[Robert Orban[_3_]]

In article ,
says...
TV audio was amplitude-modulation-based until the
aforementioned switch to stereo broadcasts(U.S. early
'80s). Europe and Asia had stereo OTA TV several
years before us - while America as usual was figuring
out which war it could get itself into...

Only SECAM (used mainly in France) used amplitude-modulated audio. NTSC
(first adopted in 1941 and updated to permit color transmission in 1953)
always used FM with +/- 25 kHz maximum frequency deviation (1/3 of that
permitted for FM broadcst stations) and 75 us pre-emphasis (same as North
American FM broadcast stations).

Because BTSC stereo is backward-compatible with all monophonic NTSC TV
sets (they would reproduce the sum of the left and right channels), the
modulation technique could not have changed from AM to FM.

--Bob Orban