Commodore 128, RGBI & GGLABS’ CGA2RGBv2

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    I have a Commodore 128 (NTSC) which has an RGBI 80-column display port, 640×200 I believe is the resolution. I am using a CGA2RGBv2 from GGLABS to convert RGBI to RGB, and have that connected to OSSC.

    At first the image didn’t look great.. the sides of the text characters looked cut off. And there was some jitter going on. After reading about other RGBI computers/consoles I started playing with H. active, H. samplerate, and Sampling Phase settings. Playing with these in addition to setting 240p/280p Line4x mode I have a nearly jitter free image and characters are the proper width – it looks great.

    My question is.. how do I know what the optimal settings are? Any other settings I should look at? And I don’t really understand what I am changing, I’m just trying stuff until I get a good picture 🙂 The wiki has some information, but any guide that really describes video signals and what I am tweaking in depth?

    I am using Dell 2007FP monitor, with an HDMI to DVI cable out of the OSSC.


    FirebrandX actually discusses how you can find optimal timings in this video –

    You’d need to find something on the C64 that generated a similar pattern, or program something I guess.


    Thanks for that video! I had to write my own program to generate a checkerboard pattern for 80-column mode 640×200 which ended up being fun.

    I have an absolutely rock solid, vibrant, jitter free image. The only “weird” part is that its absolutely fitted perfectly to the LCD screen – I’ve attached a photo. The colors in photo look bad, but in person the colors are solid and look excellent.

    Is there some way to create margins on the edge of the screen at all? So its not so tightly fit to the bezels on the screen? If I adjust H. synclen or H. backport by a value of one, I cut off either side of the screen. If anyone else has a C128 these are settings I am using:

    240p / Line2x

    H. samplerate 1016
    H. synclen 20
    H. backporch 241
    H. active 640
    V. synclen 3
    V. backport 36
    V. active 200
    Sampling Phase 11 degrees



    You need to increase and by twice the amount you decrease H.backporch and V.backporch, respectively. This keeps the picture centered and adds borders, so long as the display accepts the signal.


    For the curious, here is GEOS128 using CGA2RGBv2 via OSSC – the image is flawless in person, none of the color fringing from the photo is present.

    The GEOS128 background being a checkerboard would be an easy way for others to calibrate their OSSC

    GEOS128 via CGA2RGBv2 and OSSC

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