Showing papers on "Demodulation published in 1974"

Statistical Properties of Timing Jitter in a PAM Timing Recovery Scheme

Abstract: A new technique is presented for evaluating the performance of a popular type of timing recovery circuit for baseband synchronous pulse amplitude modulation (PAM) data signals. The timing circuit consists of a square-law device followed by a narrowband filter tuned to the pulse repetition frequency along with provision for reshaping the pulses entering the timing path (prefiltering). The output of the timing circuit is a nearly sinusoidal timing wave whose zero crossings indicate the appropriate sampling instants for demodulation of the PAM signal. For a random data sequence, the timing wave exhibits phase fluctuations which are strongly dependent on the pulse shapes entering the timing path and the passband shape of the narrow-band filter. Expressions for rms phase fluctuation in the timing wave as a function of the prefiltering and postfiltering characteristics of the filters preceding and following the square-law device are presented. These expressions have a form which is especially suitable for studying the case where the baseband PAM signal is band-limited to frequencies less than the pulse repetition frequency. A condition on prefiltering and postfiltering which gives error-free timing recovery is presented. Results obtained from some specific examples serve to illustrate several aspects of the timing recovery problem.

Carrier Synchronization and Detection of QASK Signal Sets

Abstract: A carrier regeneration loop which generates highly coherent quadrature reference signals for quadrature amplitude-shift keying (QASK) demodulation is presented. The loop employs the principle of decision feedback and has a structure analogous to a decision feedback loop for quadriphase signals suggested earlier in the literature. The error probabihty performance of QASK is computed in the presence of the noisy carrier reference signals provided by the above loop. It is demonstrated that when the ratio of data rate to loop bandwidth is 50 or greater, then for all practical purposes, ideal QASK error probability performance is achieved.

Fourier series and estimation on the circle with applications to synchronous communication--I: Analysis

Abstract: A wide variety of continuous- and discrete-time estimation problems on the circle S^1 are considered with the aid of Fourier series analysis. Measurement and diffusion update equations are derived for the conditional expectation of certain functions of the parameter to be estimated, and we investigate the use of Fourier series to obtain easily implemented optimal estimation equations. A variety of important examples--phase tracking, frequency demodulation, and phase demodulation in the presence of oscillator instabilities, additive noise, Rayleigh fading, or any combination of these--are considered.

Method of compensating for imbalances in a quadrature demodulator

Abstract: A method for correcting errors due to imbalances between the two channels of a quadrature demodulator in a radar system is shown. The contemplated method comprises generally the steps of measuring, by performing a Fourier transform on a test signal periodically impressed on the quadrature demodulator, the amplitude and phase imbalances between such channels and then deriving correction coefficients to compensate for such imbalances during operation.

Microwave relay system having auxiliary signal transmission arrangement

Abstract: A microwave heterodyne relay system transmitting a main carrier, angle-modulated wave in combination with an auxiliary signal transmission system is disclosed. The auxiliary signal transmission is accomplished by a subcarrier wave frequencymodulated with the auxiliary signal. The frequency-modulated subcarrier wave is then used to amplitude modulate the main carrier. The receiver of the relay system is branched into two paths. In the first path, the envelope of the main carrier is detected and then frequency demodulated to obtain the auxiliary signal. In the second path, the amplitude modulation of the main carrier is suppressed by an amplitude limiter before the main carrier is amplitude modulated by the locally generated frequency-modulated subcarrier wave prior to retransmission.

Direct-Detection Optical Communication Receivers

Abstract: A model that is sufficiently general to describe the predominant statistical characteristics of the output of many real optical detectors is formulated. This model is used to study the optimum receiver processing for direct-detection optical communication systems. In particular, the structures of detectors and estimators for randomly filtered doubly stochastic Poisson processes observed in additive white Gaussian noise are considered. Representations for the posterior statistics of a vector-valued Markov process that modulates the intensity of the doubly stochastic Poisson process are obtained. Quasi-optimum estimators and detectors are specified in general terms and specialized for several important applications. These include a demodulator for subcarrier angle modulation, a detector structure for binary signaling with known intensities, and a detector structure for binary signaling in the turbulent atmosphere.

