Showing papers on "Signal-to-noise ratio published in 1989"
TL;DR: A simple method is presented that should serve for estimating S/N in most cases and is generally not practical for use by a physician working in a clinical situation.
Abstract: The signal-to-noise ratio (S/N) in magnetic resonance imagining is one of the variables that must be measured when comparing the relative performance of different techniques Although various investigators and official groups have proposed different methods for measuring S/N, these are generally not practical for use by a physician working in a clinical situation The authors present a simple method that should serve for estimating S/N in most cases
343 citations
TL;DR: A procedure based on the removal of periodic noise by notch filtering in the frequency domain and the isolation of sensor noise and intrapixel variability using the semivariogram was applied easily and successfully to five sets of AVIRIS data from the 1987 flying season and could be applied to remotely sensed data from broadband sensors.
Abstract: To make the best use of narrowband Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data, an investigator needs to know the signal-to-noise ratio (SNR) The signal is land cover dependent and varies with both wavelength and atmospheric absorption, and random noise comprises sensor noise and intrapixel variability (ie variability within a pixel) The three existing methods for estimating the SNR are inadequate, since typical laboratory methods inflate, while typical dark-current and image methods deflate the SNR value The authors propose a procedure called the geostatistical method that is based on the removal of periodic noise by notch filtering in the frequency domain and the isolation of sensor noise and intrapixel variability using the semivariogram This procedure was applied easily and successfully to five sets of AVIRIS data from the 1987 flying season and could be applied to remotely sensed data from broadband sensors >
166 citations
TL;DR: It is shown that for magnetic resonance (MR) images with signal-to-noise ratio (SNR) less than 2 it is advantageous to use a phase-corrected real reconstruction, rather than the more usual magnitude reconstruction.
Abstract: We show that for magnetic resonance(MR)images with signal‐to‐noise ratio (SNR)<2 it is advantageous to use a phase‐corrected real reconstruction, rather than the more usual magnitude reconstruction We discuss the results of the phase correction algorithm used to experimentally verify the result We supplement the existing literature by presenting closed form expressions (in an MR context) for the probability distribution and first moments of the signal resulting from a magnitude reconstruction
166 citations
TL;DR: It is found that the orientation (or direction) of the cepstral vector is less susceptible to noise perturbation than the vector norm, and a family of distortion measures based on the projection between two cEPstral vectors is proposed, which have the same computational efficiency as the band-pass cepStral distortion measure.
Abstract: Consideration is given to the formulation of speech similarity measures, a fundamental component in recognizer designs, that are robust to the change of ambient conditions. The authors focus on the speech cepstrum derived from linear prediction coefficients (the LPC cepstrum). By using some common models for noisy speech, they show analytically that additive white noise reduces the norm (length) of the LPC cepstral vectors. Empirical observations on the parameter histograms not only confirm the analytical results through the use of noise models but further reveal that at a given (global) signal-to-noise ratio (SNR), the norm reduction on cepstral vectors with larger norms is generally less than on vectors with smaller norms, and that lower order coefficients are more affected than higher order terms. In addition, it is found that the orientation (or direction) of the cepstral vector is less susceptible to noise perturbation than the vector norm. As a consequence of the above results, a family of distortion measures based on the projection between two cepstral vectors is proposed. The new measures have the same computational efficiency as the band-pass cepstral distortion measure. >
166 citations
TL;DR: To improve the accuracy of the solution for (θ,φ), h+(t), h×(t), a near optimal filter is constructed for the noisy data which is deduced from the data themselves, which works for gravitational-wave bursts of any kind.
