Showing papers on "Signal-to-noise ratio published in 1988"

A transformation for ordering multispectral data in terms of image quality with implications for noise removal

Abstract: A transformation known as the maximum noise fraction (MNF) transformation, which always produces new components ordered by image quality, is presented. It can be shown that this transformation is equivalent to principal components transformations when the noise variance is the same in all bands and that it reduces to a multiple linear regression when noise is in one band only. Noise can be effectively removed from multispectral data by transforming to the MNF space, smoothing or rejecting the most noisy components, and then retransforming to the original space. In this way, more intense smoothing can be applied to the MNF components with high noise and low signal content than could be applied to each band of the original data. The MNF transformation requires knowledge of both the signal and noise covariance matrices. Except when the noise is in one band only, the noise covariance matrix needs to be estimated. One procedure for doing this is discussed and examples of cleaned images are presented. >

Survey of excitation signals for FFT based signal analyzers

Abstract: The properties of ten different excitation signals are studied to analyze their suitability as excitation signals for fast Fourier transform (FFT)-based signal and network analyzers. Their influence on the measurement time, accuracy, and sensitivity to nonlinear distortions is described. The flexibility to create a customized amplitude spectrum is investigated. With this information it becomes possible to select the best excitation signal for many applications. >

OS-CFAR theory for multiple targets and nonuniform clutter

Abstract: The performance of a cell averaging constant false-alarm rate (CA-CFAR) detector degrades rapidly in nonideal conditions caused by multiple targets and nonuniform clutter. The ordered-statistic CFAR (OS-CFAR) is an alternative to the CA-CFAR. The OS-CFAR trades a small loss in detection performance relative to the CA-CFAR in ideal conditions for much less performance degradation in nonideal conditions. A formula is given for the detection probability of the OS-CFAR when there are multiple Swerling I targets in the CFAR window, and a formula is given for the probability of false alarm in nonuniform Raleigh clutter. >

The structure of the limit cycles in sigma delta modulation

Abstract: It is shown that when the input to a sigma-delta modulator is a DC level that can be expressed as a rational number b/a, when normalized with respect to the quantizer step, the output bit string is periodic with a period that is multiple of the denominator a. On the basis of number theory, the structure of these cycles for single-loop modulators is determined and the noise contribution is computed. Around such levels the noise has two peaks, for which the maximum value and the width are proportional to the relative signal bandwidth and to the inverse of the period of the cycle, respectively. The effect of the limit cycles on the performance of the A/D and D/A converters using sigma-delta modulation is discussed. A comparison between single-loop and double-loop modulators from the point of view of this phenomena is made. >

Wavelength calibration of Fourier-transform emission spectra with applications to Fe I

Abstract: Absolute calibration requirements for Fourier-transform and classical grating spectrometers are compared. The methods and precautions needed for good Fourier-transform spectrometer calibration are illustrated by application to the iron hollow-cathode spectrum. Relative wave numbers in the Fe i spectrum can be determined with a precision of 5.8 × 10−9 (±0.00014 cm−1) for lines of good signal-to-noise ratio. The accuracy of calibration is limited by the inadequacy of available standards, by pressure shifts, and by effects of illumination, all of which are found at the 0.0005-cm−1 level. A preliminary list of 300 well-determined Fe i lines in the range 17 350–26 140 cm−1 (5760–3830 A) is presented. The absolute accuracy is believed to be ±0.001 cm−1.

