Showing papers on "Noise reduction published in 1990"

Signal-to-noise efficiency in magnetic resonance imaging

Abstract: The dependence of signal-to-noise ratio (SNR) on the analog filter, the sampling rate and the number and dimensions of voxels is derived for magnetic resonance imaging (MRI). It is shown that the object signal-to-noise ratio scales directly with the voxel volume and the square root of the number of voxels. Defining an efficiency figure of merit as the SNR divided by the square root of the imaging time, it is shown that efficiency is always improved when imaging with the lowest possible resolution (largest voxel dimensions) consistent with viewing the desired anatomical detail. The results directly imply the relative efficiency of 3-D (volume), 2-D (plane), 1-D (line) and 0-D (point) imaging techniques. It is shown that spatial averaging is an inefficient method of noise reduction in MRI. As long as voxel size is maintained constant, one can image as many pixels in the readout direction as desired with no loss in SNR; that is, the number of pixels in the readout direction has no effect on the image SNR. Further, multiple sampling of each phase encoding value (to improve SNR) has no advantage over increasing the number of pixels in the phase encoding direction while leaving the voxel size constant. Some experimental observations are given.

Adaptive box filters for removal of random noise from digital images

Abstract: Adaptive box-filtering algorithms to remove random bit errors and to smooth noisy data have been developed. For both procedures, the standard deviation of those pixels within a local box surrounding each pixel is used. A series of two or three filters with decreasing box sizes can be run to clean up extremely noisy images and to remove bit errors near sharp edges. The second filter, for noise smoothing, is similar to the 'sigma filter' of Lee (1983). The technique effectively reduces speckle in radar images without eliminating fine details.

Apparatus and method for generating echographic images

Abstract: An apparatus and method to generate low power ultrasonic, echograph images of selected stationary and moving target objects having high resolution. The apparatus and method include: an apparatus for transmitting a plurality of ultrasonic signals into a selected area of tissue, and apparatus for receiving the corresponding ultrasonic echo signals for each of the transmitted signals. A correlator autocorrelates and cross-correlates the transmitted and received ultrasonic signals. The correlated signals are summed, combined in ratios and partitioned into visibility amplitude data, visibility phase data, differential phase data, closure amplitude data and closure phase data for mapping. The preferred apparatus and method thereafter perform a non-linear image processing, either by an iterative side lobe subtraction signal processing procedure to remove signal noise and/or by an interative hybrid mapping signal processing procedure. The resulting data map yields a high resolution image of the selected target with more data and less noise. Signal processing to show motion or target object changes after noise reduction is also disclosed. In a preferred embodiment, the apparatus and method is employed as a clinical diagnostic tool for generating non-traumatic, high resolution imaging of bodily tissue.

Noise reduction using a soft-decision sine-wave vector quantizer

Abstract: Noise reduction is performed in the context of a high-quality harmonic zero-phase sine-wave analysis/synthesis system which is characterized by sine-wave amplitudes, a voicing probability, and a fundamental frequency. Least-squared error estimation of a harmonic sine-wave representation leads to a soft decision template estimate consisting of sine-wave amplitudes and a voicing probability. The least-squares solution is modified to use template-matching with nearest neighbors. The reconstruction is improved by using the modified least-squares solution only in spectral regions with low signal-to-noise ratio. The results, although preliminary, provide evidence that harmonic zero-phase sine-wave analysis/synthesis, combined with effective estimation of sine-wave amplitudes and probability of voicing, offers a promising approach to noise reduction. >

Does the maximum entropy method improve sensitivity

Abstract: Maximum entropy reconstruction has been used in several fields to produce visually striking reconstructions of positive objects (images, densities, spectra) from noisy, indirect measurements. In magnetic resonance spectroscopy, this technique is notable for its apparent noise suppression and its avoidance of the artifacts that affect discrete Fourier transform spectra of short (zero-extended) data records. In the general case where the length of the reconstructed spectrum exceeds that of the data record or where a convolution kernel is incorporated in the reconstruction, no known analytical solution to the reconstruction problem exists. Consequently, knowledge of the properties of maximum entropy reconstruction has been mainly anecdotal, based on a small selection of published reconstructions. However, in the limiting case where the lengths of the reconstructed spectrum and the data record are the same and a convolution kernel is not applied, the problem can be solved analytically. The solution has a simple structure that helps explain several commonly observed features of maximum entropy reconstructions--for example, the biases in the recovered intensities and the fact that noise near the baseline is more successfully suppressed than is noise superimposed on broad features in the spectrum. The solution also shows that the noise suppression offered by maximum entropy reconstruction could (in this special case) be equally well obtained by a "cosmetic" device: simply displaying the conventional Fourier transform reconstruction using a certain nonlinear plotting scale for the vertical (y) coordinate.

