[wa2ise]

SOme years ago I and another engineer had to design a digital chip to demodulate NTSC, PAL and SECAM. It's now the Samsung KSO127 if I recall correctly.

SECAM uses FM to modulate its color subcarrier. U is sent on one line, then V on the next, alternating. The receiver knows which is U and V as the frequency of the subcarrier is different for U and for
V. But unlike NTSC and PAL, the subcarrier amplude is constant, even in colorless areas of the picture. Not quite, they did another trick of reducing the amplitude at and near the subcarrier frequency, and letting it be higher for the higher sidebands. About 10 or so dB down at the subcarrier. Go any further and the FM demodulator loses it and you'd get noise. SO you always have cross chroma in the luma. Also any high frequency energy in the luma that happens to be near the chroma carrier can "capture" the FM demodulator and you get fake colors. There's also preemphasis of the U and V information at the encoder at the TV station and the demodulator needs to do deemphasis (cuts noise some, just like FM broadcast radio audio).

You can't separate the luma and chroma carriers too well. About the only thing you can do is notch out the spectrum around the chroma subcarriers. Comb filters wont work as the line to line correlation of NTSC and PAL isn't there.