🚀 go-pugleaf

In article <1326@mcrware.UUCP> tim@mcrware.UUCP (Tim Harris) writes:
>
>	Has anyone out there seen any plans for a homebrew Genlock device.
>To me it seems like the simpler Genlocks like the MiniGen can not be
>that difficult to make, I suppose timing would be the major issue that
>could cause trouble. 
>
I worked on one of these in my spare time for about two months once. Let me
point out that I knew almost nothing about phase locked loops when I started.
I know quite a bit more now. The first issue that needs to be dealt with is
horizontal and vertical sync. The RGB port has a horiz and a vert sync output
line. These lines double as inputs in genlock mode. When the system boots
up it checks these lines for proper horiz/vert timing signals. If they are
present, it uses them as inputs otherwise it generates its own.

	Ok, now all you have to do is seperate the sync pulses from your
external video signal. I spent a lot of time looking at various schemes for
this (I have schematics for at least 5 devices that have a sync seperator
in them. Every one does it differently.) and eventually came up with 
something that would work, but not well. This problem has been solved now,
however. Nat. Semi. came out with an 8 pin ic, the LM1881, about 6 mo. ago.
This chip seperates sync signals from even bad video signals with only 2 or
3 external components. Anyway, you should be able to use this directly with
the h/v inputs on the Amiga. 

	Now on to the harder problems. H/V sync will allow you to overlay
video, but very poorly and without color. To get a solid overlay with color,
you have to sync the amiga clock with the colorburst in the video signal.
This means you have to take a 3.579 MHZ signal and generate a 28.63 MHZ
clock from it. To do this you need some sort of PLL. I put one together
using a NE564, but despite days of work, I could never get it to lock on to
28 MHZ. Despite my center freq setting, it insisted on remaining unlocked or
locking on to 14 MHZ. If anyone tries this and gets it to work I love to
see the circuit. If you can get this working, you're at least half way there.

	Oh yeah, I forgot, you also have to get the colorburst signal out
of the ext. video signal. I also spent a while on this, and my failure to
get it working to my satisfaction is also probably why the PLL circuit
wouldn't work correctly. In any case, I haven't tried it, but the LM1881
also has a burst output that (with some sort of switching) should allow you
to pull the colorburst off of the video signal quite easily. Hmm, maybe
I'll even give this a try now that I thought of it.

	The next thing to deal with is switching. The amiga has a handy
output that switches when the color displayed is 0 (background). Hooking this
up to a 4051 CMOS analog switch will do the switching pretty well.

	That's it, unless you want RGB output as well (or own a A500 with
no color composite out to mix with the ext. video source). For that, you'll
have to get an NSTC->RGB chip. I've never tried this, but I've seen enough
postings about it to know it's not an entirely trivial task. 

	Then, of course, for 500 owners, you'll need an RGB->NSTC 
converter too.  

Anyway, as you can see, it's not a trivial project. I eventually gave up
(because of the PLL and color problems) and bought a closeout Commodore
genlock for $99. If anyone goes anywhere with this, however, I worked on it
long enough that I'd still be interested in seeing the plans.

---------------------------------------------------------------------------
Steve Ludtke
stevel@tybalt.caltech.edu              ..!cit-vax!tybalt.caltech.edu!stevel
stevel@citiago  (Bitnet)               
72335,1537      (Compuserve)                          I DO ...