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The keyed rainbow is very simply a keyed CW signal that has a frequency offset from nominal 3579545 Hz by one cycle per horizontal scan, that is, different by 15734 Hz.
Then, each keyed burst is the same amplitude as color burst, and the one that directly follows horizontal sync is used by the receiver as color burst.
The keying uses a square wave that is "on" 12 times per horizontal scan, giving a 30 degree phase shift between gated bursts. One burst occurs during horizontal blanking, the next is used as the color sync burst, and the remaining 10 are visible bars.
On a vector scope, this looks like 11 color bursts separated by 30 degrees, with the 12th burst missing. Since the phase is actually changing during the on-time of each burst, and the duty cycle is 50%, there is actually a 15 degree phase change across each gated burst, which ideally will show up as a smear of the phase, rather than a single point on the vector scope. Combined with the narrow bandwidth of the color demodulators, this results in a 12-lobed "flower petal" pattern on the vector scope, with one petal missing.
A generator conceivably could skip the nicety of blanking the burst that occurs during horzontal retrace; but I don't know if any particular designs skipped blanking or not.
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