Greyscale and Colour monitor spanning mystery

[beachycove]

This is a new one for me.

I have an Apple full page display (640x870 greyscale) that I hooked up today to two PPC towers, in combination with a variety of colour screens. All the latter had an irritating vertical line slowly going down the page, on some of which the distortion was outrageous. I then tried the full page display in combination with a Radius greyscale two page display. No problems at all doing that.

I had used the Radius greyscale with a colour monitor before on an 840av using the on-board video and a Radius Nubus card matched with the Radius monitor, without problems, so I basically expected the same behaviour on the PPC towers, which one would think should be technologically up to the task.

The machines used were a beige G3 and an 8600, both of which exhibited the same behaviour with a variety of working PCI video cards. It made no difference which monitor was hooked up to the PCI card, which monitor was 1 and 2, or which one had the menu bar.

Is there an explanation? I take it that the problem is that the greyscale is interfering with the timing of the colour monitor, but why should it do so?

 

[Trash80toHP_Mini]

The Apple Portrait Display is a known source of OUTRAGEOUS amounts of RFI, which usually show up as vertically rolling bands on any (Color?) Monitor within cat swinging distance. I had to put a grounded piece of Galvanized Sheet Metal between my (Radius OEM'd) APD and my Radius PrecisionView 2150 to keep the rolling bands at bay.

The towers are not at fault, it's the combination of monitors that'll drive you up the wall. The old Radius Two Page Display (Grayscale) is from the same era as the APD/Radius FPD and so is probably shielded adequately from its skinnier Siblings.

 

[trag]

Were the two displays close to one another? I've seen that phenomenom with dual CRT monitors when they were within a foot of each other. The things produce internal magnetic fields to direct the electron streams. They can interfere with each other if they're near to one another.

 

[Trash80toHP_Mini]

My APD was within 2-3" of my MultiScan PrecisionView 2150 when it drove me to the construction of a Faraday Fence. ATM, I've got a Viewsonic 21" MultiScan to the right and a MAG 21" MultiScan to the left of the venerable Radius PrecisionView 2150. There's not an inch between them and they show no signs of interference at all.

Getting all the cards and drivers set up on one CPU to drive all three in unison is what's driving me up the wall, ATM! 8-o

 

[beachycove]

Ahh, now that would indeed explain it. The Radius greyscale that worked "perfectly" was way across the room - too heavy at 70lbs or so to lift across for the sake of a brief test, so I just plugged in an extension cable.

It has been a long time since I read about RFI and CRT monitors in the old setup manuals. This is the first time I have actually encountered it.

I will experiment some more with screens and distances, but am cramped for space because of all the other hardware lying about.

 

[Bunsen]

I had to put a grounded piece of Galvanized Sheet Metal between

To what did you ground it? Case metal on the Mac?

 

[trag]

I will experiment some more with screens and distances, but am cramped for space because of all the other hardware lying about.

One of the ironies of the move to LCD panels. Now we're less cramped for space, but we don't need it any more because they don't interfere with each other.

Trash80, my first post was actually in response to Beachycove, not to your message. I mention it only because you replied as if my question was directed at you, which is not a problem; I just thought I'd clarify. If one looks at yours and my first posting time in this thread, it is apparent we were each composing our replies to Beachycove at the same time.

 

[Trash80toHP_Mini]

Just for clarification, I was elaborating on the description of my problematic "noisy" setup, in general, for information's sake, in light of your question.

. . . now to answer Comrade B!

It was grounded to the metal framework of the wheeled desk, which was grounded to the "clean ground line" of the 15 amp Power Circuit in the, overhead, umbilical carrying power IN, Mac Serial IN & OUT, 10bt OUT, 3 POTs lines IN, one of which connected to the Desk mounted DSL Modem which was directly connected to the 10bT Router.

Sorry, you can't see the FaradayFence, the 21" Monitor's Shade blocks it from view, but the umbilical is the gray ShopVac hose hanging straight down from the Utilities/Lighting Track above. If I have time I'll look for a better/more recent/different angle pic that shows it.

 

[beachycove]

Clearing the desk and putting about 18-20" of space between the monitors fixes the problem. Who'd have thunk it?

Having the two screens 20" apart is more like monitor gapping than spanning, however, so I will need to live with the setup for a while and see if I adjust. If not, then perhaps I will experiment with an RF shield of some sort. Does a shield require grounding and would something as simple as sheet copper or heavy aluminum foil do the trick?

 

[Trash80toHP_Mini]

You can use anything conductive, grounded material, AFAIK, a metal mesh screen works as a Faraday Cage covering.

http://en.wikipedia.org/wiki/Faraday_cage

or, for the more paranoid:

http://www.ehow.com/how_6618709_build-faraday-cage.html

8-o WWIII ! ) . . . just half-kidding. :-/