Abstract: An optical information recording and reproducing apparatus comprising: a reproduction signal processing circuit including one or more elements which have respective variable parameters for changing states of the elements, respectively at the time of read-out of data from a recording medium such that the data is read out from the recording medium by changing the states of the elements.

Abstract: A clock changeover apparatus including a reference frequency divider for dividing a reference clock by a first frequency division number; a comparative frequency divider for dividing by a second frequency division number an output clock obtained by the clock changeover apparatus; a phase comparator for comparing phase of a signal of the reference frequency divider with that of the comparative frequency divider; a clock controller for changing the output clock so as to make the phase of the signal of the reference frequency divider coincident with that of the comparative frequency divider; a clock determining member which gives an initial value of the output clock to the clock controller and is capable of selecting the initial value of the output clock arbitrarily; a changeover member for effecting changeover between the first and second frequency division numbers; and a phase lock detector for monitoring whether or not the phase of the signal of the reference frequency divider coincides with that of the compa