Abstract: A method and apparatus for communicating a voice conversation over a single telephone channel using continuously variable level quadrature amplitude modulation establishes and maintains two totally independent communication paths, each near perfect in amplitude and phase with respect to frequency over a bandwidth suitable for communication of continuously variable amplitude pulses. These pulses may be representations of any suitable bandlimited analog signal. The method establishes almost flawless QAM channels (real, imaginary), then allows switching to continuously variable amplitude analog transmissions.

Abstract: A method and system for a synchronized pseudorandom privacy modem makes use of a pseudorandom signal generator to index into a table of N signal spaces thereby altering gain and phase modifiers so or to modify both the gain and phase of portions of the data signal being transmitted from a QAM modem and thus transmit an encrypted signal pattern to the other end of the communications channel. The modem transmitter circuit includes a data scrambler, and QAM signal point mapper, connected in series, as well as a pair of mixers having inputs from the pseudorandom signal generator and functioning to modify the gain and phase of portions of the QAM signal point mapper output signal before they are pulse amplitude modulated, filtered, converted to analog form, and transmitted over the communications channel. Complementary circuit elements in the receiver, which have been synchronized with those of the transmitter by baud rate counters at both locations, demodulate and decode the received encrypted signal.

Abstract: In the transmitter of a data communication system using QAM, a plurality of trellis coders with delay units are used for forward error correction. The output of each encoder is modulated using QAM to generate sequential baud signal elements. The redundant data bits generated are distributed between several non-consecutive bauds. Likewise, at the receiver a plurality of distributed convolutional decoders are utilized to decode the received signal element. The distributed trellis decoder is self-synchronizing.

Abstract: A method of determining the data rate of signals received on a communication line is disclosed. The method analyzes the training signals which normally precede each data transmission to determine whether they contain a preselected pattern of phase reversals. The presence or absence of the pattern indicates the data rate.

Abstract: This invention provides a method of measuring the characteristics of the channel of a communication system using modems without automatic equalization. The method consists of preceeding a data string sent between modems by a set of TRAINING signals followed by a set of TEST signals. Preferably the TEST signals are chosen to eliminate intersymbol interference on the channel, so that the signals do not have to be equalized.

Abstract: In a system in which data between a master and remote location is encrypted by using the DES algorithm, the status of security devices at the remote location is monitored from a centralized security station. A message indicative of the status of the security devices is used to generate an initializing vector (IV). This IV is then used to encrypt the data from the remote to the master.

Abstract: A phase tracking loop is provided for a digital modem. The phase tracking loop rotates the equalized output of the modem by a phase corrector angle generated by a phase error detector, frequency offset integrator and phase corrector integrator. The phase error is obtained by complex multiplexing the coordinates of the received signal with a phase corrector number. The first integrator operates on the sum of the detected phase error. The second integrator operates on the sum of the output of the phase detector and first integrator. The second integrator output comprises the phase corrector angle.

Abstract: This invention describes a QAM encoder which makes use of memory means to generate the desired QAM signals. Each QAM signal corresponds to a group of bits which are defined by an angle on a standard eye-pattern. Each group is passed serially through delay circuits and at the output of each delay circuit there are look-up circuits which select a number from the memory which is a product of the sine of the angle of the group as modified by a sampling signal and a coefficient. These coefficients correspond to the coefficients of the standard transversal digital filter. The numbers generated by each look-up circuit are added to generate the output signals.

Abstract: A transmission system for transmitting a first data series, and a second data series from a first modem to a second modem using QAM or PSK type modulation techniques. The first data series is sent using standard QAM techniques such as an 8-point scheme, wherein each point has a radial component. The second data series is sent by modulating slightly the radial components of the transmitted symbols between two values in accordance with the second data series.

Abstract: This invention provides a digital data communication system which is able to combine two data streams from two data sources at the transmitter end, transmit the data and then separate the streams to two data ports at the receiver end. The system makes use of QAM or PSK modulation, and synchronization between the transmitter and the receiver is maintained by changing the radial component of every other transmitted symbol. At the receiver end the radial components of every other symbols are correlated to detect loss of synchronization. Once such a loss is detected, the receiver is adapted to self-synchronize itself with the transmitter.

Abstract: A data communication system which includes a transmitter and a receiver is used in QAM or PSK modulation. The receiver provides a signal space pattern wherein each point is defined in a coordinate system in which a first axis is the in-phase channel axis and a second axis is the quadrature channel axis in the signal space diagram. Additionally, the receiver determines the parameters for reducing the intersymbol interference of the received signal.

Abstract: A system for the generation of many-pointed QAM signal spaces by use of multiple, synchronous QAM transmitters is provided. Three individual relatively low BPS data rate transmitters have their outputs connected to different weighting multipliers. The outputs of the weighting multipliers in turn are connected to a summer which produces a composite signal. The input to the individual transmitters comprises a binary data input, the bits of which have been steered to individual ones of low BPS data rate transmitters. The composite output of the summer generates a signal space sufficient for 14,400 BPS transmission utilizing 4800 BPS transmitters.

Abstract: A modem is provided which allows two independent sets of multipoint users to share a common communication media such as a telephone channel. At each site there is provided a modem which transmits and receives two independent frequency division multiplexed data streams simultaneously over a common telephone channel. Each data stream is encoded into an eight phase PSK or QAM signal. One data stream signal is transmitted in a low frequency range and the second is transmitted in a high frequency range. Means are provided at each site to combine the data streams for transmission over a single communication line and to separate the received combined signal from the communication line.