Showing papers on "Noise measurement published in 1977"

A necessary and sufficient condition for quantization errors to be uniform and white

Abstract: In this paper, a necessary and sufficient condition is given to model the output of a quantizer as an infinite-precision input and an additive, uniform, white noise. The statistical properties of the quantization error are studied, and a detailed analysis for Gaussian distributed inputs is given.

Image restoration by Wiener filtering in the presence of signal-dependent noise.

Abstract: An optimum filter to restore the degraded image due to blurring and the signal-dependent noise is obtained on the basis of the theory of Wiener filtering. Computer simulations of image restoration using signal-dependent noise models are carried out. It becomes clear that the optimum filter, which makes use of a priori information on the signal-dependent nature of the noise and the spectral density of the signal and the noise showing significant spatial correlation, is potentially advantageous.

Maximum likelihood pitch estimation

Abstract: The problem of determining the pitch period of voiced speech is formulated as a maximum likelihood (ML) estimation problem for an unknown periodic signal in white Gaussian noise of unknown intensity. Modifications of the ML estimator for speech include removal of bias, derivation of a measure of confidence, and prefiltering for non-white noise. The pitch estimation procedure is evaluated in the frequency domain and related to comb filtering. An alternate derivation of the ML estimator, related to the procedure proposed by Lacoss for maximum likelihood spectral estimation is presented. The problem formulated is to find a filter which passes a periodic signal of period P without distortion while simultaneously suppressing, in an optimum manner, a noise signal having a known spectrum.

Audio processor for use in high noise environments

Abstract: There is disclosed apparatus for providing improved hearing acuity in the presence of high intensity background noise, such as that which exists at an industrial site. The apparatus employs an input microphone which is adapted to receive audio information including noise from the environment. The output of the microphone is applied to an expander circuit, which functions to expand the high pass audio frequency band according to the nature and intensity of the noise. In a first mode expansion is provided at a variable ratio and threshold which are selected according to environmental noise. The expanded audio signal is applied to a compressor limiter module to reduce the level and limit the amplitude of the signal within predetermined and safe limits. The compressor limiter output signal is applied to an adjustable low pass filter, which serves further to accommodate the processed signal to a particular user and to limit the frequency range of the output signals. This low pass filter has an output applied to a linear attenuator which serves to reduce the amplitude of the processed signal depending upon the particular noise levels at the site. This output signal is applied to a speaker which is located in proximity with the ear of a user to allow him to communicate relatively normally in high noise environments. Additional modes are described which employ both expansion and compression prior to limiting.

The origin and characterization of the primary signal, noise, and interference sources in the high frequency electrocardiogram

Abstract: An analysis technique was developed that enabled accurate measurement of the spectral and spatial characteristics of the predominant signal, noise, and interference components in high frequency electrocardiograms. A selective sampling technique, utilizing the isoelectric interval during the T-P segments, enabled the charactetistics of the internally generated noise and external interference to be obtained independently of the heart signal. Using this sampling technique, it was found that the electrical activity of skeletal muscles was the major source of noise in the high frequency electrocardiogram. The amounts of skdetal muscle noise present in the high frequency electrocardiograms of resting patients were measured quantitatively through the use of anesthetics and muscle relaxant drugs. In the normal resting state, the electrical activity of skeletal muscles contributed up to 15 dB of random noise to the spectra of waveforms generated by the electrical activity of the heart. Moderate, voluntary contraction of small groups of skeletal muscles increased their electrical output by 15 to 20 dB. Spectral analysis of high frequency electrocardiograms indicated that a dynamic range of approximately 66 dB is requized to adequately measure the signal and noise components present. Power spectra of high frequency electrocardiograms indicated that the 60-Hz power lines were the major source of interference Harmonics of the 60-Hz power line added significant interference components at frequencies as high as 1 kHz. The magnitude-squared coherence function between pairs of electrocardiographic leads, in conjunction with segment sampling and analysis techniques, was used to measure the spatial coherence of waveforms from electrical source associated with heart and skeletal muscle activity. The high values of spatial coherence of heart signal waveforms indicated that the body spatial transfer functions for electrical sources associated with heart activity acted as linear, noiseless transmission channels. The low spatial coherence of the waveforms generated by the electrical activity of skeletal muscles indicated that the body spatial transfer functions for electrical sources associated with skeletal muscle activity acted as noisy (nondeterministic) transmission channels. The spatial coherence of isoelectric interval waveforms was inversely proportional to both the distance between measurement sites and the amount of skeletal muscle activity.

A Directional Filter Diplexer Using Optical Techniques for Millimeter to Submillimeter Wavelengths

Abstract: An optical diplexer for injection of a local oscillator into a mixer, useful in the submillimeter and short millimeter range, is described. It has very low insertion loss for both the signal and local oscillator (L.O.) and high rejection of L.O. noise. Measured performance of a unit tested at 337 GHz indicates ~.2 dB loss for both inputs and 20 dB noise rejection.

