Noise-like Signals Research Articles

Laser–ultrasonic absorption measurements in low carbon steels

We have refined the contactless laser–ultrasound reverberation technique to measure ultrasonic absorption on small metallic samples. In this technique, a sample is supported by a holder which is ultrasonically decoupled from the sample. A pulsed laser is used to generate an acoustic pulse. After the pulse has mode converted and scattered sufficiently to fully insonify the sample, the decrease in the noise-like ultrasonic signal is recorded as a function of time using a laser-interferometer. A joint time–frequency analysis technique is used to extract an absorption spectrum from the signal. In this paper, the technique is demonstrated in a frequency bandwidth ranging from 1 to 7 MHz, and in a dynamic range of 0.003 to 0.3 dB μs −1. Measurements made on samples of three low-carbon steel grades, namely ultra low carbon (ULC), low carbon (LC), and high strength, low-alloy steels (HSLA), clearly show that ultrasonic absorption varies with steel grade. The technique was utilized to study the effect of a magnetic field on the ultrasonic absorption of an annealed ultra low carbon steel sample. It was found that magnetoelastic effects are responsible for a large fraction of the total absorption.

The use of optical fibres in radio astronomy

The low loss and ease of use of optical fibres for data transmission offers a number of advantages over traditional methods of bringing signals from radio telescopes together. Aperture synthesis techniques involve the correlation of signals from each pair of telescopes in the array. The requirements of radio astronomy systems, where the broad-band noise-like signals from each telescope have to be brought together coherently over distances as large as hundreds of km or greater in some cases, are discussed in this paper. A number of arrays around the world currently use fibres for data transmission and also for the coherent transfer of local oscillator signals. Further developments in the use of optical fibres in radio astronomy are described as well as new instruments planned for the next millennium, where fibre interconnections will be an essential part of their design.

A malleable audio representation for data compression

The boundaries between audio data compression and audio synthesis/modifications are blurring as research in both fields progresses. New results using a sines + transients + noise parametric representation allows for coding gains as efficient as the most recent nonparametric transform coding schemes. Because the audio input is decomposed into separate sinusoidal, transient, and noiselike signals, aggressive quantization can be performed independently on each signal in a perceptually meaningful manner. While the sines + transients + noise representation allows for good compression rates, it also allows for high-quality time-scale modification in the compressed domain. The multiresolution sinusoidal modeling and the Bark-band noise modeling utilized in this representation have been used in both the music synthesis and audio/speech coding arenas. The transient modeling in this representation uses similar algorithms to those used in transform coding systems.

Investigation of the Applicability of PM Noiselike Signals in SARs with Ambiguous Distance Reading

Investigation of the Applicability of PM Noiselike Signals in SARs with Ambiguous Distance Reading

Estimation of the impulse response of ultra-wide-band systems

The time and frequency algorithms of impulse response recovery in linear ultra-wide-band systems by a finite set of digital data presenting the input and output signals are proposed. The results of approbation of these algorithms by numerical simulation in the presence of additive Gaussian noise in the output signal are given. It is shown that the systems with noise-like signals at the input have the largest noise immunity in the estimation of impulse response.

Optical preamplifier receiver for spectrum-sliced WDM

Spectrum-slicing provides a low-cost alternative to the use of multiple coherent lasers for wavelength division multiplexing (WDM) applications by utilizing spectral slices of a single broadband noise source for creating the multichannel system. In this paper we analyze the performance of both p-i-n and optical preamplifier receivers for spectrum-sliced WDM using actual noise distributions, and the results are compared with those using the Gaussian approximation. This extends prior results of Marcuse for the detection of deterministic signals in the presence of optical amplifier and receiver noise. Although the methodology is similar, the results are considerably different when the signal is itself noise-like. For the case of noise-like signals, it is shown that when an optical preamplifier receiver is used, there exists an optimum filter bandwidth which minimizes the detection sensitivity for a given error probability. Moreover the evaluated detection sensitivity, in photons/bit, represents an order of magnitude (>10 dB) improvement over conventional detection techniques that employ p-i-n receivers. The Gaussian approximation is shown to be overly conservative when dealing with small ratios of the receiver optical to electrical bandwidth, for both p-i-n and preamplifier receivers.