Performance of DS/SSMA communications in impulsive channels. II. Hard-limiting correlation receivers

Abstract: For pt.I see ibid., vol.COM-35, no.11, p.1179-88 (1987). In part I it was demonstrated that impulsive channel noise can be a serious detriment to the performance of direct-sequence spread-spectrum multiple-access (DS/SSMA) communications when conventional linear correlation reception is used. Here, a hard-limiting correlator as an alternative for reception of multiple-access transmission in impulsive channels is considered. For K asynchronous binary PSK DS/SSMA users sharing a linear channel corrupted by impulsive noise that is modeled at the output of the front-end filter of the receiver, techniques are developed for analyzing bit error probabilities of this hard-limiting receiver by exact computation for short spreading sequences, by approximation for longer spreading sequences, and by asymptotic limits for infinitely long spreading sequences. Performance is compared to that of the linear correlator under a variety of conditions, showing that hard-limiting correlation reception can offer substantial improvement over conventional systems in impulsive channels. However, the linear receiver is more effective against multiple-access noise only, and so a tradeoff emerges between rejection of impulsive noise and rejection of multiple-access interference. >

Broad-band array design using the asymptotic theory of unequally spaced arrays

Abstract: The problem of designing broadband linear arrays of omnidirectional sensors is addressed. Attention is restricted to the case of shaded delay-and-sum beamforming. Broadbanding is here defined to mean that the beam-pattern function has little or no frequency dependence of peak response, main-lobe width, plateau sidelobe level, and sinespace separation between the main lobe and the plateau sidelobe. The asymptotic theory of unequally spaced arrays is used to derive relationships between beam-pattern properties and array properties. These relationships are used to translate beam-pattern requirements into functional requirements on sensor spacings and amplitude shadings. The functional requirements are then used to derive a broadband array design. In addition to the design equations, the asymptotic theory is used to derive equations for main-lobe level, sidelobe level, beamwidth, and signal-to-noise ratio (SNR) gain in isotropic noise. A specific example is presented to clarify the concepts and verify that the design procedure actually works. >

Performance of differentially coherent digital communications over frequency-selective fading channels

Abstract: The performance of binary differential phase-shift keyed (DPSK) communications over frequency-selective wide-sense-stationary uncorrelated-scattering fading channels is considered. A technique is described for obtaining bounds on the average error probability for DPSK in terms of one or two parameters obtained from multipath spread or frequency-correlation-function channel measurements. A method of approximating the performance of systems using various data-pulse shapes and systems where it is difficult to obtain a complete channel characterization is developed. Numerical examples and comparisons of the most interesting cases are given. >

Confidence intervals for the signal-to-noise ratio when a signal embedded in noise is observed over repeated trials

Abstract: The problem of estimating the signal-to-noise ratio (SNR) when repeated measurements are made of a deterministic signal embedded in random noise is considered. An estimator is described, its asymptotic distribution is derived, and a method for constructing confidence intervals is proposed. The performance of the method is evaluated using simulated evoked potential data, and an application to real evoked potential data is presented. >

Bandwidth considerations for binary phase-only filters

Abstract: A binary phase-only filter (BPOF) bandwidth and, correspondingly, the performance with respect to stochastic noise are introduced as filter design parameters. A BPOF figure of merit is defined which references the matched filter. Analytical bounds on the BPOF signal-to-noise ratio are derived. The noise performance is illustrated with simulation results. It is demonstrated through analysis and simulation that BPOFs can be designed to perform well with respect to stochastic noise.

Adaptive matched filtering for QRS detection

Abstract: The authors have investigated the use of an adaptive filter to whiten the noise in the ECG (electrocardiogram) signal and adjust the matched filter response accordingly. They applied a simple QRS detection strategy to the filtered signal and evaluated the QRS detector with ECG data containing severe motion artifact and muscle noise. Preliminary results indicate that in the presence of severe motion artifact, adaptive matched filtering for QRS detection represents a significant improvement over application of a matched filter with a white noise assumption. >

Transform coding of speech using a weighted vector quantizer

Abstract: A medium-band speech coder is proposed that uses a weighted vector quantization scheme in the transformed domain. The linear prediction residue is transformed and vector-quantized. In order to control the quantization errors in the transformed domain, adaptively weighted matching is used instead of conventional adaptive bit allocation. Therefore, the residual signal can be reconstructed by the decoder, even if the spectral envelope parameters are destroyed due to transmission errors. This coder is also capable of maintaining higher SNR (signal-to-noise ratio) performance than time-domain vector quantization coders for a wide range of computation complexities and bit rates. Coded speech is natural and unaffected by background noise. The mean opinion score for this coder at 7.2 kb/s is comparable to that of 5.5-bit log PCM coded speech sampled at 6.4 kHz. >

