546 BELL SYSTEM TECHNICAL JOURNAL 



with certain gain control criteria make possible a wide variety of signal 

 altering means to meet different requirements. 



Description of Devices in Table I 



With this introduction to the combinations of characteristics which 

 are possible it should be less difficult to distinguish between the specific 

 devices discussed in the following paragraphs, which, in addition to 

 describing the devices, contain some comments which should assist in 

 visualizing their forms and their operation. 



1. The vogad (Fig. 1) is a device which will maintain at its output 

 speech volume ^ which, over a certain range of input, is relatively 

 independent of the speech volume applied to its input and which, in 

 the ideal case, will not change its gain during periods of no speech input. 

 It makes little or no alteration in the ratios of maximum and minimum 

 instantaneous to average voltages of the speech. 



2. The volume limiter (Fig. 3) is a device which is a linear transducer 

 for all speech volumes up to a critical value, beyond which all input 

 volumes produce essentially the same output volume. It is essentially 

 different from the vogad in that its gain approaches the maximum 

 value when input is removed. 



3. The compandor (Figs. 4 and 5) is composed of a compressor and an 

 expandor. A compressor is a device whose input-output characteristic 

 on a decibel scale has a slope less than unity * and whose gain or loss is 

 variable under control of the input energy at a time rate which will 

 permit it to follow the syllabic rate of change of speech energy. Simi- 

 larly, an expandor is a device whose input-output curve has a slope 

 greater than unity and whose gain is variable at a syllabic rate under 

 control of the input energy. Thus very shortly after all input is 

 removed the gain of a compressor is maximum and the loss of an 

 expandor is maximum. The reciprocal of the compressor characteristic 

 slope is spoken of as the compression ratio, and the slope of the ex- 

 pandor characteristic is spoken of as the expansion ratio. 



4. The radio noise reducer *• ^ (Fig. 6) combines the functions of an 

 expandor which operates in the range of amplitudes where noise and 

 weaker speech sounds lie and a linear transducer which comes into play 

 for all amplitudes exceeding a critical value, which can be set to best 

 suit the atmospheric noise conditions. In other words, the radio noise 

 reducer is a limited range expandor. Inputs which are below the 

 expandor range are subject to transmission at the minimum gain. 



5. The limited range compressor (Fig. 7) is a device whose operating 

 * That is, if the input increases by x db the output increases by less than x db. 