Abstract: We develop a method for determining the source direction (θ,φ) and the two waveforms h+(t), h×(t) of a gravitational-wave burst using noisy data from three wideband gravitational-wave detectors running in coincidence. The scheme does not rely on any assumptions about the waveforms and in fact it works for gravitational-wave bursts of any kind. To improve the accuracy of the solution for (θ,φ), h+(t), h×(t), we construct a near optimal filter for the noisy data which is deduced from the data themselves. We implement the method numerically using simulated data for detectors that operate, with white Gaussian noise, in the frequency band of 500–2500 Hz. We show that for broadband signals centered around 1 kHz with a conventional signal-to-noise ratio of at least 10 in each detector we are able to locate the source within a solid angle of 1×10^-5 sr. If the signals and the detectors’ band were scaled downwards in frequency by a factor ι, at fixed signal-to-noise ratio, then the solid angle of the source’s error box would increase by a factor ι^2. The simulated data are assumed to be produced by three detectors: one on the east coast of the United States of America, one on the west coast of the United States of America, and the third in Germany or Western Australia. For conventional signal-to-noise ratios significantly lower than 10 the method still converges to the correct combination of the relative time delays but it is unable to distinguish between the two mirror-image directions defined by the relative time delays. The angular spread around these points increases as the signal-to-noise ratio decreases. For conventional signal-to-noise ratios near 1 the method loses its resolution completely.
159 citations
TL;DR: In this paper, the effect of noise, including system noise, background noise, and cell-to-cell nonuniformity (spatial noise), is mathematically treated and experimentally verified for staring-mode infrared cameras.
Abstract: . The effect of noise, including system noise, background noise, and cell-to-cell nonuniformity (spatial noise), is mathematically treated and experimentally verified for staring-mode infrared cameras. Spatial noise is shown to be dominant in high background environments (3 to 5 Am or 8 to 12 Am imagery at background temperatures greater than 0°C) even after compensation. Camera sensitivity is quantified by a contrast signal-to-noise ratio that includes the effects of system, background, and spatial noise. Past analysis of camera performance has assumed that the cell-to-cell nonuniformity can be completely removed by using nonuniformity correction techniques. We present data showing that neither variations in detector spectral response nor excess low frequency noise can be fully corrected using existing nonuniformity correction techniques. Furthermore, we show that even the small amounts of nonuniformity that persist after the application of correction algorithms will significantly degrade camera performance.
152 citations
TL;DR: In this paper, a CELP voice coder is used to reconcile differences between the estimator and the filter of a code excited linear predictive (CELP) voicecoder.
Abstract: By reconciling differences between the estimator and the filter of a code excited linear predictive (CELP) voice coder, higher quality is achieved in the output speech. The pulse amplitudes and pitch tap gain are solved for simultaneously to minimize the estimator bias in the CELP excitation. Increased signal to noise ratio is accomplished by modifying the pitch predictor such that the pitch synthesis filter accurately reflects the estimation procedure used to find the pitch tap gain, and by improving the excitation analysis technique such that the pitch predictor tap gain and codeword gain are solved for simultaneously, rather than sequentially. These modifications do not result in an increased transmission rate or significant increase in complexity of the CELP coding algorithm.
151 citations
23 May 1989
TL;DR: The colored-noise prefilter greatly enhances the quality and intelligibility of LPC output speech for noisy inputs, and it is demonstrated that such gains are unavailable with white noise assumption Kalman and Wiener filters.
Abstract: A report is presented on experiments using a colored-noise assumption Kalman filter to enhance speech additively contaminated by colored noise, such as helicopter noise and jeep noise, with a particular application to linear predictive coding (LPC) of noisy speech. The results indicate that the colored-noise Kalman filter provides a significant gain in SNR, a clear improvement in the sound spectrogram, and an audible improvement in output speech quality. The authors demonstrate that such gains are unavailable with white noise assumption Kalman and Wiener filters. The colored-noise prefilter greatly enhances the quality and intelligibility of LPC output speech for noisy inputs. >
132 citations
TL;DR: The notion of optimal phase-only filters (OPOFs) that yield improved signal to noise ratios (SNRs) with the help of several analytical examples and simulation results are introduced.
Abstract: We introduce the notion of optimal phase-only filters (OPOFs) that yield improved signal to noise ratios (SNRs). We illustrate the improvement in SNR resulting from the use of OPOFs with the help of several analytical examples and simulation results.
