info-mac@uw-beaver.UUCP (10/06/84)
From: Kevin.Dowling@CMU-RI-ROVER The Macintosh retrace rate is 22.5+ KHz, a fair amount more than standard NTSC (15.75KHz) This means that an external monitor or projection system must be cranked up to accept this higher frequency. The more obvious problem is getting the video information out of the Mac. Apple has a simple circuit to get the video out on a BNC connector for easy access to the signal. Drexel and Utah have very similar schemes to do the same thing. Electrohome Projector EDP-57 requires no retrofit to show the Mac retrace. It's just a matter of tweaking an adjustment in the projector for horizontal retrace until the picture snaps in. This probably the simplest way to go for projecting the Mac screen for a large audience. The projector is about $5K. At CMU we have a large room with 45+ Macs arranged in nice way around a center area where the Electrohome is placed in front of a screen. The Macs are at connected tables on stepped platforms in a room built for exactly this purpose. A large booth to one side is where a user consultant resides. Imagewriters are at about 10 of the stations. Electrohome, last time I called, was finishing testing a Mac interface that they're ready to market (about $200). The interface doesn't need anything inside the Mac, they take the TTL level signal and convert it to composite video. It can go in the Monitor if desired. Electrohome makes a monitor that they will retrofit. It's a 23" monitor that will run you about $800. This needs a stock retrofit. Electrohome is in Kitchener, Otario at (519) 744-7111 I talked with Paul Miller, a very helpful engineer there. The monitor is available in green or white phosphor. I have literature from Professional Data Systems. They make an interface for the Electrohome's also. But for the custom interface, monitor (Electrohome 23") and cable they want $1800. Too much, Similarly they offer the EDP-57 system for $7000. You have to ship them a Mac too. Professional Data Systems: 6416 Friars Road, Suite 117 San Diego CA 92108 (619) 291-2300 Conrac makes a studio quality monitor (23" B/W Model SNA-23/C) but this will set you back $4000. The monitor, can be tweaked easily to accept a wide range of signals. Other projection systems like the GE light-valve will also work, but again, this is beyond the realm of reasonable money. There is another possible technique called scan-conversion so you could use "normal" NTSC RS-170 monitors and the like. Broadcasters do this so they can show American TV all over the world. Scan-converters are very expensive, though, $50K or more. I would expect sometime in the future with some dual-ported memories going down in price that you could do this for a lot less. I'm curious about the pointing the camera at the screen technique that some are using. Don't you get an annoying "roll" across the image? It would probably be pretty slow but annoying nonetheless.