Showing papers on "Signal-to-noise ratio published in 1971"

Control-Loop Noise in Adaptive Array Antennas

Abstract: Adaptive array receiving antennas can be designed to sense the external noise field and to optimize the array illumination function. A substantial improvement in signal-to-noise ratio can be obtained with adaptive arrays when the external noise field is nonuniformly distributed in angle. The external noise process may be time varying and contain both discrete sources and continuously distributed sources. Two adaptive array implementations which maximize the signal-to-noise ratio are described in this paper. Expressions are derived for control-loop noise, i.e., the variance of the array element weights, and for the additional noise in the array output due to this element weight noise. It is shown that both the element weight noise and the array convergence rate are determined by the eigenvalues of the noise covariance matrix.

An Analysis of the Phase Coherent--Incoherent Output of the Bandpass Limiter

Abstract: Many applications of the bandpass limiter (BPL) involve coherent demodulation following the limiter. It is shown that as a result of demodulation, the signal mean and the noise variance are direct functions of the phase angle between the signal component passed by the BPL and the coherent reference. As a result, the relationship between the output and input signal-to-noise ratio may be significantly different than that obtained by Davenport for incoherent limiters. A study is also made of the output noise spectral density, and an approximate expressison is derived as a function of the input signal-to-noise ratio, reference phase angle, and the characteristics of the input bandpass filter to the limiter. Also discussed is the first-order signal-plus-noise probability density following coherent demodulation.

Delayed Encoding: Stabilizer for Adaptive Coders

Abstract: The decision process in source encoders can be influenced favorably by anticipating future quantizing errors and modifying the quantizer appropriately. This requires that the signal code be delayed slightly from the corresponding input signal sample. As an adaptation of an existing coder, little advantage is obtained [8]. However, the process has a stabilizing influence so that much stronger adaptation algorithms can be used to advantage, increasing the signal to quantizing noise ratio (S/N) markedly. It is believed that this fact is of general applicability, but it is shown herein only for 1-bit coders. A family of 1-bit coders (delta modulators) using exponentially adaptive step size, with two steps of integration in the feedback path has been studied using a special purpose computer facility. Such coders are ordinarily unstable and useless, but with error anticipation a measured S/N advantage of several dB over optimized adaptive coders of previous design is obtained. The study has concentrated on picture signals and an encoding which does not require a separate channel or code to signal changes in the coder. Fig. 8 compares the optimized delayed encoder operation with an optimized adaptive coder without delay. "Optimization" in the former case requires a modification of the feedback network-the use of two steps of integration instead of one. Delayed encoding is not just an improvement for existing differential coders, it promises to be a revolution in coder design.

Errors in Time Delay Measurements

Abstract: Simple delay and sum of sensors in a seismic array is an effective method for noise suppression. However, unless we have precise steering delays, much of the signal energy is lost during the beam forming process too. We have investigated possible error sources in time delay measurements, using a computerized cross-correlation procedure. Parameters perturbed are correlation window length and positioning, signal frequency content and signal to noise ratio (SNR). Our results indicate that relative low frequency waves and using the very first part of theP-signals give the most reliable and stable time delay values. High frequency bandpass filtering improves SNR, but signal correlation and the precision in beam steering corrections decrease. Significant loss of high frequency energy during beamforming seems to be unavoidable.

Data-Aided Carrier Tracking Loops

Abstract: An innovation which improves the capability and performance of certain types of phase-coherent demodulators is introduced. This innovation has application in the fields of telemetry and tracking, e.g., relay satellite systems, deep-space communications, and military communication systems. The basic idea of the innovation centers around using the power in the composite signal sidebands to enhance the demodulator's effective signal-to-noise ratio (SNR). The vehicle through which this is accomplished employs the principle of decision-directed feedback. In its simplest form it is shown, for the first-order loop, that the maximum improvement in loop SNR relative to a standard phase-locked loop is 10 dB. The paper explores and illustrates the corresponding improvement realized in detection system efficiency. Extension to higher order loops follows directly from the analysis given here.

