Showing papers on "Signal-to-noise ratio published in 1983"

ECG Enhancement by Adaptive Cancellation of Electrosurgical Interference

Abstract: A technique utilizing a combination of adaptive noise canceling and conventional signal processing is developed to enhance electrocardiographic monitoring in the operating room by reducing the noise interference that is created by an electrosurgical instrument. Significant amounts of interference are eliminated by radio frequency shielding, passive and active low-pass filtering, and optical isolation. A digital adaptive canceler using the least mean-square algorithm of Widrow and Hoff is used to reduce the remainder of the interference, yielding an improvement in signal-to-noise ratio of approximately 110 dB. Clear electrocardiograms have been obtained with electrocautery equipment in operation.

Design Methodology for ΣΔM

Abstract: The paper presents a design methodology based on correspondence between performance requirements, mathematical parameters, and circuit parameters of a sigma-delta modulator. This methodology will guide a design engineer in selecting the circuit parameters based on system requirements, in translating paper design directly into LSI design, in predicting the effect of component sensitivity, and in analyzing the operations of the sigma-delta modulator. The sigma-delta modulator is viewed as a device which distributes the noise power, determined by peak SNR, over a much broader band, compared to signal bandwidth, shapes and amplifies it, and allows filtering of the out-of-band noise. The shaping and amplification are quantified by two parameters, F and P , whose product is analogous to the square of step size of a uniform coder. These two parameters are related, on one hand, to the time constants or location of zero and poles. On the other hand, inequalities are set up between performance parameters, like signal-to-noise ratio and dynamic range, and F and P .

Performances of the Delay-Line Multiplier Circuit for Clock and Carrier Synchronization in Digital Satellite Communications

Abstract: This paper is concerned with clock and carrier synchronization in digital satellite transmissions, using the delay-line multiplier circuit. For clock recovery from baseband signal, with random data, a closed form formula is derived which gives the spectrum after multiplication, for any arbitrary pulse shape. This spectrum contains spectral lines at the clock frequency and its harmonics, and a continuous part which is the pattern noise. This pattern noise may be decomposed in noise in phase with the recovered clock, and noise in quadrature whose power spectral density is always zero at zero frequency. The effect of Gaussian noise on the channel is taken into account to calculate signal-to-noise ratio at the clock frequency as a function of the classical parameter E/N_{o} . With a modulated input carrier, the signal at the output of the delay-line multiplier may be separated into two parts: a low frequency signal that contains clock information and a bandpass spectrum signal around twice the carrier frequency that contains carrier information, when possible. Spectrum and signal-to-noise ratio in carrier recovery are studied for BPSK and offset quadrature modulation.

Optimum Filters for Estimating Evoked Potential Waveforms

Abstract: The design of time-invariant and time-varying minimum mean square error filters for processing event related brain potentials is considered. A model for taking into account the randomness associated with the signal as well as that of the noise is described. Results using simulated data show the perfonnance of the time-varying filter to be greatly superior to that of the time-invariant filter. Results of processing measured visual evoked potentials are presented.

Speech enhancement using optimal non-linear spectral amplitude estimation

Abstract: A speech enhancement system which utilizes an optimal (in the minimum mean square error sense) short-time spectral amplitude estimator is described. The derivation of the optimal estimator is based on modeling speech as a quasi-periodic signal, and on applying spectral decomposition. The optimal spectral amplitude estimator and a recently developed vector spectral subtraction amplitude estimator, are round to be nearly equivalent. The optimal spectral amplitude estimator coincides with a Wiener spectral amplitude estimator at high signal to noise ratio (SNR) values, and is found to be superior to it at low SNR values. The enhanced speech obtained by using the proposed system, is less spectrally distorted, although contains some more residual noise, than the enhanced speech obtained by using the Wiener spectral amplitude estimator, in the same system. In addition, it is free of the "musical noise" characteristic to the spectral subtraction algorithm. Both systems, the proposed one and spectral subtraction, have approximately the same complexity.

