164 BELL SYSTEM TECHNICAL JOURNAL 



stations at intermediate points as is frequently the case in railroad 

 operation. 



A unique feature is the use of opposite sidebands of the same carrier 

 frequency for opposite directions of transmission. The upper side- 

 band is used in one direction and the lower sideband in the other, the 

 carrier being suppressed. For the modulators and demodulators 

 copper oxide "varistors" are employed in place of vacuum tubes. 

 The amplifiers are single stage, employ pentode tubes and are stabilized 

 in performance by feedback. The filters have been simplified in con- 

 struction by the use of coils with a new type of core material and by 

 improved designs of paper condensers. 



The size of the new terminal has been so reduced that it occupies 

 less than 40 per cent of the space required for a Type D terminal, 

 as indicated in Fig. 1. The equipment may be mounted on racks as 

 is customary in telephone offices, or a complete terminal or repeater 

 may be mounted in a small cabinet. 



Single-channel carrier systems have been used in the Bell System 

 principally for short open-wire toll circuits. Thus, the Type D 

 systems are for the most part between 50 and 200 miles in length. 

 The Type H system, since it includes a repeater, can be used for 

 greater distances, and due to its lower cost is economical for shorter 

 distances. 



General Description of System 

 Basic System 



The basic system consists of two terminals one of which is referred 

 to as an "east" terminal and the other as a "west" terminal, as indi- 

 cated in Fig. 2. The two terminals differ only in minor respects, the 

 differences being due to the fact that at one terminal the upper side- 

 band is transmitted and the lower sideband is received, while at the 

 other terminal the reverse takes place. In order to simplify coordina- 

 tion between various types of carrier systems operating on the same 

 pole line, the frequencies between 7400 cycles and 10,150 cycles are 

 transmitted in the east to west (or north to south) direction, and 

 the frequencies between 4150 cycles and 6900 cycles in the west to 

 east (or south to north) direction. The frequency allocation of the 

 Type H system and those for the Type D and the three-channel Type 

 CS system are shown in Fig. 3. All three types may be operated on 

 the same pole line. 



The circuit arrangement is given in greater detail in Fig. 4, which 

 shows a schematic diagram of one terminal, with the exception of the 

 power supply circuit. Each terminal is made up of a transmitting 