Electronic chaos generators—design and applications

Chaotic systems are known to generate noise-like signals with a power density spectrum distributed over a certain frequency range. These signals are similar to random noise and can be used as broadband signals for system simulation and measurement, for simulation of random signals and for communication purposes. Since very simple nonlinear dynamic systems can show chaotic behaviour, its use for generating broadband signals could be an interesting and cost effective alternative to the classical application of noise from natural sources, such as resistors or semiconductor devices. For this purpose, chaotic signal generators have to be tailored to the special purposes and demands of the application. In this paper, the design of chaotic systems generating signals with prescribed probability distribution and power spectrum properties will be considered, their realization as discrete-time, digital and analogue systems will be regarded and design examples will be given. Finally some application examples of broad...

Measuring system for investigating noise-like signals

A measuring system for investigating noise-like signals is described. The arrangement can be used in robot vision systems and to record and measure electromagnetic interference.

Hearing aid with improved noise discrimination

A hearing aid that generally provides greater amplification for speech signals than noise signals. The hearing aid includes a microphone, variable highpass filter, transducer, and sensor assembly. The sensor assembly detects the amplitude of a band of low frequencies of the highpass filtered microphone signal, and provides a feedback signal to the variable filter to influence its cutoff frequency based on the characterisics of the highpass filtered microphone signal. The sensor also takes the time characteristics of the signal envelope into account, distinguishing between steady state noise-like signals and dynamically varying signals, such as speech. In response to loud, low frequency, substantially steady state signals, above a threshold amplitude, the sensor provides a feedback signal to the variable filter such that the cutoff frequency is raised. In response to softer, and/or higher frequency, and/or more dynamically amplitude varying signals, the cutoff frequency of the variable filter is raised less or left at a lower value.

Six-band sub-band coder on ECG waveforms.

An ECG sampled at a rate of 500 samples s-1 or more produces a large amount of redundant data that are difficult to store and transmit. A process is therefore required to represent the signals with clinically acceptable fidelity and with the least code bits possible. In the paper, an efficient sub-band coding method for encoding ECG waveforms is presented. Although sub-band coding has been successfully applied to speech signals, it is the first time that this technique has been applied to the encoding of ECG waveforms. A frequency band decomposition of an ECG waveform is carried out by means of quadrature mirror filters (QMF), which split the ECG spectrum into six bands of unequal width. In the lower frequency bands, which contain most of the ECG spectrum energy, a larger number of bits per sample is used, whereas in upper frequency bands, which contain noise-like signals, fewer bits per sample and the run length coding method are used. The simulation results are presented in terms of bit rates and the quality of the reconstructed waveforms. The results show that a reproduction with an average signal-to-noise ratio (SNR) of 29.97 dB can be achieved even at an average bit rate of 0.81 bits per sample.

Statistical characteristics of a noncoherent digital discriminator for the delay of a phase-keyed noise-like signal

Phase-keyed noise-like signals (PK NLS) have become extensively used in radiocommunication systems. The receivers of such systems, which are designed according to the follow-up principle, usually employ noncoherent delay discriminators which are required to determine the delay of a modulating pseudorandom sequence (PRS). In [1] a digital noncoherent delay discriminator (DNDD) of the two-channel type was proposed which may be used to solve such a problem; in [2] a method was given for analyzing its statistical characteristics.

