The present invention relates to a dual mode signal processing apparatus using a picture-in-picture (PIP) window. In particular, the present invention relates to a display apparatus and a function switching apparatus that display two views for DSPs performing different functions in use of a PIP window so that two functions are simultaneously performed. In order to achieve the such an object, embodiments of the present invention comprise at least one or more digital signal processors (DSPs) for processing different signals; a picture-in-picture(PIP) block for inputting a composite image signal, a chrominance signal, and so on, mixing two signals, and outputting one composite signal; a first signal switching unit for selecting an input of signals of the plural DSPs and outputting the selected signal to the PIP block; a second signal switching unit for selecting a composite image signal outputted from the DSPs and outputting the selected composite image signal to the PIP block; and a display unit for inputting and displaying an output signal of the PIP block. The embodiments further employ an input unit having a mode selection button and PIP-related function buttons; and a control unit for switching signals with an input of the input unit and controlling a system. Accordingly, embodiments of the present invention convert the function of the system by changes of the main view and the PIP window in the mixed display from two mixed image signals, and enables the same functions to operate with one button selection, to thereby perform combined functions with simplicity.