--- title: "FDM and its method in telephony" subject: "Communication Systems" chapter: 5 key: "semester_6/communication_systems:fdm-telephony-method" source: https://www.bctengineer.com/q/semester_6/communication_systems/fdm-telephony-method site: BCT Engineering --- # FDM and its method in telephony Communication Systems · Chapter 5: Multiplexing Techniques - Asked in: 2075 Bhadra (CS I), 2+6 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-method - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5 ## Question What is frequency division multiplexing (FDM)? Describe the method of FDM in telephony. ## Short answer Frequency division multiplexing assigns each signal a distinct frequency band for simultaneous transmission. In telephony, SSB-SC modulation places 12 voice channels of 4 kHz each into a basic group (60–108 kHz); groups then combine into supergroups and mastergroups in a hierarchy. ## Answer #### Frequency division multiplexing **FDM** is a technique in which several message signals are sent at the same time over one channel by shifting each one, with modulation, to a different non-overlapping frequency band. Guard bands separate the bands, and the receiver uses band-pass filters to separate the channels. #### Method of FDM in telephony Telephone FDM uses **SSB-SC modulation**, so each voice channel takes only 4 kHz (0.3–3.4 kHz speech plus guard band). Channels are built up in a hierarchy: | 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 | **Step 1: Basic group.** Twelve voice channels modulate carriers at 64, 68, 72, …, 108 kHz. Band-pass filters keep only the lower sidebands, which fill 60–108 kHz. **Step 2: Supergroup.** Five basic groups modulate carriers at 420, 468, 516, 564 and 612 kHz. The lower sidebands fill 312–552 kHz (60 channels); group 1 (420 kHz carrier) lands at 312–360 kHz. **Step 3: Mastergroup and above.** Five supergroups form a 300-channel mastergroup (812–2044 kHz). Three mastergroups form a 900-channel supermastergroup. ```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 -------->| ``` **Receiver.** The demultiplexer reverses each step. Band-pass filters pick out each supergroup, then each group, then each channel. Each is SSB-demodulated with a local carrier locked to transmitted pilot tones and then low-pass filtered to get the speech back. The advantages are that many calls share one cable, the hardware is analog and simple, and no sampling is needed. The drawbacks are crosstalk from non-linearity and noise that builds up over long links, which is why FDM was later replaced by TDM-PCM (T1/E1). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is FDM? A. Sending several messages simultaneously by shifting each to a different non-overlapping frequency band B. Sending messages one after another in time slots C. Sending a message with a spreading code D. Sending messages at one common frequency Answer: A. FDM modulates each message to its own band; guard bands separate them and band-pass filters recover them. (Key point: FDM principle) 2. Which carriers form a supergroup from five basic groups? A. 64, 68, ..., 108 kHz B. 420, 468, 516, 564, 612 kHz C. 312, 360, 408 kHz D. 812, 820, 828 kHz Answer: B. These carriers with lower sidebands shift the 60-108 kHz groups into 312-552 kHz. (Key point: Supergroup formation) 3. How does the FDM receiver recover speech? A. Band-pass filters separate supergroup, group, channel; SSB demodulators with pilot-locked carriers and LPF recover speech B. A single low-pass filter separates all channels C. Decommutator picks time slots D. Envelope detector on the whole band Answer: A. The demultiplexer reverses each step, using pilot tones to lock local carriers. (Key point: Receiver) 4. Why was FDM replaced by TDM-PCM? A. Crosstalk from non-linearity and noise that builds up over long links B. FDM needs more bandwidth than TDM C. FDM cannot carry telephony D. FDM needs synchronisation Answer: A. Analog FDM accumulates noise and crosstalk; digital regenerative TDM-PCM avoids that. (Key point: Drawbacks) ## Related in Communication Systems - [Short note: Frequency division multiplexing](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm) — Communication Systems, chapter 5, asked 2× - [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 in telephony and uses of filters](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-telephony-filters) — Communication Systems, chapter 5 - [FDM principle; SCPC and DAMA FDMA](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-scpc-dama) — 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 - [Frequency division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-explain) — Data Communication, chapter 6, asked 3× - [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3× - [Need for multiplexing and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-need-wdm) — Telecommunications, chapter 3 - [Comparison of FDM, TDM and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-wdm) — Telecommunications, chapter 3, asked 2×