--- title: "FDM in telephony and uses of filters" subject: "Communication Systems" chapter: 5 key: "semester_6/communication_systems:fdm-telephony-filters" source: https://www.bctengineer.com/q/semester_6/communication_systems/fdm-telephony-filters site: BCT Engineering --- # FDM in telephony and uses of filters Communication Systems · Chapter 5: Multiplexing Techniques - Asked in: 2077 Chaitra (CS I), 6+2 marks - Video: [Frequency Division Multiplexing (FDM) Explained](https://www.youtube.com/watch?v=UwWmDwbsDBs) — 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/fdm-telephony-filters - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5 ## Question Describe FDM in Telephony with its uses and structure. What are the uses of filters in FDM? ## Answer **Frequency division multiplexing (FDM) in telephony** places many voice channels, each about 4 kHz wide, side by side in frequency on one cable or radio link. Each voice channel is shifted to its own slot by single-sideband (SSB) modulation. SSB is used because it needs only 4 kHz per channel. #### Structure (CCITT FDM hierarchy) Channels are combined in stages. Each stage takes the previous blocks, SSB-modulates them onto new carriers, and stacks them in frequency. | Level (CCITT) | Made of | Voice channels | Frequency band | Bandwidth | |---|---|---|---|---| | Voice channel | — | 1 | 0–4 kHz (0.3–3.4 kHz used) | 4 kHz | | Basic group | 12 voice channels | 12 | 60–108 kHz | 48 kHz | | Supergroup | 5 groups | 60 | 312–552 kHz | 240 kHz | | Mastergroup | 5 supergroups | 300 | 812–2044 kHz | 1232 kHz | | Supermastergroup | 3 mastergroups | 900 | 8516–12388 kHz | 3872 kHz | ```text Group: 12 channels, carriers 64, 68, ..., 108 kHz (LSB of each channel kept) |ch12|ch11|ch10| ch9| ch8| ... | ch2| ch1| 60 64 68 72 76 100 104 108 kHz |<----------------- 48 kHz ----------------->| Supergroup: 5 groups, carriers 420, 468, 516, 564, 612 kHz (LSB kept) | G1 | G2 | G3 | G4 | G5 | 312 360 408 456 504 552 kHz |<-------- 240 kHz -------->| ``` #### Working 1. Each speech signal is band-limited to 300–3400 Hz by a low-pass/band-pass filter. 2. It SSB-modulates a channel carrier (64, 68, …, 108 kHz). The lower sideband is kept, giving the 60–108 kHz basic group. 3. Five groups are shifted to 312–552 kHz to form a supergroup (group 1 on the 420 kHz carrier lands at 312–360 kHz), and five supergroups form a mastergroup. 4. At the receiver, the same steps are reversed. Band-pass filters pick out each block, and SSB demodulators with locally generated carriers recover the speech. 5. Pilot tones are sent for carrier synchronisation and level control. #### Uses - Long-distance analog telephone trunks on coaxial cable and microwave links. - Satellite FDMA (SCPC) links. - The same idea is used in radio/TV broadcasting, FM stereo, and ADSL on telephone lines. #### Uses of filters in FDM - **Input low-pass filters** band-limit each message so that it fits its 4 kHz slot. - **Band-pass filters after the modulators** remove the unwanted sideband and the carrier leakage (SSB generation). They keep each channel inside its own band. - **Receiver band-pass filters** separate each channel or group from the composite signal (demultiplexing). - **Output low-pass filters** after demodulation remove high-frequency products and recover the baseband speech. - Sharp filters keep guard bands small and reduce adjacent-channel **crosstalk**. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is SSB used for FDM telephony? A. It needs only about 4 kHz per channel B. It needs no filters C. It has the highest power D. It gives digital output Answer: A. SSB halves the bandwidth compared with DSB, fitting each channel into a 4 kHz slot. (Key point: Why SSB) 2. How is the basic group formed? A. 12 channels modulate carriers 64-108 kHz, lower sidebands kept, giving 60-108 kHz B. 12 channels modulate carriers 420-612 kHz, giving 312-552 kHz C. 5 channels with upper sidebands from 0-48 kHz D. 12 channels sampled at 8 kHz Answer: A. Carriers 64, 68, ..., 108 kHz with LSB selected fill 60-108 kHz (48 kHz). (Key point: Basic group) 3. What is the role of band-pass filters after the modulators? A. Remove the unwanted sideband and carrier leakage, keeping each channel in its band B. Sample the signal C. Amplify the baseband speech D. Add guard bands digitally Answer: A. They select the wanted sideband (SSB generation) and keep each channel inside its own band. (Key point: Filters after modulators) 4. Why is each message passed through a low-pass filter before modulation? A. To band-limit it to fit the 4 kHz slot B. To raise its power C. To produce the carrier D. To add pilot tones Answer: A. Band-limiting prevents spill into adjacent channels. (Key point: Input low-pass filter) ## Related in Communication Systems - [Short note: FDM in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm-telephony) — Communication Systems, chapter 5, asked 5× - [FDM and FDM hierarchy in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-hierarchy-telephony) — Communication Systems, chapter 5, asked 4× - [FDM and its method in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-telephony-method) — Communication Systems, chapter 5 - [FDMA; FDM in telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdma-fdm-telephony-hierarchy) — Communication Systems, chapter 5, asked 2× ## Related in other subjects - [FDM hierarchy and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afdm-hierarchy-t-carrier) — Telecommunications, chapter 3, asked 2× - [Uses and scope of masonry structures](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Amasonry-uses-scope) — Concrete Technology and Masonry Structure, chapter 7, asked 5× - [Complex number multiplication using structure](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Astruct-complex-multiply) — Computer Programming, chapter 8 - [Ionosphere structure and ground wave](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aionosphere-structure-ground-wave) — Propagation and Antenna, chapter 4