Physical Limitations To Nonuniformity Correction In IR Focal Plane Arrays

Abstract: Simple nonuniformity correction algorithms currently in use can be severely limited by nonlinear response characteristics of the individual pixels in an IR focal plane array. Although more complicated multi-point algorithms improve the correction process they too can be limited by nonlinearities. Furthermore, analysis of single pixel noise power spectrums usually show some level of 1 /f noise. This in turn causes pixel outputs to drift independent of each other thus causing the spatial noise (often called fixed pattern noise) of the array to increase as a function of time since the last calibration. Measurements are presented for two arrays (a HgCdTe hybrid and a Pt:Si CCD) describing pixel nonlinearities, 1/f noise, and residual spatial noise (after nonuniforming correction). Of particular emphasis is spatial noise as a function of the lapsed time since the last calibration and the calibration process selected. The resulting spatial noise is examined in terms of its effect on the NEAT performance of each array tested and comparisons are made. Finally, a discussion of implications for array developers is given.

Architectures with improved signal-to-noise ratio in photonic systems with fiber-loop delay lines

Abstract: In a class of photonic systems, the intensity of the noise component caused by crosstalk from internal signals is investigated. The relevant systems are those that use loops of optical fiber as reentrant delay-line memory elements. Three architectural variations are shown to improve the ratio of the intensity of an internal signal to the intensity of this noise component. These variations include providing one extra memory element, minimizing the number of switches in a memory element that a photonic signal must traverse, and designing large systems hierarchically from smaller ones. These architectures, and the detailed formulas used to evaluate them, pertain specifically to a photonic time-slot interchanger, but the concepts are believed to generalize to similar photonic systems with applications in computing and signal processing. >

An improved method of embedding data into pictures by modulo masking

Abstract: An improved scheme using vertical block allocation to embed data in industrial-quality monochrome analog pictures by modulo masking is investigated. The video signal on each scan line is sampled, and a data bit is inserted into a block of three pels by a scrambling of the luminance level of only one pel in the block. The performance of the system is compared to that of a conventional system. The number of data bits embedded in an image for the proposed system is about 1.3 times as large as that of the conventional system. In addition, the SNR (signal-to-noise ratio) of the recovered image in the proposed system is increased by about 3-4 dB. >

Seismic exploration method and apparatus for cancelling non-uniformly distributed noise

Abstract: A seismic exploration method and apparatus in which non-uniformly distributed noise signals generated by wind, machinery, or surface wave propagation and received by the sensing geophones are cancelled by the utilization of adaptive filter processing. A number of horizontally sensitive geophones are disposed along the surface of the earth to detect surface waves induced in the surface of the earth at a plurality of spaced locations throughout an entire area of investigation. Similarly, a number of wind noise and mechanical noise detectors are utilized to detect airwave and noise signals at a plurality of locations throughout the area of investigation. The outputs of the surface wave detectors, the wind noise detectors and the mechanical noise detectors may then be utilized in conjunction with an adaptive filter to effectively cancel the effects of such interferences on reflected seismic wave signals at multiple locations throughout the area of investigation so that non-uniformly distributed noise signals may be accurately eliminated.

Signal-to-noise ratio of the bispectral analysis of speckle interferometry

Abstract: Monte Carlo simulations of an atmospheric phase screen, based on a Kolmogorov spectrum of phase fluctuations, were performed. Speckle patterns produced from the phase screens were used to derive statistical properties of power spectra and bispectra of speckle interferograms. We present the bispectral modulation transfer function and its signal-to-noise ratio at high light levels. The results confirm the validity of a heuristic treatment based on an interferometric picture of speckle pattern formation in deriving the attenuation factor and the signal-to-noise ratio of the bispectral modulation transfer function in the mid-spatial-frequency range. The derived modulation transfer function is also interpreted in terms of the signal-to-noise ratio at low light levels. A general expression of the signal-to-noise ratio of the bispectrum is derived as a function of the transfer functions of the telescope, the number of speckles, and the mean photon counts in the mid-spatial-frequency range.

