318 BELL SYSTEM TECHNICAL JOURNAL 



ciably masked by noise. This also is ordinarily an economic problem 

 depending on the cost of lowering the attenuation or of guarding 

 against external interference. In some cases, however, this limitation 

 is a physical one. A striking case is that of radio transmission in which 

 we have no means of controlling the attenuation of the electromagnetic 

 waves in transit to the receiving station. They may arrive at levels 

 below those of thermaP'^ noise in the antenna and other receiving 

 apparatus. Thus, even in the absence of static there is a definite use- 

 ful lower limit to the received and hence the transmitted volume. In 

 such cases the problems raised by the spread in signal intensities become 

 a matter of particular importance. Radio telephony was therefore 

 one of the fields of use particularly in view for the development of the 

 device to be described. 



Effect of Volume Control 



Until recently the only method in use for reducing the range of 

 signal intensities on radio circuits was a special operating method for 

 constant volume transmission. At each terminal the technical oper- 

 ator, with the aid of a volume indicator, adjusted the speech volume 

 going to the radio transmitter to that maximum value consistent with 

 the transmitter load capacity. 



Referring to Fig. 2, we have a diagram showing the normal relation 

 of input to output intensities of a zero loss transducer as given by the 

 diagonal line. Points ^max. and ^min. on this line indicate the ex- 

 treme values of signal intensities for sustained loud vowels covering a 

 volume range of 40 db. The effect of the volume adjustments made by 

 the technical operator is to bring all the applied volumes to a single 

 value indicated by point B in Fig. 2. The value of B could be any 

 convenient intensity. Here it is set at a value determined by trans- 

 mission conditions in the line between the technical operator's position 

 and the radio transmitter. 



As the technical operator has reduced the strongest volumes 5 db 

 and increased the weakest volumes 35 db, the result of this volume 

 control is to increase the volume range which the circuit can handle by 

 40 db. It is possible to make this adjustment for two-way transmis- 

 sion in the case of radio circuits without danger of singing because of 

 the use of voice-controlled switching arrangements ^ which permit 

 transmission in only one direction at a time. By this method of opera- 

 tion volumes initially strong or weak are delivered to the distant re- 

 ceiving point with equal margins relative to interference and the trans- 

 mission capacity of the whole system is thereby improved. 



