A signal processing apparatus of the present invention includes an input unit configured to receive a reference signal supplied from an external device, a phase detection unit configured to detect a phase difference between the reference signal received from the input unit and a clock signal, a generation unit configured to generate the clock signal with a frequency corresponding to an output of the phase detection unit, and a control unit configured to detect an error between a frequency of the reference signal received from the input unit and the frequency of the clock signal based on an output of the phase detection unit and to output information, which indicates the status of a frequency change in the reference signal, to a display device based on the detected error.