mikela@tekig3.LEN.TEK.COM (Mike Larson) (08/18/89)
[] I recently connected my A1000 to an Intec multiscan moniter (this moniter is also marketed under the brand name Samsung) using an appropriate cable (i.e. analog R,G,B, HSYNC, VSYNC, and 4 GND lines.) The Intec is a VGA-compatible moniter that accepts analog or TTL-level input. This multiscan performed flawlessly when the Amiga was displaying a non-interlaced screen. An interlaced screen, however, showed an incredible amount of flicker, much worse than my 1080. Upon closer inspection of the CRT I realized that the odd and even frames were being SUPERIMPOSED on each other rather than being displaced by half of a scan line. Question: do any multiscan moniters correctly handle interlace, or is it just this particular brand that fails? Or am I doing something else wrong? The reason I'd like to get a multiscan is that I have to let go of my 1080, and I want a moniter that will work with my A1000 now and with any future Amiga I get that may have a higher-scanrate output (like a flicker-fixer or the enhanced chipset.) Thanks, Mike Larson mikela@tekig3.LEN.TEK.COM
cmcmanis%pepper@Sun.COM (Chuck McManis) (08/19/89)
In article <4321@tekig4.LEN.TEK.COM> (Mike Larson) writes: > This multiscan performed flawlessly when the Amiga was displaying a > non-interlaced screen. An interlaced screen, however, showed an > incredible amount of flicker, much worse than my 1080. Upon closer > inspection of the CRT I realized that the odd and even frames were > being SUPERIMPOSED on each other rather than being displaced by half > of a scan line. >Question: do any multiscan moniters correctly handle interlace, or is it >just this particular brand that fails? Or am I doing something else wrong? And the answer is "not officially." To understand why this is you have to understand a bit about how interlacing is accomplished and how the multiscan feature is accomplished. First interlace : The video beam on a CRT travels to the right horizontally, and when a "line" time has passed or HSYNC occurs, it moves to the left and down by one line thickness. The "line" time is the monitors "scan rate." And can be calculated by multiplying the number of lines on the screen by the screen refresh rate. For NTSC that is 262.5 * 60 = 15750 Hz. If one of the first HSYNCs near the top of frame is half as long as normal. Then the beam will only be halfway across the screen and when it returns to the left, because it only goes half as far, it only goes "down" a half a line thickness. And since subsequent HSYNCs occur at the right time (a full line width) all subsequent lines are display "offset" by half a line thickness, and that makes them appear between the lines that are currently on the screen. And now MultiSyncs : Before the NEC multisync, monitors were "hard coded" with the scan rate, and computer hardware matched themselves to the appropriate monitor. However, NEC noticed that many programmable CRT controllers were appearing on the market, and these controllers could change their video parameters under program control. The main reason for doing this was to change the resolution of the image they were displaying. And people didn't want to buy a monitor for hi-res mono text, and one for lo-res multicolor text. So they invented the original MultiSync. The magic was that this monitor could change it's internal scan rate *on the fly* and adapt without buttons or switches to the kind of video. But how could it know what frequency to set itself to? Well, since the time between 2 HSYNCS is defined by the scan rate frequency, the tube simply monitored that time and calculated what frequency it should be at. (This sounds tough, but in fact a simple Phase Locked Loop circuit allowed it to identify the proper frequency fairly quickly.) So what happens when you send it a short HSYNC so that the beam will move halfway down ? The monitor detects this as a change in the scan rate, but notices that it only happened for one HSYNC and so writes it off as a glitch. In the mean time *it* keeps that video beam right in sync. Poof, overlayed video not interlaced video. So the moral of the story is, either buy a multiscan monitor that says "Supports Interlaced video" or try it out on a machine at the store before you buy it to make sure it doesn't "help you out" by keeping all of those video lines, lined right up for you. --Chuck McManis uucp: {anywhere}!sun!cmcmanis BIX: cmcmanis ARPAnet: cmcmanis@sun.com These opinions are my own and no one elses, but you knew that didn't you. "A most excellent barbarian ... Genghis Kahn!"