936 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



Variations in transmission response in the region near television carrier 

 cause noticeable ''smear" distortion on the received images. These varia- 

 tions can be introduced by small changes in element values of the filter 

 or equalizer sections following initial adjustment, or by other changes 

 in both transmitter and receiver. As a means of compensating for such 

 variations, two adjustable loss equalizers have been provided, each with 

 adjustable maximum loss at carrier frequency. One has a half-loss point 

 at 300 kc and the other at 80 kc from the carrier. These are adjusted for 

 minimum "smear" at periodic intervals. 



A fixed equaUzer is provided in each terminal to compensate for Joss 

 and delay distortion other than that in the filters described above. For 

 convenience, this equalization is done at modulated frequencies. It is 

 expected that additional means for periodic re-equalization of the ter- 

 minal gain and phase characteristics will be necessary to achieve 4,000 

 mile, tandem terminal transmission quality objectives. 



Pilot Frequencies Filters 



As described elsewhere,^ the L3 system has six pilot frequencies for 

 regulating automatically the transmission characteristic of the line. 

 Two of these, 7.266 mc and 8.320 mc, are in the television band. At the 

 transmitting terminal a pilot elimination filter for these two frequencies 

 is required to prevent energy in the TV signal near these frequencies from 

 disturbing the pilot levels on the system. At the receiving terminals, 

 these pilot frequencies must be removed from the TV band to avoid inter- 

 fering effects in the output TV signal. A discrimination of 50 db for 

 frequencies within 20 cycles of the two pilot frequencies is required. Also 

 at the receiver a carrier elimination filter is required to suppress the 

 residual carrier leak from the demodulator. This requires 30 db suppres- 

 sion to frequencies within 20 cycles of 4.139 mc. 



QUARTZ CRYSTALS 



fl = 7266 KC 

 fa = 8320 KC 



Fig. 16 — Simplified schematic of the pilot frequency band elimination filter. 



