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Oh, and another factor to consider: Video is "unipolar", a fancy way to say it matters if it's in phase or 180 degrees out of phase (inverted). Sync tips are the most negative part of the waveform, and white the most positive part. Though, on an RF carrier, sync tip is max amplitude modulation, and white around 10% of that. They did it that way so AGC circuits would have a constant thing to look at, when slaved to the horizontal osc, which should easily find said sync signal. Get it inverted to the way the TV circuit you're trying to inject into, and it's a total mess, no sync, and pretty much unintelligible. Unlike audio, which is "bipolar" in the sense that it doesn't matter (in mono, anyway) which way it is fed to an amp or speaker (in a stereo system, you need to be consistent channel to channel, but otherwise it doesn't matter).
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