Setting Up X11

So how do we apply this to computer displays? Let's look at the US standard NTSC system—the international PAL and SECAM systems are almost identical except for the number of lines and a minor difference in the vertical frequency. NTSC specifies 525 lines, but that includes the vertical blanking time, and in fact only about 480 lines are visible. The aspect ratio of a normal TV is 4:3, in other words the screen is one-third wider than it is high, so if we want square pixels, (one with the same height and width, which makes graphics software much simpler) we need one-third more pixels per line. This means that we can display 640 pixels per line on 480 lines. (Do these numbers look familiar? Now you know why.)

This resolution is normally abbreviated to “640*480”. PAL and SECAM have lower vertical frequencies, which allow a nominal 625 lines, of which about 580 are displayed. Either way, these values have two huge disadvantages: first, they are interlaced displays, and second, the resolution is the minimum acceptable for modern graphics displays. Older PC display hardware, such as the CGA and some EGA modes, was capable of generating these signal frequencies, but VGAs can no longer do it. This is a pity, in some ways; I'd like to have an X display on my TV in the lounge room, but my last EGA adaptor died a couple of years ago.

The first problem with these standards is interlace. It works reasonably for TVs, but it's a pain for computer displays—there's still more flicker than with a real 50 Hz or 60 Hz display. VGAs can still be run in interlace mode, but you shouldn't even think about doing so unless you're into masochism. The second problem is the resolution; nowadays, 1024*768 is a normal enough resolution, but I'm writing this on a display with 1280*1024, and many modern boards display 1600*1200. On the other hand, even 60 Hz refresh rate is barely adequate. Read any marketing literature and you'll discover that 72 Hz is the point at which flicker suddenly disappears. To get high-resolution, high-refresh-rate displays, you need some very high internal frequencies—we'll see how high further down.

Now we know that a monitor is just a glorified TV. TVs were designed to use the same circuitry for the horizontal deflection as well as generation of the high voltage required for the CRT. This simplified the hardware design at the expense of versatility loss. The flyback transformer which generates the high voltage is part of a resonant circuit which can only operate at one frequency. Run a flyback transformer off its intended frequency and it will run much less efficiently and use more power. This additional power will be dissipated in the flyback transformer and horizontal output transistor as heat. The result can be the failure of the flyback transformer, the transistor, or both.

You don't have to roll your own X configuration to burn out the monitor. Ten years ago, the standard display boards were CGAs (Color Graphics Adapter) and HDAs (Hercules Display Adapter) and they had different horizontal deflection frequencies and thus required different monitors. If you connected an HDA (18.43 kHz horizontal deflection frequency) to a CGA monitor (15.75 kHz, the NTSC horizontal deflection frequency), you could expect smoke signals within a few minutes.

In modern PC monitors the horizontal deflection and high voltage generation circuits have been separated. Called multi-sync monitors, they are capable of running at a range of deflection frequencies. Running at any of the frequencies in the specified range will work, but it is still possible to damage the monitor by running it out of the specified range. Note that just beacuse a monitor displays the image correctly doesn't mean that it is running in spec. I have a rather elderly Eizo 9500 (called Nanao in the US) which has three frequencies: (exactly) 31.5 kHz, 48 to 50 kHz, or 64 to 78 kHz. In fact, it will display at any frequency between 48 and 78 kHz, but if it were run at 57 kHz for any length of time, I would be in for a hefty repair bill. The moral of the story: Don't ever run your monitor out of spec. If your display is screwed up, there's a good chance that the frequencies are out. Turn off the monitor.

Monitors aren't the only thing that you can burn out, of course. If you try hard, you can also burn out chips on some display boards by running them at frequencies which are out of spec. In practice, though, this doesn't happen nearly as often.

Another difference between TVs and monitors is the kind of signal they take. A real TV includes a receiver, of course, so you have an antenna connection, but modern TVs also have connections for inputs from VCRs, which are usually an audio signal and a video signal. The video signal consists of five important parts: the red signal, the green signal, the blue signal, and the horizontal and vertical sync pulses. This kind of signal is called composite video. By contrast, most modern monitors separate these signals onto separate signal lines, and older boards, such as the EGA, even use several lines per colour. Unfortunately, there is no complete agreement about how these signals should work; the polarity of the sync pulses varies from one board to the next, and some boards cheat and supply the sync pulses on the green signal line. This is mainly of historical interest, but occasionally you'll come across a real bargain 20" monitor which only has 3 signal connections, and you may not be able to get it to work—this could be one of the reasons.