Author Topic: N64 S-Video... chroma crosstalk?  (Read 148 times)

Offline radorn

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N64 S-Video... chroma crosstalk?
« on: August 11, 2017, 11:23:38 am »
Hey, waiter! There's some chroma in my luma!

The most annoying outcome of this is that I get this dot-crawl like interference all over the screen instead of just the edges where composite normally fails to separate chroma from luma. But since SVideo is supposed to have them already separate, the luma channel just takes the chroma dots as part of the picture.

I use a Bt878 PCI capture card combined with DScaler. Poking arround I found this special input mode that expects a composite signal over the luma pin of the SVideo connector.
These pictures were taken from a PAL N64 that was locked up by yanking the cartridge. The point was to be able to take several shot of the exact same image using different settings on the capture card.
Here are the results:


rom_EUR - hw_NUS-001(EUR) - output_SVIDEO - input_COMPOSITE-OVER-SVIDEO - format_PAL50 - LumaAGC_OFF
It's almost grayscale, but there's a faint hint of color...


rom_EUR - hw_NUS-001(EUR) - output_SVIDEO - input_COMPOSITE-OVER-SVIDEO - format_PAL50 - LumaAGC_ON
Now with chroma AGC enabled, we get colors, although the picture quality is garbage.
I got this kind of shitty picture when I tried to feed S-Video to a PAL TV with a single SCART input. Obviously it didn't support SVIDEO at all. Now I understand why I got that kind of picture. There was a faint chroma shadow in the luma pin and the AGC circuit was amplifying it to normal levels, with results even worse than seen here.
Nevertheless, notice how there's no all-over-dotcrawl like in the pictures below. This is because decoding it as composite kicks in the chroma separation circuitry.


rom_EUR - hw_NUS-001(EUR) - output_SVIDEO - input_SVIDEO - decoded_PAL50 - ChromaAGC_OFF
Now the picture looks normal, albeit with the chroma signal causing dot patterning all over the place.


rom_EUR - hw_NUS-001(EUR) - output_SVIDEO - input_SVIDEO - decoded_PAL50 - Chroma_AGC_ON
here I activated chroma AGC, but it seems to blow up the colors a bit too much. I don't have an RGB picture to compare to, but it seems too saturated to me.

Anyway. The question is simple: Who's put chroma in my luma? Does it come out like that from the DAC? some other components between the DAC and the AV multiout? Is the cable to blame (bad shielding)? Some other possible culprit?
I'm assuming there's also some luma in my chroma, possibly affecting color decoding too, but the chroma-in-luma issue seems way more noticeable.
« Last Edit: August 11, 2017, 11:26:55 am by radorn »

Offline cgm

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Re: N64 S-Video... chroma crosstalk?
« Reply #1 on: August 16, 2017, 09:24:01 am »
Find a cable with better shielding. Cheap S-Video cables do have cross talk problems. The best way to isolate the console as the source of the problem is to somehow disconnect the chroma pin on the Multi-Out plug so the signal isn't transmitted across the cable at all. If you still get cross talk, the problem is with the console, although that is highly unlikely.

I would also try another video capture card/USB stick if available. The BT878 chip isn't exactly the pinnacle of video quality.

Offline radorn

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Re: N64 S-Video... chroma crosstalk?
« Reply #2 on: August 18, 2017, 11:07:39 am »
I'll look into better quality cable for the N64 when I have time, but, so far, I can say that it's not the card that's causing it. I have captured other S-Video sources and luma and chroma were well separated.

Writing this I just remembered that I once also experienced this strange dotting pattern while playing PS1 over RGB in a CRT.
It seemed really strange to me and I didn't understand what was going on, but now I guess it was chroma/composite leaking into the RGB wires.