824 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



2 and 3. The frequency allocation and modulation steps shown on Fig. 

 20 were chosen so that good performance could be obtained with rela- 

 tively inexpensive filtiers used for suppressing unwanted sidebands and 

 to develop lower sideband signals appearing right side up for trans- 

 mission over the high-frequency line. The carrier frequencies used in 

 the modulation steps were chosen so that some of the same filters could 

 be used in the submastergroup two and mastergroup two and three 

 modulators. The clear bands between the submastergroups and the 

 mastergroups were chosen to permit the use of economical blocking 

 filters at repeaters where circuits are to be dropped from the high-fre- 

 quency hne and to provide frequency space for the line pilots. 



It will be noticed in Fig. 19 that faciUties are provided for patching 

 spare equipment into service for maintenance reasons or in the event of 

 a failure in working units. Alarm features are incorporated in the sub- 

 master group and mastergroup units to indicate trouble conditions and 

 initiate maintenance procedures. 



The carriers required for the channel, group and supergroup units are 

 supphed by equipment developed for the LI system. The arrangements 

 for supplying the new carrier and pilot frequencies are shown in Fig. 21. 



A problem of primary importance in carrier supply design is the ac- 

 curacy of the frequencies. There is both an absolute accuracy and a 

 relative accuracy requirement. For transmission of some types of signals 

 there is a requirement that the difference in frequency between a carrier 

 at two terminals be less than 2 cycles per second. This extreme accuracy 

 is achieved by using the oscillator at one terminal to control the fre- 

 quency of oscillators at other points. A line pilot generated at the 

 terminal in which the master oscillator is located is used as a reference 

 frequency at points along the line where other terminals are located and 

 by this means carriers are held to a relative accuracy of =tl part per 

 30 million. 



In order that requirements for high quality television transmission 

 may be realized on a 4,000-mile circuit, the output of the pilot supply 

 must be extremely constant with both time and temperature changes. 

 Deviations in the magnitudes of line pilots are maintained to less than 

 0.05 db. 



Since a failure in the L3 carrier and pilot supply could cause inter- 

 ruption to service on an extremely large number of circuits, many pre- 

 cautions have been taken to make the equipment reliable. In addition, 

 standby units, which are automatically switched in place of the regular 

 units in the event of failure, are provided to improve the over-all re- 

 liability of the system. 



