Showing papers on "Noise (signal processing) published in 1970"

Statistics of Sound Propagation in the Ocean

Abstract: The statistics of transmission fluctuations in the ocean are examined theoretically for both multipath and scattering processes. It is shown that, when both are present, multipath propagation dominates the fluctuations and leads to quite broad statistical distributions. In such cases, the mean and standard deviation for logarithmic measures such as transmission loss differ considerably from corresponding mean‐square measures. Results for a single tone have been generalized to include multitones, as a model of ambient noise. Statistics of a signal combined with such noise have been derived; the standard deviation of the combination may be several decibels higher than the 5.6 dB of signal alone. The distributions of logarithmic quantities are generally found to be log transformations and combinations of chi‐square distributions, and not log‐normal distributions as commonly supposed.

The capacity of the Gaussian channel with feedback

Abstract: In this paper we provide a rigorous proof that feedback cannot increase the capacity of the channel with additive colored gaussian noise by more than a factor of two. We also give a tighter bound showing that any increase in capacity is less than the normalized correlation between the signal and noise. It is further shown that gaussian signals and linear feedback processing will achieve capacity. The practical implications are that (i) feedback should be used to simplify encoding and decoding since there is little to be gained in the way of increased capacity and (ii) the various proposed schemes which use linear feedback are doing the correct thing.

Techniques for producing low-noise, improved efficiency holograms.

Abstract: Photographic processing techniques for producing high efficiency, low noise phase holograms on silver halide emulsions have been investigated. Control of the emulsion and maintenance of the integrity of the holographic record throughout all processing were identified as prime requisites for high quality holographic reconstructions. Holograms have been obtained which yield approximately 30% diffraction efficiency and signal to scattered noise (S/N) in excess of 50:1 in the reconstruction of a diffuse wavefront. Holographic interferometry and ghost imaging with diffuse subjects has also been achieved with these techniques.

Statistically optimal stacking of seismic data

Abstract: A theory for weighting seismic records in the stacking process has been developed from a statistical seismic model. The model applies to common‐depth‐point seismic records which have been statically and dynamically corrected; the same model applies to an ordinary stacking procedure. The model stipulates for the signal and noise components, respectively, of a seismic record that (1) the signal is coincident with and similarly shaped to the signal on other records, and (2) the noise is statistically independent of that on any other record and of the signal and has zero mean value. In accord with the model, a seismic record is completely described for the purpose of weighting by its signal scale and its signal‐to‐noise energy ratio. Several statistical procedures for evaluating these parameters for seismic field data are presented. The most favorable procedure is demonstrated with both synthetic and field seismic records.

The use of film nonlinearities in optical spatial filtering.

Abstract: Optical data-processing systems that use spatial filters for detecting two-dimensional signals are usually arranged so that the data are placed in a space domain and the signals to be detected are placed in a frequency domain as holographically recorded Fourier transforms. We propose to interchange the roles of the data and the signal by introducing the data into the system as a Fourier transform hologram and by placing the signal in the space plane. Furthermore, the nonlinear behavior of the film used to record the Fourier transform hologram can be used to good advantage. These modifications combine to give a system that (1) acts as an adaptive filtering system because the noise suppression part of the matched filter is based on a specific noise sample rather than on averages of many noise samples, (2) is less sensitive to the position of the element that must be changed to perform a search for the scale or orientation of the signal, and (3) may reduce the aberrations that are sometimes caused by spatial carrier-frequency functions. Experimental results support the theoretical conclusions.

Fixed-pattern noise in photomatrices

Abstract: Two MOS photomatrix configurations, voltage sampling and recharge sampling, have been compared with regard to sources of fixed-pattern noise. Voltage sampling provides a high-amplitude low-impedance photosignal, with FPN primarily due to threshold variation in the amplifying MOST at each element. Recharge sampling is used for large high-yield rapidly scanned arrays, with FPN caused mostly by variations in spurious capacitive breakthrough. Production peak-to-peak signal to FPN ratios are 20:1 for voltage sampling and 50:1 for recharge sampling.

