Wednesday, February 02, 2011

FM Multiplexing

Unlike AM, FM has very comfortable spacing. FM Stations are 200 KHz apart, AM stations on the other hand are half that at best. (In fact AM stations can overlap.) One of the things a FM radio station does with all this extra space is multiplex. We don't ordinarily think of FM as multiplexed, but the way it encodes stereo certainly qualifies.
Multiplexing - The simultaneous transmission of multiple signals on the same channel
The FM stereo signal consists three parts, thus qualifying as "multiple." The first part is the sum of the left and right channels. This is simplified as L+R. This is a mono signal. If you have an old component amplifier from the 1970s it may have a mono switch and what it does is to only rectify this part of the signal. More here.

The second part is L-R,  a modulated 38 Khz subcarrier signal. It uses double sideband suppressed carrier (DSBSC) modulation. In DSBSC the amplitude modulated frequencies are symmetrically spaced above and below the suppressed carrier. But unlike AM radio, the wave carrier is not transmitted. So all that power is distributed between the sidebands. That improves coverage.In a simpel sense, this is an AM modulation of the subcarrier.

The third part is a 19 Khz pilot tone. 19 Khz is half of the 38 Khz subcarrier signal.  The relative percentage of modulation put into the pilot is 10%. It's like a reference tone, used for synchronization. Without the 19 kHz pilot tone a stereo receiver would ignore signals in the 38-39 kHz range. A guard band of  4 kHz above and below the pilot tone (15-23 kHz) is used to protect the pilot tone from interference.  The primary cause of interference in Fm radio would be the baseband audio signal from 50 Hz-15 kHz and the lower sideband from 23-53 kHz. (If you're wondering where RDS fits in, that's set at 57 kHz.)

So in your stereo receiver the L+R and L-R signals are combined for the Left channel [L+R] + [L-R] = 2L.  For the right channel they're just subtracted [L+R] - [L-R] = 2R.  It's actually elegant and simple, that's why it works so damn well and why it's been beating out AM radio for 30 years.