586 BELL SYSTEM TECHNICAL JOURNAL 



Since even with these oscillators absolute isochronimc sannot be 

 maintained indefinitely without readjustment, WHO was chosen as the 

 reference frequency station and WOC was provided with means by 

 which its carrier frequency could be brought into exact isochronism 

 with that of WHO. In order that the operator of W^OC could easily 

 determine the degree of isochronism, a monitoring receiver was set up 

 at a point midway between the stations and the program received 

 there was transmitted back to station WOC by wire line. A departure 

 of the two stations from isochronism is shown by a slow variation in the 

 level of the program received and the operator can then make the ad- 

 justment necessary to restore the stations to isochronism. The nicety 

 of this adjustment can best be appreciated by the fact that a complete 

 revolution of the control dial varies the carrier frequency at WOC by 

 but one part in a million. 



It was found difficult at times to determine the beat frequency re- 

 sulting from a lack of isochronism on account of the masking effect of 

 the rhythm or beat of a musical program. The receiver was therefore 

 equipped with a small audio-frequency oscillator which was arranged 

 to modulate completely the incoming carriers received from the two 

 stations. These combined modulated carriers are detected in the 

 usual way and the output from the receiver is then an audio tone, the 

 level of which is directly proportional to the resultant of the combined 

 carriers. This tone overrides the program and is transmitted back to 

 the station where even very slow changes in its level are easily detected 

 by the operator. This also has the advantage that the degree of iso- 

 chronism can be determined before any program is broadcast and 

 that any necessary readjustment to restore isochronism can be made 

 during silent periods in the program. This tone is required only at the 

 time of adjustment, and relays at the monitoring point have been ar- 

 ranged for remote control from station WOC by which the audio 

 oscillator can be turned on whenever desired. These relays also permit 

 the operation of either of two receivers and permit the setting of the gain 

 of either receiver at the proper level for day- or nighttime reception. 

 The control panel at the station, shown in Fig. 6, is equipped with 

 supervisory signal lamps which indicate the position of these relays. 



The equipment at the monitoring point, show^n in Figs. 7 and 8, is 

 mounted on a single relay rack and includes the loop antenna which has 

 been made sufficiently unidirectional to permit the obtaining of an ex- 

 act balance between the signal strengths received from the two sta- 

 tions. The two radio receivers with their associated audio oscillators 

 and the relay control panel complete the equipment at the monitoring 

 point. The rack is arranged for the complete enclosure of all the equip- 



