Speech scrambler:-
A speech scrambler is a security device that attaches to the mouthpiece of a telephone. It scrambles the speech so that casual eavesdroppers cannot understand the message. The voice message is assumed to have a spectrum falling between -5 kHz and 5 kHz. The magnitude of the spectrum is depicted in figure as a triangular shape.
The actual shape may be different but, since the message is real, the shape is always symmetric about the vertical axis. The scrambler works by multiplying the signal by a continuous wave of 20 kHz and then passing the result through a perfect high-pass filter that only accepts frequencies of |f | ≥ 20 kHz. The result is then multiplied by a signal of 25 kHz and then passed through a perfect low-pass filter that only accepts frequencies of |f | ≤ 20 kHz. The result y(t) is the scrambled voice message.
(a) Draw a diagram of the magnitude of the spectrum of the signal, say x(t), at the point right after the high-pass filter.
(b) Draw a diagram of the magnitude of the spectrum of y(t).
(c) How can the scrambled signal be unscrambled?