S/N and error rate evaluation for an optical FSK-heterodyne detection system using semiconductor lasers

Abstract: Receiving performance is evaluated for an optical FSK-heterodyne detection system in which semiconductor lasers are used as both an FSK transmitter and an independent local oscillator. Noise and error rate are measured under feedback stabilization of IF signal frequency and electrical equalization of semiconductor laser FM modulation frequency characteristics. The minimum received signal power of -44 dBm, which is about 2 dB better than that in IM direct detection, is achieved at a 10-9error rate for a 200 Mbit/s signal. Excess errors for FSK signals result from frequency broadening of the laser spectrum. Both AM and FM quantum noises in the lasers are primary factors which determine system performance. Error rate characteristics in an optical FSK direct detection system, in which a Michelson interferometer is employed as an optical frequency discriminator, are compared with the above results.

Dual energy computerized tomography system

Abstract: A dual energy method for CT scanning. Scanning is carried out at low and high energy levels, S 2 and S 1 respectively, each at a preselected dose level. Detected electronic signals (I 2 ,I 1 ) are digitized. A look up table is used to generate a pair of values from each pair of signals, one a photoelectric component and the other a Compton component. Each of these values are subjected to spatial filtering by a convolution filter function for each signal specially designed to increase the signal to noise ratio in the resultant combined image by exploiting the correlation in noise in the two values.

Comparison of Several Techniques to Obtain Multiple-Look SAR Imagery

Abstract: Synthetic aperture radar imagery is contaminated by coherent speckle noise. In order to improve interpretability, multiple-look images are usually generated. In this paper, we examine the performance of several classes of techniques to generate such multiple-look imagery. The detailed impulse responses and the signal-to-speckle noise ratios obtained by the various techniques are presented. It is found that by using several types of image domain filters, the signal-to-speckle noise ratio can be improved over the conventional discrete mixed-integrator while keeping the equivalent rectangular resolution constant.

Improving resolution for autoregressive spectral estimation by decimation

Abstract: In this paper we present a method for efficiently improving the resolution of autoregressive spectral estimation algorithms. We derive the exact autoregressive spectrum for K complex sinusoids in additive white noise. From this equation resolution boundaries are constructed which give the resolution in terms of the model order and the signal-to-noise ratio. Simulation results are used to compare the resolution boundaries for decimated and undecimated spectra. Our results demonstrate that decimation by D with a model order M yields the same resolution as a model order MD used with the undecimated signal, and that decimation reduces the computation.

Method and apparatus for enhancing video displays

Abstract: An NTSC composite color video signal is dematrixed and its RGB components are digitized so that each image or input frame is represented by a 512×512 pixel array. One high resolution output field is generated for each input frame by deriving a plurality of subpixel values for each input pixel. The subpixel values for a given pixel are derived by examining the nearest neighboring pixels and by utilizing enhancement algorithms represented by data in lookup tables. Signal to noise ratio improvement is achieved by comparing each given pixel value to the values of its neighboring pixels and then deciding whether or not to keep the given pixel value or to replace it with the median of it and its nearest neighbors. The subpixel values are fed through a digital to analog converter where the appropriate synchronization is added so that the analog outputs of the three branches of the device conform to the RS 343A format.

Adaptive beam forming using a cascade configuration

Abstract: A new technique for adaptive beam forming based on the Davies cascade array configuration [11] is presented. First of all it is shown that an n-element linear array can be reconfigured as a number of subarrays of two elements each. This two-element array is capable of steering one null in an arbitrary direction while maintaining a desired gain in some other prespecified look direction. To adaptively perform this function at any cascade stage, the inverse of a (2 × 2) correlation matrix of the signal vector at a subarray is computed and substituted in the expression for the constrained optimum weight vector. Similar processing is performed at subsequent stages of the cascade configuration. At each stage of adaptation a noise source contributing maximum power at that stage is nulled out. Simulation studies carried out verify the validity of this approach and demonstrate the fast convergence property of the proposed technique.

Digital modem having a monitor for signal-to-noise ratio

Abstract: The present invention pertains to a digital modem utilizing QAM-type modulation having a signal-to-noise monitor which utilizes some of the components of a standard modem without interfering with its normal operation. The monitor averages the absolute value of the difference between the signal received by the modem and the ideal signal as represented on a typical signal constellation. In order to minimize the effect of phase and amplitude errors the monitor only handles the innermost signals of the constellation and ignores all the other signals.

