34 BELL SYSTEM TECHNICAL JOURNAL 



72-line picture used in the two-way television-telephone installation of 

 the American Telephone and Telegraph Company in New York.* 

 Here the object to be transmitted is definitely restricted to the human 

 face, which tills the whole field of view, and is adetjuately rendered by 

 the 4,sS00 image elements used. 



The gap between the 4.000 elements of this image and the 350,000 

 considered abo^■e is enormous, not only in figures, but in terms of 

 technical j)ossil)ilit\' of bridging. I^\cn if we are forced to content 

 ourscUes with relatively simple t>i)es of scenes for television trans- 

 mission, still the fact must be s(]uarely faced that a very much larger 

 numbci' of image elements nuist be transmitted than h.is thus far been 

 found possible; and a far wider frequenc\' band utilized than has e\er 

 been used in any communication problem. Now the situation is, 

 simply stated, that all parts of the television system arc already having 

 serious difficulty in handling the 4,500-element image. Consequently, 

 a major problem in television progress is to develop means to extend the 

 practical frequency range. 



It will be worth while to survey briefly the points in a television 

 system where ditiiculty is now encountered when the attempt is made 

 to increase the amount of image detail and the accompanying band of 

 transmitted frequencies. Consider in turn the scanning discs at 

 sending and receiving ends, the photoelectric cells, the amplifying 

 systems, the transmission channels, the receix'ing lamps. 



In the scanning disc at the sending end, which we shall assume 

 arranged for direct scanning, increased detail means either loss of 

 light or increase in the size of the disc. In either case, the factor of 

 change involved is large. For instance, if the number of scanning 

 holes is doubled in a disc of given size, providing four times the number 

 of image elements, the holes must be spaced at half the angular distance 

 apart, and twice the number of holes, imagined placed end to end, 

 must be included in this half diameter scanning field. The holes will 

 therefore be of one-quarter the diameter or 1/16 the area. The light 

 falling on the photoelectric cell at any instant is the light transmitted 

 by one hole; in this case, 1/16 the amount with the disc of half the 

 number of holes. In general, the light transmitted by the disc to the 

 cell decreases as the square of the number of image elements. If the 

 disc is enlarged so as to hold the transmitted light unchanged, its 

 diameter increases directly as the number of image elements. It is 

 obvious that any considerable increase in the number of image ele- 

 ments — such as ten times — demands either enormously increased 

 sensiti\'eness in our photo-responsive de\'ices, or cjuite fabulous sizes of 



I Bell System Techuieal Jounuil, July !<),>(), p. 448. 



