Showing papers on "Noise measurement published in 1982"

The prediction of helicopter rotor discrete frequency noise

Abstract: An accurate prediction of the noise produced by helicopters requires a good understanding of the noise generating mechanisms involved Such an understanding can lead to controlling the noise of existing helicopters by avoiding noisy regimes of flight or by redesigning the main and tail rotors The present investigation is concerned with approaches which are suitable for the calculation of discrete frequency noise of helicopter rotors The governing differential equation of acoustics used in a consideration of acoustic formulations is the Ffowcs Williams-Hawkings (FW-H) equation Attention is given to a method reported by Farassat (1981), a method developed by Succi (1979), and a procedure discussed by Woan and Gregorek (1978)

A Fast and Reliable State Estimation Algorithm for AEP's New Control Center

Abstract: This paper presents an evaluation of some previously proposed and newly developed state estimation algorithms. One new algorithm, which employs a constant, decoupled gain matrix and certain other simplifying approximations, is shown to be clearly superior to the other methods considered; accordingly this algorithm has been chosen for use in AEP's new control center. The selected algorithm is fast, reliable and can process a measurement set consisting of line flows, bus injections and voltage magnitudes. Test results are given for several real networks and some standard IEEE test systems (having as many as 460 buses and R/X ratios ranging from 0.1 to 2). The network conditions used in the tests range from light load to severe contingencies with low voltages and large phase angles. A wide variety of practical and hypothetical measurement sets are considered.

Power System Tracking State Estimation and Bad Data Processing

Abstract: In this paper the tracking of the time yarying power system static state is analysed. The relative importance of the small measurement noise and the eventual occurrence of gross error in the measurements is used as a design criterion for the development of fast tracking estimators. The basic approach uses a pre-estimation bad data detection/ elimination scheme based on the exponential smoothing of past estimations and logical checks followed by an estimation stage. Three different estimation algorithms were tested: plain weighted least squares, quadratic square-root and linear criterion. Numerical results showing the performance of the three estimators in a simulated test case are also presented.

Nonlinear Restoration of Noisy Images

Abstract: The restoration of images degraded by an additive white noise is performed by nonlinearly filtering a noisy image. The standard Wiener approach to this problem is modified to take into account the edge information of the image. Various filters of increasing complexity are derived. Experimental results are shown and compared to the standard Wiener filter results and other earlier attempts involving nonstationary filters.

A technique for improving detection and estimation of signals contaminated by under ice noise

Abstract: Recent analyses of FRAM II arctic data have shown that under ice ambient noise can be at times highly impulsive and non‐Gaussian. The analyses included time domain statistical measurements which were consistent with previously reported results of experiments made within the Canadian Arctic Archipelago. New findings in the frequency domain based on skew, kurtosis, and cumulative distribution function estimates, also indicate the existence of strong non‐Gaussian noise. It is known that the ability to detect and estimate signals contaminated with non‐Gaussian noise using conventional processing is degraded compared with optimum techniques which utilize knowledge of the noise statistics. Results comparing the performance of conventional and nearly optimum signal processing methods are presented using the FRAM II data.

System identification techniques for adaptive noise cancelling

Abstract: A general form of a noise canceller is derived, using a signal modeling approach The proposed structure combines two infinite impulse response filters: a noise canceller and a line enhancer A recursive prediction error algorithm is used to adaptively estimate the filter coefficients Preliminary simulation results on the performance of the generalized adaptive noise canceller are presented

Optimal Detection of Digitally Modulated Signals

Abstract: In this paper optimal detectors are derived and analyzed for the general class of digitally modulated signals in which the sequence of symbols is unknown a priori and information data are not of interest. The detectors test the signal present condition in background white Gaussian noise versus the null condition of noise alone. Particular attention is focused upon cases in which the SNR per symbol is low compared to unity. The models employed herein are sufficiently general to include most forms of spread-spectrum signals as well as other digital type communication signals.

