TTL RGB ==> PC

[Jules Richardson]

Pete Turnbull wrote:
> 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. 

Heh - oops :-)  I'm thinking that taking monitors overseas with me isn't much 
of a long-term solution though; they'll fail eventually and patching them up 
US-side is less than ideal.

Having a gadget available that's either readily obtainable within the US or 
easily buildable seems like a better long-term solution; it's just a question 
of whether the slight (typical) frequency differences / screen geometries 
between the two markets are going to cause a problem in finding something that 
works off the shelf.

> Alexis' idea about the circuit should work too.  His surmise about 1V
> pk-pk for VGA is correct

I didn't think many VGA displays would sync down to the frequencies used by 
the older kit, though?

> 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.

Yep, I've done the  LS86 'trick' before for combining syncs - but lots of the 
old kit I have outputs composite sync, and for driving something like VGA I 
expect I'll need separate syncs. Of course I suspect I can tap into them 
somewhere within the circuitry of whatever system's hooked up to the display 
(ISTR that splitting a composite sync signal is a bit complicated, but it's 
been a while)

> 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.

Hmm, I've not tried that. The 19" Hercules LCD I have might work I suppose 
(and it's so awful for normal work that it's getting dumped for a good 
old-fashioned CRT soon as I get my stuff across the pond anyway)

cheers

Jules