Showing papers on "Background noise published in 1986"

An Adaptive Detection Algorithm

Abstract: A general problem of signal detection in a background of unknown Gaussian noise is addressed, using the techniques of statistical hypothesis testing. Signal presence is sought in one data vector, and another independent set of signal-free data vectors is available which share the unknown covariance matrix of the noise in the former vector. A likelihood ratio decision rule is derived and its performance evaluated in both the noise-only and signal-plus-noise cases.

Noise in solid state devices and circuits

Abstract: Mathematical Methods Noise Characterization Noise Measurements Thermal Noise Shot Noise Generation - Recombination Noise Flicker Noise or 1/f Noise Noise in Particular Amplifier Circuits Mixers Miscellaneous Problems Appendixes Index.

Exact maximum likelihood parameter estimation of superimposed exponential signals in noise

Abstract: A unified framework for the exact maximum likelihood estimation of the parameters of superimposed exponential signals in noise, encompassing both the time series and the array problems, is presented. An exact expression for the ML criterion is derived in terms of the linear prediction polynomial of the signal, and an iterative algorithm for the maximization of this criterion is presented. The algorithm is equally applicable in the case of signal coherence in the array problem. Simulation shows the estimator to be capable of providing more accurate frequency estimates than currently existing techniques. The algorithm is similar to those independently derived by Kumaresan et al. In addition to its practical value, the present formulation is used to interpret previous methods such as Prony's, Pisarenko's, and modifications thereof.

The statistical performance of the MUSIC and the minimum-norm algorithms in resolving plane waves in noise

Abstract: This paper presents an asymptotic statistical analysis of the null-spectra of two eigen-assisted methods, MUSIC [1] and Minimum-Norm [2], for resolving independent closely spaced plane waves in noise. Particular attention is paid to the average deviation of the null-spectra from zero at the true angles of arrival for the plane waves. These deviations are expressed as functions of signal-to-noise ratios, number of array elements, angular separation of emitters, and the number of snapshots. In the case of MUSIC. an approximate expression is derived for the resolution threshold of two plane waves with equal power in noise. This result is validated by Monte Carlo simulations.

The Effect of the Common Process Noise on the Two-Sensor Fused-Track Covariance

Abstract: This note deals with the effect of the common process noise on the fusion (combination) of the state estimates of a target based on measurements obtained by two different sensors. This problem arises in a multisensor environment where each sensor has its information processing (tracking) subsystem. In the case of an ?-s tracking filter the effect of the process noise is that, over a wide range of its variance, the uncertainty area corresponding to the fused estimates is about 70 percent of the single-sensor uncertainty area as opposed to 50 percent obtained if the dependence is ignored.

Ambient noise in the sea

Abstract: : By ambient noise we mean the prevailing, sustained unwanted background of sound at some spot in the ocean. It excludes momentary, occasional sounds, such as the noise of a close-by passage of a ship or of an occasional rain squall. It is the background of noise, typical of the location and depth where a measuring hydrophone is located, against which a signal," such as the sound of a submarine or the echo from a target, must be detected. Ambient noise also excludes all forms of self-noise, such as the noise of current flow around the measurement hydrophone and its supporting structure, and obviously must exclude all forms of electrical noise. Thus, ambient noise is what is left over, so to speak, after identifiable, occasional noise sources are accounted for.

Rate responses of auditory nerve fibers to tones in noise near masked threshold

Abstract: The rate responses of auditory nerve fibers were measured for best frequency (BF) tone bursts in the presence of continuous background noise. Rate functions for BF tones were constructed over a 32‐dB range of levels, centered on the behavioral masked thresholds of cats. The tone level at which noticeable rate changes are evoked by the tones corresponds closely to behavioral masked threshold at all noise levels used (−10‐ to 30‐dB spectrum level). As the noise level increases, the response rate to the background noise approaches saturation, and the incremental rate response to tones decreases. At high noise levels, the rate responses to tones of low and medium spontaneous rate fibers are larger than those of high spontaneous rate fibers. Empirical statistics of auditory nerve fiber spike counts are reported; these differ from those expected of a Poisson process in that the variance is smaller than the mean. A new measure of discharge rate is described that allows rate changes to be expressed in units of a standard deviation. This measure allows tone‐evoked responses to be interpreted in terms of their detectability in a signal detection task. Rate responses of low and medium spontaneous rate fibers are more detectable than those of high spontaneous rate fibers, especially at high noise levels. There appears to be sufficient information in the rate response of a small number of auditory nerve fibers to support behaviorally observed levels of detection performance.

