Why does an FM Subcarrier need two side-bands and a pilot signal?

Question

I know that an FM radio broadcast has a breakdown of:

0-15Khz : L+R channel audio
19Khz: pilot signal
23-53Khz : two side bands containing L-R audio.

I understand how a circuit encodes and decodes these signals to produce the correct audio. My question is why they couldn't have just stuck the L-R audio in a range from 19Khz-34Khz? Why need a special pilot signal and why need to amplitude modulate a 38Khz signal to obtain two sidebands? What's the point?

Answer 1

The pilot signal is used to regenerate the carrier for decoding the double sideband supressed carrier signal. They're using AM double sideband supressed carrier to simplify the receiver design. They could have used single sideband but that was the way the standard was put together at the time. SSB transmitter are a more expensive and were very expensive when this standard was written.



http://en.wikipedia.org/wiki/FM_broadcasting#FM_stereo