THE L3 SYSTEM — TELEVISION TERMINALS 939 



the modulated output signal is delivered to the L3 line facilities over 

 75-ohm coaxial cable. 



An input low-pass filter and a video clipper circuit place ceilings on 

 the maximum transmitted signal bandwidth and signal amplitude, re- 

 spectively. The chpper circuit is required to protect the 600 telephone 

 channels from inadvertant overloads due to excessive television signals. 

 Normal amplitude signals are not affected by the clipper. 



Following are a video amplifier and the modulator together with the 

 required carrier supplies and the carrier level control. This latter device 

 accurately regulates the peak carrier magnitude in the output signal 

 and thereby preserves the desirable maximum modulation. 



Following the modulator is the vestigial band filter which, together 

 with its delay equahzer, shapes the double sideband modulator output 

 into the vestigial transmission band. The flat loss of these networks re- 

 quires that amplification be provided in the carrier frequency band to 

 increase the signal level. The first of two high frequency flat gain am- 

 plifiers in the transmitter restores the signal amplitude. 



The pilot band elimination filter is next provided to remove tele- 

 vision signal energy and other possible interference from particular 

 frequencies allocated to the L3 line pilot signals. 



At this point occur the predistorter network for pre-emphasizing the 

 high frequency signal components and a mop-up equalizer. The mop-up 

 equalizer is to provide means for periodic correcting of the transmission 

 characteristic. The high-frequency amplifiers, particularly, change their 

 transmission characteristic as the vacuum tubes age. An additional high 

 frequency amplifier is provided to recover the loss of the foregoing net- 

 works and deliver a proper signal level to the line equipment. 



R3 Receiving Terminal 



The receiver demodulates the signal as transmitted over the L3 line 

 facilities to recover the video signal for transmission over local video 

 circuits. As has been discussed the demodulation is a homodyne process 

 utilizing a local carrier regenerated from information contained in the 

 transmitted signal. 



A block diagram of receiver components is shown in Fig. 19. The first 

 components are a group of networks; the restorer to compensate for 

 transmitter predistortion, the fixed and variable mop-up equaUzers to 

 compensate for deviations in the receiver frequency characteristic, and 

 a pilot eUmination filter to remove L3 pilot frequencies from the signal. 

 A high frequency amplifier provides amplification to compensate for 

 the network losses. 



