Showing papers on "Demodulation published in 1976"

The Capture Effect in FM Receivers

Abstract: In this paper a theoretical explanation of the capture effect is given by calculating the instantaneous frequency of the output signal of a limiter when two frequency modulated (FM) signals are present at the limiter input. When this signal is applied to a demodulator with unlimited bandwidth, the output signal of the demodulator proves to have an extreme capture effect. When however the demodulator bandwidth is limited, the capture effect is shown not be be extreme. This phenomenon is explained and possibilities are given to minimize the capture effect. Some of the results of measurements on limiters and demodulators are given in this paper; they prove that a weak capture effect can be obtained. A method of calculating the degree of capturing is included.

Jointly adaptive equalization and carrier recovery in two-dimensional digital communication systems

Abstract: In this paper, we describe a novel receiver structure for two-dimensional-modulated, suppressed-carrier data signals. The receiver consists of a passband equalizer followed by a demodulator which compensates for frequency offset and phase jitter; the demodulator's phase angle is provided by a data-directed, carrier recovery loop, which is shown by analysis and simulation to be capable of tracking relatively high frequency phase jitter. A derivation of the receiver parameters is presented, based on a gradient algorithm for jointly optimizing the equalizer tap coefficients and the carrier phase estimate, to minimize the output mean-squared error. System performance is related to carrier phase-tracking parameters by analysis. Computer simulations confirm the feasibility of the receiver structure.

Frequency/space diversity data transmission system

Abstract: A frequency/space diversity data transmission system includes a transmitter for transmitting two or more distinct RF signals having the same modulation from different antennas separated a wavelength or more, the frequency spacing of the carrier signals being at least ten times greater than the frequency of the modulating signal, and a broad band receiver including a video diode detector and a frequency demodulator which recovers the modulating frequency from the carrier

Color video signal recording and/or reproducing system

Abstract: A color video signal recording and reproducing system comprises a circuit for recording and reproducing a recording signal onto and from a recording medium, a modulator for frequency modulating a carrier by a luminance signal separated from an input video signal, a first converter for frequency converting a carrier chrominance signal separated from the video signal into a band which is lower than a band occupied by the frequency modulated luminance signal, a gating circuit for passing the frequency converted carrier chrominance signal with an interval of one track scanning period in which one track on the recording medium is scanned, a first mixer for mixing the frequency modulated luminance signal and the intermittent frequency converted carrier chrominance signal from the gating circuit, where the first mixer alternately produces a mixed multiplexed signal in which the frequency modulated luminance signal and the frequency converted carrier chrominance signal are multiplexed, and a signal solely comprising the frequency modulated luminance signal, for every one track scanning period, and the output signal of the first mixer is the recording signal, a demodulator for demodulating the frequency modulated luminance signal which is separated from a reproduced signal, a processing circuit for processing the intermittent frequency converted carrier chrominance signal which is separated from the reproduced signal, into a continuous carrier chrominance signal in its original band, and a second mixer for mixing the outputs of the demodulator and the processing circuit, to obtain a reproduced color video signal.

Signal demodulator including data normalization

Abstract: A digital demodulator for recovering a baseband signal from a selectable bit rate PSK modulated signal. Carrier recovery and bit timing recovery are accomplished by using the digital equivalent of an analog in phase-quadrature (I/Q) phase locked loop. The recovered I and Q components are directed through matched filters to remove unwanted out-of-band signal components. A normalization loop responds to the output of the filters to provide normalized signals whose amplitudes do not significantly vary with changes in data rate or signal to noise ratio. Circuitry is provided for tailoring the parameters of a loop filter used in the carrier recovery loop in accordance with the data rate of the incoming signal. A lock detector is provided for determining the signal-lock status of the demodulator, and for controlling the operation of a carrier sweep control. Several numerically controlled oscillators are disclosed for use in the carrier recovery and bit timing recovery loops.

Correlative converter between a 2n -ary code sequence and a 2n+1 -phase carrier pulse sequence

Abstract: A novel modulator converts a sequence of N'ary codes representative of an element P of a set (0, 1, 2, . . . , N - 1) into a sequence of 2N-phase carrier pulses, where N = 2 n , n being a positive integer. A phase shift in the carrier signal between each carrier pulse and the next preceding one is selected from Pπ/N and (P + N)π/N in compliance with a prescribed law of correlation between the N'ary codes in the sequence. Alternatively, the carrier signal is given in each carrier pulse a phase selected in the manner specified. For convenience of resorting to the prescribed correlation law, the N'ary code sequence may be converted into N trains, corresponding to the respective elements P, of three-level signals, 0 and ±1. In the train corresponding to a particular element, the three-level signals are successively produced with the prescribed correlation law, such as with resort to a bipolar or duobinary technique. A novel demodulator converts the 2N-phase carrier pulse sequence into the original N'ary code sequence.

