Pete Turnbull pete at
Mon Nov 5 16:47:40 CST 2007

On 05/11/2007 12:49, Jules Richardson wrote:

> No, I don't need a PC. I just need *something* in the US that can cope 
> with typical UK-style picture formats (typically 'home' micros in the UK 
> will be designed around PAL signals at 625 lines interlaced I expect, 
> whereas I expect US micros of the 80s were geared more toward NTSC 
> displays of 525 lines)
> I'm not sure if VGA will cope either - I don't think a typical VGA 
> monitor will sync down to the frequencies involved (i.e. converting TTL 
> to the necessary 'analogue' RGB of VGA is the easy bit :-)

It's a pity you didn't ask this yesterday when you were here: I could
have shown you my workshop monitor.  It's an Acorn AKF53 connected to an
ordinary 4-way KVM switch (only the Video part used) to which, in turn,
are connected a BBC B's RGB output, a PC's VGA card, an Archimedes, and
something else (usually).  All work.

I can't remember what modes an AKF12 or ADF32 support, but if one of
them is similar to my AKF53, that's your choice.  If it works for a
Beeb, it should work for other machines that have similar RGB outputs.

Alexis' idea about the circuit should work too.  His surmise about 1V
pk-pk for VGA is correct; I've used a similar circuit to do exactly
that, and something not too distant to combine all the signals to make
composite video (I could have shown you the ISA card for that too,
though it only uses the ISA bus to get power).  If you need to merge
hsync and vsync, a simple XOR gate (1/4 of an LS86, for example) will do
well, but often you can get away with just combining them, wire-or
style, possibly with a couple of small-signal diodes.  Some monitors
don't mind the hsync disappearing from the composite signal during the
vsync pulse; others prefer it to be present (inverted, if you use an XOR).

Have you tried any flat-screen monitors?  Some of them are designed to 
work as TV displays, and will handle horizontal rates in the 15kHz range 
as well as signals in the VGA (etc) ranges.

Pete						Peter Turnbull
						Network Manager
						University of York

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