Video Hardware, Part 1 - Dot Pitch (CRTs)

Another important specification that denotes the quality of a given CRT monitor is its dot pitch, which is controlled by the design of the shadow mask or aperture grille inside the CRT. A shadow mask is a metal plate built into the front area of the CRT, next to the phosphor layers. It has thousands of holes that are used to help focus the beam from each electron gun so that it illuminates only one correctly colored phosphor dot at a time. Because of the immense speed of screen rewriting (60–85 times per second), all dots appear to be illuminated at the same time. The mask prevents the electron gun from illuminating the wrong dots.

In a monochrome monitor, the picture element is a screen phosphor, but in a color monitor, the picture element is a phosphor triad—which is a group of three phosphor dots (red, green, and blue). Dot pitch, which applies only to color monitors, is the distance (in millimeters) between phosphor triads, measured from the center of a phosphor dot in a given triad to the same color phosphor dot in the next triad. Screens with a small dot pitch have a smaller space between the phosphor triads. As a result, the picture elements are closer together, producing a sharper picture onscreen. Conversely, screens with a large dot pitch tend to produce images that are less clear.Figure 15.7 illustrates dot pitch.

Figure 15.7 Dot pitch is the distance between each group (triad) of red, green, and blue (RGB) phosphors. A smaller dot pitch helps produce sharper, clearer images.

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Note -Dot pitch is not an issue with LCD portable or desktop display panels because of their designs, which use transistors rather than phosphor triads.

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The original IBM PC color monitor had a dot pitch of.43mm, which is considered to be poor by almost any standard. Smaller pitch values indicate sharper images. Most recent monitors have a dot pitch between.25mm and.30mm, with state-of-the-art monitors down to.24mm or less. To avoid grainy images, look for a dot pitch of.26mm or smaller. Be wary of monitors with anything larger than a.28mm dot pitch; they lack clarity for fine text and graphics. Although you can save money by buying monitors with smaller tubes or a higher dot pitch, the trade-off isn't worth it.

Monitors based on Sony's Trinitron picture tubes and Mitsubishi's DiamondTron picture tubes use an aperture grille, which uses vertical stripes (rather than a shadow mask) to separate red, green, and blue phosphors. This produces a brighter picture, although the stabilizing wires shown in Figure 15.8 are visible on close examination. Monitors using an aperture grille–type picture tube use a stripe pitch measurement instead of dot pitch. An aperture grille monitor stripe pitch of.25mm is comparable to a.27mm dot pitch on a conventional monitor.

Some of NEC's monitors use a variation on the aperture grille called the slotted mask, which is brighter than standard shadow-mask monitors and more mechanically stable than aperture grille–based monitors (see Figure 15.8).

Figure 15.8 Aperture-grille picture tubes (left) have their phosphors arranged in vertical stripes with one or two reinforcing wires, depending on CRT size. NEC's CromaClear slotted mask picture tube design (right) provides many of the benefits of both the shadow-mask and aperture-grille designs.

The dot pitch or stripe pitch measurement is one of the most important specifications of any monitor, but it is not the only specification. You might find the image on a monitor with a slightly higher dot pitch superior to that of a monitor with a lower dot pitch. There is no substitute for actually looking at images and text on the monitors you're considering purchasing.

This chapter is from Upgrading and Repairing PCs, 16th edition,by Scott Mueller. (Que Books, 2004, ISBN: 0789731738).  Check it out at your favorite bookstore today. Buy this book now!