Many signals are digitally generated or transmitted as square waves. It is often desirable to convert these signals into sine waves. For example, the 350Hz, 440Hz, 480Hz, and 620Hz telephone supervisory tones transmitted over fiber-optics may appear at curb-side as square waves. To be used in telephone equipment it is desirable to convert the square waves into low-distortion sine waves. This can be done with a simple filter.