Patent GB-1275850-A (GB)
Title: High speed digital receiver and transmission system
Inventor: GIBSON EARL DOYLE
IPC: H04L25/03
Filed: 1970-12-16
Abstract: 1275850 Amplitude, frequency and phase modulation NORTH AMERICAN ROCKWELL CORP 16 Dec 1970 [11 Feb 1970] 59781/70 Heading H4P [Also in Divisions G4-G6] A data communication system comprises a receiver for a modulated analogue signal, the receiver consisting of a demodulator, a transversal equalizer responsive to the demodulated signal to provide a first signal, a summing means for receiving the first signal, means for sampling the output of the summing means at a rate equal to the rate at which data is received, a decision means responsive to the sampled output to determine its polarity and/or amplitude and to provide a binary output indicative thereof, and a feedback loop which receives the binary output and operates on it to produce a second signal which is fed to the summing means to be subtracted from the first signal thereby to cancel intersymbol interference, noise etc. Transmitter.-Digital binary data is fed into a shift register various stages of which are connected via switching transistors and weighted resistors to a summing amplifier. The output of the amplifier is smoothed and used to modulate a carrier wave derived from an oscillator via a counter acting as a divider. The modulator generates a pure single distortionless sideband. The signal is then passed to a summing amplifier and, via an impedance matching line driver, to a transmission channel which may be a telephone line. Receiver.-The signal received from the transmission channel is filtered by an ordinary analogue filter designed to attenuate noise frequency components and demodulated. A carrier recovery device is provided which includes a phase locked loop to track the carrier frequency and drive the demodulator. The demodulated signal then passes to a transversal equalizer which consists of a tapped delay line. An adjustable gain amplifier is connected to each tap and the outputs of the amplifiers are passed to a summing amplifier. The transversal equalizer may be adjusted manually by feeding its output to a CRO which enables the gain settings of the amplifiers to be monitored. The output of the summing amplifier is sampled at a rate equal to the data transmission rate and fed to a discriminator which generates a digital binary output in accordance with the polarity and/or amplitude of the sampled signal. The discriminator may be of any known construction; a particular embodiment is described in connection with multilevel signalling (see modifications below). The output of the discriminator is serially fed into a shift register, respective stages of which are connected to adjustable gain amplifiers (or resistors). The amplifier outputs are connected to a summing amplifier the output of which is connected to the summing amplifier 48 where it is subtracted from the output of the transversal equalizer. Modifications.-In a modification digital quaternary (or other multilevel signals) may be transmitted. The specification describes a device used in the transmitter for converting a digital binary signal into a digital quaternary signal in which the binary input is selectively switched to pass alternate bits to a respective one of two shifts registers. Corresponding bits in each register are connected to an associated logic circuit having four reference inputs, the output of the logic circuits being passed via scaling amplifiers to a summing amplifier, the output of which is passed to the modulator. The discriminator used for a quaternary signal in the receiver may be a conventional A/D converter. In an embodiment the analogue input signal is successively compared in each quaternary bit period with three reference voltages. The first reference voltage is OV and if the input is positive a "binary one" is generated. The signal is then compared with + and - 2 volts in order to determine the polarity of the second bit of the binary pair which correspond to the single quaternary input signal. The feedback loop circuit includes a device similar to the converter in the transmitter (see above) for producing a signal which is fed to the summing amplifier 48 for subtraction; alternate binary bits (derived as above) being fed to two shift registers, like stages of which are connected to level discriminating logic circuits. Further details.-The specification describes the system in some detail, particularly the filtering, modulating and shaping processes. The transmitter may include two D/A converters. The outputs from the converters are shifted in phase relative to each other, passed to a modulator where they modulate a carrier frequency. The receiver includes a phase modulator for retarding or advancing the carrier phase, a phase retard circuit for retarding the phase of the phase modulator output, and a second demodulator for receiving and demodulating the modulated analogue signal at the retarded carrier rate. A difference circuit provides a control signal to the phase modulator indicative of the difference between the demodulated signals from the first and second demodulators which varies the output of the phase modulator so as to minimize the difference.