early Jones ETC keyer w/ schematics
This was an early Dave Jones keyer. Rich Brewster documented and built it. We referred to this as the black box keyer because it was in a black box. This keyer moved around a bit, at times it was at the center, at times it was at Ralph's studio. I first used this keyer as a student working at the ETC in the converted photo lab studio (at that point the Center was still on Court Street in Binghamton). Years later when I was working for the TV center I lobbied to install it back into the ETC studio. This was before we had added the keyers that were built through the Four-Board project so this was the best keyer in the studio at the time. I modified it's logic inputs so that it had +-5 inputs for normal reverse and key on/off as I had done with my own keyers. I don't recall if I added a force A and force B, though that is present in the documentation (the logic mod is from a later keyer design). As per the mod: there is no need for switches for force A and force B.
For anyone who is unsure, briefly, a keyer is a device that will layer one video image over another (also called matting). There are two dominant flavors of keying, luminance (luma) keyers and chrominance (chroma) keyers. Luma keyers use the luminance levels of the video clip source to determine how to create the matte, while chorma keyes use color information of the clip video to create the matte. This is a luma keyer. It is hard-edged meaning it used a comparator to create the clip signal (rather than a soft edged keyer, which would use a high gain amplifier to create the clip signal and usually would offer an edge softness adjustment). This keyer had a clip select switch that allowed easy selection between the three sources and thereby determining internal and external keys (external being a key where neither video source layer is also used to determine how to cut between the sources).
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Those are definitely my hand drawn schematics. I probably did build this, too.