Showing papers on "Noise measurement published in 1979"
TL;DR: A stand-alone noise suppression algorithm that resynthesizes a speech waveform and can be used as a pre-processor to narrow-band voice communications systems, speech recognition systems, or speaker authentication systems.
Abstract: A stand-alone noise suppression algorithm is presented for reducing the spectral effects of acoustically added noise in speech. Effective performance of digital speech processors operating in practical environments may require suppression of noise from the digital wave-form. Spectral subtraction offers a computationally efficient, processor-independent approach to effective digital speech analysis. The method, requiring about the same computation as high-speed convolution, suppresses stationary noise from speech by subtracting the spectral noise bias calculated during nonspeech activity. Secondary procedures are then applied to attenuate the residual noise left after subtraction. Since the algorithm resynthesizes a speech waveform, it can be used as a pre-processor to narrow-band voice communications systems, speech recognition systems, or speaker authentication systems.
4,862 citations
02 Apr 1979
TL;DR: This paper describes a method for enhancing speech corrupted by broadband noise based on the spectral noise subtraction method, which can automatically adapt to a wide range of signal-to-noise ratios, as long as a reasonable estimate of the noise spectrum can be obtained.
Abstract: This paper describes a method for enhancing speech corrupted by broadband noise. The method is based on the spectral noise subtraction method. The original method entails subtracting an estimate of the noise power spectrum from the speech power spectrum, setting negative differences to zero, recombining the new power spectrum with the original phase, and then reconstructing the time waveform. While this method reduces the broadband noise, it also usually introduces an annoying "musical noise". We have devised a method that eliminates this "musical noise" while further reducing the background noise. The method consists in subtracting an overestimate of the noise power spectrum, and preventing the resultant spectral components from going below a preset minimum level (spectral floor). The method can automatically adapt to a wide range of signal-to-noise ratios, as long as a reasonable estimate of the noise spectrum can be obtained. Extensive listening tests were performed to determine the quality and intelligibility of speech enhanced by our method. Listeners unanimously preferred the quality of the processed speech. Also, for an input signal-to-noise ratio of 5 dB, there was no loss of intelligibility associated with the enhancement technique.
1,352 citations
02 Apr 1979
TL;DR: It is shown spectral subtraction can be implemented in terms of a nonstationary, multiplicative, frequency domain filter which changes with the time varying spectral characteristics of the speech.
Abstract: Spectral subtraction has been shown to be an effective approach for reducing ambient acoustic noise in order to improve the intelligibility and quality of digitally compressed speech. This paper presents a set of implementation specifications to improve algorithm performance and minimize algorithm computation and memory requirements. It is shown spectral subtraction can be implemented in terms of a nonstationary, multiplicative, frequency domain filter which changes with the time varying spectral characteristics of the speech. Using this filter a speech activity detector is defined and used to allow the algorithm to adapt automatically to changing ambient noise environments. Also the bandwidth information of this filter is used to further reduce the residual narrowband noise components which remain after spectral subtraction.
200 citations
TL;DR: In this article, the authors compare the noise properties of conventional and energy selective X-ray computed tomography (XCT) images and show that the conventional image is a subset of the energy selective data and demonstrate that if this is done at the optimal display energy, the resulting conventional image will have the same noise as that produced by a conventional system with the same dose.
Abstract: We compare the noise properties of conventional and energy selective X-ray computed tomography. The images produced by the systems are not directly comparable so we discuss their relationship and show that the conventional image is a subset of the energy selective data. We describe how to form a conventional image from the energy selective images and demonstrate that, if this is done at the optimal display energy, the resulting conventional image will have the same noise as that produced by a conventional system with the same dose. But the energy selective system also extracts all the energy dependent information so it produces more information for the same dose than a conventional system.
128 citations
TL;DR: In this article, a noise source is identified in a cold jet at M = 0·98 by using the causality method proposed by Ribner and Siddon, which includes measuring the correlations between the velocity fluctuations inside the jet (these velocity fluctuations are measured by means of a laser velocimeter) and the far field acoustic pressure.
112 citations
02 Apr 1979
TL;DR: This paper shows the method of the indentification of the components of the noisefield from the eigenvectors and eigenvalues of the spectral density matrix of the signals received on the array.
