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Undercolor/850104/Notes on RFI

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UnderColor, Volume 1, Number 4, February 1, 1985

  • Title: Notes on RFI
  • Author: Martin Goodman, M.D.
  • Synopsis: What and how.
  • Page Scans: Link


The Color Computer arrives from Tandy with only one way of putting its video signal on a screen television set. This is known as an RF (radio frequency) interface. This has certain advantages, as most folks own color TV sets. Unfortunately, this link to the screen has several problems.

By virtue of the fact the link is via a radio signal it is vulnerable to RFI (Radio Frequency Interference). RFI commonly appears as moving wavy lines on the screen. Such lines often look like rapidly changing moiré patterns. A second problem arises from using a color image. Color video information of the sort that the CoCo uses is intrinsically "mushy" and blurry. Thus, if one is trying to use a hi-res text screen (51 or greater characters per line) the image is often marginally readable at best.

The first problem (that of RFI) is solved by using a color monitor rather than a TV to display the CoCo's signal. As the CoCo itself is not set up for monitors, a hardware adapter must be installed. Tandy Corp. does not provide such an adaptor [adapter]. (It does make an "educational" CoCo without the TV coupler but with a color composite video output.) There are many third-party suppliers that supply adapters for existing CoCos. If you own a new model (CoCo 2) and wish to display a color image on a color monitor, Computerware and Mark Data products both supply devices that will do the job. Mark Data’s product offers the option of producing a monochrome signal as well. If you own an older CoCo, Moreton Bay Software has a product that provides both a color and monochrome signal from it. Such devices cost between $25 and $40. Color monitors cost between $200 and $300. For use with the CoCo, virtually all brands of color monitor are equally suitable. Base your choice on price, styling, and (if you can arrange to see it) how the CoCo's signal looks on the screen.

Pay absolutely no attention whatsoever to electronic specifications such as bandwidth in megahertz and horizontal dots resolution. Such nonsense is utterly irrelevant to your choice. Be sure to specify a "color composite monitor," not an RGB type monitor.

In all cases you'll need to feed a separate audio signal into a separate audio input on your monitor. All video adapters

mentioned above provide such a signal. While using a color composite video monitor will eliminate all RFI, you will probably be as plagued as ever by hi-res text screens looking

blurry. To solve this problem you need to get a monochrome signal and feed it to a monochrome monitor. Computerware, Mark Data Products and Moreton Bay all supply devices

capable of producing a monochrome signal from a Color Computer. I personally designed the product from Moreton Bay and tests have indicated that it provides the cleanest

monochrome signal of all such devices. It is probably the most-used brand by those on the Color SIG.

Monochrome monitors cost between $75 and $150. They come in green, amber, and black and white screen varieties. Green is said to be most likely to produce eye strain. Amber or black and white are felt to be superior for prolonged viewing. A 12-inch diagonal monitor is the typical size used for a personal system. Once again, technical specs are irrelevant.

When you put up a CoCo monochrome signal on a monochrome monitor, text will be extraordinarily crisp and sharp, even in 85-column mode. I personally feel that it is impossible to use the CoCo for word processing without a monochrome monitor. Unfortunately, there is no way to improve on the tendency of adjacent colors to blur into one another

using a color video monitor. This is intrinsic in the color composite video process. RGB video would solve this, but no

RGB video adapters are or ever will be commercially made for the CoCo. Indeed, I know of only one person who ever made a proper RGB video adapter for the CoCo, and it was very complex.

The system I recommend for most is a monochrome monitor for use with text, relying on the RF signal for use with color. This is cost effective.

Most monochrome drivers for the Color Computer will let you use the existing RF signal. Computerware's driver for the old CoCo ("Video Plus") is, however, an exception. It

completely destroys the RF signal when you install it. Moreton Bay‘s (my) driver for the old CoCo occasionally causes this problem too, but on that driver it can be easily fixed by disconnecting a single wire. (end)