---
title: "Short note: Pre-emphasis and de-emphasis"
subject: "Communication Systems"
chapter: 3
key: "semester_6/communication_systems:short-note-pre-de-emphasis"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-pre-de-emphasis
site: BCT Engineering
---

# Short note: Pre-emphasis and de-emphasis

Communication Systems · Chapter 3: Angle Modulation

- Asked in: 2081 Chaitra, 5 marks; 2080 Chaitra (CS I), 5 marks; 2074 Bhadra (CS I), 4 marks; 2072 Asoj (CS I), 4 marks (asked 4 times)
- 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/short-note-pre-de-emphasis
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-3

## Question

Write a short note on pre-emphasis and de-emphasis.

## Short answer

Pre-emphasis boosts high-frequency message components at the FM transmitter to combat parabolic noise; de-emphasis attenuates them at the receiver to restore the original signal. Standard time constant is 75 µs (f₁≈2.1 kHz) in US FM, improving output SNR by about 10–13 dB without extra bandwidth.

## Answer

**Pre-emphasis** is the boosting of the high-frequency part of the message at the FM transmitter before modulation; **de-emphasis** is the matching attenuation of those frequencies at the receiver after demodulation.

**Why they are needed**

- In FM, the output noise power spectral density rises with the square of frequency (parabolic noise spectrum): $S_N(f) \propto f^2$.
- Audio signals have most of their energy at low frequencies; the high-frequency parts are weak.
- So the high audio frequencies have the worst SNR. Pre-emphasis raises them above the noise before transmission; de-emphasis restores the original balance and, at the same time, cuts the high-frequency noise.

**Circuits**

```text
 Pre-emphasis (high-pass)   De-emphasis (low-pass)
   +--[ R ]--+                o--[ R ]--+----o
 o-+         +--+--o                    |
   +--||-----+  |                      === C
       C        > r                     |
 o--------------+--o          o---------+----o
```

- Pre-emphasis: $H_{pe}(f) = 1 + j\dfrac{f}{f_1}$, where $f_1 = \dfrac{1}{2\pi RC}$.
- De-emphasis: $H_{de}(f) = \dfrac{1}{1 + jf/f_1}$, so $H_{pe}H_{de} = 1$ and the message is undistorted.
- Standard time constant $RC = 75\ \mu$s ($f_1 \approx 2.1$ kHz) in USA FM broadcasting; 50 µs in Europe and many other countries.

```text
 Gain
  ^      pre-emphasis
  |          ___/
  |      ___/
  |-----/---------- 0 dB
  |      \___
  |          \___ de-emphasis
  +-----+------------> f
        f1=2.1 kHz
```

**Benefits**

- Improves output SNR of FM by about 10–13 dB for audio.
- No extra bandwidth needed (for normal audio content).
- Also used in phonograph recording, tape recording and TV sound.

## 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 FM?
   A. FM noise power density rises as f^2 while audio is weak at high frequencies, so high audio frequencies have poor SNR
   B. AM noise is flat but FM has no noise
   C. To increase the carrier bandwidth
   D. To remove the pilot tone
   Answer: A. Parabolic FM noise hits the weak high audio frequencies hardest; boosting them before transmission improves their SNR. (Key point: Reason for emphasis)

2. Where are the two networks placed?
   A. Both at the receiver
   B. Pre-emphasis at the receiver, de-emphasis at the transmitter
   C. Pre-emphasis (high-pass) at the transmitter before modulation, de-emphasis (low-pass) at the receiver after demodulation
   D. Both after the modulator
   Answer: C. Pre-emphasis boosts highs before modulation; de-emphasis cuts them after the discriminator. (Key point: Location of the networks)

3. Why is the message not distorted overall?
   A. The noise is also boosted equally
   B. Hpe(f)·Hde(f) = 1, so the two cancel for the signal
   C. The bandwidth is doubled
   D. The corner frequency is zero
   Answer: B. Hpe = 1 + jf/f1 and Hde = 1/(1 + jf/f1), whose product is 1, while de-emphasis attenuates the noise. (Key point: Why the message is undistorted)

4. What is the standard time constant in USA FM broadcasting and the typical SNR gain?
   A. 10 µs; about 1 dB
   B. 75 µs (f1 ≈ 2.1 kHz); about 10–13 dB
   C. 500 µs; about 30 dB
   D. 75 ms; about 3 dB
   Answer: B. RC = 75 µs gives f1 = 1/(2πRC) ≈ 2.1 kHz, with 10–13 dB improvement. (Key point: Standard values)

## Related in Communication Systems

- [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
- [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
- [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