54 



BELL SYSTEM TECHNICAL JOURNAL 



The carrier channels are separated from the voice frequency circuit 

 on the same pair of wires by means of a high-pass and low-pass filter 

 combination as shown on Fig. 3. Several different filter sets are 

 available for this purpose differing from each other in the frequency 



400 



Fig. 4- 



800 



1200 1600 2000 2400 



rREOUENCY -CYCLES 

 -Typical overall transmission-frequency characteristic 



2800 3200 



band which is desired in the voice circuit. Where the voice circuit is 

 an ordinary message circuit the filter will have a cutoff around 3 kc. 

 Where it may be used for program transmission a filter set having a 

 cutoff above 5 kc is provided. For still wider program bands there is 

 a filter set cutting off above 8 kc. The use of this latter filter would, 

 of course, require the sacrifice of the lowest carrier channel since it 

 would be overlapped by the program band. 



An important feature of the system is the method of stabilizing 

 the overall transmission. Ahead of the terminal transmitting amplifier 

 in each direction of transmission there is connected to the circuit an 

 oscillator which generates a pilot current. This pilot current has a 

 frequency adjacent to the band of the middle channel. The oscillator 

 is designed to have a relatively high degree of stability with respect 

 to output and frequency. At the output of each repeater and at the 

 receiving terminal the pilot frequency is selected by a high-impedance 

 bridging filter, which has little effect on the through transmission, 