Abstract: Research directed towards the improvement of underwater passive listening technique has led to powerful methods but which require the knowledge of the spatial coherences of the sources and of the background noise They are sensitive to the shape of the wavefront of the signals from the sources But they are also sensitive to the spatial coherence of the background noise : this is shown in this paper in particular for the method of the indentification of the components of the noisefield from the eigenvectors and eigenvalues of the spectral density matrix of the signals received on the array
107 citations
NEC1
TL;DR: In this paper, the authors proposed a method for measuring the noise parameters of a gate-length GaAs MESFET (NE38806) for linear noisy two-ports by introducing a computational method for evaluating measured admittance errors.
Abstract: Conventional methods for noise parameter measurement for linear noisy two-ports have been improved by introducing a computational method for evaluating measured admittance errors. Derivation and comparison with a conventional method are given. Noise parameters of a packaged 0.5-mu m gate-length GaAs MESFET (NE38806) were successfully measured using the proposed technique.
89 citations
TL;DR: In this article, a method to determine the noise parameters of microwave linear two-ports (transistors) is presented, which is based on a two-channel noise temperature measuring system and an analycal data procesing procedure.
Abstract: A method to determine noise parameters of microwave linear two-ports (transistors) is presented which is based on a two-channel noise temperature measuring system and an analycal data procesing procedure. As compared with the one-channel measurements and the graphical processing techniques, the method offers advantages from both accuracy and experiment viewpoints. Experimental verification related to noise parameters determination for a microwave transistor as function of frequency in S band are reported.
64 citations
TL;DR: A rather simple adaptive nonlinear receiver structure is described which attempts to adapt to the unknown prevailing noise environment and is shown to provide fairly efficient and robust performance in a wide variety of non-Gaussian narrowband noise environments.
Abstract: A class of nonlinear receiver structures is described for the detection of weak signals in non-Gaussian narrowband noise. In particular, the concept of a locally optimum receiver structure is extended to the ease of narrowband signal and noise models. A useful class of non-Gaussian narrowband noise models is developed for which the locally optimum receiver implementation is explicitly determined. These structures are shown to provide considerable improvement over conventional linear receiver structures. The basis of comparison is taken as the asymptotic relative efficiency (ARE). Unfortunately, the locally optimum receiver requires explicit {sl a priori} knowledge of the underlying noise distribution. To circumvent this difficulty a rather simple adaptive nonlinear receiver structure is described which attempts to adapt to the unknown prevailing noise environment. This adaptive receiver is shown to provide fairly efficient and robust performance in a wide variety of non-Gaussian narrowband noise environments.
42 citations
TL;DR: In this article, the authors give theoretical and experimental results concerning the behaviour of the signal-to-noise ratio in presence of modal noise considering the power from the exit face of a short sample of a graded-index multimode optical fibre.
Abstract: Modal noise in analogue and digital optical fibre systems has been demonstrated to be a significant effect. In this letter, we give some theoretical and experimental results concerning the behaviour of the signal-to-noise ratio in presence of modal noise considering the power from the exit face of a short sample of a graded-index multimode optical fibre.
40 citations
Patent•
26 Mar 1979
TL;DR: In this paper, the non-linear transfer characteristic element is used to measure the noise present in the difference signal and control the gain of the variable-gain element accordingly, and the nonlinear element is such that above a predetermined value a constant predetermined minimum attenuation factor applies.
Abstract: Reduction of noise in a television signal is achieved by comparing signals from successive scans to produce a difference signal. Low amplitude differences are assumed to represent noise and are attenuated. Differences above a predetermined level are assumed to represent movement and are not so attenuated. The attenuation is achieved by a multiplier which receives as multiplier the output of a non-linear transfer characteristic element. The non-linear element receives the differences after rectification and after passing through a variable-gain element. The non-linear element is such that above a predetermined value a constant predetermined minimum attenuation factor applies. A noise measurement circuit measures the noise present in the difference signal and controls the gain of the variable-gain element accordingly. Several noise-measurement circuits may be provided, a selected one of which is used in dependence upon the magnitude of the input signal to the apparatus.
TL;DR: In this article, the effect of weathering or aging of conductors on noise levels is examined and the effectiveness of the concept of asymmetric conductor bundling for noise reduction is assessed.