Correlation system for pseudo-random noise signals

Abstract: An improved correlation system for coded signals wherein a signal is known in pattern but is unknown in timing and is bi-phase modulated on a suppressed carrier signal and correlation is accomplished with a time reference signal without the necessity for search and coherent recovery of the suppressed carrier signal.

On optimal soft-decision demodulation

Abstract: Wozencraft and Kennedy have suggested that the appropriate demodulator criterion of goodness is the cutoff rate of the discrete memoryless channel (DMC) created by the modulation system; the criterion of goodness adopted in this paper is the "symmetric" cutoff rate which differs from the former criterion only in that the signals are assumed equally likely. Massey's necessary condition for optimal coherent demodulation of binary signals is generalized to M -ary signals. It is shown that the optimal demodulator decision regions in likelihood space are bounded by hyperplanes. An iterative method is formulated for finding these optimal decision regions from an initial "good guess." For additive white Gaussian noise (AWGN), the corresponding optimal decision regions in signal space are bounded by hypersurfaces with hyperplane asymptotes; these asymptotes themselves bound the decision regions of a demodulator which, in several examples, is shown to be virtually optimal. In many cases, the necessary condition for demodulator optimality is also sufficient, but a counterexample to its general sufficiency is given.

Application of Passband Decision Feedback Equalization in Two-Dimensional Data Communication Systems

Abstract: We describe and experimentally evaluate a receiver structure for two-dimensional data communication systems based on passband decision feedback equalization and data-directed carrier recovery The forward and feedback tap coefficients and the demodulator's carrier phase angle are adjusted adaptively; the carrier recovery and demodulation operation, as in the case of a linear equalization receiver, can compensate for rather severe frequency offset and phase jitter Experimental results are reported of simulations of the receiver on voiceband telephone channels

Receivers for pulses of different widths

Abstract: A data transmission system for transmitting data and subsidiary information by means of pulses of different widths corresponding to a multiple of a signal propagation time delay of a NAND gate. The system makes it possible to simplify the construction of the data transmission device to a considerable degree and at the same time to transmit data at a high speed, and includes a variety of circuits for generation and demodulation of the pulses.

Analysis of a gradient algorithm for simultaneous passband equalization and carrier phase recovery

Abstract: A two-dimensional receiver structure has been proposed, incorporating two innovations: passband equalization, which mitigates intersymbol interference, and data-directed carrier recovery and demodulation following equalization, which enables compensation of carrier frequency offset and phase jitter, but does not require transmission of a separate pilot tone with the data signal. The receiver is fully adaptive; the adjustment of the equalizer tap coefficients and of the estimate of the current channel phase shift is based on a gradient algorithm for jointly minimizing the mean squared error with respect to those parameters. In this paper, we analyze the dynamic behavior of the deterministic gradient algorithm (where channel parameters entering into the gradient expression are assumed known in advance). The corresponding estimated gradient algorithm (where these parameters are initially unknown) has previously been studied experimentally, but is not treated here. The first part of the present study concerns system start-up (or transient) response when the channel's phase shift is fixed. Examination of the analytical solution leads to the qualitative conclusion that, if the equalizer tap adaptation coefficient β is small relative to the phase-tracking coefficient α, the added phase estimation feature does not strongly affect the start-up behavior of the passband equalizer under typical operating conditions. Indeed, if the equalizer tap coefficients all start at zero, their evolution in the deterministic gradient algorithm is completely unaffected by the phase-tracking loop. The second situation analyzed is the steady-state response of the system to a constant carrier frequency offset. In this case, the phase-tracking loop is found to reduce the resulting rate of rotation of the equalizer taps to about β/(α + β) of the original frequency offset. As a result, the degradation in system mean squared error due to frequency offset is typically quite small. The final analysis is of the response of a linearized version of the receiver structure to sinusoidal phase jitter. When the channel's linear

Demodulation method and circuit for discretely modulated ac signals

Abstract: Demodulation of an ac signal that has been modulated by discrete changes in its amplitude, frequency or phase, is effected by first feeding the modulated ac signal through an amplitude limiter and hence through a bandpass filter tuned to a preselected frequency of the ac signal. The resulting bandpass-filtered ac signal defines a variable amplitude envelope which varies in response to amplitude, frequency or phase modulations of the input ac signal. Amplitude variations of the envelope of the bandpass-filtered ac signal are detected by a switching circuit whose operation has been termed "self-synchronous," because it is synchronized to the frequency of the ac signal prior to the passage of the ac signal through the bandpass filter. More particularly, the switching circuit is opened and closed in response to the ac signal appearing at the input to the bandpass filter, to cause alternate, like-polarity half cycles of the bandpass-filtered ac signal to be selectively conducted to a low-pass filter. Filtering by the low-pass filter extracts modulation information from the detected envelope thereby completing the demodulation process. By virtue of the "self-synchronous" detection, the demodulation process is less prone to false detection of modulation information when noise or other spurious signals accompany the modulated ac signal, and is faster in responding to information-bearing changes in the modulated ac signal.

