Gerardus Cornelis Petrus Lokhoff

Apparatus for recording an audio signal with protection against re-recording thereof, and recording apparatus which indicates the presence of such protection

Low-frequency components of the left and right channels of the audio signal to be recorded are derived by means of filters (3, 4). First and second auxiliary signals are added to such low-frequency components in such a way that during reproduction of the audio signal so recorded, including the first and the second auxiliary signals added thereto, by means of two loudspeaker units arranged in a stereo configuration, the two auxiliary signals are not audible. However, they will inhibit re-recording by apparatus for recording audio signals which are not copy-protected in the form of the aforesaid two auxiliary signals, or will cause such apparatus to indicate that the audio signal to be re-recorded is protected.