562 BELL SYSTEM TECHNICAL JOURNAL 



Transmission Requirements 



The effects of extraneous line disturbances may or may not be 

 particularly objectionable in a specific case, depending upon their 

 magnitude and form and also on the nature and use of the received 

 picture. Furthermore, the predominance of the recorded disturbance 

 may also be affected by normal variations in the adjustments of the 

 telephotograph equipment. It is not practicable, therefore, to estab- 

 lish precise limits for the transmission requirements. The following 

 values are mentioned as illustrative of the order of magnitude for 

 some of the more important requirements applying to circuits used 

 with the new telephotograph equipment, and which experience has 

 shown will give generally satisfactory results. 



(a) Line Net Loss 



Abrupt variations in line net loss of 0.2 decibel or greater usually 

 will produce a noticeable change in shade of the received picture. 

 However, a gradual variation in net loss occurring over a period of 

 minutes is less objectionable and in many instances a change of as 

 much as 2 or 3 decibels during a transmission can be tolerated. 



(6) Noise 



Noise of a single-frequency type is likely to be recorded in the 

 received picture as an objectionable moire pattern if the difference 

 between the maximum signal and interference energy is less than 

 50 decibels. However, if the interference energy is distributed over a 

 relatively wide frequency band an energy difference of about 35 

 decibels usually can be tolerated. 



(c) Delay Distortion 



Delay distortion introduced by the circuit, if of sufficient magnitude, 

 may produce multiple outlines along the edges of objects or lines in 

 the received picture and result in a loss or general masking of picture 

 detail. In order that this effect may be inappreciable in pictures 

 received with the new telephotograph equipment it is desirable that the 

 maximum deviation in envelope delay throughout the useful frequency 

 band (1,200 to 2,600 cycles per second) be less than ± 300 micro- 

 seconds. 



D-C. Control Circuit 



Sudden small variations in line net loss are normal on toll cable 

 circuits in the United States as the result of the stepping of the regu- 

 lating network relays, which, under control of a pilot wire regulator, 

 compensate for the effect of temperature changes on the attenuation 



