--- title: "Narrowband vs wideband FM; stereo FM broadcasting" subject: "Communication Systems" chapter: 3 key: "semester_6/communication_systems:nbfm-wbfm-stereo-fm" source: https://www.bctengineer.com/q/semester_6/communication_systems/nbfm-wbfm-stereo-fm site: BCT Engineering --- # Narrowband vs wideband FM; stereo FM broadcasting Communication Systems · Chapter 3: Angle Modulation - Asked in: 2077 Chaitra (CS I), 2+6 marks - Video: [Performance comparison of NBFM and WBFM in Communication Engineering by Engineering Funda](https://www.youtube.com/watch?v=x0yFW0xh4yE) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/nbfm-wbfm-stereo-fm - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-3 ## Question Differentiate between Narrow Band FM and Wide Band FM. Describe stereo FM broadcasting with its block diagram and spectral details. ## Short answer NBFM has modulation index β < 0.3, bandwidth ≈ 2fm; WBFM has β >> 1, bandwidth ≈ 2(Δf+fm). Stereo FM transmits a composite baseband: L+R (0–15 kHz), 19 kHz pilot, and L–R DSB-SC on 38 kHz subcarrier (23–53 kHz). A mono receiver uses L+R; a stereo decoder recovers L and R via matrix. ## Answer #### NBFM versus WBFM | Point | Narrow band FM | Wide band FM | |---|---|---| | Modulation index | $\beta \ll 1$ (< 0.3) | $\beta \gg 1$ | | Bandwidth | $\approx 2f_m$ (like AM) | $\approx 2(\Delta f + f_m)$ | | Sidebands | Carrier + one pair | Many significant pairs | | Max deviation | Small (~5 kHz) | Large (75 kHz in broadcast) | | Noise performance | Little better than AM | Much better SNR | | Use | Mobile radio, police, Armstrong first stage | FM broadcast, TV sound | #### Stereo FM broadcasting Stereo FM transmits left (L) and right (R) audio on one carrier, while mono receivers can still receive the program. **Encoder (transmitter)** ```text L --+-->(+)--L+R-->[LPF]----------->( Σ )--> | ^ ^ ^ to FM R --|----+ | | mod. +-->(-)--L-R-->[LPF]-->[Bal.mod]--+ | ^ (R) ^ 38k | [19k osc]->[x2] | | | +--19 kHz pilot----+ ``` 1. Matrix makes $L+R$ and $L-R$, each limited to 15 kHz and pre-emphasized. 2. $L-R$ DSB-SC modulates a 38 kHz sub-carrier (balanced modulator). 3. 38 kHz is obtained by doubling a 19 kHz oscillator; the 19 kHz pilot is added at low level for receiver synchronization. 4. The composite signal $$ m(t) = (L+R) + (L-R)\cos(2\pi\,38\text{k}\,t) + A_p\cos(2\pi\,19\text{k}\,t) $$ frequency-modulates the RF carrier (peak deviation 75 kHz). **Spectrum** ```text | L+R pilot L-R DSB-SC |______ | ____ ____ | | | | | | | +------+------+---+----+--+----+---> f (kHz) 0 15 19 23 38 53 ``` **Decoder (receiver)** ```text FM +->[LPF 0-15k]------------L+R--+->(+)--> 2L demod | | ^ out --+->[BPF 23-53k]-->( X )->[LPF]-+ | | ^ L-R | | | | 38k +--|-->(-)--> 2R +->[BPF 19k]-->[ x2 ] | (pilot) L-R ---+ ``` - LPF gives $L+R$ (mono output). - Pilot is filtered and doubled to 38 kHz; the 23–53 kHz band is coherently detected to give $L-R$. - Adding and subtracting give $2L$ and $2R$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do narrow band and wide band FM differ in modulation index and bandwidth? A. NBFM: beta >> 1, B about 2(Delta f + fm); WBFM: beta << 1, B about 2fm B. NBFM: beta << 1, B about 2fm; WBFM: beta >> 1, B about 2(Delta f + fm) C. Both have B = 2fm but differ in carrier D. NBFM has many sideband pairs; WBFM has one pair Answer: B. NBFM has a small beta and AM-like bandwidth; WBFM has many significant sidebands. (Key point: NBFM vs WBFM index) 2. Which set of components forms the composite stereo FM baseband signal? A. L+R (0-15 kHz), 19 kHz pilot, and L-R DSB-SC around 38 kHz B. L+R at 38 kHz, L-R baseband, and a 19 kHz pilot C. L and R sent as two separate FM carriers D. L-R baseband, R at 38 kHz DSB-SC, and a 38 kHz pilot Answer: A. The sum L+R stays at baseband for mono compatibility, L-R is DSB-SC on 38 kHz (23-53 kHz), and a 19 kHz pilot is added. (Key point: Composite signal) 3. Why is a low-level 19 kHz pilot transmitted? A. To carry the L+R signal for mono sets B. To let the receiver regenerate a phase-locked 38 kHz carrier for detecting L-R C. To increase the FM deviation to 75 kHz D. To act as the pre-emphasis reference Answer: B. Doubling the recovered pilot gives a 38 kHz carrier in phase with the transmitter's, needed for coherent detection of L-R. (Key point: Purpose of 19 kHz pilot) 4. Why can a mono receiver still play a stereo broadcast properly? A. It demodulates the L-R DSB-SC signal and ignores L+R B. It subtracts the pilot from the composite signal C. Its 0-15 kHz low-pass filter picks out L+R, which is the full mono program D. It decodes the 38 kHz subcarrier with a fixed oscillator Answer: C. L+R occupies 0-15 kHz at baseband, so a simple LPF recovers the mono signal and the higher components are filtered out. (Key point: Mono compatibility) ## Related in Communication Systems - [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× - [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 - [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× - [PSD–autocorrelation relation; stereo FM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apsd-autocorrelation-stereo-fm) — Communication Systems, chapter 3 - [Stereo FM transmitter and receiver; output of mono receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Astereo-fm-mono-receiver) — Communication Systems, chapter 3 - [Digital vs analog communication; DCS block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-vs-analog-dcs-block) — Communication Systems, chapter 1 ## Related in other subjects - [OFC block diagram vs microwave link](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-block-diagram-vs-microwave) — Propagation and Antenna, chapter 6 - [OFC block diagram vs cable](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-block-diagram-vs-cable) — Propagation and Antenna, chapter 6 - [Block diagram of electric drive; series vs shunt starting](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adrive-block-diagram-motor-starting) — Utilization of Electrical Energy, chapter 2 - [Energy vs power spectral density](https://www.bctengineer.com/question.md?key=semester_4%2Fsignals_and_systems%3Aenergy-vs-power-spectral-density) — Signals and Systems, chapter 3