Showing papers on "Signal-to-noise ratio published in 1981"

Generalized image combinations in dual KVP digital radiography.

Abstract: Dual energy basis decomposition techniques apply to single projection radiographic imaging. The high and low energy images are non-linearly transformed to generate two energy-independent images characterizing the integrated Compton/photoelectric attenuation components. Characteristic linear combinations of these two basis images identify unknown materials, cancel known materials, and generate synthesized monoenergetic images. The problems of intervening materials and material displacement are solved in general for a wide class of clinical imaging tasks. The basis projection angle identifies one from a family of energy selective imaging tasks, and such performance measures as the contrast enhancement factor (CEF) and signal to noise ratio (SNR) are expressed as functions of this angle. Algorithms for the decomposition of high and low energy measurements are compared and experimental images are included.

An overview on the time delay estimate in active and passive systems for target localization

Abstract: Sonar and radar systems not only detect targets but also localize them. The process of localization involves bearing and range estimation. These objectives of bearing and range estimation can be accomplished actively or passively, depending on the situation. In active sonar or radar systems, a pulsed signal is transmitted to the target and the echo is received at the receiver. The range of the target is determined from the time delay obtained from the echo. In passive sonar systems, the target is detected from acoustic signals emitted by the target, and it is localized using time delays obtained from received signals at spacially separated points. Several authors have calculated the variance of the time delay estimate in the neighborhood of true time delays and have presented their results in terms of coherence function and signal and noise autospectra. Here we analyze these derivations and show that they are the same for the case of low signal-to-noise ratio (SNR). We also address a practical problem with a target-generated wide-band signal and present the Cramer-Rao lower bound on the variance of the time delay estimate as a function of commonly understood terms such as SNR, bandwidth, observation time, and center frequency of the band. The analysis shows that in the case of low SNR and when signal and noise autospectra are constants over the band or signal and noise autospectra fall off at the same rate, the minimum standard deviation of the time delay estimate varies inversely to the SNR, to the square root of the product of observation time and bandwidth, and to the center frequency (provided W^{2}/12 f\min{0}\max{2} \ll 1 , where W = bandwidth and f_{0} = center frequency of the band). The only difference in the case of a high SNR is that the standard deviation varies inversely to the square root of the SNR, and all other parameter relationships are the same. We also address the effects of different signal and noise autospectral slopes on the variance of the time delay estimate in passive localization.

Imaging and Target Detection With a Heterodyne-Reception Optical Radar

Abstract: A mathematical system model for a compact heterodyne-reception infrared radar is developed. This model incorporates the statistical effects of propagation through atmospheric turbulence, target speckle and glint, and heterodyne-reception shot noise. It is used to find the image signal-to-noise ratio of a matched-filter envelope-detector receiver and the target detection probability of the optimum likelihood ratio processor. For realistic parameter values it is shown that turbulence-induced beam spreading and coherence loss may be neglected. Target speckle and atmospheric scintillation, however, present serious limitations on single-frame imaging and target-detection performance. Experimental turbulence strength measurements are reviewed, and selected results are used in sample performance calculations for a realistic infrared radar.

The Correction of I and Q Errors in a Coherent Processor

Abstract: A method is presented for correcting the gain and phase imbalances and the bias errors of the in-phase and quadrature channels of a coherent signal processor [1] by means of coefficients which are derived from measurements of a test signal. The residual errors after correction depend upon the signal-to-noise ratio (S/N) of the test signal and the degree of filtering used in deriving the correction coefficients.

Analysis of discrete implementation of generalized cross correlator

Abstract: A common discrete implementation of the cross correlator uses a parabolic fit to the peak when the delay is not an integral multiple of the sampling period. This correspondence analyzes and assesses the pitfalls of this approach. It is shown that this yields a biased estimate of the time delay, with both the bias and variance of the estimate dependent on the location of the delay between samples, SNR, signal and noise bandwidths, and the prefilter or window used in the generalized correlator.