Abstract: Audible noise produced by corona during foul weather is an important consideration in the design of high-voltage transmission lines. The measurement, analysis and prediction of audible noise and the assessment of noise reduction techniques have constituted a primary area of research at Project UHV. Results of earlier short-term testing utilizing single-phase test facilities have, since 1975, been greatly augmented by longer-term field data obtained on a three-phase test line. This paper presents the results of a portion of this work The effect of the weathering or aging of conductors on noise levels is examined and the effectiveness of the concept of asymmetric conductor bundling for noise reduction is assessed.
01 Apr 1979
TL;DR: This paper describes a unique design that attacks two problem areas of LPC: noise suppression input level control and real time simulation/ test.
Abstract: This paper describes a unique design that attacks two problem areas of LPC: noise suppression input level control and real time simulation/ test The noise level design uses algorithms to digitally process speech data before input to the LPC algorithm processor The LPC processor described in the paper is based on a microprocessor design conceived specifically for speech The noise suppression and level control algorithms are performed in a separate front end processor that detects noise patterns and deletes them from the normal voice input The operational hardware system is shown to the block diagram level as well as the particular simulation/test scheme Test results are also described in this paper
TL;DR: In this article, a nonlinear theory of noise in synchronized oscillators is outlined, thus extending Kurokawa's work from small to arbitrary injection levels, and simple expressions for the various output noise and noise conversion factors in the case when the nonlinear characteristic of the active device can be described by a third-order (van-der-Pol) polynomial.
Abstract: A nonlinear theory of noise in synchronized oscillators is outlined, thus extending Kurokawa's work from small to arbitrary injection levels. The description is of phenomenological nature: it uses the describing function method of control theory for calculating the carrier waves, and the circuit theory of periodically driven nonlinear systems for an analysis of the noise sidebands. Simple expressions are derived for the various output noise and noise conversion factors in the case when the nonlinear characteristic of the active device can be described by a third-order (van-der-Pol) polynomial.
TL;DR: In this paper, the representation of a pumped exponential diode, operating as a mixer, by an equivalent Iossy network was reexamined and it was shown that the model is correct provided the network has ports for all sideband frequencies at which (real) power flow can occur between the diode and its embedding.
Abstract: The representation of a pumped exponential diode, operating as a mixer, by an equivalent Iossy network, is reexamined. It is shown that the model is correct provided the network has ports for all sideband frequencies at which (real) power flow can occur between the diode and its embedding. The temperature of the equivalent network is eta/2 times the physical temperature of the diode. The model is valid only if the series resistance and nonlinear capacitance of the diode are negligible. Expressions are derived for the input and output noise temperature and the noise-temperature ratio of ideal mixers. Some common beliefs concerning noise-figure and noise-temperature ratio are shown to be incorrect.
TL;DR: A procedure is proposed which provides a flexible compromise between estimation accuracy and computational effort and is applied to a posteriori “Wiener” filtering.
Abstract: This paper deals with the problem of separating the spectra of signal and noise in ensembles where the signal can be considered as an invariant component and the noise as a stationary additive background. Several methods are discussed and compared on the basis of a statistical analysis of the first two moments of the estimators for signal and noise spectra. As a consequence a procedure is proposed which provides a flexible compromise between estimation accuracy and computational effort. The application of this procedure to a posteriori "Wiener" filtering is compared with a more common, but time consuming, technique.
02 Apr 1979
TL;DR: A spectral subtraction technique is described, which includes a biased estimate of the noise, that does not present musical tones at the output, and an automatic speech activity detector is described and used to adapt the noise estimate to changing noise environments.
Abstract: Performance of narrowband speech communications systems, such as Linear Predictive Coding (LPC), is often severely degraded by the presence of ambient acoustic noise in the input speech signal. Spectral subtraction techniques show promise in improving the overall performance of LPC in acoustic noise environments, but typically present annoying musical tones at the output. A spectral subtraction technique is described, which includes a biased estimate of the noise, that does not present musical tones at the output. In addition, an automatic speech activity detector is described and used to adapt the noise estimate to changing noise environments.
TL;DR: An automated test set is described for characterizing the static performance of high resolution ADC's and DAC's with uncertainties of 2-4 ppm.
