--- title: "Function of basic components of a digital communication system" subject: "Communication Systems" chapter: 1 key: "semester_6/communication_systems:dcs-basic-components" source: https://www.bctengineer.com/q/semester_6/communication_systems/dcs-basic-components site: BCT Engineering --- # Function of basic components of a digital communication system Communication Systems · Chapter 1: Introduction - Asked in: 2079 Bhadra (CS II), 5 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/dcs-basic-components - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1 ## Question Briefly explain the function of the basic components of a digital communication system. ## Short answer After A/D conversion, the source encoder compresses data to reduce bit rate; the channel encoder adds error-correcting redundancy; the modulator maps bits to analog waveforms. The demodulator recovers bits, and the channel and source decoders reconstruct the original message. ## Answer A **digital communication system (DCS)** sends information as a sequence of discrete symbols. Its basic components and their functions are: ```text Source->[Source enc]->[Channel enc]->[Modulator] | Channel <- noise | User <-[Source dec]<-[Channel dec]<-[Demodulator] ``` 1. **Information source:** produces the message, analog (speech) or digital (text, computer data). Analog messages are converted to digital by sampling, quantizing and coding (A/D or PCM). 2. **Source encoder:** converts the source output into an efficient binary sequence by removing redundancy (data compression), e.g. Huffman coding, DPCM. It reduces the bit rate and hence bandwidth. 3. **Channel encoder:** adds controlled redundant bits in a known way (e.g. parity, Hamming, convolutional codes) so the receiver can detect and correct transmission errors. Code rate $r=k/n$. 4. **Digital modulator:** maps the coded bits to analog waveforms suitable for the channel, such as ASK, FSK, PSK or QAM for band-pass channels, or line codes for baseband. 5. **Channel:** the physical medium (cable, fibre, radio link) that carries the signal. It introduces attenuation, distortion (ISI), noise and interference. 6. **Digital demodulator:** processes the received noisy waveform (filtering, matched filter, sampling, decision) and gives an estimate of the transmitted bits. 7. **Channel decoder:** uses the redundancy to detect and correct errors, recovering the information bits. 8. **Source decoder:** reverses the source coding (decompression), and for analog messages converts back by D/A conversion and filtering, delivering the message to the **user (destination)**. Optional blocks are encryption/decryption for security and multiplexing/multiple access for sharing the channel; synchronization is needed at the receiver for carrier and bit timing. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The digital modulator A. Maps coded bits to waveforms suitable for the channel B. Removes redundancy C. Corrects errors D. Estimates bits Answer: A. It uses ASK, FSK, PSK, QAM or line codes to produce channel-suited waveforms. (Key point: Digital modulator function) 2. A channel encoder converts k information bits into an n-bit codeword. The code rate is A. n/k B. k/n C. n - k D. k + n Answer: B. r = k/n < 1 because redundancy is added. (Key point: Code rate) 3. At the receiver, after the demodulator the signal goes to A. Source decoder then channel decoder B. Channel decoder then source decoder C. Modulator D. Channel encoder Answer: B. Receiver blocks reverse the transmitter order: channel decoding corrects errors, then source decoding decompresses. 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