608 BELL SYSTEM TECHNICAL JOURNAL 



monitoring arrangements are provided for the telephone operator, the 

 technical operator, and the printer operator. The distant terminal 

 operator may interrupt the printer circuit with voice if such interrup- 

 tion seems expedient. In addition to the printer used by the printer 

 operator, a printer at the technical operator's position, not shown in 

 Fig. 4, is continuously connected to monitor on the system. 



It should be noted that in the arrangement shown in Fig. 4 two 

 different voice frequencies are used for transmission and two others for 

 reception, thus giving the advantages of the two-tone method of trans- 

 mission. The voice-frequency tones go out over wire circuits to the 

 transmitting station at Rocky Point where, by means of the single side 

 band suppressed-carrier method of radio transmission ^''•^^ shown in Fig. 

 4, they are changed to radio frequencies of about 60 kilocycles and am- 

 plified. The equivalent radiated power for each frequency is about 

 50 kilowatts. For signals coming from England much the same process 

 is followed at the British end, the radiated frequencies, however, being 

 different from those transmitted in the opposite direction. 



The most serious handicap in the use of printing telegraph on the 

 long-w^ave channel is noise. During the winter months little trouble is 

 experienced, but interruptions are frequent and are occasionally of 

 several hours' duration during the summer months. At the radio re- 

 ceiving stations the directive antenna systems used for telephony ^^ 

 greatly reduce the noise received. 



Another important factor in reducing receiving interference is the 

 frequency selectivity. The radio receiver itself restricts the received 

 band sharply to that required for single side band telephony, passing a 

 band about 3000 cycles wide. This is accomplished by the single side 

 band carrier resupplied receiver ^^ shown in Fig. 4. The band admitted 

 to each of the tone channel detectors beyond the receiver is narrowed 

 down to about 110 cycles by a voice-frequency filter as indicated in Fig. 

 4. It is estimated that if the printer were used continuously on the 

 long-wave transatlantic channel for the entire year, the per cent of er- 

 rors would exceed 0.1 per cent less than 12 per cent of the time and 5.0 

 per cent less than 2 per cent of the time. 



In order to obtain more accurate quantitative information regard- 

 ing the effect of noise on the transmission of teletypewriter signals over 

 radio circuits, a series of tests was carried out during 1930. For the 

 purpose of these tests a radio circuit was established between the trans- 

 mitting station at Rocky Point, L. I., and a temporary receiving station 

 at Rochester N. Y., a distance of 286 miles. 



This one-way circuit utilized at New York the transatlantic trans- 

 mitting facilities for printing telegraph described above with the ex- 



