A low frequency digital notch filter comprising two sub-filters - an all-pass-network filter (1) and a T-section filter (3) with a feedback connection from the T-filter to an input node (31), and feed forward connection from this input node to a terminal output node (33). The all-pass-network filter (1) has a transform function:A(Z)=[Z.sup.-1 +K]/[1+K.sub.1 Z.sup.-1]; andthe T-section filter (3) has throughput and tap transform functions:B(Z)=[(K.sub.3 +K.sub.2 .multidot.K.sub.4)Z.sup.-1 -1 ]/[1-K.sub.3 Z.sup.-1 ],and,C(Z)=K.sub.2 Z.sup.-1 /[1-K.sub.3 Z.sup.-1 ],where Z.sup.-1 is the unit delay operator and K.sub.1 to K.sub.4 are multiplier coefficients. The T-section filter 3 may be implemented using a combination of three multipliers (19, 21 and 23) and a delay element (25).