Patent ID: 7777896

Claim:
A signal processing apparatus comprising: an optical sensor for outputting a detection signal by detecting a surface of a recording medium on which a correction image is to be formed; and a control section configured to conduct control steps of: having said optical sensor to detect the surface of the recording medium without the correction image being formed thereon; making a frequency analysis of a first detection signal outputted from said optical sensor detecting the surface of the recording medium without the correction image being formed thereon; extracting a dominant frequency corresponding to the frequency component dominant over other values from a first analysis signal obtained by making the frequency analysis of the first detection signal; having said optical sensor to detect the surface of the recording medium with the correction image formed thereon; making a frequency analysis of a second detection signal outputted from said optical sensor detecting the surface of the recording medium with the correction image being formed thereon; deleting the component of the extracted dominant frequency from a second analysis signal obtained by making the frequency analysis of the second detection signal; and obtaining a detection signal, in which the dominant frequency component has been deleted by making reverse frequency analysis of the second analysis signal from which the dominant frequency component has been deleted.