Abstract: A recursive type digital filter receiving a digital input signal x(n) having a plurality of bits and delivering a ditial output signal y(n) satisfying the following equation, ##EQU1## where n indicates a natural number, M and N orders of time lag in the signal transference, a.sub.k and b.sub.k coefficients defined by a filter characteristic, a.sub.M and b.sub.N b being coefficients which are not equal to zero, comprises an output control circuit for delivering a digital signal indicating a positive or negative limit value in place of the digital output signal y(n) when the amplitude of the signal y(n) exceeds an allowable value. In combination with this output control circuit, the filter also utilizes a feedback signal for calculation purposes which feedback signal has its amplitude reduced from that of y(n) by a predetermined ratio. Further, an arrangement is provided for clearing registers of filter when necessary to prevent overflow oscillation.

Abstract: This invention has been made in order to prevent the malfunction due to an overflow, of a combinatorial type recursive digital filter whose feedback side is of the second or higher order. Means for detecting the overflow of the output of an arithmetic unit which executes an addition within the filter is additionally provided, and when the overflow has developed, the contents of shift registers for delay on, at least, the feedback side are reset by an output from the detecting means.

Abstract: A code converting method in which a multivalue signal x.sub.i sampled at a sampling period of mT, where m (an even number) equals 2.sup.n (n being integer) and x.sub.i is non-negative integer not greater than m, is received and subjected to density conversion to be delivered out in the form of a train of m binary signals sampled at a sampling period of T. A Z-transform Y(Z) of the binary signal to be delivered out is expressed as, ##EQU1## where q.sub.i+1 .ident.q.sub.i +X.sub.i+1 +m, and mod 2; q.sub.o =0 and where H(x.sub.i, q.sub.i, Z) represents a polynomial of the order related to Z.sup.-1 not greater than (m-1) which has non-zero terms having each a coefficient of 1(one) and which satisfies H(k, q.sub.i, 1)=k, H(k, q.sub.i, Z)=Z.sup.-(m-1) H(k,q.sub.i,1/Z) and H(k,0,Z)=H(k,1,Z) for k being an even number, and H(k,0,Z)=Z.sup.-(m-1) H(k,1,1/Z) for k being an odd number.