Amplitude Modulation, or AM, works by modulating the sound on the "peaks and valleys" of the frequency. If you were to look at the waveform of the carrier frequency, you would see it as variance in the "power levels" of the signal, going higher and lower depending on what's being sent.
Because of how AM works, you can actually hear what's being transmitted by causing an electrical arc. It's the same concept of hearing a "buzz" from high voltage power lines, except in a controlled fashion to produce sound.
The carrier frequency is way above the hearing range. So it has to be shifted to the audible range. That happens by clipping and filtering. The arcing must be doing that somehow. Probably clipping; the low-pass filtering is done by the air/your ear.
My guess is that the arc through air gap is functioning as both the detector and speaker here. I suspect that the arc can only be sustained in one direction, probably from ground to the cable, which would give us the required clipping. At the same time, I assume the arc is acting as a plasma speaker, and, as you say, the air and our ear provides the low-pass filtering.
Came here to ask this very question. Thank you. I take it then that a signal using frequency modulation or phase modulation (is that done anywhere?) wouldn't work then? Also, probably the same reason a ground loop in a home entertainment system will pick-up local AM stations. Edit: also, why does this work with the given RF frequencies which for AM are still much higher than audible sound frequencies?
All work and no play makes Sally a dull boy. Or whatever. But seriously: don't over-work yourself. I mean if you really want to, go for it, but the quality of your work will decline with you. Meet some new people, go hiking, go outside, eat better, live better, work better, and your work will look better with you.
yeah, I probably wouldn't call "the 37signals guys" script kiddies, David Hannson created Ruby on Rails. I agree with your take on innovation, Pandora would be in the same boat.
I said the exact opposite: 37signals are not script kiddies, they are really smart, educated people, that make use of CS knowledge in building their products.
Because of how AM works, you can actually hear what's being transmitted by causing an electrical arc. It's the same concept of hearing a "buzz" from high voltage power lines, except in a controlled fashion to produce sound.