Threshold correction system in FSK transmissions

Abstract: A demodulator produces mark and space signals in response to mark and tone signals received from a radio receiver. The signals after detection are in the form of characteristic tones and are separated by corresponding band pass filters. After separation, AM detection of the mark and space signals is accomplished in envelope detectors whose output voltages are D.C. voltages. In the absence of noise, the detector voltages are essentially a rounded off version of the binary signal (mark) and a rounded off version of the complement (space) of the binary signal. These voltages are compared and their difference, a binary signal, after filtering is applied to a system which automatically computes a mark-space decision threshold for a comparator or slicer such that the slicer produces a mark output when the filtered output is above the threshold and a space output when the signal is below the threshold to achieve optimum detection of the mark-space data during fading of either one or both of the carrier waves on which such data is transmitted to the radio receiver. The optimum decision threshold is computed from the signal peaks which are stored in two peak detectors, one for the positive peak and one for the negative peak. The peak detectors are periodically discharged to allow a fresh peak to be stored and the discharge circuits obtain their cues from state changes observed at the slicer output. Hence the decisions themselves signal the timing of the discharge of the peak detectors. A delay network ensures that the charge stored by each peak detector is not caused by noise; and also a sequencing logic circuit prevents the same detector from being discharged twice in a row thereby providing additional immunity to noise.

Statistical Properties of Timing Jitter in a PAM Timing Recovery Scheme

Abstract: A new technique is presented for evaluating the per- formince of a popular type of timing recovery circuit for baseband synchronous pulse amplitude modulation (PAM) data signals. The timing circuit consists of a square-law device followed by a narrow- band filter tuned to the pulse repetition frequency along with pro- vision for reshaping the pulses entering the timing path @refiltering). The output of the timing, circuit is a nearly sinusoidal timing wave whose zero crossings indicate the appropriate sampling instants for demodulation of the PAM signal. For a random data sequence, the timing wave exhibits phase fluctiations which are strongly de- pendent on the pulse shapes entering the timing path and the pass- band shape of the narrow-band filter. Expressions for rms phase fluctuation in the timing wave as a function of the prefiltering and postfiltering characteristics of the filters preceding and following the square-law device are presented. These expressions have a form which is, especially .-. suitable for studying the case where the-kas+d , tion frequency., A condition on prefiltering and postfiltering which gives error-free timing recovery is, presented. Results obtained from some specific examples serve to illustrate several aspects of the timing recovery problem. -_-_- PAM signal is band-limited to freciuencics l&stliah tliepulse repeti- . 4- -

Modulation system with carrier control

Abstract: System for controlling the magnitude of an RF carrier signal that is concurrently being amplitude-modulated, whereby there is provided an amplitude-modulated signal that is compatible for reception either as a conventional AM signal or as an SSB signal. The instantaneous power of a carrier signal is adjusted in response to the instantaneous power of a modulation signal available to modulate the carrier signal. The carrier signal power is allowed to fluctuate between a maximum value and a minimum or no-modulation value, which may provide a minimum carrier level notwithstanding the absence of a modulating signal.

Phase tracking and demodulation with delay

Abstract: Algorithms are described for estimating carrier phase by a maximum-likelihood procedure similar to the Viterbi algorithm for decoding a digital sequence. These algorithms avoid the constraint of physical realizability imposed by a phase-locked loop and allow a processing lag in estimation. An approximate analysis suggests that threshold extension can be achieved thereby. Computer simulations demonstrate the improved performance for demodulation of FM and for tracking of phase modulation due to dynamic maneuvers.

Demodulator for amplitude-and-phase modulated signals having control means for reducing phase and amplitude errors

Abstract: Amplitude-and-phase modulated signals, as received, are represented by corresponding points on a reference signal plane of points to which results of demodulation, phase errors, and amplitude errors are preliminarily assigned. Responsive to the received signals, a detector produces position signals representative of the corresponding points. Responsive to the position signals, a demodulation and error-detection circuit produces result signals, phase error signals, and amplitude error signals representative of the results of demodulation, phase errors, and amplitude errors assigned to the corresponding points. Responsive to control signals specifying a predetermined area of the reference signal plane and to the position, phase error, and amplitude error signals, a selection circuit produces phase and amplitude control signals, which correct the reference signal plane to reduce the phase and amplitude errors of the position signals. After correction, the result signals become the demodulated signals.

Feedforward error correction in interferometer modulators

Abstract: Modulation distortion due to phase modulation error in an interferometer type modulator is minimized by extracting a portion of the phase modulated carrier frequency signals in the two branches of the interferometer and sensing the resulting amplitude modulation in an auxiliary interferometer circuit. The modulation signal is compared with the input modulation signal and a weighted error signal is formed. The latter is then used to impress an error correcting phase modulation on the carrier frequency signals.

