Showing papers on "Noise measurement published in 1991"

Filtering of colored noise for speech enhancement and coding

Abstract: Scalar and vector Kalman filters are implemented for filtering speech contaminated by additive white noise or colored noise, and an iterative signal and parameter estimator which can be used for both noise types is presented. Particular emphasis is placed on the removal of colored noise, such as helicopter noise, by using state-of-the-art colored-noise-assumption Kalman filters. The results indicate that the colored noise Kalman filters provide a significant gain in signal-to-noise ratio (SNR), a visible improvement in the sound spectrogram, and an audible improvement in output speech quality, none of which are available with white-noise-assumption Kalman and Wiener filters. When the filter is used as a prefilter for linear predictive coding, the coded output speech quality and intelligibility are enhanced in comparison to direct coding of the noisy speech. >

Constrained iterative speech enhancement with application to speech recognition

Abstract: The basis of an improved form of iterative speech enhancement for single-channel inputs is sequential maximum a posteriori estimation of the speech waveform and its all-pole parameters, followed by imposition of constraints upon the sequence of speech spectra. The approaches impose intraframe and interframe constraints on the input speech signal. Properties of the line spectral pair representation of speech allow for an efficient and direct procedure for application of many of the constraint requirements. Substantial improvement over the unconstrained method is observed in a variety of domains. Informed listener quality evaluation tests and objective speech quality measures demonstrate the technique's effectiveness for additive white Gaussian noise. A consistent terminating point of the iterative technique is shown. The current systems result in substantially improved speech quality and linear predictive coding (LPC) parameter estimation with only a minor increase in computational requirements. The algorithms are evaluated with respect to improving automatic recognition of speech in the presence of additive noise and shown to outperform other enhancement methods in this application. >

1/f frequency noise effects on self-heterodyne linewidth measurements

Abstract: The effects of a 1/f frequency noise on self-heterodyne detection are described, and the results are applied to the problem of laser diode linewidth measurement. The self-heterodyne autocorrelation function and power spectrum are evaluated for both the white and the 1/f components of the frequency noise. From numerical analysis, the power spectrum resulting from the 1/f frequency noise is shown to be approximately Gaussian, and an empirical expression is given for its linewidth. These results are applied to the problem of self-heterodyne linewidth measurements for coherent optical communications, and the amount of broadening due to 1/f frequency noise is predicted. >

Aeroacoustics of Flight Vehicles: Theory and Practice. Volume 1: Noise Sources

Abstract: Methodology recommended to evaluate aeroacoustic related problems is provided, and approaches to their solutions are suggested without extensive tables, nomographs, and derivations. Orientation is toward flight vehicles and emphasis is on underlying physical concepts. Theoretical, experimental, and applied aspects are covered, including the main formulations and comparisons of theory and experiment. The topics covered include: propeller and propfan noise, rotor noise, turbomachinery noise, jet noise classical theory and experiments, noise from turbulent shear flows, jet noise generated by large-scale coherent motion, airframe noise, propulsive lift noise, combustion and core noise, and sonic booms.

Method and apparatus for continuously measuring volumetric flow

Abstract: Method and apparatus for adapting the volumetric flow rate measurement system thaught by Yelderman in U.S. Patent No. 4,507,974 to clinical environments by reducing the effects of physiological noise. Signal processing techniques are used to characterize the background noise power spectrum of the system under test so that the effect of noise aberrations on the measured data may be eliminated and so that the predominant periodicities of the background noise spectra may be avoided. The invention further cross-correlates the input data of the system with the corresponding output data of the system to determine weighting values for each input frequency so that data collected at noisy frequencies is discounted. Low frequency noise or drift is also removed from the output signal by fitting the average of the output data to a quadratic function which is then subtracted from the original output signal, point by point. Finally, the processed data is fit to a lagged normal distribution curve in the frequency domain to solve for the parameters necessary to mathematically model the transfer function of the system. Given the input to the system as well as the frequency domain transfer function and noise data of the system, the volumetric flow of the system then may be determined for virtually any input even in very noisy environments.

