INDICIA L RESPONSE OF TELEPHONE RECEIVERS 143 



The indicial response shows more emphatically than frequency response, 

 the importance of damping and the oscillations which arc to be avoided, or 

 reduced to a minimum. It also shows that the effect of delay is closely re- 

 lated to attenuation of the higher frequencies, and that frequency of cut-off 

 is inversely proportional to the time delay, for a given t\pe of receiver circuit. 



There is a noticeable similarity between the appearance of the frequency 

 response and the indicial response curves, and in many cases one curve is 

 approximately the image of the other. As an cxamj)lc of this, the three pairs 

 of linear curves show the similarity of indicial and frequency response for 

 constant velocity, constant acceleration, and constant am.plitude devices, 

 as depicted by the three curves denoted by 1, 2, and 3 in which the three 

 moving-coil instruments are assumed to be controlled by (1) a predominance 

 of acoustic resistance behind the diaphragm, (2) a mass controlled system, 

 and (3) a stiffness controlled system. In either case, the fundamental shape 

 of the curves is such that the indicial response is the image of the frequency 

 response in its general character. 



The two lower curves. Fig. 6, indicate the effect of a sharp cut-off versus 

 a gradual one. In terms of indicial response, the gradual cut-off appears to 

 be the better of the two, a principle which is widely accepted in television 

 and telegraph transmission. 



Experimental Measurements 



The oscillographic measurements of indicial response, together with cor- 

 responding frequency response measurements of telephone receivers, are 

 shown in Figs. 7, 8, and 9. The oscillograms on the left. Fig. 7, show the 

 type of data which constitute indicial response as compared with the more 

 familiar frequency response on the right. 



Curve 1, Fig. 7, represents a moving-coil receiver similar to that calculated 

 in Fig. 3, and constitutes the standard of performance which can be obtained 

 by this particular system of measurement. Each division of the oscillogram 

 represents .001 second, a somewhat faster film speed than is usual for the 

 string oscillograph. * 



Curve 2 shows the characteristics of a magnetic bipolar type of receiver 

 having a frequency range of 3000 cps with a fairly sharp cut-off at this fre- 

 quency. The acoustic circuits of this receiver serve to damp the resonance 

 of the diaphragm and extend the range from 1600 up to 3000 cps. The 

 oscillogram shows a partially damped but still somewhat oscillatory condi- 

 tion which is due to the receiver. 



With all damping circuits removed, we obtain the characteristic of curve 

 3, a simple diaphragm resonance, which is similar to the earlier type of re- 

 ceivers of the magnetic type. Curve 2 represents a real improvement over 



