Michael J. Mahon wrote:
I understand your preference, but I think you would find it much simpler in reality to build an "Apple scan doubler" that grabbed the dot clock from the Apple (AUX slot?), sampled the composite video signal (it's digital, after all), and "played it back" at 2x the scan frequency for a VGA monitor, using an appropriate RGB conversion. This would produce an "emulator style" VGA display, which, I expect, would satisfy many users--though perhaps none perfectly. ;-)
Ummm, I admit I've not much experience with video processing but I'm not sure it's so "simple" to build a scan doubler.
The chrominance information in a composite signal is encoded via a phase offset from the colour burst. This is not trivial to recover, and certainly can't be done by sampling at the dot clock rate. It is possible to generate a limited number of colours using 'digital' logic but that requires a clock 4 times (for example) the burst frequency.
In any case, if you're sampling at the dot clock rate, then you can't reproduce any analogue artifacts such as blurring, enlarging due to contrast, etc since you only have a single value sampled at the 'centre' of the pixel.
At least that's how I understand it.Regardless of all this, my decision to do it this way was heavily influenced by the fact that I have a few other features (video and non-video related) that I'd like to incorporate on an expansion card.
So I'd be mucking about with an FPGA in an Apple slot... Regards, -- Mark McDougall, Engineer Virtual Logic Pty Ltd, <http://www.vl.com.au> 21-25 King St, Rockdale, 2216 Ph: +612-9599-3255 Fax: +612-9599-3266