Aeroacoustics of large wind turbines

Abstract: This paper reviews published information on aerodynamically generated noise from large horizontal axis wind turbines operated for electric power generation. Methods are presented for predicting both the discrete frequency rotational noise components and the broadband noise components, and results are compared with measurements. Refraction effects that result in the formation of high-frequency shadow zones in the upwind direction and channeling effects for the low frequencies in the downwind direction are illustrated. Special topics such as distributed source effects in prediction and the role of building dynamics in perception are also included.

Second-order sigma-delta modulation for digital-audio signal acquisition

Abstract: The authors compare the second-order sigma-delta ( Sigma Delta ) modulator to several alternative modulator architectures in the context of digital-audio signal acquisition. Design details and experimental results are presented for a 1 mu m CMOS implementation that does not require error correction or component trimming to achieve virtually ideal 16 b performance at a conversion rate of 50 kHz. The experimental modulator is a fully differential circuit that operates from a single 5 V power supply and does not require the use of precision sample-and-hold circuitry. With an oversampling ratio of 256 and a clock rate of 12.8 MHz, the modulator achieves a 98 dB dynamic range and a peak signal-to-(noise+distortion) ratio (SNDR) of 94 dB. Measurements and simulations of discrete noise peaks in the output spectrum that result from limit-cycle oscillations are also presented and discussed. >

Speech enhancement based conceptually on auditory evidence

Abstract: A new idea, enhancing speech based on auditory evidence, is explored for the problem of enhancing speech degraded by stationary and nonstationary additive white noise. Distinguishing different objectives for heavy and light noise interference, two related algorithms are developed. For speech degraded by heavy noise, the improvement in signal-to-noise ratio (SNR) is as high as 12 dB; for lightly noisy speech, the improvement is modest and decreases as the SNR of the noisy speech increases. Quantizing noise is used to assess the capacity for reducing nonstationary noise using these algorithms; a significant reduction of such noise and an improvement in speech quality are achieved. The advantages of the proposed algorithms for speech enhancement include no need for prior knowledge of the noise and only a modest computational requirement. >

System for separating speech from background noise

Abstract: A digital signal processing system applies an adaptive filtering technique to sequences of energy estimates in each of two signal channels, one channel containing speech and environmental noise and the other channel containing primarily the same environmental noise. From the channel containing primarily environmental noise, a prediction is made of the energy of that noise in the channel containing both the speech and that noise, so that the noise can be extracted from the mixture of speech and noise. The result is that the speech will be more easily recognizable by either human listeners or speech recognition systems.

Spatial diversity processing for underwater acoustic telemetry

Abstract: A large increase in the reliability of shipboard or stationary underwater acoustic telemetry systems is achievable by using spatially distributed receivers with aperture sizes from 0.35 to 20 m. Output from each receiver is assigned a quality measure based on the estimated error rate, and the data, weighted by the quality measure, are combined and decoded. The quality measure is derived from a Viterbi error-correction decoder operating on each receiver and is shown to perform reliability in a variety of non-Gaussian noise and jamming environments and reduce to the traditional optimal diversity system in a Gaussian environment. The dynamics of the quality estimator allow operation in the presence of high-power impulsive interference by exploiting the signal and noise differential travel times to individual sensors. The spatial coherence structure of the shallow water acoustic channel shows relatively low signal coherence at separations as short as 0.35 m. Increasing receiver spacing beyond 5 m offers additional benefits in the presence of impulsive noise and larger-scale inhomogeneities in the acoustic field. A number of data transmission experiments were carried out to demonstrate system performance in realistic underwater environments. >

Three-dimensional analysis framework and measurement methodology for imaging system noise

