Method and apparatus for preparing recorded program material to prevent unauthorized duplication by magnetic tape recording

Apparatus for preparing recorded audible program material which does not interfere with normal playback of the material but which renders detectable any unauthorized copies of the material which are made by magnetic tape recording with conventional high frequency bias, said apparatus comprising:

• means for generating a high frequency modulated signal which varies in a predetermined manner over a selected spectrum of frequencies;
• and
• means for recording the modulated signal together with the audible program material;
• whereby copying of the recorded material onto magnetic tape will produce on the tape detectable interference signals between the modulated signal and the conventional recording bias.

A method for preparing recorded audible program material upon magnetic recording tape which renders detectable any unauthorized copies of the audible program material upon a magnetic recording tape which are made by magnetic tape recording with a conventional high frequency bias signal, said method comprising:

• recording the audible program material upon a first magnetic recording tape for providing a predetermined information passband of frequencies of such recorded material;
• generating a square wave high frequency modulatable signal having a quiescent frequency lying above said passband;
• recording said modulatable signal on the tape together with the audible program material;
• sensing the presence of passages of program material suitable for audibly masking the presence of a modulated signal in the upper end of said passband;
• when said masking passages of program material are available, frequency modulating said square wave high frequency modulatable signal to produce frequency excursions thereof down into the upper frequencies of said passband;
• whereby copying of the recorded audible program material onto another magnetic tape will produce on the tape copy detectable interference signals between the modulated signal and the conventional recording bias signal;
• and
• whereby the modulated signal is impossible to be removed in the process of making such tape copy.

A method for preparing recorded audible program material on a magnetic recording tape comprising:

• recording the audible program material on said tape for providing a predetermined information passband of frequencies of such recorded material, said recording being applied by a first magnetic recording head having a predetermined gap width,
• generating a square-wave high frequency modulated signal which varies in a predetermined manner over a selected spectrum of frequencies higher than said information passband of frequencies,
• said square wave having a transition time which is 1/6 or less of the dwell time thereof, and
• recording said square-wave modulated signal on said tape using a second magnetic recording head having a narrower gap width than said first head and being positioned for engaging said tape after said recorded material has been applied by said first head.