Eigenstructure methods for direction of arrival estimation in the presence of unknown noise fields

Abstract: In eigenstructure methods for direction of arrival estimation of signal wavefronts, the additive noise is assumed to be spatially white, i.e., of equal power and uncorrelated from sensor to sensor. When the noise is nonwhite but has a known covariance, we can still handle the problem through prewhitening. However, there are no techniques presently available to deal with completely unknown noise fields. In this paper, we propose a solution to this unknown noise covariance problem for the case when the noise field is invariant under two measurements of the array covariance; situations where this assumption is valid are not uncommon in sonar applications. In fact, the idea has been used in certain so-called "despoking" algorithms for conventional beamformers. Results of computer simulations carried out to compare the performance of the new algorithm to earlier methods are also presented.

A new application of adaptive noise cancellation

Abstract: A new application of Widrow's adaptive noise cancellation (ANC) is presented in this paper. Specifically, the method is applied to the case where an acoustic barrier exists between the primary and reference microphones. By updating the coefficients of the noise estimation filter only during silence, it is shown that ANC can provide substantial noise reduction with little speech distortion even when the acoustic barrier provides only moderate attenuation of acoustic signals. The use of the modified ANC method is evaluated using an oxygen facemask worn by fighter aircraft pilots. Experiments demonstrate that if a noise field is created using a single source, 11 dB signal-to-noise ratio improvements can be achieved by attaching a reference microphone to the exterior of the facemask. The length of the ANC filter required for this particular environment is only 50 points.

Performance Analysis of the Censored Mean-Level Detector

Abstract: The censored mean-level detector (CMLD) is an alternative to the mean-level detector that achieves robust detection performance in a multiple-target environment by censoring several of the largest samples of the maximum likelihood estimate of the background noise level. Here we derive exact expressions for the probability of detection of the CMLD in a multiple-target environment when a fixed number of Swerling II targets are present. The primary target is modeled by Swerling case II, and only single-pulse processing is analyzed. Optimization of the CMLD parameters is considered, and a comparison to other detectors is presented.

Low-frequency noise in low 1/f noise dc SQUID's

Abstract: We demonstrate that the low‐frequency noise in our edge junction dc superconducting quantum interference devices, with a basic 1/f flux noise of 2×10−12 Φ20/Hz at 1 Hz, can all be accounted for in terms of junction critical current fluctuations. A novel modulation readout scheme is able to cancel the effect of junction critical current fluctuations and reduce our total noise to 1×10−12 Φ20/Hz at 0.1 Hz, a level that is three times lower than the lowest flux noise ever previously reported at this frequency.

Synthesis of 2-D separable denominator digital filters with minimum roundoff noise and no overflow oscillations

Abstract: This paper presents a new expression of the variance of roundoff noise in 2-D separable denominator digital filters described by Roesser's local state-space model. The covariance matrices and noise matrices necessary to analyze the variance of roundoff noise under the 1_2 norm scaling are obtained as the solutions of Liapunov equations. The synthesis problem of 2-D separable denominator digital filters with minimum roundoff noise are formulated under the 1_2 norm scaling. The synthesis method of 2-D minimum noise realizations under the 1_2 norm scaling is proposed by applying the minimization technique of roundoff noise in the 1-D case. Furthermore, it is shown that 2-D minimum noise realizations are rotated and scaled balanced realizations. Using this property, 2-D minimum noise realizations are proved to be free of overflow oscillations under zero input conditions, if their second-order modes are distinct.