Abstract: Modern imaging sensors incorporate complex focal plane architectures and sophisticated post- detector processing. These advanced technical characteristics create the potential for multi- component noise generation which can exhibit effects temporally as well as along the vertical and horizontal image directions. Such complex three-dimensional (time, vertical, horizontal) noise cannot be adequately treated by previous mathematical analyses developed for simpler system designs where detector noise was predominant. In a parallel sense, earlier methods for noise measurement are no longer satisfactory. A new methodology has been developed at C2NVEO to characterize the noise patterns exhibited by advanced thermal imaging systems. The methods represents a significant expansion of the standard techniques to characterize thermal system noise. This paper explains the principles behind the 3-D noise methodology and the methods used. It also describes how this methodology is implemented in a laboratory measurement environment.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fourteen-decade photocurrent measurements with large-area silicon photodiodes at room temperature.

Abstract: Recent improvements in commercial silicon photodiodes and operational amplifiers permit electrical noise to be reduced to an equivalent of 0.1 fA of photocurrent when a measurement time of 400 s is used. This is equivalent to a photocurrent resulting from fewer than 800 photons/s, and it implies a dynamic range of 14 orders of magnitude for a detector circuit. We explain the circuit theory, paying particular attention to the measurement bandwidth, the causes of noise and drift, and the proper selection of circuit components. These optical radiation detectors complement the primary radiometric standards. These detectors may replace photomultiplier tubes that have been used traditionally and or that were too costly to be used.

1/f noise in n- and p-channel MOS devices through irradiation and annealing

Abstract: The 1/f noise of n- and p-channel MOS transistors was investigated through irradiation and biased anneals. While the increase in noise during irradiation is similar for both types of devices, the noise differs significantly in response to biased anneals. In particular, the noise decreases with decreasing Delta V/sub ot/ during positive-bias anneals in nMOS transistors but increases during positive-bias anneals for pMOS transistors. Conversely, negative bias anneals increase the noise in nMOS devices but decrease the noise in pMOS devices. These results are explained in terms of majority carrier trapping and detrapping at oxide defects near the Si/SiO/sub 2/ interface. Under normal operating bias conditions (positive bias for nMOS and negative bias for pMOS), the 1/f noise of both n- and p-channel transistors decreases through postirradiation annealing. >

Shot noise limited optical measurements at baseband with noisy lasers (proceedings Only)

Abstract: This paper describes a simple, all-electronic noise cancellation scheme which allows wideband, shot noise limited optical measurements at baseband, with noisy lasers, in many kinds of optical systems. With this system, it is usually possible toachieve the performance of a complex heterodyne system with a much simpler homodyne approach. Although it is similarto earlier differential and ratiometric techniques, its noise cancellation performance is much better, and it is highly effec-tive at modulation frequencies up to tens of megahertz. The basic idea is to subtract photocurrents directly, under feed-back control, to cancel excess noise (i.e. noise above the shot noise level) and spurious modulation of the beam. A sampleis split off from the beam at the laser and detected with a photodiode similar to the main detector at the system output.Most optical systems and detectors have very wide temporal bandwidths and excellent linearity; thus at all frequenciesof interest, the sample photocurrent has exactly the same instantaneous fractional excess noise fluctuations as the laserbeam itself, with no differential gain or phase. If a fraction of the sample photocurrent is subtracted from the main de-tector output, with feedback controlling the division ratio to keep the DC component of the result at zero, the excess noisecancels identically. The actual noise cancellation bandwidth is very wide, and does not depend on the feedback band-width, only on that of the differential bipolar transistor current divider. Two noise-cancelled outputs are available, onea high-pass filtered version of the signal imparted by the optical system, the other, a low-pass filtered voltage related tothe log ratio of the intensities of the two optical beams. The noise floors of the outputs depend only on the shot noiseof the signal beam. Measurements using a prototype of the device are presented which demonstrate this performance.2. INTRODUCTIONExcess noise, spurious modulation, and power drift in lasers are common problems in optical measurements. In gaslasers, the noise levels can easily reach 50 dB above shot noise, even quite far from DC. Since the noise and spurious

