What happens if...

... if you take a modern CRT monitor, a couple of function generators, connect them, and vary the horizontal and vertical sync (drive?) frequencies continuously (within specified ranges)?

So far, I've seen only specific resolutions and corresponding scan frequencies (often only implied).

If the monitor "doesn't mind", what happens if a frequency varies fairly fast? (Surely, a change in resolution is a step function, perhaps with garbage until it settles down...)

Seems to me that CRT monitor scan circuits probably don't contain any high-Q resonant circuits or equivalents, and that when multifreq. monitors were first designed, perhaps the approach was to "broadband" their scan circuits.

I assume there are something like scan-amplitude-feedback circuits to keep the raster size within reason.

Maybe I should sub. to sci.electronics.design, and post this query there. :)

Regards, and TIA!

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Nicabod =+= Waltham, Mass.
Reply to
Nicholas Bodley
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They rarely care about V - anything within the spec'd range should work.

For H, some monitors have had more than one range with gaps between them.

It's not clear what you are trying to do but you will probabl just have to test it with your candidate monitor.

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Reply to
Samuel M. Goldwasser

Driving a CRT monitor outside its design range might damage it. I would stick within the spec'd ranges and frequency combinations before experimenting outside them.

Reply to
William Sommerwerck

If you bypass the blanking and sync circuits and apply your external drive as deflection, you get this:

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Of course this assumes the gun(s) is biased on all the time for sufficient brightness of the spot.

Note that there must be some frequency relationship between the horizontal and vertical inputs in order to have a stabile scan or your eye won't be able to follow it's path. It will continually shift with the difference in phase between the two inputs.

Reply to
JB

That's what I expected.

Had also suspected that; I think I once encountered a monitor with a relay; I could hear it click when changing resolutions. Considering how horizontal scan circuits work (sorry to say, I never completely learned), those for TV surely do not seem to be broadband at all. Not sure I'd call them "tuned", but designed to run only over a narrow range of frequencies, it seems.

Fwiw, the Megascan (?) monochrome monitor (4096 x 3840, or something close, ran H scan at 240 kHz, as I recall. Yoke was amazing -- seemed to be like a "stator yoke", but windings extended out about 9 inches, like petals of a flower; cooling, maybe?) Res was 300 dpi, just about sure. Astonishing. No jaggies visible at all.

Mainly, just wanting to learn more; replies greatly appreciated.

Thanks much, Sam!

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[e]
Reply to
Eraklik

Sam,

Sorry about this; couldn't e-mail you although I wanted to include you. Message was posted to the group, though. Essentially, an ack. and thanks, with a bit of interesting stuff. Remembered your filter only after posting, and totally understand. Shouldn't have done an all-nighter!

Gory details: Only fairly recently did I switch to Linux only (after accidentally deleting my partition table (MBR) and losing Windows). Pan newsreader doesn't link to Opera, which I use for e-mail.

Thanks, again. You've been of tremendous help to countless people over the years.

--
Nicabod =+= Waltham, Mass.
Reply to
Nicholas Bodley

Indeed so; thank you! However, it seems that many, possibly all modern CRT monitors detect whether the requested scan rates are within range, and refuse to display (and self-protect) if outside of range. A crash a while back made my computer put out something out of range, and I got a red warning on the screen. Rather startling!

One reason I asked for info about the DEC VRC-21WA was hope of getting scan rate limits specifically for it. (I really have tried!) For now, I'm using the limits for the one that it seems to be similar to.

Main point of my query was largely curiosity about what happens if you vary scan rates continuously within acceptable range. (I'm not likely to want to display at some quite-oddball resolution.)

I see that my newsreader is replying by e-mail, but that doesn't work; sorry. I use Opera e-mail, but Pan doesn't "know" that.

Best regards,

--
[e]
Reply to
Nicholas Bodley

Right, and as at least one other poster noted, and the S.E.R FAQ emphasizes: Attempting to run at an unsupposrted scan range may result in smoke. :( :)

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Reply to
Samuel M. Goldwasser

We're in the realm of vector graphics, but vectors are straight; Lissajous figures are nearly always curved. "Stroke writing" seems more appropriate -- think paths in PostScript. This is *very* analog!

The idea is valid, but the horizontal scan circuit is not an amplifier. The sawtooth current is created within the output stage, not created at low level and amplified by the output stage. It's in the class of what were called "pulse circuits" decades ago.

Early in the days of TV, when magnetic deflection became mandatory*, a simple brute-force power amplifier for high-frequency scanning would have been impractical, I'm essentially sure. Huge amount of power, maybe kW, likely, because a low-inductance yoke needs lots of current, while a higher-inductance yoke needs quite a high voltage.

*The HP electrostatic wide-angle deflection yoke (I'm not kidding! CRT bulb similar to that of a mag. defl. CRT) had not been invented yet.

Horizontal scan circuits for mag. defl. TVs and monitors are quite-clever circuits that, I've read, recover energy (a bit like regen. braking in a vehicle, most likely). Unless I'm badly confusen, the horizontal output tube or transistor is essentially a pulse switch, and the whole output stage has the character of a resonant circuit.

Shame on me -- I never learned, in any detail, how TV horizontal sweeps work, and would welcome a link to a good explanation. I do know that the "damper diode" is an essential part of the circuit.

If you wanted to modify a monitor to work as an X-Y display, the horizontal yoke windings would, very likely, have too low an inductance (and the vertical, perhaps too high!). There have been mag. deflection large-screen X-Y displays, but they were never fast; that's why H-P developed that large-screen short-tube X-Y display. Even in that display, large beam swings at its fastest capability required booster transistors, and iirc one couldn't have fast, repeated, full-screen deflection at its upper frequency limit. Small swings, yes.

Nothing wrong with the CRT, itself, afaik, for use as an X-Y analog display, although color convergence would be a ball. (It has been done, in an arcade game, no less. I was painfully tempted to buy a surplus color X-Y display, years ago.)

If you were to use an existing CRT for an X-Y display, you'd want a different yoke, with (probably) the same inductance for both axes.

Then, you'd have the fun of designing analog power amplifiers; probably, adapting an audio power amplifier would be a way to go, but a practical deflection yoke might have many times more inductance than any loudspeaker. A first-rate EE should be able to do it. However, don't expect much swing in the ultrasonic range, and forget 100s of kHz.

There was a low-cost video game (Vectrex?) that used an X-Y analog display with magnetic deflection; small CRT, maybe 9 inch. It could put up small rasters.

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Nicabod =+= Waltham, Mass.
who could have bought a surplus Charactron CRT, years ago.
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Reply to
Nicholas Bodley

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