Dorothy on old RCA 'CED' disc

[old_tv_nut]

Originally Posted by Penthode

I found it interesting to find that negative modulation for North American 525 line TV broadcast required a horizontal AFC circuit to help overcome false triggering with noise impulses whereas older British televisions receiving positive modulation 405 line transmissions did not. I suspect this was because the horizontal sync pulse was at the modulation envelope peak for 525 which made it more susceptible to noise? I recall weak British 405 reception on sets with horizontal AFC locked well under the noise.

Exactly correct - with an envelope detector, impulse noise goes toward max signal. With negative modulation, that means it goes into the sync region. Better NTSC set designs included a noise inverter so that impulses going beyond normal sync level would be inverted and go back toward lower signal level (away from sync, towards white). The time constants in the inverter would be much shorter than those in the sync circuit input filter, so the narrow noise pulse that remained would not affect the sync separator.