Abstract: A number of remotely mountable transmitting heads each has a transducer driven by a sweep frequency ultrasonic signal for generating an ultrasonic field. A number of remotely mountable receiving heads each processes incident doppler shifted energy reflected by a disturbance by developing therefrom a constant amplitude received signal and multiplying this signal by the ultrasonic signal to generate a pair of noise cancelling differential signals. A processor for generates the ultrasonic signal and further processes the received signals so as to detect the disturbance or system tampering.

Abstract: 1. In a secure communication system; means for producing an analog intelligence signal; means for limiting the amplitude of said signal; means for transforming portions of said amplitude limited signal occurring in time sequence into a corresponding digital signal consisting of sequentially occurring n-digit words; means for combining said digital signal with a digital keystream signal in modulo 2.sup.n fashion; and means for transforming the digital signal produced by said combining means into an analog signal.