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Who says you can't display MDA on a CGA monitor!?

vwestlife

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I just installed a Sound Blaster Pro card in my Tandy 1000SL and I was dismayed to discover that the GLX MOD file player doesn't support CGA. I could hear the music playing just fine, but all I got was a blank screen. But before I installed a VGA card, I wondered what would happen if I switched the Tandy's onboard video into MDA/Hercules mode by pressing Ctrl-Alt-Shift-V, while still using my Tandy CM-11 CGA RGB monitor. Mind you, it's not supposed to work, because MDA outputs its video on pin 7, while CGA monitors ignore this pin and use pins 3, 4, and 5 for Red, Green, and Blue, respectively.

But to my surprise, after adjusting the monitor's controls, I got a steady, clear image! The top line and left side were cut off a bit, but the majority of the screen was perfectly readable. It was even showing on the CM-11 in the obligatory green phosphor color! (click for hi-res photos):



The odd thing is that some of the vertical bar characters appeared to be gray, instead of green. It didn't photograph well, but here's a close-up:



And here's the kicker: the CM-11 monitor doesn't even have pin 7 connected to anything -- it's missing from its DE9 connector. So, clearly the Tandy is outputing MDA/Hercules video through the RGB pins... and perhaps could even be emulating the early IBM MDA cards with the "color MDA" RGB output? I know it's also outputting video on pin 7, because I've connected a real MDA monitor to it and it worked fine.

Just to prove there was no trickery involved, here's the same CM-11 monitor showing proper color CGA video -- it's squished down to the center of the screen because that's how far off I had to adjust the monitor's controls to get the MDA video to display properly (or at least as properly as it could manage):



I had the CM-11 running with the MDA signal for nearly an hour with no apparent ill effect.
 

romanon

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Because this monitor ist multisync. Try IBM 5153 monitor with MDA and you will see....nothing
 

Scali

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Interesting... I wonder what signal it is sending to the monitor, timing-wise.

On a slightly related note... My ATi Small Wonder can display MDA/Hercules on a CGA monitor by showing it as an interlaced 400-line screen at 60 Hz, with green (real MDA/CGA is 50 Hz).
It can also do the opposite: CGA on an MDA/Hercules monitor (that is without using any tools such as SIMCGA).
 

archeocomp

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My ATi Small Wonder can display MDA/Hercules on a CGA monitor by showing it as an interlaced 400-line screen at 60 Hz, with green (real MDA/CGA is 50 Hz).
It can also do the opposite: CGA on an MDA/Hercules monitor (that is without using any tools such as SIMCGA).

Well that card is really a small wonder :) I have one too..
 

deathshadow

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I thought there was some combination of multisync and original MDA's that would actually show 8 color text modes? Am I remembering that right or not?
 

vwestlife

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Because this monitor ist multisync. Try IBM 5153 monitor with MDA and you will see....nothing

The Tandy CM-11 is not a multi-sync monitor; it just happens to have a wide enough range of its V-Hold and V-Size controls to be able to lock onto an MDA/Hercules video signal. Here are the CM-11's official specs:

http://support.radioshack.com/support_accessories/doc6/6012.htm

I did some testing with my multimeter, and in MDA mode the 1000SL is putting out video signals on both pin 7 (the normal monochrome pin) and pin 4 (the CGA green pin). There are no signals on pins 3 or 5 (red or blue) in MDA, so what appears to be "white" on the monitor in MDA mode is just an artifact of the color CRT pixels.

And in CGA mode, in addition to the normal RGB signals on pins 3, 4, and 5, it is also putting out a signal on pin 7, the MDA video pin.

I wonder if this is normal behavior for most switchable CGA/MDA video chips -- to keep both the RGB and monochrome video outputs active in either mode, and only change the scanning frequencies? That certainly simplifies the design, and causes no problems in normal use, since CGA monitors ignore pin 7 and MDA monitors ignore pins 3-5.
 

tipc

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I'm a little skeptical about hearing the 1000SL outputs MDA video. In any event, the difference between the horizontal scan rates of the CGA and MDA is not that large. 15.75khz vs. 18.5khz, roughly. Back in the day they would somehow use CGA monitors to display EGA. The colors would be off, I don't feel like getting into it. But there's a much bigger difference.
Some monitors *can* do this, but it isn't always advisable. You *can* also hobble the x-ray protection circuitry I'm told. Be careful . . . and you may also be putting strains on the monitor it wasn't designed to take.
 

vwestlife

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I'm a little skeptical about hearing the 1000SL outputs MDA video.

It's listed in the specifications -- "MDA or CGA compatible":

https://magisterrex.files.wordpress.com/2011/08/blogtandy1000slrear.png

Some monitors *can* do this, but it isn't always advisable. You *can* also hobble the x-ray protection circuitry I'm told. Be careful . . . and you may also be putting strains on the monitor it wasn't designed to take.