FM receiver and demodulation circuit

Abstract: An FM receiver has a first pair of signal mixers for receiving an RF signal and a local oscillator for providing a signal to said mixers at a frequency near the center frequency of the received signal. A phase shifter shifts the phase of the local oscillator signal to one mixer by 90°. The outputs of the first pair of mixers are connected to low pass filters having cut-off frequencies equal to approximately one-half the bandwidth of the received signals. A second pair of signal mixers receive the filtered signals and an oscillator signal at a frequency equal to at least twice the cut-off frequency of the low pass filters. The oscillator signal to one of the mixers is phase shifted 90°. The outputs of said second pair of mixers are added together, amplified, limited and thereafter demodulated to provide an audio signal. Automatic frequency control is also provided for the local oscillator.

Compatible AM stereophonic transmission system

Abstract: Amplitude modulation (AM) stereophonic transmission system characterized by the modulation of a radiant energy carrier wave with two stereo related signals, each appearing as a first order single-sideband, the carrier wave being preferably also modulated with an infrasonic frequency (e.g. 15 Hz) signal indicating stereo signal presence (with such infrasonic frequency modulation being either amplitude modulated or phase modulated). As an improvement of the AM stereo transmission technique disclosed in my U.S. Pat. No. 3,218,393 the present system develops a carrier modulated with stereo related (L and R) audio signal intelligence by amplitude modulating the carrier with the summation (L + R) signal and phase modulating the carrier with an altered stereo difference (L-R) signal, the altered difference signal being developed by combining the fundamental of the difference (L-R) signal with the difference signal derived from frequency doubled L and R signals, the amplitude level of the frequency doubled difference signal being about 13% of the amplitude level of the fundamental difference signal at full stereo modulation and being a square law function of the stereo difference signal level. In the preferred embodiment the frequency doublers are of the constant gain type and the level of the second harmonic phase modulation is determined by a level squarer type circuit.

Statistical properties of timing jitter in a PAM timing recovery scheme

Abstract: A new technique is presented for evaluating the performance of a popular type of timing recovery circuit for baseband synchronous pulse amplitude modulation (PAM) data signals. The timing circuit consists of a square-law device followed by a narrowband filter tuned to the pulse repetition frequency along with provision for reshaping the pulses entering the timing path (prefiltering). The output of the timing circuit is a nearly sinusoidal timing wave whose zero crossings indicate the appropriate sampling instants for demodulation of the PAM signal. For a random data sequence, the timing wave exhibits phase fluctuations which are strongly dependent on the pulse shapes entering the timing path and the passband shape of the narrow-band filter. Expressions for rms phase fluctuation in the timing wave as a function of the prefiltering and postfiltering characteristics of the filters preceding and following the square-law device are presented. These expressions have a form which is especially suitable for studying the case where the baseband PAM signal is band-limited to frequencies less than the pulse repetition frequency. A condition on prefiltering and postfiltering which gives error-free timing recovery is presented. Results obtained from some specific examples serve to illustrate several aspects of the timing recovery problem.

System for synchronizing local pseudo-noise sequence to a received baseband signal

Abstract: The present system generates a local PN (Pseudo Noise) sequence demodulation bit stream in synchronism with the modulation sequence of a received baseband signal The system is comprised of a PN sequence generator for generating a local PN sequence signal in response to the pulse rate of a clock signal A digital delay line generates early and late phases of the local PN sequence, which phases are correlated with the modulation sequence of the received signal Early correlation causes pulses to be inserted into the train of clock pulses while late correlation causes pulses to be deleted from the train of clock pulses A phase error less than a predetermined range (dead band) results in no adjustments to the train of clock pulses

Frequency stabilized single oscillator transceivers

Abstract: A transceiver, adapted for use as either a master or a slave in a duplex pair, has a single, voltage-tunable, solid state oscillator to provide the carrier frequency wave, a small portion of which is mixed with the received wave and applied therewith to a single ended mixer; an AGC-controlled loop cancels transmitter input modulation from the receiver output. The oscillator of the master transceiver is locked to a desired carrier frequency by a feedback loop including a tuning cavity adjusted to a frequency different from that of the slave transceiver by the common IF frequency; the slave transceiver is first locked to the frequency of its tuning cavity and thereafter, upon sensing output from its IF amplifier (from the master), is switched to operate in response to AFC controlled by the received signal, such that the master and slave transceivers are locked together at frequencies differing by their common IF frequency. A single integrating amplifier provides demodulator and AFC filtering and, together with a bistable device, initial sweeping of the oscillator control voltage.

