The communication of confidential information is generally accomplished by transforming applied message waves into an unintelligible form prior to transmission. Such a transformation may be accomplished by scrambling the applied message waves in either the frequency or time domain. Scrambling, per se, does not render a signal satisfactorily unintelligible. In order to overcome this deficiency, scrambling techniques are utilized jointly with vocoder and selective speech processing methods. In a voice-excited vocoder, the spectrum channel control signals are scrambled in the frequency domain and the baseband signal is severely center clipped in order to enhance unintelligibility. A high degree of privacy during transmission is thus obtained with negligible effect upon the quality or intelligibility of speech synthesized by an authorized recipient.