I’ve got the modified CRT working pretty well as an X/Y monitor, but one thing I’d really like to do is to be able to display the composite input signal unchanged and then blend in additional tones to interfere with it. As it is now, I can toggle the X and Y deflection signals between external input and the internal signal, but using external signals to try to recreate the image on the composite input is, no surprise, effectively impossible. I can get the vertical deflection pretty close with an external 60hz sawtooth (and the results can be pretty fun), but the horizontal is way too sensitive to get anywhere near working that way.
The two approaches I can think of would be either
-tap the H and V deflection signals from the main board, attenuate them down to audio levels, blend them with the external audio inputs and rout that all through the amplifier I’m drivign the CRT with
-get some potentiometers/rheostats that could safely handle the output of the external amplifier and the internal signals, and wire them up as simple passive blend controls
The second one would be a lot simpler but I’m hesitant to try it because it seems like there might be some potential to damage things (back-current from the amp to the main board or vice versa). Any advice?
Also, the way it works now I have to use the composite input to actually have the beam appear, which I expected but a side effect is that the beam dims during the vertical blanking period, so there are dim sections or breaks in the lines that get drawn. I’ve done quite a bit of Googling but I can’t find any information about adding a direct brightness control of the beam to a modded CRT, but that seems like it would be the way to get rid of the blanking period issue (and of course the logical next step would be to build a simple VCA to modulate it and have a Z input). If anyone can point me in the right direction for that, please do! Would it be as simple as identifying the cathode pins, measuring the voltage that’s being supplied to them with a white screen being displayed at full brightness, and then building/buying a DC source that would supply continuous voltage to them and swapping that in for the composite?
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i haven’t seen anyone successfully combine an audio input with a composite sync signal using the existing deflection system, so im not sure exactly what the technical limitations would be for that – but have you considered adding your own electromagnetic coils to the tube and then driving those with your line amps? i.e. creating a wobbulator. just an idea for circumnavigating the issues if you find yourself blocked by the built in deflection system rejecting your strange sync inputs.
i know that hacked video monitor oscilloscopes can be a lot more finicky than actual xy displays due to the difference in the frequency and signal levels that the original horizontal and vertical deflection coils are built to expect. unfortunately i dont have the answers to any of your specific questions but just wanted to share to wobbulator idea as ive seen some really great effects from those. good luck!
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No worries. The wobbulator is pretty compelling but I also want to keep this thing capable of fully reverting back to a normal TV so I can use it for rescanning as part of the signal path of my setup (rather than an end-point) or vector stuff.
Honestly, adding a Z input is actually more interesting than my original question. Finding a scope with a Z input AND a removable reticule is next to impossible, and a Vectrex is getting up near $1000 USD so that’s out.
Probably the most practical way to do that would be in Puredata/GEM or something, and using the composite out on a Raspberry Pi to control the beam color and brightness, with audio outputs controlling X and Y. Might be just the thing to get me to finally get decent at PD.
These are all interesting ideas - I’m following your progress ! It’s been a while since I hacked on crts , but I always felt there was more room to explore there.
Just wanted to add that if you added electro-magnets wobulator style you can still use the tv for rescanning - when the magnets are not being used the output will be normal. Atleast s that how I remember it working for wobbulators iv seen
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I should really just suck it up and get a 3 axis vectorscope,even though I’m trying to save up enough that I can get a fancy new audio interface when my tax return comes in.
EDIT: using the composite in as a Z axis/color control would absolutely work, it’s just the actual execution of it that’s beyond me until I do some studying. I guess an old laptop with a VGA to composite converter running as a second monitor would be the easiest way to try it.
EDIT 2: the more I think about it, the more I think maybe I really should build a Wobbulator.
If this isn’t in the Google drive yet, it probably should be!
I’m not finding the the post from a few days ago where someone suggest experimenting more with electromagnet placment other than the standard Wobbulator document and they were absolutely right! I haven’t gotten to try anything myself and won’t for a while yet but I spent last night asking everyone I could find that has obbulator experience for advice and it turns out there have been a lot of improvements to it made in the last few years, and a completely new approach to the X-Y coils is one of them. It will absolutely be doable for me.
Maybe some day some who actually has the necessary skills and knowledge will develop a sort of generic CRT experimenter’s board that’s just a bare-bones video board that can be adapted to drive any arbitrary stock TV set CRT and has a few points for adding things like voltage control of the typical picture adjustments if you want that. It seems like with old TV sets the CRT is the least likely thing to break and it’s a shame that so many of them get scrapped because of some unrelated electronic failure.