A method for designing a cascade of digital filters for use in the control of an electrolytic reduction cell by specifying the frequency and time response characteristics of the transfer function of the cascade of digital filters. The digital filters are exponential filters. The number of smoothing functions (i.e filters), each having different filter parameter values, is determined according to desired performance characteristics in the frequency and the time domains. These characteristics include: smoothing quality (specifiable in terms of passband, stopband and transition band of the filter frequency response profiles), group delay, and stabilization time. The use of the cascade of digital filters to control a reduction cell. A system for controlling a reduction cell comprising the cascade of digital filters. A process for electrolytic reduction of alumina comprising a step of filtering measurements of an electrical parameter using the cascade of digital filters.