THE L3 SYSTEM — DESIGN 



787 



0.01 



0.02 



0.04 0.06 0.1 0.2 0.4 0.6 1.0 2 3 4 5 6 8 10 



DURATION OF INTERFERENCE IN SECONDS 



Fig. 3 — Television bar pattern threshold versus duration. 



general grounds. When the system is arranged for combined television- 

 message transmission, the television channel is placed above the message 

 channels so that the second harmonic of the television carrier and its 

 immediately adjacent side bands will fall at the top edge of the band 

 where the requirement is more lenient. Likewise, the line repeater noise 

 tends to rise with frequency as does the amount of noise that the tele- 

 vision channel can tolerate. Details of the frequency allocations shown 

 on Fig. 4 will be discussed in later sections. 



Pilot frequencies, indicated on Fig. 4, are transmitted to control the 

 transmission characteristic of the system as described in a companion 

 paper. ^^ The frequencies, and the power at which the tones are trans- 

 mitted, were selected on two bases; (1) where possible, frequencies used 

 for similar purposes in the LI system were selected for possible economies 

 in pilot supply design and manufacture; these are the 556, 2,064 and 

 3,096-kc pilots; and (2) the transmission of these pilots should not 

 materially degrade the signal-to-noise or load capacity performance of 

 the system. The latter requirement led to a careful study of cross-modu- 

 lation products involving the pilot frequencies to assure that message 

 and television objectives would be met. 



2.14 Repeater Performance 



The details of the amplifier design and the factors which determine 

 its performance are covered in a companion paper. For purposes of the 

 signal-to-noise design it is sufficient to know the noise power vs fre- 

 quency characteristic, the second and third order modulation coefficients 