Minimum-Variance Deconvolution

Abstract: The purpose of this paper is to make the Mendel/Kormylo minimum-variance deconvolution (MVD) procedure, that is based on a state-space model of a signal distortion system and is derived from estimation theory, more accessible to users and potential users. The following aspects of their MVD procedure are covered: how to obtain the order and parameters in a state space model of a signal distortion system; signal-to-noise ratio as a filter tuning parameter; scaled MVD equations; and, MVD filter transfer functions.

Multichannel adaptive filtering for signal enhancement

Abstract: An adaptive technique for enhancing a signal against additive noise is described. It makes use of two or more input channels containing correlated signal components but uncorrelated noise components. The various input signals need not be of the same waveshape, since the adaptive enhancer filters the inputs before summing them. The output is a best least squares estimate of the underlying signal in a chosen input channel. Adaptivity allows optimal performance even though the signal and noise characteristics differ from channel to channel and are unknown a priori. Formulas for signal distortion and output noise power are developed. The more input channels available containing correlated signal components, the better will be the system performance. Excellent performance is obtained when the sum of the filter input signal-to-noise ratios (SNR's), defined as functions of frequency, is large compared to unity at all frequencies of interest. In this case the output noise is small, the output signal distortion is small, and the output SNR is approximately equal to the sum of the filter input SNR's. As such, the multichannel adaptive signal enhancer is a generalization of the classic time-delay-and-sum beamforming antenna.

An analysis of short pulse and dual frequency radar techniques for measuring ocean wave spectra from satellites

Abstract: Scanning beam microwave radars were used to measure ocean wave directional spectra from satellites. In principle, surface wave spectral resolution in wave number can be obtained using either short pulse (SP) or dual frequency (DF) techniques; in either case, directional resolution obtains naturally as a consequence of a Bragg-like wave front matching. A four frequency moment characterization of backscatter from the near vertical using physical optics in the high frequency limit was applied to an analysis of the SP and DF measurement techniques. The intrinsic electromagnetic modulation spectrum was to the first order in wave steepness proportional to the large wave directional slope spectrum. Harmonic distortion was small and was a minimum near 10 deg incidence. NonGaussian wave statistics can have an effect comparable to that in the second order of scattering from a normally distributed sea surface. The SP technique is superior to the DF technique in terms of measurement signal to noise ratio and contrast ratio.

Signal delay and waveform estimation using unwrapped phase averaging

Abstract: Detection and estimation under poor signal-to-noise conditions may require the averaging of an ensemble of time-locked measurements. However, when there is a variation in signal delay, it is necessary to adjust the alignment of the measurements in order for averaging to produce an undistorted estimate of the signal waveform. We describe a technique where alignment is, in effect, replaced by averaging the unwrapped Fourier phases of the measurements. The overall procedure yields an estimate of the signal at its mean delay. Simulations are presented to demonstrate the applicability of the technique to the estimation of delay in visual evoked potentials.

Lesion detection and signal-to-noise ratio in ct images

Abstract: This study measured observers' ability to detect and locate focal lesions on simulated CT images. The difficulty of the detection task was manipulated by changing the difference in attenuation between the lesion and its background (contrast), the random variation in the CT values (noise) or the lesion's size. The human observers' performance was compared to that of matched filter detector, modified to include the effects introduced by the display window and the uncertainty about the lesion's location on the image. Changes in lesion contrast, lesion size and noise produced large variations in both the lesion signal-to-noise ratio (a measure of the matched filter detector's performance) and estimated measures of the observer's detection and localization ability. Changes in observers' performance were closely related to changes in lesion signal-to-noise ratio. Generally, changes in lesion contrast, lesion size or noise that produced similar values of lesion signal-to-noise ratios had equivalent effects upon the observers' performance.

Principles of the optimum lock-in averageing in DLTS measurement

Abstract: The advantages of the transient capacitance/current signal averageing by lock-in amplifier are emphasized: improved signal to noise ratio and better detectability of non exponential transients. The concept of rate window independent phase setting is introduced which overcomes the existing difficulties of the lock-in detection technique. Finally, details of a custom built DLTS apparatus are given.

