u87rpb@ecs.oxford.ac.uk (03/23/91)
Hi there, does anybody have any idea what might be involved in trying to get a mono monitor to think it's a colour monitor (ie, to display low and med-res screens in greyscale) in *hardware* ? As far as I can tell with the RGB outputs, it would probably be a question of simply hooking up a resistor net with the right `weightings' to generate a mono intensity signal. [At a guess, although I haven't put a scope on them, I imagine that the RGB and Mono lines are simple intensity signals with no additional sync information at all]. The major problem as far as I can tell would be the sync signals. What would happen if I tried driving an SM124 with a 50 Hz frame frequency (or 60 Hz, if neccesary)? I guess that I'll either get no picture at all, or get something like a two-thirds height picture. Thanks, Ray. p.s. I modified my picture size last week after reading reports about people doing this on comp.sys.atari.st.tech, reckon I've got another 20% of useful screen area now...
adamd@rhi.hi.is (Adam David) (03/28/91)
In <9103222256.AA06374@uk.ac.oxford.ecs.booth16.ecs> u87rpb@ecs.oxford.ac.uk writes: >Hi there, does anybody have any idea what might be involved >in trying to get a mono monitor to think it's a colour >monitor (ie, to display low and med-res screens in >greyscale) in *hardware* ? A circuit diagram and instructions for modifying a SM124 to analog greyscale was published in ST Computer magazine (German) some time in 1988. The mono output is mixed from RGB using a passive resistor network as suggested. >The major problem as far as I can tell would be the sync >signals. What would happen if I tried driving an SM124 with >a 50 Hz frame frequency (or 60 Hz, if neccesary)? I guess >that I'll either get no picture at all, or get something >like a two-thirds height picture. If the monitor is not modified internally to cope with the different frequencies you will fry some of the components because higher currents are involved at the lower frequencies. I did this mod back in 1988 and it works (sort of...) Maybe there were some typos or I missed the corrections in later issues. I have a SM124 that displays colour modes in mono with 2 major faults. 1) Switched to colour mode the monitor loses all horizontal linearity. Characters are nearly twice as wide on the left side of the screen than on the right. 2) Brightness control (and maybe contrast too) is disabled in colour mode. Colour mode is not very stable until the monitor is warmed up properly. The focus adjustment is disabled in mono mode (maybe in colour too, it's a long time since I looked). The actual focus seen on the screen is unstable and gradually drifts out of focus, suddenly snapping back to correct focus once in a while. If the brightness is turned down it becomes useable and if the brightness is turned up it gets really bad. Summary: If you can find a circuit that works properly then it might be worth trying. Otherwise you will be better off with a cheap mono monitor that takes a standard video signal, and mixing the colours and the sync together using resistors. Some STs have a useable composite sync signal, with others it is necessary to mix Horz & Vert together over diodes. Plea for Help: Did anyone get the SM124 modification working satisfactorily. Get in touch with me if so and let me know how you got it fixed. -- Adam David. (adamd@rhi.hi.is)