The use of noise properties in set theoretic estimation

Abstract: In most digital signal processing problems, the goal is to estimate an object from noise corrupted observations of a physical system. The authors describe how a wide range of probabilistic information pertaining to the noise process can be used in a general set theoretic estimation framework. The basic principle is to constrain the sample statistics of the estimation residual to be consistent with those probabilistic properties of the noise which are available and to construct sets accordingly in the solution space. Adding these sets to the collection of sets describing the solution will yield a smaller feasibility set and, hence, more reliable estimates. Pieces of information relative to quantities such as range, moments, absolute moments, and second and higher order probabilistic attributes are considered, and properties of the corresponding sets are established. Simulations are provided to illustrate the theoretical developments. >

On estimation of noise variance in two-dimensional signal processing

Abstract: Estimation of noise variance is an important component of digital signal processing, in particular of image processing. In this paper we develop methods for estimating the variance of white noise in a two-dimensional degraded signal. We discuss optimal configurations of pixels for difference-based estimation, and describe asymptotically optimal selection of weights for the component pixels. After extensive analysis of possible configurations we recommend averaging linear configurations over a variety of different orientations (usually two or four). This approach produces estimators with properties of both statistical and numerical efficiency.

A Technique for Quantitatively Measuring Microstructurally Induced Ultrasonic Noise

Abstract: In ultrasonic inspections of aircraft engine components, the detectability of critical defects can be limited by grain noise. This is likely to be the case for subtle defects, such as hard-alpha-phase inclusions in titanium alloys, where the difference between the acoustic impedances of the defect and host is small. A sound quantitative description of grain noise in such alloys is essential for accurate estimates of flaw detection reliability. In this work we present a method for quantifying backscattered grain noise by using positional averaging to determine the root-mean-squared (rms) noise level. The measured noise level will depend on details of the measurement system, as well as on inherent material properties of the alloy. We present a preliminary model of the noise measurement process which accounts for system effects, and we compare its predictions with experiment. We then indicate how the rms noise data can be processed to extract a factor which parameterizes the inherent noise severity independent of the measurement process.

A statistical perspective on state-space modeling using subspace methods

Abstract: The authors investigate aspects of subspace-based state-space identification techniques from a statistical perspective. They concentrate their efforts on a simple approach which is based on finding the range-space of the observability matrix of a state-space representation. The system description is then found using the shift-invariance property of the observability matrix. It is shown that this results in a consistent system description for multivariable output-error models if the measurement noise is white in time and independent from output to output. The asymptotic covariance of the estimated poles of the system is also derived. In the test case studied, the subspace technique performs comparably with the statistically efficient PE (prediction error) method, whereas the instrumental variable method does notably worse. Hence, the subspace technique may be a strong candidate for determining initial values for the optimization in the efficient PE method. >

Spatial prediction filtering in the t-x and f-x domains

Abstract: The predictability of seismic signals from nearby traces can be a powerful tool for reducing random or locally coherent noise. The choice of algorithm to reduce noise for a given application is a function of the data signal and noise characteristics. When the signal and noise are relatively consistent over a given design window, an f-x domain Wiener‐filter approach can be used. For cases in which the data are time‐ or space‐varying, a new approach using 2-D adaptive filtering in the t-x domain can be very effective. In either of these approaches, a prediction trace‐gap can often be successfully used to remove locally coherent noise when lateral signal changes are not too rapid.

Silicon condenser microphone with integrated field-effect transistor

Abstract: A silicon condenser microphone is described, which works with an integrated field-effect transistor (FET). The gate of the transistor corresponds to the membrane of the microphone. Between the membrane and the gate oxide is a small air gap. The drain current of the transistor is controlled by the deflections of the membrane. The structure, which carries the FET and which is placed beyond the membrane, can have very small lateral dimensions. This results in small values of air-gap streaming losses and high air-gap compliances, thus yielding a good acoustic behaviour. The design of a silicon microphone with suspended-gate FET is described and experimental results of frequency response and noise are presented. The measured sensitivities are in the range 0.1–1 mV/Pa, which is about 15 dB lower than the calculated values. The reduction in sensitivity is caused by the silicon fabrication process of the microphones and can be eliminated. The frequency response is smooth up to 30 kHz. The noise measurement shows a 1/ f slope, which is typical for the noise behaviour of the FET.