On The Multiplex Advantage Of Coded Source/Aperture Photon Imaging

Abstract: The vague concept of "resolution" of a detector aperture (or of the source distribution in transmission imaging) is quantified in terms of an elementary signal discrimination task. The signal-to-noise ratio that characterizes performance of the discrimination task is obtained from statistical decision theory. Defining efficiency in terms of the square of this ratio, we find the uniformly redundant array to be 15 times as efficient as a simple open aperture or source of the same area for discrimination tasks when the objects are point-like. Otherwise, it is more efficient to use the simple large open aperture or source.© (1981) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Control device responsive to infrared radiation.

Abstract: An improved device for controlling a current line to such items as interior lighting and room air conditioners includes a pyroelectric infrared detector (25) having its own preamplifier to produce an output signal in response to a person moving within the field of view of the detector. The output signal is successively amplified in two stages (30), (40) and sent to a control unit (100) to close a current line. A timing circuit (80) is included to end the signal to the control unit when no motion of a person is sensed within a predetermined time period. The device provides adequate signal to noise ratio with minimum sophisticated or expensive components.

A Ratio Method for Fluorescence Spectral Deconvolution.

Abstract: : A procedure is described for the spectral deconvolution of multicomponent fluorescence data using a ratio method. The only criteria for deconvolution is that each component must have a spectral region where it is the only emitter and that the acquired data sets have a reasonable signal to noise ratio. The ratio method described is an extension of an algorithm previously developed for deconvolution of one-dimensional infrared data. Examples of applications of this algorithm for one-dimensional and two-dimensional fluorescence data are provided. In addition, an example is given as to how fluorescence quenching can be used to aid in spectral deconvolution. (Author)

A new digital correlation flaw detection system

Abstract: A new portable digital random signal flaw detection system is described which uses a digital delay line to replace the acoustic delay line of the original random signal system. Using this new system, a comparison was made between the two types of transmit signals which have been used in previous systems—m-sequences and random signals. This comparison has not been possible with these previous correlation flaw detection systems. Results indicated that for high-speed short code operation, the m-sequences produced slightly lower range sidelobes than typical samples of a clipped random signal. For normal long code operation, results indicated that system performance is essentially equivalent in resolution and signal-to-noise ratio using either m-sequences or clipped and sampled random signals. Further results also showed that for normal long code operation, the system produces outputs equivalent in resolution to pulse-echo systems, but with the added benefit of signal-to-noise ratio enhancement.

Estimation of blood vessel boundaries in X-ray images

Abstract: A new approach to blood vessel boundary estimation is presented in this paper. By appropriately modeling the blood vessel as a dynamically evolving state vector, and by taking into account the Poisson statistics of the x-ray imaging noise, we arrive at a state-space system with a nonlinear measurement equation which includes non-Gaussian, nonadditive noise. Maximum a posteriori (MAP) smoothing equations are derived for the state vector describing the vessel, and the optimally smoothed state vector is found by a dynamic programming search. This method performs especially well in images with low signal-to-noise ratio and low sampling rate. The performance of the proposed method is demonstrated by the boundary estimates obtained by applying the algorithm to a simulated vessel and measurement data as well as to real vessel phantom measurement data at various SNRs.

Performance characteristics of a digital fluorographic system.

