Random Noise Sources Research Articles

Development of a Zero Spillover Controller for Broadband Cancellation of Noise in Ducts

In this paper, an active noise control (ANC) system for broadband cancellation of noise in ducts, where a nonacoustical reference is unavailable and thus the ANC structure is “spatially feedforward” is presented. In this structure, acoustic feedback that is known to be detrimental to system stability and performance must be taken into account in controller design. In this case, difficulties generally arise in implementing the ideal controller, or the zero spillover controller due to plant uncertainties. In the present study, an equivalent approach based on the 2-norm (H2) optimal model matching principle is proposed in controller design. The control algorithm is implemented on a digital signal processor (DSP), by finite impulse response (FIR), infinite impulse response (IIR) and hybrid filters. The experimental results indicate that the proposed controller was effective in suppressing periodic noise and random noise sources, without and with airflow.

The Microstructure of Currency Markets: An Empirical Model of Intra-day Return and Bid-Ask Spread Behavior

This paper analyzes the intra-day relationship between bid-ask spreads and “market” return volatility for U.S. Dollar/Deutschemark quotes. We are able to identify a statistically and economically significant “Reverse U-shaped” pattern in the bid-offer spread in 1996. Tests of the stability and ordering of “market” volatility, performed across several different fractions of the day, reveal that variances of intra-day returns are heterogeneous and ordered, declining around the Asian lunch break, increasing steadily during the London morning trading hours, peaking at the opening of New York to subsequently fall with the closing of the European markets. Results also indicate that “market” volatility is significantly higher during intra-day versus overnight periods. Then, we introduce a structural model that attempts to explain those empirical regularities by capturing some currency-specific features of the data: possibly asymmetric and stochastic trading cost structure, discrete directional updates and parameters’ temporal heterogeneity, and by relating the bid-ask spread to different sources of random noise. We evaluate these parameters via GMM using a set of convenient unconditional intra-day moments implied by the basic configuration of the model. Analysis of the resulting estimated patterns reveals that trading costs play a significant role in explaining the intra-day variability of bid and offer currency returns. Inventory considerations appear to be more relevant in the trading morning, while the perceived risk of arrival of informed trades seems more likely to affect the dealers’ cost structure in the afternoon. The contribution of the “true” currency risk to the total variability of posted bid and ask quotes’ returns is not surprisingly highest with the opening of the European markets.

A digital pixel image sensor for real-time readout

An experimental digital image sensor that converts analog video signals into 8-bit digital signals for each unit pixel, and reads out the signals at an operating clock of standard TV has been developed. Each pixel is equipped with a photodiode, a 1-bit analog-to-digital converter (1-bit ADC), an 8-bit pulse counter, and a signal processing circuit. The sensor system displays a two-dimensional (2-D) image in real time. The 1-bit ADC has a dynamic range of 110 dB at an operating voltage of 1.3 V. Knee characteristics and the results of an investigation into random noise sources in the circuit are also described.

Noise reduction of page-oriented data storage by inverse filtering during recording

Two novel techniques for eliminating deterministic noise from a page-oriented memory are presented. The first technique equalizes the output response of ON pixels by adjustment of the exposure of each pixel during the recording of each data page. A test image transmitted through the system measures the spatial nonuniformities, and the appropriate inverse filter is imposed upon the data page and recorded in the storage material. On readout, the output signal values are then spatially uniform, perturbed only by random noise sources. Experimental results of using this predistortion technique in a pixel-matched holographic storage system are shown. Under conditions of high volumetric density, raw bit-error-rate (BER) improvements of 6-8 orders of magnitude are obtained (from 10(-4) to <10(-10)). The second technique uses a phase shift during holographic storage to subtract from bright OFF pixels. Under conditions of low spatial light modulator contrast, BER improvements of 6 orders of magnitude (from 10(-2) to 10(-8)) are demonstrated.