1986年パームスプリングス地震の強震記録を用いた多重震源位置のインバージョン解析

upture propagation process of multiple events is identified from short-period strong motion seismograms. The North Palm Springs earthquake (ML=5.9), which occurred in southern California on July 8, 1986, provided many strong motion records in the near field. CDMG's digital records from the five close stations (Δ=10-42km) are used for inversion analysis of multiple events. Short-period records obtained in the near field are thought to be more informative for understanding the local rupture mechanism than long-period worldwide records. However, waveforms of short-period records are usually very complicated to identify rupture mechanism. In addition, near-field records tend to include many kinds of seismic waves (i. e., direct P-, S-, reflected, or surface waves) at the same moment, while in the far-field records most of these waves would be separately observed. These facts make it difficult to discern distinct phases from strong motion records. Therefore, we employ non-stationary spectra (multi filter) to identify feeble distinct phases. Then sveral phases are successfully distinguished from other noiselike signals on the specta. Especially, two distinct phases, which have consicuous peaks and similar predominant frequencies among the records, appear in the all spectra of transverse components of the five stations. We regard them as S-wave arrivals that have directly propagated from two sub-sources. Source locations and trigger times of these sub-events are inversely calculated from the arrival times. Numerical results show that these events occurred immediately after initial rupture of the mainshock, and they were located close to the fault plane and within several kilometers from the initial hypocenter. Considering a fault dimension of 16km×12km which was inferred from aftershock distribution, it can be concluded that the major ruptures causing short-period motions concentrated near the initial hypocenter and did not extend throughout the fault plane.

Gross electrocortical activity as a linear wave phenomenon with variable temporal damping, regulated by ascending catecholamine neurones.

We report critical tests for a theory of electrocortical wave processes, in which telencephalic dendritic potentials reflect the mass action of coupled oscillatory circuits exhibiting complicated and unspecified non-linearities, the whole system being driven by active cell firing. Specific assumptions were: stochastic independence for instantaneous coupling parameters in the system, an individual central tendency to the cycle time for each circuit, and the maintenance of steady state conditions. Application of the central limit theorem to the state transition matrix shows that the gross electrocortical waves should be linear waves, exhibiting a multitude of invariant resonant modes, with the natural frequencies of all the modes being clustered about a smaller number of center values. Ascending brain-stem neurones of at least the dopaminergic and noradrenergic classes should regulate both the power of noise-like signals driving the telencephalic resonant patterns, and the temporal damping of each resonance. We devised tests which involved between hemisphere comparisons of electrocortical spectra, before and after unilateral lesion of transhypothalamic ascending fibres, thus obtaining ratio power changes attributable to post lesion asymmetry of damping and driving, in modes of equivalent left-right center-frequencies. These ratio spectra were curve-fitted to an approximate theoretical expression, and the parameters obtained enabled tests of several specific predictions. Estimates of the center values for resonant mode frequencies, comparison of the relative changes in left/right phase with that expected from the ratio changes in power, and estimates of the surface-to-signal transformation of left and right signals made by a back-calculation, all conform to expectation from the theory, and are consistent across lesion of different types of ascending neurone.

Beat excitation of whistler mode sidebands using the Siple VLF transmitter

The process of whistler mode sideband generation in the magnetosphere was studied by transmitting two monochromatic signals closely spaced in frequency (Δf = 5–45 Hz) from the experimental VLF transmitter at Siple Station, Antarctica. The signals were observed following ducted magnetospheric propagation to the conjugate station at Roberval, Quebec. Sidebands up to seventh order were generated extending to ±100 Hz with respect to the average frequency of the carriers. New frequencies were observed both under conditions of little or no growth of the input signals and when one of the input signals did grow significantly. At times the sideband amplitudes exceeded the intensity of either input signal. A small signal mechanism is proposed in which emissions are triggered by each beat between the input waves but are then suppressed by the following beat. The energy from the phase bunched particles is believed to feed preferentially into the sidebands rather than causing growth of the input waves themselves. In this model, phase trapping of resonant electrons by the wave is not required. The observed process of sideband generation provides a mechanism to break down the coherence of relatively narrowband waves in the magnetosphere and may thus account for existing evidence of the transformation of relatively coherent wave packets into broader more variable bands of noiselike signals.

Analog-Filter Digital-Correlator Hybrid Spectrometer

A system is described for measuring the power spectrum and cross spectral density of wide-band (? 50 MHz) noise-like signals which occur, for example, in radio astronomy. The system utilizes a comb-filter bank followed by digital correlator processing of each filter output. The cost equation, design factors, and a sample system are described. For measurement of the spectrum at a large number of frequencies, the system cost of the hybrid system is shown to be much lower than the cost of spectrometers which utilize either a filter-bank or digital correlator alone.

