Showing papers on "Signal-to-noise ratio published in 1977"

A Nonlinear Model for the Spatial Characteristics of the Human Visual System

Abstract: Several recent papers have presented data from experimental investigations of the human ivsual system (HVS) which support the general hypothesis that the HVS is composed of spatial frequency channels. It has been pointed out, however, that a linear systems analysis of the entire system is not valid. Furthermore, a nonlinear model consisting of a log-bandpass filter produced some experimental results with deviations at high spatial frequencies. A new structure for a nonlinear mathematical model which is easily quantifiable, produces results which compare with experimental data, and has a physiological correlate is presented. The significance of this model is that the bandwidth of the visual system decreases as contrast increases. Thus the system appears to maximize the signal to noise ratio while attempting to maintain a constant " perceptual" spatial-frequency fidelity.

Optimum Performance of Suppressed Carrier Receivers with Costas Loop Tracking

Abstract: The performance of suppressed carrier receivers with Costas loop tracking is optimized by proper choice of loop arm filter bandwidth. In particular, it is shown that for a variety of passive arm filter types, there exists, for a given data rate and data signal-signal-to-noise ratio, an optimum filter bandwidth in the sense of minimizing the loop's squaring loss. For the linear theory case, this is equivalent to minimizing the loop's tracking jitter. When symbol synchronization is known, it is shown that by replacing the passive arm filters with active filters, i.e., integrate-and-dump circuits, one can achieve an improvement in carrier-to-noise ratio of as much as 4 to 6 dB depending on the passive arm filter type used for comparison and the value of data signal-to-noise ratio (SNR).

Variable code gamma ray imaging system

Abstract: A gamma-ray source distribution in the body is imaged onto a detector using an array of apertures. The transmission of each aperture is modulated using a code such that the individual views of the source through each aperture can be decoded and separated. The codes are chosen to maximize the signal to noise ratio for each source distribution. These codes determine the photon collection efficiency of the aperture array. Planar arrays are used for volumetric reconstructions and circular arrays for cross-sectional reconstructions.

Enhancement of signal-to-noise ratio in magnetotelluric data

Abstract: A new technique has been developed which can be employed for the enhancement of the signal-to-noise ratio of magnetotelluric data and/or for the establishment of a confidence limit as a criterion for final data selection.This technique functions in a consecutively cyclic process which can finally remove the noise from the autopower estimates of MT data if the crosspower estimates are noise free and, consequently, the predicted coherencies can be improved. When noise exists in both autopower and crosspower estimates, the cyclic operation can establish a confidence limit which reveals a direct measure of incoherent noise content and thus is a more reliable criterion than the predicted coherency itself for data selection.The technique is tested using actual MT data and the result shows the adequacy of the technique. It selects useful data from those originally having moderate or low predicted coherencies; thus it appears to be important when the predicted coherency of the majority of the collected data is not high.

Detectability In The Presence Of Computed Tomographic Reconstruction Noise

Abstract: If the standard filtered backprojection algorithm with a filter of the form g(f) = |f|h(f) is applied to noisy projections, all of which have a noise power spectral density (NPSD), Sproj(f), then the resulting computed tomographic (CT) reconstruction has a two dimensional NPSD of the form, S(f) ~ |f||h(f)|2 Sproj(f). For proper reconstruction, h(f) must approach a non-zero constant as f 0. Provided Sproj(f) is constant, i.e. white projection noise, the CT noise at low frequencies is supprbssbd by the |f| factor. This low frequency suppression results in a long range negative spatial correlation of the CT noise. If white noise is spatially averaged over a circle of diameter d, then the variance in the averaged values will behave as a2 ~ d-2. For CT noise the variance drops faster than d-2. Simple signal-to-noise ratio considerations suggest that the dependence of minimum detectable contrast upon the diameter of the circle to be detected could be significantly differ-ent in the presence of CT noise than in that of white noise. Simulated reconstructions of a suitable detectability pattern demonstrate these differences may not exist unless the image is spatially smoothed before observation. It is pointed out that the pixel width used in the image display should be from 1/3 to 1/2 the width of the point spread function in order to avoid discrete binning problems.

