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Post by mrbombermillzy on Nov 25, 2019 19:34:52 GMT
Hi all.
Basically, they are using an old IBM MC6845 CRTC based chip, which is very similar to the C128 VDC (well close enough to be able to effectively achieve the same results).
However, dont everyone get too excited, as its only for NTSC composite output. That, and the H and V resolutions get a bit hammered. However, it may be good news for any of our American friends here!
Just thought I'd share here, as it looks like this hasnt been discussed here yet.
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Post by Pyrofer on Nov 25, 2019 20:03:22 GMT
The composite output on the 128 is monochrome only. I have no idea if you can abuse the chip enough to get colour out of that.
That said, my Video Mux has S-Video and composite output from the RGB signal, so it might be possible to do something with that. Sadly not many people have those so probably not worth it.
I did consider making an addition to the video DAC that would have different palettes selected with the user port or another signal from the 128. I am not sure how much use that would be, and again would need people to buy special hardware so not wide spread use.
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Post by mrbombermillzy on Nov 25, 2019 21:42:16 GMT
For some reason, I keep thinking the NTSC C128's can output colour composite CGA... I must be getting rusty!  At a bare minimum though, 16 new colours are available using this method:
Im not sure if this will be consistent on non CRT output devices though.
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Post by oziphantom on Nov 26, 2019 7:03:53 GMT
using interlace mode and differing can get you a lot more shades.
Attachments:
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Post by oziphantom on Nov 26, 2019 7:08:27 GMT
I did a quick study in how it could look in a game situation. This is a CHAR MODE image not a bitmap so it still works fine on 16K VDC.
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Post by mrbombermillzy on Nov 26, 2019 9:02:00 GMT
Nice screen mockup and palette shades diagram Oziphantom!
Regular dithering should allow something like 240 colours.
Please continue Oziphantom with the mockups!
Perhaps you could swap the colours on the 2nd (interlace) frame to get an even smoother colour graduation? You would have to take a photo at a slower shutter speed to show us the actual visible colours though.
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Post by Pyrofer on Nov 26, 2019 9:28:05 GMT
I am still unsure how this will work from a real unmodified C128.
Can somebody do some tests from the mono composite out and see if ANY colour can be gained on a NTSC monitor? (Is there a similar hack for PAL?)
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Post by oziphantom on Nov 26, 2019 10:12:00 GMT
The colour through artifacts is a NTSC is so crap it works kind of deal. PAL can not use such a mode at all. However the VDC is programmable and can be put into NTSC mode, but you would need a NTSC compatible device to view it on still. It seems to mix RGBI into a single signal but the RGB combination is digital. Not sure how NTSC devices decide if something is colour or not though.. My idea was to use something like a Beagle Black board, to pull in the RGBI data, this is digital so it is really perfect for capturing, if only one could get the "new pixels" signal off the VDC externally. Then we can just use the VDC as a digital data stream. And have the beagleboard output a HDMI image with what ever colour, sprite additions we want. mrbombermillzy alternating the attributes per frame is an excellent idea.. |
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Post by mrbombermillzy on Nov 26, 2019 11:15:04 GMT
@pyro: The method I suggested would actually allow 16 colours on NTSC monochrome output 'as is', with up to ~240 with interlace temporal dither techniques. The interlace dithering methods as shown by ozi with regular RGBI output should display a higher resolution image, if not more colour as it is using proper colours. The mono method is using the colour error from a pixel pattern for colour, so resolution is much lower. Its a bit like a poor mans Amiga HAM I suppose! lol. There must still be quite a few people in UK/Europe who have TV's/Videoplayers/DVD players with NTSC signal selection as an option. Maybe the target audience isnt so low (as in some of euro/uk as well as USA). oziphantom : Do you mean something like a framebuffer? |
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Post by Pyrofer on Nov 27, 2019 8:16:21 GMT
Pin2 on the VDC is the pixelclock I think. I use it currently while developing my Scandoubler. I just clipped a wire onto that pin!
You are right capturing the pixels is easy because it's digital. it's doing anything with them fast enough that is the issue. I am still trying to get the scanddoubler working.
If you guys are saying the mono out of the VDC could drive colour on standard CRT devices (a lot of which actually do support NTSC) that would be amazing. I wonder if just seeing the 60hz frequency would be enough to make them try and decode NTSC instead of PAL or if there is more complex detection?
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