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Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-24, 19:28

path A (master clock to pixel clock) than there are on path B (master clock to color burst/chroma yellow flip-flop) Which reminds me: the CGA wikipedia article painfully needs updating in this regard. At least the 1501981 (haven't checked the earlier schematics) bases both the pixel clock and the …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-23, 16:09

It seems like you're suggesting that DOSBox should make the CGA composite colours look as much like the RGBI colours as possible Again, no. My reference is the Apple II's low resolution and double high resolution colors, which are identical to CGA's 640x200 mode from the point of view of NTSC …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-23, 10:28

If I'm understanding it correctly, the problem is this: There are some games where the art was originally made for EGA/Tandy/PCJr 160x200 16 colour mode (which has the standard RGBI colours) but which also support composite CGA (which has a completely different set of colours). That may be …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-22, 20:37

Conclusion: Make it look like it did originally instead There is no such thing. With composite color artifacting, there is just a tremendous variation of monitors, televisions with their own idiosyncracies, multiplied by the number of possible viewer-adjustable settings. It's not about improvement, …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-22, 11:33

Checkerboard" pixel patterns tend to produce alternating lines in composite mode that look like they could apply to a comb filter, but they actually don't. You can see in several of Servo's screenshots for Demon's Forge where there are alternating lines that are not affected by the filter. The …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-22, 08:28

By the way, I just noticed when taking the luma signal from the comb filter instead of the notch filter, the dark gray is no longer solid even when post-filtering the luma to get rid of vertical bands. See the picture I posted before on the last page, now with luma from the comb filter instead of …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-22, 08:14

When were comb filters first introduced in composite color computer monitors or in consumer TVs? I know nothing about computer monitors, but in TVs, it was the mid-to-late 1970s. More expensive models first, of course. Don't they amount to the same thing? (Assuming the filters are exactly matched). …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-21, 18:52

I think that happens because both Servo's capture card and your decoder are sampling the luma on the scanline and the chroma "in between scanlines" (i.e. the chroma comes from the output of the comb filter and the luma comes from the output of the notch/bandpass filter). Not quite. See below. …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-20, 22:14

Accolade evidently used a Tandy system to test Blue Angel's composite mode. The Tandy has a shift between the color subcarrier and the pixel clock of 135 degrees compared to the IBM CGA. For what it's worth, as promised earlier, a picture from the game without and with comb-filtered grays.

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-20, 19:57

I'd like to see an example of a CGA game which uses it. Blue Angels: Formation Flight Simulation uses the dark gray I mentioned earlier. I'll post screenshots later. As for the Apple II, Karateka has a floor that alternates between blue and orange, which a comb filter would filter to gray. The …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-20, 17:58

it's a classic 3-way engineering tradeoff: attractiveness, adherence to standards, ease of implementation - pick two. I'll leave that decision to others. Did that actually work on any real monitors? Given the sheer number of Apple II (and a few CGA) games using the technique, I would assume so. It …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-20, 16:36

So the best we can do is to make it as objectively correct as possible (according to the relevant standards) and then provide knobs that people can twiddle if they aren't satisfied with the outcome Okay, but a hue knob alone won't be enough to do that. You would at least have to provide the user …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-20, 11:46

Well, it can still be optimized, of course. :) But you notice that the grass in King's Quest is the same color as Graham's pants in King's Quest II , which are cyan in RGB. The booter version of King's Quest translates the same RGB colors differently to composite colors than King's Quest II . In the …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-20, 10:13

Thinking about it, I'm in favour of using the theoretical ideal NTSC-to-709 matrix since (in combination with the palette method) it's not too slow. I think that was NewRisingSun's preference too. Again, it depends on what your goal is. If you do it that way, you can always claim that you are …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 21:00

That's interesting that the saturation is supposed to depend on the color burst amplitude - I hadn't heard that before. The standards documents are written in beautifully inconvenient language. I'm quoting from SMPTE-170M: "A gated and filtered signal derived from subcarrier, called the burst, …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 19:48

That's not what happens on my TV though In any case, the CGA does not generate proper video levels, and having gone back and forth between several ways of dealing with this, I think for emulation purposes, the best way is to just normalize everything so that black really is at 0/0/0 and white is at …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 17:36

As for doing color correction the right way - for now I can only take notes from you and NewRisingSun. If there's a consensus on how to apply the proper corrections to your current code, I'll add that in too. There will be no consensus until we have decided what we actually want to depict: a …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 17:13

we'll get a 0.5V signal after the band-limiting, which will correspond to a grey much darker than 50%. As it should. If your purpose of performing the band-limiting is to replicate the blur on a monochrome monitor, then you would be right, and you would have to linearize the signals before applying …

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