Showing papers on "Noise (signal processing) published in 1985"

Measurement of signal intensities in the presence of noise in MR images

Abstract: Power spectrum or magnitude images are frequently presented in magnetic resonance imaging. In such images, measurement of signal intensity at low signal levels is compounded with the noise. This report describes how to extract true intensity measurements in the presence of noise.

Antialiasing through stochastic sampling

Abstract: Stochastic sampling techniques, in particular Poisson and fittered sampling, are developed and analyzed. These approaches allow the construction of alias-free approximations to continuous functions using discrete calculations. Stochastic sampling scatters high frequency information into broadband noise rather than generating the false patterne produced by regular sampling. The type of randomness used in the sampling process controls the spectral character of the noise. The average sampling rate and the function being sampled determine the amount of noise that is produced. Stochastic sampling is applied adaptively so that a greater number of samples are taken where the function varies most. An estimate is used to determine how many samples to take over a given region. Noise reducing filters are used to increase the efficacy of a given sampling rate. The filter width is adaptively controlled to further improve performance. Stochastic sampling can be applied spatiotemporally as well as to other aspects of scene simulation. Ray tracing is one example of an image synthesis approach that can be antialiased by stochastic sampling.

SNR and DQE analysis of broad spectrum X-ray imaging

Abstract: A general analysis of the signal to noise ratio SNR, of X-ray imaging with a broad spectrum is presented. The analysis indicates that the energy modulation of the signal together with its degree of matching by the energy response of the image receptor are significant determinants of the SNR for signal detection. This requires a generalisation of the interpretation of detective quantum efficiency, DQE, the transfer function appropriate from SNR, that will be dependent on the image detection or discrimination task. The generalised DQE is similar to the conventional DQE for the task of detecting radiation levels, but may differ substantially from it for the task of discriminating a lesion from its surround, particularly for signals of bone or iodine. The photon counter is shown to be inferior to the ideal detector for these tasks, but to be generally superior to the energy detecting scintillators used in conventional or digital radiography and computed tomography.

Automatic gain selector for a noise suppression system

Abstract: An automatic gain selector is disclosed for use with a noise suppression system which performs speech quality enhancement upon a noisy speech signal available at the input to generate a noise-suppressed speech signal at the output by spectral gain modification. The channel gain controller (240) of the present invention produces a modification signal (245), comprised of individual channel gain values, for application to a channel gain modifier (250). A particular gain table set is automatically selected from one of a plurality of gain tables (450) by a selector switch (470) and a noise level quantizer (440) in response to a multi-channel noise parameter, such as the overall average background noise level of the input signal. Then the individual channel gain values (455) are obtained from the particular gain table set in response to the individual channel signal-to-noise ratio estimate (235). Hence, each individual channel gain value is selected as a function of (a) the channel number, (b) the current channel SNR estimate, and (c) the overall average background noise level. The automatic gain selector further includes a gain smoothing filter (460) for smoothing these noise suppression gain factors on a per-sample basis thereby improving noise flutter performance caused by step discontinuities in frame-to-frame gain changes.

Detection thresholds for tracking in clutter--A connection between estimation and signal processing

Abstract: In the Kalman-Bucy filter and other trackers, the dependence of tracking performance upon the quality of the measurement data is well understood in terms of the measurement noise covariance matrix, which specifies the uncertainty in the values of the measurement inputs. The measurement noise and process noise covariances determine, via the Riccati equation, the state estimation error covariance. When the origin of the measurements is also uncertain, one has the widely studied problem of data association (or data correlation), and tracking performance depends critically on signal processing parameters, primarily the probabilities of detection and false alarm. In this paper we derive a modified Riccati equation that quantifies (approximately) the dependence of the state error covariance on these parameters. We also show how to use a receiver operating characteristic (ROC) curve in conjunction with the above relationship to determine the detection threshold in the signal processing system that provides measurements to the tracker so as to minimize tracking errors. The approach presented in this paper provides a feedback mechanism from the information processing (tracking) subsystem to the signal processing subsystem so as to optimize the overall performance in clutter.

