AMPLITUDE RANGE CONTROL 529 



pression would result in less distortion than instantaneous compression. 

 The extra frequencies formed that were not present in the original 

 wave are the envelope frequencies, so that the additional band required 

 to transmit this wave faithfully is negligible. 



Dynamic Operation 



The measurements ^ and adjustments of speech amplitudes in com- 

 mon use are made with devices that integrate the effects of the wave 

 over certain time intervals. They do this in a rather complicated 

 manner, however, so that it is difihcult to express the resulting quanti- 

 ties in terms that are generally understood. 



In the measuring instruments the rectified voltages are impressed 

 on a condenser before being sent through a meter. The readings of 

 the meter are, therefore, proportional to the voltage on the condenser 

 modified by the damping of the meter. The voltage is made up of the 

 sum of the elTects of all the instantaneous voltages that have been 

 applied to the condenser from the beginning of time to the instant under 

 consideration. These effects die out so rapidly, however, that the 

 instantaneous voltage on the condenser is practically determined by the 

 voltages received in the immediate past. The condenser may be said 

 to have a memory but a short one. In range control devices, the 

 condenser forms the voltage which determines the amplification of the 

 device. 



To distinguish this voltage on the condenser from the applied voltage 

 at any instant, we may call the former an "impression" of the original 

 wave. If the time constant RC is small we get strong impressions 

 similar to the rectified applied wave and its envelope, and if it is large 

 we get weak impressions quite different from the applied wave but 

 something like the rectified envelope. 



Figure 6 shows the impressions of the wave of Fig. 3a, using four 

 different values of time constant RC as compared to P, the period * 

 of the envelope. Figure 7 shows smoothed summation curves of the 

 impressions of Fig. 6 formed during the time P/2. Comparing this 

 with Fig. 4, it is evident that the "bunching" effect for the distribution 

 of impressions is largely between those for the rectified instantaneous 

 and envelope curves. For the longer time constants, i.e., weak im- 

 pressions, this is not the case for the weaker e.m.f.'s. 



* This is twice the duration of Fig. 3, since only half a cycle is illustrated. It is 

 assumed that C is completely discharged at the time this wave is applied. In prac- 

 tice, the rectifier impedance varies with the applied e.m.f. so that the results are 

 not as simple as in this illustration. In general, the time actions are different de- 

 pending on whether the applied e.m.f. is increasing or decreasing. 



