558 BELL SYSTEM TECHNICAL JOURNAL 



employed were glow lamps containing neon gas, the brightness of 

 which changes with sufificient rapidity to follow the incoming signals. 



The problem of synchronization was postponed in our earlier 

 development work by mounting the scanning and receiving disks 

 upon the same axle. It was later solved for the demonstration 

 apparatus by the utilization of synchronous motors controlled by 

 two frequencies, a low frequency, that of the image repetition period, 

 and a high frequency, chosen of such a value that the fraction of the 

 cycle through which transient phase displacements occurred amounted 

 in angular displacement to less than half the angular extent of a 

 single disk aperture. The synchronization control therefore called for 

 the transmission of additional currents for synchronization purposes 

 over and above the picture current. 



In order to transmit and synchronize the image signals it is necessary 

 to transmit three different frequency bands, one for the image, and 

 two for the high and low frequency synchronization controls. In the 

 demonstration of April 7, 1927, the images were sent in the wire 

 demonstration over a high quality open wire line. The synchroniza- 

 tion control was sent over two separate carrier channels of a second 

 telephone line. In addition to these lines, another line was used for 

 conveying the telephone conversation. In the radio demonstration 

 two different wave-lengths were used respectively for the image signals 

 and for the synchronization signals which were, as in the wire demon- 

 stration, carried on two different carrier frequencies. A third channel 

 was used for the voice. In the case of both wire and radio trans- 

 mission, it is quite possible to put all of the different signals upon the 

 same transmission channel, using different carrier frequencies. 



It will aid toward a clear understanding of the reasons for the 

 success of the system of television described in the following papers if 

 we summarize at this point the chief novel features to which that 

 success is due. They may be listed as follows : 



1. Choice of image size and structure such that the resultant 

 signals fall within the transmission frequency range of available 

 transmission channels.^ 



2. Scanning by means of a projected moving beam of light. 



3. Transmission only of alternating current components of image, 



4. Use of self-luminous surfaces of high intrinsic brilliancy for re- 

 construction of the image. 



5. High frequency synchronization. 



" As the succeeding papers show, the margin between the frequency range required 

 by the scanning apparatus and that which could be made available was quite liberal. 

 It appears in the light of our experience that apparatus with 60 or 70 scanning 

 holes instead of 50 might be used with the transmission facilities which were at 

 our disposal. 