Reduction of all-pole parameter estimator bias by successive autocorrelation

Abstract: Conventional all-pole parameter estimators applied to noise corrupted all-pole sequences result in biased estimates. This paper describes a procedure by which reduction of this bias is accomplished by applying pole-preserving, signal to noise ratio improving functions to the sequence. Correlation like pole-preserving functions are investigated and pole dependent signal to noise ratio improvement is described. An all-pole parameter estimator using successive application of a pole-preserving function (successive autocorrelation) is given. Comparison is made with the least squares combination of the higher order Yule-Walker equations, an approach to bias reduction reported by Cadzow. Successive autocorrelation is found to result in improved performance, with estimates of higher Q poles being most effectively enhanced.

Some Improvements in the Measurement of Variable Latency Acoustically Evoked Potentials in Human EEG

Abstract: Since averaging of acoustically evoked potentials of variable latency yields inconsistent estimates, we tried to improve the results by applying Woody's adaptive filter [8], a weli-known template-cross-correlation method. However, the results were worse and obviously less reliable than those reported by Woody. The following reasons for this behavior were found: 1) low signal-to-noise ratio and 2) large coincidences in the spectral components of EEG and of acoustically evoked potentials, enabling erroneous synchronizations of evoked potential-like EEG transients.

An Adaptive Interference Canceling Array Utilizing Hybrid Techniques

Abstract: The steady state properties of an adaptive array utilizing prior knowledge of both approximate signal arrival direction and signal characteristics are presented. The method combines the features of a directionally constrained array and an array with a self-generated reference signal. Explicit results are obtained for output signal, interference, and noise powers assuming a single interferer is present. The inclusion of a self-generated reference circuit is shown to reduce the sensitivity to pointing error typical of arrays utilizing a zero order directional constraint, the improvement being a consequence of the reduction of the desired signal component fed back to the sidelobe canceling circuit. A relationship between the degree of sensitivity reduction and the quality of the reference signal is developed. Results of computations of signal to interference plus noise ratios for a 7-element 10-wavelength nonuniformly spaced array as a function of pointing error are presented. These results show the behavior with one interferer inside and outside the beamwidth of the quiescent array and with multiple interferers for various degrees of perfection of the reference generating circuit. In all cases the computations confirm that the otherwise severe effects of small pointing errors are substantially reduced.

Steered Beam and LMS Interference Canceler Comparison

Abstract: The performance of two kinds of interference cancelers is compared, namely, a constrained steered beam interference canceler (CSBIC) and a least mean square interference canceler (LMSIC). For simplicity, this is done for a two-element array. In our comparison we use the array output desired signal-to-interference power ratio (SIR) and the desired signal-to-noise ratio (SNR). These power ratios are obtained for the CSBIC and LMSIC under two sets of conditions: 1) The error in the assumed angle of incidence for the CSBIC is small, and the LMSIC operates in a codetracking mode. 2) The error in the assumed angle of incidence for the CSBIC is large, and the LMSIC operates in a code-acquisition mode. Comparison of the corresponding power ratios obtained under these two sets of conditions then establishes the condition under which it is more desirable to use a CBSIC as compared with an LMSIC.

Matched Filters in Nerve Conduction Velocity Estimation

Abstract: The evoked response signal-to-noise ratio in peripheral sensory nerves is of the order of one or less. To reduce noise induced errors in the nerve conduction velocity measurement, signal averaging is employed. The number of responses required depends upon the signal-to-noise ratio and the acceptable error. It is proposed to use maximum likelihood estimators to improve the signal-to-noise ratio and hence, reduce the number of responses required. The matched filter and generalized cross correlator are studied and noise performance equations obtained. It is shown that the matched filter offers a 6.8 dB improvement over the conventional 5 kHz band-limiting filter. It is further shown that the matched filter as a prefilter in the generalized cross correlator gives the optimum correlator configuration for the low signal-to-noise ratio case. These results are verified with experiments.