PSK-FSK spread spectrum modulation/demodulation

Abstract: A spread spectrum link in which a transmitter section generates a different frequency signal for each unique data symbol accepted from the data source and phase shift modulates the frequency signals with a sequence of spread spectrum symbols, the frequency differences used being such that orthogonality is obtained over the receivers observation time of each spread spectrum symbol.

Modulator demodulator for binary digitally encoded messages

Abstract: In a communications system utilizing binary messages, a modulator for differentially modulating the messages, i.e. a change in the differentially encoded data is produced for every "one" in the data stream and no change occurs for every "zero" in the data stream, the differentially encoded data is modulated by a square wave signal having a predetermined number of cycles per bit of information, and the output is passed through a band pass filter to remove a substantial portion of the upper side band, and the DC. In the demodulator the modulated, differentially encoded message is applied to one input of an exclusive OR and the same message is delayed one bit and applied to a second input of the exclusive OR to remove the modulation and differential encoding, after which the message is integrated and shaped to place the message in the original binary form.

Demodulation system for digital information

Abstract: A demodulation system for digital information, recorded by an FM or PM system, which provides a clock output pulse whose pulse width is proportional to the time length of the preceding bit frame and said clock output signal is utilized for demodulating the recorded data. Said pulse width can be controlled either by an analog or a digital circuit and thus, the present invention has the advantage that the information is demodulated stably though the moving speed of the recording medium is changed or deviated.

Adjacent channel rejector

Abstract: An adjacent channel rejector for use in communication systems such as TACAN includes a high gain amplifier adapted to be driven into saturation in response to signals in the channel, a wide band filter/demodulator connected to the high gain amplifier and a narrow band filter/demodulator connected to the amplifier. The rejector further includes an amplitude comparator coupled to these filters/demodulators for determining when a desired signal is in the channel thereby providing adjacent channel rejection.

Carrier transmission system utilizing commercial power lines as transmission lines

Abstract: A transmitter circuit and a receiver circuit couplied to such other via commericial A.C. power lines both include a phase locked loop (PLL) arrangement. In the PLL arrangement associated with the transmitter circuit, sound signals are applied to a voltage-controlled oscillator included therein to produce frequency-modulated carrier signals, which in turn are transmitted to the A.C. power lines. On the other hand, the PLL arrangement within the receiver accepts the frequency-modulated carrier signals transmitted via the A.C. power lines. Demodulation of the frequency-modulated carrier signals is provided by such PLL arrangement.

Synchronous differentially coherent PSK demodulation

Abstract: A synchronous differentially coherent PSK communication signal is demodulated by applying the signal to a phase lock loop circuit whose output is sampled once every bit period in accordance with a sample control signal which is generated each time phase lock is lost as a result of the change in phase of the communication signal, as determined by comparing the phase of the communication signal with that of the phase lock loop tracking signal, phase lock being lost and restored during each bit period.

PSK modulator with reduced spectrum occupancy

Abstract: A premodulation filter is used to slow the transitions of a digital data signal to reduce spectrum occupancy. Carrier signal generator means provides in-phase and quadrature-phase carrier signals. A first modulation means receives the in-phase carrier signal for modulation by the filtered digital data signal. A second modulation means receives the quadrature-phase carrier signal and a control signal for modulation of the quadrature-phase carrier signal by the control signal. A combining means receives the modulated signals from the first and the second modulation means for providing a combined output signal. An amplitude limiter receives the combined output signal for limiting the amplitude variations in the combined signal. A circuit means receives the filtered digital data signal, and provides the control signal which adjusts the amplitude of the quadrature-phase carrier signal to complement the amplitude of the modulated in-phase carrier signal.

Compatible AM stereophonic receivers involving sideband separation at IF frequency