Delay-Doppler resolution performance of large time-bandwidth-product linear FM signals in a multipath ocean environment

Abstract: Active sonar systems that transmit large time‐bandwidth (TW) product linear frequency‐modulated (LFM) waveforms and receive echoes from targets of unknown speed can suffer considerable correlation losses that cannot be predicted from conventional (narrow‐band) ambiguity function theory. As is well known, the theory can be modified to include the effects of Doppler distortion on large TW‐product signals by correlating the received signal against a reference that is a time‐compressed version of the transmitted signal. In this paper, the effects of multipath (or target highlight structure) and Doppler on the correlation process for rectangular weighted large TW‐product LFM waveforms are examined. When the received echo contains no multipath, the correlator peak output is a maximum for the reference channel that is closest in Doppler to the target. However, in a multipath environment, the correlator output peak does not generally occur at the correct Doppler reference channel. The spreading of the cross‐ambiguity function as a result of Doppler mismatch and multipath is calculated in terms of the delay‐Doppler coordinates and magnitude of the extrema. It is shown that, under multipath conditions, correct estimates of target parameters can be recovered through integration of the correlator output.

Method and apparatus for filtering noise from data signals

Abstract: The filtering technique of the present invention cancels or minimizes noise in data signals found to be caused by dynamic variations in the drilling process It has been discovered that the measurement of surface vibrations in the drill string and their effects on certain drilling equipment such as rotary table torque contains information that can be used to remove some or all of the pressure disturbance caused by these dynamic variations (vibrations) The result of removing some or all of the dynamic variation induced noise from the measurment-while-drilling (MWD) data signal yields a better signal to noise ratio (SNR) and therefore improves decoding of the MWD signal In accordance with another important feature of the present invention, it has been found that improved SNR results from taking the first derivative with respect to time of the torque measurement This first derivative measurement more closely resembles the actual disturbance in the standpipe pressure (SPP) In addition, even greater improvements in SNR may be obtained if the torque measurement is processed by a low-pass filter to equalize the effects of the two different channels In this case, the torque measurement still better approximates the disturbance in the SPP

Image-gathering system design for information and fidelity

Abstract: Image gathering and processing are assessed in terms of information and fidelity, and the relationship between these two figures of merit is examined. It is assumed that the system is linear and isoplanatic and that the signal and noise amplitudes are Gaussian, wide-sense stationary, and statistically independent. Within these constraints, it is found that the combined process of image gathering and reconstruction (which is intended to reproduce the output of the image-gathering system) behaves as optical, or photographic, image formation in that the informationally optimized design of the image-gathering system ordinarily does not maximize the fidelity of the reconstructed image. The combined process of image gathering and restoration (which is intended to reproduce the input of the image-gathering system) behaves more as a communication channel in that the informationally optimized design of the image-gathering system tends to maximize the fidelity of optimally restored representations of the input.

Convergent Behavior Of Modulus-restoring Adaptive Arrays In Gaussian Interference Environments

Abstract: The convergent behavior of the general class of modulus-resioral (M0RE) algorithms, which encompasses the class of constant-modulus algorithms (CMAs) are derived for the case where the MORE algorithms adapt a sensor array excited by a single non-Gaussian signal of interest (SOI) and full-rank Gaussian noise and co-channel interference. The class of modulus-mapping cost functions (MMCFs) are introduced, and stationary solutions of the MMCFs are obtained for arbitrary SOI modulus, and for arbitrary background-interference covariance distribution. It is shown (subject to mild restrictions) that the MMCFs will in general have two classes of stationary solutions in the single-SOI environment: signal-capture solutions where the desired signal is captured with maximum attainable SINR, and noise-capture solutions where the desired signal is nulled by the array. These solutions are also shown to be either rainima or saddle points of the MMCF, depending only on the variation in the desired-signal modulus.

