486 BELL SYSTEM TECHNICAL JOURNAL 



during signaling would show a steady carrier frequency component and sub- 

 stantially symmetrical upper and lower sideband frequency components. 



Single-Sideband Method 



This is similar to the method just described except that: (1) the carrier 

 frequency is located near one boundar>- of the channel filters, so that during 

 signal transmission one of the sidebands is attenuated much more than the 

 other before the signals reach the line, and (2) during the spacing condition, 

 carrier current may be either absent from the line, or present with amplitude 

 less than that of marking current. (The latter condition tends to reduce 

 that part of the distortion which is due to the quadrature component.^) 



Frequency-Shift Method 



This is a method of signaling over a channel utilizing a carrier current of 

 substantially constant amplitude from a frequency-modulated oscillator. 

 The carrier current has no phase discontinuity and its instantaneous fre- 

 quency varies between two limits within the transmission band. In the 

 present discussion the two limits, symmetrically located in the transmission 

 band, correspond respectively to the marking and spacing conditions of the 

 channel. Variation of the instantaneous frequency may be abrupt or grad- 

 ual, for example, sinusoidal. Throughout this paper the reader should 

 assume that the variation is substantially abrupt except where otherwise 

 indicated. At the receiving terminal the variable frequency signals are 

 converted to amplitude-modulated signals by means of a frequency detector. 



Two-Source Method 



This is also a frequency-shift method, but it differs from that described 

 above in that it is made up by combining two on-off channels, each supplied 

 with carrier current of a different but substantially constant frequency from 

 a separate source. In the present discussion each of the two on-off channels 

 has a separate oscillator and occupies a different frequency band on the same 

 line, and the sending relays of the channels have their operating windings 

 differentially inter-connected so that one oscillator delivers current to the 

 line during marks, and the other during spaces. Thus the marking and 

 spacing signals are confined to separate frequency bands on the line. This 

 is sometimes referred to as two-band operation. The switching takes place 

 abruptly. No attempt is made to control the phases of the two sources. 

 Therefore phase discontinuities are likely to occur at the instants of switch- 

 ing, causing brief transients of varying shapes in the line current. The out- 

 put circuits of the two receiving detectors are differentially interconnected 

 to obtain polar signals for the operation of a common receiving relay. - 



