Abstract: A system and method are provided to reduce a structure's far field acoustic adiation signature. A plurality of acoustic sensors are positioned in the acoustic near field of the structure for measuring the near field acoustic radiation of the structure. A programmable controller generates a prediction of the structure's far field acoustic radiation signature using the near field acoustic radiation. The prediction is then used to generate a noise control signal applied to a plurality of acoustic radiators suspended within the structure. The radiators convert substantially all of the noise control signal to airborne acoustic energy within the structure.

Abstract: A multi-dimensional acoustic, data processing and display system arranges acoustic data in a three-dimensional matrix. The three-dimensional matrix is compressed using singular value decomposition into singular vectors and singular values. A historical database is created and maintained and is also concatenated with the three-dimensional data. This database allows reverberation and noise to be diminished and other, weaker features in the data to be enhanced. Once the data is compressed, the data can be analyzed efficiently. The singular vectors are partitioned into one or more groups on the basis of their singular values or other criteria. Certain of the compressed data elements are enhanced or diminished by modifying the singular values within each of the groups of singular vectors. Selected singular vectors are processed further by other techniques for further enhancement, detection, isolation, feature extraction and classification.

Abstract: A device is provided for reducing the number of ECG data sample points presented to a video display system to avoid overwhelming the video display screen with a number of ECG sample points and to present several seconds of data on the screen at the same time. The device ensures that the maximum and minimum values of the original ECG signal are displayed. A collection of consecutive points is broken into sub-groups which are each processed to produce representative amplitude values for each sub-group which representative values are then displayed on the display screen.In the preferred embodiment, the device includes a microprocessor with electronic memory which calculates a representative amplitude value for each sub-group. The representative amplitude value is chosen depending on whether the sub-group contains a local maximum or a minimum in the values of consecutively sampled points or whether the data points of the sub-group are part of a curve leading to a local maximum or minimum.

Abstract: Apparatus for providing a direct quantitative readout of the amplitudes of arious frequency lines that constitute the envelope of an amplitude-modulated broadband noise signal. A selected band of carrier frequencies is fed to an AGC circuit that provides a constant-input signal amplitude to a demodulator for a period of time that is greater than the period of the lowest frequency of the modulating signal for detection. The detected signal is fed to a spectrum analyzer for the direct reading of the percent modulation.

Abstract: A normalizer based on a Least Mean Square (LMS) adaptive algorithm configured to provide effective normalization when the background noise is locally non-stationary and when the target may be subject to time spread of unknown extent. The LMS algorithm used in the normalizer includes an adaptive filter in both the primary and reference inputs as a means of adapting to variations in both the signal and noise statistics. The LMS algorithm is implemented on the logarithm of the data, so that the difference minimized in the LMS structure drives the ratio of the signal power to noise power to a constant value. The algorithm can be used as a range normalizer by running it over range in each doppler bin, or as a frequency normalizer by operating across doppler in each range bin. By continually adapting to the statistics present in the data, the normalizer more effectively deals with the variations in the noise and signal statistics.

Abstract: An acoustic sensor for use in a typical atmospheric condition, which contains both winds and turbulence, such as a wind and turbulence encountered on the exterior surface of a moving airborne flight vehicle includes a probe housing having a streamlined shape and a set of indentations in the exterior surface thereof extending inwardly located at a particular longitudinal location, radial airflow passages nested in respective ones of the concave indentations, the passages merging at a central manifold of the passages, wherein the particular longitudinal location is such as to minimize noise attributable to fluctuations in the wind in a longitudinal direction, and wherein the concave indentations have indentation depths such as to minimize noise attributable to wind transverse to the probe.

Abstract: In a preferred embodiment, an accelerometer sensor system, including: a housing structure; a first acceleration sensor disposed in the housing structure at a first selected location along the vertical axis of the housing structure, the first acceleration sensor providing a first output; a second acceleration sensor disposed in the housing structure at a second selected location along the vertical axis, the second acceleration sensor providing a second output; and apparatus to combine the first and second outputs of the first and second acceleration sensors such as to simulate the output from a virtual acceleration sensor located at any selected location on the vertical axis, allowing one to reduce noise induced by pitch motion, at any desired frequency or across a desired frequency band.

