Showing papers on "Background noise published in 1987"

Low‐frequency noise in dc superconducting quantum interference devices below 1 K

Abstract: At temperatures below about 1 K, a series of dc superconducting quantum interference devices (SQUID’s) exhibited an apparent flux noise with a spectral density scaling as 1/f α, where 0.58<α<0.80. Typically, the magnitude of the noise increased as the temperature was lowered below 1 K, tending to flatten out at low temperatures with a value of 7±3 μΦ0Hz−1/2 at 1 Hz that was nearly independent of the parameters and materials of the SQUID’s. Although a large number of hypothetical sources of the noise have been eliminated, the origin remains unidentified.

ac method for measuring low‐frequency resistance fluctuation spectra

Abstract: An ac technique is described for measuring low‐frequency resistance fluctuation spectra with improved sensitivity over dc methods achieved by avoiding preamplifier 1/f noise. The technique, easily implemented with decade resistors and a lock‐in amplifier, allows the current noise of low‐resistance (r<10 kΩ) specimens to be measured to frequencies below 1 mHz. Use of a center‐tapped, four‐probe specimen geometry allows discrimination between specimen and contact noise and eliminates noise due to bath temperature variations. The technique is demonstrated in use to determine the dependence of the 1/f noise of Cr films on film area. Measurements with simultaneous direct and alternating currents provide means to study the noise of nonlinear devices and frequency‐dependent conductors.

Multigigabit-per-second avalanche photodiode lightwave receivers

Abstract: High-speed avalanche photodiodes and high-sensitivity receivers are vital components for future multigigabit-per-second lightwave transmission systems. We review theoretical and experimental performance of high-speed III-V avalanche photodiodes, and also that of multigigabit-per-second lightwave receivers using FET and bi-polar amplifiers. Particular attention is given to APD gain-bandwidth product, and to its effect on high-speed receiver sensitivity. Comparisons between measured receiver sensitivities and calculated performance are presented for bit rates up to 8 Gbit/s.

Effects on DC SQUID characteristics of damping of input coil resonances

Abstract: The possibility of improving dc SQUID performance by damping the input circuit resonances caused by parasitic capacitances is studied experimentally A high-quality dc SQUID was coupled to a first-order axial gradiometer built for neuromagnetic research, and a resistor-capacitor shunt was connected in parallel with the input coil of the SQUID Ten different RC shunts were studied with the SQUID operating in a flux-locked loop, carefully shielded against external disturbances It was found that increasing the shunt resistance resulted in smoother flux-voltage characteristics and smaller noise At best, the minimum obtainable equivalent flux noise level was one-fourth that for the unshunted SQUID The noise level is a function of the shunt resistance R/sub s/ only, except for shunt capacitance values bringing the low-frequency resonance of the input coil close to the flux modulation frequency At a constant bias current level, where the amplitude of the flux-voltage characteristics is at maximum, the equivalent flux noise varies as R/sub s//sup -07/ The results agree reasonably well with recently published predictions based on numerical simulations where the whole input circuit with parasitic capacitances was taken into account

Novel superconducting thermometer for bolometric applications

Abstract: The temperature dependence of the magnetic penetration depth in a superconductor, in the kinetic inductance limit, is proposed as a basis for a sensitive thermometer. Considered as a bolometer, the noise equivalent power from the sum of the Johnson noise and the preamplifier noise can be reduced to about 7×10−20 W/(Hz)1/2, which is approximately four orders of magnitude below currently realized values.

Fruit location in a partially occluded image

Abstract: .D IGITA Limage analysis with a modified circular ^ Hough transform proved able to locate tomatoes based on shape and not color, even when the scenes contained substantial background noise and the fruit were partially hidden from view.

