Showing papers on "Noise (signal processing) published in 1975"

Faster-than-nyquist signaling

Abstract: The degradation suffered when pulses satisfying the Nyguist criterion are used to transmit binary data in noise at supraconventional rates is studied. Optimum processing of the received waveforms is assumed, and attention is focused on the minimum distance between signal points as a performance criterion. An upper bound on this distance is given as a function of signaling speed. In particular, the pulse energy seems to be the minimum distance up to rates of transmission 25 percent faster than the Nyguist rate, but not beyond. Some mathematical aspects related to the above problem are also considered. In particular, the minimum distance is rigorously shown to be nonzero for all transmission rates. This is tantamount to showing that, in the singular case of linear prediction, perfect prediction cannot be approached with bounded prediction coefficients.

Communication systems: an introduction to signals and noise in electrical communication

Abstract: 1 Introduction 2 Signals and Spectra 3 Signal Transmission and Filtering 4 Linear CW Modulation 5 Exponential CW Modulation 6 Sampling and Pulse Modulation 7 Analog Communication Systems 8 Probability and Random Variables 9 Random Signals and Noise 10 Noise in Analog Modulation Systems 11 Baseband Digital Transmission 12 Digitization Techniques for Analog Messages and Computer Networks 13 Channel Coding and Encryption 14 Bandpass Digital Transmission 15 Spread Spectrum Systems 16 Information and Detection Theory Appendix: Circuit and System Noise

Characterization of cyclostationary random signal processes

Abstract: Many communication and control systems employ signal formats that involve some form of periodic processing operation. Signals produced by samplers, scanners, multiplexors, and modulators are familiar examples. Often these signals are appropriately modeled by random processes that are cyclostationary (CS), i.e., processes with statistical parameters, such as mean and autocorrelation, that fluctuate periodically with time. In this paper we examine two methods for representing nonstationary processes that reveal the special properties possessed by CS processes. These representations are the harmonic series representation (HSR) and the translation series representation (TSR). We show that the HSR is particularly appropriate for characterizing the structural properties of CS processes and that the TSR provides natural models for many types of communication signal formats. The advantages gained by modeling signals as CS processes rather than stationary processes is illustrated by consideration of the optimum filtering problem. We present general solutions for filters that minimize mean-square error for continuous-waveform estimation, and we discuss several specific examples for the particular case of additive noise. These examples demonstrate improvement in performance over that of filter designs based on stationary models for the signal processes.

An Optimum Linear Receiver for Multiple Channel Digital Transmission Systems

Abstract: An optimum linear receiver for multiple channel digital transmission systems is developed for the minimum P e and for the zero-forcing criterion. A multidimensional Nyquist criterion is defined together with a theorem on the optimality of a finite lenght multiple tapped delay line. Furthermore an algorithm is given to calculate the tap settings of this multiple tapped delay line. This algorithm simplifies in those cases where the noise is so small that it can be neglected. Finally as an example the transmission of binary data over a cable, consisting of four identical wires, symmetrically situated inside a cylindrical shield, is considered.

Ultrasonic imaging method and apparatus

Abstract: The ultrasonic imaging method and apparatus comprises an ultrasonic wave transducer supplied with recurrent multifrequency energy pulses for pulse insonification of an object under investigation with ultrasonic waves. Resultant echo waves from the object are directed onto the transducer for converting the same to electrical signals which are supplied to a signal processor which includes a variable bandpass filter. One or more of the filter characteristics are varied as a function of depth from which the echo signals are returned for enhanced resolution and signal-to-noise ratio of the received signal. Preferably, the filter is matched to the noise and signal spectra of the system. For A scan and B scan operations wherein reverberated acoustic pulses are derived from a range of depths a time variable filter is employed for time varying operation thereof.

Nonlinear filtering in coherent optical systems through halftone screen processes.

Abstract: Nonlinear filtering operations can be performed in coherent optical systems with the help of the halftone screen process. Theoretical and practical considerations regarding this type of system are presented. The use of these methods to achieve logarithmic filtering is emphasized. Applications to separation of multiplicative signals and noise, speckle noise reduction, and processing of radiographic images are considered. Experimental results are presented.