Abstract: An automated test set is described for characterizing the static performance of high resolution ADC's and DAC's. Measured parameters include gain, offset, linearity, and equivalent ADC input noise with uncertainties of 2-4 ppm. Measurements to full accuracy can be made at a rate up to 40/s. A 20-bit DAC serves as a comparison standard.
01 Mar 1979
TL;DR: In this paper, an acoustic wind tunnel test was conducted to examine the noise-generating processes of an airframe during approach flight, where high-lift leading and trailing edge devices and landing gear could be added.
Abstract: Acoustic wind tunnel tests were conducted to examine the noise-generating processes of an airframe during approach flight. The airframe model was a two-dimensional wing section, to which highlift leading and trailing edge devices and landing gear could be added. Far field conventional microphones were utilized to determine component spectrum levels. An acoustic mirror directional microphone was utilized to examine noise source distributions on airframe components extended separately and in combination. Measured quantities are compared with predictions inferred from aircraft flyover data. Aeroacoustic mechanisms for each airframe component are identified. Component interaction effects on total radiated noise generally were small (within about 2 dB). However, some interactions significantly redistributed the local noise source strengths by changing local flow velocities and turbulence levels. Possibilities for noise reduction exist if trailing edge flaps could be modified to decrease their noise radiation caused by incident turbulent flow.
TL;DR: In this article, a detailed statistical analysis of transformer noise data is presented, which is used to develop a prediction method for far-field transformer noise which is based only on the fully-cooled MVA rating of the transformer.
Abstract: The findings arising from a detailed statistical analysis of transformer noise data. are described. These findings are used to develop a prediction method for far-field transformer noise which is based only on the fully-cooled MVA rating of the transformer. The implications of these findings as they concern the present NEMA standard on transformer noise are discussed.
Patent•
21 Mar 1979
TL;DR: In this article, the attenuation is achieved by a multiplier (26) which receives as multiplier the output of a non-linear transfer characteristic element (24), such that above a predetermined value a constant predetermined minimum attenuation factor applies.
Abstract: Reduction of noise in an electrical, e.g. television, signal is achieved by comparing signals from successive scans to produce a difference signal. Low amplitude differences are assumed to represent noise and are attenuated. Differences above a predetermined level are assumed to represent movement and are not so attenuated. The attenuation is achieved by a multiplier (26) which receives as multiplier the output of a non-linear transfer characteristic element (24). The non-linear element receives the differences after rectification (100) and after passing through a variable-gain element (102). The non-linear element (24) is such that above a predetermined value a constant predetermined minimum attenuation factor applies. A noise measurement circuit (X) measures the noise present in the difference signal and controls the gain of the variable-gain element (102) accordingly. Several noise-measurement circuits may be provided. a selected one of which is used in dependence upon the magnitude of the input signal to the apparatus.
TL;DR: In this article, the authors used a combination of ground-based magnetometer data and Isis 2 satellite data in the form of energetic particles, auroral luminosity, and electromagnetic and/or electrostatic noise measurements to provide a comprehensive picture of processes which are taking place on field lines penetrating the auroral oval in the evening and midnight sectors.
Abstract: The magnetic signatures of field-aligned and ionospheric currents can be used to determine the geometry and strength of the electrical current systems which couple the ionosphere to the outer regions of the earth's magnetosphere. It is now recognized that the currents flowing in the evening sector are an important clue to the understanding of processes leading to explosive substorm instabilities and episodes of intense ionospheric disruption. In this report we have used a combination of ground-based magnetometer data and Isis 2 satellite data in the form of energetic particles, auroral luminosity, and electromagnetic and/or electrostatic noise measurements to provide a comprehensive picture of processes which are taking place on field lines penetrating the auroral oval in the evening and midnight sectors. We shall show that noise measurements in the frequency range 100 < f < 500 kHz are characteristic of regions of upward current flow which are found in the region of discrete auroral features. We find that the higher-energy electrons (a few keV) are associated with the higher-frequency noise. In addition, the highest noise frequencies are found in the center of discrete auroral arc regions with the frequency decreasing on either side of the center of the arc region. We believe that the source of noise in this frequency range is associated with electron beams in discrete arcs whose velocity space distribution contains a region of positive slope. We shall finally discuss the physics of the relationship among the various electromagnetic and energetic particle signatures presented in this study.
