Converting TTL monitor to Analog

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu May 10 10:04:57 2001

Hogwash!

56Hz won't produce any flicker a human can detect. It may beat with the local
lighting, but it won't flicker noticeably. Moreover, the Multisync II that I
had on my desk in '87..89 was completely capable of interlaced display at
1024x768 and, assuming moderately subdued room lighting, which I didn't have,
showed no noticeable flicker in that mode either. Whatever claims the mfg made
about it, it was very useful at 800x600, which was the mode in which I used it
most.

I'm really confused by the hubub over CGA/EGA/VGA/XGA/PGA etc performance as
discussed here. The monitor has no control whatsoever over the horizontal pixel
density it displays. It does, however, have a set of video amplifiers with
limited bandwidth. It also has limts on its sweep circuits. These influence,
and, indeed, limit, but don't control the resolutions at which it is practical
to use a given monitor.

The Multisync series of monitors produced by NEC and emulated by virtually
everyone else as PC's became popular, had multiple bands within which they would
track horizontal and vertical sync signals. In some models this was very
obvious in that they had a set of adjustable circuits on their main circuit
board obviously marked for this purpose. These did interact somewhat and it was
possible, as I learned the hard way, to make them interact such that nothing
worked correctly. However, though every monitor puts limits on (a) its video
bandwidth, (b) its horizontal sweep and (c) vertical sweep rate(s), these
certainly don't exercise any sort of precise control.

I've never seen a video board that put out digital signals to an EGA/VGA
monitor, though that doesn't mean they don't exist. However, my experience
making "EGA" monitors continue to serve through the VGA era suggests that, since
they all seem to exhibit full dynamic range on their video inputs at nominally
1-volt p-p signal levels, reinforces my belief that they're all analog at the
inputs. The fact that the IBM EGA board had a simple DAC on it, with which it
produced discretely differing analog signal levels seems to support that, as
does the fact that the same analog input produces a display on the typical "EGA"
monitor, though it is affected by the switches than many of them had, effecting,
mainly, the termination. The ONLY problem I've ever encountered, that rendered
some EGA monitors, difficult of impossible to use with VGA signal input has been
the sweep circuits. Some of them just won't allow adjustment into the somewhat
higher horizontal sweep rates. If you get them to sweep at the desired
horizontal rate, they all seem to like a 60 Hz +/- 10% vertical rate just fine.
Some "EGA" monitors won''t slow their vertical rate down enough to support
interlaced 1024x768 display, however.

Now, perhaps someone can produce a document that clears up what the problem is.

Dick

----- Original Message -----
From: "Jeff Hellige" <jhellige_at_earthlink.net>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, May 09, 2001 1:25 PM
Subject: Re: Converting TTL monitor to Analog


> On Wednesday, May 9, 2001, at 01:23 PM, Iggy Drougge wrote:
> >> The II would do unlimited colors in analog mode and up to 64
> >> colors in TTL mode. It's vertical freq range was 50 - 80 Hz with a
> >> maximum resolution of 800 x 560. I believe the 2a was the first to
> >> be able to reach 800 x 600, though the upgrade to the GS also was
> >> able to. It's horizontal freq. range was 15.5 - 35 khz.
> >
> > How do you define resolution at the monitor level? I've always been
> > able to
> > get entirely different resolutions out of monitors, and why would they
> > disagree? If it's an analogue signal, wouldn't the monitor just sweep
> > along
> > and project whatever is input?
>
> The specs I referenced above are those put out by NEC for that
> monitor. It's doubtful that the II would do be able to do 800 x 600 in
> it's vertical freq. range, which is why NEC lists it as a max.
> resolution of 800 x 560. I believe at that resolution they list it at a
> vert. freq. of 56 Hz. Even if it could take the vertical freq. low
> enough to get to 800 x 600 I wouldn't want to have to look at it. 56 Hz
> will flicker enough as it is.
>
> Jeff
>
>
Received on Thu May 10 2001 - 10:04:57 BST