Electronic and automatic selector device connected to an antenna array formed by two or more antennas

Abstract: An electronic automatic selecting device has an antenna array consisting of two or more antennas pointing in different directions for the purpose of selecting and connecting to a service receiver that antenna from which the desired radio signal is best obtained. The selection is carried out by a selecting device and a search receiver having a low frequency output, from which, when receiving a frequency modulated emission, a noise signal is obtained proportional to the S/N ratio of the signal received by the receiver. This signal steers the selecting device. In accordance with the present invention the selecting device includes a noise handling unit the input of which receives the noise voltage from the search receiver and the output of which is large when the search receiver receives a signal with a poor S/N ratio and small when the search receiver receives a signal with a good S/N ratio. There is a voltage-to-frequency converter the operating frequency of which is determined by the output voltage of the noise handling circuit, whereby the output frequency is high when the output voltage is large and is low when the output voltage is small, a pulse oscillator which oscillates at a constant frequency and which gives the search command, an electronic change-over switch which upon receiving the search command from the pulse oscillator, sequentially connects, steered by the voltage-to-frequency converter, the antennas of the antenna array to the search receiver, and which after sampling all antennas returns to zero. There is also a circuit for comparing the signals obtained from the various antennas to the input of which are fed the pulses from the voltage-to-frequency converter and the output of which produces a pulse when during a search cycle an antenna giving a signal with a better S/N ratio than any other antenna previously sampled during the search cycle is connected to the search receiver, and a memory circuit to the input of which is fed from a dividing circuit constituting a part of said electronic change-over switch, the number in binary form of each of the antennas connected to said search receiver during the search cycle. This binary number moves to the output of the memory circuit only when an output pulse is obtained from the signals comparing circuit. Finally there is an electronic switch controlled by said memory circuit and connecting the service receiver to that antenna of the antenna array from which the search receiver during the search cycle has received the signal with the best S/N ratio.

Applications of detection and estimation theory to large array seismology

Abstract: The statistical theory of signal detection and estimation has been applied to problems in large array seismology. Using this theory the structure of the optimum detector for a known signal and long observation time in additive Gaussian noise is derived. The array processing filter employed by the optimum detector is known as the maximum-likelihood filter. This filter also has the property that it provides a minimum-variance unbiased estimate for the input signal when it is not known, which is the same as the maximum-likelihood estimate of the signal if the noise is a multidimensional Gaussian process. A series of experiments was performed using data from the large aperture seismic array to determine the effectiveness of the maximum-likelihood method relative to simpler methods such as beam-forming. These results provide significant conclusions regarding the design and processing of data from large seismic arrays. The conventional and high-resolution estimation of the frequency-wavenumber spectrum of the background microseismic noise is also presented. The diffuse structure of this spectrum is shown to aid in explaining the relative performance of array processing methods.

Apparatus for improving signal-to-noise ratio in logging-while-drilling system

Abstract: The specification discloses a method and apparatus for substantially reducing the uphole noise in a logging-while-drilling system wherein a signal representative of a downhole parameter is generated down a well and is transmitted to the surface in the form of an acoustical wave in the drilling fluid, e.g., mud. Two spaced transducers measure the acoustical pressure at two points in the mudline between the pumps and the well and convert these pressures to corresponding signals. One of these signals is time shifted an amount equal to the travel time of sound in the mud between the two transducers and, after one of these signals has had its polarity reversed, the two signals are added to reduce the uphole noise substantially. By filtering one of the pressure measurement signals with a filter having characteristics related to the distortion of the flow path between the two spaced transducers, noise is further reduced. The combined signals are further filtered with a Wiener type filter which best recovers the signal.

Sensory function in multimodal signal detection.

Abstract: Five observers detected a sinusoid in noise in a two‐interval forced‐choice experiment. The signal could occur on an earphone, on an oscilloscope, or on both devices simultaneously. Detection performance was studied as related to (1) mode of occurrence of the signal(s); (2) the external noise correlation in the auditory and visual channels; and (3) the observers' a priori knowledge of the mode of occurrence of the signal. The observed improvement in sensitivity (measured in d′ units) as a function of bimodal signal presentation closely followed the predictions of a statistical summation model and was much lower than predicted by linear and probabilistic addition models. Under conditions of independence of noise in the auditory and visual channels, some improvements in sensitivity were of almost 3 dB. The improvement in sensitivity afforded by a priori knowledge of the mode of occurrence of the signal was less for bimodal signals than for unimodal signals.

