Luma Key and Multiplexing With Genlock


I am in the research stages of a project to build my own luma keyer. I have some experience with NTSC signals, but I don’t know exactly how genlock works.

I want to build a circuit which uses a comparator to switch a mux (currently looking at LMH6574) between two different inputs. One input is a camera which outputs CVBS, and the other is a vintage Leader LCG-400 pattern generator. I want to have both the camera and the composite output of the pattern generator going into the multiplexer, apply a comparator to one of the signals, and use the output of the comparator and a little bit of discrete logic to switch the mux within the current line of video to do luma keying.

I know that in order for this to work, both video signals must be synced. So, I also have a LM1881 sync separator that I can add to the signal coming from the camera. The LM1881 has a composite sync output that I think I can then use to sync the signals, since the pattern generator box has an input for genlock. But, I don’t really know how genlocking works so here I am with questions…

  • Can I just pipe the “Composite Sync Output” from the LM1881 directly to the genlock in port on the pattern generator to sync the two signals? I suppose that it might not bode well that the pattern generator has a separate but uninstalled port for “Composite Sync” next to the genlock jack…

  • Will this lose color sync between them though? Like, since there doesn’t seem to be chroma phase info coming out of the LM1881, will the signal being switched in show up with funky colors? Since I am doing intra-line switching, there will be a lot of cases where the colorburst of a particular line’s signal is being applied to color information that is being switched in from the other source when the comparator triggers the mux.

I’ve attached some snips from the LM1881 datasheet and pics of my video gen box.

Thank you!

LM1881 composite sync chart
LM1881 composite sync description

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