Signal processing apparatus and image forming apparatus

An image forming apparatus comprising:

• an image forming section for forming an image on a recording medium;
• and
• a signal processing apparatus,
• wherein the signal processing apparatus including:
• an optical sensor for outputting a detection signal by detecting a surface of the recording medium on which a correction image is to be formed by said image forming section;
• 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 dominant frequency from a second analysis signal obtained by making the frequency analysis of the second detection signal;
• and
• obtaining a detection signal wherein 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.