Abstract: Several technical characteristics of digital fluorography are discussed. A derivation is made for the noise levels in the acquisition of digital fluorographic video images. The importance of high video signal-to-noise ratio is demonstrated and discussed. A noise analysis is made for reprocessing amplified difference images from analog video disk. It is shown that under the most stringent circumstances a disk with a signal-to- noise ratio of 200:1 increases the rms noise by at most 7%. Potential artifacts associated with the measurement of iodine sensitivity for digital radio-graphic systems are discussed. A phantom is presented which eliminates such artifacts. Experimental digital fluorographic phantom images are shown.The effects of noise in the acquisition of digital fluorographic imagesIn the conversion of the x-ray fluence detected at the input phosphor of an image intensifier to a video signal, virtually all stages are statistical ones. Thus, the original uncertainty in the detection of a finite number of x-rays is potentially increased at each step. A detailed discussion of this conversion process is given by Roehrig et al.^ In addition to the x-ray detection statistics the other major noise source is the contribution from the video electronics. In this section we study how these noise components enter into a logarithmically amplified video image.A derivation of the noise levels of x-ray statistical and electrical components as a function of signal level has been given in earlier works^/3f but, because of its importance in digital fluorography, it is repeated here. The result will be used in the next section in the derivation of the noise levels that result when images stored on an analog video disk are reprocessed. We make the following definitions:V = video signal voltage when the maximum useable signal max current is read from the camera target.aE = the rms noise voltage in the video signal due to non x-ray statistical sources.SNR = V max /a hi = video signal-to-noise ratio, N = the detected x-rays per pixel corresponding to V , 0 the brightest point in the video image. maxN = detected x-rays per pixel corresponding to darker portions of the video image.= N exp(-yx) where x is incremental object thickness.a = a to gain, V such as video camera aperture, which maps N° maxV can be measured by determining the region over which the camera has a smooth output asPa function of input light level. V is the largest output level in this region just below saturation. Similarly a canxbe measured as the rms signal variation read from the camera with no light input: Thus, the SNR can be determined experimentally. We are making the approximation that a measured for darkest signal is same for all signal levels. Although in fact a increases slightly with signal this does not alter our conclusions as will be shown?The video signal read from the camera is given by:V = otN + m (1) where m represents the video noise.

Digitized longitudinal tomography.

Abstract: An analog video longitudinal tomographic system has been converted to a digital format that reconstructs the tomographic section through the use of matrixes representing the gray scale values of the analog-recorded video images. This was done so that some simple preprocessing could be used to eliminate electronic noise and thereby increase the effective signal to noise ratio. Signal to noise was also increased by the elimination of the analog image displacement circuits (ramp generators). In addition to an improvement in the diagnostic quality (gray scale) of anatomic images, a capability to image contrast differentials as low as 1% has been achieved. A display system provides the ability to use windows having both variable central levels and widths.

Correction to 'Error Performance of Differentially Coherent Detection of Binary DPSK Data Transmission on the Hard-Limiting satellite Channel'

Abstract: The error performance of differentially coherent detection of a binary differential phase-shift keying (DPSK) system operating over a hard-limiting satellite channel is derived. The main objective is to show the extent of error rate degradation of a DPSK system when a power imbalance exists between the two symbol pulses that are used in a bit decision interval. Consideration is also given to the DPSK error rate performance for the special case of {\em uncorrelated} uplink and {\em correlated} downlink noises at the sampling instants in adjacent time slots. Error probabilities are given as functions of uplink signal-to-noise ratio (SNR) and downlink SNR with different levels of SNR imbalance and different downlink SNR and uplink SNR as parameters, respectively. Our numerical results show that 1) as long as the symbols are equiprobable, the error probability is not dependent upon the downlink noise correlation, regardless of whether there is a power imbalance; 2) error performance is definitely affected by the power imbalance for all cases of symbol distributions; and 3) the error probability does depend upon downlink noise correlation for all levels of power imbalance if the symbol probabilities are not equal.

Optimizing Optical Transmitters and Receivers for Transmitting Multichannel Video Signals Using Laser Diodes

Abstract: Analog transmission using laser diodes is quite attractive for transmitting multiplexed video signals, owing to the wide modulation bandwidth of laser diodes. Optimizations for transmitting color-TV signals (7 channel frequency division multiplexed) in the VHF band using buried heterostructure laser diodes are investigated. The major transmission objectives are a carrier to noise ratio of larger than 48 dB and intermodulation products of less than -55 dB. Design curves for minimizing distortion at the transmitter are demonstrated. A modulation index of 70 percent is shown to be capable of attaining the objective. Both the avalanche photodiode (APD) and the pinphotodiode (PD) are considered for use in the receiving amplifier. Minimum receiving power of -11 dBm obtained with the PD receiver is in good agreement with theoretical values. Deterioration of signal to noise ratio due to fiber guided transmissions is also reviewed in comparison with aerial transmissions.

