166 BELL SYSTEM TECHNICA L JOURNA L 



In order to coordinate with these existing circuits and studio loops, a 

 carrier program system must be capable of duj)licating this flexibility while 

 maintaining the desired standards of quality of transmission. 



In setting an objective for the standards of transmission quality of this 

 new system the trend towards wider band widths has been recognized. 

 Most of the major networks now use a 100 to 5000-cycle band width. A 

 large part of the present audio-frequency cable facilities, however, can be 

 arranged to transmit a band from 50 to 8000 cycles. It was decided to match 

 this grade of transmission in the design of the new carrier system. For 

 the cases where still higher quality is desired, a 15-kilocycle carrier program 

 system has been developed and is now available. 



Design Features 



The 12-channel bank of message circuits forms the basic building block 

 of the broad-band carrier telephone systems. In the channel bank, each 

 of the 12 voice-frequency channels modulates one of 12 carriers spaced 4 

 kilocycles apart from 64 to 108 kilocycles. The lower sideband resulting 

 from each modulation is selected by a band filter and combined with the 

 other 11 lower sidebands to give a channel group occupying the frequency 

 space from 60 kilocycles to 108 kilocycles. This channel group is then 

 further modulated as a unit to its appropriate place on a broad-band spec- 

 trum for transmission over the line. 



In order to arrange a channel bank for program transmission, message 

 channels, 6, 7, and 8 are disabled, clearing a space from 76 kilocycles to 88 

 kilocycles in the group-frequency spectrum. In a program terminal sepa- 

 rate from the channel bank, an audio frequency program modulates an 

 88-kilocycle carrier derived from the message channel carrier supply. Its 

 lower sideband is selected by a band filter and, combined with the lower 

 sidebands of message channels 1 to 5 and 9 to 12, gives a group-frequency 

 spectrum shown diagrammatically in Fig. 1. This figure also shows the 

 same spectrum after it has been modulated with a 120-kilocycle group 

 carrier for transmission Over a type K line. Other line-frequency spectra 

 are similarly produced in type J and type L group modulators. 



The reversing and control signal in an audio-frequency program circuit 

 is a d-c. signal superimposed on the program pair. It may be applied at 

 the studio which originates the program, and conditions all of the amphfiers 

 along the line to transmit away from the originating studio. As long as the 

 signal is applied, the direction of transmission is locked so that no other 

 control station can inadvertently break the network. When the transmis- 

 sion from this studio ends, the signal is removed, and the next originating 

 point applies it. This efifects such reversals as are required for trans- 

 mission and again locks all amplifiers. By this means it is possible to use 