Motion-adaptive video noise reduction system using recirculation and coring

Abstract: A method and apparatus are provided for reducing transmission path noise within a video signal so as to provide a noise reduced picture image upon a display. The method comprises the steps of: a. extracting low level picture details and the transmission path noise from the video signal, b. recirculating a fractional component of the extracted low level picture details and the transmission path noise, c. combining the recirculated fractional component with the extracted low level picture details and the transmission path noise to cancel the extracted low level picture details and thereby separate the transmission path noise, d. subtracting the separated transmission path noise from the video signal in the absence of motion of the picture image thereof in order to noise reduce non-moving portions of the picture image of the video signal, and e. selectively coring only the moving portions of the picture image of the video signal to noise reduce those moving portions, whereby noise is reduced in non-moving portions of the picture image by signal recirculation and is reduced in moving portions of the picture image by coring.

Mixer-ejector nozzle for jet noise suppression

Abstract: An aero/acoustic model test of a mixer-ejector nozzle was conducted at the 9 x 15 foot low-speed acoustic wind tunnel at NASA Lewis Research Center. The objective of the test was to get a preliminary assessment of ejector pumping and noise reduction potential of this device for possible application in the exhaust system of an advanced supersonic civil transport. The results of the test showed that goal levels of pumping were achieved. Exit pressure/temperature traverse data showed that there was good mixing between the primary and secondary streams. Acoustics data were dominated by shock noise; jet mixing noise levels were low because of a facility limit on primary temperature. The mixer-ejector did significantly reduce shock noise relative to the baseline conic nozzle. Because the relative magnitudes of jet mixing noise and shock noise were not in the correct proportion to properly model engine noise, an Effective Perceived Noise Level (EPNL) assessment was precluded.

Active control of propeller-induced noise fields inside a flexible cylinder

Abstract: An active noise control model has been evaluated for reducing aircraft interior noise. The structural noise transmission properties of an aircraft fuselage were modelled as a flexible cylinder excited by external acoustic dipoles simulating the noise produced by twin propellers. The amplitudes of an internal distribution of monopole control sources were determined such that the area-weighted mean square acoustic pressure was minimized in the propeller plane. The noise control model was evaluated at low frequencies corresponding to the blade passage frequency and first few harmonics of a typical turbo-prop aircraft. Interior noise reductions of 20 25 dB were achieved, over a substantial region of the cylindrical cross-section, with just a few monopole control sources. The most favorable interior noise reductions were achieved when the active noise control model was used in combination with propeller source phasing.

An accurate estimation of 3-D position and orientation of a moving object for robot stereo vision: Kalman filter approach

Abstract: A Kalman filter approach for accurately estimating the 3-D position and orientation of a moving object with respect to the robot base frame is proposed. This approach significantly differs from other approaches in that motion is estimated with the uncertainty of the position of a camera taken into account. Emphasis is also given to finding a solution to the following problem of motion estimation using a long sequence of images: the images taken from a longer distance suffer from a larger noise-to-signal ratio, which results in larger errors in 3-D reconstruction and, thereby, causes a series degradation in motion estimation. To solve this problem, the authors have derived a set of discrete Kalman filter equations for motion estimation. The measurement equation is obtained by analyzing the effect of white Gaussian noise in 2-D images on 3-D positional errors, and the system dynamic equation is formulated in terms of the measurement noise in 2-D images. Simulation results indicate that the Kalman filter equations derived present an accurate model for the estimation of 3-D position and orientation, thus providing significant error reduction in the presence of large measurement noise in a long sequence of images, as well as allowing a shorter transition period for convergence to the true values. >

Noise-reducing valve construction

Abstract: The invention provides a two-part casing for a multistage valve, itself configured for noise-reduction. Further noise reduction is achieved by complaint suspension of the valve in a two-part casing wherein separable parts of the casing define a cavity for containing nested stages of the valve. Provision is made for selective adjustment of compressional loading of the compliant suspension.

