AMPLITUDE RANGE CONTROL 525 



A receiving range controller also changes variations in the trans- 

 mission medium in proportion to the control ratio. 



Part 2 — Time Actions 

 Instantaneous Control 

 A device having a given control ratio might have its gain changed 

 simultaneously with the applied e.m.f. The signal modification 

 would become greater as the control ratio departed farther from unity 

 and the modified signals would approach rectangular wave shapes 

 at the limiting control ratios. Unless instantaneous compression is 

 limited to a very small part of the signal range, an incomplete in- 

 stantaneous expandor (inverse rooter) is required at the distant end 

 which does the reverse of what is done at the transmitting end to 

 restore the signal to substantially its original form. Due to the 

 characteristics of the compressed signals, however, a transmission 

 bandwidth without appreciable amplitude or phase distortion of two 

 to three times the normal is necessary for high quality transmission. 



Rectified Control 



To avoid the necessity of transmitting such a wide band of fre- 

 quencies, as well as to permit the use of a single device without restor- 

 ing, in which case the distortion is limited to a value which is permis- 

 sible from the standpoint of a listener, practical devices do not operate 

 instantaneously. Instead, the gain is controlled by the charge on a 

 condenser, which is controlled by rectified waves. The action of such 

 an arrangement will now be discussed. 



Consider a wave formed by subtracting two sine waves equal in 

 amplitude, one having a frequency 10 per cent less than the other.* 

 A portion of such a wave is shown in Fig. 3a. This wave is equivalent 

 to a cosine wave of frequency one-half the sum of the two frequencies, 

 as shown by the instantaneous voltages of Fig. 3a, multiplied by a 

 secondary wave (envelope) of frequency one-half the difference of the 

 two original frequencies. 



The instantaneous voltages of the wave of Fig. 3a vary from a 

 positive maximum through zero to a negative maximum. Curve a of 

 Fig. 4 is a summation of most of the instantaneous e.m.f. 's of Fig. 3a 

 with respect to their occurrence. About 99 per cent of the instantane- 

 ous voltages are in the ranges shown, the remainder being in the range 

 between the upper and lower halves of Fig. 4. 



* This illustration is not directly comparable with speech, but it contains some of 

 the attributes which are comparable in this analysis, besides being readily repro- 

 ducible and relatively simple. 



