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Monitor Shielding and Background

The letters surrounding "Double the Fun with Multiple Monitors" and "More on Multiple Monitors" in TidBITS-421 and TidBITS-422 continue to stream in. Several have offered explanations of why two monitors might interfere with one another, plus solutions for interference problems.


First off, a few quick notes. Don Kleinschnitz <[email protected]> wondered how you would recover windows on a second monitor if that monitor were to go bad. The Mac should recognize if the second monitor is disconnected and move windows and icons to the remaining monitor. If it doesn’t, zapping the PRAM by holding down Command-Option-P-R at startup should help. Ill-behaved applications may still refuse to show their windows, and for that the best solution may be to record necessary settings, then move that application’s settings files to the desktop, forcing the application to build fresh ones.

Marc Schmitt <[email protected]> noted that Village Tronic markets the MacPicasso 523 video card, a 2 MB PCI-based video card with a VGA connector for about $100 in the U.S. [A predecessor to this card was a user favorite in last year’s TidBITS Holiday Gifts issue; the U.S. distributor is Software Hut (see "Rare MacPicasso Unearthed" in TidBITS-409). -Tonya]



Steven Kan <[email protected]> passed on the URL to his experiences with multiple monitors, which show graphically some of the concepts I’ve written about in the previous articles.


Finally, Joe Mithiran <[email protected]> mentioned another virtual desktop utility for the Mac, called Virtual Desktop, that’s part of Ross Brown’s AWOL Utilities. It’s been updated recently for Mac OS 8 and is worth a look if you want to simulate a large screen.

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Victor Guess <[email protected]>, who worked for a number of years as an electrical design engineer in the television industry, explained the problem with monitor interference:

The interference between monitors is caused by low frequency magnetic fields generated by the yoke coils that surround the neck of the tube. These scan the electron beam horizontally and vertically across the phosphor-coated screen. These fields are best shielded by iron or steel and the more of it the better. Try putting a thick piece of steel between the monitors. Certain steels are specially designed for magnetic shielding, and they require considerably less steel than other alloys. (Stainless steel is about the worst!) Most monitors have some built-in shielding and some newer ones have a lot more. Internal shielding is far more effective than external. Obviously external shielding isn’t too practical and it’s best to purchase well-shielded monitors.

Magnetic shielding affects both the radiated magnetic fields (a feature often mentioned in the monitor specifications) and also the susceptibility of the monitor to magnetic fields (a problem which is rarely, if ever, mentioned by monitor manufacturers). For instance, the image on my Apple 1705 wiggles a bit whenever the ceiling cable heat is active in the room downstairs.

Greg Staten <[email protected]> passes on advice for shielding monitors:

The best product for magnetic shielding is known as Mumetal. It’s what we recommend at Avid for those having interference problems between their computer monitors (shielded) and their NTSC monitors (generally unshielded). Call Magnetic Shield Corporation at 708/766-7800 and ask for their magnetic shielding catalog. They also sell a lab kit with Mumetal and steel sheets/foils for $129 that includes a magnetic field probe (only $79 without the probe).


[Though Greg had never heard of them, I found two other companies that also appear to sell magnetic shielding products: AD-Vance Magnetics and MuShield. Victor Guess also commented that bending Mumetal can hurt its effectiveness unless it’s re-annealed (heat treated) afterwards. -Adam]



Jay Nelson <[email protected]> offers a pricier solution:

I’ve been using NoRad’s JitterBox ($395 to $595) for several years, at first because my monitor was backed up against a wall at the same place where, outside, all the power for the building came in. The monitor swam constantly. When I installed NoRad’s JitterBox (which fits around the outside of the monitor), the swimming stopped.


Jon Pugh <[email protected]> passes on the historical story of multiple monitor support:

The Mac II and System 5 first supported multiple monitors (as documented in Inside Macintosh IV). Multiple monitor support was originally done with the Mac Plus, but only with special hardware and software from Radius (QuickDraw hacks written by Andy Hertzfeld). That monitor was the first full-page display – the portrait unit that was roughly the same size as the Plus itself. Apple later rolled multiple monitor support into Color QuickDraw, so only machines with Color QuickDraw support multiple monitors via the system software now, which includes the Mac II and up, but no 68000-based Macs.

Eric Baumgartner <[email protected]> comments that he hasn’t seen any research about the productivity boost from multiple monitors:

While I agree with your argument, it’s interesting to note that, as far as I can tell, there isn’t a lot of research that links increased screen real estate with productivity. A few weeks ago, a friend asked for pointers to this kind of research because a large medical center where he works is proposing a massive upgrade of its computer infrastructure. The proposal is to purchase about 3,000 new machines, which may be Dells with 14-inch monitors.

Upgrading these machines to 15-inch to 17-inch monitors will incur a significant cost. But what’s the cost of thousands of folks chugging around a smaller screen every day? A cursory search of human factors research didn’t turn up anything that addressed this issue. As someone pointed out on <comp.human-factors>, you would think that if such research existed, monitor manufacturers would be all over it.

One way to get those bigger monitors without increasing overall cost is to buy slower, low-end machines. This raises an interesting question: for your typical user (i.e. the bulk of those 3,000 users), what makes a bigger difference – 50 percent more screen space, or a 50 percent faster processor? And if it’s the former, how do we convince CFOs?

Ralph Lord <[email protected]> offers some thoughts on why multiple monitors increase productivity:

I found your recent article on using two monitors interesting and thought you might be interested in some information that explains in a more basic way why using more monitors increases productivity. It’s a simple case of information density. If you haven’t yet seen the books by Edward R. Tufte on displaying information, you must rush out and get them. In "Envisioning Information," on page 49, he writes:

"Nearly all micro/macro designs of this chapter have portrayed large quantities of data at high densities, up to thousands of bits per square centimeter and 20 million bits per page, pushing the limits of printing technology. Such quantities are thoroughly familiar, although hardly noticed: the human eye registers 150 million bits, the 35 mm slide some 25 million bits, conventional large-scale topographic maps up to 150 million bits, the color screen of a small personal computer 8 million bits."

No wonder then that you feel that more pixels is better, it is! We can obviously register more information than our screens are capable of showing and according to Tufte, a computer screen is one of the least dense information displays we deal with. One might reason that what we need is not faster processors, but screens with much higher resolution and greater size.

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Ian Blanton <[email protected]> offers consolation for those who fear Windows NT catching up too quickly:

Windows NT does support multiple monitors. The head of our PC support group and the Mail Admin (both sharp guys), tried to set up a Pentium with multiple monitors after seeing my triple monitor setup. I won’t bore you with the labours that they went through, but four hours after they started, they got it running. Unfortunately, NT treated the larger desktop as one giant monitor, with the result that status messages, etc., all popped up in the "center" of the screen, between the two monitors. They took out the video cards and gave up.

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