izumi@mindseye.berkeley.edu (Izumi Ohzawa) (04/11/91)
So, here's why you need a Color monitor which can sync to horizontal scan rate of 62-64 kHz for Color NeXTstation and NeXTdimension board. In /usr/include/nextdev/video.h, the following numbers are defined. /* 16 bit color display parameters */ #define C16_VIDEO_W 1120 /* 1120 x 832 mode */ #define C16_VIDEO_MW 1152 #define C16_VIDEO_H 832 #define C16_VIDEO_MH 910 These also apply to Monochrome MegaPixel, and NeXTdimension. As you see, 1120 x 832 are the number of VISIBLE pixels which are specified in NeXT docs and brochures. Because any CRT monitors need some finite time to move a scan beam to starting position (retrace period), you must include the time for invisible pixels for each horizontal scan line, and several tens of additional scan lines for each frame. After these overhead, "nominal" pixel resolution is 1152 pixels x 910 lines. You see these in the second set of numbers above. So, how many "nominal" scan lines do monitors have to display per second? This is the horizontal scan rate and is given by, 910 [lines/frame] x 68 [frames/sec] = 61880 [lines/sec]. So, all NeXT configurattions (Monochrome, Color station, NeXTdimension) require a monitor which goes at least to 62 kHz horizontal rate. The bottom line from this is that, most multi-sync monitors sold for use with Super VGA for PC clones can't make it that high. There are some which might work. That is why I keep writing here to check the horizontal scan rate specification, if you really insist on finding cheaper monitors. [Frankly, I don't think it's worth it. Monitors that can do 64kHz will cost quite a bit anyway although it should come down in price like everything else when people realize that GUI on less than a Megapixel display is a joke.] Now, we have 1/61880 = 16.16 micro sec to pump out 1152 pixels, of which 1120 are visible. So, as you see, horizontal pixel resolution is really dependent on how fast the video circuit pumps out pixels onto a scan line. It is not really a fixed characteristic of a monitor, but rather is determined by the pixel clock rate of the video circuitry. You can stuff more pixels if you want to onto a scan line, within the video amp bandwidth limitation. Of course, spead of the beam will smear neighboring pixels before then, probably. I just wish that the computer ads would stop using the pixel resolution as if it were a valid spec. It makes choosing alternate monitors quite difficult for most people. [Maybe that what the manufactures want.] You have a tremendous freedom in setting these numbers for a particular monitor. The numbers do matter to determine necessary phospher triad pitch, and affect the aspect ratio of a pixel (square pixel is preferrable.), but otherwise they are pretty bogus as numbers to put into spec sheets. Just for info, pixel clock rate of the NeXT video is given by, 1152 [pixels/line] x 910 [lines/frame] x 68 [frames/sec] = 71,285,760. That's about 71.3 MHz. End of VideoDisplay-101. Izumi Ohzawa [ $@Bg_78^=;(J ] USMail: University of California, 360 Minor Hall, Berkeley, CA 94720 Telephone: (415) 642-6440 Fax: (415) 642-3323 Internet: izumi@violet.berkeley.edu NeXTmail: izumi@pinoko.berkeley.edu