Time delay estimation in unknown Gaussian spatially correlated noise

Abstract: A class of methods that estimate the difference in arrival time between two signals corrupted by spatially correlated Gaussian noise sources of unknown cross correlation is presented. The methods are based on the idea of comparing the similarities between the two sensor measurements in higher-order spectrum domains (bispectrum) rather than in the cross-correlation domain. It is demonstrated that the time delay estimation techniques based on higher-order spectra suppress the effect of correlated Gaussian noise sources and therefore exhibit improved performance over generalized cross-correlation methods. Studies are shown for different lengths of data records and signal-to-noise ratios. >

Adaptive enhancement of x-ray images

Abstract: The quality of x-ray images is significantly enhanced by adjusting the x-ray system operating parameters in real time during acquisition of x-ray data to take information about the part into account adaptively. X-ray energy, x-ray flux, and integration time can all by varied independently and in combination to improve the signal to noise ratio in the image. The x-ray data from a previous subsection of the image is processed to determine optimum system operating parameters for a next image subsection. x-ray tube voltage is adjusted to change x-ray energy and keep αL close to 2 over all image subsections. X-ray tube current is adjusted to change x-ray flux and data acquisition integration time is adjusted to keep the signal to noise ratio within limits.

A millimeterwave network analyzer based scatterometer

Abstract: The Millimeterwave Polarimeter (MMP) is a network-analyzer-based scatterometer and reflectometer system that has been developed to characterize radar clutter at 35, 94, and 140 GHz. A Hewlett-Packard 8510A network analyzer is used in the MMP system as a signal conditioner and processor to facilitate real-time data reduction, to reduce the short time-delay leakage noise inherent in traditional FM/CW radar, and to further enhance the signal-to-noise ratio of the system through signal processing techniques. Operation of the system at millimeter wavelengths is achieved with upconversion and harmonic downconversion. The use of harmonic downconverters permits low-frequency signal connections between components of the system and allows easy reconfiguration in either scatterometer, bistatic, or reflection/transmission modes. >

Analysis and assessment of errors in a parallel data acquisition system for electrical impedance tomography.

Abstract: Three possible data collection configurations for electrical impedance tomography are considered. These are serial current but parallel voltage measurement or full parallel operations. Measurement errors due to stray capacitances, common mode signals and contact impedance are considered. It is concluded that a parallel system offers advantages in terms of either speed or signal to noise ratio by may give larger common mode errors.

Volume NMR coil for optimum signal-to-noise ratio

Abstract: A RF volume coil with optimized signal-to-noise ratio, for NMR use, has a reduced length L c , which is between about 0.3r s and about 1.5r s , where r s is the radius of a sample-to-be-investigated, contained within the cylindrical volume coil, with the volume coil radius r c being between about 1.0r s and about 1.6r s . the "short" volume coil has an improved SNR for a voxel located substantially on the central plane of the coil, relative to the SNR of a "normal"-lenth volume coil with L c ≧4r s .

Signal-to-noise limitations in white light holography

Abstract: A simple derivation is given for the signal-to-noise ratio (SNR) in images reconstructed from incoherent holograms. Dependence is shown to be on the hologram SNR, object complexity, and the number of pixels in the detector. Reconstruction of involved objects becomes possible with high dynamic range detectors such as charge-coupled devices. We have produced such white light holograms by means of a rotational shear interferometer combined with a chromatic corrector. A digital inverse transform recreated the object.

An adaptive filter for noise cancelling

Abstract: The author introduces a nonlinear filter for adaptive noise cancelling. The derivation and convergence properties of the filter are presented. The performance, as measured by the signal-to-noise ratio between the signal and its estimate, is compared to that of the commonly used least-mean-square (LMS) algorithm. It is shown, through simulation, that the proposed canceller has, on the average, better performance than the LMS canceller. It is based on the Pontryagin minimum principle and the method of invariant imbedding, and its computational time is about 10% that of the LMS in the cases studied, which is a substantial improvement. >