554 BELL SYSTEM TECHNICAL JOURNAL 



lens was required, unless some much more sensitive type of photo- 

 electric cell should be found. 



Assuming that means could be developed for producing an electrical 

 signal proportional to the intensity of the light, of sufficient quickness 

 to follow a rapid scanning device, and of sufficient strength either as 

 directly delivered from a photosensitive device or as amplified, the 

 next problem is that of its transmission over an electrical communi- 

 cation system. We may quickly arrive at an understanding of certain 

 of the transmission problems by reviewing the requirements for the 

 transmission of photographs. In the system of still picture trans- 

 mission now in use by the American Telephone & Telegraph Company, 

 a picture 5 in. by 7 in. in size, divided into the equivalent of 10,000 

 elements per square inch or 350,000 elements, is transmitted in 

 approximately seven minutes. This requires the transmission of a 

 frequency band of about 400 cycles per second on each side of the 

 carrier frequency. If we plan, in the transmission of television, to 

 transmit images of the same fineness of grain, it would mean that 

 what is now transmitted in seven minutes would have to be transmitted 

 in a 16th of a second, which in turn means that the transmission 

 frequency range would have to be nearly 7000 times as great. That 

 is, a band approximately 3,000,000 cycles wide would be required. 

 Bearing in mind that wire circuits are ordinarily not designed to 

 utilize frequencies higher than 40,000 cycles per second, and that 

 with radio systems uniform transmission of wide signal bands becomes 

 extremely difficult, it is seen at once that either an image of con- 

 siderably less detail than that which we have been consideri ng must 

 suffice, or else some means for splitting up the image so that it may 

 be sent by a large number of channels is indicated. 



A further theoretical requirement must also be given consideration. 

 This is that the complete television signal will consist of all frequencies 

 up to the highest above discussed, and down to zero, that is, an 

 essential part of the signal is the direct current component, furnished 

 by those parts of the scene which do not change. The problem of 

 handling the very low frequency components, presents difficulties both 

 in the vacuum tube amplifier system adjacent to the photosensitive 

 device, and in ordinarily available transmission channels. 



In any case certain fundamental transmission requirements must be 

 met. These are that the attenuation of the signals must be uniform 

 over the whole frequency range and that the speed of transmission of 

 all frequencies must be the same. Also, as in the transmission of 

 sound signals, the amount of interference or noise must be kept down 

 sufficiently not to impair the quality of the signal or picture. 



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