In order to protect audio signals against copying, it is proposed to mix the
audio
signal with at least one non-audible disturbance signal. A first disturbance signal
may be a low-frequency signal which is added to the audio signal, while a second
disturbance signal may be a high-frequency disturbance signal which is multiplied
with the audio signal. Additionally, the second disturbance signal may be modulated
by a modulating signal comprising either the original or inverted original signal,
or spoken messages indicative of an illegal copy.
