Communication privacy: A low cost digital approach
The author demonstrates support for an analog/digital system that can be scrambled while in digital form and may or may not transmit the key word for algorithm decoding. Multichannel audio and data communication is considered. In addition to digital audio and pulse code modulation concepts, frequency shift keying, frequency division multiplexing, and quadrature phase modulation are discussed. Both practical and cost aspects are considered. Commercially available integrated parts were used in the empirical work to achieve communication privacy at a low cost.
Technology and Construction Management
House, Cliff L. "Communication privacy: a low cost digital approach." In Proceedings. Institute of Electrical and Electronics Engineers 1988 International Carnahan Conference on Security Technology, Crime Countermeasures, pp. 45-49. IEEE, 1988.
University of Kentucky, Office of Engineering Services, (Bulletin) UKY BU