TI-99 display interference

Question

I had occasion to touch a TI-99 only once, circa 1985. The machine in question was part of a lineup of 8-bit machines that also included a late-model Atari and another, perhaps a Coco but I can't recall.

One thing I do recall from this experience is that the TI's display was literally filled with interference, swimming about in a fashion that forced me to turn it off to avoid barfing on the keyboard. This isn't entirely unexpected I suppose, there were several televisions right beside each other. But the Atari display was fine, as was the 3rd machine.

I'm reading about the history of TI's battles with the FCC over interference and their eventual decision to ship a Zenith TV with their machines to avoid using an RF modulator. This decision was later rescinded in 1980.

So... was the display on the TI machines always like this? If so, I can understand the FCC's concerns!

Answer 1

CRT screens used varying magnetic fields (from wire coils carrying a sawtooth-wave electrical signal) to deflect an electron beam and scan it across the screen. These fields extended outside the TV to some extent.

If you had two TVs next to each other, being driven by signals that were at slightly different frequencies, the magnetic field from one TV might distort the picture on another.

This wasn't ordinarily a problem for broadcast signals, which had to meet rigorous timing standards. The signal from a PC wasn't so rigorously controlled. If (say) the program on one TV was refreshing at the 60Hz standard, but the signal from the TI-99 on the other TV was at 60.2Hz, you'd likely see the TV image "swimming" with a period of 5 seconds.

Depending on the individual TVs involved, you might see this distortion on both, or only the one connected to the TI-99, or only the one not connected to the TI-99, or neither. It would have changed if you changed the position or angle of either TV.

I don't know if this is what you saw, but I've certainly seen it.

Answer 2

Unless you had dozens of different types of TV's with a controlled test, it's hard to really quantify how good that RF modulator was (or how bad). Also, you said you don't remember what the third computer was. Are you sure you remember it having a better display?

When I was a small child, I had a TI-99/4A and a Commodore 64C. Both of them were RF only. Maybe I was lucky enough to have a good CRT or maybe I didn't know what composite was to compare it to. But I don't recall any one being better than the other.

I now own many computers in my collection and I have a few RF modulators from TI. On one of my CRT's (some generic no-name), even RF looks good. In fact, my NES top-loader looks fantastic on that TV using RF.

So it's quite possible that you just had a CRT that didn't play well with that RF modulator.

Also, the Colecovision used the same VDP (TMS9918 in the US). I had that as a child (with the cheap RF adapter it came with) and I still don't remember it being any better or worse than the TI or Commodore.

Finally, I have an Atari 400/800/600XL/800XL/1200XL/65XE/130XE/XEGS in my collection. Just about every 8-bit Atari there is. And every one of them look like garbage on a DIFFERENT CRT I have.

I put an S-Video mod on my 800XL (super easy) and my 800 already has S-Video (separate chroma/luma). When I utilize the S-Video's on those (or even composite for that matter), it's night-and-day difference. Even though I know the Atari has good video output, the RF for MY particular setup looks bad.

So it really is several factors:

1. What kind of local interference do you have? You mentioned three TV's right next to each other.

2. What kind of cabling/shielding do you have?

3. What kind of CRT/RF modulator combo do you have? Some play nicer than others.

There are SO many variables that to say all TI's had terrible RF output is simply not correct. My experiences were much different than yours using the same computer.

Keep in mind, too, that the "R" in RF is for Radio. Literally any sort of radio interference around you can cause issues and noise.

Answer 3

You seem to be describing a TV that wasn't fine-tuned to the Channel 3 or Channel 4 VHF signal coming from the TI-99/4A. Any computer or game using the RF modulator style of connection could suffer some kind of frequency drift. And cabling issues could often hamper reception due to impedance mismatching (visible as noise), standing waves (visible as multiple images), and cross-talk or interference (visible as moire, multiple signals, or snow).

My (very late) diagnosis is to check the RF modulator, wiggle the wires, check connections, replace the wires, and move other cables away from the TI. Additionally, I would verify that the connectors to and on the TI are solid, with attention paid to the game card slot and any peripherals attached to the main system, all of which would suffer from misalignment and fatigue. Daisy chained peripherals (like TI permitted) is a disaster.

And as others have noted, the TI had a sharp screen. My family always used an old TV rather than the "monitor" that they sold, which I do understand was generally better than most old (1970s vintage?) TVs. So our image wasn't phenomenal, but good for the time.