114 citations
TL;DR: It is demonstrated that for almost all schemes of interest in practice, it is quite sufficient for the receiver to implement only four or six linear filters which represent proper reference signals, which makes possible an almost continuous tradeoff between receiver complexity and SNR (signal/noise ratio) losses.
Abstract: By separating the two complexity problems connected with optimum coherent CPM (continuous phase modulation) receivers, namely, the number of linear filters and the number of memory states required, a straightforward procedure for reducing the complexity is proposed and its usefulness is evaluated. A concise analysis of the inherent trellis encoder associated with CPM is given. It is then demonstrated that for almost all schemes of interest in practice, it is quite sufficient for the receiver to implement only four or six linear filters which represent proper reference signals. For a reduction in the number of memory states, decision-feedback sequence estimation together with minimization of the unprocessed intersymbol interference is proposed. Modifications of this procedure allow a state reduction without losses or with only negligible losses because only error events with large distances are affected. Combinations of these methods make possible an almost continuous tradeoff between receiver complexity and SNR (signal/noise ratio) losses. Several examples are considered for which evaluations of minimum Euclidean distances and simulation results are given. >
98 citations
TL;DR: A new technique based on object modeling and estimation is developed to achieve superresolution reconstruction from partial Fourier transform data and is robust with respect to Gaussian white noise perturbation to the measured data and withrespect to systematic modeling errors.
Abstract: Many problems in physics involve imaging objects with high spatial frequency content in a limited amount of time. The limitation of available experimental data leads to the infamous problem of diffraction limited data which manifests itself by causing ringing in the image. This ringing is due to the interference phenomena in optics and is known as the Gibbs phenomenon in engineering. Present tehniques to cope with this problem include filtering and regularization schemes based on minimum norm or maximum entropy constraints. In this paper, a new technique based on object modeling and estimation is developed to achieve superresolutionreconstruction from partial Fourier transform data. The nonlinear parameters of the object model are obtained using the singular value decomposition (SVD)‐based all‐pole model framework, and the linear parameters are determined using a standard least squares estimation method. This technique is capable, in principle, of unlimited resolution and is robust with respect to Gaussian white noiseperturbation to the measured data and with respect to systematic modeling errors. Reconstruction results from simulated data and real magnetic resonance data are presented to illustrate the performance of the proposed method.
TL;DR: The symbol error probability of two selection schemes, namely, maximum signal-to-noise ratio (M gamma ) selection and maximum output (MO) selection, for M-ary multidiversity reception over a Rayleigh fading channel are discussed.
Abstract: The symbol error probability of two selection schemes, namely, maximum signal-to-noise ratio (M gamma ) selection and maximum output (MO) selection, for M-ary multidiversity reception over a Rayleigh fading channel are discussed. The symbol error probability of the MO scheme is lower than that of the M gamma scheme. The more diversity receptions that are used, the larger is the difference. A simple expression of crossover average signal-to-noise ratio (per bit) is presented as a guideline for increasing the number of diversity receptions. >
23 May 1989
TL;DR: Analyses and performance predictions of an FFT-based narrowband interference suppression filter for use in PN (pseudonoise) spread-spectrum communications systems are presented and a maximum SNR filter scheme and several suboptimal thresholding algorithms are discussed.
Abstract: Analyses and performance predictions of an FFT-based narrowband interference suppression filter for use in PN (pseudonoise) spread-spectrum communications systems are presented. The received baseband signal is processed in fixed-length blocks, transformed to the frequency domain with an FFT, filtered there by using an appropriate weighting, and then transformed back to the time domain. A general expression for the symbol matched filter signal-to-noise ratio (SNR) is given in terms of signal parameters and filter weighting coefficients. An optimal (maximum SNR) filter scheme and several suboptimal thresholding algorithms are discussed. Performance predictions, based on a realistic high-frequency interference model, are presented for these algorithms and FFT-based architecture. >
TL;DR: An adaptive noise cancelling structure suitable for situations where the noise reference transducer is closely spaced relative to the primary transducers, based on two LMS delay line cancellers with cross-coupled feedback is described.
