Re: IIgs to arcade monitor experiment

[MdntTrain <js@cimmeri.com>]

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.

* My question is the same here as with Alex -- would I feed the IIgs
video direct into the opamp sans termination, or would I provide 75ohm
termination for the IIgs signal?

* I think I'm under the mistaken impression that termination is needed
to properly shape the video signal.. lol.. rather than it being to
merely control the characteristic of a "transmission line."   And
you're saying that with the "transmission lines" being so short and
the frequencies so low, that there is no real need to go crazy with
termination values.


 JS