In one aspect, a multi-port interface circuit applied to a playback apparatus which is able to switch among a plurality of input ports coupled to a plurality of source devices for playing back. Each input port has a receiver, the receiver including a front-end for receiving and processing a data stream from the source device and providing a data enable signal, and further including a content protection circuit for performing content protection according to the data enable signal. Each receiver records data enable information associated with the data enable signal of the data stream in an initial status. When one input port is selected, receivers of the other input ports operate in a power saving mode, the front-end circuits stop receiving the data stream, and the content protection circuit maintains operation according to a regenerated enable signal, which is regenerated according to the data enable information.