[Coco] Coco game engine demo

[Gene Heskett]

On Monday 04 February 2013 00:27:26 Arthur Flexser did opine:
Message additions Copyright Monday 04 February 2013 by Gene Heskett

> That's interesting and unexpected.  Any idea why they chose 60000/1001
> instead of 60 fps exactly as the NTSC standard frame rate?
> 
> Art
> 
> On Sun, Feb 3, 2013 at 11:42 AM, Richard Goedeken
> 
> <Richard at fascinationsoftware.com> wrote:
> > Art,
> > 
> > That is a very astute observation, and actually illustrates why my
> > number is correct.  The field rate for NTSC video is not exactly 60
> > fields/sec, but 59.94, or more precisely, 60000/1001.
> > 
> >> Date: Sat, 2 Feb 2013 22:34:01 -0500
> >> From: Arthur Flexser<flexser at fiu.edu>
> >> 
> >> 
> >> The frame rate seems like it would only equal 16.683 milliseconds per
> >> frame (at 60 frames per second) if you were using those jumbo seconds
> >> that are made up of 1001 milliseconds each, no?
> >> 
> >> Nitpickingly yours,
> >> 
> >> Art

That difference isn't quite exact Art, but close.  When we started black & 
white broadcasting in the late 1940's, the line & field rates were chosen 
as 60hz so any hum would be stationary, and an h-rate of 15,750 Hz.  But 
when color came along and they wanted to fit the color signal into the B&W 
signal in a way that could mathematically be separated back out, so the 
color subcarrier was chosen as 3.57954545454545 because that is as close to 
3.58 mhz as they could divide and multiply the output of a rubidium 
frequency standard which is 5000000.00000000000 hz.  That is accurate to 
about 12 or 13 decimal places.

Because of that error, and the mathematical relationships of the rest of 
the signal. the h-rate was down shifted to 15,734 hz, and the vertical rate 
was downshifted to 59.94.  The end result of all this tom foolery was that 
the color dots on a B&W screen were interlaced horizontally, hiding the 
potentially vertical lines that strong colors would have generated.

Those signals can be fairly well separated again, in a filter method called 
a comb filter, which, because our early computers were not interlaced, 
doesn't work at all well when looking at a home computers output on a color 
tv.

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Cheers, Gene
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