S video to special composite video mixer circuit for your vintage color TV
As analog over-the-air NTSC is scheduled to go away in a year or two, you'll end up using some sort of cable, sat, or ATSC tuner box to get 525i signals to feed to a TV modulator to feed your vintage color TV sets. But you might as well do a little signal processing on the digital to analog video signal to make it look better on vintage color TV sets, which all use "notch filter" chroma-luma separation circuits inside. A notch filter assumes that all frequencies in the demodulated video signal above about 3 MHz belong to the chroma subcarrier. Not always correct, as you sometimes see crawling rainbow color patterns crawling up the refree's shirt in a football game. But if you have a source of "S video", with separate luma and chroma from a set to box or DVD player, you can pre-filter the luma to get rid of the high frequency luma that causes such rainbow crawlies in the vintage color TV set. Your vintage TV set's picture will look much better with the use of this pre-filter.
I've done a pre-filter before, but the "group delay" characteristics of that wasn't that good. One wants all frequencies of a video signal to have equal time delays thru a filter or other circuit, and if the delays get too unequal, you can see mismatches of the luma and the chroma on the CRT screen. This new circuit has minimal group delay mismatches, so the colors will line up on the luma right.
This circuit doesn't have any critical LC circuits that require any tune up or alighment. And it just uses common Ls and Cs you could salvage from dead VCRs and such. The coils I used look like little gumdrops. These have the resistor style color codes, which indicate the number of uH's like resistors indicate number of ohms.
This circuit assumes that the S video source impedances are 75 ohms (virtually all are) and that the TV modulator you will feed has a 75 ohm load resistance.