--- title: "Pre-emphasis in FM broadcasting; stereo encoder" subject: "Communication Systems" chapter: 3 key: "semester_6/communication_systems:pre-emphasis-stereo-encoder" source: https://www.bctengineer.com/q/semester_6/communication_systems/pre-emphasis-stereo-encoder site: BCT Engineering --- # Pre-emphasis in FM broadcasting; stereo encoder Communication Systems · Chapter 3: Angle Modulation - Asked in: 2075 Bhadra (CS I), 3+5 marks; 2070 Magh (CS I), 2+4 marks (asked 2 times) - Video: [CT#41 Pre emphasis and De emphasis in FM system || EC Academy](https://www.youtube.com/watch?v=ZjcMZ0ORTTM) — EC Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/pre-emphasis-stereo-encoder - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-3 ## Question Why pre-emphasis and de-emphasis circuits are used in commercial FM broadcasting? Explain the functional block diagram of stereo encoder. ## Short answer Pre-emphasis boosts high audio frequencies before modulation to overcome triangular FM noise; de-emphasis attenuates them after demodulation, restoring the signal while reducing noise by 10–13 dB. A stereo encoder matrix produces L+R and L−R; L−R modulates a 38 kHz subcarrier (DSB-SC) and a 19 kHz pilot is added, forming the composite baseband signal that FM-modulates the carrier. ## Answer #### Why pre-emphasis and de-emphasis are used In FM, noise at the discriminator output is not flat: its power spectral density rises as $f^2$ (parabolic noise). Audio signals, on the other hand, have most energy at low frequencies and little at high frequencies. So the high audio frequencies have a very poor SNR. - **Pre-emphasis** (at the transmitter, before the modulator): a high-pass RC network, $H_{pe}(f) = 1 + jf/f_1$, boosts high audio frequencies so that they stand well above the noise. - **De-emphasis** (at the receiver, after the discriminator): a low-pass RC network, $H_{de}(f) = 1/(1 + jf/f_1)$, brings the boosted frequencies back to their original level. It attenuates the high-frequency noise by the same amount. - Since $H_{pe}H_{de} = 1$, the message is undistorted, but the noise is reduced. The improvement is about 10–13 dB. - Standard time constant: 75 µs ($f_1 = 2.12$ kHz) in the USA and 50 µs in Europe and most other countries. ```text Pre-emph De-emph o-+-[R]-+-o o-[R]-+-o +-||--+ | | C > r === C o---------+-o o-----+-o ``` #### Stereo encoder A **stereo encoder** forms one composite baseband signal from the left and right audio channels for FM stereo broadcasting. ```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 network:** adder and subtractor make $L+R$ (for mono compatibility) and $L-R$. 2. **LPF (15 kHz) and pre-emphasis** on both. 3. **Balanced modulator:** $L-R$ DSB-SC modulates a 38 kHz sub-carrier (sidebands 23–53 kHz). 4. **19 kHz pilot oscillator and doubler:** gives the 38 kHz sub-carrier; the 19 kHz pilot is also added at low level so receivers can regenerate the 38 kHz carrier. 5. **Adder:** composite signal $$ m(t) = (L+R) + (L-R)\cos(2\pi\,38\text{k}\,t) + A_p\cos(2\pi\,19\text{k}\,t) $$ ```text | L+R pilot L-R DSB-SC |______ | ____ ____ | | | | | | | +------+------+---+----+--+----+---> f (kHz) 0 15 19 23 38 53 ``` This 53 kHz composite signal frequency-modulates the main carrier. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are pre-emphasis and de-emphasis used in commercial FM? A. Because FM noise rises as f^2 while audio energy is low at high frequencies B. To make the carrier constant C. To suppress the pilot D. To increase the transmitter power Answer: A. High audio frequencies suffer the worst SNR, so they are boosted before and cut after transmission. (Key point: Reason for emphasis) 2. How does de-emphasis reduce noise while keeping the message undistorted? A. It adds a second noise source B. It amplifies the message C. It removes the carrier D. It undoes the pre-emphasis boost on the signal and attenuates high-frequency noise by the same amount Answer: D. Hpe·Hde = 1 for the signal, but noise added after the transmitter only sees Hde, so it is reduced by 10–13 dB. (Key point: Noise reduction) 3. What does the matrix network of the stereo encoder form? A. L×R and L/R B. L+R for mono compatibility and L−R C. L only and R only D. L+R and the pilot Answer: B. An adder and a subtractor produce L+R and L−R. (Key point: Stereo matrix) 4. In the composite signal, where do L+R, the pilot and the L−R sidebands lie? A. L+R 23–53 kHz, pilot 38 kHz, L−R 0–15 kHz B. L+R 0–15 kHz, pilot 19 kHz, L−R 23–53 kHz C. All in 0–15 kHz D. L+R 19 kHz, pilot 0–15 kHz, L−R 38 kHz Answer: B. L+R is baseband, the 19 kHz pilot sits in the gap, and L−R DSB-SC is centred at 38 kHz. (Key point: Composite signal band) ## 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 - [Need for pre-emphasis and de-emphasis in FM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aneed-pre-de-emphasis-fm) — Communication Systems, chapter 3 - [Armstrong (indirect) FM generation; pre-emphasis](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aarmstrong-indirect-pre-emphasis) — 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 - [Short note: Pre-emphasis and de-emphasis](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-pre-de-emphasis) — Communication Systems, chapter 3, asked 4× - [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 - [Functional block diagram of excitation control](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-control-block-diagram) — Power Plant Equipment, chapter 3