Magical mystery sound

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”…