Calculation of a scalar product in a direct-type fir filter

PCT No. PCT/FI94/00568 Sec. 371 Date Jan. 23, 1996 Sec. 102(e) Date Jan. 23, 1996 PCT Filed Dec. 16, 1994 PCT Pub. No. WO95/17708 PCT Pub. Date Jun. 29, 1995The invention relates to a direct-type FIR filter, a method for calculating a scalar product in a FIR filter, and a method for designing a direct-type FIR filter. Successive words of a digital input signal are delayed in a delay line having delays (50A-50D) of the duration of one word, and the scalar product between the variously delayed words derived from the delay line and the corresponding constant coefficients is calculated. In accordance with the invention, calculation of the scalar product comprises a) combining the bits of words at the input (X0) and outputs (X1-X4) of the delay line bit by bit in a network of bit-serial subtractor and/or adder elements (51-56) wherein at least one of the bit-serial elements is involved in the multiplication operation of at least two different coefficients, and b) multiplying (49A-K) the multiplication results from the network by powers of two, and summing together (45-48) the results to yield the scalar product.