148 BELL SYSTEM TECHNICAL JOURNAL 



is not important for commercial telephone service although it is of 

 importance for loud speaker use. To raise the upper limit of the 

 voice transmission range would require giving up some of these facil- 

 ities. 



Fig. 2 will give an idea of how the distortion introduced by a length 

 of non-loaded cable can be corrected by employing distortion net- 

 works or attenuation equalizers. This figure shows the transmission 

 frequency characteristic of about 10 miles of non-loaded No. 19 A. W. 

 G. cable. Curve A, in the figure, shows the characteristic when un- 

 corrected, while Curve B shows the characteristic for the circuit when 

 equipped with an attenuation equalizer. 



After choosing the proper types of telephone circuits for use in con- 

 nection with loud speakers, there remains to be considered a number 

 of other important matters. 



The maintaining of the telephone power within proper limits at 

 different points in the circuit is very important. The power must 

 not be allowed to become too weak, otherwise the extraneous power 

 induced from paralleling circuits would tend to obliterate the tele- 

 phone transmission. On the other hand, the telephone power must 

 not be amplified to such an extent that the telephone repeaters will 

 be overloaded or severe cross talk be induced into paralleling circuits. 



To keep the telephone power throughout the circuit between the 

 above limits, requires careful study and adjustment. For handling 

 regular telephone connections, the circuits are laid out and equipped 

 with repeaters at proper points so that each circuit will be able to 

 handle the varying volumes applied at the terminals when different 

 subscribers are connected without getting into serious difficulties. 

 When loud speakers are employed it is necessary to maintain the 

 volume at the terminals of the toll lines at least within these limits 

 and it is preferable to do somewhat better than this. 



With the public address system, the high quality transmitter which 

 picks up the sound at the sending end is usually associated with an 

 amplifier whose adjustment is varied, depending on the output of 

 voice currents from the transmitter. In order to obtain the proper 

 adjustment of this amplifier, it is necessary to have some means for 

 quickly indicating the volume of transmission. For this purpose, 

 there has been developed a device which is called a "volume indicator." 

 This consists of an amplifier detector working into a direct-current 

 meter. With this volume indicator connected across the output of 

 the transmitter amplifier, the volume of transmission delivered to 

 the line is indicated by the deflections on the meter. By adjusting 

 the amplifier, therefore, to keep the deflections of this meter reason- 



