system
Given the following post, respond as if you're commenting on Reddit:
human
How are radio stations able to broadcast silence and have it not sound like static?

For example, in MC Hammer's *Can't Touch This*, what is going on between the radio station and my car stereo that makes it so that there is a pause between "Stop" and ". . . Hammertime" rather than the random noise I'd hear if I tuned to an empty station?

Thanks.
gpt
Let's back up a little bit and talk about how the radio station sends *Can't Touch This* over the air. If it's an AM radio station, the transmitter broadcasts a constant radio-frequency tone at (say) 1.000,000 MHz, but *modulates* that tone: it makes the broadcast slightly stronger or slightly weaker depending on where the speaker cone in your car radio should be.  Slightly stronger - the cone in your car radio pushes out a little.  Slightly weaker - the cone pulls in a little.  Do that a lot, really fast, and you hear stuff.  

That style of modulation is called *amplitude modulation* and it's what AM stands for (my dad used to say AM stood for "awful music").  It's really easy to decode - you just run the radio signal through a diode, and then smooth it out a little - once you've done that you can just amplify it and send it to the speakers.  The problem is that, when you're close to the station, the signal is REALLY STRONG AND THE MUSIC IS NATURALLY VERY LOUD, but when you're far from the station ^^^the ^^^music ^^^is ^^^naturally ^^^very ^^^quiet ^^^because ^^^the ^^^signal ^^^is ^^^so ^^^weak. Your car radio gets over that with a special circuit called an "Automatic Gain Control" (AGC).  The AGC senses just how strong the incoming radio signal is, and adjusts a radio-frequency amplifier to higher or lower gain to keep the signal at constant strength.  The AGC circuit is constructed to adjust the gain slowly compared to the modulation -- it might take 1/10 of a second to go from quiet to loud -- so that it doesn't mess with the sound itself.

AGC works really, really well when you're tuned to a radio station.  But when you tune between stations there isn't any RF signal and it turns the gain all the way up!  It amplifies the incoming (non-)signal so much that thermal fluctuations in the receiver circuit components make a loud hissing noise come out of the speakers.

So - when the station is broadcasting silence, it is broadcasting a constant radiofrequency wave with no modulation.  That maintains the AGC circuit at whatever level it normally holds, and no sound comes out of the speakers.  When you tune between stations, there is no signal coming in the RF stage, and the AGC "turns up the volume" so high that the thermal excitation of individual electrons in the receiver circuit makes the familiar static.

FM has a different modulation scheme but the principle is the same: a radio carrier wave with zero modulation is different from no signal at all.