--- title: "Stereo FM transmitter and receiver; output of mono receiver" subject: "Communication Systems" chapter: 3 key: "semester_6/communication_systems:stereo-fm-mono-receiver" source: https://www.bctengineer.com/q/semester_6/communication_systems/stereo-fm-mono-receiver site: BCT Engineering --- # Stereo FM transmitter and receiver; output of mono receiver Communication Systems · Chapter 3: Angle Modulation - Asked in: 2065 Kartik (CS I), 6+2 marks - Video: [FM Stereo Multiplex: How One Signal Carries Left and Right Audio](https://www.youtube.com/watch?v=G2cChOhXu8w) — Baltic Lab - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/stereo-fm-mono-receiver - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-3 ## Question Explain stereo FM transmitter and receiver with the help of block diagrams. If mono FM receiver is used to receive the signal from stereo FM transmitter, what will be the output of FM receiver? Explain. ## Short answer Stereo FM transmits a composite baseband: L+R (0–15 kHz), a 19 kHz pilot, and L−R DSB‑SC (23–53 kHz) on a 38 kHz subcarrier. A stereo receiver recovers L and R using filters and synchronous detection. A mono FM receiver passes only the L+R portion, outputting a compatible monophonic signal containing both channels. ## Answer **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 ```text 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 ```text +--------+ +--------+ 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which correctly describes the stereo FM baseband spectrum? A. L+R at 0-15 kHz, 19 kHz pilot, L-R DSB-SC at 23-53 kHz B. L-R at 0-15 kHz, pilot at 38 kHz, L+R at 23-53 kHz C. L and R each AM at 19 kHz D. L+R at 23-53 kHz, L-R at 0-15 kHz Answer: A. The sum signal is baseband, the pilot is at 19 kHz, and L-R is DSB-SC on a 38 kHz subcarrier, occupying 23-53 kHz. (Key point: Composite baseband) 2. Why is a 19 kHz pilot transmitted? A. To carry the L+R signal B. To increase deviation to 75 kHz C. To act as the carrier D. The receiver doubles it to regenerate the 38 kHz subcarrier in phase for demodulating L-R Answer: D. The pilot is half the subcarrier frequency; doubling it gives the synchronous carrier needed to detect the DSB-SC L-R signal. (Key point: Pilot tone) 3. How does the stereo receiver obtain L and R? A. Add and subtract L+R and L-R in a matrix to give 2L and 2R B. Filter L and R directly with BPFs C. Square the composite signal D. Use two separate FM carriers Answer: A. (L+R)+(L-R) = 2L and (L+R)-(L-R) = 2R. (Key point: Recovering L and R) 4. What does a mono receiver output from a stereo transmission? A. Only the left channel B. Only L-R C. Nothing D. L+R, a normal monophonic signal containing both channels Answer: D. Its audio stage passes only up to about 15 kHz, so the 19 kHz pilot and 23-53 kHz L-R are not reproduced; this is why stereo is compatible. (Key point: Mono receiver output) ## Related in Communication Systems - [Stereo FM encoder and decoder block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Astereo-fm-encoder-decoder-blocks) — Communication Systems, chapter 3 - [Pre-emphasis in FM broadcasting; stereo encoder](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apre-emphasis-stereo-encoder) — Communication Systems, chapter 3, asked 2× - [Armstrong FM transmitter numerical](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aarmstrong-numerical-200khz-25hz) — Communication Systems, chapter 3 - [Armstrong FM transmitter numerical (Δf1 = 24 Hz)](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aarmstrong-numerical-24hz) — Communication Systems, chapter 3 - [Narrowband vs wideband FM; stereo FM broadcasting](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anbfm-wbfm-stereo-fm) — Communication Systems, chapter 3 - [Stereo FM encoder and decoder](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Astereo-fm-encoder-decoder) — Communication Systems, chapter 3, asked 2× ## Related in other subjects - [OFDM transmitter and receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-transmitter-receiver) — Wireless Communication, chapter 4, asked 5× - [Spread spectrum and DSSS transmitter/receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-dsss-tx-rx) — Wireless Communication, chapter 4 - [DPSK transmitter/receiver and PN sequence](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adpsk-pn-sequence) — Wireless Communication, chapter 4 - [OQPSK transmitter/receiver and π/4-QPSK](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aoqpsk-pi4-qpsk) — Wireless Communication, chapter 4