Stretching the image horizontally by 3 was initially suggested by HardWareMan to simplify the calculations, so all my attempts were relying on that method. The first revisions of the filter also have an erroneous 4 scanline pattern instead of a 6 scanline one. I think I fixed it in later revisions.
This stretching approach, however, resulted in wrong samplerate that I mentioned. Hence the exact pattern didn't match the one from a tuner or from a TV, even though without notch filter it did look close to an SVideo capture for an untrained eye.
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I never see moire patterns on other people's captures and photographs of PAL NES stuff, I've only seen them in this thread, which is what's confusing to me.
That's because everyone uses the Composite channel to obtain the signal from the console, and because of that, on flat areas, all the colors of the pattern get mixed together. Notch is done by applying some latency to every color, so they bleed together all the time. On flat areas more or less the same color pattern repeats, so they smoothly mix together. But for color edges, it becomes a few pixels of shades of one color, then a few pixels of shades of the other color, and only then it might start repeating. So those don't mix together as well, in the end you see the moire artifacts.
SVideo doesn't apply the notch filter by design, because it's supposed to get Luma and Chroma already separated. NES doesn't output that, but if we plug it into the SVideo channel, we can clearly see the exact pattern of the moire and study/simulate it first. Then we can either smooth the picture out in some way or just cancel the moire rendering for flat areas. I think that's how Blargg's filter does that.
Now here's a pack of all the shot's we've done, both from TV and from our tuners. My tuner is the USB one, the weakest, but it shows some things accurate enough to compare to, tuners from Eugene and HardWareMan are Avermedia, so they produce a picture similar to what you see at GDQ marathons.
https://www.dropbox.com/s/lsvh4nql4m3tssr/shots.7z?dl=0