A signal processing apparatus comprises a reference inserting circuit for inserting a reference waveform, preferably a ramp, at periodic intervals into an analog information signal. The reference waveform varies in the range of amplitude variations of the information signal. A reference detector circuit detects the inserted reference waveform from the signal as the latter is reproduced from a recording medium or received through a transmissions medium. The detected reference waveform and the information signal are alternately applied to a correction circuit having a which stores a signal describing the relationship between the original reference waveform and the detected reference waveform which has been affected by errors present in the apparatus. The correction circuit is periodically operative in response to detection of error-affected reference waveform and transforms the waveform of the information signal according to the stored signal.