Surveillance arrangement including a television system and infrared detector means

Abstract: The invention provides a surveillance arrangement, and in particular an airborne surveillance arrangement, in which a television display of a scene is brightened up, by means of infra-red detectors, to emphasize heat emitting targets. In one example of airborne surveillance system, a linear array of infra-red detectors is arranged to view part of a scene viewed by a television camera, with the same horizontal field of view at a desired target range. In front of the array of detectors a light chopper is provided so that the output of each detector is a modulated carrier. The output of each detector is applied to a store, via individual impedance transformers, narrow bandpass filters and demodulators. Between each demodulator and the store, individual sharp out-off low pass filters and threshold detectors may be provided to provide a degree of discrimination against spurious responses. The store is read-out under the control of a sequence timing unit which is synchronized with the synchronizing signal generator of the television camera and the video output of the latter is mixed with the output from the store prior to application to a display unit so that the display provided is a scene of an area towards which the aircraft is flying with heat emitting targets highlighted.

Device for phase synchronizing a reproduced reference carrier signal with windows specified for preselected ones of amplitude and phase modulated signal points

Abstract: A phase synchronizer for a reference carrier signal reproduced in a receiver for a combined amplitude and phase modulated signal comprises a window specifier for specifying windows for preselected ones of true signal points of which the demodulated signal is selectively representative. The phase synchronizer includes a demodulator for producing complex signals representative of coarse signal points so long as the reproduced carrier signal has a phase error which falls within the windows. Differences between the true points and the coarse points falling within the windows are calculated and summed up to produce a control signal for reducing the phase error substantially to zero.

Subscription television decoder apparatus

Abstract: This disclosure is concerned with simplified decoder apparatus for use with depressed horizontal sync scrambled television transmissions, embodying intercarrier detection of picture and sound carriers and FM demodulation of audio information including a pilot tone used to restore the video sync signal by controlling an RF gating amplifier.

Color mark detector with pulsed source and synchronous demodulation

Abstract: A detector for marks having a color which contrasts with the mark background employs a solid state light source projecting a light beam having a plurality of wave length components corresponding to a plurality of colored light wave lengths. The light beam may approach the appearance of white light. A pulse modulated detector or sensor is used for receiving light reflected from colored surfaces having contrasting colored marks thereon, wherein high ambient light level rejection is obtained and high signal to noise ratio is provided in a sensor output. The sensor output is gated to eliminate between pulse noise, and the gated output is integrated to obtain a reference signal to which the gated pulse is continuously compared. An increase or decrease in signal is used to provide a light or dark mark indication respectively as a result of the comparison. Feedback from the output suppresses signal instability which may exist at the edges of the detection threshold. Since the comparator reference is self-adjusting to the background condition on the surface carrying the marks, the detection capability is dependent upon detector sweep speed relative to the surface.

Fast FSK signals and their demodulation

Abstract: The Fast FSK is a particular kind of FSK (Frequency Shift Keying) which is useful for the transmission of digital data over an r.f. channel which is limited in both bandwidth and power. This paper describes the performance of the Fast FSK, and outlines the practical circuits which demodulate it.

Programmable data terminal set

Abstract: An operator console, a general purpose digital computer and a radio set irface are connected to perform the modulation and demodulation of the differential coherent quadrature phase shift signal used to transmit the digital information over a computer-to-computer radio link. The console is used to initiate communications and to control and monitor link operation. The computer performs all of the signal processing and control functions. The radio set interface performs analogue-to-digital (A/D) and digital-to-analogue (D/A) conversions for the received and transmitted signals, respectively. The programmable data terminal set (PDTS) also controls the data link operation and sequencing and tests its own performance during operation.

Demodulator for frequency-keyed communication system

Abstract: At the receiving end of a communication system utilizing a higher keying frequency F A and a lower keying frequency F Z for the transmission of information, a counter is stepped by clock pulses having a cadence F c >> F A . A train of short signal pulses, derived from the incoming wave and coinciding with the zero crossings thereof, is used to reset the counter whose output is delivered to a digital comparator also receiving a fixed reference number not less than and preferably equal to the count of clock pulses generated during a half-cycle of frequency F A . Whenever the output of the counter surpasses that reference number, the comparator has an output yielding, upon integration in a low-pass filter, a low-frequency wave which can be squared with the aid of a threshold circuit.

Unbalanced quadriphase demodulator

Abstract: A new and improved apparatus and method for demodulation of quaternary phase shift keyed (QPSK) data, particularly unbalanced QPSK, with phase adjustment of the output of a phase-locked loop local oscillator to reduce sensitivity to amplitude variations internal to the demodulator.