Linear Methods In Phase Retrieval

Abstract: On-orbit wavefront sensing and active alignment control are essential features of many spaceborne optical systems currently being developed. Phase retrieval is an especially appropriate wavefront sensing technique for this application, because it directly monitors system image quality and eliminates or reduces the need for auxiliary wavefront sensors. Although the general phase retrieval problem is highly complex and requires sophisticated nonlinear estimation techniques, properly selected linear methods provide satisfactory and efficient solutions to a number of important special cases. This paper discusses the per­ formance of several such linear phase retrieval algorithms. One method yields noise-optimal estimates of small wavefront errors, while a second approach can be used with arbitrarily large errors but is much more sensitive to noise. These two phase retrieval algorithms are actually special cases of a general linear algorithm that can be tuned as a function of wavefront error characteristics, measurement noise statistics, and focal plane detector geometry.Introduction the utility of linear phase retrievalOptical systems with large apertures and high resolution over a wide field of view will be required for a variety of future infrared- and visible-wavelength space sensors. The optical forms selected for these systems frequently consist of fast, off-axis mirrors that are highly sensitive to misalignments. Passive structural approaches to achieving the required opto-mechanical tolerances in such telescopes are often not feasible because sensor weight must be kept low; periodic or active on-orbit alignment sensing and control are then necessary when conventional passive approaches are inadequate. Phase retrieval from focal plane imagery is an attractive approach to alignment error sensing for this application, because it controls alignment directly on the basis of optical system performance and avoids the physical complexity of auxiliary alignment sensors, which are subject to drift or failure.Although the general phase retrieval problem is highly complex and requires sophisticated nonlinear estimation methods for its solution,1"5 three aspects of the phase retrieval application summarized above suggest that a simpler approach may prove satisfactory in this case. For near- to mid-term applications, the wavefront errors to be estimated will arise from rigid body mirror misalignments or from low-frequency mirror thermal deformations. The number of wavefront degrees freedom will therefore vary from three to approximately twenty.The phase retrieval algorithm need not obtain an exact value for wavefront error in a single application, but must instead produce satisfactory optical alignment after several cycles of alignment error estimation and correction. Point sources in the form of stars or beacons can be assumed for many potential systems. Granted these simplifying assumptions, it appears that properly devised linear phase retrieval algorithms possess sufficient accu­ racy and range to "fine tune" the alignment of optical systems with wavefront errors of up to 0.3 to 0.4 waves rms. These linear algorithms impose comparatively modest signal processing requirements, and therefore would be valuable when (as is usual) sensor signal processing requirements must be minimized.The remainder of this paper describes the theory and application of linear phase retrieval algorithms in greater detail. First, the advantages and limitations of two early attempts at linear phase retrieval are described, and then an optimal linear algorithm that a) is tunable as a function of operating conditions, b) permits performance analysis via closed-form equations of expected estimation accuracy, and c) can be specialized to the two previously described methods as limit cases, is presented. Then several additional applica­ tions of this linear phase retrieval algorithm are discussed.90

Optimization of a clip‐on SQUID current probe

Abstract: This paper presents details of an unusual clip‐on nanoammeter. The device utilizes a toroidal magnetic flux sensor which is split to allow its placement around a continuous current carrying filament, and is coupled to a sensitive superconducting current amplifier. Optimization formulas allow the design of an instrument having the greatest possible signal‐to‐noise ratio consistent with the physical volume available for the sensing element. A prototype exhibited 125‐nA current sensitivity at 25 Hz with a noise level of 40 nA/Hz1/2. The performance of this instrument verified the predictions of the theory. The optimization analysis for this probe demonstrated that the signal‐to‐noise ratio could have been doubled by a slight change in toroid dimensions. It was shown that SQUID noise is negligible for this instrument, justifying the assumption of a noiseless amplifier in the original calculation. Finally, the conditions under which an impedance matching transformer can reduce the importance of the SQUID noise...

Spatial frequency analysis and matched filtering in electromagnetic nondestructive evaluation

Abstract: The theory of electromagnetic flaw detection based on the Lorentz reciprocity theorem has been applied to quantitative modeling of the corrosion buildup problem in PWR steam generators. Formulation of the electromagnetic probe response in spatial Fourier transform domain clarifies the role of the probe as a spatial frequency filter. Examples are given of tailoring the spatial frequency response to achieve specific performance characteristics, such as liftoff noise discrimination and flaw shape characterization. The relationship of spatial frequency response to probe geometry is analyzed for two dimensional inductive and capacitive structures, and the use of signal compression for realizing good spatial resolution and improved signal‐to‐noise ratio in long probes is discussed.

New Centroid Algorithm Based upon Amplitude-Angle Signature

Abstract: A method of estimating the centroid location of a target utilizing radar scan return amplitude versus angle information is presented. The method is compared with three thresholding estimators and a first moment estimator in a computer-simulated automatic landing system. This new method is the most robust and accurate during periods of low signal-to-noise ratio. In periods of high signal-to-noise ratio the method has less error than the thresholding methods and is similar in accuracy to the first moment estimator. Furthermore, the number of pulse transmissions required to obtain a desired level of performance in noise is much less than that needed for the thresholding methods and the first moment estimator employed in this simulation.

