PRODUCTION OF TELEVISION SIGNALS 



561 



subjects in a view, the scanning process must be repeated and a new 

 picture transmitted at least every sixteenth of a second. 



Many purely theoretical methods could be, and have been, devised 

 to accomplish such a scanning process and to translate a view into 

 electrical currents or signals. Unfortunately, however, a practical 

 system of television must operate with materials and conditions as 

 they exist, and these practical limitations constitute the serious 

 problems of television. 



The high speeds and relatively large amplitudes with which any 

 television scanning mechanism must move, and the necessity for 

 synchronizing the transmitting and receiving apparatus lead to the 

 use of synchronously rotating machines as apparently the only practical 

 solution of the scanning and receiving problems. Consequently, the 



Fig. 1 — Several light sources illuminate the subject; a lens forms an image which 

 is scanned by a spiral of apertures, through which the light falls on a single photo- 

 electric cell. 



present television system has been designed to operate with con- 

 tinuously rotating mechanical parts. 



The efficiency that must be secured in the optical part of any 

 scanning method is fixed by the three following factors — the amount 

 of picture detail that is to be transmitted, the efficiency of the light- 

 sensitive cell, and the practical limit to amplifier systems. The first of 

 these factors decides the area from which light can be collected at 

 any one instant. In the present case this was fixed in an initial survey 

 of the entire television problem when it was decided to confine the 

 first attempt to the transmission of pictures as if they were made 



