A THG Primer: CRT Guide

Introduction

The most expensive part of any computer system is almost always the monitor, yet most people don't understand how their monitors work or how to shop for one. They tend to rely on a friend's advice, past experience, reviews, a salesperson's recommendations, price, or they just settle for the monitor bundled with their system. I've seen people invest hundreds of dollars on a state-of-the-art graphics board, and then connect a cheap and cheesy monitor to their system. Perhaps even more shameful are people who already own a good monitor, but don't bother to keep it properly calibrated (or have never calibrated it at all). In this article, I'll talk about the various kinds of monitors that are available and focus primarily on CRT (Cathode Ray Tube) monitors. I'll give some tips on shopping for a new monitor and how to keep it properly calibrated.

We've already covered LCD displays in the following articles so, I thought it was worth giving the good ol' CRT a run this time around:

• TFT Guide - Part 1 - Flat Panel Displays
• TFT Guide - Part 2 - Viewing Angle Technologies
• TFT Guide Part 3 - Digital Interfaces

CRT Basics

CRT displays, in the form of TV sets, have been around for about 60 years and, for the most part, their inner workings haven't changed much. The basic idea is that an electron gun at the back of the picture tube fires a beam of electrons toward the inside front of the tube, which is coated with a layer of phosphorus material. The electron beam passes through a series of strong magnets that bend its path to strike different parts of the front of the tube. When the electron beam reaches the front glass, it strikes the phosphorous coating on the inside surface, causing that spot to glow temporarily. Each spot represents a single pixel (picture element). By carefully controlling the voltage of the electron beam, the individual spots can be made to glow brighter or dimmer. Originally, black and white TV picture tubes had one electron gun and a uniform coating of phosphors. Later, multiple guns were used and the phosphors were painted in discrete dots.

To create an image, the beam sweeps across a single horizontal line (scan line) from left to right, lighting up phosphor dots and causing them to glow brighter or dimmer, depending on the voltage. The speed that a monitor draws a single scan line, called the "horizontal frequency," is measured in kilohertz (kHz). When the beam reaches the end of the line, it is turned off momentarily (called the "horizontal blanking interval"), the magnets reset, and the next line down is then painted. The process repeats, painting line after line, until the screen is filled. At that point, the beam is turned off again (called the "vertical blanking interval"), the magnets reset, and the whole process starts again at the top left of the screen. The speed that a monitor draws an entire screen, called the "vertical refresh rate" or "frequency," is measured in hertz (Hz).

In the early days of television, the engineers who designed and built TV picture tubes faced technical problems that caused them to make some compromises. First, the quality of the phosphors available in the early days was not that great, and the dots would start to fade out before the entire screen could be painted. To get around this, televisions use an interlacing technique where the screen is painted in two passes, every other line at a time. On the first pass, only the odd scan lines (1, 3, 5, ...) are painted. Then, the beam resets to the top, and the even scan lines (2, 4, 6, ...) are painted. Each pass is called a "field," and two fields combined are called a frame. In NTSC there are 60 fields (30 frames) painted per second, and in PAL TV systems there are 50 fields (25 frames) painted every second (movie film runs at 24fps, by the way). Any slower than that, and most people will begin to notice image flickering.

By the time computers came along, both the quality of the phosphors and that of the electronics had improved to the point where interlacing wasn't necessary, but since most computer work involves lots of text, resolutions had to be increased. Where a typical TV set runs at about 13.5kHz horizontal refresh at a 25 to 30Hz vertical refresh rate, most computer monitors are capable of drawing a screen at over 60kHz horizontal refresh at an 85Hz vertical refresh rate. While no two people are alike, it is probably best to try and run a CRT monitor at 85Hz or better, in order to reduce eyestrain caused by flicker (even if you can't actually see the flicker at lower frequencies).