Low‐frequency excess noise in Nb‐Al2O3‐Nb Josephson tunnel junctions

Abstract: A fabrication technique for Nb‐Al2O3‐Nb Josephson tunnel junctions is described that is an alternative to the trilayer method generally used. At 4.2 K the magnitude of the low‐frequency noise in the critical current I0 of four junctions with areas A ranging from 7.8 to 115 μm2 was characterized by S1/2I0A1/2/I0 =(14±6)pA μm/μA Hz1/2, where SI0 is the spectral density of the excess noise at 1 Hz. The noise power spectrum of one 3×3 μm2 junction exhibited a Lorentzian feature associated with the emptying and filling of a single trap in the barrier. The low level of noise makes these junctions attractive for use in superconducting quantum interference devices.

Analysis and Optimization of Millimeter-And Submillimeter-Wavelength Mixer Diodes

Abstract: An analysis of the noise of millimeter- and submillimeter-wavelength mixers with GaAs Schottky diodes is presented. This analysis accounts for the correlation of the downconverted components of the time-varying hot-electron noise in the series resistance, and is thus accurate even for cryogenically cooled mixers operated at submillimeter wavelengths. This paper shows that the terms of the series-resistance noise correlation matrix are functions of the Fourier coefficients of the squared diode current F(I/sup 2/) rather than the square of the Fourier coefficients of the diode current [F(I)]/sup 2/, as has been previously presented in the literature. The analysis is used to evaluate the optimization of cryogenic mixer diodes. It is shown that minimization of the diode's I-V slope parameter V/sub 0/ is more critical than reduction of the parasitic elements for millimeter-wavelength operation, while at frequencies above 600 GHz (Lambda<0.5 mm), the junction capacitance is the most crucial parameter. Experimental results from several research groups working with a variety of mixers are presented to substantiate these results.

Telephone with background volume control

Abstract: This invention relates to the adjustment of sound intensity (volume) from appliances which communicate audible information to users. Information carried by sound energy from an appliance competes with whatever audible noise is present in the listener's area. Some appliances have volume controls that let the user adjust sound intensity. These controls do not deal adequately with background noise that changes often, and which makes it necessary for the user to frequently reset the volume control. A setting of volume control that is appropriate at a quiet time, may make it impossible to understand a message when background noise becomes louder. Similarly, a volume control setting which is suitable when background noise is high may produce a message which is disturbingly loud when background noise is low. The present invention solves this problem of audio volume control by monitoring background noise in the listening area and automatically adjusting the sound level of the appliance accordingly. By raising the volume of sound in the presence of higher background noise, and lowering the volume of sound when background noise level is lower, the needs of the user are better met.

Generalized homomorphic and adaptive order statistic filters for the removal of impulsive and signal-dependent noise

Abstract: In this paper, we propose two new nonlinear filters for filtering signal-dependent noise, additive noise, and impulsive noise in image processing. The first filter proposed is an order statistic filter based on a generalized homomorphic transformation. The second is an adaptive order statistic filter with a variable threshold, which changes according to the noise level. Both of these filters perform well for the different kinds of noise encountered in image processing. They suppress signal-dependent noise, additive noise, and impulsive noise better than median filters, \alpha -trimmed mean filters, general nonlinear mean filters, modified trimmed mean filters, and double-window modified trimmed mean filters. They also preserve the edges of an image better than median filters and are simple to implement.

Speech/Silence segmentation for real-time coding via rule based adaptive endpoint detection

Abstract: A new algorithmic technique is presented for efficiently implementing the end-point decisions necessary to separate and segment speech from noisy background environments. The algorithm utilizes a set of computationally efficient production rules that are used to generate speech and noise metrics continuously from the input speech waveform. These production rules are based on statistical assumptions about the characteristics of the speech and noise waveform and are generated via time-domain processing to achieve a zero delay decision. An end-pointer compares the speech and silence metrics using an adaptive thresholding scheme with a hysteresis characteristic to control the switching speed of the speech/silence decision. The paper further describes the application of this algorithm to silence compression of speech in which speech segments are encoded with a low bit-rate encoding scheme and silence information is characterized by a set of parameters. In the receiver the resulting packetized speech is reconstructed by decoding the speech segments and reconstructing the silence intervals through a noise substitution process in which the amplitude and duration of background noise is defined by the silence parameters. A noise generation technique is described which utilizes an 18th order polynomial to generate a spectrally flat pseudo-random sequence that is filtered to match the mean coloration of acoustical background noise. A technique is further described in which the speech/silence transitions are merged rather than switched to achieve maximum subjective performance of the compression technique. The above silence compression algorithm has been implemented in a single DSP-20 signal processing chip using sub-band coding for speech encoding. Using this system, experiments were conducted to evaluate the performance of the technique and to verify the robustness of the endpoint and silence compression over a wide range of background noise conditions.