Performance Analysis of DPSK Systems in Both Thermal Noise and Intersymbol Interference

Abstract: In this paper a useful and mathematically rigorous method is presented for exactly evaluating the error-probability performance of a binary differentially coherent phase-shift-keyed (DPSK) system in the presence of both thermal noise and intersymbol interference. The channel filter and the receiver input signal-to-noise ratio are assumed to be given. This method suggests, first, to calculate the probability of error induced by both the thermal noise and the large interfering samples whose normalized amplitudes with respect to noise standard deviation are greater than unity, and then, to add the correction terms due to the presence of the other intersymbol interference samples. If the normalized interference amplitudes with respect to the noise standard deviation are less than unity (although the total interference power may still be greater than total noise power), then this method simply becomes that which first evaluates the error probability induced by thermal noise alone and then adds the correction terms due to the presence of intersymbol interference. For numerical computations, various recurrence relations are developed so that the probability of error can be easily obtained on a digital computer. These relations have been utilized in an example to calculate the performance of certain practical PSK and DPSK systems.

Power-noise characterization of phase-locked IMPATT Oscillators

Abstract: IMPATT diode characterization on the basis of output power and the corresponding FM noise figure over a range of operating conditions is presented. The characterization consists of families of power noise curves obtained for a phase-locked IMPATT oscillator where the supply current, load conductance, and the operating frequency are parameters. It is shown that the maximum output power and minimum FM noise are not achieved concurrently. In particular FM transmitter application, it is shown that the best performance for each type of diode was obtained when operated at less than maximum power (and at reduced efficiency) where the system benefits from the attending lower noise. Better system performance, this application, was obtained with the GaAs IMPATT diode. The power-noise characterization defines the optimum operating conditions for an IMPATT diode and provides a valid basis for the comparison of diodes for specific applications.

Bearing estimation error with a linear array

Abstract: This paper discusses the rms error in estimating the bearing of a source with a linear array in a noisy environment. The techniques used are the Cramer-Rao (CR) bound and techniques developed recently by Ziv and Zakai. These new techniques give improved results at low signal-to-noise ratio. Below a certain threshold level of signal-to-noise ratio, the error increases faster than the CR bound predicts. This error is shown to be related to the sidelobe structure of the array. The variation of bearing error with array design is considered.

Bounds on subjective performance measures for source encoding systems

Abstract: Quantizing noise is present whenever analog information is encoded into digital form suitable as an input to any digital system such as a computer or digital transmission line. The subjective impairment caused by this noise is frequently measured by the ratio of signal power to frequency-weighted quantizing noise power S/N_Y . An upper bound on S/N_Y is found such that source encoding systems will always have values of S/N_Y less than this bound. The bound has the form (in decibels) S/N_Y \leq T_B + T_P + T_S , where T_B is a constant that depends on the hit rate of the signal, T_P depends on the redundancy (or predictability) of the signal, and T_S depends on subjective considerations (as embodied in a subjectively determined frequency-weighting function). The bound is applied to source encoding systems for speech and television signals. By using the frequency-weighting function, bounds on commonly used measures of subjective impairments are possible.

Signal-to-noise ratio determination circuit

Abstract: A signal-to-noise ratio (SNR) determination circuit to determine the SNR of an input having signal components (S) within a given frequency range and noise components (N), without actual measurement of the noise (N) components. Input means receive the input, and first filter means having a frequency bandpass range different from said given frequency range are connected to the input means to convert the db level of the input SNR to another level. Bandpass limiter means having a constant signal plus noise output level are connected to the output of the first filter means, the signal-to-noise ratio of the input to the bandpass limiter means being linearly related to the dbm level of signal components at the output thereof, for a given db range of converted SNR input levels. Calibrating means are connected to the bandpass limiter means and are responsive to the signal components at the output thereof to derive the SNR of the input to the determination circuit. The SNR determination circuit is disclosed for use in a diversity receiver having a plurality of input channels. Weighting means are used to weight the channels prior to summation to provide maximum signal-to-noise ratio at the output of a combiner.

