Consider just an AM signal for right now.

I understand that an AM signal is transmitted at a specific frequency, that my receiver grabs that frequency, and that the amplitude is then modulated to convey information.

But it’s that last part that I don’t really get.

See, when you hear me speak, aren’t you understanding me by parsing (for lack of a better word) the frequency and amplitude of my voice. Music contains an amplitude and a frequency. If radio keeps ONE of those things constant (either frequency in AM or amplitude in FM) how are we hearing differences in frequency and amplitude through a radio?

With AM, if you’re sending me a sine wave at a specific frequency and just modifying the amplitude, shouldn’t I hear a tone with a specific pitch and differing volume? Ignore for a second that I can’t hear a 68kHz tone.

Check out this page at How Stuff Works. They basically explain that the sine wave gets modulated and demodulated but not how.

Here’s my guess: this might be far fetched, but I suppose that while I was modifying the amplitude of the signal, I could modify it at a particular frequency. That is, while sending you a sine wave at a frequency at 680,000Hz, I could modifying the amplitude “properly” and then modify the amplitude X times per second. That is, you’re deciphering not only the amplitude modification, but also the frequency with which I’m modifying the amplitude.

Still, that sounds like it’s missing something.

(I know I use “modify” and “modulate” interchangeably).

Take a look at the AM Detection picture. What you “hear” out of the radio is the longer wave represented by the “tops” of the sine wave that has been rectified (passed through a diode, represented by the arrow thing in a circle).

In the picture, you’d hear a single tone, in real life, you’d hear whatever is being broadcast, and would look much more irregular.

The lower picture here helps illustrate the idea…the radio wave oscillates thousands of times for each single audio wave. The blocks of green in that picture are thousands of radio waves, which together trace out the shape of an audio signal.

Here’s a followup: that original sine wave shows 14 peaks. Let’s say that is it’s frequency, 14.

The new “sine” wave shows 2 peaks. Should that be considered it’s frequency? Is that a picture of what a tone with frequency of “2” would look like, at constant volume.

In the real world, if the original signal was 680,000 Hz, might that be a picture of a tone with a frequency of 1/7th the original, a 97,000 Hz signal at constant volume?

[on preview: in light of what gorilla posted, I think I get it.]
That’s what I was trying to get at in my initial post.

Yes, you seem to have got it. The misleading aspect of the howstuffworks diagram is that the difference between the two frequencies (the ‘carrier’ and ‘signal’) isn’t great enough - one is normally thousands of times faster than the other. But it’s hard to draw that clearly

Yes, carrier frequency 14Hz (asssuming that signal is over 1 second), “data/sound” frequency of 2Hz.

In your example, same thing, except that if it was a 97Khz signal, it’d be too high to hear. Possibly useful for data though.

Gorilla Man nails it with it being too hard to draw with real world examples.

You’ve got the concept though.