Multi-Channel Telephonp: Systems 



It was pointed out under "Early Developments" that, for clear 

 transmission, telephone circuits must transmit a hand of frequencies, 

 the minimum hand used for new telephone circuits hein^ hetween 

 approximately 250 and 2,750 cycles. However, many telephone lines 

 can be made suitable for transmitting a much broader hand of fre- 

 quencies, namely, frequencies running up into the tens of thousands 

 or, by applying the latest develo{)ments, to hundreds of thousands of 

 cycles. This fact naturally raised the question whether some means 

 could not be devised for operating a multiplicity of telephone channels 

 on one circuit using this broader frequency range. 



The general idea is as old as telephony itself or older as applied to 

 telegraphy. Alexander Graham Bell's invention of the telephone 

 came, in part at least, through his experimentation in means of pro- 

 viding several telegraph channels over one circuit by using currents of 

 different frequencies. The fundamental principles of multiplex 

 telephony were early thought of and well understood. They involve: 



(1) Means for so varying a high-frequency current (called a "car- 



rier") that, with this variation, it represents the sounds to be 

 transmitted over the telephone circuit just as do the voice 

 currents produced by the telephone transmitter in the range 

 250 to 2,750 cycles. As ordinarily carried out, this involves 

 the control of the amplitude of the carrier current in pro- 

 portion to the instantaneous values of the voice-frequency 

 telephone currents, this process being known as "modu- 

 lation." 



(2) Correspondingly, means for reproducing the sounds transmitted 



by suitably operating upon the modulated high-frequency 

 current. This is done by reproducing from this current the 

 ordinary voice current (a process know^n as "demodulation") 

 and applying this voice current to an ordinary telephone 

 receiver. 



(3) Means for joining the modulated carriers of different frequencies 



so that they can be transmitted over the same telephone 

 wires, and for completely separating them from each other 

 at the receiving end by virtue of their different frequency 

 ranges so that each modulated carrier can be demodulated 

 in a separate receiving circuit and the various conversations 

 carried on simultaneously without interference. This func- 

 tion has been termed selectivity. 



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