Showing papers on "Noise (signal processing) published in 1969"

Optimal recursive estimation with uncertain observation

Abstract: In classical estimation theory, the observation is always assumed to contain the signal to be estimated. In practice, certain observations, or sequences of observations, may contain noise alone, only the probability of occurrence of such cases being available to the estimator. An example is trajectory tracking where the signal is first detected and then the estimator is allowed to process it for tracking purposes. However, any detection decision is associated with a false-alarm probability, which is the probability that the detected signal contains only noise. Minimum mean-square estimators are derived for two different forms of this problem; 1) when it is possible that the observation at any sample time contains signal or is noise alone, independent of the situation at any other sample, and 2) when the entire sequence of observations contains signal or is only noise. The estimators derived are of recursive form. A simple example is given for illustration.

Conjugate linear filtering

Abstract: Aspects of optimum filtering for complex valued random processes are presented. Ordinary linear filters are complemented with conjugate linear filters. It is found that the incorporation of conjugate linear filtering improves signal-to-noise ratio by a factor of two in matched filter receivers. For optimum least squares filtering the inclusion of conjugate processing reduces mean-square error by a factor as great as two; the improvement depends primarily on the degree of correlation between the real and imaginary parts of the signal process. The analysis utilizes special correlation properties of receiver noise. Also, in the absence of phase lock, conjugate linear processing offers no improvement. Finally, it is observed that in the Gaussian case the least squares nonlinear receiver for modulations consists of the derived linear-conjugate linear receiver followed by demodulators comparable to those used in practice.

Representation and Analysis of Time‐Limited Signals Using a Complex Exponential Algorithm

Abstract: A complex exponential algorithm developed for the representation and analysis of time‐limited signals is defined, and its evaluation with respect to conventional discrete Fourier techniques is discussed. It is shown that for a given length of a signal containing discrete frequency information, sampled at least to the Nyquist criterion, the complex exponential algorithm can often provide increased frequency resolution over standard Fourier techniques. It is also shown that the complex exponential algorithm provides an improved mechanism over Fourier techniques for interpolation between points in a sampled signal containing discrete frequency components in the presence of broad‐band noise. The effects of noise in the complex exponential technique and the computational difficulties associated with the present complex exponential algorithm are also discussed.

Signal Processing for a Cocktail Party Effect

Abstract: A binaural listener has the ability to concentrate on speech from a particular location while suppressing speech from other locations (binaural cocktail party effect). Similarly, an array of microphones can be used to achieve an enhancement of intelligibility coming from a particular (on‐center) location. This paper discusses various processing schemes for the microphone outputs in order to achieve this discrimination. In particular, one nonlinear process, studied using an analog system employing subtraction, full‐wave rectification, and addition, results in reproduction of the on‐center speech without change, and in distortion and reduced level of an off‐center speech source. Because of the distortion, the suppression appears to be greater than for linear processing. In addition, this process eliminates completely any off‐center impulsive noise which is nonoverlapping at the four microphones. It has been pointed out that this process is equivalent to an instantaneous selection of the two microphone outpu...

Digital fsk/psk detector

Abstract: The digital FSK/PSK detector demodulates digital data from a frequency shift keyed modulation signal (FSK) or a phase shift keyed modulation signal (PSK) for moderate data rates including at least 1200 bits per second and is capable of operating over switching networks. The digital FSK/PSK detector comprises a binary amplitude quantizer, a clock, a digital time quantizer, a digital delay coacting with an Exclusive-OR circuit for detecting the digital data signal from the FSK/PSK modulated signal and a digital filter and smoothing circuit for eliminating undesirable noise from the digital data signal.