Optimum length transmission line discriminator with low noise detector

Abstract: Measurement apparatus using an optimum length transmission line discriminr is disclosed which allows near carrier frequency modulation to be measured in the very high and ultra high frequency regions without the use of a microwave type cavity resonator or a tuned resonant circuit. Amplitude modulation rejection through direct current biasing of a quadrature detector enhances near carrier FM measurement. Signal energy is routed through two channels to the quadrature detector. A reference channel provides attenuation and phase shifting, and a signal channel comprising the optimum length line provides FM discrimination. The phase detector is direct current biased to separate amplitude modulation noise from near carrier frequency modulation noise.

FM noise threshold extension demodulator apparatus

Abstract: Apparatus for lowering the noise threshold at which FM and other angle modulated signals may be demodulated is described wherein an oscillating limiter provided by an amplitude limiter having a bandpass filter in regenerative feedback relationship therewith is connected ahead of the demodulator. The bandpass filter is made tunable in response to a steering signal developed by processing the frequency proportional signal at the output of the demodulator to attenuate high frequency components thereof which are outside the baseband of the signals and adjusting the amplitude of the signal so that the center frequency of the bandpass filter is caused to substantially match the frequency of the frequency modulated signal. The steering signal circuit has electrical inertia, thus during click events the oscillating limiter will oscillate at a frequency which matches the best available estimate of the instantaneous frequency of the modulated signal, thereby minimizing both impulse noise (clicks) and discontinuities in the output, frequency demodulated (base band) signal during a click event.

Carrier lock detector

Abstract: Apparatus for indicating a phase lock condition between an incoming multiphase carrier signal and a local oscillator in a phase-locked loop demodulator. A phase error signal is generated by phase comparing the carrier signal and the local oscillator signal. The phase error signal is rectified and the result detected for a range of values that indicate a phase lock condition. Phase lock exists when the phase error signal is within selected voltage ranges. After detection, the error signal is low-pass filtered and compared to a predetermined threshold value to generate a phase-lock indication signal. In the last two steps, an indication signal is generated to signify that the phase error signal is in the range of voltage values corresponding to phase lock for a greater percentage of time than it is within this range with the loop is not phase locked.

Method and apparatus for closed loop testing of first and second modulators and demodulators

Abstract: A method and apparatus are disclosed for closed loop testing of first and second modulators and demodulators from the modulated signal side of the first modulator and first demodulator. During ordinary communication, the first modulator and second demodulator are connected in series and the first demodulator and the second modulator are connected in series. In response to each test initiate signal, a loop is established between the second modulator and the second demodulator on their modulated signal side. Upon reception of some test initiate signals, a loop is established between the first modulator and first demodulator on their demodulated side and they are disconnected from the second demodulator and second modulator. Thus, the first modulator and demodulator can be tested from their modulated signal side when a loop is established between them; and the first and second modulators and demodulators can be tested when a loop is established between the second modulator and demodulator but not between the first modulator and demodulator.

Automatic noise reduction system of FM stereo receiver

Abstract: Disclosed is an automatic noise reduction system of FM stereo receiver wherein the input level and carrier frequency deviation of an FM stereo transmission signal received by an antenna are detected and, when at least either one of the input level and carrier frequency deviation decreases, the separation between first and second audio outputs of a stereo demodulator for demodulating the FM stereo transmission signal is decreased and simultaneously high frequency components on the first and second audio outputs are attenuated.

Vestigial-sideband transmission system for synchronous data signals

Abstract: A vestigial-sideband transmission system for synchronous data signals. The transmitter is provided with a filtering and modulating circuit for generating a vestigial-sideband channel signal which at the location of its carrier frequency and at the location of a frequency which is spaced from this carrier frequency by a distance equal to one half of the clock frequency of the data signals is a double-sideband modulated signal. In the receiver the reference carrier for coherent domodulation and the reference clock signal for regeneration can be simply recovered from the transmitted vestigial-sideband channel signal itself.

An Automatic Control System for Compensating Cross-Polarization Coupling in Frequency-Reuse Communication Systems

Abstract: In frequency-reuse communication systems using orthogonally polarized waves, depolarization due mainly to rain and Faraday rotation (in satellite systems) reduces channel isolation. The control system described, incorporating a network suggested by T. S. Chu, automatically compensates for cross-polarization coupling without using iterative methods or computations. Control voltages are generated on the receive side by phase-sensitive demodulation of beacons contained in the communication channels. Slightly modified conventional tracking receivers may be used for the demodulation. The system can be used for the reception of dual linear or dual circular polarizations without modification. An analysis of the control system behavior is given and a method for automatic initial balancing of the systemindependent of its initial state-is proposed.