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Short note: Stereo encoder

Short answer

A stereo encoder forms L+R and L-R signals, low-pass filters and pre-emphasizes them, modulates L-R onto a 38 kHz subcarrier via DSB-SC, adds a 19 kHz pilot, and sums them to produce composite baseband m(t)=(L+R)+(L-R)cos(2π·38000t)+K cos(2π·19000t) that frequency-modulates the carrier. Total baseband bandwidth is 53 kHz.

Write a short note on stereo encoder.

A stereo encoder (stereo multiplexer) combines the left (L) and right (R) audio channels into one composite baseband signal for FM stereo broadcasting, in a way that mono receivers can still use.

Block diagram

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

Working

  1. A matrix circuit (adder and subtractor) forms L+RL+R (mono-compatible sum) and L−RL-R (difference).
  2. Both are low-pass filtered to 15 kHz and pre-emphasized.
  3. L−RL-R is DSB-SC modulated in a balanced modulator onto a 38 kHz sub-carrier, giving sidebands from 23 to 53 kHz.
  4. The 38 kHz sub-carrier is made by doubling a 19 kHz crystal oscillator. A small 19 kHz pilot (about 10% deviation) is added so the receiver can regenerate the 38 kHz carrier in phase.
  5. The composite signal is
m(t)=(L+R)+(L−R)cos⁡(2π 38000 t)+Kcos⁡(2π 19000 t)m(t) = (L+R) + (L-R)\cos(2\pi\,38000\,t) + K\cos(2\pi\,19000\,t)

Spectrum of composite signal

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

The total baseband bandwidth is 53 kHz, which then frequency-modulates the carrier. A mono receiver uses only 0–15 kHz (L+RL+R); a stereo receiver recovers L−RL-R as well and forms LL and RR by adding and subtracting.

✓ Answer checked by Claude Opus 5.5 (high effort)

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