Envelope versus Microstructure in the Fusion of Dichotic Signals

Abstract: An attempt was made to assess the effect of an imposed amplitude envelope on the binaural fusion of dichotic tone signals and noise signals. The envelope was imposed by an electronic switch at three rise/decay rates—10 msec, 25 msec, and 50 msec. The duration of the signals was such that there was no constant amplitude portion for any signal. The tonal signals covered a range from 250 to 2000 Hz. The noise was either high passed or low passed at 1000 Hz. Only the imposed envelope was delayed at one ear—not the microstructure of the signal. For tonal signals, the maximum envelope delay time for which the signal still yields a single stationary image is definitely a function of the envelope slope, being approximately 5–7 msec for the 10‐msec slope; 10–12 msec for the 25‐msec slope; and 15–20 msec for the 50‐msec slope. For the two lesser slopes, there is also an envelope delay range for which the image moves across the head from leading ear to lagging ear. For the signals with noise microstructure, behavior of the image is more complex than for tones, but a number of tentative principles emerge, most of them amenable to further testing: (1) The position of the signal is predominantly determined by the interaural envelope delay. (2) Correlated microstructure (noise from the same noise generator) definitely increases the likelihood that a signal with large interaural envelope delay will fuse into a single image, even though the delay and the microstructure should yield different positions for the image. (3) Signals presented at higher sensation level tend to split into two at smaller envelope delays when microstructure information indicates the signals are different. (4) For the type of signal used here, maximum single‐image envelope delays are greater for low‐frequency microstructure than for high‐frequency microstructure. For these signals, singleness of image and position of image appear to be analyzed separately, the information being combined later into a single perceptual impression.

Identification of Mechanical Sources of Noise in a Diesel Engine: Sound Originating from Piston Slap:

Abstract: The sound produced by the piston slap of a diesel engine is investigated by standard methods of sound and vibration analysis augmented by correlation of incidence of individual transient components. This latter technique makes use of information obtained from the tuned outputs of the noise signal, focusing attention on the incidence of single-frequency transients.The contribution of piston slap to the overall noise emitted by an engine is studied analytically by examining equations for the transverse motion of the piston and experimentally by varying engine parameters such as piston-cylinder clearance, gudgeon-pin offset, and piston lubrication.By considering the various distinct characteristics, an insight is gained into the mechanism of noise generation and radiation.

Energy Detection of a Random Process in Colored Gaussian Noise

Abstract: A likelihood receiver for a Gaussian random signal process in colored Gaussian noise is realized with a quadratic form of a finite-duration sample of the input process. Such a receiver may be called a "filtered energy detector." The output statistic is compared with a threshold and if the threshold is exceeded, a signal is said to be present. False alarm and detection probabilities may be estimated if tabulated distributions can be fitted to the actual distributions of the test statistic which are unknown. Gamma distributions were fitted to the conditional probability densities of the output statistic by equating means and variances, formulas for which are derived assuming a large observation interval. A numerical example is given for the case in which the noise and signal processes have spectral densities of the same shape or are flat. The optimum filter turns out to be a band-limited noise whitener. The factors governing false alarm and detection probabilities are the filter bandwidth, the sample duration, and the signal level compared to the noise. Two sets of receiver operating characteristic curves are presented to complete the example.

Signal transmission method and system

Abstract: A signal transmission system enabling exchange of the information content of message signals with substantial reduction of communication channel noise effects thereof. Message signals are converted into other signals actually transmitted. Received signals are reconverted to provide said message signals, the conversion and reconversion being of such nature as to effect substantially uniform distribution of channel noise in message signals derived from said received signals.

Optimum Arrays and the Schwartz Inequality

Abstract: Optimum processors for detection of known and random signals in noise using an array of sensors are derived. It is shown how the Schwartz inequality may be used to solve the problem of maximizing the detection index in both cases. This approach provides both the optimum filter functions and the performance of the optimum processor simply and directly. Narrow‐band signal assumptions are avoided.

