Here's the thing:
The original NTSC (National Television Systems Committee) laid down the original US monochrome 525 line TV standard in 1941.
Every article I can find states that it used 4.5MHz FM sound.
HOWEVER, every US TV circuit diagram I can find from the 1940s shows an AM sound detector, much like the one used by British 405 line TVs.
Also my 1947 edition of Donald Fink's "Television Engineering Handbook" makes no mention of FM sound either.
So at some point they must have switched to FM, but when was it?
(I can't find any access to 1950s and later circuits).
NTSC has unusually narrow (25kHz) deviation for the sound, which would make sense, as that would allow reasonable FM slope detection for earlier sets designed for AM sound.

Hmmmm...
I don't know, but I think I know who does.

I'll ask earlytelevision.org and post the answer here.

You can get circuits for pre and post war US TVs on that site. Also some UK sets.

Answer from ETF:


Hi Ian,

Yes, the 1941 standard included FM sound. However, all prewar sets that I'm aware of were built before the standard was adopted, so they had AM sound.

Several of the prewar sets that I've seen had modifications for FM sound, including a couple of RCA sets that had what looked like factory made kits.

You are correct about slope detection. The sets on display at the museum use slope detection for sound, as did most sets in use before the war.

My comment: Not much public TV development was done during WW2. So there was probably an element of "do we really need to change over to this new-fangled thing just yet, when what we have works fine?"

Fink's book was published well after 1941. It describes the NTSC system in considerable detail, and it calls it NTSC. Nowhere is FM sound mentioned; it gives details pertaining to AM sound only. I've also seen quite a few circuits from the mid to late 1940s; every single one of them has a simple diode detector for AM sound. I get the feeling the notion of NTSC having FM intercarrier sound from day #1 is a factoid.

It's true that broadcast TV stagnated for the duration of the war, but an enormous amount of effort went into military applications. There were actually radio controlled flying bombs using miniaturised image orthicon tubes, but they turned out to be far too difficult to control. Closed circuit TV was also extensively used in the Manhattan project for remote monitoring of the highly radioactive plutonium processing plants.

I wouldn't be at all surprised if FM intercarrier sound was a post-war addition.