Image processing by simulated annealing

Abstract: It is shown that simulated annealing, a statistical mechanics method recently proposed as a tool in solving complex optimization problems, can be used in problems arising in image processing. The problems examined are the estimation of the parameters necessary to describe a geometrical pattern corrupted by noise, the smoothing of bi-level images, and the process of halftoning a continuous-level image. The analogy between the system to be optimized and an equivalent physical system, whose ground state is sought, is put forward by showing that some of these problems are formally equivalent to ground state problems for two-dimensional Ising spin systems. In the case of low signal-to-noise ratios (particularly in image smoothing), the methods proposed here give better results than those obtained with standard techniques.

Low power, high resolution digitizing system with cordless pen/mouse

Abstract: A cordless pen emits a directional electric field from the tip of a conductive pen cartridge. The pen tip is capacitively coupled to embedded major X and major Y conductors in a digitizer tablet, which are scanned to determine which three X conductors and which three Y conductors have the greatest resulting signal levels. A microprocessor calculates the precise pen location from the three largest X signals and the three largest Y signals. A resistive path to ground is provided for each X and Y conductor. A plurality of minor conductors are disposed between each pair of X and Y conductors. A resistive divider circuit linearly distributes the voltage across the pair of conductors among the minor conductors therebetween, and improves the capacitively coupled signal levels and also "localizes" noise signals. The frequency of the pen signal is modulated by several pen switches. Circuitry in the digitizing tablet discriminates the pen frequency to decode the commands represented by switch closures between data scanning operations. In one embodiment of the invention a keyboard is placed on the digitizer tablet. The keyboard includes a groud plane through which a plurality of conductive tips actuated by keys extend to capactively couple an electrical signal to embedded X and Y conductors. The microprocessor interprets the scanned data to determine the identity of each depressed key.

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.

Signal Recovery from Noise in Electronic Instrumentation

Abstract: Low-pass filtering and visual averaging multiple time averaging and drift phase-sensitive detector methods spectral view of signal recovery 1/F noise frequency response calculations frequency-domain view of the phase-sensitive detector digitation and noise magnitude determination for transient signals of known shape and timing measurement of the time of occurrence of a signal transient.

Locally Optimum and Suboptimum Detector Performance in a Non-Gaussian Interference Environment

Abstract: Since the normally assumed white Gaussian interference is the most destructive in terms of minimizing channel capacity, substantial improvement can usually be obtained if the real-world interference environment (non-Gaussian) is properly taken into account. In this paper, the performance of the locally optimum Bayes detector (LOBD) is compared to the performance of various ad hoc nonlinear detection schemes. The known results are reviewed, and then it is demonstrated that these theoretical results may be misleading due to the assumptions that are required in order to derive them analytically. For a particular type of broad-band impulsive noise, the critical assumptions of "sufficiently" small signal level and large number of samples (large time-bandwidth product so that the central limit theorem applies) are removed; the first analytically, and the second by computer simulation. The thus-derived performance characteristics are then compared, especially as the signal level increases. One result is that there are situations where the bandpass limiter outperforms the LOBD as the signal level increases; that is, the locally optimum detector may not remain "near optimum" in actual operational situations.

Electronic noise-reducing system

Abstract: A method and apparatus for reducing noise from a near-field noise source sent together with signals from a far-field source. The method uses an adaptive shaping filter and a summer, in conjunction with a directional reference sensor and a primary sensor which have at least a common sensing element therebetween. The directional reference sensor situated between the near-field noise source and the far-field signal source, rejects the broad-band signal but accepts the broad-band noise and feeds this noise into a reference channel of the adaptive filter. The primary sensor accepts both the far-field signal and near-field noise with equally sensitivity. The primary sensor feeds into the primary channel of the adaptive filter. The adaptive filter system subtracts the noise in the reference channel from the signal-plus-noise in the primary channel, thus producing an output having a greatly improved signal-to-noise ratio.

Effects of estimated noise covariance matrix in optimal signal detection

Abstract: There is loss of efficiency when an estimated noise covariance matrix is used in the place of the unknown true noise covariance matrix in the construction of the optimum filter for signal detection. In the case of detecting a single signal specified by a real or a complex vector, we investigate the extent of this loss by obtaining an exact confidence bound for the realized signal-to-noise ratio. We also give an estimate of this ratio which is useful in optimum selection of features. Some of these results are extended to the case of discrimination between a number of given signals.

