Importance

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Abstract

An anti-copy system which prevents copying of tapes and records and other source material by placing a degrade signal on an analog source signal to be recorded on a recording device. One embodiment of the system includes a pulse width modulation (PWM) circuit that generates a pulsed signal in a frequency range from near the upper end to well above the range of hearing. The PWM pulsed signal is modulated by the source signal to be recorded. The PWM signal becomes the degrade signal. This degrade signal is keyed so that it is combined with the source signal only when the frequency of the source signal exceeds a desired threshold. The combined source signal and degrade signal are recorded on the original recording. This original recording can be played back without the presence of the degrade signal impacting the audible quality of the reproduced source signal. However, attempts to make taped copies of the original recordings cause the degrade signal to beat against the bias oscillator frequency of the tape recording head, thereby creating harmonic signals that result in distortion and noticeable alteration of the reproduced source signal that are unpleasant and objectionable to the listener. The anti-copy system may be used with stereophonic recordings, monophonic recordings, or even digital recordings of the type wherein an audible tone burst is used to represent desired digital data.

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First Claim