Showing papers on "Noise measurement published in 1981"

A Natural Pixel Decomposition for Two-Dimensional Image Reconstruction

Abstract: In two-dimensional image reconstruction from line integrals using maximum likelihood, Bayesian, or minimum variance algorithms, the x-y plane on which the object estimate is defined is decomposed into nonoverlapping regions, or "pixels." This decomposition of an otherwise continuous structure results in significant errors, or model noise, which can exceed the effects of the fundamental measurement noise.

Phase noise of single-mode diode lasers in interferometer systems

Abstract: Measurements have been made of the phase noise of six different types of single‐mode diode laser in an unbalanced Michelson interferometer, as a function of optical path difference. The frequency dependence of the phase noise has also been determined. Possible origins of the frequency fluctuations which result in the phase noise are discussed. The detrimental effect of the phase noise on the sensitivity and dynamic range of optical fiber interferometer sensors is briefly discussed.

Performance of the generalized cross correlator in the presence of a strong spectral peak in the signal

Abstract: Under consideration is the effectiveness of various windowing functions in the generalized correlator when a strong spectral peak, i.e., a sinusoid, is present in the signal. The windows W HBII (ω) and W SCOT (ω) or W R (ω) avoid the ambiguity problem that is encountered by the other windows when sinusoids are present in the signal.

Model for the power spectral density of quantization noise

Abstract: A model is given for the spectrum of the noise produced by passing a signal through a uniform quantizer. The model requires knowledge only of the amplitude distribution of the derivative of this signal. The model is compared with experimental results for a sinusoidal input signal and the sum of two sinusoids. Conditions are given under which the quantization noise spectrum is white.

Measurement of Losses in Noise-Matching Networks

Abstract: The noise contribution of an input-matching network to a low-noise amplifier is equal to the inverse of the network's available gain. The available gain of various networks at 4 GHz was computed from high-accuracy S-parameter measurements. The available gain of a typical tuner was experimentally found to be a strong function of its tuning, which shows that "back-to-back" measurements of two tuners to obtain the loss of each tuner can be inaccurate. Measurement of the available gain of an amplifier's input-matching circuit is shown to give quick insight into its minimum noise contribution before the actual amplifier stage is built.

Two procedures for estimating internal noise

Abstract: Internal noise was measured in two paradigms simultaneously. In one method the listener’s agreement in choosing the same masker as the one in a pair that sounds most signal‐like is used to estimate internal noise [D. M. Green, Psych. Rev. 71, 392–407 (1964)]. In the other the increment in detection performance on those trials having identical as opposed to different maskers is used to estimate internal noise [R. A. Siegel, unpublished Master’s thesis, M.I.T. (1979)]. Most results place the estimate of additive internal noise as nearly equal to the external noise variability. The estimates from the agreement method can be adversely affected by uncertainty regarding the observation interval and interval biases, whereas the estimates obtained with the detection method are highly sensitive to measurement errors.

Tracking Accuracies with Position and Rate Measurements

Abstract: Tracking accuracies for the radial component of motion are computed for a track-while-scan radar system which obtains position and rate data during the dwell time on a target These results will be of interest to persons developing trackers for pulse Doppler surveillance radars. The normalized accuracies, computed for a two state Kalman tracking filter with white noise maneuver capability, are shown to depend upon two parameters, r = 4?0/?aT2 and s = ?dT/?0. The symbols ?0 and ?d are the position and rate measurement accuracies, respectively, ?a is the standard deviation of the white noise maneuver process and T is the antenna scan time. The scalar tracking filter equations are derived and numerical results are presented. Lower steady state tracking errors plus the earlier attainment of steady state accuracies are the direct consequence of incorporating the rate measurements into the tracking filter.

Noise Identification and Removal in Positron Imaging Systems

Abstract: The noise (scattered and random) generated in two positron emission imaging systems has been studied and mathematically modeled as an exponential convolving function. Compton scattering is presumed to be a dominant factor, as opposed to random events. Deconvolution of the sensed data has enabled us to eliminate noise and to obtain images which can be quantified. Further, these images have improved diagnostic quality.

Improving the quality of a noisy speech signal

Abstract: In this paper we discuss the problem of reducing the noise level of a noisy speech signal. Several variants of the well-known class of “spectral subtraction” techniques are described. The basic implementation consists of a channel vocoder in which both the noise spectral level and the overall (signal + noise) spectral level are estimated in each channel, and the gain of each channel is adjusted on the basis of the relative noise level in that channel. Two improvements over previously known techniques have been studied. One is a noise level estimator based on a slowly varying, adaptive noise-level histogram. The other is a nonlinear smoother based on inter-channel continuity constraints for eliminating the so-called “musical tones” (i.e., narrow band noise bursts of varying pitch). Informal listening indicates that for modest signal to noise ratios (greater than about 8 dB) substantial noise reduction is achieved with little degradation of the speech qualify.

