A radio receiver receives an AM or FM transmission of a speaking voice. Place a rogue transmitter near the receiver to alter the speech received: cancel out the original signal and transmit a modified signal at the same frequency.
The terrorist application is to mess with communications between aircraft and air traffic control: e.g., altering a spoken runway number to cause collisions (e.g., Tenerife).
A tricky problem is the rogue transmitter needs to transmit the canceling signal y= -f(x) instantaneously (or sooner, depending on geometry). This seemingly impossible task can be done by predicting the future signal, for which a great amount of research has already been done in voice compression.
How would one cancel an "s" sound? Speech compression tells us the human ear cannot distinguish one "s" from another, but how can we predict the high frequency oscillations of a specific "s", especially the phase? Maybe it's impossible.
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