Instantaneous Compandors

A compandor is characterized by compression followed by expansion. To achieve noise reduction by compandor action,* compression is applied before and expansion after the noise exposure. By compression one means that the effective gain which is applied to a signal varies as a function of the magnitude of the signal, the effective gain being greater for small than for large signals. In the process of expansion the effective gain also varies as a function of the signal but is greater for large than for small signals. There are two general classes1 of compandors, "syllabic" and "instantaneous." For many years, because of theoretical and practical reasons, only the syllabic type was used to any appreciable extent. Although utilized primarily in special situations-" 5 , syllabic compandors have in these instances served to improve telephone operation by providing a substantial noise advantage. More recently 6 " ll the instantaneous type also has begun to find important applications to time-division systems. Since an instantaneous compandor produces effective gain variations in response to instantaneous values of the signal wave, the instantaneous type is well adapted to pulse systems. Moreover, in time-division pulse-modulation systems, one instantaneous compandor usually serves a plurality of channels thereby affording additional economies. THEORY