Multichannel adaptive filtering for signal enhancement

Abstract: An adaptive technique for enhancing a signal against additive noise is described. It makes use of two or more input channels containing correlated signal components but uncorrelated noise components. The various input signals need not be of the same waveshape, since the adaptive enhancer filters the inputs before summing them. The output is a best least squares estimate of the underlying signal in a chosen input channel. Adaptivity allows optimal performance even though the signal and noise characteristics differ from channel to channel and are unknown a priori. Formulas for signal distortion and output noise power are developed. The more input channels available containing correlated signal components, the better will be the system performance. Excellent performance is obtained when the sum of the filter input signal-to-noise ratios (SNR's), defined as functions of frequency, is large compared to unity at all frequencies of interest. In this case the output noise is small, the output signal distortion is small, and the output SNR is approximately equal to the sum of the filter input SNR's. As such, the multichannel adaptive signal enhancer is a generalization of the classic time-delay-and-sum beamforming antenna.

Integral expression for transforming signal-dependent noise into signal-independent noise

Abstract: A simple proof is given for an integral expression that permits the transformation of signal-dependent noise into signal-independent additive noise. This expression is used to derive transformations for film-grain noise that does not necessarily follow the square-root law and for fully developed speckle noise.

Handbook of aircraft noise metrics

Abstract: Information is presented on 22 noise metrics that are associated with the measurement and prediction of the effects of aircraft noise. Some of the instantaneous frequency weighted sound level measures, such as A-weighted sound level, are used to provide multiple assessment of the aircraft noise level. Other multiple event metrics, such as day-night average sound level, were designed to relate sound levels measured over a period of time to subjective responses in an effort to determine compatible land uses and aid in community planning. The various measures are divided into: (1) instantaneous sound level metrics; (2) duration corrected single event metrics; (3) multiple event metrics; and (4) speech communication metrics. The scope of each measure is examined in terms of its: definition, purpose, background, relationship to other measures, calculation method, example, equipment, references, and standards.

Gamma-Induced Noise in CCDs

Abstract: Analysis and measurement of gamma-induced noise in CCDs (Charge Coupled Devices) provides an approach to study of energy deposition patterns in solids. The results should be of interest to those concerned with predicting the biological effects of ionizing radiation (e.g., chromosome" aberrations), as well as for those interested in semiconductor devices. The CCD appears to be a valuable tool in these areas of microdosimetry. The theory of energy deposition and distribution in CCD structures is developed, and test results are shown which indicate model validation. The model described permits the determination of the noise as a function of pixel size, chip material, and incident photon energy.

Characterization of Frequency Fluctuations by Crosscorrelations and by Using Three or More Oscillators

Abstract: Summary The individual stability of an oscillator can be reached by using two auxiliary oscillators and comparing the three oscillators twotwo. The direct application of this principle gives erroneous results if the three comparisons are not made simultaneously. However the results are affected by the measurement system noises. New methods are proposed for comparing three oscillators. They use the concepts of crossvariance and crosscorrelation. They enable to characterize individually the oscillators and they reduce the influence of spurious noise sources, when uncorrelated. Therefore better resolution can be achieved in the frequency or phase fluctuation measurements.

Error Probability for Fading CPSK Signals in Gaussian and Impulsive Atmospheric Noise Environments

Abstract: Recently the performance of digital communication systems in the presence of impulsive atmospheric noise has been evaluated assuming a noise model which is in excellent agreement with experimental results. We evaluate the bit error probability for coherent phase-shift keying (CPSK) signaling assuming the same atmospheric noise model but considering a more reasonable representation of the communication channel which accounts for the simultaneous presence of Gaussian (always present) atmospheric noise and signal fading.

New technique for the direct measurement of core noise from aircraft engines

Abstract: A new technique is presented for directly measuring the core noise levels from gas turbine aircraft engines. The technique requires that fluctuating pressures be measured in the far-field and at two locations within the engine core. The cross-spectra of these measurements are used to determine the levels of the far-field noise that propagated from the engine core. The technique makes it possible to measure core noise levels even when other noise sources dominate. The technique was applied to signals measured from an AVCO Lycoming YF102 turbofan engine. Core noise levels as a function of frequency and radiation angle were measured and are presented over a range of power settings.

A performance analysis of adaptive line enhancer-augmented spectral detectors

Abstract: This paper discusses the receiver operating characteristic performance of Adaptive Line Enhancer (ALE) augmented spectral detectors for sinusoidal signals in both stationary white Gaussian noise and in nonstationary noise. The detectors considered are based on the discrete Fourier transform (DFT) and include both cases with and without incoherent integration. Analytical expressions are provided for the detector output probability density functions in the stationary noise case. Extensive Monte Carlo simulation results are used to verify these expressions, and to treat the nonstationary noise case.

Measurement of Audible Noise from Transmission Lines

Abstract: The audible noise produced by corona on high-voltage transmission iines has several characteristics which differentiate it from other more often encountered community noises, and which warrant a specialized document describing techniques for the measurement of transmission-line noise. This document covers the general characteristics of ac and dc transmission-line noise, instrumentation for its measurement, measuring and reporting procedures, and precautions in measurement.

