According to an embodiment, a signal processing apparatus includes a first switch, a second switch, a corrector and a circuit. The first switch selects one signal from a first signal group including a first signal and a first constant envelope signal to obtain a first selected signal. The second switch selects one signal from a second signal group including a second signal and a second constant envelope signal to obtain a second selected signal. The corrector corrects a characteristic of at least one of the first and second selected signals to obtain a first corrected signal and a second corrected signal. The circuit generates an output signal from the first corrected signal and the second corrected signal.