Broadcast NTSC did actually modulate the color signal into a "comb-shaped" frequency spectrum that was overlaid onto the complimentary "comb-shaped" spectrum of the B+W signal, because they didn't want to have to re-allocate higher broadcast channel bandwidths to be able to support color (and therefore have fewer channels available). The comb-shaped nature of the spectrum is an artifact of the discrete lines the picture signal is made up of, and the pause between each line when the electron beam reset itself and prepared to draw the next line. The fact they were able to figure out how to do color in the same bandwidth in a backward-compatible way is actually kind of insane, and I've seen it mentioned somewhere as one of the, if not THE most impressive engineering achievements of the 20th century.
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