--- title: "Digital vs analog communication; DCS block diagram" subject: "Communication Systems" chapter: 1 key: "semester_6/communication_systems:digital-vs-analog-dcs-block" source: https://www.bctengineer.com/q/semester_6/communication_systems/digital-vs-analog-dcs-block site: BCT Engineering --- # Digital vs analog communication; DCS block diagram Communication Systems · Chapter 1: Introduction - Asked in: 2080 Bhadra (CS II), 2+6 marks - Video: [Introduction to Analog and Digital Communication | The Basic Block Diagram of Communication System](https://www.youtube.com/watch?v=qhjj6WG7Rgc) — ALL ABOUT ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/digital-vs-analog-dcs-block - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1 ## Question List the differences between digital communication system and analog communication system. Draw functional block diagrams of DCS and explain the significance of each major blocks. ## Short answer Digital systems use discrete symbols, offer high noise immunity via regenerative repeaters, error control, security, and easy TDM multiplexing but require more bandwidth. The DCS block diagram includes source encoder (compression), channel encoder (error correction), modulator/demodulator, channel, and synchronization blocks. ## Answer #### Digital vs analog communication | Point | Analog communication | Digital communication | |---|---|---| | Signal | Continuous in time and amplitude | Finite set of symbols (e.g. 0/1) | | Noise immunity | Low; noise accumulates | High; regenerative repeaters remove noise | | Error control | Not possible | Error detection and correction codes | | Security | Hard to encrypt | Easy encryption | | Multiplexing | FDM | TDM, easy integration of voice, video, data | | Bandwidth | Less | More (e.g. PCM voice 64 kb/s) | | Hardware | Analog circuits, drift | VLSI, DSP, cheap and flexible | | Quality measure | SNR | Bit error rate (BER) | #### Functional block diagram of a digital communication system ```text Source->[Format]->[Source enc]->[Encrypt]->[Channel enc] | [Modulator]<-[Multiplex]<--+ | CHANNEL <- noise, fading | [Demod/Detect]->[Demux]-->+ | Sink<-[Format]<-[Source dec]<-[Decrypt]<-[Channel dec] ``` #### Significance of each block 1. **Information source and formatting:** the source gives analog or digital data. Formatting converts it to digital form by **sampling, quantization and coding (PCM)**, producing a bit stream. 2. **Source encoder:** removes redundancy to reduce the bit rate, e.g. Huffman coding, DPCM, MP3, JPEG. This saves bandwidth and power. 3. **Encryption:** scrambles data with a key so that only authorized receivers can read it; gives privacy and authentication. 4. **Channel encoder:** adds controlled redundancy (parity, Hamming, CRC, convolutional codes) so that the receiver can **detect and correct errors** caused by noise. 5. **Multiplexer / multiple access:** combines several users' bit streams (TDM, CDMA) to share one channel efficiently. 6. **Modulator:** maps bits onto waveforms suitable for the channel; baseband line codes (NRZ, Manchester) or band-pass schemes (ASK, FSK, PSK, QAM). It also allows efficient radiation and frequency allocation. 7. **Channel:** the physical medium (wire, fibre, radio). It attenuates, distorts (ISI) and adds noise and interference. 8. **Demodulator / detector:** includes filtering (matched filter), sampling, synchronization and decision devices to recover bits from the noisy waveform with minimum error. 9. **Receiver-side blocks (demultiplex, channel decoder, decryption, source decoder, formatting):** reverse the transmitter operations: separate users, correct errors, decrypt, expand compressed data and convert back to analog (D/A, filtering) for the user. 10. **Synchronization** (carrier and symbol timing) runs throughout the receiver and is essential for correct detection. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why do digital systems have better noise immunity over long distances? A. Regenerative repeaters remove noise at each hop B. They use less bandwidth C. They are analog D. Noise cannot enter a digital channel Answer: A. Since only discrete levels exist, repeaters regenerate clean symbols and noise does not accumulate. (Key point: Noise immunity) 2. The source encoder in a DCS A. Adds redundancy to correct errors B. Removes redundancy to reduce the bit rate C. Encrypts data D. Combines users Answer: B. Compression (Huffman, DPCM, MP3) saves bandwidth and power. (Key point: Source encoder role) 3. The channel encoder adds A. Controlled redundancy for error detection and correction B. A carrier C. Noise D. Compression Answer: A. Parity, Hamming, CRC or convolutional codes let the receiver detect and correct errors. (Key point: Channel encoder role) 4. Quality of a digital system is measured by A. SNR only B. Bit error rate C. Carrier frequency D. Antenna gain Answer: B. Digital quality is expressed by BER, whereas analog uses SNR. 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