Digital filters are described for filtering blocks of data values. In order to reduce edge effects, symmetric data extension at the edges of the blocks is used. Multiplexers 34, 40, 46, 52 are used to pass data values between non-adjacent delay elements 36, 38, 42, 44, 48, 50 at leading and trailing edges of a data block to effect symmetric extension. The delay elements can be multisample delay elements and data being filtered can be multiplexed data.