542 BELL SYSTEM TECHNICAL JOURNAL 



It will be recognized that if the application or removal of the control 

 energy is retarded, the action of the control circuit may be made quite 

 different on transient inputs than on steady state inputs. It will 

 appear later that this is the important distinction between some of the 

 devices to be discussed and that fundamental differences in their 

 functioning are thus brought about. 



Referring to the figures once more it will be noted that some control 

 devices are connected to the transmission path at the input to the 

 vario-losser. These are known as "forward acting" control circuits. 

 Other controls, connected at the vario-losser outputs, are known as 

 "backward acting" control circuits. This is simply convenient 

 terminology to indicate whether the control energy is progressing in the 

 same direction as the main transmission or is progressing in a backward 

 direction after traversing the main path, usually through a vario- 

 losser. Some backward acting controls function to measure the 

 output of the devices containing them and to make whatever adjust- 

 ments are required. Others are placed in that position to take 

 advantage of the vario-lossers in the transmission paths, i.e., such 

 controls could be replaced by combinations of forward acting controls 

 and extra vario-lossers. 



In Table I, nine of the volume controlled devices * which have been 



developed for various commercial and experimental uses are listed with 



the functions of voltage, time, and frequency which are employed to 



obtain their respective performances. There is, of course, some 



latitude in the choice of these functions for any one device. Pending 



more complete description of the different types in the following 



paragraphs this table should be viewed as illustrating the general 



character of the different circuits and also the range of the variables 



which already have been employed. For example, it will be seen that 



instantaneous voltage of the signal wave, its short time average value, 



peak power, syllabic variations, and long time average power have all 



been used as criteria of gain settings in different circuits. Some 



devices change their adjustments only when critical values or ranges 



are exceeded, while others vary somewhat with every syllable if 



speech, for example, is being transmitted. Some are linear transducers 



to all but low or high amplitudes while others reduce or increase the 



output range from that at the input. It will be seen that proper 



choices of times for gain increase and gain decrease in combination 



* The names employed do not follow an entirely logical classification, but they 

 are given here because they have had considerable usage. For the same reason the 

 term volume controlled devices is used, although to be strictly correct it might better 

 be sound energy controlled devices, for example, for not all the devices operate in 

 accordance with volume as measured by the well-known class of visual reading meters 

 called volume indicators. 



