36 BEI.L SYSTEM TECHXICAL JOURXAL 



referred to, the neon must be assisted by a frequently renewed ad- 

 mixture of hydrogen, which again cannot be expected to increase the 

 frequency range indefinitely. In the scanning disc, as at the sending 

 end, increasing the number of image elements rapidly reduces the 

 amount of light in the image. With a plate glow lamp of given 

 brightness, the apparent brightness of the image is inversely as the 

 number of image elements. 



From this rapid survey, it is clear that at practically every stage in 

 the television system, we encounter serious difficulties when a large 

 increase in image elements is contemplated. It is not claimed that 

 these difficulties are insuperable. One of the chief uses of a tabulation 

 of difficulties is to aid in marshalling the attack upon them. But the 

 existing situation is that if a many-element television image is called 

 for today, it is not available, and one of the chief obstacles is the difficulty 

 of geiterating, transmitting, and recoverijig signals extending over wide 

 frequejicy hands. 



One alternative, which prompted the experimental work to be 

 described below, is the use of multiple scanning, and multiple-channel 

 transmission. The general idea, which is obvious from the name given 

 to the method, is to divide the image into groups of elements, the 

 various groups to be simultaneously scanned, and to transmit the 

 signals from the several groups through separate transmission channels. 

 In place of apparatus to generate and transmit a frequency band of n 

 cycles, we arrange m scanning processes each to provide frequency 

 bands of njm cycles width ; njm being chosen as within the limits set by 

 the available practical elements of a television system. It will appear 

 that the method which has been developed does provide an image of 

 manyfold more image elements than heretofore, and may make easier 

 the problem of transmission over practical transmission lines. 



Description of a Three-Channel Apparatus 

 The multi-scanning apparatus which has been constructed and 

 given experimental test uses scanning discs over whose holes are 

 placed prisms of several different angles. At the sending end, the 

 beams of light from successive holes are thereby diverted to different 

 photoelectric cells. At the receiving end, the prisms similarly take 

 beams of light from several lamps and divert them to a common 

 direction. The mode of action of the prisms is illustrated in Fig. la, 

 where a three-channel arrangement is shown, which is that actually 

 used in the experimental apparatus. In the figure, the disc holes are 

 shown disposed in a spiral, at such angular distances apart that 

 three holes are always included in the frame/. Over the first hole of a 



