5()2 BELL SVSLKM TELIIMCAL JOURNAL 



may eventually grow large enough to break the sheer (if the system employs 

 such a device) while in the latter case the reduced pulse slope and the spread- 

 ing out of time bounds may also bring about transmission disaster. In 

 both cases these growing distortions successively reduce the margin that it 

 is necessary to provide for noise and interference. To circumvent such 

 effects, the pulses may be reshaped at all or some repeaters. Reshaping 

 consists of measuring the information conveyed by the pulse (in the time or 

 amplitude dimension) and sending out a new pulse of standard shape pos- 

 sessing that measured characteristic in time or amplitude. This process is 

 distinctly different from regeneration as practiced in quantized systems; in 

 general, reshaping can only be counted upon to confine the rate of accumula- 

 tion of noise, interference and crosstalk to that of power addition from span 

 to span. 



In FM systems any distortion which results in amplitude "modulation" 

 of the FM wave may be treated with limiting at each repeater to prevent 

 such amplitude variation from accumulating and breaking the hmiter. Like 

 pulse reshaping, this measure does not stop the accumulation of disturbance 

 to the intelligence. Certain kinds of distortion may be combated by double 

 FM.'^ 



Reshaping (or, in the case of FM, limiting) may be employed to conserve 

 power in the systems having an improvement threshold. Without reshap- 

 ing, the minimum repeater power is the marginal'^ value for the total noise 

 accumulated from all spans. If reshaping is practiced at each repeater the 

 power need be marginal for the noise from only one span. More bandwidth 

 must be used, then, in exchange for the lower power; and, while this in turn 

 increases the marginal power, the result is a net power saving. Tables II 

 and III of Section III illustrate this point and Section VIII illustrates its 

 application to metallic circuits. 



The Radio Repeater 



Repeaters for relaying television signals must achieve low distortion and 

 we will take a current design and assume that such a repeater represents a 

 basis for discussing the transmission of multiplex telephony by non-quantiz- 

 ing methods. This repeater employs, in the two-way api)lication, four 

 antennas and two frequencies as shown in Fig. 4. It is proposed to transmit 

 5-mc video television signals by FM in bands spaced 40 mc. The repeater 

 employs double detection and the band separation is effected mainly by the 



'^ Leland E. Thompson, "A Microwave Relay System," Proc. I.R.E., Vol. 34, Decem- 

 ber, 1946, pp. 936-942. 



'* By marginal power is meant that power which just safely exceeds the improvement 

 threshold power. For a given noise level, minimum power is achieved when the bandwidth 

 imjirovement factor yields the required signal-to-noisc ratio iu the channel with the power 

 that is marginal for that bandwidth. 



