FDM in telephony and uses of filters

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.

 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.