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Back in the tube era they would change the horizontal and vertical scanning rates slightly for color TV. I believe one of the things that started to reduce that practice was VCR timecode being used in broadcast automation....The time code was based on frame count and it probably would be hard to handle it accurately every time if it changed between color and monochrome.
I suspect color network logo watermarks in the programming contributed to the constant burst practice, since if the CEO tells you the watermark has to be in color and not change no matter what the program is then the burst has to stay on.
IIRC in the 70s didn't PBS do a thing where they made their bursts ultra precise so NIST could use them as a lab calibration reference?..If that was the case they may have had to keep burst on during monochrome shows to make that work, and may have been doing constant burst before cable.
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