Re: IIgs RGB to VGA experiment

["js@cimmeri.com" <js@cimmeri.com>]

Michael J. Mahon wrote:
>
> More seriously, I suspect that changing the voltage is causing some
> variation in sampling oscillator frequency, and allowing it to lock
> to some submultiple of the IIgs dot clock.  It would be much easier
> to find this oscillator and just "tune" it, rather than changing the
> operating voltage of the entire board--which, at 4.1v, has to be
> making some parts quite marginal in operation.

I haven't a clue, but I notice as I decrease voltage from 5.0 (or raise
it), the displayed image on the CRT screen goes through these little
cycles where it looks like I'm adjusting phase on an LCD monitor.
"Submultiple" seems an apt word for what I'm seeing.

Converter works fine down to 4v... starts dimming after that.


> > Now, the problem still exists that the converter is going from
> > 15.734khz -> 30.9khz... the Multisync reports 31khz x 60hz.   I think
> > we would much rather have 31.5khz so as to be compatible with ALL VGA
> > monitors -- not the select few that can sync down to 31khz.
>
> Perhaps, but 2:1 is a "magic" ratio, and changing it slightly can
> easily result in very visible artifacts.  (There is no good technical
> reason why a monitor that works at 32kHz won't work at 31kHz except
> that the designer didn't expect it.)

One of us is confused... so I'll repeat.  I would much prefer 31.5khz
because it IS 2x 15.75khz.   How or why these guys chose a factory
preset 30.9khz from 15.75khz does bewilder me.

My IBM VGA monitor is a fixed frequency monitor, and does not go below
31.5khz.  The Multisync does but may be having some slight issues with
30.9khz... thus causing the slight shimmy I was seeing (until I lowered
the voltage to the converter.)

The only remaining problem is what appears to be something of a shadow
phase.. where there are about 8 black, shadowy verticle bars...  and
they move as I adjust the voltage to the board.   So at one voltage,
most of the chars in a line of 80col text will look normal, but other
chars have some of their edges darkened.  Adjusting the voltage
brightens THOSE edges, but then other edges go dark.    On a solid
color screen, the bars are more noticeable.

Btw, I did boot into GSOS to see what hi-res looks like.  Blech.
Though MUCH better than composite->VGA, you could see EVERY line (being
640x480).   Is this because you need a fat dot pitch in order to blur
the lines together?   If so, not sure I want to even go to using VGA!
lol   Though the GR colors are very nice, the hi-res colors suck!

Speaking of fat dot pitch, I got the Commodore 1084 cable made and
tried that.  While a nice monitor for viewing composite television, and
flexible with all the inputs, it's clearly very inferior to the IIgs
RGB monitor.. in every respect except screen size.

Speaking of screen size, what the heck does the IIgs have such fat,
screen wasting borders for?  rrrrrggg.   Had to turn 1084 screen knobs
to near maximums just to get some decent visible screen estate.




> > My next test will be to try using the J02 input -- which is the RGBHV
> > one -- because of it's advertised much greater flexibility (should do
> > 15.75khz->31.5khz).  First, I will see if it will sync up only using
> > the H line with the IIgs' composite sync... as many monitors do.  If
> > that doesn't work, I'll have to build a gizmo using the LM3881N chip to
> > give me the pseudo HV signals to feed the input.  I think this small
> > chip can easily be piggybacked or flown over the converter board
> > somewhere.
>
> I'm a little amazed that this J02 input is anything more than a
> slightly rewired version of the J01 input.  In other words, I would
> expect both inputs to share exactly the same video processing, and
> that the "difference" you see in the specs is just a typo.

I did look and noticed that the RGB pins on J01 and J02 are in fact
tied together.  However, the SYNC lines are not.   The S line (from
J01) goes to one area of circuitry, and the H+V lines (from J02) go to
another.


> Any chance you could post some pictures for comparison?

I don't know that I have enough light or right kind of dig camera to
take screen shots.. but I might try later if I can get some time and
some better results to show.

~ J