There was no audible whine from the monitor's oscillator, like what you get when trying to display a video signal a monitor or TV can't handle, such as trying to display Super VGA video on a regular VGA monitor.
 

vwestlife

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Ironically I just had a monitor burn up... but not the CM-11, thankfully. It was an old EGA monitor that I rescued from the recycling center. Right after I had adjusted all its controls and got it working perfectly, about 15 minutes after I turned it off, I heard a hissing and crackling sound and a huge plume of smoke came out the top of it! :eek: The funny thing is, instead of smelling like burnt electronics, it smelled like burning wood!
 

mikey99

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Ironically I just had a monitor burn up... but not the CM-11, thankfully. It was an old EGA monitor that I rescued from the recycling center. Right after I had adjusted all its controls and got it working perfectly, about 15 minutes after I turned it off, I heard a hissing and crackling sound and a huge plume of smoke came out the top of it! :eek: The funny thing is, instead of smelling like burnt electronics, it smelled like burning wood!

Sounds like either a line suppression capacitor or an electrolytic capacitor in the PS blew up, might be an easy fix :)

modem7's site has some info about the line suppression capacitors here:
http://www.minuszerodegrees.net/failure.htm
 

vwestlife

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Ok, but why do you assume that has anything to do w/x-ray protection? And I thought transformers are the things that whine . . .

Correct, the flyback transformer is what makes the whine. In normal use it will be nearly inaudible. But if you start to push the limits of what the transformer can do, it will whine quite loudly. And if you go too far, the X-ray protection circuit will kick in and will either reduce the high voltage to keep it from going too far above its normal range (usually 25 kV) or will shut down the HV entirely.

Like I said, many people here have experienced that happen when trying to drive a VGA monitor with a resolution or refresh rate higher than it can handle; the result usually is a scrambled, rolling picture and audible whine from the flyback transformer, or a complete shutdown of the picture. So if driving a 31.5 kHz VGA monitor with a 38 kHz Super VGA signal (21% higher than its design spec) doesn't fry the monitor or zap you with X-rays, then driving a 15.75 kHz CGA monitor with an 18.43 kHz MDA signal (17% higher than its design spec) won't either.
 

misterblack

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Heh and I came here thinking this thread was about reprogramming the 6845 on an MDA card to output 15.7khz signal. (No idea if this is possible due to clock issues.) Still a neat trick on the Tandy though.
 

Trixter

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There are many things about this thread that are raising my hackles. I'm going to solve your issues with software:

Here is a patched version of GLX that works with CGA: ftp://ftp.oldskool.org/pub/misc/Software/Audio/GLX_CGA.zip

Here is a potentially better modplayer for 8088 systems anyway: ftp://ftp.oldskool.org/pub/misc/Software/Audio/MODMXT4.zip
This is still in development, but is a direct competitor to GLX in terms of mixing speed, and can actually outperform GLX in some circumstances (if you set GLX too fast, it produces noise or locks up, but if you set MODMXT4 too fast, it will simply drop instruments to keep up). More information on its ongoing development and testing is here: https://www.vogons.org/viewtopic.php?f=62&t=66350&start=40

And if you really want to do something silly, MODMXT4 supports the GUS on 8088 systems (by using only PIO to transfer samples instead of 16-bit DMA).
 

KC9UDX

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I always wonder if a flyback transformer can really whine. Usually ceramic disk capacitors make very good high frequency audio speakers, but not potted transformers.
 

Eudimorphodon

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Back in the day I experimented with overdriving an NEC Multisync GS (which normally topped out at VGA frequencies) to 800x600, and it would start making some pretty ominous noises even at 800x600@56hz. More of a rumble than a whine, wherever it was coming from.
 

vwestlife

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I made this thread before I knew about TANTRAKR, which can play MOD files through the built-in DAC in the later Tandy 1000 models. For example, on a 1000SL (NEC V30 @ 8 MHz) it can play a 4-channel MOD file at a sampling rate of 18 kHz:


TANTAKR has separate 8086 and 286 versions, the latter of which can also run on the NEC V30.
 

Hugo Holden

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I wouldn't suggest even trying, as it may kill the monitor. It's not supposed to handle 18 Khz.

You cannot kill a 5153 by sending it any kind of abnormal sync pulse. The reason is that like TV sets & video monitors it has its own independent H & V scan oscillators that keep the range of possible scan frequencies tight, they only "lock" to incoming syncs if they are in range, so you could feed them noise as syncs and still no issue. This is unlike the 5151 which does not have its own H scan osc and will be easily killed with the wrong H sync.
 
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