586 BELL SYSTEM TECHNICAL JOURNAL 



system, the two details will be displaced relatively to each other along 

 the line of scanning and, in most cases, if this shift is appreciable, 

 some change in the shape of the signal wave defining each detail 

 results with further increase in the distortion. The same relative 

 shift would occur if the narrow detail were located upon the broader 

 one, in which case such a shift would be more apparent. It would 

 seem reasonable to expect then that differences in the envelope time 

 of transmission comparable to a whole picture element (about 28 

 microseconds in the demonstration apparatus) would be noticeable. 



In most images very few details will have signal shapes, as in this 

 special case, in which the frequency components are concentrated in 

 narrow frequency bands. An abrupt change in signal strength, for 

 instance, is represented by components distributed over the whole 

 frequency range. We can imagine these frequencies divided into any 

 arbitrary number of groups, each of which determines a wave form. 

 When these wave forms are added together, they will reproduce the 

 original abrupt change in signal strength. If, however, they are sent 

 through a system in which the envelope delays for the different groups 

 are unequal, the individual wave forms will be relatively displaced 

 and will no longer combine correctly. As a result the image is blurred. 

 For some types of phase distortion the effect appears as an oscillatory 

 transient following sudden changes in intensity. 



It was furthermore found by experiment that the limit of ± 10 

 microseconds was not necessary for the lower frequencies. Reference 

 to the delay characteristics of the transformers described in the latter 

 part of this paper shows that in the lower part of the frequency 

 scale deviations from the nearly uniform value of delay at the upper 

 frequencies appear of magnitude greater than 100 microseconds. 

 When the signal was sent through these transformers, however, there 

 was no observable distortion of the image. The requirements are 

 therefore much more lenient at the low frequencies. 



In the terminal apparatus the problem of meeting the above out- 

 lined phase transmission requirements was not a very serious one. 

 The circuits involved are such that when a flat amplitude-frequency 

 characteristic had been secured the phase distortion was also negligible. 



Section III. Terminal Circuits for Sending and Receiving 

 Television Signals 



The preceding sections have discussed the methods by which an 

 object, the image of which is to be transmitted, is made to control the 

 time variations in a light, thus giving a luminous signal wave, and the 

 means by which the image may be reconstructed with the aid of an 



