Patent ID: 6882490

Claim:
A method of modifying a programme signal to provide protection against copying on a magnetic tape recorder, the method comprising the steps of: receiving a programme signal; frequency analysing the programme signal to determine at least one frequency-dependent characteristic of the programme signal; adaptively selecting a desired interference type in dependence upon the said characteristic; generating an anti-copy signal by the complex division: D ( j ω)= N ( j ω)/ R ( j ω), where D(jω) is the frequency-domain representation of the desired anti-copy signal to be generated; N(jω) is the frequency-domain representation of the desired interference type to be produced on replay of an illegally-copied tape; and R(jω) is the frequency-domain representation of an assumed tape recorder response; and by converting D(jω) to the time domain; and adding to the programme signal the anti-copy signal to provide an output signal; wherein the anti-copy signal is such as to produce the desired interference when recorded on a tape recorder with a high-frequency writing head bias but is such as to be masked by the programme signal when the output signal is reproduced without being so recorded.