Short Papers Optimum Filter for Minimization of Image Registration Error Variance

Abstract: It is shown that when two images are registered by cross-correlating one image with a functional of the other image the minimum registration error variance results when the operation performed cofresponds to that of matched filtering.

Some statistical aspects of digital Wiener filtering and detection of prescribed frequency components in time averaging of biological signals

Abstract: A statistical analysis of digital a posteriori Wiener filtering as applied to time averaging techniques for biological signals is presented. The authors show that when ap.w.f. is applied to the average signal hardly any effect can be expected, where as when applied to the individual responses, ap.w.f. improves the signal to noise ratio. The applied analysis leads to a simple test to check whether a prescribed frequency component is present.

Arrangement for detecting light sources

Abstract: An arrangement which enables the presence in an observed field of a light source whose radiation spectrum is known to be detected with a high signal to noise ratio. It includes an acousto-optical tunable filter controlled to scan through the expected spectrum of the source periodically and preferably linearly. The photo-detected signals are processed by a high-pass filter which removes the D.C. component due to ambient interference radiation and provides a periodic signal which correspond to the observed light source. The arrangement applies, in particular, to the angular location of luminous targets.

Threshold extension fm demodulator apparatus for wide band width fm signals

Abstract: A tunable oscillating limiter threshold extension demodulator capable of demodulating video and other wide band width FM signals is described. A steering signal is generated to tune the oscillating limiter in response to the video output signals from an FM discriminator to which input signals from the oscillating limiter are applied. The steering signal is generated so as to be in proper phase relationship to track the frequency of the FM signal for modulating frequencies within certain critical regions and is reduced in amplitude over another region where proper phase relationship is not achievable because of unavoidable delays in the system. The steering signal generating circuit may be provided by a filter having a phase response characteristic such that the total phase lag of the system is small at the low frequency end of the band width and about 360° at the high frequency end of the band width. The filter presents substantial attenuation (in the form of a notch) in the frequency region of the band width where the phase delay may be about 180° and would otherwise provide improper oscillating limiter steering action. In the case of color television signals, for example, significant spectral components of the signal, specifically the components due to the sync and equalizing signals are present in the low frequency end of the signal band width while the significant color or chromatic subcarrier spectral component is at the high frequency end of the signal band width. Accordingly, when applied to tune the band pass filter of the oscillating limiter the frequency to which the oscillating limiter is tuned is caused to substantially match the instantaneous frequency of the color television modulated FM signal at the frequencies of these significant components and the noise threshold in terms of the minimum signal to noise ratio which the demodulator can operate properly is thereby improved.

Photodetector with improved signal-to-noise ratio

Abstract: A novel photodetector circuit utilizing both the generated photovoltage, as well as its inverse, to improve the signal-to-noise ratio at the optical receiver output.

Improving linear prediction analysis of noisy speech by predictive noise cancellation

Abstract: The analysis of speech using Linear Prediction is reformulated to account for the presence of acoustically added noise and a technique is presented for reducing its effect on parameter estimation. The method, called Predictive Noise Cancellation (PNC), modifies the noisy speech autocorrelations using an estimate of present background noise which is adaptively updated from an average all-pole noise spectrum. The all-pole noise spectrum is calculated by averaging autocorrelations during non-speech activity. The method uses procedures which are already available to the LPC analyzer, and thus is well suited for real time analysis of noisy speech. Preliminary results show signal to noise improvements on the order of 10 to 20 db.

Weight Jitter Phenomena in Adaptive Array Control Loops

Abstract: An analysis of weight jitter in adaptive array control loops is presented. lt is shown that steady state weight jitter and statistical dependence between input signal and weight processes are related factors. A Markov characterization for the first-order adaptive array loop is presented and Fokker-Planck techniques are used to obtain the steady state probability density of the weight process for the case of a one-loop array. From this density the mean and the variance of the weight process are obtained and the dependence of these statistics upon loop and input signal parameters is explored.