Wide-band optical communication systems: Part I—Time division multiplexing

Abstract: The purpose of this paper is to consider the theoretical error performance of several digital formats suitable for use with a mode-locked laser source, to discuss the problems of implementing these formats, and to consider the design of an illustrative system. Several techniques for time multiplexing are also described. It is assumed that the transmission medium introduces only additive noise and that the detection process is signal shot noise limited. Among the formats examined the practical choices which can achieve highest information rates with lowest probabilities of error are binary, differential phase shift keying, and binary pulse position modulation. For purposes of illustrating state-of-the-art performance the design of a 4000-mile repeatered binary pulse code modulation intensity modulated system is outlined.

Operating characteristics and a priori probability of the signal

Abstract: Two experiments in auditory signal detection produced changes in the operating characteristic as the a priori probability of signal occurrence, p(SN), was varied. The signal was a sinusoid of 1,000 Hz presented for 250 msec against a continuous background of noise. In Experiment 1 three values of p(SN)—0.25, 0.50, and 0.75—were paired with each of three signal intensities. In Experiment 2 the signal intensity was fixed and p(SN) was assigned values of 0.10, 0.25, 0.50, 0.75, and 0.90. On normal-normal coordinates, operating characteristics were fitted to the points obtained from the 4-point rating scale used by the listeners. Such operating characteristics may be specified by two parameters: ds, an index of detectability related to d’, and m, its slope. While ds was found to be independent ofp(SN), m was found to be a joint function of p(SN) and signal intensity. These results are discussed against the background of the theory of signal detectability.

Acoustic presence detection method and apparatus

Abstract: A method and means for passively detecting presence of noise radiating dees in a noise environment by determining the number of times per time sample that a received noise signal upwardly crosses a preselected threshold level, comparing this number with the number of upward crossings caused by a standard noise signal or background noise signal, and applying a predetermined ratio criteria. The noise sensors in the detection system may comprise a directional acoustic transducer in combination with an omnidirectional acoustic transducer, and acoustic transducer in combination with an artificially generated noise signal, or a single acoustic transducer.

Automatic transversal equalizer system

Abstract: The invention is directed to a system for automatically adjusting the multiple gain taps in a transversal equalizer so as to minimize the errors in a received signal caused by intersymbol interference and noise. In a preferred embodiment, the sign of a delayed replica of the sampled input signal is multiplied by the sign of a derived error signal to form an incremental signal, which signal is used to increment a selected gain tap in the transversal equalizer by a fixed amount and in a direction determined by the sign of the incremental signal. This type of procedure is applied to each of the tap-gains of the transversal equalizer to drive the kth tap-gain, for example, to the value that minimizes the cross-correlation function. WHERE Rho J IS THE ERROR IN THE JTH SAMPLE OF THE SYSTEM''S PULSE RESPONSE AS SEEN AT THE EQUALIZER OUTPUT AND HJ K IS THE (J-K)TH SAMPLE OF THE SYSTEM''S PULSE RESPONSE AS SEEN AT THE EQUALIZER''S INPUT. The derived error signal is formed by comparing the actual system''s output signal with a computed desired signal, with the error signal being the difference between the two.

Electroacoustic Characteristics of Marine Seismic Streamers

Abstract: To specify intelligently an unambiguous tow‐noise level in marine seismic exploration, the electroacoustic characteristics of the streamer must be understood. In this paper, these characteristics are examined as a basis for an industry‐wide standard for specifying streamer tow‐noise level. Sources of tow noise—including electrical, ambient, flow, radiated, and mechanically induced—are examined and the important parameters that control their amplitude spectrum are presented. The theoretical bases for various means of reducing the components of tow noise are analyzed and compared with experimental results. With this background of tow‐noise sources and noise‐reduction schemes, the noise signal is traced from the hydrophone/seawater interface to the recording system. The hydrophone array’s amplitude response and phase response are determined from the transfer characteristics of the coupling circuit and the recording system. Finally, a method for specifying tow‐noise level is proposed. The specified tow‐noise ...