TL;DR: In this paper, the influence of free-stream turbulence in the low-frequency broadband noise radiation from model rotors has been experimentally investigated and the experimental data show good agreement with an ad hoc model based on unsteady aerodynamics.
Abstract: Low-frequency broadband noise generated by model rotors is attributed to the interaction of ingested turbulence with the rotor blades. The influence of free-stream turbulence in the low-frequency broadband noise radiation from model rotors has been experimentally investigated. The turbulence was generated in the M.I.T. anechoic wind tunnel facility with the aid of bipolar grids of various sizes. The spectra and the intensity of the low-frequency broadband noise have been studied as a function of parameters which characterize the turbulence and of helicopter performance parameters. The location of the peak intensity was observed to be strongly dependent on the rotor-tip velocity and on the longitudinal integral scale of turbulence. The size scale of turbulence had negligible effect on the intensity of low-frequency broadband noise. The experimental data show good agreement with an ad hoc model based on unsteady aerodynamics.
TL;DR: In this article, the authors reviewed the ways in which impulsive noise is generated and methods by which it may be characterized, and selected parameters suitable for an assessment of system performance, and design considerations were given for equipment suitable for the measurement of these parameters and the effect of limitations in the equipment on the measured parameters.
Abstract: After reviewing the ways in which impulsive noise is generated and methods by which it may be characterized, parameters suitable for an assessment of system performance are selected. Design considerations are given for equipment suitable for the measurement of these parameters and the effect of limitations in the equipment on the measured parameters is described. Methods of overcoming these limitations are discussed.
TL;DR: In this article, a component method of airframe noise prediction is used to predict levels of operational and proposed aircraft airframe noises to assess the contribution of air-frame noise to community noise levels.
Abstract: A component method of airframe noise prediction is used to predict levels of operational and proposed aircraft airframe noise to assess the contribution of airframe noise to community noise levels. This is done after first evaluating the prediction method using newly acquired detailed measurements from full-scale aircraft and models. In the course of the evaluation, modeling techniques of airframe noise sources are examined with attention to scaling. Finally, when used to predict approach airframe EPNLs, the levels fell about 10 EPNdB below current noise regulations and about 5 EPNdB below proposed noise regulations.
01 Feb 1979
TL;DR: In this article, structural and acoustic finite element coupled models of a single engine general aviation aircraft were used to determine the importance of engine induced structural borne noise and determine the necessary modeling requirements for the prediction of structural borne interior noise.
Abstract: Structural borne interior noise in a single engine general aviation aircraft was studied to determine the importance of engine induced structural borne noise and to determine the necessary modeling requirements for the prediction of structural borne interior noise. Engine attached/detached ground test data show that engine induced structural borne noise is a primary interior noise source for the single engine test aircraft, cabin noise is highly influenced by responses at the propeller tone, and cabin acoustic resonances can influence overall noise levels. Results from structural and acoustic finite element coupled models of the test aircraft show that wall flexibility has a strong influence on fundamental cabin acoustic resonances, the lightweight fuselage structure has a high modal density, and finite element analysis procedures are appropriate for the prediction of structural borne noise.
TL;DR: In this article, two procedures for determining the flow noise level are described, and guidelines for sound measurements in the presence of flow and for the use of a microphone with a slit-tube under unusual temperature conditions or in gases other than air.
01 Apr 1979
TL;DR: In the limit of white observation noise this paper develops a modified adaptive algorithm which incorporates a priori or measured noise power information so that the generated frequency estimates are unbiased.
Abstract: The application of an adaptive prediction error filter to autoregressive spectral analysis has been proposed by Lloyd Griffiths and is in use for the measurement of the frequencies of sinusoids It is well known that additive observation noise can bias these frequency estimates, thus limiting the useful applications of Griffiths technique to low noise environments. In the limit of white observation noise this paper develops a modified adaptive algorithm which incorporates a priori or measured noise power information so that the generated frequency estimates are unbiased. Unlike other bias removal techniques recently discussed, this technique can work with multiple sinusoids. The performance of this algorithm will be demonstrated through computer simulations and its convergence behavior will be discussed.