Abstract: An apparatus and method for clutter elimination from signals which digitizes a signal to generate n samples of the signals. M basis functions (such as various levels of filters) are fit to the n samples. An i+1 basis function of the m basis functions is iteratively subtracted from each of the n samples, and the difference is stored in an i difference signal until the i+1 difference signal is less than a threshold. In various embodiments, the threshold may be an absolute noise floor (R.sub.O.sup.min) preset by a manufacturer or a user. In other embodiments, the threshold may be based upon an absolute or relative predictor error of the signal once the basis function (filter) has been removed. The i difference signal may be used for performing a frequency estimate of the signal.

Abstract: In a preferred embodiment, an accelerometer sensor system, having a first axis of rotation at a first given frequency of periodic oscillation thereabout, which accelerometer sensor includes: a first acceleration sensor; a platform to which the first acceleration sensor is fixedly attached; and the first acceleration sensor being vertically displaced from the first axis of rotation by a first selected distance such that the noise received by the first acceleration sensor, due to the periodic oscillation at the first frequency, is reduced.

Abstract: A side-looking fish finding apparatus transmits a sonar pulse approximately horizontally into a body of water, and then monitors echoes from the pulse. The time interval between successive sonar pulses is varied in a random manner in order to permit two or more similar units to be used in close proximity to each other without significantly interfering with each other. An individual echo is treated as the detection of a fish if it has a magnitude above a predetermined threshold, if it has a length within a predetermined range bounded by upper and lower limits, if it is preceded by a leading space interval free of other echoes and having a predetermined duration, if it is followed by a trailing space interval free of other echoes and having a predetermined duration, the duration of the trailing space interval being less than the duration of the leading space interval, and if an equivalent echo is received in response to each of the next four successive sonar pulses.

Abstract: For suppressing reflections of an acoustic reception signal at a transmitting/receiving ultrasound transducer, the ultrasound transducer is connected to a compensation circuit which supplies the ultrasound transducer with a compensation transmission signal during reception, which has a waveform similar to the acoustic reception signal, but with a polarity opposite that of the acoustic reception signal. A summing stage is connected in a reception channel of the ultrasound transducer, the summing unit having a first input supplied with a compensated reception signal, formed by a combination of the acoustic reception signal and the compensation transmission signal. A second input of the summing unit is connected to the compensation circuit, which supplies a signal at the second input which removes the contribution made by the compensation transmission signal from the compensated reception signal, so that the output of the summing unit generates a reception signal devoided of the compensation transmission signal.

Abstract: A multi-dimensional acoustic data processing and display system arranges acoustic data in a three-dimensional matrix. The three-dimensional matrix is compressed using singular value decomposition into singular vectors and singular values. A historical database is created and maintained and is also concatenated with the three-dimensional data. This database allows reverberation and noise to be diminished and other, weaker features in the data to be enhanced. Once the data is compressed, the data can be analyzed efficiently. The singular vectors are partitioned into one or more groups on the basis of their singular values or other criteria. Certain of the compressed data elements are enhanced or diminished by modifying the singular values within each of the groups of singular vectors. Selected singular vectors are processed further by other techniques for further enhancement, detection, isolation, feature extraction and classification.

Abstract: Signal processing apparatus and methods for elimination of undesirable clutter signals from desirable signals. This is performed without the use of filters by subtracting orthonormal basis functions from signal samples until the appropriate degree of clutter removal is achieved. Estimates of frequency information such as Doppler shifts due to fluid flow may thus be achieved with superior discrimination of true flow signals from sources of clutter or artifact, greater sensitivity to low flow rates, minimal computational effort and with fewer samples than the prior art.

Abstract: A method and apparatus are provided to adaptively form an optimum signal-free reference used to cancel near-field noise in an adaptive plate-noise cancellation system. First, second and third pressure sensors are positioned to detect any near-field plate-radiated noise. The first sensor is positioned closest to the plate, the third sensor is positioned furthest from the plate, and the second sensor is positioned between the first and third sensors. An acoustic far-field projector generates a plurality of broadband signals from a plurality of incidence angles. Each broadband signal is projected at an amplitude indicative of a noise-off condition. Outputs from the first and third sensors are combined to form a dipole responsive to each broadband signal). A feedback system is operatively associated with the dipole and the second sensor.

