Abstract: 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.

Abstract: Disclosed is a system for synthesizing speech from stored signals representative of words precoded in accordance with phase vocoder techniques. The stored signals comprise short-time Fourier transform parameters which describe the magnitude and phase derivative of the short-time signal spectrum. Speech synthesis is achieved by extracting the stored signals of chosen words under control of a duration factor signal, by concatenating the extracted signals, by operating on the phase derivative parameters to effect a desired speech pitch change, by interpolating the magnitude parameters of the short-time Fourier transform in response to the pitch and duration changes, and by decoding the resultant signals in accordance with phase vocoder techniques.

Abstract: Ideally, when a signal is quantized, the range of the quantizer should be matched to the power of the signal. Disclosed herein is adaptive quantizer apparatus which utilizes a particular quantizing scheme for those communications where the power of the signal to be quantized is unknown, but remains constant for the duration of a communication. The amplitude range of an adaptive quantizer is adjusted during an initial training period to a value appropriate to the power of the applied input signal. This is accomplished by successively altering the amplitude range by a multiplicative quantity that depends upon the amplitude of the quantizer output signal and upon the time elapsed since the onset of the training period.