Gaps between characters on video output

jibberjabber · Post by **jibberjabber** » Sun Dec 13, 2009 5:14 am

More VIC-20 display woes. Today I fired up my CR VIC and there are gaps between the characters. It's almost as if the vertical lines on the display are not aligned with each other.

Apologies if this has been covered before. I'm pretty sure I've seen similar pictures, but after searching the forums today I can't find anything relevant. The VIC is PAL, and the output is via a composite video lead. Everything else works, it's just the video output is a tad messy. I don't remember it being like this before. I'm sure I'd have noticed it.

Any suggestions as to what might be wrong, and what I could do to fix it would be appreciated. I know that I could do the SVIDEO mod, but to be honest I'd rather know that it was going to work before I started cutting connections on the PCB!

Thanks

Mike · Post by **Mike** » Sun Dec 13, 2009 5:59 am

Today's LCD TVs do a heavy amount of after processing to produce the output. It could quite well be, that your TV gets hiccups on the composite signal of your VIC.

The chroma signal of the VIC is especially bad behaved. Normally it is supposed to be a sine wave at a certain frequency, with the phase signalling the hue, and amplitude a certain saturation. That sine wave could effectively be filtered out by a band pass, even on the oldest colour TV sets. Unfortunately, the VIC produces a square wave chroma, which in principle can deliver the same colour information - but its harmonics cross-talk to the luminance signal, so vertical lines are distorted.

The display algorithms on modern TVs can mistake these harmonics for detailed luminance information, and produce an even worse result by bad luck.

First I'd try if your VIC still can produce a "normal" looking output on an older CRT. This way you can exclude that your VIC is at fault.

Then, you should still consider the S-Video mod. That way, the TV set doesn't need to separate luma & chroma with filters, and won't mistake parts of the chroma signal for luminance information.

Vic20-Ian · Post by **Vic20-Ian** » Sun Dec 13, 2009 6:37 am

Try turning off any Digital Noise Reduction on the TV.

Try a Scart cable to take the image from the Vic monitor outputs rather than through an RF Modulator.

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jibberjabber · Post by **jibberjabber** » Sun Dec 13, 2009 6:39 am

Thanks Mike. You're spot on. Here's the same output on an old CRT.

It is actually a new TV / Monitor. Annoyingly I had been using a LG TV at work so I could make sure the display from the VIC was okay with it. The new LG I purchased is a slightly different model. I guess they've changed the image processing filters. From fiddling with the settings I can improve the display somewhat by turning the sharpness filtered right down, but it's more of a fudge than a fix.

I think the S-Video mod is still the way forward anyway. Anything I can do to improve the picture is going to be worth it.

Thanks,