Showing papers on "Signal-to-noise ratio published in 1985"

Unified snr analysis of medical imaging systems

Abstract: The ideal observer signal to noise ratio (SNR) has been derived from statistical decision theory for all of the major medical imaging modalities This SNR provides an absolute scale for image system performance assessment and leads to instrumentation design goals and constraints for imaging system optimisation since no observer can surpass the performance of the ideal observer The dependence of detectable detail size on exposure or imaging time follows immediately from the analysis A framework emerges from comparing data acquisition techniques, eg reconstruction from projections versus Fourier methods in NMR imaging, and time of flight positron emission tomography (TOFPET) versus conventional PET The approach of studying the ideal observer is motivated by measurements on human observers which show that they can come close to the performance of the ideal observer, except when the image noise has negative correlations-as in images reconstructed from projections-where they suffer a small but significant penalty

Quantitative data processing of parallel recorded electron energy-loss spectra with low signal to background

Abstract: The performance of a multielement array detector for measuring small spectral features superimposed on a large background has been examined. In order to have the sensitivity of detection limited only by counting statistics, two factors have to be critically considered: the nonuniform spatial response of the array detector, and the possible addition of noise by the collection system. For the special case discussed here, energy‐loss spectroscopy of 100‐keV electrons, the added noise was small. The detective quantum efficiency (DQE) was measured to be DQE=0.87. Several techniques reducing the effect of the nonuniform response were tested: normalizing the spectra with an experimentally measured response curve, electronic differentiation to reduce the background, dynamic gain correction, and two methods of experimentally averaging the spatial response by measuring a sequence of repositioned spectra. Preservation of the statistical information present in a representative energy‐loss spectrum is shown to be feas...

A New Time Domain Technique for Velocity Measurements Using Doppler Ultrasound

Abstract: A new technique for determining the Doppler frequency shift in a phase-coherent pulsed Doppler system is presented. In the new approach, the Doppler frequency shift is given directly in the time domain in terms of the measured I and Q components of the measured Doppler signal. The algorithm is based on an expression for the instantaneous rate of change of phase which separates rapidly varying from slowly varying terms. It permits noise smoothing in each term separately. Since the technique relies solely on signal processing in the time domain, it is significantly simpler to implement than the classic Fourier transform approach. In addition, the algorithm can be shown to give rigorously accurate values for instantaneous frequency and outperform the Fourier transform approach in poor signal-to-noise environments. Experimental results are presented which confirm the superiority of the new domain technique.

The Role of AM-to-PM Conversion in Memoryless Nonlinear Systems

Abstract: A generalized proof is presented that AM-to-PM conversion can only degrade, never improve, the intermodulation-noise performance of memoryless nonlinear systems with random input signals having even probability density functions, and a measure of degradation is defined. It is also shown for such signals that AM-to-PM conversion causes a deterministic constant phase shift to be added to the argument of the signal component at the output but has no other effect on its phase. This class of inputs includes one or the sum of several PSK signals, as well as large ensembles that can be modeled as Gaussian noise. The latter are dealt with by using Bussgang's theorem on input-output cross correlation. In the proof, Bussgang's theorem is extended to the complex case, to include phase as well as amplitude nonlinearities, yielding a complex version of the theorem. For Gaussian inputs it is shown that the undistorted signal and the intermodulation noise at the output of such systems are uncorrelated.

The effect of phase-shifter errors on the performance of an antenna-array beamformer

Abstract: The paper considers the random phase errors in the phase shifters which are used in an antenna array to steer the beam in the look direction, and analyzes the effect of these errors on the performance of the optimal processor which maximizes the output SNR by deriving the expressions for the output signal power, residual interference power, output SNR, and the array gain as a function of the variance of these errors. The paper also considers the phase quantization error which arises in the digital phase shifters and shows how the performance of the optimal processor depends on the number of bits of the digital phase shifters.

