Stereo FM encoder and decoder block diagram

Stereo FM carries left (L) and right (R) audio on one FM carrier as a composite baseband signal, keeping compatibility with mono receivers.

Encoder

 L --+-->(+)--L+R-->[LPF]----------->( Σ )-->
     |    ^                            ^ ^  to FM
 R --|----+                            | |  mod.
     +-->(-)--L-R-->[LPF]-->[Bal.mod]--+ |
          ^ (R)                 ^ 38k    |
                  [19k osc]->[x2]        |
                      |                  |
                      +--19 kHz pilot----+

• Matrix (adder/subtractor): forms $L+R$ and $L-R$.
• LPF (15 kHz) and pre-emphasis: band-limit each audio signal and boost high frequencies.
• 19 kHz pilot oscillator: crystal-controlled reference.
• Frequency doubler (×2): gives the 38 kHz sub-carrier, phase-locked to the pilot.
• Balanced modulator: DSB-SC modulates $L-R$ on 38 kHz, producing 23–53 kHz.
• Adder: sums $L+R$, the DSB-SC $L-R$ and a low-level 19 kHz pilot into the composite signal, which drives the FM modulator.

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

 |  L+R      pilot    L-R DSB-SC
 |______       |    ____    ____
 |      |      |   |    |  |    |
 +------+------+---+----+--+----+---> f (kHz)
 0     15     19  23   38      53

Decoder

FM    +->[LPF 0-15k]------------L+R--+->(+)--> 2L
demod |                              |   ^
out --+->[BPF 23-53k]-->( X )->[LPF]-+   |
      |                   ^      L-R  |  |
      |                   | 38k       +--|-->(-)--> 2R
      +->[BPF 19k]-->[ x2 ]              |
         (pilot)                  L-R ---+

• FM discriminator: recovers the composite signal.
• LPF (0–15 kHz): extracts $L+R$ (mono signal).
• Narrow BPF (19 kHz): extracts the pilot.
• Doubler (×2): regenerates a 38 kHz carrier in phase with the transmitter's.
• BPF (23–53 kHz): selects the $L-R$ DSB-SC signal.
• Product detector and LPF: coherent detection with 38 kHz gives $L-R$.
• Matrix: $(L+R)+(L-R) = 2L$, $(L+R)-(L-R) = 2R$.
• De-emphasis and AF amplifiers: restore flat response and drive the left and right speakers.