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Short note: FDM in telephony

Short answer

FDM in telephony combines voice channels (300-3400 Hz) into 4 kHz slots using SSB-SC modulation. Hierarchy: 12 channels form group (60-108 kHz), 5 groups form supergroup (60 channels, 312-552 kHz), 5 supergroups form mastergroup (300 channels, 812-2044 kHz), 3 mastergroups form supermastergroup (900 channels, 8.5-12.4 MHz). Requires sharp filters, stable oscillators, pilot tones.

Write a short note on FDM in telephony.

FDM in telephony combines many voice channels onto one wideband link (coaxial cable, microwave) by giving each a separate frequency slot. It was the standard in analog long-distance telephone networks.

Principle

  • Each voice signal is band-limited to 300–3400 Hz and given a 4 kHz slot (the extra 900 Hz is a guard band).
  • Each channel SSB-SC modulates (usually LSB) a different subcarrier; SSB halves the bandwidth compared with DSB.
  • The outputs are added and sent together; at the receiver band-pass filters separate them and synchronous demodulators recover each voice.
ch1->[SSB mod f1]->[BPF]-+
ch2->[SSB mod f2]->[BPF]-+->(sum)-> line
 ...                     |
ch12->[SSB f12]->[BPF]---+

Hierarchy (ITU-T/CCITT)

LevelMade ofChannelsBand
Basic group12 voice1260–108 kHz
Supergroup5 groups60312–552 kHz
Mastergroup5 supergroups300812–2044 kHz
Supermastergroup3 mastergroups9008516–12388 kHz

In the basic group, carriers are 64, 68, ..., 108 kHz with LSB selection.

Requirements: sharp SSB filters (crystal/mechanical), stable carriers derived from one master oscillator, pilot tones for level and frequency control, and linear amplifiers to avoid intermodulation crosstalk.

FDM telephony has largely been replaced by digital TDM (E1/T1, SDH), but the principle remains in radio and cable systems.

✓ Answer checked by Claude Opus 5.5 (high effort)

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