WIDE-BAND OPEN-WIRE PROGRAM SYSTEM 363 



many as six branches are supplied but generally only two or three taps 

 are utilized. 



To accomplish this branching out at a bridging station a resistance 

 network multiple is provided, having six outlets. This network 

 multiple is shown on Fig. 11. To annul the loss of the network a 

 single stage amplifier is connected in front of it. This network mul- 

 tiple and amplifier are mounted on the same panel forming a single 

 integral unit. The network multiple is so proportioned that if any 

 one of the branches is accidentally opened or short-circuited the other 

 branches are affected to only a minor degree. The amplifier is ad- 

 justed so that the gain from the input terminals to any of the output 

 branches is zero. The bridging amplifier is normally inserted imme- 

 diately in front of the line amplifier. As in the case of the line 

 amplifier mentioned above, high inductance coils are utilized in order 

 to keep phase distortion at a minimum. A resistance adjustment is 

 provided in the grid circuit in order to adjust the high-frequency 

 characteristic of this amplifier to the desired value. 



The gain-frequency characteristic of the bridging amplifier is prac- 

 tically identical with the corresponding characteristic just described 

 for the line amplifier, while the delay is even less. 



Predistortion 



The means utilized to accomplish the predistorted transmission 

 referred to earlier includes the provision of a so-called predistorting 

 network at the sending end of a program circuit and a restoring 

 network in each branch which supplies a broadcasting station. The 

 predistorting network introduces a large loss at low frequencies with 

 a decrease in loss as the frequency is increased. By introducing 

 suitable amplification immediately behind the predistorting network 

 the resultant effect is to raise the high-frequency transmission relative 

 to the low-frequency transmission by the difference in loss between 

 the 1,000-cycle loss of the predistorting network and its higher fre- 

 quency loss. The restoring network characteristic is the inverse of 

 the predistorting network. These two networks are 600-ohm constant 

 impedance type structures. The restoring network is shown schemat- 

 ically in Fig. 12. The predistorting network is generally similar to 

 this, having different constants and a slightly different arrangement 

 of elements. On Fig. 13 are shown the loss-frequency characteristics 

 of the predistorting and restoring networks and a third characteristic 

 which is the sum of these two. As may be noted this latter character- 

 istic has a constant value throughout the frequency range. 



