Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site watcgl.UUCP Path: utzoo!watmath!watcgl!dmmartindale From: dmmartindale@watcgl.UUCP (Dave Martindale) Newsgroups: net.video Subject: Re: monitor query: composite color input vs. RGB Message-ID: <3000@watcgl.UUCP> Date: Fri, 24-Aug-84 02:50:25 EDT Article-I.D.: watcgl.3000 Posted: Fri Aug 24 02:50:25 1984 Date-Received: Fri, 24-Aug-84 04:31:24 EDT References: <918@bbncca.ARPA> Organization: U of Waterloo, Ontario Lines: 30 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.