RCA 1861, NTSC and a DEC VR-201

From: allisonp_at_world.std.com <(allisonp_at_world.std.com)>
Date: Fri Sep 24 14:21:01 1999

> Isn't it? Are you absolutely sure about that? I am almost certain that
> the Rainbow video output is _very_ close to the TV scan rates, and that
> drives a VR201 with no problems. It's also a plain composite video signal
> -- there are no separate syncs on the connector.

The VR201 is not tolerent of the wider timing that monitors allow and
also of sloppy levels. It was designed to be the tube for a host of
systems and designed to a narrower spec. the reason for that is it runs
on Pro3xx, DECmate and a few other things without adjustment so it's one
part and no adjustments for centering external. Feed it gook and it does
bad things.

On the other hand the 1861 is sloppy as well get out and relies on
consumer TVs that will generally synce or can be adjusted (back then
Hhold and Vhold were common knobs!). Also some of the comac 1861
implmentations were rather poor about output levels and impedences. I've
seen one that would not drive a tube unless it presented a ~75ohm
resistive load due to a lack of a DC path to ground at the drive end
(crummy circuit!).

I've used Barco and other mono monitors with DECmates and the like and
they output NTSC. Unlike common monitors the VR201 is rather strict
about getting good stuff to give good stuff. It was made that way so that
it could be plugged into any dec system with no adjustment other than
brightness and be in sync and centered.

> Sure it is!. The VR201 is pretty much the same circuit as a number of
> composite video monitors that I've worked on. The only odd features are
> the input connector (a DA15, also connected to an RJ11 for the keyboard)
> and the fact that it needs a 12V DC power supply.

Your right but still worng in the sense I applied it. It also does not
have a Hhold external, Vhold external or access externally to toher than
brightness or contrast. It was factory adjusted to expect a strict timing
for a properly centered image. It can be re adjusted to accept different
but it will look poor on a DECMATE until adjusted back.

Hence the comment that a Vr201 is not a generic monitor, though it's
_possible_ to use it as one assuming the drive source does not take to
many liberties.

Allison
Received on Fri Sep 24 1999 - 14:21:01 BST