. The Bell System technical journal . ccomplished with an error rate of the order that hasbeen implied for data transmission, even in the worst channel. 3. There is a relatively wide range of performance of the system overdifferent Nl circuits, and the average performance is sensibly betterthan that under the limiting conditions which have been considered. Distribution of Signal in Allocated Bandwidth A more extensive discussion of the use of bandwidth is given in Sec-tion , below. However, a few specific points are appropriate here onthe band use in telegraph channels. The spectrum o
. The Bell System technical journal . ccomplished with an error rate of the order that hasbeen implied for data transmission, even in the worst channel. 3. There is a relatively wide range of performance of the system overdifferent Nl circuits, and the average performance is sensibly betterthan that under the limiting conditions which have been considered. Distribution of Signal in Allocated Bandwidth A more extensive discussion of the use of bandwidth is given in Sec-tion , below. However, a few specific points are appropriate here onthe band use in telegraph channels. The spectrum of the original voice frequency telegraph system, based Table IV — Use of Frequency Spectrum in Telegraph Channel AM FS 1. Channel spacing 2. Nominal effective bands 3. Roll-off band (both sides) 4. FM swing 5. Guard band (both sides) 170 74 37 0 59 170 cycles 74 26 70 0 PRIVATE LIXE DATA TKAXSMISSIOX 14G7 ^- 170 Xy 2 X NOMINALEFFECTIVEBAND74-Aj ~0\ 170\ ■*\ NOMINALEFFECTIVEBAND X37 \j FM SWING _ ♦70Oj ROLL- IOFF130). FREQUENCY ^-*> (a) AM (b)Fs Fig. 3 — Utilization of telegraph channels. on 170 cycles between carriers, was conservatively developed for the60 word per minute speed of the time. The 100 word per minute speedhas used up some of this conservatism. The use of frequency shift in thesame channels has, however, used up the spectrum space even more. An outline of the band allowances is given in Table IV, and illustratedin Fig. 3. Item 2 of the table is based on the 100 word per minute speed,using double sideband. On this basis, the number is equal to the num-ber of bits per second. This is the minimum double sideband over whichthat number of bits can be transmitted, according to the Xycjuisttheory. Each such sideband is sometimes called a nominal effectiveband. In practice various allowances are necessary over this minimum. In the first place a roll-off is necessary because filters are not infinitelysharp, and in addition the nature of the modulatio
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