---
title: "Need for pre-emphasis and de-emphasis in FM"
subject: "Communication Systems"
chapter: 3
key: "semester_6/communication_systems:need-pre-de-emphasis-fm"
source: https://www.bctengineer.com/q/semester_6/communication_systems/need-pre-de-emphasis-fm
site: BCT Engineering
---

# Need for pre-emphasis and de-emphasis in FM

Communication Systems · Chapter 3: Angle Modulation

- Asked in: 2080 Chaitra, 4 marks
- Video: [Communication Engineering - Pre-emphasis and De-emphasis](https://www.youtube.com/watch?v=VnRppcBZ2Nc) — Dr. Sapna Katiyar
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/need-pre-de-emphasis-fm
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-3

## Question

Why do we need pre-emphasis and de-emphasis circuits in FM? Explain.

## Short answer

FM demodulator output noise power increases as f², while audio signals are weak at high frequencies, causing poor SNR. Pre-emphasis (high-pass filter) boosts highs before modulation; de-emphasis (low-pass filter) after demodulation restores the original spectrum and attenuates noise, giving about 10–13 dB SNR improvement.

## Answer

Pre-emphasis and de-emphasis are needed in FM to improve the signal-to-noise ratio at high audio frequencies.

1. **Noise in FM rises with frequency.** The FM discriminator output noise PSD is $S_{N}(f) \propto f^2$ (triangular noise voltage, parabolic noise power). So high-frequency audio components get much more noise.
2. **Audio signals are weak at high frequencies.** Most speech and music energy is below 1–2 kHz.
3. Together, these make the SNR of the high audio frequencies very poor.

**Solution**

- **Pre-emphasis** at the transmitter: a high-pass RC network boosts high audio frequencies before modulation, $H_{pe}(f) = 1 + jf/f_1$.
- **De-emphasis** at the receiver: a low-pass RC network after demodulation, $H_{de}(f) = 1/(1 + jf/f_1)$, restores the original response and attenuates the high-frequency noise at the same time.
- Since $H_{pe}H_{de} = 1$, the signal is unchanged but the output noise falls, giving about 10–13 dB SNR improvement. Standard time constant: 75 µs (USA) or 50 µs (Europe), $f_1 = 1/(2\pi RC) \approx 2.1$ kHz for 75 µs.

```text
 Gain   pre-emph  _/
  |           __/
  |----------/--------- 0 dB
  |          \__
  |             \_ de-emph
  +---------+---------> f
           f1
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does FM discriminator output noise vary with frequency?
   A. Its power density is constant
   B. Its power density falls as 1/f
   C. Its power density rises as f^2 (parabolic)
   D. It is zero above 15 kHz
   Answer: C. Noise voltage is triangular, so noise power density is parabolic, hurting high audio frequencies. (Key point: Noise in FM)

2. Why do high audio frequencies have especially poor SNR?
   A. Audio energy is mostly at low frequencies, so highs are weak while noise is largest there
   B. Audio is strongest at high frequencies
   C. The carrier is weakest there
   D. De-emphasis is absent
   Answer: A. Weak signal combined with the highest noise gives very poor SNR. (Key point: Audio spectrum)

3. Which pair is correct?
   A. Pre-emphasis low-pass at receiver; de-emphasis high-pass at transmitter
   B. Pre-emphasis high-pass at transmitter; de-emphasis low-pass at receiver
   C. Both are high-pass
   D. Both are low-pass
   Answer: B. Pre-emphasis boosts highs before modulation and de-emphasis attenuates them after demodulation. (Key point: Function of each network)

4. For a 75 µs time constant, f1 = 1/(2πRC) is about
   A. 2.1 kHz
   B. 21 kHz
   C. 75 kHz
   D. 212 Hz
   Answer: A. 1/(2π × 75 µs) ≈ 2.12 kHz. (Key point: Corner frequency)

## Related in Communication Systems

- [Limiter-discriminator FM demodulation; need for pre-emphasis](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Alimiter-discriminator-pre-emphasis) — 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 (indirect) FM generation; pre-emphasis](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aarmstrong-indirect-pre-emphasis) — 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×

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