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Macrovision was a delicate...and ingenious...balance of the proper +100 units of video and -40 units of sync in the AGC world. They would shove a bunch of 140 units of white bar video in to the vertical interval at their varying levels. The AGC would choke and try to adjust to bring the video back to 100 units but the underlying sync would shrink in proportion to the adjustment in AGCland. Bad sync equals bad copy.
Little black boxes are around to strip the sync and pass the AGC'd video. Or you can use any Betamax of the era. They put new sync on anything and you could copy that.
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“Once you eliminate the impossible...whatever remains, no matter how improbable, must be the truth." Sherlock Holmes.
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