BAND WIDTH AXD TKANSMISSIOy PERFORMANCE 531 



the curve of Fig. 19 at 69 db audio signal-to-noise ratio which would be 

 appropriate to yield 60 db when the "noise" is marginal at 3 db below the 

 FM wave power instead of 12 db. A different procedure is required for 

 the narrow band entries of Table I\'. Here the emphasis on conservation 

 of bandwidth leads to a two-frequency repeater plan with tolerance of 

 similar system interference 44 db down. A 60 db audio signal-to-inter- 

 ference ratio can be met under these conditions with moderate swings for 

 which the equivalent noise representation of the interference is not valid. 

 Tlie result is considerably influenced by the channel loading and we have no 

 impeccable method of calculating the necessary bandwidth. We estimate 

 that a bandwidth of 22.5 mc, with jS = 14.5 mc, will satisfy the require- 

 ments for all except unusually adverse loading conditions. 



III. B.A.ND Width .a.nd Power Tables 



The information contained in the curves of Fig. 9-20 has been used in 

 preparing Tables II and III, which show what can be done with the various 

 systems when bandwidth is used freely. The prime objective studied here 

 is the conservation of peak transmitted power. In Table II the audio 

 channel must meet message circuit requirements-^ while, in Table III, a 

 much better grade of performance— more than sufficient for transmission 

 of high fidelity musical programs— is stipulated. We have prepared Ta- 

 ble III (as well as Table \) on the basis of replacing the 1000 4-kc. message 

 channels of Table II with 250 16-kc. channels. Since we have available 

 established load rating theory only for message circuits, we have omitted 

 FDM and FDM-FM from Table III (and Table V) . The values of Table II 

 are based on a nominal 60 db ratio of signal-to-noise, but it is assumed per- 

 missible to meet this in the pulse systems by using 22 db of instantaneous 

 companding so that only 38 db signal-to-noise ratio is actually required 

 within the compandor. The PCM systems provide for a 39 db circuit 

 within the compandor, corresponding to 6 binary digits, 3 quaternary digits, 

 2 octonary digits or one 64-ary digit. No allowance is made for the accu- 

 mulation of quantizing noise arising when several PCM links are connected 

 in tandem at voice frequency. In practice, 7 binary digits might be used. 

 This would provide for several links and would permit slightly more com- 

 panding. Table III assumes a 75 db signal-to-noise ratio and no compand- 

 ing and is referred to here as a "program" circuit. We use such a high-grade 

 circuit to illustrate more emphatically how the system preferences depend 



25 We do not pretend to deal fully with the involved matter of system requirements 

 distinguishing between kinds of noise and interference or crosstalk that appear in message 

 channels. We merely assume that the power of the separate types of disturbances con- 

 sidered must be individually 60 db below that of a full load test tone under the worst 

 specified transmission condition. 



