Re: C65 on Ebay

Hi!,


On 2017-11-04 14:31, Mia Magnusson wrote:
> ...
> But there is some slight difference in the crystal frequency afaik,
> maybe that also affects the RF modulator. 

There is, and it does affect how the color is encoded indeed.

For a PAL machine, the clock is 4.43361875 * 8 / 5 ~= 7,093MHz. For an
NTSC one, it is 3.57954 * 2 = 7.15908MHz. As you can see even though the
machines don't produce composite color by themself, they were still
designed so that their pixel clocks could be in sync with the color
subcarrier frequency of the respective composite video standards (to
have a chance to avoid the floating color problem and similar artifacts).

Neither I have seen an NTSC A520, yet (quick googling tells me it does
exist). I have seen and modified a PAL A520, to show the video signal of
an Amstrad CPC (another RGB machine) using a projector that only
accepted composite. The PAL A520 does take the master clock (and
consequently, the pixel clock) of the Amiga into account. In the
schematics, you can see that the circuit is based on a MC1377 composite
encoder. The MC1377 needs a color subcarrier clock (PAL or NTSC,
respectively) to work. In the A520, this oscillator part of the MC1377
slightly deviates from the one seen in the original application note -
it effectively syncs the color subcarrier oscillator to the clock it
gets from the machine.

http://members.iinet.net.au/~davem2/overclock/A520-standard.png

Speaking of the RF modulator itself, that one just needs to be a
different part for the PAL and NTSC A520 (--> different channel
allocation and width, different sound subcarrier frequency etc. etc.).

> Most TV's are rather tolerant
> because the sync frequencyes from a VCR's can vary quite a bit if the
> tape for some reason (temporary) runs too slow or too fast.

For video sync, yes. For color subcarrier, less likely. I'd guess the
PAL A500 probably produces a black and white image with an NTSC A520 and
vice versa. Even if the picture of the machine succeeds to show up in
colors, it must produce noticeable color image floating artifacts.


Levente

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