Abstract: A full duplex data modem for general application, e.g., cable system signal distribution, terrestrial or satellite radio link, or the like includes transmit and receive portions for permitting reliable communications between plural stations coupled to the common communications link.In accordance with the present invention, a fixed value is applied to a modem adapted in a test mode to speak with itself, and the number of transitions counted at the modem receiver output, to measure the error rate of an entire system.

Abstract: A system including a data modem having transmitter and receiver sections. The modem transmitter includes a multi-level data encoding system, a stable carrier generator and a selected transmission frequency, and a double sideband suppressed carrier modulator to modulate the multi-level signals on the carrier. All filtering to shape the signals in the transmitter section is done prior to modulation and, in the receiver section, after demodulation. Signals from several modem transmitters, each occupying its own assigned band, are multiplexed with two pilot carriers spaced by a predetermined frequency difference. The modem receiver includes a circuit to separate one of the carriers from the received band and mix that carrier with signals in a limited part of the band to shift the frequency of signals in the limited part of the band to an intermediate frequency (IF) band. A desired signal can be selected by frequency-shifting the IF signals so that the carrier frequency of the desired signal is at zero.

Abstract: Apparatus for effecting relative positioning between two mechanical members in accordance with digital (or analog) commands includes a standing wave supporting, electrical wave propagating structure terminated other than by its iterative impedance. The wave structure is mechanically fixed to one mechanical member, and a probe is coupled to the wave structure and fixed for translation with the other mechanical member.A voltage controlled oscillator excites the wave structure developing a standing electrical wave therein -- the wave pattern node positioning being determined by the input digital command. Feedback servomechanism apparatus then responds to the potential induced in the probe (or, by superposition, that on the wave structure) for positioning the probe at the requisite standing wave null point.In accordance with one aspect of the present invention, the wave propagating structure is sealed, and the probe disposed external thereto, to preserve measuring accuracy.