302 BELL SYSTEM TECHNICAL JOURNAL 



FS transmission is essentially immune from effects of non-selective level 

 variation, even when extremely rapid, and in this characteristic displays 

 its most outstanding advantage over AM. 



Operation Over Wire Circuits 



A wire circuit usually provides a transmission medium having a low noise 

 level with slow and relatively small variations in attenuation. Such cir- 

 cuits, when equipped with suitable automatic gain control, allow stable 

 operation with AM telegraphy and but little improvement could probably 

 be obtained by using FS. The choice between AM and FS under such rela- 

 tively ideal conditions becomes one of economic considerations of the ter- 

 minal equipment and carrier supply. However, when FS is applied to 

 multichannel systems the problem of interchannel interference requires 

 attention. For wire circuits having high noise levels or sudden changes in 

 attenuation the use of FS instead of AM provides considerable improvement 

 and in severe cases the FS method may be a necessity for satisfactory opera- 

 tion. Wide band FS operation with its sharper breaking distortion-versus- 

 noise-level characteristic gives a low value of rms-to-peak distortion which 

 would be especially advantageous for tandem operation. However, the 

 necessary frequency space for wide-band operation is not usually economi- 

 cally justified for wire line operation. 



Operation Over Radio Circuits 



For operation over radio circuits providing stable conditions similar to 

 those on wire circuits the FS method does not show a great advantage over 

 the AM method. In the case of long distance telegraphy in the H.F. range, 

 however, FS shows a marked advantage over AM. This is because of the 

 rapid fading and high noise conditions which commonly prevail in the 

 H.F. region. The amount of rapid variation in marking level that an AM 

 system can accommodate is less than the difference between marking and 

 spacing levels that an FS system can tolerate. In the worst case of selective 

 fading the level differences between the mark and space frequencies might 

 approach values equal to the short time level swings of a single frequency, 

 but in general would be less. A given condition of selective fading thus 

 causes less distortion in an FS system than in an AM system. FS allows 

 the use of narrow bands without much loss in signal quality in the presence 

 of fading, whereas AM does not. FS therefore is essential for satisfactory 

 operation of closely spaced narrow band H.F. radio channels. Where fre- 

 quency space is not restricted and wider bands are used to permit consider- 

 able frequency drift, the improvement afforded by FS over AM is materially 

 less. To obtain optimum results from an AM system, however, requires 