Statistically Constrained Inverse Electrocardiography

Abstract: This paper examines the feasibility of utilizing statistical constraints on the inverse potential model to determine the potential distribution over a 4 cm sphere surrounding the heart from perturbed torso potentials. These perturbed torso potentials reflect instrumentation, quadrature, electrode placement, and heart position uncertainties. This work is an extension of the authors' previous work which concluded that it is not feasible to determine this same potential distribution using unconstrained solutions. However, the results of the present work indicate that with the use of approximate signal and noise covariance matrices, it is possible to achieve estimates of this potential distribution with an average sum squared error of twenty-five percent. Further, the estimation of the signal and noise covariance matrices can be accomplished with a knowledge of heart geometry, torso geometry, The approximate measurement error, and a rough estimate of the time an average section of myocardium is depolarized, but without an a priori specification of the activation sequence.

Phase tracking network

Abstract: A phase tracking network comprising a phase correction network for correcting at least some of the phase error of an input signal to provide a phase corrected signal having components in first and second channels and a detector responsive to the phase corrected signal for providing a detected signal having first and second components in the first and second channels. An error calculator provides a phase correction signal back to the phase correction apparatus which responds to the phase correction signal to correct the phase error of the input signal. The phase error signal contains substantially no noise from the first channel at least when the first component of the detected signal is zero and contains substantially no noise from the second channel at least when the second component of the detected signal is zero. This is accomplished by utilizing the error calculator to provide a first control signal which represents a first fixed value, zero, and a second fixed value when the first component of the detected signal is ideally positive and not zero, ideally zero, and ideally negative and not zero, respectively. The error calculator similarly provides a second control signal for the second component of the detected signal.

Base-line noise and detection limits in signal-integrating analytical methods. Application to chromatography

Abstract: The uncertainty in the determination of the time integral of analytical signals due to base-line noise is calculated for specified noise types of frequent occurrence. The influence of time constants, integration time and analog or digital filters on the precision is considered. The optimal conditions for maximum precision in chromatography without advanced data handling are determined.

Heart beat detector

Abstract: A heart beat detector for use in emergencies and especially under adverse conditions of noise and vibration, of the type adapted to continuously monitor the ECG, extracting the QRS complexes and signalling the occurrence of heart beats by easily perceptible, light signals. The detector includes an inter-electrode impedance sensor; for automatically ascertaining whether adequate conductivity exists between the body of the patient and the electrodes of the detector applied to said body. According to a specific embodiment of the invention the device comprises a sub-system adapted to indicate the activity of the batteries of the instrument, the state of the electrodes and of the other circuits.

Method and apparatus for error reduction in digital information transmission systems

Abstract: A method and apparatus for reducing idle channel noise, cross talk and quantizing error noise in a digital information transmission system designed for operation over a predetermined frequency spectrum and having an analog-to-digital converter, an information transmission medium, and a digital-to-analog converter all located between a transmitting station and a receiving station. A controlled noise signal having an amplitude lying in the range from about 1/4 to 1/2 of the magnitude of a quantizing interval of the ADC and DAC and a frequency content concentrated at (n+1/2) times the ADC and DAC sampling frequency but outside the system frequency spectrum is added to the analog information input signals prior to conversion to digital form. The injected controlled noise signal predominates whenever the amplitude of spurious noise signals is less than the amplitude of the controlled noise signal and the level of the input to the ADC is near a quantizing interval transition point so that the subsequent analog output signals from the DAC have frequencies concentrated outside the frequency spectrum of the system. The DAC output signals are subsequently filtered by a post sampling filter having a pass band coextensive with the system frequency spectrum so that the controlled noise signal components are removed before coupling to the receiving station.