Speech enhancement system having dynamic gain control

Abstract: An arrangement for a speech enhancement processor which maintains the processed speech at a constant level regardless of large changes in the associated noise level. The composite speech and noise signal is applied to a first AGC circuit and then to a speech enhancement system which removes tonal, impulse, and wideband noises from the signal. The extracted noise power estimates are subtracted from the constant amplitude signal to provide a gain control signal value to which the gain of a second variable gain amplifier is inversely proportional. The amplifier multiplies the processed speech output from the enhancement system and, because of the variable gain control, provides an output speech signal having short-term amplitude levels which correspond to those of the input speech signal, and having a constant long-term amplitude level.

Voice operated switch

Abstract: A voice operated switch for use in noisy environments is described. The switch includes independent amplifiers for separately amplifying frequency band limited speech and noise signals. The independently amplified speech and noise signals are rectified, applied through time constant circuits, and are applied to a differential amplifier. Variations in the resulting signal are detected to determine the present or absence of speech, and control pulses are generated. The control pulses operate a switch device.

Detection of temporal gaps in bandlimited noise: Effects of variations in bandwidth and signal‐to‐masker ratio

Abstract: Thresholds were measured for the detection of a temporal gap in a bandlimited noise signal presented in a continuous wideband masker, using an adaptive forced‐choice procedure. In experiment I the ratio of signal spectrum level to masker spectrum level (the SMR) was fixed at 10 dB and gap thresholds were measured as a function of signal bandwidth at three center frequencies: 0.4, 1.0, and 6.5 kHz. Performance improved with increasing bandwidth and increasing center frequency. For a subset of conditions, gap threshold was also measured as bandwidth was varied keeping the upper cutoff frequency of the signal constant. In this case the variation of gap threshold with bandwidth was more gradual, suggesting that subjects detect the gap using primarily the highest frequency region available in the signal. At low center frequencies, however, subjects may have a limited ability to combine information in different frequency regions. In experiment II gap thresholds were measured as a function of SMR for several signal bandwidths at each of three center frequencies: 0.5, 1.0, and 6.5 kHz. Gap thresholds improved with increasing SMR, but the improvement was minimal for SMRs greater than 12–15 dB. The results are used to evaluate the relative importance of factors influencing gap threshold.

Regional arrays and optimum data processing schemes

Abstract: In the 19609s, Lincoln Laboratory, Texas Instruments, and Teledyne-Geotech research groups were assigned tasks related to the design of the large-aperture arrays LASA and NORSAR resulting in a number of sophisticated processing techniques for additional signal-to-noise ratio (SNR) enhancements relative to conventional delay-and-sum processing (or beamforming). At that time, these techniques (essentially variations of Wiener filtering theory) did not prove a great success, and part of the lame was attributed to inadequate computing power for handling large data volumes in a nontrivial manner. However, with the present advent of miniarrays and relatively much faster computers, optimum array processing techniques are again in vogue. In this respect, we have examined different weighting schemes based on the noise structure as manifested in the noise covariance matrix using data from the prototype NORESS array in Norway. Results are as follows: (i) with strong correlations between sensors the processing gain is obtained essentially by deleting “redundant” sensors; (ii) with strong-to-moderate negative correlations, SNR gain in excess of N is obtained by giving large weights to a few sensors to ensure destructive interference; (iii) with weakly structured noise conventional beamforming is very efficient as expected; and (iv) observed noise correlation curves exhibit rather strong spatial variation, i.e., depend on both phase velocity and azimuth. Also the time stationarity of the noise covariance matrix is rather weak, thus diminishing the expected gain from the optimum weighting schemes in a real-time context. Some experiments were also performed with a simplified maximum-likelihood filtering processor, and in this case approximate N gains were obtained even for strongly correlated noise. Because processing gains implicitly reflect sensor geometry, criteria for array configurations are discussed in the light of the results obtained from the optimum processing experiments. Essentially, an array aperture of 3 km, like that of NORESS, represents high-pass filters with a lower cut-off at about 2 to 3 Hz while signal decorrelation and signal beamforming losses ensure that the array acts as a low-pass filter with a cut-off at about 6 Hz. However, using subsets of the array9s original 25 sensors with correspondingly smaller apertures, the array9s operational bandwidth can efficiently be shifted toward higher frequencies, say 4 to 8 Hz. Besides array aperture, the sensor interspacing distribution is an important parameter for judging array performance. SNR spectra for events with signal paths in oceanic and active tectonic belt regions peak at about 3 Hz, and for shield areas between 5 to 7 Hz. Finally, future array developments are discussed in view of recent initiatives within the seismological community concomitant with advances in communication and microprocessor technologies.

