An Epiphany

I woke up this morning, with an immediate realization of truth. I was wrong about the chroma dot pattern. Not that it doesn’t exist, because it’s there. And not that it’s not unique in style to the PCE, because it is (compared to the snes and Genesis). But that I believed there was a mechanism internal to the VCE that help produce this pattern on the Luma channel, when the Chroma channel needed. As if, to emphasize this pattern.

I guess my subconscious noticed that there was no mechanism mentioned in the schematic. That, and some other things of late (namely designing my own composite output system on an engineering level, on an FPGA).

I originally tested the chroma dot pattern to see if it was just chroma bleeding into luma frequency, by removing one of the subcarrier modulated signals. Looking at the signal in pure B&W (green in on componet), I originally didn’t see a difference.

So immediately waking up (and I mean immediately), I took the lid off my Duo and cut the both traces to the paired chroma sub channels. Then hooked it to my captured card. B&W white signal like I had expect. Ok, so far so good. Then I plugged composite into the old tek scope, to make sure it was the both subcarrier lines I cut and not the Color Burst line. The scope verified that the CB was indeed on the back porch.

So, the pure XOR dot pattern going down the screen makes sense, in the fact that the pixels (of the chroma dot pattern) look exactly as on then off. Not like the SNES or NES. Duh – because the CB is alternated exactly 180 degrees out of phase every other scanline.

Phase alternating was meant to directly tell comb filters the difference between two scanlines (three in fact, that’s why the SNES and NES use a three scanline phase shift group). The comb filters know how to separate chroma frequency bleed into the Luma frequency, because of the phase shifts. And because the Genesis external encoders do not do this, there’s no shift on the chroma dot pattern and it’s pretty much impossible to pull the additional detail from the Luma channel, after narrow frequecy filtering subcarrier frequency. It makes perfect sense 😀

Why is that really important? Because I had originally thought making an s-video mod for the PCE was useless – because I had thought part of the chroma dot pattern was partially or fully driven on the Luma channel. But it’s NOT, this means you CAN do beautiful s-video out on the PCE without ANY encoder chip… and without looking the real PCE colors (because an external s-video chip doesn’t convert the RGB to YUV as same as the table in the VCE). That’s fantastic news. So much so, that I’m gonna get a second amp for the chroma signal, and make an s-video mod myself 😀

It’s also possible to make component video from this too. But that requires a bit more circuitry. You need to demodulate the amplitude modulation of both chroma channels. I haven’t tested it but, if the VCE pattern schematic is any indication, the rate at which the sine wave frequency is amplitude modulated actually might just be the dot clock of the VCE (the dot clock that the VDC sees/is feed). If it is (which I have a strong feeling it is), than that means you have full chroma resolution for component too. Well, assuming your demodulation circuit is up to snuff. That’s pretty freaking awesome.


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