the Trinitron, the in-line electron beams and the vertical phosphor stripes yield better horizontal resolution than the Delta CRT. The vertically slotted aperture mask allows the CRT faceplate to be vertically flatter than a Delta CRT, but not as flat as a Trinitron CRT. The length and weight of the RCA in-line CRT is also intermediate between a Delta and a Trinitron CRT.
Of course, all three types of color CRTs have continually undergone refinements since their introductions, but it is probably safe to say that a color monitor with an in-line CRT is superior to one with a Delta CRT.
Black Matrix vs. Black Screen
Two CRT improvements that people often confuse with one another are black matrix and black screen.
Black matrix is an improvement to the Delta-configuration CRT. In it, the space in between the phosphor dots is a deep shade of black. This absorbs scattered light and increases picture contrast. It also allows the use of wider electron beams while avoiding overlap, increasing picture brightness.
Black screen is an improvement to CRTs in all types of monitors. The screen or faceplate of the CRT consists of gray glass instead of clear glass. The darkened faceplate absorbs ambient light twice: first as it enters the faceplate, and again as it reflects back out again from the glow of the phosphors. Light emitted by the phosphors, however, is only absorbed once. So even in a brightly lit room, the screen maintains the picture’s contrast.
Resolution is a measure of how much picture information, or detail, a monitor can display. You can measure resolution from the top to the bottom of the screen (vertically), and from left to right (horizontally).
The 483 lines traced by a CRT represent the maximum amount of picture information the monitor can display between the tope and the bottom of the screen, and is called the maximum vertical resolution. All TVs and monitors that conform to the NTSC standard used in the U.S., Japan, and a few other countries) have the same maximum vertical resolution.
The amount of picture information available across the width of the monitor’s screen is called the horizontal resolution. It is not limited by the NTSC standard, and can vary greatly from monitor to monitor. This is the type of resolution quoted by video equipment manufacturers in their marketing brochures.
When matching a monitor with other video equipment, it’s always a good idea to select a monitor whose horizontal resolution is slightly better than the equipment. That way, you’ll be sure to see all the clarity the equipment can offer without spending an arm and a leg for resolution you’ll never see.
A simple method for demonstrating a monochrome monitor’s horizontal resolution is to display special images on the screen. First we connect a high-resolution, black-and-white video camera to the monitor. Then we place in front of the camera the photograph of a white picket fence with about 400 skinny pickets and equally skinny black spaces between them. We turn the switch on and look at the monitor’s screen. The electron beam traces identical lines composed of regularly spaced black and white dots, and the entire image of the 400-picket fence appears on the screen. If the image is crisp and clear, and the edges of the pickets and the shadows that separate them are sharp and well defined, we can say that the monitor has a resolution of at least 400 lines. A professional resolution chart offers a similar but more accurate method to determine horizontal resolution.
Figure 5 shows a simplified version the monitor’s picket-fence image and the corresponding variations of the electron beam’s intensity. The steep vertical slopes of the beam-intensity changes. The faster a monochrome monitor is able to vary its electron-beam intensity, the better its horizontal resolution.
Since color CRTs use discrete phosphor dots or stripes instead of a continuous coating like monochrome CRTs, their resolution varies, based on how close together the dots or stripes are. so color monitor manufacturers sometimes quote a specification called dot pitch or stripe pitch in the marketing brochures. Dot or stripe pitch is simply the distance between each dot or stripe. The smaller the distance, the smaller the dots–hence, better vertical and horizontal resolution.
When you break it down into its components, you can easily see what a marvel of technology the video monitor really is. But like many other everyday wonders, we tend to take it for granted, simply expecting it to light up with a perfect picture every time we hit the "on" switch.
Hopefully, this little tutorial will help you when it’s time to make your next monitor purchase. And who knows? Instead of taking it for granted, it may even cause you to gaze in wonder the next time you turn on your TV.
Sidebar: Monitor Features
- Y/C Inputs The increasing popularity of Super-VHS and Hi8 video equipment has produced an upsurge in color monitors that offer Y/C input connectors along with the traditional composite input connectors. A composite video signal fed into a color monitor is filtered and separated into brightness (Y) and color (C) signals. The Y/C input bypasses this filter, allowing the signal to pass unmolested into the monitor.
- Autodegausser A color CRT’s metal shadow mask or aperture grille can eventually become magnetized by the earth’s magnetic field or by stray magnetic fields from nearby electrical devices. If this happens, the grille or mask will deflect the electron beams to hit the wrong phosphors, resulting in poor color reproduction. To prevent this, some monitors have an autodegausser that automatically demagnetizes the shadow mask or aperture grille every time you activate the monitor’s power switch.
- Underscan This is a feature on some high-end monitors that, with the flip of a switch, allows you to shrink the picture and make the borders visible. This is handy for spotting unwanted objects, like an overhead microphone, at the edge of a scene. It’s also used to spot VCR playback flaws such as edge jitter or flagging.
- Pulse-cross This feature exists on some high-end monitors. It displays the edge and bottom of the picture in the middle of the screen and allows you to see the synchronizing and tracking portions of the video signal. At a glance, a video engineer can check the health of the video signal with this feature.
- Blue-gun Only This is another high-end feature for calibrating a color monitor. It activates the CRT’s blue electron gun (or beam) and disables the red and green guns. To calibrate the monitor’s color, you feed a color-bar signal into the monitor, activate the blue-gun only and adjust the color controls until every other bar displays the same intensity of blue.