Offset-amplitude variation and controlled-amplitude processing

Abstract: The partition of plane seismic waves at plane interfaces introduces changes in seismic amplitude which vary with angle of incidence. These amplitude variations are a function of the elastic parameters of rocks on either side of the interface. Controlled‐amplitude processing is designed to obtain the true amplitude information which is geologic in origin. The offset‐amplitude information may be successfully used to predict the fluid type in reservoir sands. Various tests were carried out on a seismic profile from the Gulf Coast. The processing comparison emphasized the effects and pitfalls of trace equalization, coherent noise, offset, and surface‐related problems. Two wells drilled at amplitude anomaly locations confirmed the prediction of hydrocarbons from offset‐amplitude analysis. Furthermore, controlled‐amplitude processing provided clues in evaluating reservoir quality, which was not evident on the conventional relative amplitude data.

Investigation of basic imaging properties in digital radiography. 3. Effect of pixel size on SNR and threshold contrast.

Abstract: The effect of pixel size on the signal-to-noise ratio (SNR) and threshold detection of low-contrast radiologic patterns was investigated theoretically for digital radiographic systems. The SNR based on the perceived statistical decision theory model, together with the internal noise of the human eye-brain system, was calculated by using two-dimensional displayed digital signal spectra and noise Wiener spectra. Threshold contrasts were predicted from the calculated SNR for various combinations of object size and shape, pixel size, resolution, and noise. Predicted threshold contrasts agreed well with those determined experimentally in an observer performance study. The threshold contrast of small objects increased substantially as the pixel size increased beyond 0.2 mm. For pixel sizes of 0.1 and 0.2 mm, however, the threshold contrasts were similar. Since a digital system is not shift invariant, a range of threshold contrast results for a small object and a large pixel, depending on the alignment of the object position relative to the sampling coordinates.

Electronic counter measure system utilizing a digital RF memory

Abstract: An active radar jamming system using a digital radio frequency memory (DRFM) as the central subsystem in the generation and detection of specific signal characteristics. Associated systems control the operation, data processing, and loading of the DRFM. Signals generated by the jamming system are either repeated replicas of the received radar signal stored in the DRFM, or newly generated signals constructed from data loaded into the DRFM by the associated subsystems. Such associated subsystems generate appropriate data for providing carrier and noise RF signals. Systems are also provided for determining the presence of phase coding and chirp modulation in the received radar signal data contained in the DRFM, and for detecting the angle-of-arrival of the radar signal.

Some experimental and theoretical results using a new adaptive filter structure for noise cancellation in the presence of crosstalk

Abstract: The application of adaptive filters in noise cancelling often requires the relative placement of the two transducers at a distance that necessitates a large order filter in order to obtain an adequate output signal-to-noise ratio. A new adaptive filter structure is introduced that permits a closer placement of the transducers and that allows the cancellation of noise in the presence of crosstalk. Algorithms are developed for the new transversal and lattice structures. Simulations show considerable improvement in mean-square error over that obtained with standard noise cancelling algorithms.

Effects of masker waveform and signal-to-masker phase relation on diotic and dichotic masking by reproducible noise.

