Re: [stella] problems with monitor syncing

Subject: Re: [stella] problems with monitor syncing
From: Chris Wilkson <ecwilkso@xxxxxxx>
Date: Tue, 3 May 2005 15:03:31 -0400
It's hard to tell much from the pictures.  But here's my guess...

The VGA horizontal frequency is ~ twice that of the TIA:  31kHz vs. 15.7kHz
The vertical frequency is also somewhere "near" twice as fast, though
there is a lot more variation due to the software control of the TIA's
vertical timing.

If you're not already doing it, you're going to need a line buffer.
The TIA should write video to the buffer, and the buffer should be
played twice to the VGA output.  I think the easiest way to do this
is the following:  during TIA scanline n, the TIA fills buffer A.
During TIA scanline n+1, the TIA fills buffer B while buffer A is
being read by the VGA output (twice), etc.

There's a lot of hardware overhead, and the SYNC signals are tricky,
because the 2 horizontal frequencies are not integer multiples.  But
it works.  I'm sure there are more elegant solutions for building a
non-integer line buffer, but you get the idea.

I ran into similar problems (in reverse) a long time ago when I tried
to double the 2600 clock speed and output it to a TV.  The first thing
I tried was masking every other hsync pulse.  I wound up with a display
similar to yours.  IIRC, I had 4 psuedo-copies of Combat on screen at
once.  But I did finally make it work, so keep hacking at it!  :)


On Mon, 2 May 2005, Adam Wozniak wrote:

> Hash: SHA1
> These are images of my FPGA TIA driving my VGA monitor.  (Space Invaders,
> if you can't tell.)
> It looks as though the monitor cannot sync to the horizontal frequency
> I'm giving it.  The image is totally stable vertically (i.e. no rolling
> or anything), but it looks as though each line is wrapped around the
> screen twice (one and a half times?).  Look closely at the score; note
> the large gaps between pixels, both horizontally and vertically.
> Unfortunately I don't have a scope handy to measure frequencies.
> Is it possible my hsync is correct, but that my monitor sucks?  Or have
> I done something else wrong?
> - --
> adam@xxxxxxxxxxxxxxxx
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