COAXIAL CABLE SYSTEM FOR TELEVISION TRANSMISSION 451 



from 2400 down to 1570, is passed to the second demodulator where a 

 carrier of 2376 kc. is appHed. The lowest frequency of the lower 

 sideband, 1570 kc, is converted to 806 kc, becoming the highest 

 frequency of the final demodulated band. The frequencies from 2352 

 to 2400 kc. of the sideband before the second demodulation have been 

 attenuated somewhat by the high-pass filter following the second 

 modulator, and the second demodulating carrier, 2376 kc, falls in 

 the middle of this attenuated band as shown in inset No. 1. Fre- 

 quencies extending about 24 kc. above the carrier are inverted by the 

 demodulation, and superimposed upon the corresponding frequencies 

 just below the carrier. The magnitude and phase of these components 

 are proportioned by the high-pass filter and an equalizer so that the 

 overall result, when they are superimposed, is an essentially flat trans- 

 mission band from to 806 kc. 



The above steps of modulation involved a number of difficulties. 

 In the first place the signal level must be carefully controlled so that 

 on the one hand it does not sink into the background noise, while on 

 the other hand it must not be raised to such high levels that unwanted 

 modulation products are produced in too great magnitude. The first 

 modulator presents some special problems. It must accommodate all 

 frequencies from to 806 kc. In order to eliminate the carrier, it 

 must be balanced to a very high degree — about 80 db in this case. 

 The reason the carrier must be so completely wiped out is that the 

 frequency component of the signal is identical with the carrier at the 

 output and hence the true d-c value of the signal must be exceptionally 

 free from carrier interference. 



Referring to Fig. 12, the next piece of apparatus is a band filter to 

 eliminate the video signal and cut off the top edge of the band. Then 

 follows the 2nd modulator which is quite conventional. A low-pass 

 filter is next and is very important as it performs part of the function 

 of cutting off and adjusting the vestigial sideband. Then follows an 

 amplifier, a predistorting network to partially equalize the amplitudes 

 of the different components of the signal, an aperture equaHzer to 

 correct for the fact that the scanning spot is of finite size, a terminal 

 equalizer to make up for irregularities in the overall setup and other 

 amplifiers. 



The carrier apparatus at the sending end is shown on Fig. 13. 

 It is mounted in rather conventional form except for the 1st modulator 

 which was arranged to minimize the effect of low-frequency vibrations. 

 At the receiving end about the same apparatus is required in the 

 inverse order and will not be discussed in detail. 



