Abstract: Circuitry for decoding differential phase shift keyed signals utilizes a counter, having a capacity selected such that in an integration interval having a duration equivalent to one-half the period of a reconstructed carrier, a full count and resultant rollover identifies a decision threshold for information descriptive of the received signal. At the conclusion of the integration interval, the contents of the counter represent half a full count thereof for no phase error in the reconstructed carrier. Phase error in the carrier is determined during a subsequent interval by counting up of the same counter to achieve a reset condition thereof. The counter is counted up until occurrence of a rollover signal, which signal is used to prevent passage of any further pulses to the counter and to maintain the counter in an all zero state.

Abstract: In a system in which a central modem individually polls a number of remotely located modems for data, an adaptive equalizer in the central modem is initially trained to minimize communication link interference from a remote modem and develops a set of equalizer coefficients based on the initial training which is transmitted to the remote modem and stored there for future use while the central modem goes on to do the same with other remote modems. When a remote modem is polled, the stored equalizer coefficients are used to set the adaptive equalizer of the remote modem so the adaptive equalizer of the central modem does not have to be retrained for each individual remote modem.

Abstract: A two-wire, carrier-type communication system is provided which enables simultaneous two-way transmission over two-wire circuits in the same spectrum. Carrier signals in quadrature are used to allow separation of signals within the same frequency spectrum. The carrier recovered at each subscriber station is used to demodulate the received signal and, after being phase shifted 90.degree., is applied as carrier to the transmitter modulator. The transmitters at the subscriber stations are thus the same as that at the central office but are locked to the receiver and 90.degree. out of phase. A bidirectional control phase adjustment network at the central station adjusts the phase of the signal received thereby until the received carrier is 90.degree. out of phase with the transmit carrier.

Abstract: A pair of full duplex modems (12, 14), each at one end of a two-wire voice grade telephone channel (16), simultaneously transmit synchronous data at high speeds in both directions. In each modem, the simultaneous transmission and reception are accomplished by frequency division. The high speed operation is obtained by odd/even bit stream division and full response binary to ternary level coding coupled with nine level quadrature amplitude modulation and correction framing techniques.

Abstract: A partial response QAM Modem is provided wherein a transmit signal is generated by modulating band limited, multilevel, partial response precoded data with inphase and quadrature carriers in inphase and quadrature phase channels, respectively, and summing the modulated outputs. The received signal is processed by a passband automatic complex equalizer. The equalized signal is delayed and then demodulated with recovered inphase and quadrature carriers to recover the inphase and quadrature phase baseband signals. The recovered baseband signals are sampled at appropriate instants to produce estimates of the transmitted symbols. These symbols are partial response decoded and processed by error correctors to improve the overall system performance. The error corrector outputs are decoded to convert the multilevel estimated data to binary data and the binary data of the two channels is summed to obtain estimates of the binary data transmitted.

Abstract: A system is provided for correcting phase jitter and frequency offsets in QAM modems. The system comprises (i) a narrow bandwidth, decision directed, phase-locked loop for tracking the average phase and frequency offsets followed by (ii) a phase jitter corrector located outside of the loop. The phase error signal for the jitter corrector is produced by passing the phase error signal of the phase-locked loop through a low pass filter designed to pass the expected jitter frequencies. Inphase and quadrature signals from the loop are delayed to compensate for the delay of the low pass filter, and the delayed signals, together with the filter output, are applied to a phase shifter which provides phase jitter correction using a rotational transformation.

Abstract: A phase-locked demodulator for demodulating a phase-difference modulated carrier having a repeated phase offset increment introduced therein. The demodulator includes, in common with conventional phase locked demodulators, a mixer having a first input connected to receive the modulated carrier and a reference carrier generator such as a voltage controlled oscillator (VCO) connected in a phase-locked feedback loop forming the second input to the mixer. In order to reduce the number of output levels, the VCO is tuned to a frequency Fr=Fc.+-.Fb[(.alpha.)/360] where Fc is the carrier frequency, Fb is the baud rate and .alpha. is the repeated phase offset. In this way the phase offset increment introduced into the modulated carrier is substantially cancelled out thereby leaving only the phase changes containing data information.

Abstract: A system is provided for extracting the proper carrier phase from a single sideband suppressed carrier signal. A received signal is equalized and then product demodulated by estimated in-phase and quadrature carriers produced by a voltage controlled oscillator (VCO). The outputs of the in-phase and quadrature product demodulators are low-pass filtered producing corresponding in-phase and quadrature baseband signals. An estimator compares the in-phase baseband signal with reference levels, samples the output of the comparing circuit at the band rate and generates a sequence of estimated data signals. The error signal, which forms the input to the VCO, is produced by multiplying the recovered quadrature baseband signal with the estimated data signals and is proportional to the product of the square of the low pass filtered baseband signal and the sine of the difference between the phases of the received carrier and estimated carrier.

Abstract: A multi-phase, multi-amplitude level modulation method, and modulator, is provided which produces a three-level, ten-phase modulated carrier. The carrier is modulated in three amplitude levels, viz., off, intermediate and high and in ten phase positions, to produce sixteen signal states, one state corresponding to the carrier being off, five intermediate amplitude level states having five equally spaced phase positions, and ten high amplitude level states having ten equally spaced phase positions. A number of different four digit binary coding schemes for the sixteen signal states are disclosed together with other different coding and modulation techniques.