Performance of an ADPCM/TASI system for PCM speech compression

Abstract: This paper contains a summary of performance measurements for a digital speech interpolation (DSI) system called adaptive differential PCM with time assignment speech interpolation (ADPCM/ TASI). This system is used to compress the output of two or more T1 PCM terminals into a bit rate normally requited by one such terminal. The performance measurements presented are as follows. 1) Signal-to-noise power ratios (SNR) for a 1004 Hz tone signal measured by a transmission impairment measuring test set (TIMS). 2) Idle channel noise performance measured by a TIMS. 3) SNR for speech measured by actual computation. These measurements were made for a variety of traffic loading conditions, and a number of different input power levels. The ADPCM/TASI system was constructed by using an AMD 2900 bit slice microprocessor architecture, but was designed to operate on only one speech channel. The presence of other active channels was simulated programatically. The performance of the system when voice band data signals occupy some of the T1 channels is discussed and is reflected in the measurements. An audio tape was made to demonstrate the subjective performance of the system with and without errors in the transmission medium. The objective performance of the system is compared to the D3 channel bank specifications.

Signal to noise ratio improvement for rip detectors

Abstract: The frequency of the AC signal used to detect the integrity or lack of same in a conveyor belt is reduced to be in the 25 KHz. to about less than 200 KHz. and most preferably in the 50 KHz. to 100 KHz. frequency range. When such AC signals are capacitively coupled to antennas carried by a conveyor belt past a rip detector station, the ratio of the magnitude of such signal relative to that of the cross coupled noise, in particular, and other electrical noise in general is significantly increased. The invention relates to an apparatus and method in which signals in such frequency range are used.

A practical procedure for estimation of instantaneous frequency

Abstract: The instantaneous frequency of a frequency-modulated signal may be estimated using the zero crossings of the signal. Furthermore, the procedure is simple and gives good results when only samples of the signals are available and when the signal-to-noise ratio is above about 12 dB. An example is given.

New Developments In Digital X-Ray Detection

Abstract: Experiments and calculations are used to list the properties of integrating X-ray detectors important in digital applications. This list includes costs, detective quantum efficiency, non-linear effects, signal to noise ratio, and noise equivalent number of photons. Indicated are the critical areas, where improvements are still possible. Naturally all future improvements on the detector side will relieve constraints for the X-ray source and will improve scanner versatility and image quality.

Digital Imaging For Radiation Therapy Verification

Abstract: A linear diode array was used to scan a radiation field for pulsed high energy x-ray imaging. With a signal of about 50 pico-coulombs of charge from each diode for each beam pulse, the signal to noise ratio for an image of an open field was 119. An image of a porcine femur in a wax phantom was obtained by scanning the transmitted radiation. The resultant electronic image was superior in quality when compared to the standard radiotherapy port film. The simplicity of the detector system has demonstrated the feasibility of constructing an electronic portal imaging system for clinical use.

Demodulation technique for rip detector signals

Abstract: Synchronous demodulation using a lock in amplifier technique improves substantially the signal to noise ratio in a conveyor belt rip detector. The lock in amplifier technique uses an analog multiplier to form a product of a received AC transmitter signal, i.e. that received by the rip detector receiver from an antenna carried by the conveyor belt, and an AC reference signal, which has the same frequency as the transmitted AC transmitter signal, i.e. that transmitted by the rip detector transmitter. A shift in the phase of the AC transmitter signal as it is coupled from the transmitter to the receiver requires a corresponding shift in the phase of the AC reference signal so that the latter is tuned to the same phase as that of the received AC transmitter signal. The invention lends itself to computer control and self diagnosis of faults of the overall rip detector system. To effect such fault detection not only is there produced an AC transmitter signal and an AC reference signal, but there also is produced a test signal representative of proper operation of the transmitter, and this parameter is checked to learn whether a rip has occurred causing an antenna not to be sensed at an expected location on the belt or a fault has occurred in the rip detector equipment.