Re: Innovative Amiga genlocking

Den Mon, 6 Nov 2017 21:12:10 +0100 skrev silverdr@wfmh.org.pl:
> 
> > On 2017-11-06, at 21:04, Gerrit Heitsch <gerrit@laosinh.s.bawue.de>
> > wrote:
> > 
> >>> What pulses? Making hsync and vsync inputs instead of outputs?
> >>> It's done via software (*). XCLKEN only switches the clock signal
> >>> from internal crystal to external input, nothing more.
> >>> 
> >>> (*) Should be Bit 1 in BPLCON0 Register at $dff100
> >> That's the internal DDR I thought needed to exist if XCLKEN
> >> doesn't affect the sync pins data direction. Now the good question
> >> - it's been ages ago, but... I /think/ I didn't have to run any
> >> software in order to genlock the machine. Would that mean that we
> >> (both smf and me) were wrong about how the genlocking was done? Or
> >> is there a way to detect _XCLKEN state via software and the rest
> >> is done by KS?
> > 
> > I see XCLKEN go only to the 74F258 on the A500+ schematics. And if
> > you think about it, you don't really need external resync, you can
> > use hsync and vsync in output state to sync up with the source by
> > varying the clock supplied to the system until everything matches.
> > Kind of a PLL.
> 
> That's exactly what I understand Mia wrote and I rephrased: "Meaning
> you have to slow down or speed up the Amiga clock and wait for the
> syncs to align their phases. This takes time. Not much but still.
> Then you need to constantly monitor the two for drifting and react
> accordingly by either speeding up or slowing down - basically a form
> of PLL."

Really old tellys, like from the time where active components were
expensive (valves/tubes and the early transistor TV's) almost did that.
But it usually took far less than a second. Although there were no
pixel clock, they slowly synced up their local oscillators for hsynk
and vsynk and you could se the picture roll around for a short moment.

> If that's how it actually works then I stand corrected. I (and as I
> understood smf too) thought it was done by supplying the reference
> pulses to Amiga so that it "knows" when to start the line/field. I
> thought that was the purpose of having the possibility of sync pins
> to act as inputs.

I thought so too before I got hold of an actual genlock. :)



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