Abstract: Apparatus and methods for informing the central unit of the transmission requirements of a remote unit in a bi-directional data transmission system that facilitates communications between a central unit and a plurality of remote units using a frame based discrete multi-tone (DMT) transmission scheme. The method includes the step of transmitting, using a fast access transmission mode, a communication access request from a selected first remote unit to the central unit during any symbol period. The communication access request includes a unique remote unit identifier associated with the selected first remote unit and is transmitted from the selected first remote unit on at least one unused sub-channel using a modulation scheme that does not require equalization to decode at the central unit.

Abstract: A method of correcting for the impairments of phase jitter and frequency offset imposed by a transmission medium and of a clock frequency difference between a transmitting and a receiving multicarrier modem is disclosed. The method comprises two modes: a training mode that uses pilot tones, and a data mode that uses random data modulated onto many carriers. During the training mode the frequencies of the phase jitter are estimated, and the parameters of a correcting signal are initialized; during the data mode the correcting signal is used to compensate for the impairments, and then is updated from an estimate of the phase error.

Abstract: A method of correcting for the impairments of phase jitter and frequency offset imposed by a transmission medium and of a clock frequency difference between a transmitting and a receiving multicarrier modem is disclosed. The method comprises two modes: a training mode that uses pilot tones, and a data mode that uses random data modulated onto many carriers. During the training mode the frequencies of the phase jitter are estimated, and the parameters of a correcting signal are initialized; during the data mode the correcting signal is used to compensate for the impairments, and then is updated from an estimate of the phase error.

Abstract: Previously recommended methods of transmitting facsimile data on the telephone network resulted in periods of time when there is no signal present in the network; one consequence of this was that the retraining time required for the facsimile modem's receiver after each quiet period was undesirably long. This problem is solved by the high-speed transmitter sending two or more holding tones whenever it is not sending training or facsimile signals; these tones prevent re-routing of a telephone connection and allow the high-speed receiver to continue to track data clock and carrier frequencies, thereby considerably reducing the time required for retraining.

Abstract: A switched-capacitor modulator for quadrature modulation is disclosed. The modulator of the present invention provides a means for simultaneous modulation of two carrier signals in quadrature by two input signals in such a way that the gains of the two modulations are exactly the same. In accordance with the invention a switched-capacitor modulator is provided having a plurality of input signal sources for quadrature amplitude modulation. The invention provides for a single common switched-capacitor network with multiple switches or multiple switched-capacitor networks for each incoming modulating signal. A switched-capacitor network utilized to form and modulate a carrier signal for shaping the carrier to reduce sidebands of the third and fifth harmonics in the modulated signal is also disclosed.

Abstract: Disclosed is a constrained adaptive equalizer for use in a digital communication system receiver combining transversal and decision feedback equalization techniques. The constrained adaptive equalizer uses a constraint on the taps of the tranversal section of the equalizer which has both leading and lagging taps. These constraints force the transversal equalizer to correct only the leading echoes of a received digital signal in a communication system and treat them as if they resulted only from phase distortion. The constrained adaptive equalizer minimizes noise amplification while reducing intersymbol interference. The transversal segment of the constrained adaptive equalizer approximate the phase equalizer by constraining its multiplying coefficients i.e. taps, in accordance with the symmetry of the differential delay characteristic of the transmission medium around a signal carrier frequency.

Abstract: Novel apparatus and techniques for the modulation of a signal from one frequency band to another frequency band are disclosed. An integrator is changed between the inverting and non inverting modes, each mode occurring with each half cycle of a carrier signal. The modulating signal is sampled and applied to the integrator to provide the modulated signal. The modulator circuit components are fully suitable for fabrication on a single substrate.

Abstract: In a data transmission system, in which asynchronous-to-synchronous data conversion at the transmitter end of a data link at times removes stop bits from, and at other times adds stop bits, to a character, a receive buffer at the receive end of the data link which converts the synchronous data to asynchronous data. The receive buffer investigates the end of each character for missing stop bits and reinserts a stop bit where it finds a lack of one, and adds other stop bits to speed up the data rate from the nominal synchronous data bit transmission rate to the nominal asynchronous data bit output rate. The inserted stop bits are of a constant bit width which is the same as the character identification bit width, and the entire asynchronous data output stream has a constant bit rate.

Abstract: A full duplex communication system including a pair of full duplex modems. Each modem has both a call originate mode of operation and a call answer mode of operation. In the call originate mode outgoing data is transmitted by modulation of a carrier frequency F2 and incoming data is received by demodulation of a data modulating carrier of frequency F1. When in the answer mode of operation, the data is received on frequency F2 and transmitted on frequency F1. Means are provided for injecting a carrier frequency F6 in the outgoing data transmission path. The frequency F6 is near the midpoint between F1 and F2. The amplitude is such that the distribution of power within the transmission band is balanced so that the power in the frequency band set aside for single frequency signaling systems does not exceed the power in the remainder of the transmission band.

Abstract: The disclosed full-duplex communication system includes a pair of full-duplex modems, one at each end of a two wire circuit. Each of the modems includes a transmitter for transmitting over the two wire circuit a stream of synchronous data at a first carrier frequency, and a receiver for receiving over the two wire circuit a stream of synchronous data at a second carrier frequency. The receiver includes means for coherently demodulating the stream of synchronous data. In a preferred embodiment, an asynchronous-to-synchronous converter converts asynchronous input data into a stream of synchronous data for transmission by the transmitter. Also, the preferred embodiment transmits the data and receives the data as double side band suppressed carrier quadrature amplitude modulation. The system includes a remote test capability for automatically looping back test data for comparison with the transmitted test data.