Abstract: The proportions of hits and false alarms were estimated for the detection of a 500‐Hz sinusoidal signal in each of 25, reproducible samples of wideband, white, Gaussian noise. The effects of signal phase were investigated under diotic (MoSo) and dichotic (MoSπ) conditions and compared to the predictions of two major models of binaural hearing. Averaging the data over samples obscured important across‐sample and across‐subject differences in performance. The proportions of hits and false alarms for individual noise samples presented under the MoSo condition were highly correlated with those for the same noise samples under the dichotic MoSπ condition, suggesting that the cues determining performance under these conditions are related. Signal‐to‐masker phase had a large effect on the proportion of hits under the MoSo condition, but only a small effect under the MoSπ condition. The Vector model predicts a large effect of signal phase under the MoSπ condition, and is, therefore, imcompatible with this aspect of the data. The expected value of the decision variable of the EC model is independent of signal phase. However, when the variance of the decision variable is also considered, the EC model does predict changes in the proportion of hits with the phase of the signal, comparable to those observed here. Further, it was shown that, if minor changes in the form of the EC model’s decision variable or in the distribution of the internal noise parameters are assumed, the expected value of the decision variable also changes with the phase of the signal.

Use of a nonstationary temporal Wiener filter in nuclear medicine.

Abstract: The use of the Wiener filter is proposed for temporal filtering of nuclear medicine dynamic studies. This filter adapts to the signal and noise levels of each pixel activity curve in a dynamic study to produce an “optimal” suppression of noise, while maintaining the signal content of the curve. The filter is derived to be a simple function of the power spectrum of the time-activity curve. Examples of its use for temporaly filtering gated blood-pool studies for cine viewing and functional image formation are shown.

Analysis of biomedical signals by means of linear modeling

Abstract: The recording and subsequent analysis of electrical signals of physiological origin constitutes an important aspect of current biomedical research. A versatile method for the analysis of such signals is based on linear, i.e., autoregressive (moving average) modeling. These techniques are based on fitting a hypothetical model to the signal under observation. These models are capable of generating the original signal by a linear combination of past observations and past and present noise samples. High resolution spectral estimates can be obtained in this way. Also, the often small number of model coefficients offer a concise description of the signal and may be used for classification purposes. Other applications entail the detection of nonstationarities, data-compression, and signal enhancement. In this review, linear modeling methods for the analysis of electroencephalograms, electro- and phono-cardiograms, electromyograms, and gastrointestinal signals are surveyed.

QANI Trellis-coded signal structure

Abstract: A signal structure for high speed Trellis coded data communications selects each signal point in the structure to attain a low probability of error in the presence of both additive noise and multiplicative noise. Viterbi decoding is modified to take account of the multiplicative noise energy content of the received signal.

Low bit-rate pattern encoding and decoding capable of reducing an information transmission rate

Abstract: In an encoder operable in response to a discrete pattern signal divisible into a sequence of segments to produce an output code sequence, each segment is produced during a frame and specified by representative excitation signals extracted from each segment. The representative excitation signals may be representative pulses placed in a selected one of subframes formed by dividing the frame with reference to a spectral parameter and a pitch parameter extracted from each segment. Alternatively, the representative excitation signals may be either a combination of the representative pulses and a noise or a noise alone. The representative pulses and the spectral parameters may be subjected to interpolation. In a decoder for decoding the output code sequence into a reproduction of the discrete pattern signal, the representative pulses are interpolated to arrange excitation pulses in all of subframes of each frame and to produce an excitation vocal source signal. The excitation vocal source signal may also be produced by the use of a decoded noise. A synthesizing filter circuit is driven by the excitation vocal source signal to produce the reproduction.

Signal restoration for linear systems with weighted inputs. Singular value analysis for two cases of low-pass filtering

Abstract: The a priori knowledge that the input of a linear system is weighted by some kind of profile function can be exploited for restoring the output signal by means of the singular value technique. The authors determine the analytic expression of the singular system for two typical cases in which the system behaves like a low-pass filter and the profile function has the same form as the impulse response of the filter. The analysis leads us to predict the performance of the restoration process in the presence of noise. It is shown that under suitable conditions the resolution of the restored signal can be twice that of the unrestored output.

Noise rejection in a sonic transducer

Abstract: A method of time domain noise rejection in a sonic transducer is disclosed in which all signal reception is prevented between application of a periodic electrical pulse and reception of the corresponding electrical signal. An inhibit signal, produced for a substantial portion of the time interval between application of an electrical pulse and reception of the corresponding electrical return signal, blocks all noise that may occur to provide a false indication of a return electrical signal during the time of the inhibit signal.