The Measurement of Noise in Microwave Transmitters

Abstract: A tutorial review of the basis for transmitter noise measurements shows that noise is best described and measured as AM and FM noise. The determination of RF spectrum is done by calculation after the AM and FM noise are known. The contribution of AM noise to RF spectrum shape is determined by the power spectral density shape of the AM noise. The contribution of FM noise to RF spectrum is to make the shape that of an RLC circuit resonant response rather than a delta function with a sideband structure. The measurement of AM noise is done with a direct detector diode. The measurement of FM noise for frequencies above 5 GHz is done with a discriminator based on a one-port cavity resonator. The measurement of FM noise below 5 GHz is done with an improved transmission line discriminator which is described in detail. Measurement of low-power low-noise signal sources is made posbible with an injection-locked oscillator for a preamplifier to the discriminator. The most widely used baseband analyzer is the constant bandwidth superhetdrodyne wave or spectrum analyzer. Most differences in measurement results are resolved by understanding the baseband analyzers. At least the baseband spectrum analysis of transmitter noise measurements can be automated with worthwhile savings in time and improvement of documentation.

Measurement of far field combustion noise from a turbofan engine using coherence functions

Abstract: Coherence measurements between fluctuating pressure in the combustor of a YF-102 turbofan engine and far-field acoustic pressure were made. The results indicated that a coherent relationship between the combustor pressure and far-field existed only at frequencies below 250 Hz, with the peak occurring near 125 Hz. The coherence functions and the far-field spectra were used to compute the combustor-associated far-field noise in terms of spectra, directivity, and acoustic power, over a range of engine operating conditions. The acoustic results so measured were compared with results obtained by conventional methods, as well as with various semiempirical predictions schemes. Examination of the directivity patterns indicated a peak in the combustion noise near 120 deg (relative to the inlet axis).

Signal/noise ratios for domain-referenced tests

Abstract: Using the assumption of randomly parallel tests and concepts from generalizability theory, three signal/noise ratios for domain-referenced tests are developed, discussed, and compared. The three ratios have the same noise but different signals depending upon the kind of decision to be made as a result of measurement. It is also shown that these ratios incorporate a definition of noise or error which is different from the classical definition of noise typically used to characterize norm-referenced tests.

Robust Detectors for Signals in Non-Gaussian Noise

Abstract: The robustness of suboptimal nonlinear detectors for known discrete-time signals in non-Gaussian noise is investigated. The measure of robustness used is the degradation of asymptotic relative efficiency (compared to a linear detector) from that achieved by an optimal nonlinear detector. The first order density of non-Gaussian noise is modeled as a mixture of a small variance Gaussian background noise pdf and a large variance impulsive pdf.

Guidelines for Preparing Environmental Impact Statements on Noise

Abstract: : Guidelines are proposed for the uniform description and assessment of the various noise environments potentially requiring an Environmental Impact Statement for Noise. In addition to general, audible noise environments, the report covers separately high-energy impulse noise, special noises such as ultrasound and infrasound, and the environmental impact of structure-borne vibration. Whenever feasible and practical, a single-number noise impact characterization is recommended, based on the new concept of level-weighted population: i.e., the summation over the total population of the product of each residential person times a weighting factor that varies with yearly day-night average sound level outside the residence of that person. A sound-level weighting function for general impact and environmental degradation analysis is proposed, based on the average annoyance response observed in community response studies; this weighting function is supplemented by an additional weighting function at higher noise environments to quantify the potential of noise-induced hearing loss and general health effects. The evaluation of the environmental impact of vibration is derived from existing or proposed ISO standards. The report explains and justifies the procedures selected and gives examples of their application.

The Super-Schottky Diode

Abstract: The super-Schottky-barrier diode, a superconductor-semiconductor tunneling junction, has been established as the most sensitive detector of microwaves. These record sensitivities were obtained in both the video and mixing modes of operation. Measurements at X-band have yielded a video NEP of 5 x 10/sup -16/ W/Hz/sup 1/2/ and a mixer input noise temperature of 6 K. The super-Schottky mixer provides a front-end component for ultralow-noise receivers that is superior in bandwidth to available parametric and maser amplifiers and yet has a comparable noise temperature. This article reports the design, fabrication, and measurement of Pb on p-GaAs super-Schottky diodes which perform as nearly ideal low-noise mixers at 9 GHz.

Effects of external boundary layer flow on jet noise in flight

Abstract: The effects on jet flow of the external boundary-layer flow emanating from the trailing edge of an engine cowl in flight has been shown to be the main reason for the disparity between predicted and experimental results obtained from flight measurements. Flight simulation experiments indicate that the external boundary-layer flow tends to shield the jet flow in flight. This in turn modifies the jet noise source in flight and consequently the radiated noise from aircraft in flight. Close to 0/ = 90° and in the forward quadrant, this study indicates that the far-field jet noise and its spectrum scales approximately with the absolute jet velocity instead of the relative velocity as has been assumed in the existing prediction models.

Core Noise Measurements on a YF-102 Turbofan Engine

Abstract: Core noise from a YF-102 high bypass ratio turbofan engine was investigated through the use of simultaneous measurements of internal fluctuating pressures and far field noise. Acoustic waveguide probes, located in the engine at the compressor exit, in the combustor, at the turbine exit, and in the core nozzle, were employed to measure internal fluctuating pressures. Spectra showed that the internal signals were free of tones, except at high frequency where machinery noise was present. Data obtained over a wide range of engine conditions suggest that below 60% of maximum fan speed the low frequency core noise contributes significantly to the far field noise.

