channel limit S/N = 67 dB Average of 8 channels Worst channel limit Use optical attenuator in this region 8-Channel FM/FDM system S/N Ratio versus receiver sensitivity Figure 17.13 Link performance of an FM system. (Courtesy of ADC Video Systems.). Table 17.1 Link Budgets for AM Fiber Links Total Total Total Distance Distance Fiber Fiber Splice Splice Loss/km Loss/2 km Budget Margin Mileage, Losses, and Margins - 1310 nm 12.40 19.96 0.5 dB 0.1 dB 10.98 13.00 15.15 24.38 0.4 dB 0.1 dB 10.97 13.00 17.00 27.36 0.35 dB 0.1 dB 10.94 13.00 Mileage, Losses, and Margins - 1550 nm 22.75 36.61 0.25 dB 0.1 dB 10.98 13.00 Calculation of Video S/N for FM System. Given the CNR for a particular FM system, the S/N of a TV video channel may be calculated as follows: SNRw = K + CNR - 10 1.6AF BF (17.13) where K = a constant (~23.7 dB) made of weighting network, deemphasis, and rms to p-p conversion factors, CNR = carrier-to-noise ratio in the IF bandwidth, BIF = IF bandwidth, BF = baseband filter bandwidth, and AF = sync tip-to-peak white (STPW) deviation. 17.5 DIGITAL TRANSMISSION OF CATV SIGNALS With AF = 4 MHz, BIF = 30 MHz, and BF = 5 MHz, the SNRw is improved by approximately 34 dB above CNR. Example. If the C/N on an FM fiber link is 32 dB, what is the S/N for a TV video channel using the given values. Use Eq. (17.14): S/N = 23.7 dB + 32 dB + 10 log(30/5) + 20 log(1.6 X 4/5) = 23.7 + 32 + 7.78 + 2.14 = 65.62 dB. 17.5 DIGITAL TRANSMISSION OF CATV SIGNALS 17.5.1 Approaches There are two approaches to digitally transmitting both audio and video TV signals: trans- port either raw, uncompressed video or compressed video. Each method has advantages and disadvantages of which some are application-driven. For example, if the objective is digital to the residence/office, compressed TV may be the most advantageous. 17.5.2 Transmission of Uncompressed Video on CATV Trunks Video, as we discuss in Chapter 16, is an analog signal. It is converted to a digital format using techniques that are similar to the 8-bit PCM covered in Chapter 6, 