On Apr 22, 5:53 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
The sync level and impedance is fine--for short lines (and anything
over 500 Ohms has to be a short line ;-)--the only requirement is
that the load (the monitor) not load down the source, so any load
impedance more than 5x the source impedance is fine.
* Ok, this one I understand from working with tubes. ;-)
The RGB signals are more interesting. 4vpp is a pretty high level
for an analog input, so I'd check to be sure that it isn't actually
TTL.
* It is indeed analog. On the monitor side, the signal comes in,
sees a 5k terminator, then a series 4.7k before entering as the base
of a simple 3 stage transistor amplifier controlling the CRT cathodes
-- very direct CRT control needing relatively high input voltages to
start out with. 4-5V is standard for analog arcade monitors, strange
to us computer guys used to 0.75V-1V.
If it is analog, then you'll need some gain (and possible a polarity
inversion). The right answer is three video op-amps, but since the
load impedance is so high, you don't actually need much current, just
voltage gain. I'd start with a quad op-amp with a unity-gain frequency
in the hundreds of megahertz (which isn't too hard), and build either
non-inverting or inverting gain-of-4 amplifiers. You can probably
find these in a single package and operating with a single (+5) supply.
Again, for short cables, you don't really need to terminate the RGB
cables, which will typically provide you with 2x the terminated
voltage, so you may actually need only 2x amplifiers--so make the
feedback resistors easy to change or even variable to set the
contrast.
* Ok, so you're suggesting the same thing as Alex... except you appear
to be saying that I wouldn't need to add 75 ohm termination on the
monitor side (Alex described impedance matching the op-amp outputs
with 68 ohm series, and 75 ohm parallel on the monitor side). The
cable from video amp to monitor will be no longer than 6ft.