Recursive digital filter comprising a circuit responsive to first sum

Abstract

In a recursive digital filter for dealing with data words given by two's complement representation in a common word format comprising a sign bit, an integer bit, and a predetermined number of fractional bits, an overflow detect and correct circuit is supplied with simultaneously produced sign bits of bit-serial first sum, feedback, and second sum data words and with the integer bit of the second sum data word and detects overflow in the second sum data word to produce, for use in the circuit, an overflow detect pulse indicative of presence or absence of overflow. In either event, the circuit produces an overflow-free data word for use in the filter. When overflow is detected, the circuit produces a polarity decision pulse that decides polarities of the overflow-free bits. Otherwise, the circuit determines the overflow-free bits directly by the corresponding bits of the second sum data word. Use is preferred of a first and a second timing signal which specify time slots for the sign bit and a prescribed bit, respectively, of each serial data words and which are for directly deciding the overflow-free fractional bits by the polarity decision pulse and for producing a second polarity decision pulse for direct decision of polarities of the overflow-free sign and integer bits. A memory having a plurality of memory areas is preferred for production of the overflow detect and the polarity decision pulses and of the overflow-free bits.