Reflection noise in index-guided InGaAsP lasers

Abstract: We report a detailed study of the excess noise induced in index-guided InGaAsP laser structures by reflection feedback. The phenomena of high-frequency noise (1-5 GHz), low-frequency noise (< 100 MHz), and intensity fluctuations are shown to have a common physical origin in the unusual instability of the coupled laser-external cavity system. After a randomly occurring light intensity drop, the light output recovers in 10-15 steps, each corresponding to an external cavity roundtrip (high-frequency noise); the total recovery time corresponds to the low-frequency noise. The instability, and the subsequent noise, can be suppressed under conditions of very strong feedback such as obtained for lasers with anti-reflection-coated facets. The reflection noise characteristics are shown to be largely independent of the laser structure and structure modifications such as distributed feedback.

Annealing of ion‐implanted resistors reduces the 1/ f noise

Abstract: The conductivity fluctuations in 1‐MeV, boron‐implanted layers in silicon have been investigated at room temperature with the anneal temperature as a parameter. The 1/ f noise parameter α has been calculated from conductance, Hall voltage, and noise experiments. Annealing of the implants causes, in addition to an increase in the number of electrically active impurities, a decrease in the 1/ f noise parameter α by a factor of at least 50. The α value is proportional to eΔE/kTan, with an annealing activation energy ΔE=1.1 eV in a temperature range of 722 K

Frequency and intensity noise in injection-locked semiconductor lasers: Theory and experiments

Abstract: Analytical expressions for the power spectral densities of intensity and frequency noise of single-mode semiconductor lasers operating in a regime of injection locking are derived by appropriately taking into account the spontaneous emission processes into the lasing modes of both the master and slave lasers. They show how the noise spectra of the slave are influenced by the value of the injected power, by the difference between the emission frequencies of the master and slave optical cavities, and how they are correlated to the noise properties of both the master and the free-running slave. In particular, the very low frequency part of the frequency noise of the slave turns out to coincide with that of the master within a certain frequency region whose range increases as the values of the injected signal does, too. We also present measurements of the power spectral densities obtained by means of an experimental apparatus similar to that described in [1] and show how the experimental results are accounted for by the present theory.

The Nth-root stack; theory, applications, and examples

Abstract: Multichannel geophysical data are usually stacked by calculating the average of the observations on all channels. In the Nth-root stack, the average of the Nth root of each observation is raised to the Nth power, with the signs of the observations and average maintained. When N = 1, the process is identical to conventional linear stacking or averaging. Nth-root stacking has been applied in the processing of seismic refraction and teleseismic array data. In some experiments and certain applications it is inferior to linear stacking, but in others it is superior. Although the variance for an Nth-root stack is typically less than for a linear stack, the mean square error is larger, because of signal attenuation. The fractional amount by which the signal is attenuated depends in a complicated way on the number of data channels, the order (N) of the stack, the signal-to-noise ratio, and the noise distribution. Because the signal-to-noise ratio varies across a wavelet, peaking where the signal is greatest and approaching zero at the zero-crossing points, the attenuation of the signal varies across a wavelet, thereby producing signal distortion. The main visual effect of the distortion is a sharpening of the legs of the wavelet. However, the attenuation of the signal is accompanied by a much greater attenuation of the background noise, leading to a significant contrast enhancement. It is this sharpening of the signal, accompanied by the contrast enhancement, that makes the technique powerful in beam-steering applications of array data. For large values of N, the attenuation of the signal with low signal-to-noise ratios ultimately leads to its destruction. Nth-root stacking is therefore particularly powerful in applications where signal sharpening and contrast enhancement are important but signal distortion is not.

Towards a model for discrimination of broadband signals

Abstract: The conventional model for broadband discrimination assumes that resolution is limited by peripheral internal noise that is statistically independent across channels. In this paper, we extend this model in a number of directions. In particular, we compute, compare, and discuss the effects of interchannel correlation and central noise on the sensitivity index d’, for discrimination of overall level and discrimination of spectral shape.