Signal detection in the presence of weakly dependent noise--I: Optimum detection

Abstract: The problem of designing robust systems for detecting constant signals in the presence of weakly dependent noise with uncertain statistics is considered. As in Part I of this study, which treated optimum detection in weakly dependent noise, a moving-average representation is used to model the dependence structure of the noise process, with the degree of dependence being parameterized by the averaging weights. Weak dependence is then modeled as the situation in which quantities depending to second or higher order on the averaging weights can be considered to be negligible. Uncertainty in the noise statistics is introduced within this framework by allowing a general type of uncertainty in the univariate statistics of the independent sequence that drives the moving average. To find robust detectors for signals in this type of weakly dependent noise environment, related results concerning robust location estimation in an analogous dependent situation are applied to modify a robust detection system for the corresponding independent-noise case. It is argued here that a robust detector for this dependent noise model is characterized by a least favorable noise distribution which coincides with the distribution that is least favorable for the corresponding independent-noise case. However, the resulting detector design for dependent noise differs from that for independent noise; in particular, the robust detector for dependent noise is based on a linearly corrected version of the influence curve that defines the independent-noise robust detector. The worst-case performance of the proposed robust detector relative to that of the independent-noise robust detector is also analyzed, with the conclusion that the performance of the proposed technique is better, to first order in the averaging weights, in this respect. A modification of this robust detector is also proposed which eliminates some practical disadvantages of this system while retaining equivalent performance to first order. The specific situation of contaminated Gaussian noise is treated in order to illustrate the analysis.

Some Noise Comparisons of Data-Collection Arrays for Emission Tomography-Systems Having Time-of-Flight Measurements

Abstract: The noise performance of an emission tomography system having time-of-flight measurements is shown in several examples to be superior for a confidence-weighted data array compared to a most likely position data array. The examples range from a point to a planar distribution of radioactivity, and include a crude model of the left ventricle of a heart containing radioactive palmitate.

Noise reduction system for television signals

Abstract: In a system for noise reduction in a television signal utilizing a full frame picture store to enable averaging of corresponding signals of successful frames for noise reduction and having also a motion detector, the signal representing motion is passed through a combination of a low-pass filter and a threshold circuit of which, respectively, the cut-off frequency and the threshold value are controllable as a function of the noise amplitude. As a result, at low noise content of the input signal, no disturbing loss of sharpness of moving edges occurs, whereas at higher noise amplitudes, although noise reduction is at the cost of some loss of edge sharpness, the annoying effects of having noise reduction controlled by the noise itself, are avoided.

ARMA Modeling of Time Series

Abstract: A method for efficiently generating a rational model of a wide-sense stationary time series is presented. In this method the autoregressive parameters associated with an ARMA model consisting of q zeros and p poles are optimally chosen with the selection being based on a finite set of time series observations. This selection is made so that a set of Yule-Walker equation approximations are ``best'' satisfied. The resultant autoregressive parameter estimates have the desired statistical feature of being unbiased and consistent. This estimation method has been found to provide a modeling performance which typically equals or exceeds that of contemporary alternatives. Moreover, this method is amenable to a computationally efficient adaptive solution procedure. The autoregressive parameters characterizing the resultant ARMA model estimate can serve the role of decision variables in pattern classification schemes. For example, these parameters can be utilized in determining whether or not a member(s) of a given signal class is contained within a noise corrupted measurement signal. This approach has been found to be particularly effective in Doppler radar and array processing applications in which one is looking for the presence of spectral lines (i.e., sinusoids) in the measurement signal.

Automatic Noise Temperature Measurement through Frequency Variation

Abstract: The dependence of two-port noise temperature on the source reflection factor does not lend itself to easy automated measurement. This paper shows that a noise analysis performed over a small frequency interval centered about the frequency of interest and with a source circuit having fast phase variations leads to a straightforward solution of the problem. The conditions for applying the procedure are broad enough to enable measuring most components like transistors and amplifiers over the entire microwave range. An example of practical implementation is presented.

The Exact Noise Figure of Amplifiers with Parallel Feedback and Lossy Matching Circuits

Abstract: Exact formulas for the noise parameters and noise figure of amplifiers with parallel feedback and lossy input and output matching circuits are derived. The formulas which take into account the thermal agitation of all circuit elements are applicable to feedback and lossy match amplifiers, as well as amplifiers that employ both principles simultaneously.

Estimating Noise Variances by Using Multiple Observers

Abstract: Unknown variances of the noises that excite a time-invariant, linear dynamic system and/or in the observation of its output can be estimated by use of multiple observers. One observer is needed, in general, for each unknown variance. Each observer is time invariant and has different gains from the others. It is shown that each unknown variance is a linear combination of the variances of the residuals of the observers. The required estimates of the noise variances are obtained by using the measured variances of the residuals. The method presented in this paper is illustrated by an application to determining noise parameters in a ring laser gyro.

