--- title: "Analog vs digital source; atmospheric noise" subject: "Communication Systems" chapter: 1 key: "semester_6/communication_systems:analog-vs-digital-source-atmospheric-noise" source: https://www.bctengineer.com/q/semester_6/communication_systems/analog-vs-digital-source-atmospheric-noise site: BCT Engineering --- # Analog vs digital source; atmospheric noise Communication Systems · Chapter 1: Introduction - Asked in: 2077 Chaitra (CS I), 2+2+4 marks - Video: [Different type of Noise- Extra-terrestrial Noise and Atmospheric Noise](https://www.youtube.com/watch?v=JAJEoPjXQzY) — Signals Systems - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/analog-vs-digital-source-atmospheric-noise - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1 ## Question List out the differences between analog source and digital source. What are the effects of atmospheric noise in analog communication system. Mention the advantages of analog communication over digital communication system? ## Short answer Analog sources produce continuous waveforms, while digital sources output discrete symbols. Atmospheric noise from lightning causes impulsive interference, degrades AM severely, and accumulates along repeaters; FM is much less affected. Analog advantages include lower bandwidth, simpler hardware, no quantization noise, graceful degradation, and no conversion delay. ## Answer #### Analog source vs digital source | Basis | Analog source | Digital source | |---|---|---| | Output | Continuous in time and amplitude | Finite set of discrete symbols | | Values | Infinite possible values | Countable (e.g. 0/1, alphabet) | | Examples | Microphone, thermometer, camera | Computer, keyboard, ADC output | | Description | Waveform, bandwidth | Symbol rate, entropy (bits/symbol) | #### Effects of atmospheric noise on analog communication Atmospheric (static) noise is produced mainly by **lightning discharges** and other natural electrical disturbances in the atmosphere. It travels to the receiver like radio waves. - It appears as **impulsive crackling** or "static" in AM radio and as spots on analog TV pictures. - It is strongest at **low and medium frequencies** and falls as frequency increases; it is negligible above about 30 MHz. - Since it is **additive**, it directly changes the amplitude of the received signal; **AM** is badly affected because the information is in the amplitude. FM is far less affected because a limiter removes amplitude variations. - It **reduces the SNR** and fidelity of the received message and limits the usable range of LF/MF/HF links. - In an analog system the noise cannot be separated from the signal once added, and repeaters amplify it along with the signal, so it **accumulates** over the link. - It is worst in the tropics and in summer/rainy seasons when thunderstorms are frequent. #### Advantages of analog over digital communication 1. **Less bandwidth:** e.g. a voice channel needs about 4 kHz in AM-SSB versus 64 kbit/s (about 32 kHz or more) in PCM. 2. **Simpler and cheaper hardware:** no ADC/DAC, coding or complex synchronization. 3. **No quantization noise:** the signal is represented exactly, apart from added noise. 4. **Graceful degradation:** quality falls slowly as SNR drops, instead of the sudden failure (cliff effect) seen in digital links. 5. **Real-time with no processing delay** (no coding or buffering latency). 6. Natural signals are already analog, so no conversion is needed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement correctly distinguishes an analog source from a digital source? A. Analog source is described by entropy, digital by waveform bandwidth B. Analog source output is continuous in time and amplitude; digital output is a finite set of discrete symbols C. Analog source outputs a finite set of discrete symbols; digital outputs continuous values D. Both have a countable set of values but differ in bandwidth Answer: B. An analog source (microphone) gives continuous waveforms with infinitely many values; a digital source gives discrete symbols, described by symbol rate and entropy. (Key point: Analog vs digital source) 2. Why is AM badly affected by atmospheric noise while FM is much less affected? A. Noise is additive and changes amplitude; AM carries information in amplitude, while FM's limiter removes amplitude variations B. FM operates below 30 MHz where atmospheric noise is weak C. FM signals are digital so noise has no effect D. AM uses a wider bandwidth than FM Answer: A. Lightning noise adds to the signal amplitude. AM information is in amplitude, whereas an FM receiver limiter clips amplitude variations. (Key point: Effect of atmospheric noise) 3. Atmospheric noise is strongest in which frequency region? A. Only at microwave frequencies B. Above 30 MHz, in the VHF/UHF range C. It is equally strong at all frequencies D. Low and medium frequencies, below about 30 MHz Answer: D. Atmospheric noise is strong at LF/MF/HF below about 30 MHz and negligible above it. (Key point: Frequency range of atmospheric noise) 4. What is meant by the 'graceful degradation' advantage of analog communication? A. It needs more bandwidth but gives better quality B. It has zero noise C. Quality falls slowly as SNR drops instead of a sudden failure like the digital cliff effect D. It needs no amplifier Answer: C. Analog quality deteriorates gradually with falling SNR, while digital links can fail abruptly (cliff effect). 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