A Self Bit Synchronizer Matched to the Signal Shape

Abstract: A suboptimum self bit synchronizer is considered that matches the signal shape, i. e., operates making a clever use of the knowledge of the signaling waveform. The noise performance of this system is analyzed for high signal-to-noise ratios, and an expression is obtained for the variance of the timing errors. This variance is compared with that of systems proposed by other authors on the basis of equal closed-loop noise bandwidth and equal signal shape. It is found that the use of this matched synchronizer enables appreciable timing jitter reductions.

Measurement of signal-to-noise ratio in hologram reconstructions, by vibration interferograms

Abstract: This paper reports the use of vibration interferograms reconstructed from time-average holograms to measure signal-to-noise ratios in hologram reconstructions. Comparisons are made between black silver holograms and two common types of bleached holograms, both of which are shown to offer improvement over black silver.

Threshold Analysis of Phase-Locked-Loop Demodulators Using Most Likely Noise

Abstract: A new method for determining the number of spikes per second, and hence the threshold carrier-to-noise ratio (CNR), of the phase-locked-loop demodulator is presented. This technique can also be applied to the frequency demodulator with feedback. This method is applicable when nonwhite noise is present at the input of the demodulator and also when the carrier is frequency modulated. Computer processing is employed so that rapid comparisons of the phase-locked-loop (PLL) systems can be made. The results show the spikes per second and the threshold CNR, without modulation, for the first-, second-, and third-order PLL and FM discriminator. Experimental results are then presented.

Signal-to-noise ratio improving device for receiving systems having two wave collectors

Abstract: A signal-to-noise ratio improving device for processing narrow frequency band signals received by two wave collectors (transducers or antennas) and respectively delivered at one and the other of two input terminals in the form of identical useful signals on each of which a different quasi-stationary noise is superposed. The device forms the sum and difference of the noiseaffected signals received at said input terminals and the said difference is processed in a circuit comprising amplitude regulating means, first and second correlators each having two inputs to one of which is fed the said sum and to the other of which are applied, for said first correlator, said amplituderegulated difference and, for said second correlator, the latter said difference delayed by a fixed time interval. The correlators deliver at their respective outputs signals proportional to the time average of the product of the signals applied to their two inputs. The averaged signals are respectively applied to a first input of each one of two multipliers, to a second input of which are respectively applied said amplitude-regulated difference and said delayed amplitude-regulated difference. The outputs of the multipliers deliver new signals which are applied to the inputs of an adder, and the output signal from said adder is applied to an input of a subtractor, to the other input of which said sum of noise-affected signals is applied. The output of the subtractor delivers the improved signal.

Detector for reading bar codes on moving articles and having improved signal-to-noise ratio

Abstract: A detection system is provided of the type used for reading bar coded indicia on articles passing on a moving conveyor and for emitting a corresponding output signal to be used in subsequently manipulating the articles. To reduce noise in the output signal and so to provide an improved signal-to-noise ratio and more accurate manipulation of the articles in response to the code, the detection system is arranged to be selectively responsive not only to the frequency characteristic of the spacing of the coded bars, relative to their speed of passage, but also similarly responsive to the frequency characteristic of the width of the bars in the code. This predetermined detection frequency is established in the coded indicia by giving each individual bar a uniform width dimension, preferably equal to the spacing between adjacent bars. The detector includes a notch filter passing this predetermined frequency so that wide band noise and spurious signals developed at other frequencies will be eliminated from the output signal.

Spectrum-shape discriminating on/off target indicator

Abstract: A spectrum-shaped discriminating on/off target indicator which indicates if a signal simultaneously meets prescribed conditions on power level and spectrum shape. Signal power level and spectrum width are simultaneously measured and a DC logic output is obtained only for input signals which have a signal-to-noise ratio equal to, or higher than a preset minimum level though the signal "noise bandwidth" meets the prescribed conditions.

Fluctuating Target Detection in Clutter Using Sidelobe Blanking Logic

Abstract: In an earlier paper, Maisel [6] considered two-channel detection systems using a sidelobe blanking logic when a nonfluctuating target was present. This paper is an extension of the earlier work to include fluctuating targets. The Swerling I, II, III, and IV models are considered when single-pulse detection is of interest. An adaptive threshold procedure is also briefly discussed whereby the probability of false alarm at any given resolution cell is maintained constant, even though the input clutter level may vary from cell to cell or from beam position to beam position. Useful data are presented for detection probabilities in the range 0.5 to 0.9, for false alarm probabilities in the range 104 to 10-8, and for a false detection probability of 0.1 for a sidelobe target yielding an apparent signal to total noise power density ratio of 13.0 dB in the main beam receiver.

