PAL red/green color moire, not in NTSC

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rmzalbar
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Re: PAL red/green color moire, not in NTSC

Post by rmzalbar »

banman wrote: Wed Apr 29, 2020 9:34 am I noticed alternating vertical bars. I then reconnected the same C64 setup to an old Pioneer Plasma TV. This Pioneer set has seen better days, some of cells aren't working correctly.

Interestingly I observed no vertical bars. Maybe you can give some thought on this.
The image wasn't as sharp as the Hisense LCD. The Pioneer screen is about 5x larger than the Hisense LCD TV set. This may be a factor at play.

Maybe the issue is as eslapion says a feature of the way TV sets / monitors display the incoming signal......

In both cases I used the RF cable and attempted to tune the sets as close as I could to each other.

Where ever I manually tuned the Hisense LCD TV set the vertical bars were present.
The bars look like the AEC noise the VIC-II has become notorious for. The reason it's *become* notorious is that it's often more noticeable now in modern displays than it did in the classic days. See, modern display design assumes that you'll be receiving modern signals generated to better standards that yesteryear - from cable TV, recent DVD players, etc - and so their goal is to give a high-definition rendering of that without filtering, attenuating or smearing it as much as older displays did. Hence, the flaws are more noticeable. On an 80s CRT, you'd see them, but not in such eye-popping detail. Still, modern TVs use various methods of decoding and digitizing legacy analog inputs typically with intentional noise filters applied, some of which may hide the AEC noise better than others.

On top of this, you are mentioning that tweaking the RF tuning a bit tends to make the noise more or less visible. This is an artifact of how the RF modulation encodes the composite signal and audio together into a carrier wave. TV's, old AND new, used a automatic gain control to try to make detail visible out of the signal, and sometimes if you tune a little either side of ideal you'll get overamplification of the luma changes (looks like oversharpening) in an attempt to amplify part of the signal the decoder's circuitry sees as 'not good enough.'

Current receivers tend to do a relatively poor job with RF output from old devices mainly because they are more selective and are expecting a stronger and more tightly-tuned input signal, which is why, although it seems paradoxic that the best new stuff would do a worse job with old equipment, it does.


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Re: PAL red/green color moire, not in NTSC

Post by rmzalbar »

So I checked out Adrian's RF modulator mods for cleaning up video, and that's one of the best videos he's done in my opinion. He removes some filtering by clipping two capacitors and bypassing an inductor which sharpens the video and reduces color smearing by a lot. I did these mods on my 250407 this evening and the difference is large! Definitely worth doing on the long boards, and you don't have to give up composite or RF output. The changes are easy to do right from the top, don't even need to take your motherboard out of the case or remove the bottom shield.

He describes how rainbow patterns can get into high frequency areas of graphics due to luma leaking into the chroma where they are mixed in the RF modulator to get composite video, which I already suspected.

He also claims to have seen a small amount of luma leakage into the chroma at the VIC-II itself, which he successfully filtered with a capacitor.

So now I have a new avenue to explore. In the Atari 8-bit world, similar mods are done to clean up the video there (the XL machines have much worse video output than ours do out of the box) and it's pretty common to install a switch to disable the composite mixing for better Y/C quality. Both my Ataris have that switch. I'm glad Adrian did similar legwork on the C64 at last.

I'm going to add a similar luma filter to deal with the VIC-II luma to chroma leakage, and put a switch in to isolate luma from chroma at the RF modulator too and see what that does. I'm pretty sure now that the red/green color bars effect I'm seeing in my breadbin, that I've also seen in other PAL breadbins on images around the web are partly just inherent to the 6569 VIC-II, and the 85xx silicon is likely improved in that respect which is why the short board model I have doesn't exhibit those bars (also a different RF modulator version in those, newer and PAL specific.)

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eslapion
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Re: PAL red/green color moire, not in NTSC

Post by eslapion »

rmzalbar wrote: Sat May 02, 2020 11:15 am By TOLBerizing it, of course!
Oh yeah, that works.
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Re: PAL red/green color moire, not in NTSC

Post by banman »

Hi rmzalbar and eslapion,

I saw this interesting YouTube video today.....


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