[BC] 19 kHz pilot tones

[Jim Tonne]

It has always been an FCC requirement that the
pilot tone be within 2 Hz of target.

The receivers would indeed "light the light" if
very far off.   Some would respond if several
hundred Hz off.

But having a pilot tone very far off would be a sure-fire flag to the 
FCC that something was wrong.  So would amplitude modulation of that 
tone if one of the early-style composite-waveform clippers was in use.

- JimT