Investigation of continuously traversing microphone system for mode measurement

Abstract: The continuously Traversing Microphone System consists of a data acquisition and processing method for obtaining the modal coefficients of the discrete, coherent acoustic field in a fan inlet duct. The system would be used in fan rigs or full scale engine installations where present measurement methods, because of the excessive number of microphones and long test times required, are not feasible. The purpose of the investigation reported here was to develop a method for defining modal structure by means of a continuously traversing microphone system and to perform an evaluation of the method, based upon analytical studies and computer simulated tests. A variety of system parameters were examined, and the effects of deviations from ideal were explored. Effects of traverse speed, digitizing rate, run time, roundoff error, calibration errors, and random noise background level were determined. For constant fan operating speed, the sensitivity of the method to normal errors and deviations was determined to be acceptable. Good recovery of mode coefficients was attainable. Fluctuating fan speed conditions received special attention, and it was concluded that by employing suitable time delay procedures, satisfactory information on mode coefficients can be obtained under realistic conditions. A plan for further development involving fan rig tests was prepared.

Performance of an Automated High Accuracy Phase Measurement System

Abstract: : A fully automated measurement system has been developed that combines many properties previously realized with separate techniques. This system is an extension of the dual mixer time difference technique, and maintains its important features: zero dead time, absolute phase different measurement, very high precision, the ability to measure oscillators of equal frequency and the ability to make measurements at the time of the operator's choice. For one set of design parameters, the theoretical resolution is 0.2 ps, the measurement noise is 2 ps rms and measurements may be made within 0.1 s of any selected time. The dual mixer technique has been extended by adding scalers which remove the cycle ambiguity experienced in previous realizations. In this respect, the system functions like a divider plus clock, storing the epoch of each device under test in hardware.

Enhancement of sinusoids in colored noise and the whitening performance of exact least squares predictors

Abstract: The extraction of sinusoids in white noise using least squares predictors has attracted a lot of attention in the past, mainly in the context of the adaptive line enhancer (ALE). However, very few results exist for the practical colored noise case or for the whitening performance of the predictors. We use a matrix formulation to derive the optimal least squares coefficients and frequency response of the D-step predictor or ALE for sinusoids (real or complex) in additive colored noise. Several cases are considered, and in particular, new formulas for the amplitude gain are obtained. In low-pass background noise, the amplitude gain of the sinusoids becomes essentially a monotonically increasing function of their frequency, and a decreasing function for high-pass noise. For the whitening application, signal-to-noise ratio (SNR) bounds of the output are derived when the input is a white signal plus a sinusoidal interference. We also give a state-space model and stochastic interpretations of our analysis of the D-step predictor, providing connections to other related areas. To enable filtering of nonstationary complex inputs, as well as multichannel and multiexperiment data, a complex vector version of the ladder algorithm is presented that can be used to implement the ALE, noise cancelling, and noise inversion for narrow-band interference rejection.

White noise of MOS transistors operating in weak inversion

Abstract: A theory is given for the thermal noise of MOS transistors in the weak inversion regime. For MOS transistors with low surface-state density, the relation for the thermal noise in saturation is shown to be the same as a shot noise relation. The theory is compared with measurements and the origin of the white noise in MOST's operating in weak inversion is discussed.

A generalized comb filtering technique for speech enhancement

Abstract: Because of speech signals nonstationarity, usual comb filtering of noisy speech signals results only in a modest improvement in signal to noise ratio, and only in a small perceptual reduction of structured noise or interference. A generalized comb filtering technique, which applies a time-varying weighting to each pitch period, is mathematically analyzed and shown to be capable of breaking up the noise structure, in addition to comb filtering. This is found to provide a meaningful perceptual improvement when the noise or interference are structured. The mathematical analysis is facilitated by using a polyphase network model of the generalized comb filter. Design constraints and rules are developed and several filter families are proposed. Computer simulation results are discussed.

Evaluation of a digital subtraction angiography unit.

Abstract: The operating characteristics of a digital subtraction angiography unit were evaluated. Measurements included image intensifier and system linearity, uniformity, quantum and system noise, and iodine contrast. Various approaches to noise measurement were analyzed and tested, leading to the following conclusions: the video level should be maximized; changes in signal-to-noise ratio (SNR) due to changes in x-ray exposure do not necessarily reflect changes in quantum SNR alone, due to dependence of video levels on x-ray exposure; an understanding of the separate contributions of quantum and video SNRs to the total SNR guides proper selection of variables to minimize patient dose and x-ray tube heat loading.

