WIRE TRANSMISSION SYSTEM FOR TELEVISION 619 



teristics were known. From these various data, the permissible 

 deviations of the delay characteristic for still picture transmission 

 were determined, and dividing these figures by 50, the ratio of the 

 rate of transmission in picture elements per second in the two cases, 

 the limits for the television circuits were obtained. In this way it 

 was decided to attempt to keep within ±10 microseconds, if possible, 

 with outside limits of ± 20 microseconds. Check tests of these limits 

 were made with the television apparatus in the laboratory by trans- 

 mitting the currents through various known networks, and noting 

 the effect on the received image. 



Unlike the attenuation requirements, the delay requirements for 

 television are not the same over the entire frequency range, but are 

 much more lenient in the lower frequency range, as was shown by 

 experiments in the laboratory. A physical picture of the reason for 

 this may be obtained by reference to Fig. 1. 



Arrows show direction of motion of light spot 



A B 



Fig. 1 



Fig. 1a shows a picture placed in position before the sending machine, 

 consisting of a piece of cardboard the same size as the image-area 

 which can be transmitted, the upper half of the cardboard being 

 colored black, while the lower half is white. As has been explained in 

 the paper by Messrs. Gray, Horton and Mathes, the picture is scanned 

 by a spot of light which moves from left to right in successive lines, 

 tracing 50 horizontal lines across the picture in one sixteenth of a 

 second. The first 25 of the lines lie on the black and the remaining 

 25 on the light part of the picture. The process is repeated 16 times 

 per second, each repetition of 50 lines giving one complete cycle of 

 black and white. The frequency components in this case are multiples 

 of 16 cycles. A transient which blurs the picture outline over a 

 given number, n, of picture elements (downwards) corresponds to a 

 time interval equal to the time of tracing n lines, i.e., w/800 second. 



Now consider Fig. 1b. Here the picture has been rotated 90 degrees. 



