930 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



Filters and Equalizers 



A low-pass filter is required before the video signal is modulated, to 

 limit the bandwith of the signal, eliminating possible sources of disturbing 

 cross products, both in the modulator and in the repeaters of the L3 

 system. This filter has a cut-off at 4.3 mc and provides over 40 db dis- 

 crimination to all frequencies greater than 4.8 mc. It consists of three 

 m-derived sections and introduces about 1.3 microseconds of envelope 

 delay distortion near its cut-off frequency. This is equalized after modu- 

 lation by the delay equalizer to be described later. 



Following the modulator is the vestigial sideband filter which passes 

 the upper sideband, and provides 60 db discrimination against all fre- 

 quencies in the lower sideband less than 3.7 mc. The large discrimination 

 is required to avoid interference with the telephone channels during 

 transmission over the coaxial Une. This filter provides a controlled loss 

 characteristic to frequencies in the band 3.64 to 4.64 mc which satisfies 

 the requirement for vestigial sideband transmission. The response func- 

 tion for this band is shown on Fig. 2. A flat transmission characteristic 

 including the effective pass band loss of the video LP filter is maintained 

 over the entire upper sideband from 4.6 to 8.44 mc. A second low pass 

 filter after the modulator provides at least 50 db discrimination against 

 third and higher harmonics of the TV carrier (4.139 mc). 



A four section high-pass filter designed by the insertion loss method^ 

 is used to supply 50 db of the discrimination at frequencies less than 



3456 789 10 11 



FREQUENCY IN MEGACYCLES PER SECOND 



12 



Fig. 9 — Insertion loss of the transmitting terminal filters. 



