Author Topic: Commodore 128 to Video Converter  (Read 2629 times)

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Offline RobClaggy

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Commodore 128 to Video Converter
« on: November 18, 2021, 10:22:25 pm »
I have a Commodore 128 (that of course has two outputs, an RGB, and a composite.  I bought a GBS-8200 video converter to allow me to connect the C128 to newer monitors and such.  This is the second different type of converter I have bought!

None of them seem to work.  I mean, the CGA side is no big deal.  But I want to output the RGB signal on an LCD HDTV that I own.  When I plug the RGB into the 8200 I get just lots of lines and distortion.  Nothing I can really make heads or tails out of.

So with that a bust I tried a different tactic. When I look the GBS-8200 manual, it quite clearly shows how to wire the RGB direct into the pins on the harness.  So I chopped up an old RGB cable I had and soldered the pins directly.  Same exact thing.  Lots of lines and distortion and no signal.

Can someone shed some light on what I am doing wrong here?  I don't see any other configuration options that are relevant.  I know both the monitor and the C128 are working!  I guess the converter could be bad, but my first one (different brand even) didn't work either.
 

Offline james_s

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Re: Commodore 128 to Video Converter
« Reply #1 on: November 18, 2021, 11:14:25 pm »
What kind of monitor are you using? There is more than one type of RGB. The RGB output on old computers used separate lines for each color and one each for horizontal and vertical sync. The RGB input on some monitors that uses three RCA connectors is luminance and sync on the green one and then difference signals for red and green. If you get garbled lines you might not be getting the sync.
 

Offline m k

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Re: Commodore 128 to Video Converter
« Reply #2 on: November 19, 2021, 01:59:59 pm »
Lots of lines and distortion feels more than no signal.

Since input and output are both RGBHV only sync fequency seems to be a possible precepted problem.
(NTSC vs. PAL is probably not the one)

The link below indicates that the needed input is RGBC when GBS-8200 booklet has modes YUV/RGBS/RGBHV.
(RGBC and RGBS are equal, RGsB is SoG)

https://sites.google.com/site/h2obsession/CBM/C128/rgbi-to-vga
Aneng-Appa-AVO-Fluke-General Radio-Heathkit-Herbert Arnold-HP-Kaise-Kyoritsu-Mastech-Simpson-Tektronix-YFE
(plus lesser brands from the work shop of the world)
 

Online Benta

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Re: Commodore 128 to Video Converter
« Reply #3 on: November 19, 2021, 08:07:55 pm »
The GBS-8200 actually only supports analog RGB, not digital as with CGA.
A resistor network to do the level shifting is possible.

 

Offline RobClaggy

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Re: Commodore 128 to Video Converter
« Reply #4 on: November 19, 2021, 08:58:15 pm »
The GBS-8200 actually only supports analog RGB, not digital as with CGA.
A resistor network to do the level shifting is possible.

Wait.  Can you explain this in a little further detail?
 

Offline RobClaggy

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Re: Commodore 128 to Video Converter
« Reply #5 on: November 19, 2021, 08:58:59 pm »
Lots of lines and distortion feels more than no signal.

Since input and output are both RGBHV only sync fequency seems to be a possible precepted problem.
(NTSC vs. PAL is probably not the one)

The link below indicates that the needed input is RGBC when GBS-8200 booklet has modes YUV/RGBS/RGBHV.
(RGBC and RGBS are equal, RGsB is SoG)

https://sites.google.com/site/h2obsession/CBM/C128/rgbi-to-vga

This sounds like what I need!
 

Online Benta

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Re: Commodore 128 to Video Converter
« Reply #6 on: November 19, 2021, 09:17:38 pm »
The GBS-8200 actually only supports analog RGB, not digital as with CGA.
A resistor network to do the level shifting is possible.

Wait.  Can you explain this in a little further detail?

Google is your friend:
https://gglabs.us/node/2022

CGA output is 5 V TTL levels, analog RGB needs 0.7 VPP levels. You're overdriving the inputs of your converter totally. No wonder that it's freaking out.
 

Offline m k

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Re: Commodore 128 to Video Converter
« Reply #7 on: November 21, 2021, 11:59:41 am »
The GBS-8200 actually only supports analog RGB, not digital as with CGA.
A resistor network to do the level shifting is possible.

And its whole named purpose is CGA/EGA/YUV to VGA.
The converter even has a special 5-pin CGA/EGA connector.

How this should be taken?

YUV is analog SoG.
YPbPr is the same with gamma correction.

So SW button is a sync mode selector.
But YUV is not RGB so it must be a color space selector too.

Back in the day VGA resolutions were available before 15-pin D-sub became standard.

Anyway, RGBI needs that intensity signal and is't not available anywhere.
I'd say that component video to VGA converter is much better name, even that it excludes higher frequencies.
Aneng-Appa-AVO-Fluke-General Radio-Heathkit-Herbert Arnold-HP-Kaise-Kyoritsu-Mastech-Simpson-Tektronix-YFE
(plus lesser brands from the work shop of the world)
 

Offline james_s

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Re: Commodore 128 to Video Converter
« Reply #8 on: November 22, 2021, 04:31:55 am »
A critical detail here is that this thing is designed to convert the signal from arcade game boards to VGA. In the classic arcade world "CGA" and "EGA" refer to the resolution/scan rates only, the monitors are invariably analog RGB. A "CGA" arcade monitor is not compatible with a CGA PC monitor, only the resolution and refresh rate is the same. IIRC the video levels are higher than 0.7V of VGA though.
 


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