Signal-to-noise ratio measurement

Abstract: System and apparatus for the measurement of signal-to-noise ratio in electrical signals. An adjustable meter is set to the signal level or known fraction thereof and a square wave is added to the signal for display on an oscilloscope. A pair of vertically displaced oscilloscope traces, both containing the signal and the noise, are displayed and the square wave amplitude is adjusted to move the traces into contiguous or tangent relationship. The measured adjusted square wave amplitude is the signal-to-noise ratio of the electrical signal.

Error Probabilities Due to Impulsive Noise in Linear and Hard-Limited DPSK Systems

Abstract: A method previously presented by the authors for the evaluation of error probabilities in digital systems when impulsive noise is the main cause of incorrect decisions is here applied to differential phase-shift keying (DPSK) modems. Specifically, the receiver impulsive characteristic, which is proportional to the error rate, is evaluated for binary DPSK systems both in the linear and hard-limited modes of operation. Two encoding systems are considered, in-phase encoding and quadrature encoding, and it is shown that they yield essentially the same performance, at least when the binary symbols are equally likely. The results are compared with the performance of phase-shift keying (PSK) and it is found that, in the presence of impulsive noise, DPSK systems have, for the same SNR, an error rate which is nearly twice the error rate obtained for PSK. However, the dependence of error rate on SNR is generally a slowly decreasing function so that a considerable improvement in SNR is required to improve performance.

On the Performance of Some Sequential Multiple-Decision Procedures

Abstract: The performance of several sequential procedures for the following multiple-decision problem is investigated. Samples from k random processes (or populations) are available, k at a time (one from each process), to a receiver or data processor. One process contains a signal; the other k - 1 are statistically identical noise. The receiver is to select the odd process (locate the signal), with prescribed probability of error. The optimal receiver makes the selection in minimum average time. Analytical and Monte Carlo computations were performed under the hypothesis that the processes sampled are Rayleigh; however, a method for extrapolating results to other cases is given. The parameter k is allowed to vary from 2 to 1000.

Transmitter-Receiver Optimization for Active Signaling over Undersea Acoustic Channels

Abstract: A performance coefficient corresponding to an extended form of the signal-to-noise ratio at the output of an estimatorcorrelator detector is introduced and discussed. A suboptimum approach is then considered to the detection of signals transmitted through a purely random channel and corrupted by additive Gaussian noise, with arbitrary signal-to-noise ratios at the channel output. An estimator-correlator detector is postulated and the linear filter that maximizes the performance coefficient is determined. This performance coefficient is then once more maximized with respect to the transmitted waveform and the results of this analysis are applied to cases characterized by arbitrary large signal-to-noise ratios and Gaussian quasi-stationary undersea acoustic channels.

A 36-Mbit/s Television Codec Employing Pseudorandom Quantization

Abstract: The paper describes an experimental system constructed to allow investigation of pseudorandom dither signals for coarse quantization of 625-line monochrome television siginals. The system employs a separate encoder/decoder for the television synchronizing pulses so that the picture signal can be allowed to occupy the full nonsaturating range of the video encoder. Dither samples are added to the video signal before coding and subtracted at the decoder and are negatively correlated to maximize the picture signal-to-noise ratio according to a subjective noise weighting function. The power spectral distribution of the quantizing noise is derived and, assuming this to be picture independent and random, pre- and deemphasis networks are used to increase the signal-to-noise ratio. Empirical methods are used to determine the network parameters. The results of preliminary subjective experiments with the 3 bits/sample pseudorandom codec are given for a sampling frequency of 12 MHz and indicate the practical limitations of a theory which predicts a high standard of picture quality.

Apparatus for improving the signal-to-noise ratio of a received signal

Abstract: Device for improving the signal/noise ratio of a common signal received on three aerials utilizing correlation between the sum and difference values of combinations of the signals from the three aerials to eliminate or substantially suppress the noise received with the common signal.

