Processing a coded signal formed so that multiple signal points, having individual signal values consonant with coded values, are arranged in accordance with a predetermined order. A signal restoration unit calculates an average value for signal values at signal points immediately preceding and following a predetermined signal point. The predetermined signal point comprises the coded signal. The signal restoration unit calculates, for selected preceding signal points, a signal value difference for the two selected signal points, and obtains first difference values. The signal restoration unit calculates, for selected following signal points, a signal value difference for the two selected signal points, and obtains second difference values. The signal restoration unit corrects the average value by employing either one or a plurality of either the first or the second, or both first and second difference values to obtain a restored value for a signal value at the predetermined signal point.