Patent ID: 7170922

Claim:
A digital roll-off filter comprising: a shift register having a predetermined number of tap outputs and holding input digital data while delaying said input digital data in accordance with a data period sequentially; a memory holding tap coefficients respectively corresponding to sampling points set on a desired impulse response waveform having roll-off characteristics, a sampling point being of an oversampling period corresponding to 1/m of said data period, m being a positive integer, said memory also holding tap coefficients each corresponding to a point intermediate between said sampling points immediately adjacent and sandwiching a boundary of said data period; a counter operative to count in accordance with said oversampling period and reset in accordance with said data period; a period variation detection circuit detecting variation in said data period in response to a maximal value output from said counter; tap coefficient selectors equal in number to said predetermined number of tap outputs and provided for said predetermined number of tap outputs, respectively, said counter outputting a value shared by said predetermined number of tap coefficient selectors, said period variation detection circuit outputting a detection output shared by said predetermined number of tap coefficient selectors, each said tap coefficient selector receiving from said memory m tap coefficients corresponding to m sampling points included in said data period of said tap output corresponding thereto and said tap coefficient corresponding to said point intermediate between said sampling points immediately adjacent and sandwiching the boundary of said data period, each said tap coefficient selector repetitively, sequentially selecting said m tap coefficients in response to said value output from said counter, and when said period variation detection circuit detects variation in said data period each said tap coefficient selector further selecting said tap coefficient of said point intermediate between said successive sampling points preceding and following said variation to interpolate said coefficient before and after said variation; a multiplication circuit providing respective multiplications of respective data of said predetermined number of tap outputs of said shift register by tap coefficients respectively selected by said predetermined number of tap coefficient selectors; and an addition circuit adding said multiplications together for output as a filter output.