 IMPAIRMENTS AND PERFORMANCE MEASURES These values are based on a VSB-AM (vestigial sideband, amplitude modulation) with an 87.5% modulation index. The values for S/N should be compared to those derived by the Television Allocations Study Organization (TASO) and published in their report to the US FCC in 1959. Their ratings, corrected for a 4-MHz bandwidth, instead of the 6-MHz bandwidth that was used previously, are shown in Section 16.4.2. Once a tolerable noise level is determined, the levels required in a CATV system can be specified. If the desired S/N has been set at 43 dB at a subscriber TV set, the minimum signal level required at the first amplifier would be -9 dBmV + 43 dB or -16 dBmV, considering thermal noise only. Actual levels would be quite a bit higher because of the noise generated by subsequent amplifiers in cascade. It has been found that the optimum gain of a CATV amplifier is about 22 dB. When the gain is increased, IM/Xm products become excessive. For gains below this value, thermal noise increases, and system length is shortened or the number of amplifiers must be increased-neither of which is desirable. There is another rule-of-thumb of which we should be cognizant. Every time the gain of an amplifier is increased 1 dB, IM products and "beats" increase their levels by 2 dB. And the converse is true: every time gain is decreased 1 dB, IM products and beat levels are decreased by 2 dB. With most CATV systems, coaxial cable trunk amplifiers are identical. This, of course, eases noise calculations. We can calculate the noise level at the output of one trunk amplifier. This is Nv = -59 dBmV + NFdb, (17.6) where NF is the noise figure of the amplifier in decibels. In the case of two amplifiers in cascade (tandem), the noise level (voltage) is Nv = -59 dBmV + NFdb + 3 dB. (17.7) If we have M identical amplifiers in cascade, the noise level (voltage) at the output of the last amplifier is Nv = -59 dBmV + NF + 10 log M (17.8) This assumes that all system noise is generated by the amplifiers, and no