Tuned optical receivers for microwave subcarrier multiplexed lightwave systems

Abstract: An analysis of tuned optical-receiver noise performance for microwave subcarrier multiplexed lightwave systems is presented. The effect of correlation between the gate and the drain HEMT noise sources and the design of tuning networks to obtain partial noise cancellation were investigated. Anoptimization algorithm is used to determine the tuning elements values for minimizing noise of 16 dB for a 60-video-channel subcarrier multiplexing (SCM) system, and 12 dB for a 120-channel system, which allows a significant increase in passive optical network distribution capacity. Design results for tuned front-end receivers encompassing the effects of p-i-n, HEMT, and SCM band parameters are presented. >

Noise reduction based on microphone array with LMS adaptive post-filtering

Abstract: This letter presents a self-adapting noise reduction system which is based on a 4-microphone array combined with an adaptive Wiener filter. The LMS algorithm is used for adaptation. This filtering structure allows a simple implementation in the time domain on a sample-by-sample basis.

Decoder for subsampled video signal

Abstract: Disclosed is a MUSE decoder for a pixel signal sampled in accordance with Multiple Sub-nyquist Sampling Encoding (MUSE). An interframe interpolation circuit (142) applies a pixel signal Sg which is not subjected to noise reduction processing to an intrafield interpolation circuit (18′) for motion picture processing through a signal line (L5). A pixel signal delayed, which is outputted from delay circuits (24a, 24b) is subjected to a required noise reduction processing by an adder 60. Since the intrafield interpolation circuit 18′ receives the pixel signal Sg which is not subjected to the noise reduction processing, an adverse influence on a motion picture, which may be caused by the noise reduction, is prevented even in case that the delay circuits (24a, 24b) are shared for motion picture processing and still picture processing.

Delta-sigma converter with bandpass filter for noise reduction in receivers

Abstract: Analogue signals are converted to digital data by use of a Sigma-Delta modulator including a pair of bandpass filters and a feedback loop including a digital to analogue converter. The tendency of such a circuit to instability is reduced or eliminated by making small variations in the phase of a clock controlling feedback pulses relative to a clock controlling data output pulses. As an alternative, corrections may be made to the feedback pulses by an additional feedback loop providing pulses for correcting the feedback pulses.

Multiple widths yield reliable finite differences

Abstract: Many edge finders extract the signs of finite differences of image intensity values. Camera noise renders many of these signs unreliable. Previous algorithms for reducing noise are difficult to analyze, fail to detect faint or closely packed features, or handle restricted classes of features. The author proposes taking finite differences with a range of separations between data points, and choosing the narrowest response with statistically reliable sign. Fine detail is then detected by narrow operators. Faint features are filled in by wide operators, which can more reliably distinguish low-amplitude boundaries from noise. It is shown, both theoretically and empirically, that this method out-performs traditional Gaussian smoothing. Measurements of noise in a real camera system are also presented. >

Direct dynamic motion vision

Abstract: A method is presented for estimating the structure of a scene and the motion of an observer relative to the scene from a sequence of images. Two key features distinguish this method from previous solutions to this problem: no computation of optical flow is required, which leads to considerable speedup, and a Kalman filtering algorithm takes advantage of the entire sequence and leads to noise reduction. Dense depth is estimated by the Kalman filter and is obtained from motion by a least-squares method. No assumptions about surface structure or motion are made. Experimental results on real images are presented. >

Digital image noise reduction of luminance and chrominance based on overlapping planar approximation

Abstract: The present invention reduces noise in digital photographic images based on the assumption that images may be decomposed into two types of regions, smooth regions and edge regions. Smooth regions are areas of the image lacking any sharp detail, such as blue sky. Edge regions are regions containing sharp detail, such as edges and textured regions (such as grass). The present method reduces noise in the smooth regions by a mathematical blurring technique based on least squares regression. The blurring does not degrade the sharpness of the image, because there are no sharp details in the smooth regions. Edge regions are left undisturbed to maintain sharpness, but the noise is less noticeable in those regions than in the smooth regions. The method operates upon the luminance and chrominance component of a digital image.

