JHSangster@DOCKMASTER.NCSC.MIL (02/24/90)
Oops, too bad this "reply" function won't let me fill in a Subject..
For Ronnie Hughes: IDC's phone number is... (815) 968-2228 (voice).
BBS number is 968-2229 and fax is 968-6888
Address is 1220 Rock Street, Rockford, IL 61101-1437. Info accurate as
of December 1987, hope it's close enough. -=-=-=-=-=-=-=-=-=-=-=- For
Nick DiMasi and others, info on 800XL chroma mod and whatnot.
Mapping The Atari, Revised Addition, page 237, shows the connector
pinouts for the "800 and XL/XE's" as follows:
Audio output 3 1 Composite luminance
Composite chroma 5 4 Composite video
2
Ground
The "notch" in the connector is at the top, opposite pin 2 (ground).
In actuality, Composite Chroma is NOT brought out in the 800XL, and
presumably also not in the XE series. The mod that was posted, which I
think comes from the Feb. 1986 issue of ANTIC, shows a 200-ohm resistor
brought out from the junction of the existing R67 and R68 to pin 5 of
the monitor jack. For those of you who don't have the whole schematic
committed to memory, R68 is a 3000-ohm (orange-black-red) resistor which
is coupled by C55 (100pF capacitor) from the video amp (Q3) output where
it connects to the modulator at pin 1. R67 is a 75-ohm re- sistor which
leads from the color amp (Q5) out through C54 and L7 to the composite
video output (monitor jack pin 4).
Here's an attempt to render the diagram. Turn on your PostScript
interpreters...
Q3 (video amp)
emitter -------/\/\/\/-----\_/\_/\_/\_/-----+---------> Modulator pin 1
R54, 1200 ohms L6, "2" (uH?) |
|
= C55, 100pF
|
Z
R68, 3000ohms Z
Z
| 200 ohms (new chroma out)
+---/\/\/\/\-----> Monitor
| (added) Jack, Pin 5
| Q5 color amp |
emitter -----------------------------------+---/\/\/\/\--||--- - -
R67 C54
75 ohms .001 uF
I'm not entirely certain why the resistor value chosen was 200 ohms. If
you are driving 75 ohm cable with it, you might find that the picture is
sharper if you use something in the vicinity of 47 to 75 ohms. I'd try
47 or 56 ohms myself, to give as close to 75 ohms source impedance as
humanly possible. HINT: emitter followers at low frequencies exhibit
an emitter output source impedance of around 26 ohms at room
temperature. Add that to the resistor value to get a rough idea of your
actual source impedance.
Incidentally, I thought I sensed some confusion out there. The output
that the Atari 8-bitters provide (incl XL/XE with the above mod) is your
choice of composite video, composite luminance, or separate chroma.
Composite video is made up by combining chroma and luminance signals and
putting the chroma in at the color subcarrier frequency of 3.579545 MHz.
The signal is "double sideband", and the lower sideband falls well down
into the band occupied by the luminance signal, which starts at DC and
extends upward. If you could let it go up to 6 MHz or so, you would
have REAL SHARP picture resolution. Obviously, with color stuck in
there, you have to do something to limit the bandwidth of both luminance
and chroma signals so they will not spill over and interfere with each
other. Actually, there is a bit of black magic to it, and the system
actually has been made to work better than theoretically possible, sort
of like the bumblebee or the analog magnetic tape recorder, neither of
which can possibly accomplish what they do.
Anyway, by keeping the luminance and chroma on separate wires, you don't
have to limit their bandwidth, you can preserve it and have an
incredibly sharp picture.
What any monitor must do, however, is to take the chroma and the
luminance signals and "decode" them, using, typically, a chip called an
"NTSC Decoder". (NTSC, of course, stands for "never twice the same
color".) An RGB monitor, on the other hand, expects the decoding to
have already been done for it. To hook an RGB monitor to an Atari
8-bitter, you must supply your own NTSC decoder circuit, to convert the
composite chroma and luminance into the R, G, and B signals.
According to a cousin of mine who is both an Atari 800XL fan and the
chief engineer of WAAY-TV in Huntsville, AL (and also a former
professional video special effects equipment designer), if you buy an
off the shelf NTSC decoder chip and hook it up in the obvious way, you
will likely run afoul of some funny "matrixing" that is done in the chip
which will give you slightly incorrect colors. Apparently the tubes
used in TV sets want something other than pure R, G, and B signals, and
that's what the typical chips give them -- something other. My cousin
says that the right way to build the device you need is with separate
balanced modulator chips and some fine-tuning adjustments that will let
you balance the colors correctly.
If anybody REALLY wants to build such a device, I will put you in touch
with my cousin and a friend who I think built one under his tutelage.
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Finally, the joystick ports... whew, had to sweat a little to find
this, it is on page C-1 of the 400/800 hardware manual.
1. Forward Input 6. Trigger Input
2. Back Input 7. +5 Volts
3. Left Input 8. Ground
4. Right Input 9. "A" Potentiometer input
5. "B" Potentiometer input
Physical layout looking at the jack 1 2 3 4 5
o o o o o
o o o o
6 7 8 9
I hope this answers at least all the questions I was trying to help
with!
-John Sangster SPHINX Technologies, Inc. (617) 235-8800