VIDEO INVERSION AND YOU!
Using Older Nintendo Monitors in New Conversions
Copyright June 1, 1992 by Randy Fromm
The images you see on any television screen are made from combinations of
red, green and blue; the three "primary colors" of light. By combining
these three primary colors in different combinations, we can make any
color we want. For example, combining any two of the primary colors will
create one of the three "secondary colors." Red and blue combine to make
magenta. Red and green mix to form yellow while blue and green blend
together to create a light turquoise color known as cyan (pronounced sigh-
an with the accent on the last syllable.) Combining all three colors
makes white while the absence of all color creates black.
Video Controls the Color
Most of you are familiar with the red, green and blue "video" wires that
connect the logic board to the monitor in a videogame. On the JAMMA
connector the three video outputs of the logic board are at pins 12, 13
and N. These wires carry the three "video signals" from the logic board
to the video input connector of the monitor.
The video signals control the video amplifiers in the monitor which, in
turn, regulate how much red, green and blue will show on the screen.
Higher voltages at the monitor's video input connector will make a color
very bright (tentative new AMOA standards set maximum video output
voltage from the logic board at 2 volts.) Lower voltages will produce
correspondingly dimmer colors. Naturally, if the video voltage is 0,
there will be no color at all.
Let's say, for example, that we want to display a picture of a brightly
colored parrot on the screen. As the monitor scans the area of Polly's
red head, only the red video signal will have any voltage on it. Since we
want our parrot to have a bright red head, we crank the red video voltage
up all the way to 2 volts. The green video signal and the blue video
signal will both be at 0 volts.
As the monitor scans the yellow ring of feathers around the parrot's
throat, the green gun is turned on in addition to the red gun. The
combination of red and green creates Polly's bright yellow collar.
The green body comes next, so only the green video output will carry a
signal now. But Polly's body is not as bright as his head so the green
video signal will be at a somewhat lower voltage; perhaps only a volt and
a half or so. This will produce a dimmer, darker green color.
Polly's wings are pure blue. Finally, his tail combines bright red with
bright blue to create a beautiful magenta. Here, the green video is at 0
volts with the red and blue video at 2 volts each.
Has This Happened to You?
You swap one Nintendo monitor for another (or install a Nintendo monitor
in a non-Nintendo videogame or use a Nintendo monitor with a conversion
kit) and the colors come out very strange. Objects that are supposed to
be blue are now bright yellow. Green things have turned a lovely shade of
magenta. Even the completely black background has suddenly changed to
bright white!
What happened? Re-read that last sentence above for an important clue. If
the black part of the screen has turned white, the color signals must be
working backwards! Instead of turning ON each color by applying a
positive voltage to the video inputs, the Nintendo's red, green and blue
circuits are being turned OFF! Likewise, when the video input voltage is
0, the colors are turned on full blast. In other words, low voltage (or
no voltage) turns each color on while higher voltage turns the colors
off!
It's called "negative video" and all the early Nintendo games were
engineered this way. Although the negative video system works perfectly
well, it clearly did not become the industry standard as all other
videogames produce normal, positive video signals. Only Nintendo used the
negative video system.
What happens if you run a positive video signal into the negative video
input of a Nintendo monitor? Let's try it with Polly and see what happens.
Polly's head is supposed to be red but now the red video amplifier will
be deactivated by the high video voltage and the blue and green video
amplifiers will be turned on full instead. Polly's head will be bright
cyan.
Polly's yellow collar has turned blue while his blue wings are now yellow.
His green body is now magenta and his tail is green. Everything is just
the opposite of what it should be! They don't call it negative video for
nuthin' you know. The picture will look like a color film negative!
Converting Positive to Negative
Nintendo games have two different monitors. One type is manufactured by
Sharp (not to be confused with Sharp Image.) The other is manufactured by
Sanyo. It's easy to tell the difference between the two. The Sharp has a
row of adjustments in the front of the monitor, just under the picture
tube. The Sanyo adjustments are on a separate little printed circuit
board at the back of the monitor.
In some versions of the Sanyo monitor, you will find a small printed
circuit board mounted on the right side of the monitor. It's called a
"video inverter" board. The video inverter board will flip the video
voltage upside-down, changing positive video into negative video or
negative video into positive! In this case, the video inverter board will
let you use the Sanyo monitor in any of today's standard positive video
systems.
During my Arcade School in Orlando, FL last month, one of the students
brought in an old tempest cabinet that had been converted into an
Arkanoid. The old X-Y monitor had been replaced with a Sanyo (with an
isolation transformer added to power the monitor. See RePlay January 1990.
) The game looked fantastic; especially with that dark black glass that
Tempest had covering the screen.
To use the Sanyo monitor (which requires negative video) the positive
video output of the Arkanoid logic board is connected to the signal input
of the video inverter board. The inverter changes the positive video into
negative video. The inverted video is then connected to the video input
of the monitor and everything looks perfect!
If you have a Sanyo monitor without the inverter board, there's still a
way that you can use it with positive video. As a matter of fact, the
monitor was originally designed to accept positive video but was modified
by the removal of a small handful of components and the installation of a
few jumper wires.
To convert the Sanyo monitor to accept normal, positive video:
1. Remove the wire jumpers that have been installed in place of
transistors TR201, TR202, TR203.
2. Remove R213, R214, R215.
3. Install the following components:
a. R201, R202, R203 100K
b. R210, R211, R212 100K
c. R213, R214, R215 820 ohm
d. R217, R218, R219 560 ohm
e. TR201, TR202, TR203 2SC1815 or equiv.
f. C202 220uf 16V
g. D210 1N4004
Pretty simple, huh? To make things even easier for you, a complete kit of
parts including a schematic diagram and step-by-step instructions is
available from Zanen Electronics 3109 2nd. St. Lubbock, TX 79415 (806)
793-6337. The cost is just $4.95. Ask for kit number 910. It's a great
deal so go back into the shop right now and look at those Nintendo
monitors you have sitting on the shelf. If they're Sanyo, you should be
able to use them in just about any videogame.
A entire video inverter board is also available from Louise's Harness
Shop 712 County Road 58 Prattville, AL 36067 (800) 365-1941 FAX (205) 365-
4435 It's called the Color Conversion PCB and sells for just $22.00.
My thanks to Larry Rosenthal and monitor mavin Aubrey Jones for their
research assistance.
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RANDY FROMM YMNP18A@prodigy.com
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