TL;DR: Numerical evaluations using the image of a tank indicate that using such a filter can provide an improvement in SNR of ~5 dB over the conventional binary phase-only filter (BPOF), superior to the 1-dB improvement obtained for that image by varying the threshold line angle (TLA) in filter binarization.
Abstract: An efficient algorithm for designing a ternary valued filter yielding the highest signal to noise ratio (SNR) is utlined. Numerical evaluations using the image of a tank indicate that using such a filter can provide an improvement in SNR of ~5 dB over the conventional binary phase-only filter (BPOF). This is superior to the 1-dB improvement obtained for that image by varying the threshold line angle (TLA) in filter binarization. Simulation results are presented. They agree with the numerically computed SNRs.
TL;DR: A polarity thresholding algorithm that has recently been developed for split-spectrum processing for ultrasonic coherent noise reduction is theoretically analyzed to evaluate its performance and some experimental results of SNR enhancement obtained with this algorithm are presented.
Abstract: A polarity thresholding algorithm that has recently been developed for split-spectrum processing for ultrasonic coherent noise reduction is theoretically analyzed to evaluate its performance. The probability density function (PDF) of the output of the algorithm is derived and used to calculate the theoretical signal-to-noise ratio (SNR) enhancement and the receiver operating characteristics. The performance limits of the algorithm are also established. Some experimental results of SNR enhancement obtained with the polarity thresholding algorithm are presented. >
Journal Article•
TL;DR: Pattern synthesis based on either optimization of signal-to-noise ratio or noise minimization is developed and a planar array example applied to a constant-amplitude solid-state phased array is discussed.
Abstract: Pattern synthesis based on either optimization of signal-to-noise ratio or noise minimization is developed. The connection between adaptive arrays and adapted pattern shape is used in synthesizing linear array patterns with freedom to adjust both amplitude and phase. Phase-only pattern synthesis is described in connection with suppressing interference or clutter in proportions of space. A planar array example applied to a constant-amplitude solid-state phased array is discussed. >
TL;DR: Using a generalized likelihood ratio test, it is proven that, for a symmetric noise probability density function, the detection performance is asymptotically equivalent to that obtained for a detector designed with a priori knowledge of the noise parameters.
Abstract: The problem of detecting a signal known except for amplitude in non-Gaussian noise is addressed. The noise samples are assumed to be independent and identically distributed with a probability density function known except for a few parameters. Using a generalized likelihood ratio test, it is proven that, for a symmetric noise probability density function, the detection performance is asymptotically equivalent to that obtained for a detector designed with a priori knowledge of the noise parameters. A computationally more efficient but equivalent test is proposed, and a computer simulation performed to illustrate the theory is described. >
TL;DR: In free space optical data transmission systems illumination of the receiver antenna by background radiation will decrease the signal to noise ratio and expressions for that degradation are derived both for direct and for heterodyne/homodyne receivers.
Abstract: In free space optical data transmission systems illumination of the receiver antenna by background radiation will decrease the signal to noise ratio. We derive expressions for that degradation both for direct and for heterodyne/homodyne receivers. Examples are given for cases where the sun, the moon, the earth, and Venus illuminate earth orbiting receivers operating at wavelengths of 0.85 μm, 1.3 μm, and 10.6 μm. Direct detection receivers will typically suffer a degradation of between 5 and 15 dB at λ = 0.85 μm and λ = 1.3 μm when illuminated by the sun. Heterodyne/homodyne receivers at 10.6 μm degrade more with sun radiation (typically 4 dB) than at the smaller wavelengths (≈0.3 dB). The moon, earth, and Venus cause negligible reduction of signal to noise ratio.
TL;DR: An intercept receiver which uses a transform-domain-processing filter is described, which detects direct-sequence binary-phase-shift-keyed (DS-BPSK) spread-spectrum signals in the presence of narrow band interference by employing adaptive narrowband interference rejection techniques.
