THE L3 SYSTEM — TELEVISION TERMINALS 



917 



smaller will be the total television band required. However, both the 

 cost of band shaping filters and the difficulty of maintaining satisfactorily- 

 low values of vestigial sideband quadrature distortion increase as the 

 vestigial band width is reduced. The compromise of these factors re- 

 sulted in the choice of a 500-kc vestigial sideband. 



Another choice made was to transmit the television signal in the 

 upper part of the L3 band and the message channels in the lower part. 

 This allocation was determined by considering the noise distribution in 

 the transmission band together with the modulation distortion, (har- 

 monic distortion), produced by the repeaters. By transmitting television 

 in the upper part of the band a minimum modulation distortion is 

 achieved since the harmonics of the television signal largely fall outside 

 the transmitted band or at high frequencies where their effects in the 

 picture are relatively less visible than low frequency distortion. This 

 factor outweighs the higher noise level in the upper part of the band. 



With respect to the television carrier location, it is placed at the bot- 

 tom of the television band at 4.139 mc, with the vestigial sideband 

 extending down to 3.64 mc and the main sideband extending upward to 

 8.50 mc. Alternatively the carrier could have been located at the top of 

 the L3 band with a main lower sideband and vestigial upper sideband, 

 but this choice would be disadvantageous because of higher noise levels 

 and poorer repeater gain stability near the top edge of the transmitted 

 band. 



The final allocation of frequencies is shown in Fig. 1. Just below the 

 television band from about 3 mc to 3.5 mc is a dead space. This fre- 

 quency space is needed for the filters which are employed to separate the 

 telephone signals from the television signals at television and telephone 

 dropping points. A very high value of loss is required of these filters in 

 their attenuating bands and if this is built up over too small a frequency 



« < n 2 (TV CARRIER) 

 O CD to ^t 



Q 



telephone band 

 (600 channels) 



8 



TELEVISION BAND 



TRANSMISSION 

 ALLOCATIONS 



2 



2000 3000 4000 5000 6000 7000 

 FREQUENCY IN KILOCYCLES PER SECOND 



PILOTS 



Fig. 



sion. 



1 — Frequency allocations for L3 combined television-telephone transmis- 



