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The Beatles tried and failed to invent a spoiler signal that would prevent
their music being recorded at home, but that doesn’t stop others having a go.
Rakvere, a company based on the Isle of Man, reckons it can add an inaudible
signal to a sound recording that spoils any copy made with a conventional tape
recorder (GB 2348736). Its system analyses the original sound and adds
high-pitched tones at frequencies that are masked by the music. Normal play is
not affected, but when someone tries to re-record the music on tape the tones
mix with the high-frequency “bias” signal which tape recorders use to shake up
the magnetic particles. This creates audible lower-frequency tones that spoil
the recording.