168 BELL SYSTEM TECHNICAL JOURNAL 



modulators such as were then current in telephone practice. In the 

 case of grid-current modulators, the necessary non-linear characteristic 

 is obtained by so constructing the input circuit that the voltage be- 

 tween the grid and filament of the modulating tubes does not vary 

 directly with the voltage impressed upon the modulator input, while 

 plate modulation is suppressed ; in the case of plate-current modulators, 

 on the other hand, there is a linear relation between the grid-to-filament 

 voltage and the voltage impressed upon the modulator input, but 

 advantage is taken of the fact that the plate current does not vary 

 directly with the grid voltage to secure the desired modulation effect. 

 The reason for using grid-current modulators of this type was that the 

 increased power output secured thereby made it possible to produce 

 the required output levels without auxiliary amplifiers. 



The trial double-modulation system performed satisfactorily under 

 commercial conditions although the upper group or remodulated 

 system was somewhat less satisfactory than the standard. This wide 

 range system has not been used, however, partly because of reduced 

 demand due to economic conditions and partly because advances in 

 the art of filter design now make possible a considerable extension on a 

 single modulation basis; furthermore, the increasingly wide use of 

 carrier telephone circuits makes it desirable to restrict the band width 

 used by a voice-frequency telegraph system to the frequency range 

 normally assigned to a telephone channel. 



Signal Distortion 



Improvements in transmission amount essentially to reductions in 

 signal distortion. The principal sources of such distortion ^^' ^^ are: 



Type of 

 Distortion Source 



Characteristic Filter characteristics. 



Wave shaping characteristic of detectors. 

 Bias Variations in circuit net-loss. 



Battery variations at terminals. 



Variations in carrier-current generator voltage. 



Gradual frequency changes. 



High-resistance sending-relay contacts. 



Asymmetrical relay adjustments. 

 Fortuitous Noise. 



Lightning. 



Functional switching operations. 



Change in repeater gain with load. 



Intermodulation of several channels in repeaters. 



Infiltration from adjacent telegraph channels. 



Relay contact troubles. 



Irregularities in relay operation. 



Rapid variations in carrier frequencies. 



