Classic Computer Magazine Archive COMPUTE! ISSUE 42 / NOVEMBER 1983 / PAGE 191

How To Improve The TV Quality Of The Commodore 64

Jim Butterfield, Associate Editor

If you own a 1701 Video Monitor, you may not be getting maximum video quality. Here's an alternative hookup that produces a remarkable picture. Even if you don't use a 1701, you can still use some of these ideas to improve your computer's picture quality on a TV or monitor.

The Commodore 1701 Video Monitor is an attractive 13-inch color monitor with good color definition and good sound. But most Commodore 64 users miss a bet: they hook it up via the front connections.

There seems to be a rumor that you can't use the connections at the back unless you have an 8-pin video connector. Not true – you can get a magnificent picture from the traditional 5-pin interface.

Inside A Monitor

There are two parts to a video signal: the brightness and the color. Most monitors mix them together to produce a "composite" video signal. Inside a monitor – or television set – the two signals must be split apart once again before they can be used.

The color (or chrominance) signal is carefully designed so that it can be mixed in with the brightness (or luminance) and later separated. The system isn't perfect, however, and there's always a trace of the color signal left in the screen brightness.

Traces of the chrominance signal left in the brightness can cause viewing trouble. Depending on the foreground and background colors, a finely checkered pattern can appear on the screen. To make matters worse, this pattern interferes with the normal pixel resolution of the screen, and every second character on the screen will look smeared.

For some colors, this isn't a problem. Other color combinations look bad. But the whole problem can be solved by not mixing chrominance and luminance; instead, deliver them on separate wires to the monitor.

By the way, there's another method used to deliver signals to video monitors. It's called RGB, for Red/Green/Blue; it uses three signal wires, one for each color. However, this method is not available for use with the Commodore 64.

Hooking It Up

There are two different video signals available on the 5-pin DIN connector on the 64. The signal on pin 4 is called Video Out: it's a composite video signal containing both luminance and chrominance. On pin 1, we'll find the luminance signal: a sharp, black-and-white signal with no color component. If you connect pin 1 to the 1701's luminance connection, and pin 4 to the nearby chrominance connection (they are both on the back of the monitor), you'll get a picture of marvelous quality.

I'm amazed to find that the necessary cable doesn't come in the box with the monitor. The connector that comes with the monitor is wired for the front connection. This is fine for both the VIC and the 64, but the 64 can do far better on the rear connection. At the present time, the cable isn't provided; you'll have to wire one up or buy one.

If you'd rather not wire your own cables, you can buy a general-purpose "octopus" connector that brings out all five pins to differently colored plugs. The color codes don't seem to be universally consistent, but on the ones I tried, the most common arrangement seemed to be: red for luminance, white for chrominance (video out), and black for audio. Remember – throw the little switch at the back of the monitor to energize the back connections.

How is it that we can use a composite video signal as chrominance? Because the chrominance connection throws out any luminance that may be mixed into the signal.


Let's talk for a moment about why the picture is so sharp on a properly hooked-up monitor. It has to do with two aspects of television standards.

First, the color signal is "modulated," or coded, using a high frequency signal at slightly over 3.58 megacycles per second. That's a TV standard: it was designed long ago so that we could decode the color signal and separate it from the brightness. If we didn't take out the color signal (and we can never remove it completely), we would get a pattern of fine dots on the screen. These dots would not be too noticeable on a conventional television picture, but would interfere with our perception of computer characters.

Second, television color has been carefully designed to be less sharp than the black-and-white part of the picture. It turns out we can't detect color sharpness as accurately as black-and-white; so the television engineers deliberately take out the sharp color edges to allow them to design the television signal more efficiently. The technical term for this, by the way, is lower bandwidth.

So the sharpness is always in the black-and-white, or luminance, part of the signal. And the chrominance signal is not only less sharp, but also contains an extra frequency that will degrade the picture. No wonder we would prefer not to mix them.

The strange interrelationship of sharpness and color leads to another odd thing. If you ever draw high-resolution pictures on the 64, you are advised to make lines at least two pixels wide. Why? Because extremely thin, sharp lines get partly mixed into the color signal, and you'll get a slight but annoying "color smear" on these lines. But it won't happen on a rear-connected 1701 monitor.

If You Don't Have A 1701 …

Even if you don't have the 1701 monitor, you can make use of the information on how the video signal works.

If you have a conventional color monitor, or just a color TV set, you can try for a sharper picture. The objective here is to put more luminance into the video signal. We do this by making a connection between pin 1 and pin 4 on the video connector of the 64. If you have a monitor, you can connect the two pins within the cable. If you have a TV set, you must make up a video plug with the two pins strapped together; even though the signal doesn't go out through this connector, the balance between luminance and chrominance will change. In either case, you'll need to readjust the color controls to get a satisfactory picture; and you might not even like the results. If you'd rather not do your own cabling or soldering, have your local computer or TV service store do the job for you.

You can also make a significant improvement on a black-and-white monitor, which you might use for such things as word processing or financial calculations where color doesn't matter. Now that you know about pin 1, which contains the luminance signal only, you can use it for a crisp black-and-white picture.