Comparison of methods for determining specific acoustic impedance

An investigation was conducted to explore potential improvements provided by alternate implementations of two impedance-measurement methods, known as the two-microphone and multipoint methods. This investigation is part of a continuing technology development to find more efficient and convenient methods for impedance measurements in harsh environments at high frequencies. The two alternate methods are compared with other methods to determine relative merits. As expected, the “best method” depends upon trade-offs between convenience and accuracy. The single-microphone method eliminates one of the two microphones typically used in the two-microphone method. The single-microphone method should be useful where mechanical constraints allow only one microphone to be placed into the test environment. It is found to be quite accurate with a single discrete frequency source, but the most inaccurate with a random noise source. An alternate implementation of the multipoint method, achieved by replacing the single-tone source with a pseudo-random noise source, requires significantly less time with minimal loss of accuracy. Typical agreement between this implementation and the accepted standard is within 0.1 ρc units. However, slightly larger deviations at frequencies above 2.5 kHz indicate challenges in the proposed extension to high-frequency (up to 25-kHz) applications.

An optoelectronic CMOS circuit implementing a simulated annealing algorithm

An original optoelectronic implementation of simulated annealing is presented. A compact and simple optical system provides a chip with arrays of independent random noise sources. The silicon chip is composed of a mesh of computing cells. Each cell includes both analog and digital circuits and includes two photosensors. A detailed analysis of this cell is given including a presentation of the design constraints. A 4/spl times/4-cells prototype chip was implemented in a 1 /spl mu/m CMOS digital technology and was successfully operated at 20000 iterations per second. The measurements and characterization of this chip made possible the successful design of a 600-cells chip also presented. These results demonstrate the video-rate application of simulated annealing to early vision tasks.

Modification of two methods for determining acoustic impedance.

Results of an investigation to modify two impedance measurement methods (two‐microphone method, TMM; and multipoint method, MPM) to improve efficiency and convenience are presented. The TMM is modified by eliminating the need for one of the microphones in situations where only one microphone can be placed into the test environment. The MPM modification uses pseudorandom noise to significantly reduce the time necessary to complete acoustic impedance measurements. These modifications are the results of on‐going technology development at NASA Langley Research Center to simplify and increase accuracy of measurements in harsh environments. Evaluations of efficiency and accuracy requirements of each of the normal incidence acoustic impedance measurement techniques in use at NASA Langley Research Center are provided to determine which method should be used for a given problem. The single‐tone source implementation of the MPM is found to be the most accurate, but the most time‐consuming technique currently in use. A pseudorandom noise source implementation of the MPM requires significantly less time, with minimal loss of accuracy. The single‐microphone method (SMM) is shown to be accurate with a single discrete frequency source, but degrades in accuracy with a random noise source.

Pulse noise in thin film disk recording systems

The effect of media noise on pulse amplitude and pulse shape variation has been investigated at a thin film disk drive. In a new approach total amplitude noise is divided into a reproducible (deterministic) part and a random part. At higher pulse interference position jitter is identified as source of additional random amplitude noise. From pulse shape analysis it is concluded that flying height and transition length are the reasons for the observed amplitude variations.

Acoustic intensity measurements in the presence of low Mach number flow

Abstract : Acoustic intensity is an important analysis tool since it provides wave propagation directionality along with absolute magnitude, and it can be measured in the near or far field of a source. Acoustic intensity measurements acquired in the presence of a mean flow are susceptible to errors due to the effects of the flow noise on the sensor. To determine if this error could be accurately quantified, intensity measurements were acquired, with the standard two microphone cross spectral technique, in a sound field that contained both mean flow and an independent random broadband noise source. The microphones were flush mounted, at several different separation distances, in the test section of a wind tunnel that provided the desired flow conditions, while a large speaker provided the independent random noise source. The error calculations were based on a technique that had already been derived theoretically and published, but had not been proven experimentally. The experiments performed validate that the error is indeed a bias error and that it can be accurately quantified. In addition, accurate quantification of the error is not limited to one-dimensional sound fields that contain only plane waves, and the method can be easily extended to tow or three directions with complex wave propagation. Acoustic intensity, Low mach number, Flow, Two microphone, Cross spectral, Wind tunnel.

