At 10:57 -0700 7/9/11, Chris M wrote: >I recall seeing the mac video being designated a "composite" signal. Composite >or NTSC is what's fed into a video monitor (not TV, unless it accepts >composite input). Composite imposed on a carrier signal is what's displayed on >channel 3 or 4. Composite just means everything is combined into 1 signal >(video and syncs) and fed on 1 wire. Some monitors have composite sync >(horizontal + vertical). Sync on green monitors have the composite sync signal >fed into the green input. If you had a monitor that need a SOG signal, you can >add the vertical and horizontal syncs to the green line w/a resistor. I've >done it. > But the Mac's composite signal is likely a different animal as Doug pointed > out. It might help to scope out the point at which the video signal is > created, that is where the separate components of the signal are added > together - to see if the Mac's composite video can be altered to conform to > the NTSC standard. This might be harder on a Mac then say an old IBM PC, > being first of all there is no crt controller in a Mac. All the video is > created by firmware programming of the 68k plus presumably any glue contained > in custom logic chips (glue usually designates ancillary logic gates/chips > like 74ls*** that you'll see all over most motherboards, and occasionally are > implemented in custom chips. 1 custom chip (fpga,gal,pal,...) can do the work > of dozens of discrete logic chips. This glue is needed to provide additional > logic that the microprocessor or crt controller, both being general purpose > ic's in some sense, don't provide, so you can customize a piece > of microprocessor based logic to your specific needs. CRT controllers are > also microprocessors in a broad sense, having internal registers and clocks > and whatnot. Video cards these days have enormous horsepower, and are > utilized by hackers to perform brute force repetitive tasks like password > guessing. The firware/flashware is reworked to make the crt controller logic > act like a standard micro. Wicked). > This would be a terribly interesting project, and a very educational one, > assuming someone was willing to get way down and dirty. If someone were able > to isolate the components of the video signal (i.e determine at what point > they were spit out out of some pin or whatever) I'm thinking some > semi-special purpose ic could be used to morph these *bare* components into a > true composite signal. Maybe I'm just nuts though. >
In the days of the Mac Plus which was a black and white, not gray scale, machine the only video conversion needed is to read out one bit at a time into an appropriate amplifier to drive the grid of the CRT. Synchronizing the readout to the vertical and horizontal drive of the magnetic yoke is pretty simple and a few bits were wasted during flyback intervals. I'm not sure if the CRT board synchronized itself to the motherboard or if it was the other way around. I remember coding the Mac Plus using a second screen buffer into which you could write. When a new screen was finished you could just tell the monitor to swap buffers to get what looks like an instant redraw. The developer version of Hypercard allowed that. Audio goes to a speaker and a phone jack and was never mixed with the video in a composite output. Some third party may have made an NTSC, National Television Standards Committee, conversion to allow for a second monitor but I never saw it. Apple's earlier Apple II line did put out NTSC video and you could run a TV monitor from that but even there I don't think the audio was ever mixed with the video. It was the SE, system extendable, series that first had an internal plug-in capability. That made it possible to drive a lot of monitors probably including third party color NTSC composite. Radius was a major supplier of cards for that. NTSC is a 14.5 or so kHz horizontal scan rate with a 30 Hz vertical refresh rate. Video bandwidth is about 4 MHz. The horizontal lines are interlaced with all odd lines delivered on one vertical pass while the even lines are delivered on the next. That tends to eliminate perceived fluctuations in TV sets. Color NTSC has other things added. All things considered it's probably easier to replace the mother board with a PC104 sized single board computer that runs at 500 MHz and already has facilities for driving that 9 inch monitor. But that would take you into Linux and wouldn't be anything like Mac OS 4 and Quickdraw. Not the OP's intention. -- --> From the U S of A, the only socialist country that refuses to admit it. <-- -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/