Phase Noise Reduction in FET Oscillators by Low-Frequency Loading and Feedback Circuitry Optimization

Abstract: Optimization of low-frequency Ioading allows reduction of 1/f converted noise in FET oscillators. Moreover, an appropriate low-frequency feedback between drain and gate gives an improvement over previous results. This improvement is explained by taking into account the properties of the noise autocorrelation function.

Iterative speech enhancement with spectral constraints

Abstract: A new and improved iterative speech enhancement technique based on spectral constraints is presented in this paper. The iterative technique, originally formulated by Lim and Oppenheim, attempts to solve for the maximum likelihood estimate of a speech waveform in additive white noise. The new approach applies inter- and intra-frame spectral constraints to ensure convergence to reasonable values and hence improve speech quality. An extremely efficient technique for applying these constraints is in the use of line spectral pair (LSP) coefficients. The inter-frame constraints ensures more speech-like formant trajectories than those found in the unconstrained approach. Results from speech degraded by additive white Gaussian noise show noticeable quality improvement.

Influence of some practical factors on background extrapolation in EELS quantification

Abstract: SUMMARY The detectability of a minor element with EELS in a STEM is of great interest. This paper first describes errors resulting from a conventional background extrapolation scheme and other faults in the collected spectra which in effect greatly reduce the seemingly good signal to noise ratio of a spectrum and therefore impose much more stringent statistical counting requirement on spectra if a small amount of element is to be quantified with good accuracy. Secondly, the systematic variation of the index r of the power law background fitting curve of an ionizatoin edge tail is discussed. Its effect on the detectability of a minor element is analysed. It puts a lower limit on the detectability of an element, however many counts are accumulated. Also a scheme is proposed of some non-conventional linear least squares fits by which one might suffer from less quantification error due to background extrapolation and achieve lower detectability of a minor element.

Resistance noise in nonlinear resistor networks.

Abstract: Cohn s theorem is extended to the case of circuit elements with the nonlinear I-V characteristic V rI'. This simplifies the study of resistance noise in nonlinear resistor networks. Exact exponent inequalities are derived. Fractal and percolating structures are considered. The infinite number of exponents necessary to characterize completely the electrical properties of linear and nonlinear percolating networks are calculated within the Migdal-Kadanoff approximation.

Generation of noise by electronic iteration of the logistic map

Abstract: A new circuit that generates low-frequency noise is presented. This circuit is based upon an electronic realization of the logistic map system. The noise power density spectrum of the output was measured and confirmed to be approximately white. We also note the operation of the circuit as a voltage-programmable 2-, 3-, or 4-valued oscillator.

Speech recognition with a noise-adapting codebook

Abstract: Speech recognizers trained in quiet conditions but operating in noise usually have poor accuracy. This paper reports two methods for improving the accuracy of an LPC vector-quantization speech recognizer by adapting the vector codebook to noisy conditions. First, each codebook vector is changed to reflect the way people speak in noise. Second, the estimated spectrum of the background noise is added to the codebook vectors. These ideas have been tested on a total of 2400 utterances of digits recorded in a car by 4 speakers. A baseline word spotter similar to NTT's SPLIT system was modified by adapting its vector codebook to noise. This adapted codebook, when used with a new word decision criterion, yields error rates at least 4 times lower for noisy conditions. The accuracy is significantly better than without codebook adaptation techniques.

