DIGITAL FILTER

PURPOSE:To form frequency characteristics relating to an amplitude and group delay to be symmetrical arithmatically to the center frequency, by setting the ratio of the center frequency being an arithmetic mean of the upper limit and lower limit of a band to a sampling frequency to 1:4. CONSTITUTION:A sampling pulse generator 2 generates a sampling pulse determining a time discrete interval in order to convert a time continuous signal into the time discrete signal. A filter arithmetic section 3 calculates in accordance with a transfer function on the Z plane. A sequence control section 4 controls the sequence of the filter arithmetic section 3. Then, the ratio of the center frequency being the arithmetic mean of the upper limit and lower limit frequencies of the band to the sampling frequency from the sampling generator 2 is set to 1:4. Thus, the frequency characteristics of a band pass and a band stop filter become arithmetically symmetrical to the center frequency.