248 BELL SYSTEM TECHNICAL JOURNAL 



long loop of the circuit is compared with transmission over a local distor- 

 tionless circuit. Each type of circuit thus is rated as to the maximum 

 volume it can transmit without noticeable distortion. The highest volume 

 which can be permitted without excessive crosstalk into other program or 

 message telephone circuits is also investigated, and whichever limit is the 

 lower determines the maximum allowable working volume for service. The 

 range between the maximum permissible volume and the noise level on very 

 long lengths of the present program circuits is about 45 or 50 decibels, 

 except under some conditions on certain open-wire sections. On the indi- 

 vidual links making up the long circuit, the range is 10 or 20 decibels greater 

 than this. 



Attenuation and Delay Distortion 



Another important consideration is the amount of attenuation and delay 

 (or phase) distortion to be permitted within the transmitted frequency band. 

 It is the practice to equip program circuits with adjustable attentuation 

 equalizers. By means of these once the desired frequency band has been 

 chosen the deviation in attenuation at any frequency within that band, 

 compared with that at 1,000 cycles, can be adjusted within close limits. On 

 very long circuits, however, experience has shown that even with automatic 

 regulating features and careful operation residual variations which may 

 amount to several decibels may develop as a result of changing temperature 

 and other conditions. These variations are kept within tolerable limits 

 by readjustment of the equalizers from time to time. 



Associated with the attenuation distortion is another effect detrimental 

 to program quality, namely, differences in time of transmission for different 

 frequency components of the signal. In practice, circuits tend to have a 

 lower velocity of transmission near the edges of the frequency band than 

 in the middle portions. This results in frequency components near the 

 edges being delayed as compared to the middle portions of the band. This 

 difference in time of transmission is called delay distortion of the circuit. 

 Careful listening tests have shown that it becomes noticeable if, at the 

 highest transmitted frequency, the delay is more than eight milliseconds 

 greater than at 1,000 cycles, and if, at 100 cycles, it is more than about 

 15 milliseconds greater than at 1,000 cycles. It is controlled by careful 

 attention to the design of loading system.s, amplifiers, repeating coils, and 

 all other elements of the circuit. Since such small amounts of over-all 

 delay distortion are detectable and since networks frequently have 100 or 

 more amplifiers in tandem between an originating point and the broad- 

 casting stations on the more distant portions of the networks, it is necessary 

 that the delay distortion of all individual components of a network be held 

 to exceedingly close limits. Accumulations of residual delay distortion 