Determination of signal and noise statistics using correlation theory

Abstract: A seismic trace may be represented as the sum of a signal and noise series. Each of the series may further be represented by convolution of a finite wavelet and a random series. With this representation, and provided that the signal and noise are uncorrelated, it is possible, in theory, to extract signal and noise statistics from two adjacent traces of a reflection seismogram. Some experiments are shown on model seismic traces, and it is shown that within the time‐duration of the seismic wavelet, the estimates of signal and noise statistics are reasonable for low signal‐to‐noise ratio. There remains, however, the problem of determining the optimum time lengths of the estimates.

Optimization of the Performance of a Digital-Data-Transition Tracking Loop

Abstract: The steady-state behavior of a data-transition tracking loop, used as a bit synchronizer in a phase-coherent receiver, is considered. Optimization of mean-square phase noise and mean time to first cycle slip is performed when the average power of the reference cross-correlating signal is constrained. It is shown that by adjusting the quadrature channel gain along with the integration interval, a significant improvement in phase noise and cycle slip performances can be achieved over that system which integrates in the quadrature channel over the full symbol period. All the results are derived for a first-order loop filter merely to indicate the approach to the problem and the relative value of optimizing the system.

Optimum sampling and smoothing conditions for digitally recorded spectra

Abstract: Optimum sampling and smoothing conditions for digital recording of spectral lines are derived for a system consisting of a spectrometer with CR-filter output, a sampling instrument for measurement of the spectrometer output and a processor applying a quadratic smoothing polynomial on to the sampled signal. Relevant experimental parameters for smoothing of the noise process, for the integral square error and for the signal-to-noise ratio will be discussed, and a set of practical rules to obtain efficient signal enhancement is given.

The capacity of linear channels with additive Gaussian noise

Abstract: The standard method of computing the mutual information between two stochastic processes with finite energy replaces the processes with their Fourier coefficients. This procedure is mathematically justified here for random signals w,(ω) square-integrable in the product space t × ω where t ∊ [O, T] and w is an element of a probability space. A natural notion of the sigma field generated by w, (ω) is presented and it is shown to coincide with the sigma field generated by the random Fourier coefficients of w,(ω) in any complete orthonormal system in L 2 [O, T]. This justifies the use of Fourier coefficients in mutual information computations. Capacity is calculated for finite and infinite-dimensional channels, where the output signal consists of a filter (general Hilbert-Schmidt operator) operating on the input signal with additive Gaussian noise. The finite-dimensional optimal signal is obtained. In the infinite-dimensional case capacity can be approached arbitrarily closely with finite-dimensional inputs. The question of the existence of an infinite-dimensional signal which achieves capacity is considered. There are channels for which no signal achieves capacity. Some results are obtained when the noise coordinates are independent in the eigensystem of the filter.

Sampling and Synchronous Detection for Studying Fast Low Intensity Light Signals

Abstract: Superimposed modulation and lock‐in techniques have been successfully applied in order to minimize statistical fluctuations and to eliminate the low frequency internally generated noise in a sampling apparatus for fast, repetitive light signals detection. Some improvements are suggested here for applying the described technique to the study of ir fast light pulses, by means of electro‐optical sampling.

Detection and estimation of signals in noise when one or both are non-Gaussian

Abstract: An examination is made of the basic principles and results of the theory of detection and estimation of signals in noise, which is not limited to the condition that the useful signal and noise be Gaussian and that the noise be additive. Formulas are obtained [(23) and (25)] for likelihood ratios which are useful in the Markovian as well as in the non-Markovian case. The results are specialized for the case of diffusion noise and fixed but unknown signal parameters, when it is possible to effectively utilize the theory of conditional Markov processes. Estimation by the quasi-linear theory is also discussed, the applicability of which is limited not by the Markovian condition, but by the condition of high a posteriori accuracy. In conclusion, a generalization is given of the theory for the case of adaptive detection and estimation, when the a priori information is replaced by learning. In this case, application of the theory of conditional Markov processes makes it possible to obtain, besides the previous equations of Gaussian approximation, similar equations for the unknown parameters.

