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Old 22 June 2012, 18:48   #1
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Changing video port master clock

Okay, anyone who's seen my posts lately knows I've been looking into getting smooth scrolling, V-Sync locked output at both 50hz and 60hz on my CD32. On my PAL machine I've noticed that when it's set to 60hz NTSC the vertical sync comes out as 59.28hz. This is too far out of spec for my TV to get a V-sync lock and so it stutters.

I was chatting to a friend about this and he just e-mailed me with some interesting information:-

Just been digging around (reading the Amiga Technical Reference Manual if you must know ) and I noticed something :- The 28MHz master clock is slightly different for PAL and NTSC (everything derives from this). However, I also remembered it's possible to supply your own master clock on the Amiga via the video port IIRC… I think you can guess where I'm going with this… I'm guessing someone must have tried it, so I'm having a dig/further read. The pins are XCLK and XCLKEN btw. My theory is it might be possible to provide the NTSC frequency to a PAL machine via this, and correct the slight difference (presumably at the loss of PAL TV compatibility, since the colour clock would no longer match). It's quite possibly not workable, but I remember wondering about it "back in the day". That machine is still my favourite computer ever

So, is there anything in this or is it barking up the wrong tree? I wouldn't have a clue where to start with what hardware mod was required, but I am very interested.
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Old 23 June 2012, 18:50   #2
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It's basically what a genlock does. This might not work with non os-friendly software, but I haven't tried it myself.
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Old 23 June 2012, 21:37   #3
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Yeah we were thinking Genlocks, but I never owned one, I've no idea how they work. Can anyone elaborate further?
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Old 24 June 2012, 13:58   #4
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Using a 28.63636 MHz oscillator and feeding it into XCLK (pin 1) should work. Connect XCLKEN to ground with a 100 ohm resistor to enable this mode. 28.63636 MHz oscillators are available as they are 8x the NTSC colour burst frequency, commonly used with video devices.

The slight difference in the Vsync timing will be down to the difference between the 28.63636 MHz crystal of an NTSC and the 28.37518 MHz used on PAL machine. I don't have a copy of SMPTE 170M here but I remember you multiply and divide the reference clock many times to make the line/field sync timings from the one, tight, reference clock.

There are some details on Genlocks in the Amiga developer CD 2.1

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Old 24 June 2012, 18:10   #5
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Ah so we were thinking on the right lines at least. This could be built into a SCART cable then? Sadly I've no soldering/hardware construction skills so I guess the project will forever remain just an idea, especially as I seem to be in the minority when it comes to noticing stuttery scrolling anyway!

If anyone fancies constructing this cable for me I will reward you financially and with a spot of honour on my amazing website, lol
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