488 BELL SYSTEM TEClIXfCAL JOURNAL 



filters and associated repeating coils. The two-source arrangements each 

 occupied two such bands with midband frequencies spaced 170 cycles apart, 

 except in the case of the narrow-band two-source arrangement which occu- 

 pied two bands having characteristics similar to curve C with midband 

 frequencies spaced 120 cycles apart. Curve B represents the loss vs. fre- 

 quency characteristic of the wide-band frequency-shift arrangement, having 

 approximately twice the band width of curve A. These characteristics were 

 all measured between 600-()hm terminations, without adjacent channel 

 filters present. 



Oii-Ojf Terminal Apparatus 



Figure 2 shows in block form the different circuit arrangements tested, 

 together with the location of the carrier frequencies in the transmitted bands. 

 In the on-off arrangement, the oscillator transmitted 1955-cycle current to 

 a modulator which contained a polar telegraph relay controlled by signals 

 from the local sending loop. During spacing signals this relay short-cir- 

 cuited the carrier supply, and during marking signals it allowed the carrier 

 current to flow through the sending band-pass filter to the adjustable resist- 

 ance line. 



When it was desired to measure the effect of interference from other chan- 

 nels working in adjacent pass bands, their terminal equipment was added by 

 connecting the line sides of their sending or receiving filters to the common 

 sending or receiving bus (See dashed lines designated "bus" in Fig. 2), so 

 that all the channels would transmit over the same line. 



After the carrier signals passed through the receiving filter connected to 

 the output of the line, they were converted by a detector-amplifier into direct 

 current for operating a polar receiving relay, which, in the absence of incom- 

 ing signals, was held on its spacing contact by local biasing current. The 

 receiving relay contacts transmitted into the local receiving loop. 



Single-Sideband Terminal Apparatus 



In testing the single-sideband arrangement shown in Fig. 2, the terminal 

 equipment of the on-off arrangement was used and the carrier frequency was 

 placed 43 cycles above midband, at 1W8 cycles. In some of the tests the 

 modulator was modified to transmit during spacing intervals a carrier cur- 

 rent 6 db below its marking value. Since the loss in the receiving filter was 

 greater at the edge of the band than at the center, it was necessar}' to in- 

 crease the gain of the linear detector-amplifier in order to have the same 

 change of current in the line winding of the receiving relay of the single side- 

 band arrangement as in the on-off arrangement. A further increase in the 

 detector-amplifier gain was necessary when the single-sideband arrangement 

 was operated with spacing carrier 6 db below the marking carrier. 



