254 BELL SYSTEM TECHNICAL JOURNAL 



2-wire line without permitting them to go into the outgoing leg of 

 the 4-wire circuit. The currents coming from the 2-wire line go into 

 both sides of the 4-wire circuit but travel on the receiving leg only 

 until they meet with a repeater which, being directed against them, 

 prevents further travel. The amount of amplification which can be 

 maintained in such a circuit is dependent upon the effectiveness of 

 the balance maintained between the real 2-wire line and the artificial 

 line or network at the hybrid coil. The transatlantic circuit was set 

 up for initial two-way experiments in accordance with this procedure. 

 Since the east-bound and west-bound channels were on different 

 frequencies, the selectivity of the receiving sets prevented any cross- 

 fire from the local transmitter into the local receiving circuit. 



Since it is necessary to deliver signals to the distant receiving 

 station of the maximum possible amplitude in order to maintain a 

 favorable signal-to-noiste ratio, it was essential that the transmitters 

 be kept loaded up to full output even though the voice currents 

 coming from the speakers might vary widely due to differences in 

 voices and differences in attenuation of connected 2-wire circuits. 

 This was done by changing the gain in the repeaters, the operation 

 being carried out by a control operator in a manner similar to that 

 employed in broadcasting stations. In order to maintain the overall 

 gain around the circuit constant to avoid singing difficulties, it was 

 necessary to change the amplification at the receiving end in such a 

 manner as to compensate for changes at the transmitting end. 



Experimental operation of the system on this basis was hindered by 

 the fact that the two frequency bands being employed for the two 

 oppositely directed channels were also being used by a number of 

 radio telegraph stations, some of these being so powerful as to produce 

 interfering signals which very seriously hampered telephone conversa- 

 tion. It was evident that some arrangement must be made to enable 

 the telephone communications to be carried on in frequency bands 

 which were used by them exclusively. The fact that radio telephony 

 inherently requires a wider band for its accomplishment than does 

 radio telegraphy made it desirable to use every device available to 

 narrow the band occupied in order to reduce to a minimum the 

 necessary displacement of existing telegraph services. The employ- 

 ment of single sideband carrier eliminated transmission had already 

 cut in half the frequency space required over that which would be 

 needed if the ordinary form of modulated transmission were used. 

 In order to cut down still further the width of frequency band occupied 

 it was decided to attempt to operate both the east-bound and the 

 west-bound channels on exactly the same frequency band. If this 



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