Adaptive signal detection system

Abstract: Apparatus for detecting information signal level variations in the presence of noise or other background level variation phenomena, includes an array of sensors or transducers to convert energy to be detected to a form suitable for processing, such as representative electrical signals. The signals are sequentially sampled according to a predetermined scanning sequence of the sensor array, and are processed in parallel channels on a synchronized element-by-element basis to derive therefrom amplitude representations of level variations occurring over maximum and minimum time intervals related to intervals over which information signal level variations can be expected to occur. These amplitude representations are then compared as an indication of whether they represent actual information signal or simply noise.

Estimation of signal-to-noise ratios

Abstract: Statistical method estimates signal-to-noise ratios in an observed random voltage, such as the output of a telemetry receiver. Signals from a distant transmitting source, overlaid by noise signals, are monitored continuously.

Induction radio transmission system

Abstract: An induction radio transmission system for vehicles wherein three spaced conductors are installed in parallel along the vehicle path with a generator feeding an in-phase signal electric current to the outer of said three conductors and the signal electric current in opposite phase to the remaining center conductor. Two antenna are mounted aboard the vehicle and are respectively and cooperatively coupled with the magnetic fields of opposite phase generated by the energized conductors and are serially connected to additively combine the signal received from each antenna and cancel unwanted noise. The system may be reversed such that the antennas are energized to induce signal current flow in the conductors.

Audio signal processor

Abstract: The output signal from a microphone is fed through a preamplifier to a variable attenuator and then to an automatic volume control (AVC) circuit. A voice operated switch is connected between the output of the microphone preamp and the attenuator and activates the latter when the output signal from the preamp falls below a predetermined minimum amplitude to introduce signal attenuation at the same rate as gain is increased in the automatic volume control circuit to thereby eliminate induced noise surges during periods of silence or low amplitude input into the microphone. The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

Influence of Lower Cutoff Frequency on the Measured Variance of 1/f Noise

Abstract: Previous experimental comparisons of 1/f and Nyquist noise in carbon resistors reveal that both are normally distributed in any time interval but that 1/f noise appears to have a nonstationary variance. The present paper examines the possibility that this fluctuation is caused by insufficient sampling of the 1/f noise signal. It is hypothesized that the 1/f noise is wide‐sense stationary and the number of independent samples per measurement is found and shown to be far less than that for Nyquist noise. An experiment is then described in which the number of samples is sufficiently large to eliminate this source of variance fluctuation. The results exhibit the same spread in measured 1/f noise variance reported earlier, suggesting that an intrinsic nonstationarity does exist in this noise process when produced in carbon resistors.

Monaural detection with contralateral cue (MDCC). I. Better than energy detector performance by human observers.

Abstract: Human observers were given a monaural detection task. Information in the form of a “cue” derived in some manner from the signal was presented simultaneously to the other ear. The signal and the cue always had the same timing and spectral characteristics and were samples of low‐pass filtered noise. The cue was always presented in each possible signal interval, whether or not the signal itself was presented. Different cue conditions were used, in which the cue bore different amounts of information relevant to the signal. When the cue and signal were independently generated but identically timed and filtered, performance was indistinguishable from the no‐cue control condition, and was like that of an inefficient energy detector using a filter matched to the signal bandwidth. When the cue was an identical replica of the signal, performance was much better than that of an ideal energy detector and, for bandwidths less than 1600 Hz, approached that of an equally inefficient likelihood‐ratio detector for signal known exactly. Efficiency in both cases was of the order of 25%–30%. When the cue was derived from the signal but passed through a wide‐band 90° phase shifter, it provided an intermediate amount of information, and detection performance was also at an intermediate level. The results are taken to show that when relevant information is available to the detection mechanism, it can be used; and that the usual finding of energy‐detector like monaural performance is due to lack of more precise information about the signal at the detector.