Data processing apparatus for producing X-ray images with improved signal to noise ratio

Abstract: An X-ray image data of a patient into which an X-ray contrast medium is being administered is processed in an X-ray image data processing apparatus so as to obtain the X-ray image data having an improved S/N ratio. The processing apparatus mainly comprises a linear differential filter that is composed of a plurality of multipliers and an adder. The X-ray image data is multiplied by a plurality of differential coefficients, and thereafter the results of these multiplications are summed.

Binarily weighted D to a converter ladder with inherently reduced ladder switching noise

Abstract: A digital to analog signal converter having a resistor ladder and switching network of the R-2R type is provided with the 2R resistor legs divided into two parallel 4R resistors, the most significant bit stage replaced with a switchable terminating 4R resistor stage, and logic means provided for each remaining bit stage responsive to its own bit stage input signal as well as the MSB stage signal such that, at the mid scale, the added 4R terminating resistor is switchable in and there is no switching at the other 4R resistor legs. By reducing the switching noise at the lowest input signal level, the system signal to noise ratio is substantially increased. To further reduce switching noise, additional bit stages may be replaced by terminating resistors and the 2R resistor legs appropriately modified.

New Principle of Array Processing in Underwater Passive Listening

Abstract: : One of the main function of an underwater passive listening system is the reckoning of the number of present sources as well as the characteristic parameters of everyone. To do so, the noises transmitted by the sources are used when recorded on the sensors of an array. The basic tool is the spatial processing. The traditional tool is the classical beamforming. Then, in view of improving the performance, one came to adaptive beamforming of a sensor array. As a result, this method brings an improvement -an array gain- which is asymptotically bound by the signal to noise ratio of the source noises, when measured on a particular sensor. More recently have appeared more powerful methods called high resolution. The improvement in performance as compared to previous processing is at the cost of one more assumption on the medium. Nevertheless, these methods carry the possibility to include free parameters in the medium model, and that make more flexible the assumptions to be accepted.

Measurement system for temporal response of atomic and molecular systems using the correlation method with pseudorandomly modulated laser light

Abstract: A system to measure the temporal behavior of atomic or molecular fluorescence, which is disturbed by large noise background, has been developed. This system uses the pseudorandom modulation technique of length up to 1023, temporal resolution of approximately 83 ns, and a photon counting capacity of 107 counts/s. Data processing by a minicomputer and a hardware demodulation circuit enable quick analysis of a large volume of data. The signal‐to‐noise ratio of this measurement system is discussed with some experimental comparisons. The system is tested in the decay time measurement of I2 fluorescence.

Adaptive Array Behavior with Sinusoidal Envelope Modulated Interference

Abstract: The behavior of a LMS (least mean square) adaptive array with modulated interference is described. An interference signal with sinusoidal, double-sideband, suppressed-carrier modulation is assumed. It is shown that such interference causes the array to modulate the desired signal envelope but not its phase. The amount of the desired signal modulation is determined as a function of signal arrival angles and powers and the modulation frequency of the interference. Such interference also causes the array output signal-to-interference-plus-noise ratio (SINR) to vary with time. However, it is shown that when the desired signal is a digital communication signal, the averaged bit error probability is essentially the same as for continuous wave (CW) interference.

Signal analyzing device

Abstract: PURPOSE:To improve a signal to noise ratio and to stabilize a spectrum, by obtaining spectral moment and a pitch frequency from power spectrums obtained by performing cumulation average of short time spectrums. CONSTITUTION:An input signal from an input terminal 101 is coded into a digital signal at a specified frequency by an A/D converter 102. A Hamming window having a window length Tw is formed by a window forming device 104 and cut out. A power spectrum is obtained by a Fast Fourier converter 105. Then, the input to the window forming device 104 is delayed by a delay time Td through a delay memory circuit 103. The power spectrum is computed by the some procedure, and inputted into a cumulation average circuit 106. Thus, the cumulation average power spectrum for a short time is obtained. In a cumulation device 106, the cumulation average power spectrum for a long time about ten times the window length Tw is computed. The result is inputted to a spectral moment computing circuit 108 and a pitch extracting circuit 107.