What is the different between silent subliminal and high frequency modulated with a message?
The difference is: the first probably doesn’t work, the second definitely doesn’t work …
and high frequency modulated with a message?
That’s how radio works… The radio frequency is modulated with the audio and the receiver “detects” or “demodulates” the audio and sends it to the speakers so you can hear it.
With FM stereo the mono (L+R) is regular frequency modulation and the stereo (L-R) is embedded in a 19kHz “pilot tone”. The 19kHz tone is in the audio range but it’s filtered-out. In a mono radio the pilot is simply filtered-out and lost. In a stereo radio, the demodulated L-R audio is combined with the L+R audio to get regular left & right stereo.
high frequency modulated with a message
I read that with the comma in a different place. “High Frequency” meaning above, say 20,000Hz (ultra-sonic), and modulation to be determined. Most of the variations I’ve seen have parts of your ear responding to some version of AM. That’s the modulation you can receive on your waffle iron if you live close enough to a transmitter.
So it would make sense that you might “fool” your ear into hearing it without you knowing.
Stereo FM info is carried at 38KHz. The 19KHz. pilot is there so the radio can accurately decode the separation info. The pilot’s most important function is to light up the stereo light.