A nibble stream containing MIDI music data words and synchronous nibbles and an external audio signal are selectively converted to an audio-frequency signal, which in turn is converted to a set of PCM codes for storing it in a compact disc; and the audio frequency signal, which is demodulated from the PCM data codes, is analyzed to see which is the origin of the audio frequency signal on the basis of the signal level and what sort of modulation technique was employed on the basis of features of the audio frequency signal such as peak-to-peak intervals and similarity to reference waveforms so that the nibble stream or the external audio signal is exactly reproduced from the audio frequency signal.