Abstract: An apparatus and method for detecting, processing, and tracking sonar sigs is provided. Plane wave energy from the sonar signal source is measured at multiple points using an array of plane wave energy receptors. These measurements are processed using an inverse beamformer to generate output beam levels. These output beam levels are then processed using the spectrum normalizer to yield spatially and spectrally normalized output beam levels. The normalized beam levels are then processed using an eight nearest-neighbor peak-picker to provide plane wave peaks. Finally, the plane wave peaks are processed by a three-dimensioned M of N tracker to identify peaks within a specified limit of frequency and angle change over time. The identified peaks may be displaced or recorded for further analysis.

Abstract: A system separates unknown signals which have been combined together through unknown linear filters and for which observations at multiple sensors are made. In a two channel circuit with two inputs and two sensors, the reconstructed source signals are assumed to be decorrelated such that the cross-correlation between the reconstructed source signals is near zero. The transfer functions which represent the crosstalk processes are estimated. The output signals are detected and the transfer functions are recursively solved. A reconstruction filter is used to recover the original input signals.

Abstract: A plurality of acoustic transfer functions for a plurality of sets of different positions of a loudspeaker and a microphone in an acoustic system are measured by an acoustic transfer function measuring part. The plurality of measured acoustic transfer functions are used to estimate poles of the acoustic system by a pole estimation part, and a fixed AR filter is provided with the estimated poles as fixed values. A variable MA filter is connected in series to the fixed AR filter and the acoustic transfer function of the acoustic system is simulated by the two filters. The filter coefficients of the variable MA filter are modified with a change in the acoustic transfer function of the acoustic system.

Abstract: Transducer circuit, including a tuning inductor for removing the reactive mponent from the signal of an acoustic transducer; the inductance varies with frequency, thus permitting the transducer-tuning inductor to present a non-reactive load to the an amplifier over a broad operating frequency.

Abstract: This switch device for detecting an object comprises a transducer for emitting and receiving ultrasonic signals to and from an object, a transmitter for driving the transducer, a receiver for receiving a signal from the transducer, a gate for detecting a short-distant object, a gate for detecting a long-distant object, and a circuit for setting the timings of the gates in accordance with the detecting ranges of the object.

Abstract: A sonar sensor comprising a substantially non-corrodible housing and a fluidic gainblock disposed therein. In addition, one or more fluid-filled laminar proportional amplifiers are arranged either in series or in parallel and contained within the fluidic gainblock. Means are provided to produce a jet within each laminar proportional amplifier, which is then modulated by incoming acoustic signals. The pressure caused by diverting the jet from its equilibrium path is proportional to, but greater than, the incoming acoustic signal. Hydrophones are used to convert the amplified acoustic signal into an electrical signal. In one embodiment, the fluidic gainblock is free-flooded and open to the liquid environment in which it is used, obviating the need for pressure equalization. In another embodiment, the fluidic gainblock is provided with filtering means for prevent the accumulation of sediment within the cavities making up the laminar proportional amplifiers.

Abstract: The adaptive statistical filter providing improved performance target mot analysis noise discrimination of the present invention includes a bank of parallel Kalman filters. Each filter estimates a statistic vector of specific order, which in the exemplary third order bank of filters of the preferred embodiment, respectively constitute coefficients of a constant, linear and quadratic fit. In addition, each filter provides a sum-of-squares residuals performance index. A sequential comparator is disclosed that performs a likelihood ratio test performed pairwise for a given model order and the next lowest, which indicates whether the tested model orders provide significant information above the next model order. The optimum model order is selected based on testing the highest model orders. A robust, unbiased estimate of minimal rank for information retention providing computational efficiency and improved performance noise discrimination is therewith accomplished.