Flow rate computer adjunct for use with an impedance plethysmograph and method

Abstract: A computer adjunct is described which is used in combination with an impedance plethysmograph. The impedance plethysmograph provides signals related to conductance in a biological segment and related to deviation from a quiescent resistance level in the biological segment as body fluids are pumped therethrough in a pulsatile manner by the pumping action of the heart. The deviation of the segment resistance contains artifact signals such as those imposed by movement or breathing. The computer adjunct contains means for providing a signal indicative of heart pumping rate and circuit means for measuring the level of the artifact signals once each heart pumping cycle and generating a correction signal related thereto. The correction signal is thereafter applied to the deviation signal for reducing error induced by the artifact signals. The heart rate signal is conditioned to obtain a signal related thereto which is averaged. The corrected deviation signal is also averaged. The averaging is performed by circuitry which integrates the corrected deviation signal, thereby reducing residual cyclic noise due to the artifact signal while still affording relatively short output stabilization time. The two averaged signals are multiplied together and then combined with the segment conductance signal for providing an output signal indicating blood flow rate through the biological segment. The combination is capable of blood flow rate measurements for calf segments or thoracic segments.

Composite wavefront decomposition via multidimensional digital filtering of array data

Abstract: A solution is obtained to the problem of estimating the number, vector velocity, and waveshape of overlapping planewaves in the presence of interfering planewaves and channel noise, where previous solutions have assumed one or more of these quantities as known. A general optimum solution is not found; instead, a heuristic solution is presented along with a complete working implementation program for large scale computers. For the case where the number of waves and the vector velocities are known, the solution is optimum. The detection of waves and the estimation of their bearing, velocity, and waveshape is accomplished via digital filtering of the frequencywavenumber power spectrum, which is computed via an efficient estimator, of the array sensed data. A new approach to the multiwave estimation problem is to reduce it to a succession of single wave problems using especially developed frequency-wavenumber filters. Special attention is given throughout the study to computationally efficient approaches. The results of the paper are placed in perspective by showing how the historically important approaches to the processing of array data such as delay and sum, weighted delay and sum, array prewhitening, beam forming, inverse filtering, least mean-square estimation, and maximum likelihood estimation are related via the spatio-temporal filtering of the frequency-wavenumber spectrum. The spectral estimation, digital filtering, and the multiwave maximum likelihood estimator developments are demonstrated by the processing of a set of simulated planewaves of various bearings, velocities, and frequencies, as well as by processing electroencephalographic (brain wave) data monitored via an array of scalp electrodes.

Homomorphic filtering — theory and practice*

Abstract: The application of homomorphic filtering in marine seismic reflection work is investigated with the aims to achieve the estimation of the basic wavelet, the wavelet deconvolution and the elimination of multiples. Each of these deconvolution problems can be subdivided into two parts: The first problem is the detection of those parts in the cepstrum which ought to be suppressed in processing. The second part includes the actual filtering process and the problem of minimizing the random noise which generally is enhanced during the homomorphic procedure. The application of homomorphic filters to synthetic seismograms and air-gun measurements shows the possibilities for the practical application of the method as well as the critical parameters which determine the quality of the results. These parameters are: a) the signal-to-noise ratio (SNR) of the input data b) the window width and the cepstrum components for the separation of the individual parts c) the time invariance of the signal in the trace. In the presence of random noise the power cepstrum is most efficient for the detection of wavelet arrival times. For wavelet estimation, overlapping signals can be detected with the power cepstrum up to a SNR of three. In comparison with this, the detection of long period multiples is much more complicated. While the exact determination of the water reverberation arrival times can be realized with the power cepstrum up to a multiples-to-primaries ratio of three to five, the detection of the internal multiples is generally not possible, since for these multiples this threshold value of detectibility and arrival time determination is generally not realized. For wavelet estimation, comb filtering of the complex cepstrum is most valuable. The wavelet estimation gives no problems up to a SNR of ten. Even in the presence of larger noise a reasonable estimation can be obtained up to a SNR of five by filtering the phase spectrum during the computation of the complex cepstrum. In contrast to this, the successful application of the method for the multiple reduction is confined to a SNR of ten, since the filtering of the phase spectrum for noise reduction cannot be applied. Even if the threshold results are empirical, they show the limits for the successful application of the method.