Abstract: Amplitude modulation (AM) stereophonic transmission receivers for reception of a radiant energy carrier wave with two stereo related signals, each appearing as a first order single-sideband, the carrier wave preferably being also modulated with an infrasonic frequency (e.g. 15 Hz) signal indicating stereo signal presence (with such infrasonic frequency modulation being either amplitude modulated or phase modulated). The upper sideband and the lower sideband portions of the received signal are separately detected and through voltage comparison means are applied to the development of automatic switching control of the detector outputs to deliver as receiver outputs either stereophonically related audio signals in the presence of different upper and lower sideband detector outputs or to deliver monophonically related (i.e. combined) detector outputs in the instance of reception of a monophonically modulated carrier wave. Such voltage comparison means, operating on the respective upper and lower sideband detector outputs, is advantageously activated (i.e. gated) by the presence in at least one detector output of the infrasonic modulation signifying the presence of stereo related modulation of the carrier wave, the output from such voltage comparison means also providing, in addition to the automatic switching of the detector outputs to stereophonically related audio receiver output mode as above indicated, a visual output (suitably in the form of a stereo presence/tuning light) directly and visually indicating to the receiver operator the presence of a stereo signal, and also providing the operator with a means to tune the RF portion of the receiver accurately to the carrier frequency, the proper RF tuning being indicated by maximal intensity of the visual output. Modified forms of receiver circuitry are presented wherein, in a first instance, the IF passband is defined by a double sideband filter and the intelligence is separated into upper sideband and lower sideband portions by separate upper sideband and lower sideband filters, and, in the second instance, the IF passband is defined simply by upper sideband and lower sideband filters with a portion of the filter outputs summated and the envelope thereof separately detected to provide a monophonic audio output and an AGC signal.

Time domain laser reconnaissance technique

Abstract: Method and apparatus for producing imagery of an object having distinct vertical geometry characteristics located at a reference plane remote from a scanning laser. In particular, a laser beam is utilized to scan objects at the reference plane and the optical signal reflected therefrom, corresponding to the reflectivity of the objects and background, is detected. An electrical signal, corresponding to the detected reflectivity signal, is coupled to an RF amplifier, the output of the RF amplifier being applied to an amplitude demodulator. The output of the amplitude demodulator, corresponding to the reflectivity and vertical geometry characteristics of the object at the reference plane, is applied to a recording device to produce the aforementioned imagery.

Delta modulation and demodulation with syllabic companding

Abstract: A delta modulator and demodulator are shown using double integration in the audio feedback loop and including a separate feedback loop for direct current offset compensation. The output bit pattern is analyzed to detect when an increase in companding stepsize is required. When required, a companding control voltage is generated and integrated at a syllabic rate. This syllabic integrator has a decay circuit providing a decay time approximately three times the attack time, thus insuring good voice tracking, acceptable noise trailoff, and at the same time causing exponential decay of digital errors at a syllabic rate. The companding control voltage is applied to a nonlinear current step generator simulating a logarithmic characteristic by piecewise linear current segments. Temperature-compensated transistor circuits are used in the step generator. Impedance isolation, combined with staged temperature compensation, provides stable and reproducible nonlinear current characteristics with standard integrated circuits.

Color-picture/multichannel-sound record and recording/playback apparatus and methods therefor

Abstract: Color picture information is recorded on a video disc together with multiple channels of sound information (e.g., a pair of stereo sound signal components). Each audio signal to be recorded modulates the frequency of a respectively different sound carrier over a respectively different low frequency deviation range. Picture carrier waves are frequency modulated in accordance with composite color video signals, including a luminance component and a chrominance component (the latter comprising a modulated color subcarrier buried in the midband of the luminance component). The duty cycle of the modulated picture carrier waves is varied in accordance with the sum of the respective frequency modulated sound carriers to form a recording signal. The color subcarrier frequency, and the respective sound carrier center frequencies correspond to different odd multiples of half the line frequency. The sound carrier deviation ranges lie below (but near) the band occupied by the chrominance component, and below the first order sideband of the modulated picture carrier. The black-representative picture carrier frequency is separated from the chrominance band upper boundary by a frequency substantially corresponding to the highest video signal frequency. A disc record is developed with a pattern of depressed and non-depressed regions in its groove bottom representative of the recording signal. Playback apparatus recovers from the disc record a playback signal of the general form of the recording signal. Respective bandpass filters, with passbands centered about the respective sound carrier deviation ranges, select playback signal portions for application to respective sound carrier FM demodulators. Additional filter, with passband encompassing picture carrier deviation range and first order sideband therefor, supplies input to picture carrier FM demodulator. Video signal processing circuits, responsive to output of picture carrier FM demodulator, include respective comb filters for selectively passing respective luminance and chrominance components.

Decision-Aided Tracking Loops for Channels with Phase Jitter and Intersymbol Interference

Abstract: The theory of decision-aided carrier tracking loops as developed by Lindsey and Simon is extended to include the effects of intersymbol interference. A major theoretical result is the solution of the Fokker-Planck equation for the probability density function (pdf) of the phase error for a first-order tracking loop. This function is used to average upper and lower bounds on the demodulator error rate so that an estimate of the overall system error rate may be obtained. The modulation technique analyzed is double-sideband-pulse-amplitude pulse-amplitude modulation. Curves are presented which display the threshold performance of the system error rate at low modulation index.