540 BELL SYSTEM TECHNICAL JOURNAL 



We are to be concerned here principally with those elements allied to 

 the telephonic art, although some applications are to be found in other 

 fields. It is not intended to include those voice operated functions 

 which are essentially switching operations although the distinction in 

 some cases becomes exceedingly fine. 



Names of volume controlled devices * which have been used in 

 published papers include vogad,t' ^' ^' * compandor,!' ^' ® and volume 

 limiter.'^ Without direct comparison it may not be obvious how these 

 and similar devices differ. First the apparent similarity of several of 

 these devices will be shown in simple diagrams. Next the more 

 important characteristics of a number of devices will be presented in 

 tabular form, followed by descriptions of the different types. These 

 will then be discussed with particular emphasis on their distinctive 

 qualities, with notes on their variants which have some apparent 

 value. 



General Characteristics of Volume Controlled Devices 



In Figs. 1 to 10 are shown simplified diagrams of some of these 

 devices. While detailed descriptions of them will be deferred till later 

 it may be pointed out that all those shown contain vario-lossers, and all 

 have paths from the main transmission path to control circuits which 

 affect the vario-lossers. A vario-losser usually consists of a balanced 

 pair of vacuum tubes whose gain is changed by varying the grid bias, or 

 of a network of non-linear elements such as copper oxide or silicon 

 carbide whose loss is changed by varying a current through them. In 

 some special cases it may be a mechanically adjusted variable network. 

 The word vario-losser is thus a generic term relating to a circuit whose 

 loss or gain is controllable. A control circuit ordinarily consists of an 

 amplifier and rectifier whose direct current or alternating current 

 output bears a chosen relation to its input. Thus some control 

 circuits are marginal; they produce no control voltage till the input 

 exceeds some critical value, then produce large control voltages for 

 small additional increments of input. These are used, for example, 

 when it is desired to limit the output of a vario-losser to a definite 

 amount. Another type of control circuit produces a current or voltage 

 which is linear with input expressed in decibels. In combination with 

 a vario-losser whose gain is a linear function of control current or 

 voltage one can produce a device whose gain is a linear function in 

 decibels of the input to the control circuit. 



* See the footnote on page 543. 



t " Folume Operated Gain Adjusting ZJevice." 



X A combination of the names "Compressor" and "Kxpandor." 



