franken@bbncca.ARPA (Ed Frankenberry) (08/23/84)
Can someone explain the differences between monitors designed for use as computer displays and those intended for ordinary video? I would like to find a color monitor that accepts both separate RGB inputs (such as from a computer) and conventional (NTSC composite) video. Do computer monitors generally require greater resolution? Is one monitor suitable for both? A related question: does a standard exist for RGB video signals? Although several "IBM PC compatible" monitors are now available, I seem to recall hearing that normal (analog?) monitors wouldn't accept the (digital?) signal from the PC. Are TTL levels used at the 8-pin interface? There seems to be a lot of misinformation regarding compatibility. What's the real story? Thanks
dmmartindale@watcgl.UUCP (Dave Martindale) (08/24/84)
A good RGB monitor needs more video bandwidth than an ordinary TV. Perhaps 15-20 MHz is needed to give good resolution for pictures displayed at TV rates; ordinary TV's have perhaps 5MHz video bandwidth. Any display that accepts NTSC composite video contains an NTSC to RGB decoder, since all tubes eventually need RGB signals. Including such a decoder in a monitor intended primarily for RGB use would produce a very good NTSC monitor as well. Unfortunately, most monitors I've seen with dual RGB and NTSC inputs are really TV sets with access to the video amplifiers, but without sufficient bandwidth to make a good RGB monitor. There is a standard for NTSC composite video signals - it's called RS170A, and specifies 1V peak-to-peak signal level. There is no real standard for RGB. Some sources produce a signal which looks identical to three different channels of RS170 black&white signals - video plus sync on all three channels, 1V P-P. In other cases, only the green channel carries the sync information. In yet others, the RGB signals are nothing but luminance information and a 4th cable is used for sync. In this case, the RGB signals may be 714mV P-P (same as the non-sync part of RS170 signals) or they may be 1V P-P. Most RGB monitors are capable of being driven from most or all of these slightly-different versions. The important thing in common is that the brightness information for each of RGB is an analog voltage. The IBM PC, on the other hand, apparently sends digital signals to the monitor - they are red, green, blue, and bright. This saves building three 2-bit DACs in the PC, and three linear video amplifiers in the monitor, but gives you a monitor and computer that are incompatible with the rest of the world of analog video, and forever limits you to 15 colours. Yechh.
jcp@brl-tgr.ARPA (Joe Pistritto <jcp>) (08/28/84)
There are several systems for sending color signals to a monitor, in order of resolution (low to high) NTSC (American Television Standard) - uses 525 lines, 30Hz, each frame is actually two frames, (interlaced). This is ALWAYS sent over a composite (single) cable. This can be made from the PC's composite video via an RF modulator, (about $20 from any video store), but you will lose every thing over the PC's medium resolution mode on the color board. (and the result isn't EXACTLY NTSC so it doesn't work on all TV's, Sony's work excellently though). Top bandwith is 3.5Mhz here (due to the TV sound trap) Composite Video - Any # lines, (usually from 400 to 600, 512 is popular). Usually, this is used for 512x512 applications. Requires one cable. This is often interchangable with NTSC above at low resolutions (320x240 or so). The PC's composite video works at all resoulutions, but the high resolution stuff cannot be successfully converted to NTSC. I believe the PC produces ~12 Mhz on the RCA phono plug on the color board. RS-170 3 or 4 wire - medium resolution graphics display standard. Uses seperate R, G, and B, and often sync. Sync can also be added to the Green line, (but more and more it appears seperately). The standard bit rate for RS-170 is 12-20Mhz or some such. This is what the PC color adapter puts out on the multipin connector. If you are using high-resolution mode, USE a RS-170 monitor!!!!!. The color saturation is usually much better, (the colors aren't demultiplexed in the tube unit), and the phosphors are much more precise. The Princeton Graphics monitor is supposed to be excellent. RS-343 (3 or 4 wire) - This is FAST stuff, intended for real serious graphics work (1024x1280 at 30Hz, 512x640 at 60Hz). The bit rates here are typically over 25Mhz, up to about 36Mhz depending on screen configuration. Usually this is 4 wire, although some systems encode sync on green again. Monitors for this are made by Hitachi, Mitsubishi, and most of the high-end graphics manufacturers, minimum price is $2K or so. There ARE boards for the PC which produce this, but they are rare. I hope this helps answer your question, if not, send me mail at JCP@BRL -JCP-