Re: IIgs RGB to VGA experiment

["Michael J. Mahon" <mjmahon@aol.com>]

js@cimmeri.com wrote:
> 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.

You are correct--I was confused.

I, too, am amazed that the output scan frequency is not 2x the input
scan frequency!

> 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.

It sounds like a beat between the sample clock and the Apple dot clock.

> 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!

That's what I would expect, since the Apple video signal is pretty
low-res by VGA standards.

> 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.

The active region of 40 microseconds (about 2/3 of a line) was set
by Woz back when TVs were the standard monitor, and overscan was a
significant issue.

-michael

Parallel computing for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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