60 BELL SYSTEM TECHNICAL JOURNAL 



The transmitting amplifier is the same as that used on the receiving 

 side and in both directions of transmission in the repeater. This 

 ampHfier is capable of operating at a level 18 db above the transmitting 

 toll switchboard. 



On the receiving side, following the directional filters, is the equip- 

 ment which makes up the system of equalization and regulation. 

 This is identical with that used at the repeaters and is described more 

 fully later. The regulator functions so as to maintain a nearly 

 constant level at the output of the receiving amplifier. The band 

 filters differ from those on the transmitting side only in the frequency 

 bands which are transmitted and are the same as those used at the 

 distant transmitting terminal. The demodulator circuit is of the 

 same general type as the modulator circuit and the oscillator which 

 supplies it with a carrier frequency is practically identical to that 

 used by the modulator. However, because of the low levels at which 

 these copper-oxide units are operated, an amplifier tube is necessary 

 to restore the level to the required value at the output. The gain of 

 this amplifier is continuously adjustable over a range of about 10 db 

 so that precise adjustments of the overall circuit net loss can be made 

 on each channel individually. 



On very short non-repeatered systems the transmission variations 

 may not be great enough to require the automatic regulating equip- 

 ment. In such cases a manually operated potentiometer will be used 

 for controlling the gain. 



Repeater 



A block diagram of the repeater is shown in Fig. 9. Directional 

 filters on each side separate the two directions of transmission. As in 

 the case of the receiving terminal, the equalizing and regulating 

 equipment maintains all channels at the proper level at the amplifier 

 output. The high cut-off filter shown on the circuit in the west-to- 

 east direction limits the transmission to frequencies below 30 kc. 

 This may be desirable when a system employing still higher frequencies 

 is used on the same pair as the Type C system or on other pairs on 

 the same line. 



The repeater provides a maximum gain of about 49 db at the 

 highest frequency in the upper group of the new system and about 

 43 db at the similar point in the lower group. The exact gains at 

 different points in the frequency range are adjusted by the regulator 

 so as to compensate for the attenuation of the line section preceding 

 the repeater. 



A complete repeater with its regulating equipment is mounted on 

 a single equipment bay. A photograph of this bay is shown in Fig. 10, 



