TELEVISION TRANSMISSION OVER WIRE LINES 71 



single-sideband transmission, of course, is desirable. The actual method 

 chosen involves also a modest vestigial band since it appears impracticable 

 to select a single sideband involving video frequencies as low as 45 cycles 

 in any other way. The present coaxial amplifiers pass a band from about 

 64 kc to about 3100 kc. The region useful for television, however, appeared 

 to be somewhat less than three megacycles on account of the difficulty of 

 equalizing the delay distortion near the lower edge of this band. About 

 100 kc was allotted to obtain proper shaping of the vestigial sideband. 

 The carrier was therefore placed at about 300 kc and a net television 

 band of about 2f mc was obtained. If we attempt to move a 3 mc video 

 band up 300 kc in a single step of modulation, the result is an overlapping 

 of the sidebands which hopelessly distorts the signal. Two steps of modula- 

 tion are therefore resorted to as shown in Fig. 9. 



The energy of a television system is concentrated in the lower frequencies 

 or, in a carrier system, near the carrier. To take most advantage of the 

 coaxial system, the carrier should be at the low end where the full feedback 

 in the amplifiers is available. The four lines in Fig. 9 illustrate the four 

 stages of modulation, two at the transmitting terminal and two at the 

 receiving terminal. As can be seen the signal is first modulated with a 

 carrier of about 8 megacycles and the lower sideband, part of the carrier, 

 and a portion of the upper sideband, are selected by a band filter. This 

 signal is then modulated again with a carrier of about 8.3 megacycles and 

 the lower sideband again selected. In this position of the signal, which 

 is the position at which it will be transmitted over the coaxial line, the 

 frequency which corresponds to d.c. in the video signal is at 311 kc, the 

 main sideband extends from 311 to 3111 kc and the vestigial sideband from 

 311 kc down to 200 kc. 



The receiving terminal is in general the inverse of the transmitting ter- 

 minal and will not be discussed in detail. The sideband shaping^ is ac- 

 complished by the four filters, two at the transmitting terminal and two 

 similar ones at the receiving terminal, acting in conjunction. The result 

 is that at the final stage of demodulation the contribution from the vestigial 

 sideband when added to the contribution from the shaped portion of the 

 main sideband gives back very nearly an undistorted video signal. This 

 last stage of demodulation is accomplished in a linear detector. The car- 

 rier amplitude at the input terminals of this detector is about six db greater 

 than the amplitude of the video envelope of the modulated signal, the 

 amount of carrier which was mixed with the sidebands at the output of the 

 first modulator having been adjusted to achieve this result. The reason 

 for using this amount of transmitted carrier is the relatively narrow ves- 

 tigial sideband — 111 kc vs. a main sideband of about 2f mc. With such a 

 narrow vestigial sideband the quadrature component of the carrier en- 