A preliminary comparison between the SR‐3 propeller noise in flight and in a wind tunnel

Abstract: The noise generated by supersonic-tip-speed propellers is addressed. Models of such propellers were tested for acoustics in the Lewis 8-by-6-foot wind tunnel. One of these propeller models, SR-3, was tested in flight on the Jetstar airplane and noise data were obtained. Preliminary comparisons of the maximum blade passing tone variation with helical tip Mach number taken in flight with those taken in the tunnel showed good agreement when corrected to the same test conditions. This indicated that the wind tunnel is a viable location for measuring the noise of these propeller models. Comparisons of the directivities at 0.6 and 0.7 axial Mach number showed reasonable agreement. At 0.75 and 0.8 axial Mach number the tunnel directivity data fell off more towards the front than did the airplane data. A possible explanation for this is boundary layer refraction which could be different in the wind tunnel from that in flight. This may imply that some corrections should be applied to both the airplane and wind tunnel data at the forward angles. At and aft of the peak noise angle the boundary layer refraction does not appear to be significant and no correction appears necessary.

Rotorcraft noise prediction

Abstract: The purpose of this presentation is to give a general strategy for rotorcraft noise prediction. This strategy is expressed through a modular software system design rather than theoretical analysis of the aerocoustic phenomena. The crucial design choices in a software system design are the module interface definitions. An interface is the data that are passed from one module to another. A module takes data from one (input) interface and transforms it, through a prediction method, to another (output) interface. In system design, the method is less important than the interface. The two types of methods available may be braodly classified as empirical or analytical, although no method is purely one or the other. These two general approaches will be compared as they apply to rotorcraft noise prediction.

Comments on "Simultaneous determination of device noise and gain parameters through noise measurements only"

Abstract: A novel method of measuring the noise and gain parameters of a linear two-port is presented. This method is based on the Friis formula to determine all the noise and gain parameters solely from noise figure measurements. This results in a much simplified procedure and improved accuracy over the conventional methods. An example using a low-noise transistor at 2 GHz is shown.

Determination of Microwave Transistor Noise and Gain Parameters through Noise-Figure Measurements Only

Abstract: A novel method for measuring noise and gain parameters of linear two-ports solely from noise-figure measurements is applied here to perform noise and gain characterization of microwave transistors versus frequency and collector current in S-band. The method results in a simpler procedure and improved accuracy compared to conventional methods. In addition, a technique to estimate the loss of the input tuner of the measuring setup is presented, which yields a further improvement in accuracy. As experimental verification, the noise and gain parameters of a microwave transistor versus collector current in the 2-4-GHz frequency range are reported.

Practical Aspects of the Low Frequency Noise Problem

Abstract: Low frequency sound is omnipresent and although our ears do not readily detect it, the noise spectrum measured in even the quietest location is dominated by energy at the low end of the spectrum (Fig. I). However, very often, and certainly much more frequently than is currently recognised, the levels of low frequency noise are enough to interfere with people to a degree sufficient to cause significant misery, distress and economic penalty.

Spectral analysis of impulse noise for hearing conservation purposes

Abstract: Damage-risk criteria for impulse noise does not presently take the spectrum of the impulse into account; however, it is known that the human auditory system is spectrally tuned. The present paper advocates the extension to impulse noise of the noise dose concept which is widely used for continuous noise. This approach is based upon sound exposure instead of sound pressure. An A-weighting filter or an octave band analysis can then be used to take the spectral content of the impulses into account. The equipment needed for applying these procedures for impulse noise is an integrating sound level meter or a digital Fourier processor. Generalized spectral methods have been evaluated by means of an impulse simulation applied to a mathematical model of the human hearing mechanism. The results of this simulation agree with the most recent experiments on impulse noise and fully support the proposed rating methods. This conclusion must be emphasized as it leads the derivation of a uniform procedure for predicting loudness and damage risk for hearing which is applicable for continuous noise as well as for impulse noise.

Should helicopter noise be measured differently from other aircraft noise? A review of the psychoacoustic literature

Abstract: A review of 34 studies indicates that several factors or variables might be important in providing a psychoacoustic foundation for measurements of the noise from helicopters These factors are phase relations, tail rotor noise, repetition rate, crest level, and generic differences between conventional aircraft and helicopters Particular attention was given to the impulsive noise known as blade slap Analysis of the evidence for and against each factor reveals that, for the present state of scientific knowledge, none of these factors should be regarded as the basis for a significant noise measurement correction due to impulsive blade slap The current method of measuring effective perceived noise level for conventional aircraft appears to be adequate for measuring helicopter noise as well