538 BELL SYSTEM TECHNICAL JOURNAL 



factor of one outside the working range. Various other combinations 

 of control ratios can, of course, be employed. 



3. Pilot Channel 



In all complete compressors some form of pilot channel is necessary 

 to control the re-expansion if this is required. If the gain changes 

 are slow, the pilot channel may include an operator who changes the 

 gain of the receiving device in a manner complementary to that of the 

 sending device based on aural or visual signals. If the gain changes 

 are too rapid for the operator to follow, the receiving gain may be 

 changed automatically. 



The pilot channel itself may be a direct or alternating current of 

 variable amplitude or frequency, or in case of carrier or radio, it may 

 be the carrier frequency. In sound reproduction a pilot channel 

 might be a pilot track on the record. 



Summary 



In an amplitude range control system, the following characteristics 

 must be specified in addition to the usual repeater characteristics, to 

 determine its design and performance: 



1. The steady-state control ratio, which determines how much control 



is obtained and whether restoration can be made automatically 

 or not. 



2. The manner in which the output varies with time, following a given 



change in input. 



3. The range over which it is to function. 



In specific cases the following should also be considered: 



4. The action of the device for inputs outside the working range. 



5. If the device is a complete compressor, the type of pilot channel 



for restoring. 



6. The action of the device when signals are removed. 



Acknowledgment 



The computations used to obtain Figs. 3 to 9, inclusive, were made 

 by Miss Marian Darville. 



References 



1. "Devices for Controlling Amplitude Characteristics of Telephonic Signals," A. C. 



Norwine. Presented at A. I. E. E. Pacific Coast Convention, Aug. 9-12, 1938. 



2. "Speech Power and Its Measurement," L. J. Sivian, B. S. T. J., Vol. VIII, 



pp. 646-661. 



3. "The Compandor — An Aid Against Static in Radio Telephonv," R. C Mathes 



and S. B. Wright, B. S. T. J., Vol. XIII, pp. 315-332. 



4. "A Vogad for Radio Telephone Circuits," S. B. Wright, S. Doha, A. C. Dickieson. 



Presented at I. R. E. Convention, June 1938. 