Abstract: An intercept receiver which uses a transform-domain-processing filter is described. This receiver detects direct-sequence binary-phase-shift-keyed (DS-BPSK) spread-spectrum signals in the presence of narrowband interference by employing adaptive narrowband interference rejection techniques. The improvement in the system performance over that of conventional detection techniques is shown by presenting the results of experimental measurements of probability of detection versus false alarm for an enhanced total power detector. Also presented are certain results corresponding to detection of the spectral lines generated at twice the carrier frequency, wherein the goal is often not just signal detection, but also carrier frequency estimation. The receiver uses one of two transform-domain-processing techniques for adaptive narrowband interference rejection. In the first technique, the narrowband interference is detected and excised in the transform domain by using an adaptive notch filter. In the second technique, the interference is suppressed using soft-limiting in the transform domain. >
TL;DR: A water referencing algorithm for addressing the spectroscopic imaging problems of low SNR and main field inhomogeneities is proposed using the location of the water peak from each voxel and additional a priori information results in a parametric estimation problem.
Abstract: A water referencing algorithm for addressing the spectroscopic imaging problems of low SNR and main field inhomogeneities is proposed. Using the location of the water peak from each voxel and additional a priori information results in a parametric estimation problem. Optimum estimates of the desired metabolite concentrations can then be computed and displayed in an image format. The algorithm is shown to be very stable in the presence of noise and is insensitive to B0 inhomogeneity. A detailed error analysis as well as extensions to the basic data model are also discussed. Results from both 1H and 31P experiments are presented to verify the predicted good performance even with extremely low signal-to-noise ratio data. © 1989 Academic Press, Inc.
TL;DR: In this article, an inexpensive apparatus for measuring small rotations based on the use of an external intensity stabiliser for a laboratory He-Ne laser is discussed, and the limitations on the signal-to-noise ratio as consequence of the detector noise, shot noise and technical noise present in the system are examined.
Abstract: The change in intensity of light transmitted through an analysing polariser after reflection from a ferromagnetic surface can be related to the magneto-optic Kerr rotation. The construction of an inexpensive apparatus for measuring small rotations based on the use of an external intensity stabiliser for a laboratory He-Ne laser is discussed. The authors examine the limitations on the signal-to-noise ratio as consequence of the detector noise, shot noise and technical noise present in the system. An intensity stability of 0.05% has been achieved and has enabled them to obtain hysteresis loops from a 5 nm thick iron film with a signal to noise ratio of 40:1 for a data collection time of 100 s.
TL;DR: A time-division multiple-access system with four-level quadrature amplitude modulation point-to-point links strikes the right balance between flexibility and complexity and can support rates of at least 1 Mb/s.
Abstract: Adaptive equalization is used to ensure that the outage probability is less than 10/sup -3/ for a target bit error rate of 10/sup -4/ in buildings with RMS delay spread of up to 100 ns. A time-division multiple-access system with four-level quadrature amplitude modulation point-to-point links strikes the right balance between flexibility and complexity. It is shown that such a system can support rates of at least 1 Mb/s. >
TL;DR: In this paper, it is shown that the amplitude distribution of the phase-induced intensity noise (PIIN) can become highly nonGaussian with extremely short tails, and a given low error rate may be maintained at significantly smaller signal-to-noise (S/N) ratio values than Gaussian statistics would predict.
Abstract: It is experimentally demonstrated that when multipath optical-fiber systems are driven by single-mode sources whose linewidth is small compared with the detection system's electrical bandwidth, the amplitude distribution of the phase-induced intensity noise (PIIN) can become highly nonGaussian with extremely short tails. Consequently, a given low error rate may be maintained at significantly smaller signal-to-noise (S/N) ratio values than Gaussian statistics would predict. This effect is particularly prominent at the large S/N values at which digital communication systems usually operate. The theory of PIIN is briefly described, including a quantitative treatment of the photocurrent S/N bit-error-rate (BER) relationship in systems where both PIIN and thermal noise exist. A technique that enables the measurement of the S/N-BER relationship well into the tails of the noise-photocurrent distribution, where ordinary histogramming techniques fail due to insufficient resolving power, is also described. >
TL;DR: It is shown that the QPOF can have improved correlation response and signal-to-noise ratio with respect to the Hartley binary phase-only filter for some objects.
