Abstract: Communications apparatus includes one-for-n modem redundancy for a plurality of typically contiguously disposed modem modules. The modem modules comprise n active units each illustratively operating in a different frequency band and a redundant (spare) modem of electronically adjustable frequency. The on-line (active) modem modules each include a multiplexer which normally connects user terminal equipment associated with the module to the frequency translating and data and control signal processing circuitry in the module.When a fault is detected in one of the modems (sensed, for example, by negated module "data valid" or "clear to send" levels for receive and transmit modems, respectively), a hierarchal control network and related apparatus automatically sets the redundant modem to the proper operating frequency and substitutes the spare for the failed unit via the appropriate multiplexer.

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 data transmission system, particularly useful for transmission of computer data by radio frequency cables, makes use of a phase demodulator which eliminates the usual requirement for a coherent local oscillator for use in the demodulation process. The demodulator performs a phase-shifting operation on a phase modulated signal to arrive at an unmodulated carrier signal. The unmodulated carrier is sustained by use of a resonant circuit and used to demodulate the phase modulated signal.

Abstract: A bilateral digital signalling network couples a master station and plural remote, or satellite stations via a common communications link--as by way of an MATV coaxial cable. Any data/control signals may be interchanged by the system stations--for example sensor station and closure latches in an apartment complex security system multiplexing security signals on the MATV video program disseminating cable.The system employs sequential multi-mode remote stations. In accordance with varying aspects of the present invention, encryption verification is required and performed on transmission address and data fields; satellite stations revert to a predetermined secure state if not polled frequently enough by a master station; and structure is provided to prevent a station from failing in a "talking" condition which blocks (masks) all upstream transmission.

Abstract: Frequency response of a signal path over which frequency-spaced signals are transmitted is analyzed by sweeping relatively narrow pass band tunable receiving means across the frequency band expected to be received at the location to be analyzed. Incoming signal to which the tunable receiving means is tuned at any instant is used to control the frequency of a voltage controlled oscillator. The frequency of oscillations produced by the oscillator has a known relationship to the amplitude of the instantaneously received signals, and these oscillations are transmitted to a frequency-to-amplitude converter at a measuring location. The converter produces an output signal that has an amplitude which is a function of the frequency of oscillations applied to it. A spectrum indicator at the measuring location is connected to the converter to produce a visual display of the amplitude of signals received at the location at which the characteristics of the transmission path are being analyzed.