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As I understand it, the FCC changed the standard from 441 lines to 525 lines just before WW2, and also mandated FM for the sound carrier. Most TV horizontal circuit designs could accommodate the 20% increase, but FM sound would require (obviously) an FM demodulator, instead of an AM detector. Oh, you could "slope detect" FM with an AM detector, but that doesn't work all that well. This is before intercarrier sets were invented.
The FCC could get away with this as there weren't that many older TVs out there back then.
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