Signal detection apparatus

Abstract: A receiver for use in a system in which randomly varying targetindicating signals from a transducer are detected in the presence of noise signals which may vary in amplitude between wide limits. Advantage is taken of the fact that the spectrum of frequencies of randomly varying target-indicating signals differs from the spectrum of frequencies of noise signals. Therefore, by appropriately multiplexing and filtering the signals from a transducer, separate signals proportional, respectively, to the root mean square value of noise signals and target-indicating signals may be derived and compared to produce an output signal which changes from one level to another when target-indicating signals are present.

Monopulse radar apparatus

Abstract: A monopulse radar apparatus for the automatic tracking of a moving target comprising a receiver for the coherent detection and filtering of received echo signals without undesired noise signals and undesired signals that are caused by signal and noise components falling within the passband of the filter due to the internal clutter movement of fixed targets. The receiver comprises a signal generator that provides only desired narrowband signals to track the target when the frequency and phase of the signal generator matches that of the detected doppler signal, thereby resulting in optimum subclutter tracking.

Pulse-operated receiver

Abstract: Disclosed is a receiver for a navigation system of the Omega type which operates with pulse-modulated signals. The phase of received and local signals is compared by quantized pulses; phase equalization, commutating gate control, phase tracking, and synchronization of gate patterns with timed-sequence input signals are obtained by pulse insertion into, or deletion from, signal loops which contain phase detecting, sequential signal selecting, and readout control components. Phase coincidence is counted cumulatively and is electromechanically stored. Lane position is recorded by pulse insertion. Components are constructed and interrelated to reduce noise and improve selectivity to enhance the benefits obtained by pulsed operation control.

Method and apparatus for ensemble averaging repetitive signals

Abstract: A spectrometer or similar device is scanned repetitively to produce a series of repetitive output signals either of analog or digital form. Each output signal includes an ensemble of timedisplaced components and is identical to the other output signals except for noise. The series of output signals is ensemble averaged (time averaged) by an ensambled-averaging digital computer which scans each output signal of the series and samples each output signal at a plurality of sampling points at the same relative position in each output ensemble. Digital data for each sampling point is accumulated in a separate channel of the memory to improve the signal-to-noise ratio. An extra bit is added into the sampled data for each sampling point, such added bit being less than the least significant bit to be stored in the memory. The accumulation of added bits of each sampling point, over a series of scans, adds to zero to some number which is the same for each sampling point, whereby the digitization error is reduced.

Binaural Masking with Sine‐Wave Maskers

Abstract: The detectability of one binaural tone, masked by another of the same frequency, was measured as a function of the phase difference between them. In one condition, the tone to be detected, the signal, was the same in both ears. In another condition, the signal was inverted at one ear causing a 180° interaural phase difference in the signal. The masking tone was always the same in both ears. It was presented continuously in one experiment, and was gated on and off with the signal in another experiment. In contrast with similar work in which a noise masker was used, binaural signals inverted at one ear are less detectable in some conditions than binaural signals that are the same in both ears: a negative MLD. The negative MLD was largest (about 10 dB) in gated masking conditions and was found to be dependent on signal‐masker duration, growing smaller with longer duration. The detectability of interaurally in‐phase signals as a function of the phase difference between signal and masker can be accounted for by a simple energy‐detection model. However, no existing theory can adequately explain the results obtained with out of‐phase signals, although the results suggest the operation of a system with some of the properties of Durlach's equalization‐cancellation mechanism.

Robust detection using extreme-value theory

Abstract: The use of extreme-value theory (EVT) in the detection of a binary signal in additive, but statistically unknown, noise is considered. It is shown that the optimum threshold and the probability of error of the system can be accurately estimated by using EVT to obtain properties of the initial probability density functions on their "tails." Both constant signals and slowly fading signals are considered. In the case of a fading signal, the detector becomes adaptive. Detection of the constant signal, both with and without an initial learning period, is studied by computer simulation.