Application of the Wiener-Hermite functional method to the stochastic oscillator

The Wiener-Hermite functional (WHF) method is applied to the stochastic oscillator model under stable system conditions. The application concerns a non-linear trivariate-input/single-output problem. Gaussian random noise sources are assumed. A set of coupled integral equations is established in the frequency domain for any required approximation accuracy (WHF- N approximation), from which noise signature functions of the response can be derived. The oscillator model is explicitly treated in a second-order approximation (WHF-2 approximation) with white noise sources and the resulting steady-state value and the power spectral density of the response are compared with exact results derived by the Fokker-Planck method. The approximative results exhibit the same structure as recently found in the application of the WHF method to point reactor kinetics driven by random reactivity fluctuations.

Active acoustic attenuation system with overall modeling

An active acoustic attenuation system (200) has a first adaptive filter model M modeling the acoustic path P from an input transducer (10) to an output transducer (14), and a second adaptive filter model Q modeling the overall system from the input transducer (10) to an error transducer (16). A third adaptive filter model T models the transfer function S of the output transducer (14) and the error path E between the output transducer (14) and the error transducer (16), without an auxiliary random noise source.

Corona discharge streamer model

Starting from the motion of a charged particle in the electric field set up by the conductor system of a power transmission line, the paper develops a model for the streamers released when the voltage stress at the surfaces of conductors leads to corona discharges from them. The frequency content of streamer pulses defines the frequency spectrum of radio noise-fields radiated from transmission lines in corona. This is a definition which is achieved for the first time in analytical form in the present paper. From the basis which the waveshape of a single streamer pulse in the time domain provides, multiple point discharges are interpreted as random uncorrelated noise sources in power spectral density form. Comparisons are made between this frequency domain function and previously published corona excitation functions derived from measurement. Further validation studies are reported in which noise-interfering fields found from the streamer model which the paper develops are correlated with the results of recent measurements on a 500 kV double-circuit line energised for test purposes to 522 kV.

Broadband modeling and source localization of a PRN projector tow

A broadband ray model is used to simulate data from a deep water experiment (TAGEX 87) in which a pseudo random noise (PRN) source is towed past a 24‐element, bottom‐moored array. The source spectrum contains a series of evenly spaced lines from 60–100 Hz. Simulated shade plots of received spectrum level versus frequency and time exhibit excellent agreement with the data. A broadband matched field algorithm is used to localize the source in range as a function of time. The algorithm consists of matching the recorded cross spectrum R1R2* between two phones with simulated cross spectra S1S2* at a series of ranges. The correlations between recorded and simulated cross spectra are added coherently as a function of frequency to obtain the localization matrix entry: Lij = Σf[RliR2i*] [SljS2j*]*. Successful localizations of the PRN source are obtained at ranges from 0–30 km. [Work supported by Naval Ocean Systems Center, Code 733, under contract N00039‐88‐C‐0043‐1‐43.]

Interaural correlation discrimination and autocorrelation

The discrimination of differences in interaural correlation was examined for auditory noise sources differing internally in terms of their autocorrelation. Interaural correlation discrimination is inversely related to the autocorrelation of the sound source: lowest interaural thresholds with independent random noise sources; highest thresholds with repeated periodic noise sources. Interaural thresholds increase with higher periodicities. Periodicity leads to higher thresholds, apparently, by reducing the number of independent samples for the calculation of interaural correlation.

Free-stream turbulence: A limitation on fractal descriptions of open-flow systems

Cylinder wake velocity measurements were made at Reynolds number 100 with varying levels of free-stream turbulence. The Grassberger–Procaccia (G–P) correlation function, which is often used for estimating fractal dimension, was computed for these data, and also for data obtained by adding random noise to the iterations of the Henon map. A comparison of the two plots shows that the free-stream turbulence acts as a highly random external noise source. Since free-stream turbulence determines the details of the smaller flow scales, no low-order dynamical system will be able to model all the scales in this open flow.

