550 



BELL SYSTEM TECHNICAL JOURNAL 



area under the distribution curve above a certain point to the region 

 near that point, which is its Hmiting volume. This is illustrated in 

 Fig. 11, in which the calculated modifications of a volume distribution 

 by a vogad and by a volume limiter are shown. In the cases "without 

 volume control " and "with a vogad " the distributions are normal, and 

 the standard deviation, a, has its usual statistical significance. With a 

 volume limiter, only volumes above the limiting volume are affected, 



-15 -10 -5 5 10 15 



VOLUME IN DECIBELS FROM MEAN VOLUME 



Fig. 11 — Modification of volume distribution by use of a vogad or a volume limiter. 



and these higher volumes are redistributed according to a normal law 

 whose standard deviation is 1 decibel, as stated in the figure. 



It is also important to distinguish between a peak limiter and a peak 

 chopper. Figs. 8 and 9. Naturally they resemble one another since 

 they are intended to permit transmission of signals at higher average 

 amplitudes without excessive loading of transmission circuits. How- 

 ever, they are intended for different classes of service and hence are not 

 interchangeable except in some borderline cases. For the highest 

 grade of transmission harmonic production must be negligible and the 

 reduction in amplitude range of signals small and infrequent. Gain 

 changes must be smooth, though rapid enough to compensate for 

 practically any input wave to be expected. These characteristics are 

 found in the peak limiter now being furnished for use on program 

 networks and radio transmitters."- ^^ For services in which it is 

 desirable to maintain the signal energy at a high value to over-ride 

 noise and in which harmonic distortion must be kept low a peak 

 limiter with somewhat smaller time constants may be used. A high 

 ratio limited range compressor might be suitable in this instance. 

 This device would lower its gain a little more quickly on excessive 