Position Sensitive Gamma Ray Detectors Using Resistive Charge Division Readout

Abstract: The analysis of an orthogonal strip, two-dimensional position sensitive high purity germanium gamma ray detector is discussed Position sensitivity is obtained by connecting each electrode strip on the detector to a resistor network and the ends of the network to charge sensitive preamplifiers The difference of the voltage pulses at the output of each preamplifier is proportional to the position at which the charge entered the resistor network and the sum of the voltage pulses is proportional to the energy of the detected gamma ray The results of the analysis of the energy and spatial signal noise show that the position resolution is proportional to the square root of the filter amplifier's output pulse time constant and that for energy measurement the resolution is maximized at the filter amplifier's noise corner time constant The electronic noise filtering system and experimental noise measurement results are presented

Voice detector circuit

Abstract: A voice detector circuit including a slew-rate limited amplifier at the input thereof so as to reduce the interference with the voice detection process caused by impulse noise. The slew-rate limited amplifier is essentially transparent to audio signals, while providing severe attenuation of impulse noise components. The actual determination of the voice content of the audio signal is provided by comparing the average syllabic-rate content of the impulse noise limited, audio signal with the peak noise content thereof. Additionally, a logarithmic amplifier is included for the purpose of optimizing the separation of the syllabic-rate envelope from the audio signal.

Observation of Mechanical Nyquist Noise in a Cryogenic Gravitational-Wave Antenna

Abstract: A 680-kg, Weber-type, cylindrical, gravitational-wave detector has been operated at liquid helium temperature. The average vibrational energy in the lowest longitudinal mode at 1315.3 Hz was found to be consistent with the level of thermal noise at the antenna temperature. An effective noise temperature of 0.39 K for pulse excitation was measured. This is a factor of 20 below the lowest published values for room-temperature detectors.

Some Measured and Calculated Effects of Forward Velocity on Propeller Noise

Abstract: Test flights using a twin-engine, high-wing, light STOL transport plane powered by free turbine engines driving three-bladed propellers, were conducted at the NASA Wallops Flight Center. A ground-based acoustic measuring range was set up, and aircraft tracking information was supplied by a radar system. The noise of the unshrouded propeller was measured under static and forward flight conditions, and the measurements were compared with theory. Under static conditions, the principal noise is related to the unsteady loading associated with the interaction of the propeller with persistent turbulent eddies passing through the propeller disk. Through the use of existing prediction techniques, it was shown that changes in the geometry such as different airfoil thickness distribution can affect the radiated acoustic pressure signatures. This test program confirmed that lower propeller noise levels are produced in forward flight than under static conditions and that the most significant reductions occur at the midfrequencies which dominate perceived and A-weighted noise levels.

Apple grove 750 KV project - two year statistical analysis of audible noise from conductors at 775 KV and ambient noise data

Abstract: This second paper on audible noise from Apple Grove 750 kV Test Project presents the results of a statistical analysis covering approximately two years of data on corona generated audible noise from three test lines. Also, statistical data is given on ambient noise in foul and fair weather, along with day-night variations. A comparison of measurements of audible noise from three sizes of four conductor bundles (diameter of 1.387 inch (35.1 mm), 1.196 inch (30.4 mm) and 1.0 inch (25.4 mm)) with measurements of ambient noise indicates the difference between the corona noise and background levels.

Accurate Measurements of Spectral Density of Phase Noise in Devices

Abstract: Systematic errors larger than 10 dB can occur in the measurement of the spectral density of phase unless considerable caution is exercised. Some potential problems due to the shape of the analyzer passband and the Fourier frequency dependence of mixers are discussed. Three measurement systems are analyzed to determine the conditions under which they may be used to make spectral density measurements with an accuracy of 0.2 dB.

An Adaptive Threshold System for Nonstationary Noise Backgrounds

Abstract: The problem of detecting target signals in an ocean environment using active sonar is complicated by the nonstationary background which usually consists of both ambient ocean noise and reverberation. on. In this paper a signal processing system capable of detecting a signal in nonstationary noise is introduced. This system makes use of the mean and variance time functions of the nonstationary noise background in order to design estimation filters which will cope with the nonstationarity. Appropriate statistics of the noise and signal (tone burst) plus noise have been obtained and are used to determine the probabilities of false alarm and detection and the receiver operating characteristics.

A phase-locking technique for estimating the ensemble average of time-series data

Abstract: A digital method employing Fourier analysis is presented, which in the absence of any external synchronizing signals can, with certain restrictions, 1) reduce random noise from repetitive frames of sampled input data; and 2) average waveforms which may differ in shape and which may occur in the presence or absence of random noise. The restrictions refer to the ability to select a window width which will each time encompass all of the aperiodic waveforms or one period of a periodic waveform. There is no restriction as to where in the window any waveform may appear. The new method is called the phase-locked time (PLT) average and should prove useful in a variety of situations.