Method and apparatus for detecting speech signals in the presence of noise

Abstract: A METHOD AND APPARATUS FOR DETECTING THE INSTANTANEOUS PEAK VALUES SOF A PCM CODED VOICE SIGNAL ABOVE A THRESHOLD LEVEL AND ENERGIZING A TRANSMITTER CARRIER IN RESPONSE THERETO, THEREBY CONSERVING CARRIER POWER DURING THE PERIODS WHEN NO VOICE SIGNAL IS PRESENT. THE THRESHOLD IS ESTABLISHED AT A LEVEL WHERE THE PROBABILITY OF THE INSTANTANEOUS VALUE OF A SPEECH SIGNAL EXCEEDING ITS RMS VALUE IS MUCH GREATER THAN THE PROBABILITY OF THE INSTANTANEOUS VALUE OF A WHITE GAUSSIAN NOISE SIGNAL EXCEEDING ITS RMS VALUE FOR EQUAL OF POWER. THE CIRCUIT MAINTAINS CARRIER POWER FOR A VARIABLE DELAY OR DEFERRED HANGOVER PERIOD AFTER EACH THRESHOLD VOICE DETECTION. THE HANGOVER PERIOD VARIES FROM A PREDETERMINED MINIMUM TIME DELAY TOA MAXIMUM TIME DELAY WHICH EQUALS THE LENGTH OF THE VOICE BURST NOT WXCEEDING 150 MILLISECONDS. ALTERNATELY, BECAUSE OF THE PARTICULAR CHARACTERISTICS OF SPEECH WAVEFORMS, THE THRESHOLD VOICE DETECTIONS MAY BE CONVENIENTLY COUNTED AND THE CARRIER TRANSMITTER ENERGIZED WHENEVER THE COUNT EXCEEDS A PREDETERMINED VALUE, WHICH FURTHER REDUCE THE MRAGIN OF NOISE TRIGGERING ERROR, IN ADDITION, THE DEFERRED, VARIABLE HANGOVER PERIOD MAY BE REPLACED BY A FIXED DELAY TO FURTHER SIMPLIFY THE CIRCUITRY REQUIRED.

Magnetic memory disk drive apparatus with reduced r. f. noise

Abstract: A magnetic memory disk drive apparatus with reduced R.F. noise includes a d.c. drive motor. The disk is coupled to the motor shaft by a drive plate and a drive plate hub which have surfaces interleaved with a grounded housing to provide a shunt capacitance which reduces the noise signal produced by commutation reaching the magnetic disk.

Passive object detection apparatus including ambient noise compensation

Abstract: An ambient noise compensating arrangement for use in a passive ultrasonic vehicle detection apparatus carried by a vehicle for compensating for ambient background noise present during the detection of vehicles located in predetermined regions of surveillance rearward of the subject vehicle. Prior to the detection of the presence of a vehicle in one of the regions of surveillance, the prevailing ambient background noise is sensed by a signal-receiving horn member mounted on the subject vehicle and processed by an input signal arrangement coupled to the horn member to produce a noise voltage signal representative of the ambient background noise. The noise voltage signal is applied to a field effect transistor the resistance of which is increased in response to the noise voltage signal by an amount sufficient to essentially fully attenuate the noise voltage signal.

Circuit arrangement for reduction of high frequency noise disturbances in wide bandsignals such as video signals

Abstract: System for reducing noise in video signals by subtracting coherent noise modulation from useful signal. The high frequency component of the video signal is separated into parts above and below a given amplitude. The low-amplitude part corresponds to the noise level and is subtracted from the wide-band signal. The high-amplitude part is restored for compensation.

High voltage functional comparator

Abstract: A comparator circuit has an hysteresis relationship between its two-level output signal and the input signal. As a result, the level of the output signal is substantially independent of noise on the input signal. By changing the value of a resistor the amount of hysteresis in the circuit is changed.

Control system filtering technique

Abstract: A control system filtering technique permits heavy noise filtering of a position input signal without introduction of system instabilities. A rate of change of the filtered position signal is derived and utilized in a feedback loop to complement the signal from which the rate signal was derived, permitting heavy position signal filtering without position loop phase lag and thus effectively separating the problem of generating a tight control from the sensor noise problem.

Probing linear filters-- Signal design for the detection problem

Abstract: The problem of designing an optimum probing signal for discriminating between two known linear filters in the presence of noise is considered. It is shown that in the general case of colored noise, the optimum signal is the solution of two simultaneous integral equations. An expression for the probability of error is given, and some illustrative examples are presented.