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- From: poehland%phvax.dnet@SMITHKLINE.COM
- Newsgroups: comp.sys.atari.8bit
- Subject: 1200XL Video/Composite -> CGA/Dead 800 Color
- Message-ID: <9208132126.AA15389@smithkline.com>
- Date: 13 Aug 92 21:26:26 GMT
- Sender: daemon@ucbvax.BERKELEY.EDU
- Lines: 79
-
- 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
-
- *******************************************************************************
-