Re: [stella] Atari 2600 mods, clean video, stereo audio

Subject: Re: [stella] Atari 2600 mods, clean video, stereo audio
From: Chris Wilkson <ecwilkso@xxxxxxx>
Date: Thu, 15 Jul 1999 13:57:54 -0400 (EDT)
On Thu, 15 Jul 1999, Glenn Saunders wrote:

> I just got my VCS back from Chris Wilkson.  Although he forgot to send my
> power supply back (doh!) I had a spare handy and was able to see what he
> hath wrought.

Doh!  I also have what looks to be an insert from the Stella CD...?  Sorry!

> Jerry Jessop was wrestling with my six switcher trying to eliminate this
> weird hardware noise pattern that was showing up in the video.  According
> to Jerry, problems like these have plagued Atari itself when they were
> showing the VCS at trade shows and wanted to avoid RF and go composite.
> 
> I do believe that Chris has designed the ultimate 2600 modification because
> Chris decided to make two snips to the TIA chip itself and this eliminated
> the problem entirely!

I'm still not sure what causes this hardware ghosting effect.  And of course
I couldn't use Glenn's deck to investigate, because it didn't show up! :)
I'll tear into my deck maybe this weekend to see if I can isolate the cause...

> When I saw the display in Y/C, I have got to imagine that it is the
> cleanest display ever generated by a 2600 in history.  Now, it isn't
> perfect.  There is a vertical gridding pattern when you view it in Y/C
> which has something to do with the way the VCS times its video.  As I
> recall, many Atari 8-bits also exhibit this phenomenon while in Y/C mode,
> especially the 130XE, so it's normal.  There is also a twinge of ghosting
> in the sprites even in Y/C, but aside from my sub-standard Y/C cable, it's
> probably the theoretical highest quality output you are ever going to see.

I don't know how to eliminate the color ghosting.  Maybe in a later rev.
The vertcal bars are an expected side effect of the high color saturation
level in the chroma signal.  This could then be reduced by lowering the
saturation output and later corrected by boosting the color level on the
tv/monitor.  A single capacitor added to the existing circuit can accomplish
this.

Glenn, did you get a chance to run the video through a vectorscope?  If so,
how did the levels turn out?

> Being able to hear the 2600 in "stereo" for the first time was also a
> thrill.  In Space Invaders, the marching is on one side, and the firing is
> on the other.  In Pitfall II, the percussion is on one side, and the three
> melodies are on the other.  For Decathalon, you have a nice effect of the
> melody on one side, and the harmony on the other.  Chris set the audio
> levels so that they zero out perfectly through my mixing board.

How good is the channel separation?  I didn't have stereo capability to test
this.  Ie, if you turn the balance all the way to the left in Pitfall II, 
can you still hear the other channel?

> For games that do not use flicker, the 2600 will record well into my
> nonlinear system.  However, most games which use flicker wind up with
> certain sprites totally dropping out of the picture based on which field
> they are flickered in.  It looks as if the time base corrector throws away
> one or two fields per image, replacing them with duplicate fields.  If I do
> a traditional framegrab, the missing sprite will show up and I can step
> through the fields (useful for demonstrating how flicker works, by the way).
> 
> I'm going to have to try recording the VCS footage to tape and see if the
> deck can do a better job of preserving the flicker, otherwise I won't be
> able to show the games totally faithfully.

I like the idea of flicker demonstration.  But I hope you can get the complete
frame to show up.  I remember trying to run Solaris in PCAE on my 486-33.
I thought to myself "this game sucks!  The flicker is *terrible*!"  Of course
it's nowhere near as bad on a real 2600... :)

> If you want to see a sample screengrab, go here:

Nice!  My only test carts were Raiders of the Lost Ark, Super Baseball, and
my playfield demo cart (useful for setting luma levels).  And, of course,
Kaboom...to look for hardware ghosting.

I got so sick of looking at Raiders.... :P

> Chris says he'll be submitting an article for 2600 Connection.  I wholly
> endorse the modfication!

Cool.  I'm glad it works for you.  I'll try to write it up RSN.  And I'm
thinking of making a printed circuit board version.  If I do that, I'll
be able to do the mod for anyone who wants it!

-Chris



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