Converting TTL monitor to Analog

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu May 10 13:44:49 2001

Lets start with this one:

> > > You "can" drive monitors all over the map, eventually you will destroy
them
> > > if you allow the horizontal output driver to over heat. However, getting
an
> > > acceptable signal out of one is more constrained.
> > >
> >While that's true, it's hard to do.
>
> Your correct of course, with 10 year old technology it is somewhat
> difficult. With a modern video card for the PC its trivial.

I'm completely confused by what you say here.

What I meant with my remark that it's difficult to do, is that it's difficult to
damage a monitor by driving it "all over the map" as you put it. There are
monitors that can be damaged by the absence of an appropriate sync signal.
Those are fixed-frequency types, however, and they've mostly been damaged
already, or they have a home where they are fed the correct signals.

The fact is, this discussion has wandered around without any reference to steer
it. I agree, that IF there were such a thing as a digital-input monitor, which
may exist, it might take a video signal that's digital in nature. I've said,
however, that I've never seen one, and I've had, not hundreds, but, over a
hundred monitors here that have all happily produced correct-looking and
thoroughly useable displays from a nominally 1v p-p analog signal as is produced
by a typical VGA/SVGA or whatever you want to call that adapter. I've had
exactly ONE CGA (card) and compatible "color" monitor in house, and, after
viewing the display on the essentially new monitor, promptly disposed of it, so
I've not examined those. I've had little interest in the EGA types, except in
that you can buy them for $5 and tweak them to work with a VGA card. That means
that I can spend $5 and a little time and then give them to a charitable
organization together with an essentially junk-able computer and subsequently
write the computer off. Moreover, some of those that I donated nearly 10 years
ago are still serving those organizations.

I've given these things to friends for use by their kids, and to folks otherwise
unable to afford a computer for use in learning to use a computer. Back in '92
or so, I bought a box of surplus 3-1/2" diskettes and found they were sets of
Autocad-v12 diskettes, so I gave a set to a fellow who'd been disabled due to a
construction accident, along with a computer with a tweaked EGA monitor on it,
and that one's still working. I've just not encountered the problem that is
apparently at the center of this TTL-monitor thing. You'd think I'd have bumped
into at least one.

If somebody has a schematic and manual for one of these "digital" monitors, I'd
be interested in looking it over. It makes little sense to put a digital signal
where an analog one will work, but this is IBM we're dealing with, right?

I've seen monitors with switchable termination resistors that respond quite
nonlinearly when the terminations are turned off, but those were monochrome
types.

Now ... see below, plz.

Dick
----- Original Message -----
From: "Chuck McManis" <cmcmanis_at_mcmanis.com>
To: <classiccmp_at_classiccmp.org>
Sent: Thursday, May 10, 2001 10:29 AM
Subject: Re: Converting TTL monitor to Analog


> At 09:10 AM 5/10/01 -0600, Dick wrote:
> > >
> > > Color monitors have a color mask, and that color mask has a 'dot pitch'
> > > which defines where you can display pixels. If you attempt to display more
> > > pixels on a line than there are holes in the mask, then you will get
> > > banding artifacts.
>
This suggests that there are precise "spots" that one has to hit. in order to
generate an appropriate display. It also seems to suggest that if one move the
right margin of the display, leaving the number of pixels unchanged, but
increases the dot-clock, one should see evidence of this effect. Moreover,
since this is a fixed characteristic of the front-face of the CRT, altering the
vertical or horizontal size should make it apparent as well.
>
> > >
> >I've seen no evidence of this. If this were the case, then the fact that
many
> >display systems use inexpensive 1000 ppm oscillators would cause enough
> >variation that it would be obvious on those monitors having this feature.
> >Moreover, horizontal phase adjustment and vertical height/horizontal width
> >adjustment would be impractical.
>
> Evidence of what? The shadow mask or the interference patterns created by
> displaying at something close to the dot pitch? Take a 17" monitor (13.5"
> across) with a .28 dot pitch. That's about 91 dots per inch (25.4mm/inch /
> 0.28mm/dot) which gives a top resolution of 1228.5 dots. Now run it at 1280
> x 1024, see the "moire" pattern. ? Now try it at 1600 x 1200, its even
> weirder.
>
I've just now fiddled with the adjustments on my video card, which allows slight
horizontal and moderate vertical frequency alterations to be made in software.
I've also done this while fiddling with the vertical and horizontal adjustments.
I see no evidence of banding or any other anomalous behavior.
>
> >Amplifier bandwidth issues are a factor in the practical use range of a given
> >monitor. However, that has nothing to do with whether a monitor is analog or
> >digital in nature.
>
> All monitors are analog, they all have amplifiers to convert from the
> user's video input into a voltage that can affect the brightness of the
> electron beam. Now in "TTL" or "Digital" monitors there amplifiers respond
> in a very non-linear way to the input. Thats the difference. Once it gets
> to the actual video electronics they are darn near all the same.
>
I don't doubt any of this, but I have reservations about the purported existence
of "digital" monitors outside the monochrome world.
>
> Amplifier bandwidth will determine how "cleanly" the colors shift from one
> to the next, colors are produced by a set of three intensities that in turn
> become beam intensities, which in turn excite three phosphors to become
> light intensity. There is also a frequency response component in the phosphor.
>
No disagreement here.
>
> > >
<snip>
>
> --Chuck
>
>
>
Received on Thu May 10 2001 - 13:44:49 BST