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BELL SYSTEM TECHNICAL JOURNAL 



high-pass filter to eliminate, from the input to the carrier system, 

 noises below about 200 cycles, such as telegraph harmonics, 20-cycle 

 ringing, 60-cycle power, etc., which may be present on connected voice- 

 frequency circuits. Otherwise these noise frequencies, which are below 

 the voice range, would modulate and pass through the terminal to load 

 unnecessarily the carrier repeaters along the line, as well as to inter- 

 fere with the level indications of the pilot channels. 



VOICE BANDS 



VOICE BANDS 



Fig. 1 — Frequency allocation. 



From the terminating equipment the circuit loops through jacks 

 which have paralleled contacts for reliability. The level at this point 

 is - 13 db compared with the transmitting toll switchboard, which 

 level is expected to be generally used in the Bell System for all multi- 

 channel carrier telephone systems. Then comes the channel modula- 

 tor which consists of four copper-oxide discs, each three-sixteenths of an 

 inch in diameter, potted in a small can. This makes a very simple and 

 inexpensive modulator which is much more satisfactory than tubes. 



