450 



BELL SYSTEM TECHNICAL JOURNAL 



sideband from 2376 to 3182. The carrier itself is eliminated in the 

 balanced modulator. The output of this modulation is passed through 

 a filter, but because the two sidebands touch each other at 2376 kc, 

 the filter cannot cut ofif all the upper sideband. At the output of 

 this filter there is thus the lower sideband plus a small amount of the 

 lower part of the upper sideband. The upper sidebands from all 

 subsequent modulations are readily eliminated by filters because of 

 the wide separation. 



TRANSMITTING END 



RETAINED u.. UPPER _j 



V^ SIDE-BAND "^ 



PASSED LOWER_ J,^_ .pcTFD- J 



mu. 



2376 (CARRIER) 

 (FIRST MODULATION) 



LOWER SIDE-BAND 

 TRANSMITTED OVER LINE 



REJECTED 



-- UPPER >| 



SIDE-BAND ! 



2520 (CARRIER) 

 (SECOND MODULATION) 



RECEIVING END 



°"° 2376 (CARRIER) ^^^^ "' "" 



(SECOND demodulation) 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 1 1 — Schematic diagram illustrating the processes of modulation and demodulation 

 used in transmitting television signals over the coaxial line. 



The carrier for the second modulation is 2520 kc, and the lower 

 sideband extends from 950 down to 144 kc. plus a vestigial upper 

 sideband remaining from the first modulation which extends down to 

 120 kc. The high-pass filter following this modulation is accurately 

 designed to pass with controlled attenuation a group of frequencies 

 just above 144 kc. and the vestigial sideband. The resulting single 

 sideband, extending from 120 to 950 kc, is then passed over the 

 coaxial cable. 



At Philadelphia the received signal, together with a carrier of 

 2520 kc, is applied to the first demodulator, and the lower sideband. 



