556 BELL SYSTEM TECHNICAL JOURNAL 



General Outline of Means Employed in the Present 

 Television System 



It has been pointed out above that if the goal which we set in 

 television is the transmission of extended scenes, with a large amount 

 of detail and hence made up of an exceedingly large number of 

 elementary areas, we meet with the necessity for transmission channels 

 of a character which are not now available. In the present develop- 

 ment it was decided at the start to restrict our experiments to a size 

 and grain of picture which, if the scanning and re-creating means 

 were developed, would be capable of transmission over practical 

 transmission channels, either wire or radio. This restriction fortu- 

 nately leaves us with the possibility of meeting what was felt to be 

 the typical problem of a Telephone Company, namely, the trans- 

 mission of a human face in a television system used as an adjunct to a 

 telephone system. Taking, as a criterion of acceptable quality, 

 reproduction by the halftone engraving process, it is known that 

 the human face can be satisfactorily reproduced by a 50-line screen. 

 Assuming equal definition in both directions, 50 lines means 2500 

 elementary areas in all. 2500 elements transmitted in 1/16 second 

 is 40,000 elements per second. The frequency range necessary to 

 transmit this number of elements per second with a fidelity satisfactory 

 for television cannot be calculated with assurance in advance. An 

 approximate value can however be arrived at from a study of the 

 results obtained in still picture transmission. In pictures transmitted 

 by the system already referred to, individual faces contained in a 

 square space 3^ inch on a side are quite recognizable.^ Taking the 

 ratio of this area to the area of the whole picture, and using the 

 frequency range figure already deduced, for a complete 5 in. by 7 in. 

 picture, it appears that a band of 20,000 cycles would be sufficient to 

 transmit such an image in 1/16 second.^ These considerations led to 

 the choice of a 50-line (2500-element) image as one which would be 

 both satisfactory as to detail rendering, for our purposes, and as 

 calling for frequency transmission requirements sufficiently low to 

 give a good margin of safety in existing single communication channels. 



As a method of scanning, the method which is probably mechanically 

 simplest, namely, that of rotating disks with spirally arranged holes, 

 proposed by Plotnow ^ in 1884, was chosen. In accordance with the 



* Cf. Fig. 18 of the paper referred to (Reference 2). 



^ A factor which this analogy does not cover is that if the image is moving so that 

 it falls on several discrete scanning elements in rapid succession a very material 

 apparent increase in the fineness of the image structure results. This effect is 

 similar to that by which the relatively coarse-grained individual images in a motion 

 picture film fuse to give smooth appearing pictures. 



« Plotnow, D. R. P. 30105, 6.1, 1884. 