Abstract: A novel totally integrated divers face mask and ultrasound underwater voice communication apparatus having integral transducer-transmit-receive-power source enclosures of low profile, flexible hands-free-voice or push-to-talk operation while submerged, and noise-cancelling microphone features for reducing bubble and vibration noise.

Abstract: A survival detection system distinguishes a low frequency survivor generated signal component from ambient low frequency signal components in a seismic sensor signal output. A twin-tee filter circuit a has its input coupled to the sensor signal output and it has a plurality of interconnected charging resistor and capacitor components with an output circuit coupled thereto to generate the detected survivor signal component.A CMOS switch is connected in each charging current path between the associated resistor and capacitor in such path to control the capacitor charging currents. A bandwidth control circuit applies an adjustable feedback voltage from filter output circuit to the twin-tee filter.A variable duty cycle generator controls the on-off duty cycle of the CMOS switches to control the effective charging current path resistance and thereby the capacitor charging currents and the tuned or center frequency of the filter.

Abstract: An active adaptive noise canceller that inserts delays in the weight update logic of an adaptive filter employed by the canceller to make the filter stable. It has been found that there is a great deal of flexibility regarding the selection of the delay values. This insensitivity permits designing the delays in advance, and not having to adjust them to different situations as they change, thus no longer requiring a training mode. The canceller dramatically reduces the amount of hardward needed to perform active adaptive noise cancelling, and eliminates the need for the training mode, which in some applications, including automobiles, for example, can be as objectionable as the noise sources that are to be suppressed.

Abstract: Apparatus for detecting sonar signals embedded in noise includes a neural network trained to detect signals in response to the slope of amplitude rank ordered noise corrected powers. A detector detects an analog waveform. Means samples and digitizes the analog waveform to obtain digital samples which in turn are passed through a cosine window. The digital samples are Fourier transformed into conjugate sets of complex numbers representing amplitude and phase. One conjugate set of the complex numbers are discarded, and the remaining complex numbers ranked according to frequency. The sum of the square of the real and imaginary component of each of the remaining complex numbers in a frequency band are provided to obtain a corresponding series of representing estimated power ranked by frequency over the band. The noise contained in subbands of the band is estimated.

Abstract: An ultrasonic energy detector advantageously supplies both frequency and amplitude information which is related to the ultrasonic frequency and the amplitude of the detected ultrasonic energy. Frequency division is employed to reduce the relatively broad range of ultrasonic frequencies into a narrower audio frequency range while still providing information useful to identify the ultrasonic energy by frequency detection. The effect of anticipated background noise at all levels of gain selected is eliminated by subtracting a signal representative of the anticipated background noise and proportional to the selected predetermined gain from the amplitude of the detected ultrasonic energy.

Abstract: A floating sensor system is provided to detect very low frequency pressure ignals (down to 0.01 Hz). This system detects pressure fluctuations or pressure signals of interest in the ocean or other body of water in the presence of unwanted pressure signals generated by surface wave induced motion. A drifting sensor surface float follows the surface waves and in turn moves a suspended pressure sensor vertically, such that it detects the wave motion as a change in static pressure which then constitutes a noise source. A correlation circuit and a logic circuit discriminate between a first composite signal, comprised of the pressure signals of interest and wave motion noise, and a second reference signal, comprised only of the wave motion noise to provide an output signal transmitted to a monitoring station.

Abstract: The method and the device are designed to cause the signal of each hydrophone of an antenna to be rid of the component due to the vibrations of a wall close to which the antenna is placed. To this end, a correction signal is deducted from the signal of the hydrophone. This connection signal takes account of the vibrations of the wall measured by sensors placed on the wall, in the vicinity of the antenna. The study of the circuit which enables this correction signal to be given is done by resolving the vibration of the wall into its dominant modes of deformation.

Abstract: A beam former method and an apparatus therefor in an ultrasonic imaging system, in which switching of delay elements is performed by means of delay switches in a section in which no ultrasonic wave transmission/reception is performed and switching noise generated in this section is stored as a noise signal into a memory. The stored noise signal is subtracted from a reception signal which is obtained through ultrasonic wave transmission/reception and which contains switching noise superimposed thereon to thereby obtain a reception signal from which the noise has been eliminated.

