454 BELL SYSTEM TECHNICAL JOURNAL 



motor and its speed cannot be kept very constant. By means of an 

 auxiliary lamp and photocell, the frequency with which one scanning 

 line followed another — ^approximately 5760 per second — was obtained. 

 This was modulated with the 72 kc. mentioned above and transmitted 

 over the line as a lower sideband at 66.24 kc. At the receiving end 

 by demodulation, the exact line speed is obtained and used to drive 

 the horizontal sweep. The vertical sweep is obtained by generating 

 the 240th subharmonic of this — namely 24 c.p.s. 



Line Equalization and Test Results 



Returning now to the line transmission problem, the signals which 

 might be transmitted in the general case are indicated in Fig. 14. 

 They include the pilot channels used for automatic transmission 

 regulation at 60 kc. and 1024 kc. For convenience, one telephone 

 channel with a carrier at 64 kc. is indicated as an order wire, and a 



PILOT FREQUENCY 

 I rSYNCHRONIZING CARRIER , PILOT 



Ijr-CARRIER FREQUENCY 



->{(*- ORDER-WIRE CHANNEL I 



]W [<-PROGRAM CHANNEL T 



III I k TELEVISION CHANNEL M 



i 



60|| , 84 120 FREQUENCY IN KILOCYCLES PER SECOND ^^° '^^^ 



6472 



'I' 

 66.24 



Fig. 14 — Frequency allocation for a television transmission system with associated 



control circuits. 



wide-band program channel with carrier at 84 kc. to transmit the 

 sound. These, of course, could be provided with ordinary telephone 

 facilities. The base frequency of 72 kc. and the disk synchronizing 

 sideband at 66.24 are also included. For the television signal itself 

 the band from 120 to 950 kc. is provided. Actually in the tests to 

 Philadelphia, automatic regulation was not needed and a separate 

 wire line was used for synchronization. 



It was necessary to provide networks and equalizers to insure that 

 the coaxial line did not distort the ultimate image due to unequal 

 attenuation, resulting in amplitude distortion, or to unequal time of 

 transmission, causing phase distortion. The actual attenuation char- 

 acteristics of the line ^ and the overall result were shown above in 

 Fig. 3. The requirements for phase distortion are rather difficult to 

 meet. The details in the scanned picture result in various frequencies 

 of the electrical signal, and if these details are to appear in the repro- 

 duced picture in the same relative position as in the scanned picture, 

 it is essential that all frequencies be received in very closely the same 



