[Coco] again: CoCo RGB to VGA conversion

[Kevin Diggs]

Hi,

	I don't think trying to get it sharp is the problem. The
problem is that a VGA monitor is to sharp.

	I don't see how buffering a "field" will do any good? Unless
you are playing a game, the display is changing slowly (at least
relative to the screen refresh rate). This degenerates into scan
doubling. Only with a lot more complexity to get it.

	The statement that "scan doubling" makes pixels double
height is nonsense. Keep in mind we are filling the same vertical
space. We are just doing it with twice as many scans (that are half
as thick). The pixels will look different. You will be able to clearly
see that they are made up of two scans. This will probably have
some psychological impact on "how they look".

	I do agree that without special processing the display will
look different from the CrapMaster-8. It would probably be possible
to add some horizontal "fuzz" to more closely simulate the CM-8
(perhaps adding a 1F capacitor to the output lines would match the
CM-8). Adding vertical fuzz would be MUCH more difficult. And
almost certainly not worth the cost. Another possible solution is
to take a piece of plexi-glass and blur it using sandpaper. Put
this on the monitor when using it for coco displays!

	I would gladly accept the vertical blockiness that scan
doubling will cause to get the increased horizontal sharpness.

	I am fairly certain that the coco is NOT interlaced. There
is only 1 field.

					kevin
> 
> Yes, it will always be blocky which is why I said that the only way to get
> it sharp is to increase the resolution of the CoCo to produce at least a
> true res (not scan doubled) to 640x400. Of course the GIME can't produce
> this so we're stuck with an output resolution of 640 x 200. Now to simply
> scan double to create 640 x 400 video scanlines simply takes the 640x200 and
> makes the pixels double-height ( multiplying the vertical resolution by 2).
> This is what I mean by making it look even more blocky. Also, because a VGA
> monitor is sharper, a pixel will have a distinct squareness to it unlike the
> lower resolution monitor which still creates a reasonably round pixel (plus
> blurring) with the blank gaps in between.
> 
> There are 2 tricks that may be possible:
> 
> The CoCo to VGA converter has to do more than scan double, it has to fill
> the blank scanlines with valid data. This was the idea behind when I said to
> create a device that captures 1 entire CoCo frame, keeps it in a buffer then
> displays the next frame to the VGA but using scanlines from the buffered
> image to fill the blank scanlines, this giving a full 640x400 resolution (in
> other words a true VGA). Unfortunately, this circuit would be very complex
> and expensive so it will never be a viable project.
> 
> The other idea is a de-interlacer. Sockmaster has found that using the 1986
> version of the GIME, it can be fooled to do interlace video (640x400
> interlaced). A circuit that de-interlaces to a standard VGA monitor could do
> this. Unfortunately, the 1986 GIME also has a few more bugs that were
> corrected in the 1987 version and I suspect most people are using the 1987
> GIME.
> 
> I run the MESS emulator with the scanlines option on (it simulates the blank
> scanlines to make it look like a real CoCo on a CM8 type monitor). I find it
> looks much better, not sharper, but less blocky that when run without
> scanline (scandoubling). Graphics with scandoubling starts creating the
> appearance of a much lower resolution because of the emphasis of square
> pixels.
> 
> Anyway, that's my spin on it!   :)
> 
> Nick.
> 
>