Measurement of the frequency dependent impedance of major station equipment

Abstract: High-voltage direct-current (HVDC) converter stations generate radio frequency (RF) electromagnetic (EM) noise due to valve firing. The noise propagates into the AC and DC switchyards and along their corresponding transmission lines. This noise can affect the performance of adjacent communication, control, and computer equipment, and it can interfere with carrier system operation. Therefore, it is important to measure, predict, and mitigate the EM noise and interference. Measurements on equipment are necessary for the purpose of determining these impedance characteristics. A description is given of the instrumentation, improved measurement procedures, a systematic measurement program, and equipment representation concepts. All these have been developed and applied successfully in practice in the course of a project sponsored by the Electric Power Research Institute (EPRI). Transformer-related examples are used to illustrate the more relevant features of the study. >

High-speed, low-noise, fine-resolution TDI CCD imagers

Abstract: It is well demonstrated that the CCD TDI mode of operation provides increased photosensitivity, relative to a linear CCD array, without the sacrifice of spatial resolution. However, in order to utilize the advantages which the TDI mode of operation offers, attention should be given to the CCD TDI design and tradeoffs which exist between the pixel pitch, dead space between pixels in the horizontal direction, high speed, high photosensitivity, high spatial resolution and wide dynamic range. For example, a 2000 pixel ThI with an MTF of 0.5 will have an effective spatial resolution of only 1000 pixels. Other tradeoffs also are present, such as high speed versus power dissipation which exist both in the array and in the on-chip output video amplifier. Further, noise considerations exist at the video on-chip output amplifier. These include high speed which demands high bandwidth, Johnson noise, 1/f noise, reset noise, and output signal charge to voltage sensitivity. In this paper we shall describe the novel approach which we used in the design of a 2048 x 96 TDI CCD imager. We shall show how we addressed successfully these design tradeoffs and issues. Details about the analysis and design of a high speed on-chip output amplifier will also be presented. In addition, we shall explain how the use of buried channel MOSFET's enabled us to address and solve the power dissipation, speed, and noise issues.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Improvements to the noise reduction neural network

Abstract: Two experiments aimed at improving the performance of a noise reduction neural network are described. One approach is to split the last single affine transformation of the noise reduction neural network into two affine transformations and to adaptively control these transformations for noise components and speech components. The other is to further split the last single affine transformation to yield 22 affine transformations tuned for abstract concepts, or phonemes, and to adaptively control these transformations according to certain selection criteria. The latter is a refinement of the former. Both approaches are based on the fact that the speech component is more easily separated form the noise component in the second hidden layer output than in the physical input space of the network. >

Noise suppression in vehicles

Abstract: An integrated noise reduction and communication and/or entertainment system for vehicle use includes one or more signal sources, selected for example from radio tuners (2), microphones (7), cassette decks (6) or the like, providing electrical outputs to be mixed with an electrical output from an adaptive noise reduction unit (14), in a balanced mixer (18), the mixer output being supplied to one or more loudspeakers (22, 22A) located at suitable positions in the vehicle cabin.

Implementation of a charge integration system in a low-background application

Abstract: This paper describes an implementation of a recently developed charge amplifier, called Integration Amplifier, in the UKIRT 7 channel spectrometer (CGS2), resulting in the conversion of the CGS2 from the traditional transimpedance amplifier to a commercial charge integration scheme. Also described is a new data acquisition system; the news system allowed the implementation of a noise reduction algorithm which is a mojor factor in the improved sensitivity of CGS2. The noise reduction algorithm is presented together with a system diagram of CGS2.

Noise and modulation performance of Fabry-Perot and DFB semiconductor lasers with arbitrary external optical feedback

Abstract: A unified study of the noise and modulation characteristics of Fabry-Perot and DFB semiconductor lasers with arbitrary external optical feedback and arbitrary external cavity length is presented. By performing a small-signal analysis and taking into account the nonlinear gain saturation effect, generalisations of previous analytical results and novel features are presented in the following topics: intensity and frequency noise, intensity and frequency modulation responses, chirp reduction, frequency and damping rate of relaxation oscillations.

A nonlinear digital filter using multi-layered neural networks

Abstract: A nonlinear digital filter utilizing multilayered neural networks is proposed. This filter significantly reduces random noises superimposed on signals which contain sharp edges, while preserving their sharpness. In addition, degradation of the capability for noise reduction due to the increase of the noise power is greatly suppressed compared to previously proposed nonlinear filters. The high performance of this neural filter is demonstrated in computer simulations and actual image processing. When the noise power is small, the performance of the neural filter is almost the same as that of the epsilon -filter, which corresponds to a simplified neural filter; however, when the noise power is large, the effectiveness of the neural filter is clearly demonstrated. >