There are 100 possible FM channels between 88.1 and 107.9 MHz. These are allocated in 200 kHz increments. Because of limited bandwidth a radio station can only transmit audio which is up to 100 kHz. But hey, that's still much wider than the range of human hearing. In practice, FM audio is usually restricted to 15 kHz. This leaves a lot of spectrum for other uses. FM Stations are permitted to place subcarriers in this unused portion of the spectrum.
Here's what's on that carrier wave that you can hear:
Normal baseband audio: consists of the right and left audio mixed together (R+L). This is so that when listening on a mono receiver, you hear both channels of sound.
Difference signal: This is harder to imagine, it's the difference between the right and left channels. When a songs in stereo both ears are getting very similar but not exactly the same audio. This is just the parts that are different or (L-R). This is transmitted on a 38 kHz subcarrier using FM modulation.
Mind blowing isnt' it? FM radio does not broadcast the Left and Right Channels. It's not a damn think like the channels on a cassete tape. Here is how how stereo audio is decoded. Of course, as much of a genius as Edwin Armstrong was, it was nto his system that gave us stereo audio. The Armstrong system was rejected by the FCC because it did not allow sub-carrier services. The the Zenith system has gone on to become the standard method in most countries.
The Armstrong system was more noise resistant I understand. Instead today's stereo FM signals are far more susceptible to noise and multipath distortion than mono FM signals. This is due to several factors, including the following:
1. the addition of the two sidebands of the difference subcarrier to the baseband signal increases the noise bandwidth of the signal by a factor of three (9.5 dB) as compared with a mono signal.
2. The pre-emphasis is applied to the audio signals results in the pre-emphasis acting in the wrong direction on the lower sideband of the difference subcarrier, i.e. decreasing the level as the frequency rises, which will have a further deleterious effect on the S/N of the difference signal. yadda yadda , I know thats too technical. Btu I cant think of a way to simplify it.
So heres what we got and how it works:
If you fed the baseband audio into a receiver's antenna input it will demodulate the signal. This demodulated signal would be the R+ L audio. If you combine this with the Difference (R-L) signal you get a mess, but that's kind of how it works. What it does is add the R+L and R-L signals, leaving just the right (R) channel. the remainder being the LEft channel left (L) channel. ta-dah!
The upside of this complexity and reduced reception qualityis that mono and stereo stations can all operate with this system as can all FM tuners. Also side bands can exist and aren't those nifty. More tomorrow!