H/sub infinity / fixed-lag smoothing filter for scalar systems

Abstract: The solution of the H/sub infinity / optimal linear fixed-lag smoothing problem is considered using a polynomial matrix description for the discrete system. The smoother is obtained from the solution of a linear equation and a spectral factorization calculation. The pole-zero properties of the optimal smoother are obvious in the polynomial representation, and insights into the measurement noise rejection properties of the smoother are obtained. Allowance is made for both dynamic cost weighting and colored measurement noise. The polynomial form of the smoothing filter may be incorporated into a self-tuning algorithm and a simple adaptive smoother is discussed. >

Low-frequency noise in depletion-mode SIMOX MOS transistors

Abstract: Low-frequency noise measurements in depletion-mode SIMOX MOSFETs are reported. A simple model provides a reliable interpretation of the low-frequency noise in multi-interface depletion-mode transistors. An experimental procedure to separate noise contributions of front and back interfaces from noise due to bulk carrier fluctuations is described. The noises generated in the thin Si film and at the two Si-SiO/sub 2/ interfaces can be identified and characterized independently in terms of bulk properties and interface trap densities. Single-level traps at the back interface and defects in the volume are detected in high-temperature annealed materials. >

Speech detection apparatus not affected by input energy or background noise levels

Abstract: A speech detection apparatus capable of reliably detecting speech segments in audio signals regardless of the levels of input audio signals and background noises. In the apparatus, a parameter of input audio signals is calculated frame by frame, and then compared with a threshold in order to judge each input frame as one of a speech segment and a noise segment, while the parameters of the input frames judged as the noise segments are stored in the buffer and the threshold is updated according to the parameters stored in the buffer. The apparatus may utilize a transformed parameter obtained from the parameter, in which the difference between speech and noise is emphasized, and noise standard patterns are constructed from the parameters of the input frames pre-estimated as noise segments.

A vector approach for noise parameter fitting and selection of source admittances

Abstract: Simple vector concepts can be used in the determination of noise parameters from measured data. The use of such concepts leads to a simplification in the least-square fitting algorithm, complete determination of the admittance loci that produce ill conditioning, and simple criteria for the selection of source admittances that minimize the sensitivity of the noise parameters to experimental error. The sensitivity of the noise parameters to small perturbations in the reflection coefficients is compared for a group of source admittances presented in previous work. The results show that a great reduction in the error of the noise parameters can be achieved by properly selecting the source admittances. >

FET noise model and on-wafer measurement of noise parameters

Abstract: A noise model of a microwave FET (M.W. Pospieszalski 1988, 1989) is verified with on-wafer S-parameters and noise parameter measurement data. An excellent agreement between the model prediction and measurement results is obtained for a wide range of FET bias. It is shown that the equivalent drain temperature is a very strong function of the drain current, while the equivalent gate temperature is a very weak function of the drain current and, within the measurement error, it is equal to the ambient temperature for small drain current. >

Evaluation of the effect of noise on subjective image quality

Abstract: Basic considerations are given about the effect of static and dynamic noise on the contrast sensitivity of the eye and a numerical evaluation is made of published data on this subject. The results of this analysis are used to extend the square-root integral method for the evaluation of subjective image quality in such a way that it also can be applied to describe the effect of noise. Using an adapted version of the square- root-integral, calculations are made for experiments with pictures with various amounts of white noise found in literature. From a comparison between measurements and calculations, it appears that the image quality calculated in this way shows a very good correlation with the measurents.