One-dimensional processing for adaptive image restoration

Abstract: A one-dimensional (1-D) approach to the problem of adaptive image restoration is presented. In this approach, we use a cascade of four 1-D adaptive filters oriented in the four major correlation directions of the image, with each filter treating the image as a 1-D signal. The objective of developing restoration techniques using this approach is to improve the performance of two-dimensional (2-D) techniques for image restoration. This differs considerably from previous 1-D approaches, the objectives of which have typically been to approximate a 2-D approach for computational reasons and not to improve its performance. The approach is applied to existing 2-D image restoration algorithms to illustrate its main advantage, namely its capability to preserve edges in the image while removing noise in all regions of the image, including the edge regions. Experimental results with images degraded by additive white noise at various SNR's (signal-to-noise ratios) are presented.

On the probability density of signal-to-noise ratio in an improved adaptive detector

Abstract: The main advantage of our previously presented [10] principal-component method of adaptive detection, in comparison with the method based on the inverse of the estimated covariance matrix, is that much less data is required to produce a given, needed level of SNR with high probability. In this paper we present an outline of the derivation of the approximate probability density of the SNR for this adaptive detector. To simplify the derivation we assume that the noise consists of a strong gaussian rank-one-covariance interference component plus a component of white gaussian background noise. The approximations and the final probability density are tested through simulation.

BPSK system with sounder to combat Rayleigh fading in mobile radio communication

Abstract: A simple method is proposed to remove random FM from a fast Rayleigh faded binary phase shift keying (BPSK) signal and the great improved performance of reducing the irreducible error is presented. In the mobile radio environment, there happens inevitably a fast Rayleigh fading which causes random FM. The phase error of the reference signal due to random FM will degrade the bit error rate (BER) performance, even if the signal-to-noise ratio is sufficiently high. In the realization of reliable mobile communication, removal of this random FM is an unavoidable theme. Proposed strategy is to transmit a sounding continuous wave (CW) together with a BPSK signal, each phase being kept orthogonal and each power being kept the same. This CW wave is also used as the reference signal in coherent reception. The BER performance of this system is worse by 3 dB than that of the coherent BPSK in a slow Rayleigh fading environment.

Resolving power of signal subspace methods for finite data lengths

Abstract: The signal subspace algorithm, based on functions of the eigenvectors and eigenvalues of a data covariance matrix, is often used as a "high resolution" parameter estimator. In this paper, the resolving power of a signal subspace method is studied. By employing the statistical distributions of the eigenvectors of a sample covariance matrix, a measure of the expected resolving power of the MUSIC source direction estimator is obtained. The analysis shows that the ability of the MUSIC algorithm to resolve two closely spaced sources incident on an array of sensors is strongly linked to the observation time, the signal to noise ratio, and the separation between the sources.

Thresholds in frequency estimation

Abstract: It is well known that frequency estimation exhibits a threshold effect: as the SNR dips below a certain value (while the number of data points remains fixed) the estimate's variance increases rapidly, and the Cramer-Rao bound is no longer attainable. In this paper, a simple, explicit formula for this threshold will be derived, for the case of a single complex sinusoid in white Gaussian noise. Since the presence of multiple frequencies raises this threshold, the formula serves as a lower bound for more complex problems. Because of the duality between time series and sensor arrays, the threshold is also applicable to angle of arrival estimation of a plane wave. Frequency estimation thresholds have been studied for some time. For instance, in Van Trees classic textbook, "Detection, Estimation, and Modulation Theory", plots of SNR versus estimation variance are shown which exhibit obvious thresholding behavior. Previous studies, however, lack formulas for the threshold locations. The case to which the formula applies (one complex sinusoid) is clearly the simplest to analyze. Unfortunately, more complex signal scenarios (such as one or more real sinusoids) are more commonplace. The methodology employed in deriving our formula is sufficiently flexible so as to suggest that similar thresholds are attainable for more complex signals.

Analysis of a Digital EMG Signal Processor in Dynamic Conditions

Abstract: The performances of digital electromyographic signal processors in dynamic conditions are determined by evaluating the root-mean-square value of the estimation error, including bias and variance of the estimates. The results thus obtained are valid for a wide class of myoprocessor schemes. The analysis has been used to optimize myoprocessors with moving-average smoothing filters, the length of which will be adapted to the local variability characteristics of the force in order to obtain a minimum overali estimation error.