Coherent optical processing method and system having improved signal-to-noise ratio utilizing polarizing filters

Abstract: A coherent optical processing method and system for improving signal to noise ratio. A coherent optical beam which can be polarized or unpolarized is split into two component beams which are then polarized to two separate orthogonal states of polarization. Each component beam is operated upon to impart information thereto or sent through a transmission channel during which operation and transmission a small portion of each component beam is unavoidably scattered and depolarized. Each component beam is thereafter optically filtered with a polarizer aligned with the polarization state of each component beam, thereby removing approximately half of the depolarized noise portions of each component beam. The filtered component beams are then recombined, forming a polarized beam with a noise component depolarized with respect to the polarization beam. The combined polarized beam is again filtered in a polarizer aligned with the polarization state of the combined beam to reduce the depolarized noise portion again by about half. The resultant polarized beam has a signal to noise enhancement of about four by the inventive system.

Phase-Locked Loop Bandwidth and Signal-to-Noise-Ratio Requirements for Deep-Space Communications

Abstract: Properly designed phase-locked loops play a key role in deep-space communication links. The interrelationships among the loop band-width, acquisition time, and the loop signal-to-noise ratio are established for a given probability of acquisition. The Doppler rate tracking capability of the second-order loop is considered. The probability of time to lose lock is derived by the Fokker-Planck techniques. The results of the study are used to determine the loop bandwidth and the signal-to-noise ratio requirements of Venus and Jupiter probes' communication links.

Magnetic pickup with feedback circuit

Abstract: A magnetic pickup having an electronic switch circuit for generating a high level digital signal having improved signal form and an improved signal to noise ratio. The switch has an input transistor connected to a sensing coil and an output transistor which generates a digital output signal. The circuit further includes a feedback connection between the output transistor and the input transistor to stabilize operation for low level voltages induced in the pickup coil.

Method and apparatus for increasing seismic signal-to-noise ratio

Abstract: An improved technique for enhancing the signal-to-noise ratio in the automatic processing of seismic data is provided according to the present invention. Signals from horizontally and vertically oriented geophones, representing two functions of time, are separately recorded. The product of these two signals is formed as a signal representative of a third function of time. The product signal thus formed is then used in various ways as an ''''enhancement function'''' signal, to attenuate noise components and enhance seismic signal components. One technique for utilizing the enhancement function signal involves half-wave rectifying the enhancement function signal and then multiplying the rectified signal by the original signal from the vertically oriented geophone. Another technique involves using the enhancement function signal to produce a gating signal to cancel the original output signal from the vertically oriented geophone whenever the enhancement function signal is other than negative.

Thermal Imaging Performance of Passive Infrared Scanners

Abstract: An analytical base for assessing the performance of thermal mapping systems suitable for remote environmental sensing is provided. For systems that operate in a pulsed mode and are limited by photon noise, the signal-to-noise ratio can be expressed simply in terms of an effective signal irradiance which involves the whole spectral region of interest, and a single number which is the system's noise-equivalent irradiance (NEI) at the peak-response wavelength of the detector. A compact expression of the NEI is derived for background fluxes representative of situations encountered with contemporary radiation-shielded detector packages operating in the earth's atmosphere. In conjunction with the definition of an effective signal irradiance for extended targets, the concept of a figure of merit for passive thermal-imaging equipment is introduced, and implications for system tradeoffs are discussed. The analysis is applied to evaluate an 8- to 14-?m airborne scanner from the point of view of angular resolution and thermal sensitivity; limitations imposed by optical diffraction effects, detector time constant, and detector size requirement are also considered. Angular resolutions of, say, 0.1 to 0.2 mrad exemplify the capabilities of the current generation of infrared imaging sensors.

On the signal-to-noise ratio of nuclear magnetic resonance oscillator spectrometers

Abstract: General expressions are obtained for the amplitude noise and frequency noise of steady-state oscillators. The nmr signal-to-noise ratios in the am and fm modes are then calculated. It is found that fm detection of nmr signals is insensitive to 1/f oscillator noise. Some measurements of noise and signal-to-noise ratio are reported and are shown to be in agreement with theory.