Simultaneous stochastic inversion for geomagnetic main field and secular variation: 2. 1820–1980

We apply the method of simultaneous stochastic inversion for the geomagnetic main field and secular variation to data from the period 1820–1980, producing two time‐dependent field models, one for the period 1900–1980, the other for 1820–1900. We introduce several adaptations of our original formalism of the inverse problem: we choose to represent the time dependency of the field by an expansion in Chebychev polynomials; we apply an alternative regularization condition in the time domain; and we adopt a different method from that used in our earlier study for solving the resulting system of equations. Whereas our earlier study used only twentieth century observatory data, we consider almost all the available data from the time interval 1820–1980. We examine the effects of the crustal field on our observations and find that existing methods of accounting for these fields as sources of random noise are inadequate in two circumstances. The first is sequences of measurements made at one particular site (for example, at permanent magnetic observatories), in which case the crustal field produces temporal correlation of the errors in the observations: we devise a method for accounting for these correlations. The other is measurements made at satellite altitude, the crustal field producing spatial correlation of the errors. We show how, using a very simple statistical model of the crustal field, these correlations can be calculated. Rather than attempting to account directly for these correlations (which would require, without further assumptions, massive computation) we instead limit our data selection to points sufficiently well separated in space that the effect of the crustal field is minimal. We are able to fit the data to within our estimates of the noise in the data, yet the resulting models have lower power in the secular variation than previously postulated by some authors. Many of the features in the field at the core‐mantle boundary described in recent work by Bloxham and Gubbins are seen most clearly; indeed this time‐varying representation of the field reinforces many of their earlier conclusions.

Active noise control on systems with time-varying sources and acoustical elements

An active noise control system has been developed based on a system identification concept using a recursive adaptive filter to model the direct and feedback elements of an acoustic system. An independent random noise source is used with a second adaptive filter to provide a model of the output transducer and error path transfer functions. This system has the ability to continuously cancel narrow band and broadband noise. The system identification configuration combined with adaptive filter techniques enables the system to respond quickly and accurately to changes in time-varying sources, such as frequency or amplitude, and changes in the acoustical system, such as temperature. Noise reduction results will be presented. The system capabilities will be demonstrated on a variety of time-varying sources and time-varying acoustical elements. The ability of the system to track these changes in real-time as well as implications for actual applications will be discussed.

A stochastic model of sea-surface roughness. I. Wave crests

In this paper we develop a two-scale model of sea-surface roughness, in which for the first time the randomness of both long and short waves is taken fully into account. The model includes long–wave-short-wave interactions, dissipation of the short-wave energy by breaking, and regeneration by the wind. This leads to an integral equation for the short-wave steepness, which is solved by iteration. The effects of wind speed and of long-wave steepness upon the distribution of roughness at the long-wave crests are calculated and discussed. Also the effect of a band-width parameter for the long-wave spectrum. A random noise source can be included.

Detection and Correction of Small Abscissa Errors

The availability of quantifying procedures such as least-squares fitting and correlation makes it possible to suppress noise which occurs at spatial frequencies higher than those of the analyte signal. However, these calculations do not discriminate against low-frequency interferences, whether by random noise or other sources. One of the significant contributions to low-frequency interfering signals in Raman as well as in other types of spectroscopic measurements is uncertainty in the establishment of the abscissa position. Having found this to be a limiting factor in certain measurements, we have been interested in finding a procedure that would allow efficient recognition and correction of these errors.

The effects of cathodically protected pipelines on aeromagnetic surveys

Many types of cultural noise are found in present‐day high‐resolution aeromagnetic data, and such noise must be removed if surveys flown in areas of existing cultural features are to be interpreted properly—particularly areas of oil and gas production. One of the worst causes of cultural noise in such areas is the dc electrical current passed through pipelines to prevent corrosion. This practice is called “cathodic protection,” and it is in common use worldwide; however, there is little or no discussion of the resulting magnetic anomalies in the geophysical literature. Cathodically protected pipeline (CPP) anomalies are particularly troublesome at the level of airborne surveys because they fall off slowly as the first power of the distance, rather than as the square or cube of the distance as in most sources of random cultural noise. However, the curve shapes of CPP anomalies are readily calculable: they depend upon the strike directions of the pipeline and the aeromagnetic profile, and on the inclination and declination of the Earth’s magnetic field. The amplitude of CPP anomalies depends upon the height of the magnetometer above the pipeline and the strength of the current in the line. Standard curves were plotted for several CPP anomalies due to long, straight, horizontal pipelines, worldwide, and were used to verify the existence of CPP anomalies in production survey lines and tie lines for an aeromagnetic survey at the magnetic latitude of Oklahoma, USA.