or 3251 M FM video 3251D FM video modulator demodulator 3251 M 3251D FM video FM video modulator demodulator 3251 M 3251D FM video FM video modulator demodulator Optical Optical transmitter receiver 3251 M 3251D FM video FM video modulator demodulator 3251 M 3251D FM video FM video modulator demodulator RF from RF to audio audio 3251 M system FM video system 3251D FM video modulator demodulator 3251 M 3251D FM video FM video modulator demodulator Figure 17.11 FM system model block diagram for the video transmission subsystem. (Courtesy of ADC Video Systems, Inc.) typical FM/fiber hub. Figure 17.13 shows the link performance of an FM system and how we can achieve an S/N of 67 dB and better. As illustrated in Figure 17.11, at the headend, each video and audio channel must broken out separately. Each of these channels must FM modulate its own subcarrier (see Figure 17.10). It should be noted that there is a similar but separate system for 17.4 HYBRID FIBER-COAX (HFC) SYSTEMS FM band Fiber Video Optical FM video & downstream S/C audio 4.5 MHz Ch. 1 splitter demodulator Audio Video FM video & S/C audio 4.5 MHz demodulator Audio Ch. 4 Emergency alert RF splitter RS-232C RS-232C modem modem RF combiner Video FM video & Ch. 1 4.5 MHz S/C audio Fiber Audio modulator Optical upstream combiner Video FM video & S/C audio 4.5 MHz Audio modulator Ch. 4 Figure 17.12 A typical FM/fiber hub serving both upstream and downstream directions. the associated aural (audio) channels with 30-MHz spacing starting at 70 MHz. These audio channels may be multiplexed before transmission. Each video carrier occupies a 40-MHz slot. These RF carriers, audio and video, are combined in a passive network. The composite RF signal intensity-modulates a laser diode source. Figure 17.12 shows a typical fiber/FM hub where this technique is utilized. Figure 17.13 illustrates the link performance of an FM fiber-optic system for video channels. Table 17.1 shows typical link budgets for an HFC AM system. COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION) Best 