Abstract: For detecting a quickly or sharply varying signal (the event) against a background of a more slowly changing ambient signal (the background), the envelope detector inputs the signal and full wave rectifies it. It sends the signal to the background filter and event filter, each of which includes an RC circuit. The background filter and event filter each independently process the signal to be evaluated in different manners based on the intrinsic frequency characteristics of the event signal, and of the background signal. The outputs of the two filter sections are sent to the decision section. Based on its processing, the decision section outputs a signal in one of two states, one corresponding to detection, and the other corresponding to non-detection.

Abstract: An apparatus for preventing a direct turning-in of the supersonic wave from the opening of the sender to the opening of the receiver in a supersonic wave device for a vehicle in which an object placed near the vehicle body id detected by the supersonic wave. A cylindrical turning-in preventive member made of foamed material having discrete bubbles is arranged at the opening of the sender or the receiver.

Abstract: Acoustic sensor device with noise suppression, in particular for sensors arranged on a noise-generating aggregate, using a double sensor (M1, M2) located between a noise source (S) and an object emitting the sound to be picked up, one of the sensors (M1) being directed towards the noise source (S) and supplying to a control circuit (R) a noise signal which controls an anti-noise source (A) in such a way that it generates an anti-phase sound p.sub.A for compensating the noise p.sub.S, the other sensor (M2) being directed toward the object to be measured and supplying a measuring signal in which the noise fraction is highly weakened and the useful sound fraction is barely weakened, so that the acquisition range of the sensor (M2) is extended as a result of this reduction of sound fractions as a function of the direction of arrival.

Abstract: An active, relatively high gain, low power consuming band pass filter circuit centered at approximately 6 KHZ and having a pass band of 100-200 HZ. In a disclosed glass breakage sensor, the filter couples sensed ambient sounds of breaking glass to associated detection, driver and annunciator means. Signals of appropriate duration and frequencies indicative of breaking glass are thereby monitorable to define and induce an alarm condition within a security system.

Abstract: A switch device is disclosed for detecting an object which comprises a transducer for emitting and receiving ultrasonic signals to and from an object, a transmitter for driving the transducer, a receiver for receiving a signal from the transducer, a gate for detecting a short-distant object, a gate for detecting a long-distant object, and a circuit for setting the timings of the gates in accordance with the detecting ranges of the object.

Abstract: In acoustic level sensing apparatus, in which pulses of acoustic energy are transmitted by an electrically energized transducer towards a surface to be sensed, and the electrical output from the transducer following the pulse is digitized and analyzed to detect a return echo from the surface, so as to produce a digitized echo profile. The digitzed echo profile is enhanced by removing narrow peaks likely to correspond with certain types of spurious echo, and narrow valleys which may represent fragmentation of a wanted echo. Echoes are identified by comparing the profile with a time varying threshold generated derived from a linear regression performed on part of the threshold, various different types of comparison being performed and their results combined prior to selecting the most likely true echo on the basis of the combined results.

Abstract: A method for suppressing interference signals during operation of ultrasonic proximity transducers suppresses the interference signals that are generated by continuous noises. Repeated ultrasonic pulses are transmitted from an ultrasonic proximity transducer, and the received signals are scanned. The scanned values of the received signal are converted into digital values. After storage, the digital values are compared with a step function having one or more step sections of constant value serving as a time-dependent interference noise threshold so that the usable echo signal can be determined and separated from the interference signals. With further evaluation, the echo transit time of the usable echo signal is determined and provided to a control and evaluation unit. By suppressing continuous noises, and further suppressing stochastic interference signals, the usable echo signal is reliably filtered from a reception signal that is overlaid with interference signals.

Abstract: An ultrasonic system for identifying the location of personnel within a multiple room complex. The system includes ultrasonic transmitters adapted to be transported by the personnel. The transmitters periodically transmit pulse code modulated (PCM) transmitter signals representative of a digital code sequence characteristic of the person, and formed by a plurality of bit positions which are separated from adjacent bit positions by predetermined time intervals. Ultrasonic receivers for receiving the transmitter signals are positioned within each room of the complex. The receivers detect bits of the code sequence only during window periods corresponding to expected bit positions. Detected code sequences are compared to predetermined code sequences, and receiver signals are produced as a function of the comparison. A central station is coupled to receive the receiver signals, and provides a visual indication of the rooms in which the persons transporting the transmitters are located.

