Auto focus frequency conversion filter for ccd imagers having different

Abstract

A sampling frequency conversion filter circuit permits the same auto focus circuit to be used with CCD imagers having different numbers of pixels per line. The conversion filter converts a digital signal of a first sampling frequency into a second sampling frequency and for providing a digital signal of the first sampling frequency with a predetermined frequency characteristic includes a delay circuit for delaying an input digital signal of the first sampling frequency by a predetermined amount; a first flip-flop for taking in the input digital signal of the first sampling frequency in synchronism with a clock of the second sampling frequency; a second flip-flop for taking in the digital signal of the first sampling frequency delayed by the predetermined amount through the delay circuit with a clock of the second sampling frequency; an adder for adding outputs of the first and flip-flops; and control circuits for controlling inputs of the first and second flip-flops to obtain from the adder circuit an output equivalent to a result obtained by over-sampling the digital signal of the first sampling frequency at a frequency that is a common multiple of the first sampling frequency and the second sampling frequency, and then re-sampling it at the second sampling frequency via a filter having the predetermined frequency characteristic.