[Coco] again: CoCo RGB to VGA conversion

[Nickolas Marentes]

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

It's not sharpness that is the problem, it's blockiness.

>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.

I don't think you understand when I say buffering. Let me make it clearer..

How many horizontal scanlines does the CoCo3 produce?  Answer = 225 max
What is the horizontal resolution of standard VGA (not SVGA or XGA)?
Answer = 480

So how are you going to display 480 horizontal scanlines (full screen) on a
31Khz VGA monitor when your CoCo can only produce a max of 225?

Scandoubling.

You take each scanline produced from the CoCo3 and send it out twice. 2 x
225 = 450.

This effectively makes every pixel double height or if you like, fills the
vertical space between each pixel with an extra scanline which is a copy of
the previous one.

What is ideally needed to avoid scandoubling is to replace the doubled-up
scanline with a fresh scanline made up of new data. It's complex to explain
but essentially we're creating an interlaced display that is not interlaced
(this won't make sense but maybe someone else can describe it better than
I).

>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".

A pixel is not defined as one scanline. A pixel can take up as many
scanlines as it takes to display the graphic resolution. In this case, the
CoCo3's resolution is unchanged, only the number of scanlines being used to
represent it. The scanlines are closer together (but there is still a fine
black division between them) which makes each pixel appear larger and
"squarer".

>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!

You're trying to be funny right?  :)

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

I know what you mean, I have a CM-8 and it is fuzzy on the horizontal
resolution. It is afterall a budget 15khz analogue RGB monitor. I also have
an Amiga 1942 monitor that displays .21 dot pitch (same as most VGA
monitors) and the horizontal resolution is great!

The vertical blockiness would give me the sh*ts. Graphics actually look
"lo-res". I guess everyone has a personal preference.

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

Maybe someone can clarify this. The coco sends a frame 60 times per second.
Each frame is overlayed over each other the same. What Sockmaster has found
is that he can fool the 1986 GIME into dropping down a scanline, into the
black region. By alternating 2 frames of data with this scanline skip, he
simulates an interlaced VGA resolution display (15Khz still). (He write a
terminal program that utilizes this mode called Twilight Term). This is why
I thought a de-interlacer circuit would then convert it to a nice
flickerfree VGA 31Khz output.

Maybe Sockmaster can clarify?

Nick

Nick