h little triangles on top. For example in the 15-supergroup assembly shown in Figure 4.21, the pilot tone is at 1552 kHz. A pilot tone provides a comparatively constant amplitude reference for an automatic gain control (AGC) circuit. Frequency division multiplex equipment was designed to carry speech telephony. The nature of speech, particularly its varying amplitude, makes it a poor prospect as a reference for level control. Ideally, simple single-sinusoid (a sine REVIEW EXERCISES wave signal), constant-amplitude signals with 100% duty cycles provide simple control information for level regulating equipment (i.e., the AGC circuit). 16 Modern FDM equipments initiate a level-regulating pilot tone on each group on the transmitting side of the circuit. Individual level-regulating pilots are inserted on each supergroup and other frequency configurations. The intent is to maintain the system level within +0.5 dB. Pilots are assigned frequencies that are part of the transmitted FDM spectrum yet do not interfer with the voice channel operation. They are standardized by CCITT and are usually inserted on a frequency in the guard between voice channels or are residual carriers (i.e., partially suppressed carriers). 4.5.4 Comments on the Employment and Disadvantages of FDM Systems FDM systems began to be implemented in the 1950s, reaching a peak employment in the 1970s. All long-haul (long-distance) broadband systems, typically line-of-sight microwave, satellite communication, and coaxial cable systems, almost universally used FDM configurations. One transcontinental system in North America, called the L-5 sys- tem, carried 10,800 voice channels on each cable pair. (Remember the system is four-wire, requiring two coaxial cables per system.) There were 10 working cable pairs and one pair as spare. The total system had 108,000 voice channel capacity. Few, if any, new FDM systems are being installed in North America today. FDM is being completely displaced by TDM systems (i.e., digital PCM systems, see Chapter 6). The principa