a optical recording / playback apparatus projects a laser beam on to a recording medium to read out precoded address data and record or read out data in the data recording area of the recording medium . the reflected light from the recording medium is detected by photosensors which produce currents from the received light intensity , and the currents are converted into voltage signals by current - to - voltage converting amplifiers and the signals are summed by a summing amplifier . the summing amplifier has a switching means , by which the gain is brought to unity or near - unity value during the period of reading the data recording area , so that the summed output has a smaller difference of voltage levels for the address area and data recording area even though the detected light intensity has a large difference between these portions . the output of summing amplifier is fed through a high - pass filter prior to the zero - cross detection , and a smaller differentiation pulse resulting from the smaller voltage level difference enables the reliable readout of address data .