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I think that the simplest way to explain this is to say that most professional analog recorders and certainly all digital recorders expect to see burst in the composite NTSC video signal they record. Equipment from the era of the TK-30A generally did not produce a composite signal, i.e., one that was made up of video and sync. The camera is fed with horizontal and vertical drive signals and, as you mentioned, sync was added later in the switcher. It is true that you can produce a composite signal from a TK-30A by applying composite sync to the appropriate input on the CCU, but to do a full color signal that more modern recorders would require, burst is also needed to lock the color circuits. Thus, by applying color sync, rather than "sync" alone, a composite color signal is produced even though it only carries black and white information. The obvious choice for this color sync would be "Black Burst" or NTSC composite black video.
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