BAND WIDTII AND TKANSM ISSION I'ERI'OKM ANCE 547 



that systems of dilTereat vulnerability require different guard space to pro- 

 tect against adjacent-band crosstalk. In Section VII it is concluded that 

 with binary PCM-AM the band spacing could perhaps be as small as 1.5/ To 

 with realizable filters and gates. Tq is the time allotted to one pulse. 

 Underlying the meaning of B as used heretofore in connection with PCM- 

 AM, is the relation Tq = 2/B. If the band spacing is to be taken as LS/To , 

 the band spacing may be expressed in terms of B as 0.75 B. In other words, 

 the band spacing factor U may be reduced to 0.75. 



In the case of ideal FM systems the receiving frequency discrimination 

 need not suppress adjacent radio signal bands to anywhere near the degree 

 required of co-channel interference, provided the near edges of the adjacent 

 signal bands differ by more than the width of the baseband filter. We do 

 not, of course, assume ideal apparatus and have been rather liberal in guard 

 space allowance. 



The following band spacing factors, to be used with the bandwidths of 

 Tables I\' and V, are considered realizable and consistent with the shape 

 of the spectrum to be transmitted. A reduction to practice would likely 

 lead to somewhat different factors but these will suflice for our illustra- 

 tive Tables \T and VII. In the non-regenerative systems, the spacing 

 factor required to protect against interference from new frequencies required 

 at a junction is, perhaps, less than is required to protect against interference 

 at ever\^ repeater on a long route. A small economy in occupancy could 

 properly be invoked on this account in some cases but, in the interest of 

 simplicity, this has been neglected, and the same factor U will be associated 

 with every frequency required. 



System of Table \T Factor U 



FDM 2.5 



FDM-FM 3 



PPM-AM 2 



PPM-FM 2 



PAM-AM 2 



PAM-FM (narrow band) 2 



PAM-FM (wide band) 1 . 5 



PCM-AM (64-ary) 2 



PCM-AM (quaternary) 0.9 



PCM-AM (binary)i2 0.75 



PCM-FM (64-ary) 2 



PCM-FM (octonary) 1 . 5 



PCM-FM (quaternary) 1.5 



'2 The experimental system described by Meacham and Peterson (ioc. cit.) employs a 

 spacing factor of 1.12. 



