TYPE-O CARRIER SYSTEM GUU 



transmitting and receiving group units to translate the sidebands be- 

 tween the frequencj'^ range of the channel band filters and the correct line 

 frequency allocation. The group receiving unit contains the directional 

 filter for separating the four-channel transmitting and receiving groups 

 at line frequencies. INIultiple points are indicated for the connection of 

 other O carrier systems on the same pair, 



Channel Unit 



A block diagram of the channel unit is shown on Fig. 8. As indicated 

 by the dashed line, the channel unit is comprised of four parts which 

 are interconnected by plugs and jacks. These are: 



1. The Compressor Sub-Assembly. This unit contains the compressor 

 and a tei'minating arrangement to permit the system to be used for 

 foin*-wire termination, or for two-wire termination at non-gain locations, 

 i.e., those mthout SAvitching pad control. 



2. The Expandor Sub- Assembly. This unit contains the expandor as 

 well as the signal transmitting and receiving equipment. 



3. The Carrier Frequency Sub- Assembly . This unit contains the trans- 

 mitting and recei\'ing modulators, and is arranged to receive the plug-in 

 transmitting and receiving channel band filters. 



4. The Transmitting and Receiving Band Filters. These are combined 

 in a single plug-in unit. 



Items 1 and 2 are practially identical to the corresponding sub-assem- 

 blies for N carrier. Each channel receives its carrier supply for the trans- 

 mitting side from an oscillator in the twin channel unit. On the receiving 

 side the carrier is obtained by selecting and amplifying the transmitted 

 carrier. 



The frequencies indicated on Fig. 8 are the same for all systems, and 

 apply to two of the four channels in the group. Figs. 4 and 5 show go and 

 return channels in high and low frequency assignments in the same 

 system. However, as sho^vn on Fig. 9 covering the OB system, the fre- 

 quency assignments applying to the channel band filters are above any 

 of the line frequencies and are in the frequency range from 180 to 196 

 kc for l)oth transmitting and receiving channel band filters. 



Transmitting and receiving channel band filters are paired in a single 

 plug-in unit. In order to reduce the number of kinds of paired channel 

 band filters (transmitting and receiving in the same plug-in luiit) the 

 pairing has been done in a special way. If, for example, transmitting 

 assignment 180-184 were paired with receiving assignment 180-184, 

 etc., four different kinds of paired filters would result. Instead, assign- 

 ment 180-184 is paired with assignment 192-196, and by making the 