Seti and serendipity

The serendipitous detection of an ETI signal during the course of regularly scheduled radio astronomical observations has been an intriguing possibility for the past two decades. For some, this has been enough. For others, unsatisfied with such a passive approach, it has been enough to attempt detection through unconventional applications of radio astronomical instrumentation. Had detections occurred during such searches, they too would have been serendipitous to the development of the field of radio astronomy. For still others, use of equipment intended for the detection of weak, noise-like signals from astrophysical sources has not been enough. SETI-specific instrumentation having unique capabilities for the efficient detection of a moderately broad range of artificial signals has been designed and partially constructed. As a result, it is now possible to consider what serendipitous discoveries of an astrophysical nature might result from the use of this equipment in regularly scheduled SETI observations. The SETI-specific instrumentation will view the Universe through a set of observational filters which have never before been systematically employed by astronomers. In this paper, a particular model of what constitutes a truly new and unique astrophysical phenomenon is used to predict the likelihood of an astrophysically serendipitous discovery during the course of the proposed SETI microwave observing program. Having considered what SETI may do for astronomy, we return to the more traditional approach to serendipity and discuss what a newly designed background software package for the Very Large Array of radiotelescopes can do for SETI. This project is the logical follow-on to the symbiotic use of data contained in stored radio maps from the Westerbork Synthesis Radio Telescope, which was described at the IAA CETI Review Symposium during the 1981 IAF Congress in Rome.

Fundamental limitations in passive time delay estimation--Part I: Narrow-band systems

Time delay estimation of a noise-like random signal observed at two or more spatially separated receivers is a problem of considerable practical interest in passive radar/sonar applications. A new method is presented to analyze the mean-square error performance of delay estimation schemes based on a modified (improved) version of the Ziv-Zakai lower bound (ZZLB). This technique is shown to yield the tightest results on the attainable system performance for a wide range of signal-to-noise ratio (SNR) conditions. For delay estimation using narrow-band (ambiguity-prone) signals, the fundamental result of this study is illustrated in Fig. 3. The entire domain of SNR is divided into several disjoint segments indicating several distinct modes of operation. If the available SNR does not exceed SNR <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> , signal observations from the receiver outputs are completely dominated by noise thus essentially useless for the delay estimation. As a result, the attainable mean-square error <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\bar{\epsilon}^{2}</tex> is bounded only by the a priori parameter domain. If SNR <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> < SNR < SNR <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , the modified ZZLB coincides with the Barankin bound. In this regime differential delay observations are subject to ambiguities. If SNR > SNR <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> the modified ZZLB coincides with the Cramer-Rao lower bound indicating that the ambiguity in the differential delay estimation can essentially be resolved. The transition from the ambiguity-dominated mode of operation to the ambiguity-free mode of operation starts at SNR <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and ends at SNR <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> . This is the threshold phenomenon in time delay estimation. The various deflection points SNR <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> and the various segments of the bound (Fig. 3) are given as functions of such important system parameters as time-bandwidth product (WT), signal bandwidth to center frequency ratio (W/ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> ) and the number of half wavelengths of the signal center frequency contained in the spacing between receivers. With this information the composite bound illustrated in Fig. 3 provides the most complete characterization of the attainable system performance under any prespecified SNR conditions.

A short note on the reduction of digitisation errors

The conditions under which systematic digitisation errors do and do not vanish are examined, and it is shown that in certain circumstances they can be eliminated entirely by the addition of a suitable noise-like signal to the data before digitisation.

Noise generator for a polyphonic tone synthesizer

First Page

Estimation of time-varying co-ordinates of ion beams

A method is presented to estimate the time-varying co-ordinates of a continuous, low-energy ion beam. The method is noncontacting and exploits noise-like signals induced by the beam on electrostatic induction electrodes. Results of some preliminary experiments are also shown.