Noise-Loading Analysis for a General Memoryless System

Abstract: An expression for the noise power ratio of a memoryless nonlinearity with finite output power is given. This generalizes previous results which were restricted to nonlinearities with finite Taylor series expansions. Simpler proofs of these earlier results are presented.

Interferometer Design for Elevation Angle Estimation

Abstract: Radars that are developed for the purpose of monitoring aircraft landings in the terminal air traffic control system can be designed to exploit the relatively high signal-to-noise ratio that characterizes the power budgets calculated for such a link. An interferometer using a pair of low gain antennas can be used to obtain passive coverage over a targe azimuth and elevation sector. A large baseline can be used to obtain the desired elevation angle estimation accuracy. In this paper an optimal tradeoff between the width of the subarray aperture and the width of the interferometer baseline is performed that achieves a specified elevation angle estimation error while minimizing the overall height of the interferometer configuration. The algorithm searches through the class of antenna patterns that can be synthesized from so-called finite impulse response, linear phase digital filters. For the specific problem of designing an elevation sensor for monitoring landing aircraft on final approach, the elevation angle can be estimated with no more than 1-mrad rms error when the aircraft is within ± 60° azimuth, 2.5° to 40° elevation, using two 7-wavelength subarray antennas spaced 8 wave-lengths apart. The design of a separate sensor for resolving the interferometer ambiguities is formulated as a hypothesis testing problem and solved using statistical decision theory. A bound on the probability of an ambiguity error is derived that accounts for the effects of ground reflection multipath and receiver noise.

Optimal Recursive Processing in Phase-Coded Waveform Radars

Abstract: A recursive algorithm is proposed for target detection in pulse-compression radars. This method performs a least square estimate of the radar-channel impulse response, yielding range-sidelobe elimination The algorithm considered becomes particularly simple when pseudorandom binary sequences are employed. In such a case, the losses of signal-to-noise ratio, with respect to the matched filter, are evaluated and the performance degradations due to a Doppler residual, for an imperfect frequency tuning, are computed.

Optimum speech classification and its application to adaptive noise cancellation

Abstract: In an attempt to develop a more robust vocoder an adaptive noise-stripping Wiener filter is used to prefilter the noisy speech. In order to adapt to quasi-stationary noise a speech classifier is developed that detects the presence of silence (noise alone), unvoiced speech or voiced speech. During the silent intervals the noise statistics and the corresponding Wiener filter are up-dated resulting in a decision-directed adaptive structure.

Image Noise and Smoothing in Computed Tomography (CT) Scanners

Abstract: The question of what spatial frequency response gives optimum image quality in computed tomography (CT) images is studied. We have developed a technique for smoothing these images by using a modified back projection filter with attenuated high frequency response. The results are images with a blurred appearance but dramatically reduced image noise. On processing raw data from the EMI head scanner in this way, grey and white matter distributions are clearly visualized with no increase in radiation dose. These results are analyzed in terms of the Weiner spectrum and correlation properties of CT noise. In particular, a "noise equivalent aperture" is defined in a way analogous to that of Shade. It is shown that the prediction of the simple Rose model of noise sampling can not explain the improvement produced by smoothing, even when the correlated character of the noise is taken into account.

A Useful Analytical Method for Discrete Adaptive Delta Modulation

Abstract: -In this paper a simple expression is derived for the formula of variable quantizing step size. This formula can be used to improve the following ability of the discrete adaptive delta modulation codec. Then it is shown that the previously published formulas for video signal can be reduced to this expression. Next, clarifying the relation between the first-order difference \xi of input signal and the mechanism of appearance of the slope overload noise by using this expression, the quantizing noise is classified into granular noise, the first class slope overload noise and the second class slope overload noise. Then these noises are simply expressed as a function of \xi . Finally, using this result, the signal-to-noise ratios for sinusoidal input and Gaussian input are obtained and in consequence the utility of our simple analytical method is confirmed.

