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Video and data projection

Why won't our iMacs work with our LCD projectors?

Apple iMac DV

There's nothing wrong with your projectors, but unfortunately, that doesn't mean they'll work properly with an iMac.

In designing the iMac DV (that is, the model with an RGB output, introduced in 1999), Apple decided to depart from the usual practice of computer manufacturers. Most Mac and PC models use a true multisync monitor and allow you to display multiple resolution signals at a variety of refresh rates (i.e. 800 x 600, 1024 x 768, etc. at about 70 - 80 Hz. (Note that the refresh rate is also known as the scan rate vertical, and that's how you'll find it specified in our projector catalog.) Thus there is no problem sending a signal to a large-screen monitor, LCD or DLP projector from most computers.

With the iMac, however, Apple allowed only three display choices:

640 x 480 (VGA) resolution at 117 Hz refresh rate.
800 x 600 (SVGA) resolution at 95 Hz refresh rate.
1024 x 768 (XGA) resolution at 75 Hz refresh rate.

This caught a lot of manufacturers by surprise. Very few monitors or projectors will sync up at 117 Hz, and the majority will not sync at 95 Hz. (Almost any projector will sync up at 75 Hz.)

There are at least three solutions to the problem.

1. Go to your iMac "Monitors" control panel and switch to 1024 x 768 at 75 Hz.

That seems simple enough, but there's a hitch—hopefully minor. Assuming that you, like most educators, are using native 800 x 600 projectors and assuming your projectors can show 1024 x 768 (a few cannot), your 1024 x 768 image will be "compressed" or "resized" to 800 x 600. The quality of the final image will depend on the quality of the compression or resizing algorithm built into your projector, but it will be at least somewhat compromised.

2. Pick up a piece of shareware called "SwitchRes" or "SwitchResX." This simple software allows the iMac DV to output a signal at a lower refresh rate—low enough to allow almost any monitor or projector to work. SwitchRes works well, but there's one hitch: since the iMac's built-in monitor will not work with the lower refresh rates, you'll essentially turn off your built-in monitor when you "turn on" your projector using SwitchRes.

You can download the program from www.tucows.com (Search for "SwitchRes"). You can also go the the vendor site at www.madrau.com. If all else fails just search the web for "Switchres" with your favorite search engine. This is a shareware program. The cost, if you keep it is $15.00.

3. If you're planning to buy new projectors, be careful that your new projectors will handle a 95 Hz vertical scan rate. (Check with your dealer for recommendations, or check the "scan rate, vertical" listings in our on-line catalog.)

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