Abstract: An active phase quieting target highlight detector using phase and magnitude detection. In contrast to classical magnitude-only detection schemes, the present invention, by combining phase detection, can discriminate sets of spatially-ordered highlights due to a target, from clutter or multipath echoes, along with nonsmoothed magnitude or envelope detection, that can better discriminate target structure due to reception of echoes from shorter active pulses. False target detection and multipath detection are reduced while detection or target structure is retained by the time domain detection scheme of the invention.

Abstract: An echo canceller in a system, having a long impulse response such as in an acoustic system, which employs a fast Fourier transform to transform input data represented in the time domain into signals represented in the frequency domain to reduce calculations. To solve the problem of a long delay, the impulse response length is divided into a plurality of blocks. Each block then has a decreased number of samples within each block. Thus, a fast Fourier transform and finite impulse response type digital filtering are effected, so that the processing delay is decreased while the amount of calculations is kept small.

Abstract: A narrow band noise cancellor and signal correllator which obtains an increased signal to noise ratio of a received signal from a multi sensor array; the resulting output signal is at double the input frequency. A pair of narrow band similar input signals are added, and subtracted, and the resulting signals are multiplied together. The positive and negative product terms of the signal are separated and the positive term inverted, and the resulting separated signals are individually added with the squared representation of one of the input signals. The resulting added signals are summed and the sum is passed through a narrow band filter having its center frequency at double the input frequency.

Abstract: A channel adaptive active sonar is disclosed, wherein the sonar transmit waveform is adaptively selected in accordance with the measured sonar channel scattering function to reduce the response of the sonar to unwanted reverberation while preserving the response to signal returns. As a result, the sonar target detectability is improved over sonar systems which do not adapt to the variable properties of the ocean channel. The sonar is operable in two modes, the channel measurement mode and the normal active mode. In the channel measurement mode, probe signals are transmitted to test the channel response. From the sonar receive signals resulting from the transmitted probe signals, the channel scattering function is estimated. The transmit waveform for the sonar active mode is then designed to provide a waveform whose ambiguity function overlaps minimally with the estimated channel scattering function.

Abstract: A hydrophone mounted on the hull of a ship comprises an electromechanical ansducer made of void-containing ceramic material having a high piezoelectric sensitivity to water-borne acoustic sound signals, a transducer made of solid ceramic material having a low piezoelectric sensitivity to hydrostatic acoustic signals, both transducers having similar piezoelectric sensitivity to shipboard noise transmitted via the ship's hull and transducer mounting; and means for sensing and manipulating voltage signals generated in response to water-borne acoustic signals and mount-transmitted shipboard noise such that the mount transmitted noise signals are largely cancelled out and a relatively noise-free signal representing hydrostatically transmitted sound is obtained.

Abstract: A system is provided for the detection of an unsuitable water condition in which either large globules of solid material or bait fish suspended in the water beneath a vessel adjacent the hull of the vessel are detected and the associated depth sounding device is inhibited from providing a depth output. This eliminates the display or annunciation of erratic depth data, or the use of erratic data for closed or open loop control systems. In one embodiment, the unsuitable water condition is sensed through the detection of returns in a short water column adjacent the hull. In a further embodiment, returns from the water column coupled with receiver gain being open above a predetermined threshold provide the indication of the presence of the unsuitable water condition.

Abstract: A sensor for creating images containing range depth information is disclosed. The sensor comprises a gated energy transmitter used to "illuminate" the object to be imaged, a gated integrating imaging energy receiver to produce "raw" images, a processor system to combine the raw images from the receiver to produce a output image, and a timing system to control the gate timing of the transmitter and gate timing of the receiver. In operation, two raw images are created with differing time relationships between the transmitter gating, receiver gating and integrated energy readout. One raw image is a reference image containing no range information while the second image contains range information along with unwanted information such as reflectivity variations. These raw images are processed to produce a final output image in which the unwanted information is mainly cancelled producing an output "range" image. This output image is a one or two dimensional array of data.

