570 BELL SYSTEM TECHNICAL JOURNAL 



carrier current. The resultant modulated bands - of frequencies pass 

 through a band filter allowing only the desired band to pass to the 

 transmitting amplifier, thence this band passes through a so-called 

 directional filter and a high-pass filter to the line circuit. The high- 

 pass filter last referred to, in association with its complementary low- 

 pass filter, forms a so-called line filter set whereby the regular voice 

 range currents are separated from the higher frequency carrier current 

 at both terminal and repeater offices. 



The other two carrier channels function similarly, and the several 

 modulation bands of carrier frequencies join the first channel in passing 

 through the common amplifier and directional filter circuit to the line. 

 At the repeater point the group of bands comprising the three channels 

 passes through the high-pass line filter circuit, thence through a 

 directional filter and line equalizer to the amplifier circuit and outward 

 through the directional and line filter circuit to the next line section. 

 At the farther terminal the combined carrier currents pass through the 

 directional filter and are again amplified in the receiving amplifier. 

 At the output of the amplifier the different carrier channel bands of 

 frequencies are selected one from another by the band filters, thence 

 they pass to the demodulator circuit, are demodulated to their original 

 form and then pass from the output connection of the hybrid coil to 

 their respective terminations. 



Circuit Arrangements at Terminals. Figure 3 shows diagram- 

 matically in somewhat greater detail the terminal of the type "C" 

 system. The modulator circuit consists of a two-tube "push-pull" 

 grid-bias vacuum tube circuit in which the carrier frequency is balanced 

 out. A separate oscillator tube circuit of exceptional frequency 

 stability supplies the carrier. The frequency allocation requires the 

 transmission of only the upper or lower sideband frequencies, and the 

 band filter at the output selects the desired band, rejecting the other 

 products of modulation as well as the amplified voice frequencies 

 which are incidentally transmitted through the modulator unit. This 

 sideband current in conjunction with the corresponding currents of 

 the other two sidebands of the outgoing channels passes through the 

 common amplifier. This is a two-stage vacuum tube unit having 

 four tubes in the output circuit arranged in parallel push-pull con- 

 nection to insure the required load carrying capacity. 



The circuit then leads through a directional filter of either low-pass 

 or high-pass type which distinguishes between the band groups of 



2 For a discussion of modulation see E. H. Colpitis and O. B. Blackvvell, "Carrier 

 Current Telephony and Telegraphy," A. I. E. E. Transactions, V. 40, 1921, pix 

 205-300; R. V. L. Hartley, "Relation of Carrier and Side Bands in Radio Trans- 

 mission," Bell System Tech. Jl., V. 2, April 1923, pp. 90-112. 