Lower Bounds on Worst Case Probability of Large Error for Two Channel Time

Abstract: A lower bound on the worst case probability of large error for two channel time delay estimation using random signals is developed. The bound is based on the minimum probability of error Mary hypothesis test using a maximum likelihood estimator (MLE). The bound can only be evaluated approximately; however, it can be determined arbitrarily well via simulation if the MLE can be instrumented. The bound is evaluated in detail for uniform, lowpass signal and noise spectra and large bsndwidth time product. It is then compared with another lower hound based on a binary hypothesis test, with computer simulation results for an approximate MLE instrumentation, and with an upper bound established via computer simulation for a correlator. For a uniform, lowpass spectrum the M-ary bound is shown to be very nearly a greatest lower hound over the full range of input signal to noise ratio. Further, the correlator is shown to he very nearly an optimal instrumenta- tion in the sense of reaching the minimum probability of large error.

Enhancement of Raman Spectra Obtained at Low Signal-to-Noise Ratios: Matched Filtering and Adaptive Peak Detection

Abstract: Two methods of Raman spectral peak enhancement are described. The matched-filter approach employs an optimal filter to maximize the signal-to-noise ratio at its output. The impulse response of the matched filter is the reversed version of the profile of the spectral peak to be enhanced; therefore, the effectiveness of the matched filter depends on a priori knowledge of the peak's shape. The adaptive peak detector (APD) utilizes a filter having a time-varying impulse response. APD performance is relatively insensitive to peak shape, but is instead contingent on a number of user-specified parameters. Background is provided to acquaint the reader with signal processing terminology, and practical guidelines are given for computationally inexpensive computer implementations of both techniques. Examples are shown which demonstrate the effects of these two enhancement methods on simulated Raman spectra.

Characterisation of digital radio signals

Abstract: A method of characterizing a modulated carrier signal by generating electrical signals which represent arithmetic ratios and which can be used to construct a histogram indicative of the quality of the modulated carrier signal. For a digitally modulated carrier signal, time spaced samples of in band amplitude dispersion of the signal are generated and first counts are made of the number of occurrences of particular values of in band dispersion, over a range of such values. The error ratio of the decoded modulated carrier signal is also monitored and seconds counts are accummulated, of the numbers of occurrences of respective values of in band amplitude dispersions which at least substantially coincide with the value of said error ratio crossing a predetermined level. The ratio of each second count to each corresponding first count is then taken and these ratios are then representative of the probability that, at the respective in band amplitude dispersion value, the value of the error ratio will cross the predetermined level. When practicing the method with analogue modulated carrier signals, values of the signal to noise ratio are employed instead of the values of error ratio. A method of the invention permits construction of a histogram which pictorially represents the quality of the modulated carrier signal.

CMOS 8b 25MHz flash ADC

Abstract: THE IMPORTANCE O F CMOS flash A/D converter^"^'^ with superior cost-effectiveness and VLSI logic compatibility has increasingly been recognized for such applications as high-speed data acquisition and digital video-signal processing. However, they demand both a high conversion rate of above 20MIlz and a high-resolution of more than 8b; which have only recently been achieved with CMOS technology. This has been due to the lack of CMOS comparators suitable for use in this flash converter, which requires not only more stringent device-ratio matching for higher-resolution, but also high-speed and highdensity. This paper will cover a monolithic CMOS 8 b video-speed flash A/D converter using a high-speed offset-reduction comparator technique, which has experimentally shown the capability of a high-resolution of up to 9b and above at high videospeed. The converter was implemented by standard 2 p silicongate CMOS technology including only one capacitor process (AI-poly Si).

