702. 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



picked off by a narrow band crystal filter. This same carrier is supplied 

 to the receiving side of the channel units for demodulating the associated 

 sidebands. 



Group Transmitting Unit 



The OB group transmitting unit is shown on Figure 11. It receives the 

 four sidebands and two transmitted carriers and places them in the 

 proper high or low line frequency assignment. The transmitting group 

 unit, depending on the optional connection to the group oscillator (Fig, 

 11), can be either a high group transmitting unit or a low group trans- 

 mitting unit. 



For convenience the noise generator is contained in the group trans- 

 mitting unit. On very quiet circuits this noise source provides a means of 

 masking crosstalk. In ordinary usage the noise thus provided is not 

 noticeable on the circuit, but is sufficient to reduce the chance of hearing 

 intelligible crosstalk to a small value. 



Group Receiving Unit 



The OB group recei\dng unit is shown on Fig. 12. It is comprised of 

 an amplifier and a regulator-modulator arrangement equipped with plug- 

 in filters. The same basic arrangement is used for all receiving group 

 units, as well as for all repeaters. Only the plug-in filters, and the fre- 

 quencies from the associated oscillators are different. The directional 



CARRIER SUPPLY 



TO TRANSMITTING- 



MODULATORS 



TO RECEIVING 

 BAND FILTERS 



OSCILLATOR 



184 KG 

 (or 192 KC) 



-Wv 



AA^ 



PICKOFF 



FILTER 



192 KC 



(or 184 KC) 



CONTROL 

 AMPLIFIER 



CARRIER SUPPLY 

 TO RECEIVING 

 MODULATORS 



TRANSMITTED 



CARRIER 

 ADJUSTMENT 



REGULATING 

 AMPLIFIER 



TRANSMITTED 

 CARRIER 



TO OTHER TWIN-_ 

 CHANNEL UNIT 



Fig. 10 — T}^pe-0 twin channel unit. 



■AAV' 



— »- 



— 'WV-i 



180-196 KC 



