694 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



to provide sufficient regulator range to accommodate line variations due 

 to wet or icy lines. The repeater and receiving group regulator range 

 common to four channels is in the order of 40 to 50 db, or approximately 

 four times the regulating range of an N repeater. The range of the 

 twin-channel regulator is comparable to the N individual channel regu- 

 lator, but the O regulator is shared by two channels forming adjacent 

 sidebands of the same carrier. 



The use of a single sideband imposes more severe requirements on 

 channel band filters. The use of a material called ferrite, in combination 

 with a crystal, affords an efficient channel band filter in a small space 

 when compared to previous single sideband channel filters employing 

 air-core coils. Ferrite coils are employed in a coil-condenser type of filter 

 to provide separation for the various four-channel groups. While the N 

 system employs only receiving channel band filters, O has filters in 

 both the transmitting and recei\dng terminals. 



The O system employs the double modulation principle for all groups. 

 This arrangement permits the use of only four channel band filter de- 

 signs for all of the 32 channel frequency allocations. The frequency range 

 for these basic channel bands has been selected to provide the most 

 economical overall filter design. 



The use of die-castings has been extended in a number of ways. No- 

 table among these is a die-cast framework, used in both the terminal 

 and the repeater. The plug-in technique has been expanded to provide 

 plug-in filters for channel and group band filters. 



DESCRIPTION OF THE SYSTEM 



The system will be described first by block schematics, second, by 

 transmission characteristics, and finally by photographs. This descrip- 

 tion will show representative features rather than describe the system 

 completely. 



While the description wdll cover the complete O plan, it should be 

 pointed out that the OB system is the first to be made available. It will 

 be followed by the other systems. 



In the schematic description, where a unit is common to all systems 

 the designation is "Type 0." Where the arrangement is different for 

 the several systems, a particular designation is used, such as "Type 

 OB," etc. 



Schematics 



The O modulation plan is shown on Fig. 5. The single-sideband chan- 

 nel filters for all groups are in the frequency range from 180 to 196 kc. 



