Stereo FM transmitter and receiver; output of mono receiver

Stereo FM sends two audio channels, left (L) and right (R), on one FM carrier in a way that is compatible with mono receivers. It uses frequency-division multiplexing of the sum and difference signals.

Stereo FM transmitter

L,R->+--------+ L+R ---------------->+-----+
     | Matrix |                      |     |
     | (+, -) | L-R ->[Balanced ]--->| Sum |->[FM ]-> out
     +--------+       [modulator]    |     |  [mod]
                           ^ 38 kHz  |     |
       [19 kHz osc]->[x2]--+         |     |
            |                        |     |
            +---- pilot 19 kHz ----->+-----+

1. A matrix forms $L+R$ and $L-R$ (both 30 Hz – 15 kHz).
2. $L-R$ DSB-SC modulates a 38 kHz subcarrier, occupying 23–53 kHz.
3. A 19 kHz pilot (half the subcarrier frequency, about 10% deviation) is added so the receiver can regenerate 38 kHz in phase.
4. The composite baseband signal is

$$m(t) = [L+R] + [L-R]\cos(2\pi\, 38\text{k}\, t) + K\cos(2\pi\, 19\text{k}\, t)$$

and it frequency-modulates the carrier (total deviation 75 kHz).

Baseband spectrum: L+R: 0-15 kHz | pilot 19 kHz | L-R DSB-SC: 23-53 kHz.

Stereo FM receiver

     +--------+     +--------+ L+R  +--------+--> L
FM ->| FM     |--+->| LPF    |----->| Matrix |
     |receiver|  |  | 15 kHz |      | (+,-)  |--> R
     +--------+  |  +--------+      +--------+
                 |  +--------+  +-----+  ^ L-R
                 +->| BPF    |->|Prod.|--+ (via LPF)
                 |  |23-53kHz|  | det.|
                 |  +--------+  +-----+
                 |  +--------+  +----+   ^ 38 kHz
                 +->|BPF 19k |->| x2 |---+
                    +--------+  +----+

1. The FM detector gives the composite baseband signal.
2. A 15 kHz LPF gives $L+R$.
3. A 23–53 kHz BPF and a synchronous (product) detector with the regenerated 38 kHz carrier (pilot ×2) give $L-R$.
4. The matrix forms $(L+R)+(L-R) = 2L$ and $(L+R)-(L-R) = 2R$, fed to the two speakers.

Output of a mono receiver

A mono FM receiver's audio stage passes only up to about 15 kHz. The $L-R$ signal at 23–53 kHz and the 19 kHz pilot are outside its audio band (and are removed by its de-emphasis and audio filtering). So the output is only

$$L + R$$

i.e. a normal monophonic signal containing both channels. This is why the system is called compatible: stereo broadcasts can be received on old mono sets without loss of programme content.