WIDE-BAND TRANSMISSION OVER COAXIAL LINES 677 



Demodulation 



On the receiving side the modulation process is reversed. The 

 apparatus units are similar to those used on the transmitting side, and 

 are similarly arranged. Figure 16 illustrates this for the case of double 

 modulation. 



Carrier Frequency Supply 



In systems operating at higher frequencies it is necessary that the 

 carrier frequencies be maintained within a few cycles of their theoretical 

 position in order to avoid beat tones or distortion of the speech band. 

 Separate oscillators of high stability could, of course, be used for 

 the carrier supply but it appears more economical to provide carriers 

 by means of harmonic generation from a fundamental basic frequency. 

 Such a base frequency may be transmitted from one end of the cir- 

 cuit to the other, or may be supplied separately at each end. 



Television 



The broad band made available by the line and repeaters may be 

 used for the transmission of signals for high-quality television. Such 

 signals may contain frequency components extending over the entire 

 range from zero or a very low frequency up to a million or more cycles."' 

 The amplifying and transmitting of these frequencies, particularly the 

 lower ones, presents a serious problem. The difficulty can be over- 

 come by translating the entire band upward in frequency to a range 

 which can be satisfactorily transmitted. To effect such a shift, the 

 television band may first be modulated up to a position considerably 

 higher than its highest frequency and then with a second step of modu- 

 lation be stepped down to the position desired for line transmission. 



This method is illustrated in Fig. 19 for a 500-kc. television signal 

 band. The original television signal is first modulated with a rela- 

 tively high frequency, two million cycles in this case (Ci). The lower 

 sideband, extending to 1500 kilocycles, is selected and is modulated 

 again with a frequency of 2100 kilocycles (G). The lower sideband 

 of 100 to 600 kilocycles is selected with a special filter so designed that 

 the low frequency end is accurately reproduced. The television 

 signal then occupies the frequency range of 100 to 600 kilocycles as 

 shown on the diagram and may be transmitted over a coaxial or other 

 high frequency line. At the receiving end a reverse process is em- 

 ployed. The same method using correspondingly higher frequencies 

 may be used for wider bands of television signals. 