Noise Cancellation for Brainstem Auditory Evoked Potentials

Abstract: A technique for cancelling background noise in sensory evoked potential recordings is described. Since sensory evoked potentials are generally much smaller than the background noise, a large number of individual responses must be averaged. The proposed technique uses a second recording channel to estimate the background noise; a cancellation filter function is derived for each individual response in the average. The filter can vary from response to response; however, it does not attempt to adapt to long-term changes in the noise. The technique is intended for situations where the noise is intermittent or rapidly varying, as is often found in the operating room and intensive care unit. The technique was evaluated by a simulation using a known brainstem auditory evoked potential signal plus EEG samples obtained from a patient in the intensive care unit during an evoked potential test. The results showed that the technique could provide improvements in signal-to-noise ratio of 3-6 when the dominant noise component was external electrical interference. The effect was less when the background noise was muscle activity or spontaneous EEG, but there was still significant improvement. It was also found that the improvement obtained was approximately equal to the improvement that could be predicted from the formulation of the filter function.

MR imaging technology: maximizing the signal-to-noise ratio from a single tissue.

Abstract: The pulse-sequence equations for spin-echo magnetic resonance imaging were used to determine interpulse delay times that give the highest signal-to-noise ratio from a single tissue This theoretical result was then verified experimentally using 1-, 2-, and 5-mM/l copper sulfate solutions imaged on a 015-T resistive system Theoretical analysis determined the spin-echo interpulse delay times that maximize the signal-to-noise ratio from a single tissue as TEopt = TEmin, the minimum echo delay time permitted by the system, and, to a good approximation, TRopt = 127 T1 + 190 TEmin, with T1 the longitudinal magnetic relaxation time of the tissue Phantom measurements of the signal-to-noise ratio in a typical imaging system confirmed the theoretically determined TRopt values to within 7%

Double lock-in detection for recovering weak coherent radio frequency signals

Abstract: A single radio frequency lock‐in amplifier reduces broadband noise, but not rf pickup of the same frequency as the signal. If this pickup noise is at least 14 dB stronger than broadband noise, after both have passed through the lock‐in, then the signal‐to‐noise ratio can be improved by applying the lock‐in output to a second, low‐frequency lock‐in which is synchronized to an independent modulation of the signal. Weak coherent radio frequency signals buried in both rf pickup and broadband noise can be recovered by using this double lock‐in method, as demonstrated in a plasma diagnostics experiment.

Phase measurements using pseudo-random code

Abstract: Signal phase delay may be accurately measured by transmitting a pseudo-random noise coded reference signal, deriving in-phase and quadrature components of the received coded signal and then separately decoding the in-phase and quadrature components. This technique effectively compresses the transmitted signal pulse to a very narrow width (thereby simplifying discrimination of separate pulses with similar arrival times) while magnifying the pulse amplitude (thereby enhancing the signal to noise ratio).

Digital line receiver

Abstract: A stream of high-speed bipolar signals transmitted along standard twisted pair telephone wiring are detected by a circuit that includes a pair of differential amplifiers each having a signal input connected directly to one conductor of the twisted pair and a reference input connected through a peak voltage detector to the opposite conductor. The amplifiers function as an input signal comparator which generates a corresponding bit stream of logical ones in response to the bipolar signals that exceed 50% of the average peak voltage input. A threshold bias voltage corresponding to positive going signals on each conductor is generated by the detector and charges separate capacitors that connect each reference input to its conductor. The bias voltage and bipolar signals input to each amplifier are algebraically summed which doubles the differential input signal and results in an increased signal to noise ratio of about six decibels.

Precorrelation Filter Design for Spread-Spectrum Code Tracking in Interference

Abstract: Optimal design of a precorrelation filtering system for use in noncoberent direct-sequence code-tracking loops in the presence of colored noise is described, utilizing a linearized theory of loop operation. The design produces a bias-free loop with an odd S-curve, for which the rms tracking error is minimized. As a byproduct, one arm of the tracking loop provides an optimized signal for data-detection purposes. Widely applicable performance calculations, including rms tracking jitter, mean time to lose lock, and average detection signal-to-noise ratio, are given and illustrated, based on nonlinear models of loop operation.

Low cost storing of two electrical biosignals from DC to 20 kHz at more than 80 dB dynamic range.

