ough The filters Fc1 and Fc2 remove the unwanted products from the space between the voice frequencies (0.3 to 3.4kHz) to economi- demodulation process before presentation to the user or to the next cally filter the carrier, pilots, outband signalling tones, etc. that are lower order of translation. positioned in the gaps between the speech signals. The FDM system represents the most efficient use of bandwidth. Where transmission bandwidth is at a premium then the channel With only the lower sideband of the modulation products trans- spacing may alternatively be based on 3kHz (voice bandwidth 0.2 mitted the system limitations of bandwidth and overload are maxi- to 3.050kHz). This provides a 4/3 increase in channel capacity but mised. is only achieved with increased cost of the channel translation stage. 20.3.3 Group and supergroup History With reference to Figure 20.2 the channel translating section con- The first UK FDM cable systems started service with a 12 channel verts 12 voice frequency channels (or 16 if 3kHz channelling) and system between Bristol and Plymouth in 1936. (Young, 1983) assembles them into a basic 'group' in the range 60 to 108kHz. F1-3.4 F2-3.4 F1-0.3 F2-0.3 LINE FREQUENCY SPECTRUM Figure 20.1 Frequency division multiplexing 20/4 Frequency division multiplexing CHANNEL GROUP To Mastergroup SUPERGROUP HYPERGROUP TRANSLATING TRANSLATING Assemblies Fig 3 4 TRANSLATING TRANSLATING Speech Channels Group 0.3 3.4kHz 60 108 kHz 15 X SG Hy ypergroup 12X HG 60 MHz System. (10 800 Channel) Translating 312 026 kHz Translating 4 404 59 580 kHz Supergroup HG 13 312 552 kHz SG 16 Television Translating 18 MHz System Translating 312 16 924 KHz Speech Channels 16 X SG 4 MHz System 0.2 3.05kHz Translating 60 420 12 MHz System 3X HG 312 12 336 kHz Translating 1.3 MHz System 5 X SG 60 300 kHz VF DATA 12 60 Channel Translating VF TELEGRAPH "n"Groups Systems 120 Channel "n" X SG 50 Baud .100 Baud Systems Translating Open Wire or Symmetrical Cable links Coaxial Cable Radio , or Satellite Links. Figure 20.2 FDM hie