490 BELL SYSTEM TECHNICAL JOURNAL 



marking frequency was 2195 cycles which were equally spaced from the 

 midband frequency, 2125 cycles. In this arrangement a limiter was always 

 used at the receiving terminal. The discriminator was adjusted to be 

 linear over twice the frequency range of the previously described discrimina- 

 tor, and the slope of the new discriminator characteristic was adjusted to give 

 the same marking or spacing output as before. The wide-band frequency- 

 shift arrangement was also tested with low-pass filters associated with the 

 detector outputs which had a cut-off frequency (about 58 cycles) low enough 

 to give a distortion vs. speed characteristic close to that obtained on the 

 normal-band frequency-shift arrangement. The loss of each of these low- 

 pass filters was about 10 db at 58 cycles. Since each filter consisted of only 

 one section the cut-off was gradual. 



Two-Source Terminal Apparatus 



The sending circuits of the two-source arrangement shown in Fig. 2 in- 

 cluded separate oscillators of different frequency for marking and spacing 

 signals. Two sending relays were operated in synchronism by the signals in 

 the sending loop. The relay contacts were so connected that marking car- 

 rier was transmitted to a marking band-pass filter and spacing carrier was 

 cut off from a spacing band-pass filter, or vice versa. Thus either the 

 marking or the spacing carrier frequency was transmitted to the line at 

 any instant. At the receiving end of the line the incoming signals flowed 

 through two receiving filters, one passing the marking current and the other 

 passing the spacing current. When a limiter was not used, the outputs of 

 these filters were connected directly to separate amplifiers, detectors and 

 low-pass filters. The rectified marking and spacing signals were combined 

 differentially, passed through a push-pull d-c amplifier, and operated the 

 receiving relay just as in the frequency-shift arrangement. When a limiter 

 was used the outputs of the receiving filters were recombined and passed 

 through the limiter, after which they were agam separated by means of 

 additional band-pass filters whose losses were about half those of the receiv- 

 ing band-pass filters. The two-source arrangement was tested both with and 

 without limiter when the loss characteristics of the spacing and marking 

 paths were each similar to curv^e A of Fig. 1 and had midband frequencies of 

 1785 and 1955 cycles, respectively. 



The two-source arrangement with limiter also was tested with filters 

 having loss characteristics for the spacing and marking paths each similar 

 to curve C of Fig. 1, and having midband frequencies of 1980 and 2100 

 cycles, respectively. This arrangement occupied a frequency band approxi- 

 mately 12/17 that used for the two-source arrangement with filters having 

 characteristics similar to curve A. 