Abstract: In combination with any signal processor system which uses an R-C integra type energy detector to recognize the presence of electrical signals such as those that are proportional to a received acoustic ping signal, the turnaround time stability improvement comprising a timing initiation signal recognition means, a validation means, first and second one-shorts for A/D conversion, and AND gate means for receiving the timing initiation signal and the validation signal and outputting a valid ping timing signal only when both are true, said valid ping timing signal in turn causing subsequent actions normally initiated by this event to occur, such as a transponder system incorporation the present invention projecting a return ping into the water.

Abstract: There is provided a coordinates input apparatus comprising: a vibrator drive controller to output a vibrator drive signal obtained by synthesizing a plurality of pulse trains whose phases differ; a vibration pen to generate a vibration from a vibrator which generates the vibration in response to the vibration drive signal output from the vibrator drive controller; a vibration propagating plate made of a transparent acrylic or glass plate to propagate the vibration generated by the vibration pen; a plurality of vibration sensors, attached to the vibration propagating plate, for detecting the vibration generated by the vibration pen at a plurality of positions; and a coordinate value calculation/control circuit for calculating the vibration propagating times from the vibrations detected by the vibration sensors at a plurality of positions, thereby obtaining the coordinate values on the vibration propagating plate on which the vibration pen is located from the propagating times calculated.

Abstract: A digital echo canceller [1] with a good protection against double-talk comprises a first filter [9] for the echo cancellation proper, a second adaptive filter [11] for making continuously adapted filter coefficients available to the first filter [9] and gate means [15] which will only apply the adapted filter coefficients to the first filter [9] if certain conditions are satisfied. In the echo canceller [1] a specific filter combination is used, in which the first filter [9] is a time-domain programmable filter (TDPF) and the second filter [11] is a frequency-domain block-adaptive (FDAF), and further means [20,20(1)] are provided that are arranged in cascade with the gate means [15] for transforming the frequency-domain filter coefficients [W(p;m)] of the FDAF [11] into time-domain filter coefficients [w(i;m)] for the TDPF [9].

Abstract: The level of signal noise caused by fluid flow at a pressure transducer, such as a hydrophone or geophone, is determined by measuring the electric field induced when the fluid flow through a magnetic field. The electric field, which is proportional to flow rate, is measured by electrodes positioned about the transducer. The pressure due to the flow is derived from the flow rate voltage by a digital filter which may be adaptive, to allow for varying flow conditions, or which may employ a predetermined electric field/flow noise relation. This flow noise is then subtracted from the received signal, the resultant signal then being substantially noise free.

Abstract: In acoustic level sensing apparatus, in which pulses of acoustic energy are transmitted by an electrically energized transducer towards a surface to be sensed, and the electrical output from the transducer following the pulse is digitized and analyzed to detect a return echo from the surface, so as to produce a digitized echo profile. The digitized echo profile is enhanced by removing narrow peaks likely to correspond with certain types of spurious echo, and narrow valleys which may represent fragmentation of a wanted echo. Echoes are identified by comparing the profile with a time varying threshold generated derived from a linear regression performed on part of the threshold, various different types of comparison being performed and their results combined prior to selecting the most likely true echo on the basis of the combined results.

Abstract: In a fluidborne or structureborne sound system, a method and apparatus are rovided for actively canceling transient acoustic noise. A high-speed controller utilizes a weighted combination of open and closed-loop inputs to provide a correction signal to a cancellation source. The cancellation source introduces a canceling acoustic wave that is equal in amplitude but 180.degree. out of phase with respect to the acoustic noise in order to cancel the acoustic noise within the system. The open-loop input is provided by a database containing a predicted, off-line model of the acoustic noise. The closed-loop input is provided by a combination of 1) an input signal generated by the input acoustic wave and feedback from the cancellation source as measured by an input sensor and 2) an error signal generated at the output of the system as measured by an error sensor. The weighted combination of the open and closed-loop inputs is chosen to minimize the error signal.