Speech detection and recognition apparatus for use with background noise of varying levels

Abstract: A speech detection system compares the amplitude of an audio signal during successive time periods with speech detection thresholds, and generates an indication of whether the signal contains speech. It derives a background amplitude level from portions of the signal which it indicates do not contain speech, and improves its speech detection by altering the amplitude of the audio signal relative to the speech detection thresholds as a function of this background level. Preferably the background amplitude level is a moving average, which is repeatedly recalculated and repeatedly used to alter the relative amplitude of the audio signal and the detection thresholds. The apparatus uses a measure of the variability of the background amplitude to improve its speech detection. It generates start-of-speech and end-of-speech indications when the amplitude crosses respective thresholds for specified numbers of frames. The background amplitude level is calculated from frames which precede the start-of-speech indication by a predetermined amount and which follow the end-of-speech indication. The invention also provides a speech recognition system which compares the amplitudes an audio signal against the amplitudes of acoustic models of vocabulary words to determine which vocabulary words correspond to the signal. The system compensates for background noise by using the background amplitude level, described above, to alter the audio signal amplitudes relative to the acoustic model amplitudes.

Charge redistribution and noise margins in domino CMOS logic

Abstract: Domino CMOS gates suffer from an inherent noise margin problem as a result of charge redistribution between parasitic capacitances at internal nodes of the circuit under specific input conditions. This charge redistribution effect can destroy the noise margin and cause glitches at the output of a domino gate. This paper deals with circuit and layout techniques which can help in alleviating the problem. A simple analytical noise margin model for domino gates is discussed. The model is useful to monitor the noise margins of a domino gate while dimensioning the devices in the gate to obtain a specified gate delay. A new technique which allows the design of domino gates with a very large fan-in (typically 20 or more inputs), while maintaining good noise margins and acceptable gate delays, is presented. This technique is useful in, for example, decoder circuits.

Investigation of basic imaging properties in digital radiography. 7. Noise Wiener spectra of II-TV digital imaging systems.

Abstract: We used Wiener spectral analysis in order to investigate the different noise sources and the effects of various parameters such as pixel size, image intensifier (II) field size, and exposure level on the noise in an II–TV digital system. The digital Wiener spectra in terms of relative x‐ray intensity were determined directly from the digital noise data in terms of pixel values, by use of the characteristic curve of the imaging system. From averaged, subtracted, and/or combination images, the amount of structure mottle relative to the amount of quantum mottle was estimated. We found that a substantial amount of structure mottle was included in our II–TV digital subtraction angiography system, whereas the electronic noise of the TV system was quite small relative to the quantum and structure mottle. The effects of time jitter on the noise in single‐frame images (consisting of multiple video frames) and in subtracted and averaged images were also investigated.

Noise compensation for speech recognition using probabilistic models

Abstract: There are significant applications of speech recognition where a poor signal-to-noise ratio seriously degrades performance. In many cases the noise properties are constant or change only slowly. Klatt has proposed a solution based on noise masking and Bridle et al. one on noise marking. Our approach extends their techniques to make fuller use of the available data and we apply our method to recognizers based on whole-word pattern matching using both DTW and Markov model algorithms. The method also gives improved training where it is convenient for this to be done in a noisy environment, or where training in noise is essential because of voice quality changes that are a consequence of the noise.

On the Measurement of Noise Parameters of Microwave Two-Ports (Short Paper)

Abstract: A novel procedure for determining the noise parameters of microwave two-ports is introduced. In this procedure, the computations necessary to find the noise parameters from the set of measurements of noise temperature (noise figure) are greatly simplified. The assessment of accuracy with which the noise parameters can he determined from a given set of measurement data is straightforward.

Low-frequency noise and discrete charge trapping in small-area tunnel junction dc SQUID's

Abstract: We present measurements of the noise properties of very small‐area, high current density tunnel junction dc superconducting quantum interference devices (SQUID’s). The low‐frequency noise spectra display broad curves and bumps consistent with the presence of individual Lorentzian features superimposed on a background which is always much flatter than 1/f. When these features are most prominent, the voltage across the SQUID exhibits discrete switching behavior which we attribute to the trapping and untrapping of single electrons within the tunneling barrier. This observation, along with other evidence presented, suggests that critical current fluctuations displaying tunneling kinetics dominate the low‐frequency noise of these ultrasmall devices.

Magnetic equilibrium noise in spin-glasses: Eu0.4Sr0.6S.

