From: poehland%phvax.dnet@SMITHKLINE.COM
Newsgroups: comp.sys.atari.8bit
Subject: 1200XL Video/Composite -> CGA/Dead 800 Color
Date: 13 Aug 92 21:26:26 GMT

Whew, there seems to be a lot of questions lately about video.  Being of the
extremely nearsighted persuasion, video has been one of those subjjects near &
dear to the eyes of the 8-Bit Alchemist.  Or at least it used to be, before I
became a Managing Editor Person  *sigh* .  OK, here's my $.02:

Video in the 1200XL is a nasty beast, the output circuits are more complex
than in virtually any other 8bit product, comprising something like 6-8
transistors including feedback loops & other yucky things.  I have no idea
how well it works, having never seen the video output of a 1200XL.  I gather
1200XL owners aren't all that happy w/their video (some of them anyway).

I regard video hacking more as art than science, because much of what you 
observe on the display is so very subjective.  I use words like smear, grain,
blur, bloom, fuzz, streak, & artifact to describe the various defects I look for
when I subjectively analyze display quality.  But those words are meaningless
to somebody else who hasn't also seen exactly the same phenomenon I see when I
use that particular term.  And really poor displays can combine some or all of
those defects to give a screen that an untrained eye would be unable to
describe in the quantitative sense required for a followup electronic analysis.
Add to that, you must FIRST ascertain that what you are seeing is indeed being
generated by the computer & not the monitor itself.  Sheesh!

We 8bitters aren't the only ones suffering.  The composite output on my STe
is pure crap.  From an electronic engineering point of view, there is virtually
no excuse for the video atrocities Atari visited upon the composite outputs
of all its products, its sheer incompetence.  No one at Atari EVER understood
analog video.  God bless the Tramiel Entente.

Sorry, Jeff, you're a little off-base RE: your comments about my Super Video
REV 2 upgrade.  In S-V REV 2, the color/composite/mono outputs ARE all impedance
matched (to 75 ohms or pretty close thereto).  The switch has nothing to do 
with impedance matching but instead serves the purpose of enabling the composite
output, which would otherwise be disconnected as a result of improvements made
to the luma circuit.  The problem is that having composite connected while you
are using mono degrades mono quality via color clock leakage which makes mono
look grainy.  The switch is there for folks who don't have a luma-chroma monitor
& must use composite to get color.  For mono users, using the switch to
disconnect the composite output cleans up the mono display noticeably.

Kevin Chase inquired about converting C-V to CGA.  I think I fielded that
question about 6 times/yr when I was at Current Notes.  The answer is: NO,
it is NOT easy to do.  Full analog C-V consists of: red, blu, green, H-sync,
V-sync, Intensity, & audio, all encoded into one signal.  You would have to
decode, isolate, & clean up the individual signals, then send them all back out
individually in their digital representations w/suitable impedance buffering.
Not trivial.  Over the past couple years both Radio-Electronics & Popular
Electronics have published any number of mod articles, most of which don't
produce all the signals I would like to see on the output.

Man, if EVER CSS were seeking a new product idea, this HAS to be it.  This
suubject of analog -> digital video conversion just keeps coming back again &
again, it won't go away.  (Bob Puff, I hope you are reading this!)

Forget about going into your 8bit & extracting digital video signals from some
place on the board.  The color signals are there, but not all the required
basic video signals are available.  They are locked up in the GTIA & not brought
out to pins where you can get at them.

There ARE possibilites for directly extracting TTL mono from the XEP-80.  For
personal reasons, however, I don't want to get into that & will not answer any
questions about it.

And finally there was J. Katz's inquiry about dead red in his 800.  Sounds to
me like a possible failure of the CTIA or one of the output buffer components.
I can't recall if the 800 uses a 4050 to buffer some of the video signals; if
so replacing that chip could be the fix.  Output video circuits in the 800
are almost as complex as in the 1200XL.  I'd look for a bad coupling cap or a
bad transistor somewhere in that mess.  Most 800 machines are >10 years old,
it's time for electrolytic failures to start showing up.  There's a pot in there
to adjust the color balance.  Corrosion/crud on that pot could be the culprit.
Treatment of the pot with Cramolin followed by realignment of the color (using
COLORBAR or TESTBILD software) is the prescription there.

Gee, this was fun!  Now back to this boring magazine stuff......

     - BEN POEHLAND
       The Alchemist

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