Abstract: A quad-phase-only filter (QPOF) is introduced. It is shown that the QPOF can have improved correlation response and signal-to-noise ratio with respect to the Hartley binary phase-only filter for some objects.
TL;DR: The technique advocated in this paper makes use of an inexpensive, non-invasive phonocardiographic (phono) transducer which facilitates safe long-term patient monitoring and reduces the number of erroneous estimates during periods of low signal to noise ration (SNR).
Abstract: The technique described makes use of an inexpensive, non-invasive phonocardiographic (phono) transducer which facilitates safe long-term patient monitoring. A variable comb filter applied to the frequency domain is used in order to take full advantage of the harmonic content of fetal heart signals. Real time estimation of FHR has been achieved on pre-recorded phono signals lasting eight hours. Recordings with a reasonable signal quality were analysed and some of the results are given. Advanced signal processing techniques followed by Artificial Intelligence (AI) algorithms reduce the number of erroneous estimates during periods of low signal to noise ratio (SNR). The resulting FHR time series is stored on the host computer for further processing, display and parameter extraction.
11 Jun 1989
TL;DR: In this paper, the error probability at large signal-to-noise ratios of rate-1/2 convolutionally encoded CPFSK (continuous phase frequency-shift keying) with an optimum non-coherent detector on an additive white Gaussian noise channel was considered.
Abstract: The authors consider the error probability at large signal-to-noise ratios of rate-1/2 convolutionally encoded CPFSK (continuous-phase frequency-shift keying) with an optimum noncoherent detector on an additive white Gaussian noise channel. The performance is given in terms of a parameter called the minimum squared normalized equivalent Euclidean distance which plays the same role mathematically as the minimum squared normalized Euclidean distance used for coherent detectors. It is shown that by introducing convolutional coding the error performance is significantly improved. The authors propose a decoding algorithm for these convolutionally encoded CPM schemes which is based on a limited tree search algorithm and uses the maximum-likelihood decision rule for noncoherent detection. Computer simulations show that the degradation in error performance compared to the performance of the optimum coherent Viterbi detector is less than 1 dB with a relatively simple noncoherent detector on an additive white Gaussian noise channel for most of the schemes considered. >
23 May 1989
TL;DR: The study shows that removing less than the full amount of noise and whitening it improves spectral estimation and speech device performance.
Abstract: The authors present the results of a study designed to investigate the effects of subtractive-type noise reduction algorithms on LPC-based spectral parameter estimation as related to the performance of speech processors operating with input SNRs of 15 dB and below. Subtractive noise preprocessing greatly improves the SNR, but system performance improvement is not commensurate. LPC spectral estimation is affected by the character of the residual noise which exhibits greater variance and spectral granularity than the original broadband noise. The study shows that removing less than the full amount of noise and whitening it improves spectral estimation and speech device performance. Techniques and performance results are presented. >
TL;DR: In this article, a survey of ambient noise data taken under winddominated conditions in shallow water shows substantial differences in spectrum level (often greater than 10 dB) under the same windspeed and sea state conditions.
Abstract: A survey of ambient noise data taken under wind‐dominated conditions in shallow water shows substantial differences in spectrum level (often greater than 10 dB) under the same windspeed and sea‐state conditions. This area‐dependent effect, which is evident even in long‐term averaged data, is caused by differences in ocean bottom properties, water depth, and sound‐speed profile. A shallow‐water, wind‐noise model is used to remove propagation effects from selected data sets that cover a broad range of bottom types, water depths, and sound‐speed profiles. These reduced data exhibit less variability, and it is concluded that the model is capable of predicting the effect of the shallow‐water environment on the spectrum level of wind‐dominated ambient noise. The results also suggest that commonly used universal ambient noise curves that do not take into account environmental differences are inferior to the model predictions in shallow water.