Adaptive expander for telephones

Abstract: An adaptive expander for telephones reduces the gain of a transmitting amplifier in proportion to the intensity of the background noise. Gain reduction occurs when the user is not speaking into the handset microphone. When the user is speaking, however, the gain of the transmitting amplifier is restored to its normal level. Noise is distinguished from speech via long-term averaging of the microphone output signal, and a circuit that precludes the magnitude of the long-term average voltage from ever exceeding the short-term average voltage. The present invention is used in full duplex arrangements where simultaneous transmission in both directions is possible rather than in half duplex arrangements, such as a speakerphone, where transmission occurs only in one direction at a time.

Noise due to Brownian motion in ultrasensitive solid-state pressure sensors

Abstract: Noise due to Brownian motion of diaphragm in ultrasensitive solid-state capacitive and piezoresistive pressure sensors operating at sub-millimeters of mercury pressures in a gaseous ambient is considered. The statistical properties and spectral characteristics of the noise are obtained as functions of the diaphragm dimensions, temperature, and applied pressure. The results show that the Brownian equivalent pressure noise is substantially less than has been previously reported and is well below 1µmHg for most practical cases of interest. Thus it is not a limiting factor in setting device performance when compared to circuit noise sources.

High-Resolution Signal and Noise Field Estimation Using the L 1 (Least Absolute Values) Norm

Abstract: In this paper a new method for obtaining a quantitative estimate of an acoustic field consisting of a set of discrete sources and background noise is described. The method is based on the L 1 (least absolute values) norm solution to an underdetermined system of linear equations defining the Fourier transform of the signal series. Implementations of the method with either equality or inequality constraints are presented and discussed. The much faster and more compact equality constraint version with a provision for modeling the noise field is recommended in practice. Experience with real data has shown the necessity of correcting for an observed Gaussian decay on the covariances. A simple means of estimating this effect and taking it into account in the signal estimation procedure is discussed, and the implications of this effect in high-resolution beamforming are considered. The effectiveness and versatility of the L 1 method indicate that it has a useful role in high-resolution signal estimation.

Adaptive noise reduction in aircraft communication systems

Abstract: In many military environments, such as fighter jet cockpits, the increasing use of digital communication systems has created a need for robust vocoders and speech recognition systems. However, the high level of ambient noise in such environments makes vocoders less intelligible and makes reliable speech recognition more difficult. One method of enhancing the noise-corrupted speech is adaptive noise cancellation. In previous research, this method was tested in a simulated cockpit environment, yielding impressive results. However, in new simulations, reflecting more realistic conditions, adaptive noise cancellation has been less successful. Spectral analysis of the data showed that the diffuseness of the ambient noise fieid, along With the microphone characteristics, has a significant effect on the performance of adaptive noise cancellation.

Effects of tilt and offset of signal field on heterodyne efficiency.

Abstract: The effects of tilt and offset on the heterodyne efficiency are discussed for optical fields with Gaussian field distributions. An aperture is used in the input plane of the system to decrease the background noise which is incident on the signal. The numerical results show that in optimum conditions of the system and beam fields, the tilt must be less than ~10−4 deg and the offset about one-fifth of the input aperture to obtain more than 60% of the ideal values of the efficiency.

Detection of narrow-band sonar signals using order statistical filters

Abstract: In detecting a narrow-band signal in noise in a sonar environment, an estimation of the background noise power has to be made so that a detection threshold can be set. Here, we consider a variety of nonlinear estimators based on order statistics. The performances of these methods in providing an estimate of the noise power and in detecting a narrow-band signal in noise have been analyzed and have been found to be robust even under the interference of other signals.

Adaptive Median Filtering for Impulse Noise Elimination in Real-Time TV Signals

Abstract: A two-dimensional (3 × 3) median filter with controlled turn-on is employed in an adaptive fashion for selective removal of impulse noise interference in real-time television signals. Various filters and threshold conditions are selected from programmed PROM's as a function of impulse noise detected and counted during a vertical interval window period. The adaptive median filter is used in conjunction with a satellite FM communication downlink system and can effectively improve the onset of threshold performance by about 3 dB carrier-to-noise ratio. The particular FM system discussed employs a multiplexed analog component, MAC, formatted TV signal. Use of the filter for standard NTSC composite signals is indicated.