Alternative Monitor for VAXmate

[Paul Koning]

> On May 23, 2020, at 9:45 AM, Rob Jarratt via cctalk <cctalk at classiccmp.org> wrote:
> 
> As it looks like I am not going to be able to repair the monitor board for
> my VAXmate I am wondering if I can do anything with the outputs from the I/O
> board to drive an external monitor instead.
> 
> 
> 
> The connector to the monitor board has RGB+Intensity outputs at TTL levels.
> The horizontal sync has a frequency of 26.6KHz, active low with the high
> voltage 3.7V, Vertical sync is 60Hz. I don't believe that corresponds to any
> known standard, does it?

I assume it's the same as the Pro video.  That might not exactly match standards.  But the intensity output is an acceptable 480i monochrome video signal.  I've fed mine into a video capture device and into a TV display, both work.

> I had a go at building this
> http://www.dasarodesigns.com/wp-content/uploads/2014/03/pet-composite-video-
> adapter.jpg feeding its output to a composite to VGA device to see if it
> would convert it to VGA, but no luck.

VGA has double the horizontal frequency, so that's going to be problematic.  What should work is to convert the RGB signal to "component video" and feed that into a TV monitor that has such an input with 480i support.  There are some simple circuits for doing this in design notes from various chip companies.  One that uses a triple video amp is nice except that the device only comes in a microscopic size SMT package.  Older devices should work too, though.

One complication is sync.  The Pro has sync on the monochrome signal as well as on the green one, but if you convert the RGB to component video, sync needs to be on the "Y" output and G is only a portion of that, so it probably won't be good enough.  The intensity signal could be used as Y, except that in the case of the Pro it isn't done right -- it should be a weighted sum of R/G/B with most of the weight on G, but instead DEC simply made it R+G+B (equal weights).  

I'd like to build a circuit to do this, with a sync separator, some video amps to do RGB->Y/Pb/Pr and reinsert the sync.  Maybe in a few months.

	paul