Doppler radar automatic signal detection apparatus

Abstract: The electrical output of a noncoherent doppler radar is applied to a noise AGC system which varies the channel gain so as to maintain substantially constant the level of random noise, thereby counteracting the noise-level distorting effects of AGC in the radar I.F. amplifier. Variations in the radar output due to changes in strength or spread of clutter signals are reduced by a clutter control circuit including a self-adapting filter which automatically changes its lower-frequency cutoff skirt so as to increase the attenuation of clutter signals when the energy of the interfering clutter-signal components tends to increase. After passing through the noise AGC circuit and clutter control circuit in series, the energy of the resultant signal varies substantially only due to the presence or absence of desired moving-target-produced received signals, and is applied to an energy-responsive threshold circuit to produce automatically an output signal indicative of moving target presence or absence. Either the noise AGC circuit or the clutter control circuit can be used by itself without the other.

Optical Processing of Underground Sound Waves in Seismic Exploration for Oil

Abstract: When coherent light from a laser beam is passed through a transparency on which pictorial information is impressed, the information itself acts as a complex grating to produce a Fraunhofer diffraction pattern which is the two-dimensional Fourier transform, or power spectrum, of the information in the picture. By properly obstructing the light in the transform, one can filter out undesired spatial frequencies or directions of inclinations in the original picture. The most widespread commercial application of this principle has been to the analysis and filtering of seismic reflection records made in exploration for oil. Removal of undesired noise from seismic recordings can often be accomplished more economically by optical filtering than by more conventional digital and electrical analog techniques. Examples are demonstrated where optical filters have substantially improved the quality of seismic data. Use of optical transforms for analysis and solution of geological problems is also illustrated. The most unique advantages of optical processing are in economy, flexibility, and convenience of monitoring. Principal disadvantages are limited dynamic range, and difficulty in constructing filters that behave as gradational rather than step functions.

System for measuring nonlinearity of a signal-transmitting network

Abstract: The output of a random-noise generator is modulated with the aid of two fixed reference frequencies G1, G2 (one of which may be zero) to yield two mutually transposed but correlated noise bands overlapping in a frequency range which substantially coincides with a band of signal frequencies to be transmitted through a test circuit to be examined for nonlinearity. With the sum or difference of any two correlated signal frequencies fi'', fi'''' in the two noise bands equal to a fixed beat frequency F G2 + OR G1, the amplitude of a signal of this beat frequency in the output of the test circuit receiving the two wholly or partly coincident noise bands is a measure of the nonlinearity of that test circuit throughout the signal band. A broadband of signal frequencies may be substituted for the noise frequencies if the system is to be tested in actual operation.

Detection system for control information

Abstract: Disclosed herein is a detection system for control information transmitted from the sending side to the receiving side in a broadcast system, in which the control information is detected in consideration of the following two conditions: (1) if a control signal representative of the control information is intermitted by the control at the sending side or by noise, the control signal is deemed continuous in case of the intermitted time of the control signal is less than a predetermined first time (t); and (2) if a signal deemed continuous exceeds a predetermined second time (T) longer than the first time (t), this signal is decided as the control information.

Speech-noise discriminating constant pulse width squelch

Abstract: A squelch circuit for a receiver wherein the detected signals at the discriminator are amplified and converted to pulses of constant period or width. The pulses are detected and used to develop a control voltage to actuate an audio or transmitter switch. A decrease in the noise signal due to the presence of a proper radio frequency signal will decrease the pulse rate, increase the control voltage and actuate the switches allowing the audio signal to pass through the audio stages of the receiver and activating an associated transmitter.

Measurement of the Velocity and Attenuation of Ultrasonic Pulses in the Presence of Noise

Abstract: The well developed techniques of coherent detection and signal averaging may be used to extend the range and accuracy of measurements of ultrasonic velocity and attenuation. Significant improvements in signal‐to‐noise ratio are obtained by averaging a coherently detected echo train in a signal averager such as a multichannel analyzer or boxcar integrator. A standard circuit of this type is described and analyzed. Comparison of coherent detection and rectification shows the inherent advantages of coherent detection for signal‐to‐noise enhancement.