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BELL SYSTEM TECHNICAL JOURNAL 



downward behind the disk at such a speed that the successive holes 

 throw their light in successive rows across the picture one above 

 another. The film moves one frame for each revolution of the disk. 

 A photosensitive surface mounted behind the film picks up the light 

 transmitted through it, and produces a complex electric current 

 corresponding to the variations of light in the picture. Figure 7 is a 

 photograph of the housing in which the disk is mounted. This 



PHOTO-SENSITIVE 

 SURFACE AND 

 ELECTRON 

 AMPLIFIER 



Fig. 6 — Schematic diagram of the mechanical scanning arrangement used for 



television testing. 



scanning arrangement produced a picture of 240 lines, 24 frames per 

 second. It was recognized that 24 frames per second were not suffi- 

 cient to avoid flicker but this choice simplified the scanning apparatus 

 and it was believed would not interfere with engineering tests. 



Signal Frequency Range 



In order to understand what frequency is required to transmit an 

 image scanned in this way consider the diagram shown in Fig. 8. 