Abstract: Mesure simultanee, par une nouvelle technique magnetometrique, du bruit lie aux fluctuations magnetiques et de la susceptibilite ac comme fonctions de la temperature; mise en evidence de la validite du theoreme de fluctuation-dissipation au-dessus et au-dessous de la temperature de gel; particularites des variations dans le domaine des basses frequences

Role of spontaneous emission in the dynamics of mode locking by synchronous pumping

Abstract: Numerical simulations showing the effect of spontaneous background noise in the dynamics of mode locking by synchronous pumping are presented. We demonstrate that this can cause macroscopic perturbations in the mode-locked pulses under nominally steady-state conditions1 and that the cavity length for which this effect is minimized is shorter than that required for generation of single short pulses.

The noise properties of high electron mobility transistors

Abstract: A simple analytic model for the HEMT, based on the Pucel theory for MESFET's, is developed which can be used to calculate the noise properties of the transistor. Good agreement between calculation and experiment is found. The dependence of noise temperature on gate length and channel thickness is presented.

Steady state analysis of the generalized sidelobe canceller by adaptive noise cancelling techniques

Abstract: Narrow-band adaptive noise cancelling techniques are used to study the generalized sidelobe canceller (GSC), a general form of linearly constrained adaptive beamforming structure In an environment which consists of a look-direction signal, one jammer, and additive receiver noise, exact expressions are derived for the Wiener solution, the steady state output signal-to-interference-plus-noise ratio (SINR 0 ), and performance improvement due to adaptation (PIA), defined as the ratio of SINR 0 after adaptation to SINR0 before adaptation These expressions are in terms of the signal directions and power levels, an arbitrary array geometry, and a general signal blocking matrix Next, easily evaluated scalar equations for PIA are given for two classes of constraints The first is constant gain in the look direction, and the second is constant gain plus a main beam zero first derivative in the look direction Under the further assumption of an equally spaced line array, even simpler equations for PIA result, and are used to show that for jammers arriving outside the heamwidth between first nulls (BWFN) region of the unadapted beampattern, the introduction of the additional main beam zero first derivative constraint leads to negligible degradation in PIA

Transit-Time Effects in the Noise of Schottky-Barrier Diodes

Abstract: Room-temperature noise measurements at 2.2, 12, and 97.5 GHz were performed on commercial silicon Schottky-barrier diodes and are shown to agree with the model presented in this work. This model is an extension of earlier work by van der Ziel on infrared detection in Schottky-barrier diodes. In the theoretical analysis, the electrons participating in the charge-transport process across the barrier are subdivided into four groups based on their initial velocity. The contribution of each group to the device conductance, susceptance, and current spectral intensity was incorporated including the effects of the transit time. By taking each of these effects into account, an accurate model which applies over a wide range of bias and frequency has been developed. Although the emphasis of this model has been on the high-frequency performance, the model also gives the correct results in the low-frequency limit.

Low-noise power-efficient oscillators: theory and design

Abstract: The paper proposes a new linear theory with experimental verification which shows how the spectrum of a high-efficiency oscillator varies with the amplifier noise figure (F), the voltage gain of the amplifier (G), the unloaded quality factor of the resonator (Qo), the loaded quality factor (QL) and the total RF power in the oscillating system (PFED). By defining the parameters, F, G, PFED, QL and hence QL/Qo fundamentally and precisely, the theory shows that F, G and QL/Qo are interdependent. The noise equation is therefore expressed in only a few variables, F, Qo, QL/Qo and PFED. Optimum operating conditions occur when the differential of sideband noise with respect to QL/Qo = O, so that if F is assumed to be constant, minimum sideband noise occurs when QL/Qo = 2/3. Many LC oscillators therefore operate far from the point at which minimum sideband noise occurs. By defining the oscillator power as the total power in the system, the variation of sideband noise power with DC input power can be described, permitting the optimum design of oscillators with minimum sideband noise for any given DC input power. Based on this theory a new highly efficient oscillator configuration has been designed satisfying this criterion. This configuration also reduces the pulling effect of the load sufficiently to permit its use directly as a transmitter. Experimental verification of the theory has been obtained at 1 MHz between the limits 0.09 < QL/Qo < 0.93. A 150 MHz low-noise oscillator, designed using the same approach, has been successfully demonstrated.