Means for reducing nuclear radiation-induced fluorescence noise in fiber-optics communications systems

Abstract: Photons from nuclear radiation are absorbed by fiber optics and the mater of the fibers have their energy levels elevated causing fluorescence. The fluorescence is a noise component which interferes with light signals axially passing through the fiber optics. By using a narrow bandwidth interference optical filter, centered about the communication frequency, the noise interference may be substantially reduced so that an acceptable signal-to-noise ratio is achieved. A tuned optical filter will compensate for drift of the center communication frequency.

Signal-to-Noise Ratios in Self-Biased-Pwer-Law Amplifiers with Saturation

Abstract: The relation between the output signal-to-noise ratio (SNR) and the input SNR is presented for the bandpass self-biased third-law amplifier with saturation, when the input is composed of sinusoidal carrier and zero-mean stationary narrowband Gaussian noise. It is found that significant improvement in the output SNR at low input SNR's can be achieved by the self-biased third-law amplifier with saturation operated in class A. The results obtained are also verified experimentally.

Optimum waveform design and its effect on the peak shift compensation

Abstract: An equalization method of reducing peak shift caused by the intersymbol interference is presented to obtain high density digital magnetic recording First, a new ideal frequency characteristic is determined to get an impulse response free from peak shift Then, using the equalizer that gives this response, the extent of the peak shift compensation is simulated with a computer When a 2200BPI (FM) disk unit is used to double the bit density with MFM, it is found that the peak-shift in the worst case is only 84% of the full phase margin Here, other parameters such as RPM, remain constants However, as the application of the equalizer increases the noise power, signal to noise ratio of the input signal is required to be sufficiently high When the equalizer is applied to a 4040BPI (MFM) disk unit at the same density, phase margin for data separation increases, because the decrease in peak shift seems to be greater than the increase in the noise power

Maximum Theoretical Angular Accuracy of Planar and Linear Arrays of Sensors

Abstract: A study has been made of the maximum theoretical accuracy in the angular location of a radiating object that is achievable by using a planar or linear array. The elements are assumed to have identical radiation patterns and the complex voltages observed at their ports are assumed to be subject to phase measurement errors, having normal probability density. An optimum scheme for the statistical extraction of the parameters defining the direction is established. It consists of combining the observed phases linearly with weights depending upon the element locations. It is shown that the presence of thermal noise, for sufficiently high signal-to-noise ratio, does not change the structure of the estimator. Comparison with conventional multiple interferometric techniques indicates the superiority of the proposed scheme. Finally, a limited numerical study on a small linear array vertically located on a reflecting terrain is performed. Although in such a situation the scheme proposed is not the theoretical optimum, it leads to errors that, for most directions of the target, are smaller than those found for the same array when using conventional multiple interferometer techniques.

MTI noise integration loss

Abstract: MTI signal processing correlates the independent input noise and thus degrades detection performance when the MTI pulses are integrated. Using simulation techniques, this degradation called the MTI noise integration loss is calculated for small and intermediate sample sizes.

Electronic detection apparatus for leaks

Abstract: This invention is an improved electronic detection system for leaks in articles such as can tops and the like comprising a special ultrasonic microphone-amplification system for listening for leaks in articles, a signal averaging network to receive the signal produced by the amplifier and to improve the signal to noise ratio, an enable circuit which is synchronized with the pressurization of the article to be leak tested and which triggers the signal averaging network and a comparator to receive the averaged signal from the signal averager. The comparator produces a logic pulse for rejecting leaky articles when the averaged signal produced by the signal averaging network is above a predetermined level.

Probability of Detecting a Fluctuating Target Immersed in Both Noise and Clutter

Abstract: The probability of detecting either a Swerling 1 or Swerling 2 target immersed in both Rayleigh-distributed noise and log-normally distributed clutter is calculated. Results are presented which demonstrate the effect of noise-to-clutter ratio, signal-to-noise ratio, and number of pulses integrated on the detection statistics.