--- title: "Reasons for modulation; noise vs distortion vs interference" subject: "Communication Systems" chapter: 1 key: "semester_6/communication_systems:modulation-reasons-noise-distortion-compare" source: https://www.bctengineer.com/q/semester_6/communication_systems/modulation-reasons-noise-distortion-compare site: BCT Engineering --- # Reasons for modulation; noise vs distortion vs interference Communication Systems · Chapter 1: Introduction - Asked in: 2070 Magh (CS I), 2+4 marks - Video: [1.2 Noise, distortion and interference | Communication System IOE](https://www.youtube.com/watch?v=kTfHdgxg5QY) — Saurab Poudel - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/modulation-reasons-noise-distortion-compare - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1 ## Question Define modulation and explain the reasons for modulation. Compare noise, distortion and interference. ## Short answer Modulation varies a high-frequency carrier’s parameter with the message. Reasons: practical antenna size, multiplexing, avoiding mutual interference, channel matching, and noise reduction. Noise is random, always present; distortion is deterministic waveform change from imperfect response; interference is unwanted deterministic signals from other sources. ## Answer #### Modulation **Modulation** is the process of varying some parameter (amplitude, frequency or phase) of a high-frequency **carrier** in accordance with the instantaneous value of the message (modulating) signal. The message is then carried at the carrier frequency, e.g. AM, FM, PM, ASK, FSK. #### Reasons for modulation 1. **Practical antenna size:** an antenna must be about $\lambda/4$ long. For a 3 kHz audio signal $\lambda=100$ km, needing a 25 km antenna; at 1 MHz carrier the length is only about 75 m. 2. **Multiplexing:** different messages are placed on different carriers (FDM), so many stations share one medium without mixing. 3. **Avoid mixing of signals:** all audio lies in 20 Hz–20 kHz; without modulation all stations would interfere. 4. **Matching to the channel:** the signal is moved to a band where the channel has low loss and suitable propagation (e.g. ionospheric, line-of-sight). 5. **Noise reduction:** wideband methods such as FM and PCM trade bandwidth for better SNR. 6. **Narrowbanding:** the ratio of highest to lowest frequency becomes close to 1, so one antenna and amplifier design works over the whole band. 7. **Long-distance radiation:** high-frequency signals radiate efficiently and travel farther. #### Noise vs distortion vs interference | Point | Noise | Distortion | Interference | |---|---|---|---| | Nature | Random, unpredictable | Deterministic waveform change | Unwanted signal from another source | | Cause | Thermal agitation, shot effect, atmosphere | Imperfect system response (non-linearity, non-flat amplitude or phase) | Other transmitters, machines, crosstalk | | Exists without signal? | Yes | No, vanishes with signal | Yes | | Spectrum | Usually broad (white) | Harmonics or changed spectrum of the signal | Specific frequencies of the other source | | Example | Hiss in a radio | Clipping in an overdriven amplifier | Adjacent station heard in a radio | | Remedy | Filtering, low-noise devices, more power | Equalizers, linear design | Shielding, filtering, frequency planning | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Modulation is: A. Varying a carrier parameter according to the message B. Amplifying the message C. Filtering noise D. Sampling the message Answer: A. Amplitude, frequency or phase of the carrier follows the message. (Key point: Modulation definition) 2. For a 3 kHz signal, lambda is 100 km. Quarter-wave antenna length is: A. 25 km B. 100 km C. 75 m D. 1 km Answer: A. lambda/4 = 25 km; at 1 MHz carrier it is about 75 m. (Key point: Antenna size) 3. Which has no existence without the signal? A. Noise B. Interference C. Distortion D. All three Answer: C. Distortion vanishes when the signal is absent; noise and interference remain. (Key point: Noise vs distortion) 4. Why do FM and PCM reduce noise effects? A. They use less bandwidth B. They trade bandwidth for better SNR C. They increase carrier amplitude D. They remove thermal noise Answer: B. Wideband modulation exchanges bandwidth for SNR. (Key point: Noise reduction) ## 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 vs interference; minimizing effects; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-interference-minimize-modulation-need) — 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 - [Distortion, noise, interference; four types of noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adistortion-noise-interference-four-noise) — 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 - [Reasons: slot numbers and speed vs size](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aslot-and-speed-reasons) — Electric Machine Design, chapter 4, asked 2×