Acoustic noise suppression in the context of a perceptual model

Abstract: An acoustic noise suppression algorithm has been developed which suppresses noise from speech by first filtering it into a set of signals which approximate the loudness components perceived by the auditory system. These signals are generated by passing the input stimulus waveform through a filter bank with frequency bandwidths which approximate the ear's critical bandwidths. The noise on each signal is then suppressed using spectral subtraction techniques in a domain of simulated perception. This approach to noise suppression retains the intelligibility produced by spectral subtraction methods while eliminating the accompanying musical quality.

A Fast Implementation of a Minimum Variance Estimator for Computerized Tomography Image Reconstruction

Abstract: Most present day computerized tomography (CT) systems are based on reconstruction algorithms that produce only approximate deterministic solutions of the image reconstruction problem. These algorithms yield reasonable results in cases of low measurement noise and regular measurement geometry, and are considered acceptable because they require far less computation and storage than more powerful algorithms that can yield near optimal results. However, the special geometry of the CT image reconstruction problem can be used to reduce by orders of magnitude the computation required for optimal reconstruction methods, such as the minimum variance estimator. These simplifications can make the minimum variance technique very competitive with well-known approximate techniques such as the algebraic reconstruction technique (ART) and convolution-back projection. The general minimum variance estimator for CT is first presented, and then a fast algorithm is described that uses Fourier transform techniques to implement the estimator for either fan beam or parallel beam geometries. The computational requirements of these estimators are examined and compared to other techniques. To allow further comparison with the commonly used convolution-back projection method, a representation of the fast algorithm is derived which allows its equivalent convolving function to be examined. Several examples are presented.

A Two-Channel Picture Coding System: I--Real-Time Implementation

Abstract: The two-channel system previously reported has been implemented in hardware using system parameters appropriate to consumer television. The basic system divides the spectrum into lowand high-frequency spatial components. The lows are coarsely sampled and finely quantized and the highs finely sampled and coarsely quantized using a companded, randomized quantizer. The purpose of this experiment was to demonstrate the potential of the system for low-cost practical application, to study the effect of the character of the randomizing noise, and to ascertain that there were no deleterious effects due to interlace, motion, or input noise. Theoretical noise calculations were qualitatively confirmed.

Impulse noise reduction system for TV receivers

Abstract: A television receiver having circuitry for reducing the effects of impulse noise in the video signal is controlled by impulse noise detection circuitry responsive to the accompanying sound signal. Since impulse noise is relatively broadband it will occur simultaneously in both the broadcast picture and accompanying sound signals. Amplitude variations of the frequency modulated sound signal are indicative of the presence of noise, which variations are detected and utilized to generate a control signal whenever the noise amplitude exceeds a predetermined threshold.

Integrating acousto-optic channelized receivers

Abstract: The subject of this paper is acousto-optic channelized receivers with large noncoherent processing gain. A receiver model is developed and the output statistics are derived for signal plus noise input. Noise equivalent bandwidths and signal detection sensitivity are calculated. The implications of large average noise power are discussed. Experimental measurements are in agreement with the theory presented.

Algorithms for Detecting M-Dimensional Objects in N-Dimensional Spaces

Abstract: Exact and approximate algorithms for detecting lines in a two-dimensional image space are discussed. For the case of uniformly distributed noise within an image space, transform methods and different notions of probability measures governing the parameters of the transforms are described. It is shown that different quantization schemes of the transformed space are desirable for different probabilistic assumptions. The quantization schemes are evaluated and compared. For one of the procedures that uses a generalized Duda-Hart procedure and a mixed quantization scheme, the time complexity to find all m-flats in n-space is shown to be bounded by O(ptm(n-m)2), where p is the number of points and t is a user parameter. For this procedure more true flats in a given orientation have been found and the number of spurious flats is small.

Error analysis of time delay estimation using a finite integration time correlator

Abstract: A methodology is presented for analyzing the error associated with time delay estimation using a finite integration time correlator, processing waveforms received at two separate sensors. The type of signal considered is a sinusoid whose amplitude is randomly modulated. The signals are assumed to be imbedded in additive Gaussian noise. Before they are correlated, the received waveforms are converted to a lower center frequency by mixers whose local oscillators are assumed to contain phase noise. By direct calculation in the time domain, the variance of the error in the time delay estimate is shown to be a function of integration time, signal-to-noise ratios, signal and noise bandwidths, and phase noise variance. The phase noise is shown to limit the accuracy of the time delay estimate. However, without phase noise, using the methodology the accuracy is shown to approach that obtained by the maximum likelihood estimator.

Gate current dependence of low-frequency noise in GaAs MESFET'S

Abstract: Characteristics of low-frequency noise generators in ion-implanted GaAs MESFET's on semi-insulating substrates were determined using measurements at 300°K and 105°K in the frequency range of 10Hz to 50KHz. The noise magnitude shows strong dependence on gate leakage current and its spectral response is a combination of both 1/f and 1/f2types. The high-field gate-leakage current dependence of excess LF noise suggests the tunneling of electrons into deep level defects and their subsequent thermal emission to the conduction band.