566 BELL SYSTFAf rFXIIXICAL JOURNAL 



of 1.5/ To to 1.7/7'(i can be expected to have satisfactorily small crossfire 

 for binary PCM. Pulse 1 1 and a spacing of 1 .5/To were assumed for binary 

 PCM-AM in Tables \'I to \III. 



Figure 26 shows the enx'elopes of r.f. pulses produced by passing flat- 

 topped r.f. pulses or r.f. spikes through r.f. hlters. These envelopes are the 

 baseband shapes produced by wide-band envelope detectors. If baseband 

 pulses are shaped by baseband filters the resulting pulses are the same as 

 shown for pulses 1 to 7, but for pulses 8 to 11 the tails turn out to be over- 

 shoots passing smoothly through zero instead of reaching zero cusp-wise. 

 If these pulses are used to modulate the amplitude of a carrier in a product 

 modulator, the cusps in the envelope are produced as shown, but if they 

 are used to modulate the frequency of a carrier the baseband pulses produced 

 by frequency detection retain their smooth transition through zero. In 

 PAM-FM relatively wide gate pulses could be centered at time zero, Tq , 

 2To , etc., and the inter-pulse crosstalk would be partially balanced out by 

 partial cancellation of positive and negative contributions. By the use of 

 biases in the AM case a similar result could be obtained. Our tables, 

 assuming pulse 4 spaced 2 To , do not reflect this possibiUty of operation. 



Delay Line B.\lancing 



Techniques have been developed^ • ■*" in which the received pulse train is 

 spUt into two or more branches, after detection to the baseband, and recom- 

 bined with suitable delays, attenuations and polarity reversals. Such a 

 procedure is effective in reducing the pulse tails or hangover and its use 

 has been especially valuable in experimental PAM and PAM-FM systems. 

 While this device may be regarded as a kind of phase and amplitude equalizer 

 (comprising as it does only linear, passive elements) the result may be a 

 pulse shape slightly more desirable than those obtained from simple but 

 "ideal" networks, shown in Fig. 26. Our judgment that pulses of shape 4, 

 spaced 27^0 , should be used in PAM-FM may be slightly pessimistic if this 

 kind of balancing is used. 



More significant reductions of inter-pulse interference may be sought 

 by the method suggested by MacColH^ (which is more than "equalizing") 

 but this method, like the PCM method of Appendix III soon makes pre- 

 posterous demands on the transmission medium and upon the transmitted 

 power. 



\ III. Transmission Over Metallic Circuits 



In radio relay transmission we have assumed a span length of 30 miles 

 and have assumed span losses in keeping with the microwave antenna art 



"• V. D. Landon, loc. cit. 



""W. D. Boothroyd and E. M. Creamer, Jr., "A Time Division Multiplexing System," 

 Paper presented at winter general meeting, A.I.E.E., New York, Jan. 31, 1949. 

 « U. S. Patent No. 2,056, 284 Oct. 6, 1936 issued to L. A. MacColl. 



