A signal processing apparatus monitors the dubbing state at a higher-speed dubbing of digital video data or audio data. The signal processing apparatus includes a first storage device containing an MPEG system stream as digital data, a second storage device containing an MPEG system stream as the digital data, and a decoder unit for carrying out a decoding process for the digital data read from the first storage device. The decoder unit is constructed to carry out a discrete decoding process for decoding the system stream as digital data transferred from the first storage device to the second storage device at a higher rate than the normal playback transfer rate.