Abstract: An instrumentation for storing electrical analog signals (DC to 20 kHz) was composed from a video recorder and a slightly modified pulse code modulation processor. As these components are mass products of the consumer electronics industry, the system price could be kept below 5000 DM. The setup allows one to register simultaneously two signals at DC to 20 kHz at a dynamic amplitude resolution of 16 bit [corresponding to a signal to noise ratio (S/N-ratio) of 96 dB] and AC-signals on the audiochannels of the stereo-video recorder. This is exemplified by recordings of insect chemoreceptors.

Boundary detection in speckle images

Abstract: Optimal statistical procedures are formulated for detecting object boundaries in speckle noise imagery. Although speckle noise is found in many applications, our principal interest is in synthetic aperture radar (SAR). The first procedure described is parametric. The ratio of local neighborhood averages is thresholded, which is a statistically natural approach since the noise is often modeled as multiplicative. A nonparametric procedure based on a linear rank statistic is also described, which can be shown to be locally most powerful (among rank tests). Examples are given, and comparisons are offered. The parametric scheme performed slightly better than the rank-order method in the examples, but the inherent robustness of the latter may recommend it for the practical application.

HF spectral occupancy and frequency planning

Abstract: The paper gives measurements of spectral occupancy for each ITU-defined user allocation, across the whole HF spectrum. Such measurements are being made currently during stable ionospheric conditions about midday and midnight, at the times of the winter and summer solstice, over a range of sunspot numbers. An application is given in which the occupancy results for a particular service (fixed in this case), are plotted as contours for direct comparison with the conventional OWF predictions. Thus it is possible to choose a frequency based on expected occupancy, as well as on predicted signal/atmospheric noise. The frequency choice for a particular 1500 km link is considered, when it appears beneficial to operate below the predicted OWF during daytime.

Signal-To-Noise-Ratio Measurements On Two High-Resolution Screen-Film Systems

Abstract: Measurements of the performance of two high-resolution radiographic screen-film systems at 50 and 30 kVp are given in terms of the noise equivalent number of recorded x-ray quanta (NEQ) and the detective quantum efficiency (DQE). Additionally, the MTF, contrast transfer function (CTF), and noise power spectra (NPS) of these imaging systems are presented. The NEQ, DQE, CTF, and NPS are shown to be strong functions of both spatial frequency and exposure. The shapes of these surfaces are significantly different from those published for general-purpose radiographic systems; these data provide a useful context within which screen-film system performance can be interpreted.

Noise Evaluation and Filter Design in CT Images

Abstract: Filtering CT images to remove noise, and thereby enhance the signal-to-noise ratio in the images, is a difficult process because CT noise is of a broad-band spatial-frequency character, overlapping frequencies of interest in the signal. We present a measurement of the noise power spectrum of a CT scanner and show that some form of spatially variant filtering of CT images can be beneficial if the filtering process is based upon the differences between the frequency characteristics of the noise and the signal. For evaluating the performance, we used a percentage standard deviation, an index representing contrast, a frequency spectral pattern, and several CT images processed with the filter.

Magnetoresistive random access cross-tie memory architecture and signal processing system

Abstract: A magnetoresistive random access memory array and signal processing system which provides an improved signal to noise ratio. The basic memory complex associated with the storage of a single binary digit is a quad of memory elements or cells which are addressed in complementary fashion. The enhanced read-out signal discrimination is had by utilizing a double-correlated double-sampling differential signal processing system in which complementary data is serially passed through the same path with repeated high-speed differencing which greatly reduces nonuniformity fixed patterns as well as correlated low frequency temporal noise.

New Approaches in the Adaptive Array Processing Field

Abstract: This paper presents an overview of modern array processing methods which have appeared recently. They are called “high resolution methods” because theoretically their resolving power increases with the observation time up to infinity, whereas it is limited by the signal to noise ratio in previous methods. This property is due to a more acute modeling of the medium. Besides the finite observation time, their limitations come from discrepancies between the model and the actual medium. Nevertheless, methods can be proposed to reduce their sensitivity.