Carrier-modulated coherency monitoring system

Abstract: A radio information system is described which uses the exact mathematical tios and the unique periodic angular coherence of the residual RF carrier and the digitally encoded modulation spectral components of the electromagnetic wave as emitted by a source transmitter (or transmitters) to enhance system performance and to provide new instrumentation and measurement capabilities at the receiver apparatus. The digital modulation synchronizing code occupies a wide frequency bandwidth and is encoded for precise repetitive transmission to provide, for example in a satellite navigation system, a means of accurate synchronization and measurement of time and range parameters. Many encoded sequences of either discrete or pseudo-random types may be used as the synchronizing code. However in the description herein the pseudo-random noise (PRN) codes are used for their added advantage in spread spectrum discrimination against intentional interference. The total or composite RF signal typically also includes non-interfering information or message modulation.

Discrete optimal linear smoothing for systems with uncertain observations

Abstract: The smoothing filter and smoothing error covariance matrix equations are developed for discrete linear systems whose observations may contain noise alone, where only the probability of occurrence of such cases is known to the estimator. An example of such a system arises in trajectory tracking, where the signal is first detected and then is processed by the estimator for tracking purposes. The results apply to any detection decision process, however, any such decision is associated with a false alarm probability, which is the probability that the detected signal contains only noise. The present results together with the earlier work of Nahi on prediction and filtering give a complete treatment of the discrete linear estimation problem for systems characterized by uncertain observations. These results, of course, reduce to well-known formulations for the classical estimation problem in the case where the observation is always assumed to contain the signal to be estimated.

Wideband receiving system including multi-channel filter for eliminating narrowband interference

Abstract: In a returned wave object detection receiving system, for receiving a wideband signal, means are provided for attenuating narrowband interference. A returned signal is split such that equal power broadband signals are applied to a plurality of channels. In each channel, the broadband signal is coupled through a narrowband filter and a linear amplifier in parallel with a detector and noise correlator. The narrowband signals are recombined in an adder having an output coupled to utilization means. Disabling means are coupled in each channel for disabling one narrowband channel in response to narrowband interference therein.

Apparatus and method for reducing effects of amplitude and phase jitter

Abstract: Means and method are provided for correcting for the effects of amplitude and phase jitter, in the presence of noise on quadrature signals transmitted over a channel at regular intervals as quadrature pairs. The signal values of respective signal elements of sequentially transmitted quadrature pairs, as received are measured. Responsive to such measurement a first decision is made as to which pair of quadrature signals was transmitted. The measured value and value from the first decision for the same signal are compared for each signal and as a result of such comparison measures of the amplitude and phase errors occurring during transmission. The measures of amplitudeand of phase errors are averaged over respective pluralities of intervals. The measured signal values are transmitted over a line and modified to compensate for the average values of the phase errors and of the amplitude errors. The signal values so modified are used as the basis of a second decision as to the elements modified.

A digital technique for analyzing a class of multicomponent signals

Abstract: There are numerous occasions in applied sciences where there is need to analyze multicomponent signals formed by a linear superposition of functions having the same shape and location but different widths and amplitudes. A fast digital technique capable of producing the spectrum, i.e., the distribution of component amplitudes versus component widths, of such experimental data is presented in this paper. The method described does not require any a priori numerical information regarding the composition of the signal, can be easily and efficiently implemented on digital computers, can be automated, and is essential for an automated interpretation of these types of laboratory data. The technique is based upon a nonlinear change of variables followed by a deconvolution. A low-pass filtering is necessary in the final stage of data processing to reduce the effect of computational and experimental noise. As a main example of the practical implementation of this technique within a laboratory environment, the paper details its usage in connection with pulsed NMR measurements on malignant tissues, where the data have the form of a superposition of exponential decays.

Digital phase detector

Abstract: A digital system for measuring a phase relationship between two noisy signals of a given period wherein the phase difference measurement may lie in the range of from 0 to 360 degrees. The noisy signals are amplified and limited so that the processed signal only contains noise in the time vicinity around the zero crossing points. Time interval counters are utilized to measure the time delay between signals and the time period of the reference signal. Phase relationship may be determined by multiplying the ratio of the two digital numbers produced by 360°. Errors in digital outputs are minimized by digitally integrating the counter input during the noisy zero crossover periods. The measurements are completed within two period times of the signals being measured and may be repeated in each of the succeeding two period times.

