INDICIAL RESPONSE OF TELEPHONE RECEIVERS 139 



measurement are related, and how they may theoretically be converted one 

 into the other, provided in the case of frequency response the magnitude and 

 phase are both known. 



General Applications 



The use of indicial response as a tool in telephone receiver studies is par- 

 ticularly adapted to the study of transients. Since all voice and sound trans- 

 mission, particularly that of orchestra! music, may be regarded as essentially 

 a transient problem, it is appropriate that we visuaUze the effects on the 

 complex wave forms of any distortions which may be present in the trans- 

 mission apparatus. The indicial response will, in general, depart from the 

 ideal square form, and the amount of this departure may be regarded as 

 indicative of the relative faithfulness of wave form reproduction by ap- 

 paratus having different frequency characteristics. An examination of 

 these departures should therefore be helpful as a supplementary method of 

 appraising the relative merits of different frequency response characteristics. 

 The effect, for example, of small resonance peaks or dips upon transients 

 is very forcefully shown in the form of the indicial admittance. The de- 

 parture from squareness of a particular system may often be improved by 

 use of the proper shape of frequency characteristic. 



The use of a closed coupler when measuring telephone receivers is par- 

 ticularly adapted for such studies, because the disturbing effects of de- 

 ficiencies at the low frequencies due to leakage may thus be eliminated. 

 Interpretation by inspection then becomes a matter of observation of the 

 various types of departures at the higher frequencies from the ideal form. 



Since listening tests do not always agree with interpretations of physical 

 measurements of steady-state frequency response, it often becomes a matter 

 of interest to obtain different criteria of judgment in which the weight given 

 to the various frequencies may be judged by the relative effects of irregulari- 

 ties in various parts of the frequency spectrum upon the indicial response. 



Apparatus and Method of Testing 



Various forms of apparatus may be used for receiver testing with square 

 waves. Square-wave generator circuits have been published both for audio^ 

 and video' frequency use, involving vacuum tube circuits which overload at 

 low voltages. For low speeds using low-frequency waves of the order 

 60 cps, a simple mercury switch operated by an oscillator gives very satis- 

 factory results. 



The square-wave voltage is introduced across a small part of the resistance 

 termination as shown in Fig. 4, the whole resistance termination being 

 matched to the magnitude of the receiver impedance at 800 cps. The re- 



