---
title: "Distortion, noise, interference; four types of noise"
subject: "Communication Systems"
chapter: 1
key: "semester_6/communication_systems:distortion-noise-interference-four-noise"
source: https://www.bctengineer.com/q/semester_6/communication_systems/distortion-noise-interference-four-noise
site: BCT Engineering
---

# Distortion, noise, interference; four types of noise

Communication Systems · Chapter 1: Introduction

- Asked in: 2068 Jestha (old course), 6+4 marks
- Video: [Types of Signal Impairment: Attenuation, Distortion & Noise | Computer Network Course | Lecture 17](https://www.youtube.com/watch?v=4O6yyHxJDDI) — Mind Your Exam Channel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/distortion-noise-interference-four-noise
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1

## Question

With examples, differentiate between distortion, noise and interference. Briefly explain any four types of noise encountered in communication.

## Short answer

Distortion is deterministic waveform change from system imperfections (e.g., amplifier clipping). Noise is random unwanted energy from natural/device sources (e.g., thermal hiss). Interference is contamination from other man-made signals (e.g., crosstalk). Four noise types: thermal (kTB), shot (2qI_dcB), flicker (1/f), and transit-time (high-frequency).

## Answer

#### Distortion, noise and interference
- **Distortion** is the change in waveform caused by the imperfect response of the system itself. Example: an overdriven audio amplifier clips peaks, adding harmonics; a long cable attenuates high frequencies more than low, smearing pulses.
- **Noise** is random, unwanted electrical energy from natural or device sources. Example: hiss in a radio from thermal noise of the RF amplifier; static from lightning.
- **Interference** is contamination by unwanted signals from other man-made sources. Example: a nearby FM station heard on your channel; power-line hum (50 Hz) picked up by audio cables; crosstalk in telephone lines.

| Point | Distortion | Noise | Interference |
|---|---|---|---|
| Origin | Within the system | Natural/device, random | Other man-made signals |
| Nature | Deterministic | Random | Usually deterministic |
| Present without signal | No | Yes | Yes |
| Spectrum | Harmonics or altered signal spectrum | Broad, often white | Specific frequencies |
| Predictable | Yes | No, only statistically | Mostly |
| Remedy | Equalizer, linear design, pre-distortion | Filtering, low-noise amplifier, more power | Shielding, filtering, frequency planning |

#### Four types of noise
1. **Thermal (Johnson) noise:** caused by random motion of electrons in any resistor or conductor above 0 K. Its power is $P_n=kTB$ and voltage $V_n=\sqrt{4kTBR}$; spectrum is white. It is the main internal noise in receivers.
2. **Shot noise:** occurs in active devices (diodes, transistors, tubes) because current is made of discrete charges arriving randomly at a junction. Mean-square current $\overline{i_n^2}=2qI_{dc}B$. Its spectrum is also nearly flat.
3. **Flicker (1/f) noise:** found at low frequencies (below a few kHz) in semiconductors and carbon resistors, due to surface and crystal imperfections. Its PSD varies roughly as $1/f$, so it matters in audio and DC amplifiers.
4. **Transit-time (high-frequency) noise:** at very high frequencies the carrier transit time through a device becomes comparable to the signal period, giving noise that rises with frequency and limits the device's upper frequency.

Other types worth naming: **external noise** such as atmospheric (lightning static, below about 30 MHz), extraterrestrial (solar and cosmic noise) and industrial (man-made, from ignition, motors, switching).

## Check yourself

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

1. Which statement correctly distinguishes distortion from noise?
   A. Distortion is random and present without signal
   B. Distortion arises within the system and disappears when the signal is removed
   C. Distortion comes from other transmitters
   D. Distortion has a flat white spectrum
   Answer: B. Distortion is deterministic, caused by the system itself, and absent without a signal; noise is random and present always. (Key point: Distortion vs noise)

2. Hearing a nearby FM station on your tuned channel is an example of
   A. Thermal noise
   B. Harmonic distortion
   C. Interference
   D. Shot noise
   Answer: C. Interference is contamination from other man-made signals at specific frequencies. (Key point: Interference example)

3. Thermal noise power in a bandwidth B at temperature T is
   A. 2qI B
   B. kTB
   C. 1/f
   D. kB/T
   Answer: B. Thermal (Johnson) noise from electron motion has P = kTB and a white spectrum. (Key point: Thermal noise)

4. Shot noise arises because
   A. Electrons move randomly in a resistor
   B. Current consists of discrete charges arriving randomly at a junction
   C. Lightning produces static
   D. Carrier transit time is comparable to the signal period
   Answer: B. Discrete charge carriers crossing a junction randomly give mean-square current 2qI_dc B. (Key point: Shot noise cause)

## Related in Communication Systems

- [Noise vs interference vs distortion](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-interference-distortion) — Communication Systems, chapter 1
- [Distortion vs noise vs interference](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adistortion-noise-interference-diff) — Communication Systems, chapter 1
- [Noise, distortion, interference; internal and external noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-types-internal-external) — Communication Systems, chapter 1
- [Noise, interference, distortion; thermal and high frequency noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-effects-thermal-hf) — Communication Systems, chapter 1
- [Reasons for modulation; noise vs distortion vs interference](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodulation-reasons-noise-distortion-compare) — Communication Systems, chapter 1
- [Short note: Distortion and interference](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-distortion-interference) — Communication Systems, chapter 1

## Related in other subjects

- [Noise, thermal noise, delay distortion](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Anoise-types-thermal) — Data Communication, chapter 1
- [Short note: Interference in mobile communication](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ainterference-mobile-communication) — Wireless Communication, chapter 2
- [Noise and two-wire to four-wire converter](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Anoise-two-four-wire-converter) — Telecommunications, chapter 2
- [Two-wire vs four-wire communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atwo-vs-four-wire) — Telecommunications, chapter 2