Abstract: In a recording/playback system, increased information is achieved by 4 level biased magnetic recording where the maximum amplitude 4 level recording signal drives the medium's magnetization into a nonlinear region of its transfer function. The bias does not eliminate distortion at the maximum signal input level, however the system's signal to noise ratio is improved due to an increase in the amplitude of the playback signal resulting from the increased recording level. The nonlinear mapping capability of a neural network provides equalization of playback signals distorted due to the record/playback nonlinearity. The 4 level recorded signals provide a factor of 2 in information storage compared to binary recording, and quadrature amplitude modulation (QAM) combined with the 4 level recording technique provides an additional factor of 2, for a factor of 4 in the information content stored.

Abstract: A covert communication apparatus exploits chaotic dynamics in the multi-dimensional phase space with a continuous number of states and wide internal spectral bandwidth. The information modulates a chaotically generated signal, and the modulated chaotic signal is shaped for compatibility with the specific constraints demanded by the communication channel. In an analog embodiment, the chaotic signal spectrum is band limited to be compatible with the bandwidth requirements of the transmission link, and in a digital embodiment the word length and baud rate of the transmitted digital data are selected for similar transmission link compatibility. After transmission over the communication link, the received signal is applied to a receiver chaotic signal generator substantially identical to the transmitter chaotic signal generator and the chaotic signal is recovered. This chaotic signal is used to demodulate the received signal for recovery of the information.