Improving Cheap & Nasty Monitor Switchers

article from Dave's Amiga Hardware Page

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Due to the increasing numbers of Amigas with graphics cards, many Amiga users are finding a need to switch between their existing ECS/AGA Amiga output and their graphics card output. While some cards, such as the earlier CV64 and CV643D have inbuilt switchers, most other cards do not. Some users have got around the problem by using a more expensive monitor with two inputs and using the switch provided on the front panel. Other users without this facility have had to get a monitor switchbox, commonly used in PC systems. Unfortunately these cheaply made boxes degrade the video signal - especially at the higher scan-rates and resolutions, causing a blurred image or "ringing" in light to dark transitions in the picture - or both.

The problem is caused by inductance and capacitance added to the video signal lines, an effect not unlike that introduced by the interference suppression components added to all display cards these days. In the case of the switchbox, it's quite random, and usually much larger, causing visible problems. Because the inductance in the switchbox interacts with the capacitance in the cable leading from the switchbox to the monitor, the first thing to do is to reduce capacitance as much as possible in this cable. This is done by using low capacitance cable (sometimes called "low loss") Get the best you can find. Don't skimp on it. Next you will have to make it as short as possible. This will either mean making up your own cable from scratch (using high quality co-ax for the R, G, B, signals) or using a high quality commercially made cable and cutting one end off and fitting a new connector on the cut off end. Make sure the cable is no longer than absolutely necessary. Remember to note which wire in the cable goes to which pin. In any case it is a "straight through" cable, ie, pin 1 on one end goes to pin 1 on the other, with pins 1, 2 and 3 being the cores of low capacitance cables and 6, 7 and 8 being the shields of 1, 2 and 3 respectively. The other wires on the other pins are of ordinary wire. You should note a major improvement with this new cable. If there are still problems, you will need to modify the switchbox as outlined below.

Most monitor switchboxes are nothing more than a metal box with a 15-pole switch inside and appropriate wiring to hook it all up. With the lid off, the wiring looks like a birds nest, and is always longer than necessary. So the first thing to do is to identify the wires running from pins 1, 2 and 3 on each of the VGA connectors into the switch. These need to be made as short as possible. Unsolder one end of each wire at a time, cut it short and solder it back. If you wish you can do it to all 15 wires on each connector, however, it's only pins 1, 2 and 3 which are important.

The next thing to do is to fix up the ground return wiring in the switchbox. Usually it is also run through the switch through similar long wires. The ground wires actually don't need to be switched, so they can be run directly from the input connectors to the output connector. They also do not need to be isolated from each other. Therefore, I disconnect all wiring from pins 6, 7 and 8 and connect them all directly together on all connectors. Use the shortest, thickest wire you can easily use as well. The idea is to reduce capacitance, inductance and resistance in the ground return as much as possible. Be careful and avoid shorting the pins on the connectors as they are rather close to one another.

I have performed these mods on a 4-way box on my system, and now there is no visible degrading of the display - even at the highest scan-rates. Note that I did not find it necessary to specially shorten the leads from the various display cards going into the switchbox, however, I do strongly recommend using low loss cable here also.

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First Published: February 5th 1999

Dave's Amiga Hardware Page