Flux locked loop

Abstract: A flux locked loop, employing a squid in connection with the production of the locked-loop error signal, includes circuitry which renders the locked loop more immune to disturbance inputs such as a ground-loop noise and interference transients and which facilitates a scale factor adjustment of a sub-carrier drive signal for the squid. To minimize ground-loop noise disturbance effects, a portion of the circuitry in the feedback path of the locked loop includes a voltage-responsive converter circuit arrangement providing a current-drive feedback signal for coupling to the squid. To minimize interference transient disturbance effects, a portion of the circuitry in the forward path of the locked loop includes a circuit arrangement having a non-linear transfer characteristic defining opposed substantial attenuation regions separated by a generally linear gain region. To facilitate adjustment of the scale factor to a desired value, a portion of the circuitry provides for an open-loop operation in which a test-mode oscillating signal is coupled to the squid along with an operating-mode modulation drive signal. This causes a heterodyning process to take place such that the frequency spectrum of the electrical signal does not contain frequency components at or near the frequency of the test-mode oscillating signal under circumstances in which the modulation drive signal has been adjusted to the desired value.

CW radar system

Abstract: A self telemetering miss-distance radar method and system of high sensitivity and free of second-time-around echoes is provided by phase modulating a continuous wave signal with the output of a repetitive code source such as a pseudo-random noise generator. That signal is transmitted as the radar signal. Returning echoes are applied to one input of a product detector to whose other input a sample of the unmodulated CW signal may be applied. The mixer output contains the product of the CW signal and the echo. The echo includes the CW signal modified to include the noise code and Doppler shift. That product signal is applied to a second product detector to which a delayed version of the noise code is applied. If the code in the echo arrives at the second product detector in unison with the delayed code, the output level will be high. If not, it will be low. The second detector output, the second product signal, is passed through a Doppler filter. If there was unison in the codes, the Doppler signal output will be large. In some applications, the first product detector may be omitted. Doppler filter output can be used to frequency modulate the CW oscillator to develop a telemetering signal, or it can be used to generate digital signals which are added to the pseudo-random noise code generator output. In either event, the radar signal is modulated with current miss-distance information so that it serves as a telemetering signal for application to a separate antenna or to the radar antenna.

Motor control system, particularly to reduce idling speed

Abstract: To reduce noise and idling speed of electric motors, particularly series motors, the speed-torque characteristic of the motor is dropped with respect to the loaded speed-torque characteristic, by sensing no-load operation of the motor and deriving a sensed signal, the sensed signal being applied to controlled switches to abruptly change the effective voltage applied to the motor between values corresponding to the no-load speed-torque characteristic, or the load speed-torque characteristic and resulting in the desired speed, in dependence on the value of the sensed signal. Preferably, the operating condition of the motor is sensed by an inductive transducer, and a speed change is obtained by phase control, or half-wave rectification.

Low noise electronic circuit, transducer using the same, and method

Abstract: An electronic circuit and method used in conjunction with transducers for eliminating noise signals which includes an AC source of excitation for first and second transducer coils, called pickoff coils located in first and second signal channels. Output from the pickoffs is supplied to detectors disposed in each of the signal channels. The detected pickoff output signals are filtered and connected to a summing circuit wherein the internally generated noise from the AC source appearing as common mode signals in both signal channels is cancelled and differential mode signals produced in said channels are added to produce an output.

Simple schemes for measuring autocorrelation functions

Abstract: Several methods for obtaining the autocorrelation function of fluctuating signals are discussed. The efficiency of data processing and the signal−to−noise ratio obtained for a given time of data acquisition using each method is compared to that obtainable with a full (ideal) autocorrelator. Two simple methods utilizing a signal averager (CAT) were tested experimentally with a variety of noise sources and the results compared with the theoretically predicted correlation functions.