Re: [stella] Megacart, video capture

Subject: Re: [stella] Megacart, video capture
From: Chris Wilkson <ecwilkso@xxxxxxx>
Date: Fri, 12 Jul 2002 12:49:02 -0400 (EDT)
On Fri, 12 Jul 2002, John Saeger wrote:

> > On a more serious note, what are the space limitations in z26?
> 32KB for a bankswitch scheme is pretty much the max right now.  We do
> megaboy which is 64KB but it's pretty ugly.  But like Eckhard, I'm pretty

Is it a PC memory limitation, or a time limitation, or what?  I'm sorry
but I don't know anything about how z26 is written.

> much motivated by games, or hardware or something.  But let's say,
> hypothetically speaking, that for some reason I decided that I wanted to
> implement the megacart right now.  How would I test it without a game?
> There needs to be something I can run.

Yep, that's pretty understandable.  Can't blame a guy for poking you
though.  ;)

> before.  I built the Tom Clancy s-video mod for my 2600 a long time ago and

Cool!  Does it have torpedo bays?  Where does Harrison Ford sit?
(sorry...I've been wanting to make that joke for a loooonnnnggg time!)

> it works great with video monitors, but recently I got an Asus deluxe video
> card that has video capture.  When I try to capture vcs images, I get things
> like this:

Heh.  Heh heh.  heh hehehheeee!   Heeehaaahahaa! Muahahahaaaaa!!!!!
Cough!  Cough!


This is a problem with the mod.  There's nothing you can do about it unless
you change the circuit.  Basically, it's crosstalk between luma and chroma.
This is the problem that I've been fighting for about 2 years now.  Check
out the captures I took last night, using my mod-in-progress.  These were
taken with an ATI Radeon 7500 All in Wonder, BTW.

You'll note that compared with the RF picture, the composite looks really
nice, except around the bottom of the ledges.  But the S-video is horrible.
I know why this is happening in my circuit.  I just haven't found a nice,
cheap way of fixing it yet.  There were a couple of expensive fixes, but I
wouldn't want to pay for it.

> As you can see, the chroma signal doesn't seem to last for the entire color
> clock period and there are these vertical stripes.  I guess we don't see
> them on the old monitors that I use because of their limited bandwidth.  I'm
> thinking maybe the Asus capture circuit has too much bandwidth.

A "perfect" chroma signal is a sinewave.  What you're seeing is the peaks and
valleys of the chroma sinewave being added to the luma levels.  What kind
of monitors are you using?  A C= 1702 will difinitely pick this up.  So will
a 1084s.  :(

> Do you have
> any ideas on how to do a 2600 video mod that doesn't have this problem?
> Something that displays correct colors smoothly even if the video
> monitor/capture circuit has lots of bandwidth?

Heh...I'm working on it.  ;)


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