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BELL SYSTEM TECHNICAL JOURNAL 



If the smallest picture element to be transmitted is a single block 

 then the distribution of light and shade over the block is unimportant. 

 The average brightness over the block is what counts. Obviously, 

 a simple approximation is a sine wave as shown at the bottom of the 

 picture. This wave has 3^ cycle for each block and is as high a 

 frequency as there is any profit in transmitting for this diagram. 



The top frequency needed for such a picture can then be calculated. 

 The number of square elements in the picture computed above is 



■240 X -3- ELEMENTS 



TOP FREQUENCY = 240 X 240 X -|- X ^ X 24 =806.4 KILOCYCLES 



Fig. 8 — Diagram illustrating the resolution of an image into picture elements and the 

 derivation of maximum frequency required for transmission. 



67,200. As each of these elements represents 3^ cycle, this figure is 

 divided by 2, giving 33,600 cycles per frame. As similar frames are 

 reproduced 24 times a second, the result is 24 X 33,600 or 806,400 

 cycles per second. In a real moving picture, other frequency compo- 

 nents may exist at all other frequencies from 800 kc. down to and 

 including direct current. The direct current or zero frequency 

 component controls the general level of brightness of the picture. 

 Where the general level of brightness changes slowly, it results in a 

 component of very low frequency. A composite picture can be 



