568 BELL SYSTEM TECHNICAL JOURNAL 



of the same frequency is required for "demodulation." Advances in 

 the art of designing vacuum tube oscillators of great frequency stability 

 have made it possible to insure that these oscillators, which may be 

 hundreds of miles apart, remain sufficiently close together in fre- 

 quency so that no noticeable impairment in quality of transmission 

 results. 



In the matter of the frequency allocation of the channel bands, 

 the type "C" system possesses one of the essential features of the 

 older type "B" system, that is, the use of dififerent carrier frequencies 

 for transmission in opposite directions. Comparative experience with 

 the type "A" system which, by means of high-frequency line and 

 network balance, employed the same frequency band for the opposite 

 directional paths of the channel led to the conc'usion that the systems 

 which avoided the high-frequency balance requirement were most 

 desirable. Also the problem of intermediate repeater amplification is 

 simplified where the opposite directional frequencies are thus separated 

 and grouped. Furthermore, the crosstalk problem between different 

 systems on the same pole line is greatly simplified for reasons which 

 will be discussed later, and a greater total number of channels may 

 usually be obtained on the same pole line. 



The single sideband transmission employed reduces by about one 

 half the frequency band that would otherwise be required for each 

 channel. The carrier is not transmitted, as the presence in the system 

 of carrier currents of the large magnitude required for a "carrier 

 transmitted" system not only requires greater amplifier load capacity 

 at the repeaters, but may increase the possibility of troublesome cross- 

 talk and noise interference. The selectivity requirements of the band 

 filters would also become more severe to keep the carrier of one channel 

 out of the other channels in the system. 



A Complete System. The simplified layout of a complete system is 

 shown on Figure 2. It will be noted that it includes apparatus at a 

 terminal, a line circuit, a repeater station, a second line circuit and 

 apparatus at a second terminal. Obviously, the total line length 

 between terminals may be extended by the use of a greater number of 

 repeaters. 



At each end there are the terminations of the three carrier channels 

 1, 2 and 3, and the regular voice circuit 4. These terminations appear, 

 of course, at the long distance switchboard in the same office or in a 

 different office from the carrier terminal. When a subscriber is 

 connected to one of the terminations, for example, No. 1, speech 

 currents pass through the three-winding hybrid coil, thence into the 

 modulator circuit where they are caused to modulate high-frequency 



