Showing papers on "Noise reduction published in 1979"

Enhancement of speech corrupted by acoustic noise

Abstract: This paper describes a method for enhancing speech corrupted by broadband noise. The method is based on the spectral noise subtraction method. The original method entails subtracting an estimate of the noise power spectrum from the speech power spectrum, setting negative differences to zero, recombining the new power spectrum with the original phase, and then reconstructing the time waveform. While this method reduces the broadband noise, it also usually introduces an annoying "musical noise". We have devised a method that eliminates this "musical noise" while further reducing the background noise. The method consists in subtracting an overestimate of the noise power spectrum, and preventing the resultant spectral components from going below a preset minimum level (spectral floor). The method can automatically adapt to a wide range of signal-to-noise ratios, as long as a reasonable estimate of the noise spectrum can be obtained. Extensive listening tests were performed to determine the quality and intelligibility of speech enhanced by our method. Listeners unanimously preferred the quality of the processed speech. Also, for an input signal-to-noise ratio of 5 dB, there was no loss of intelligibility associated with the enhancement technique.

A spectral subtraction algorithm for suppression of acoustic noise in speech

Abstract: Spectral subtraction has been shown to be an effective approach for reducing ambient acoustic noise in order to improve the intelligibility and quality of digitally compressed speech. This paper presents a set of implementation specifications to improve algorithm performance and minimize algorithm computation and memory requirements. It is shown spectral subtraction can be implemented in terms of a nonstationary, multiplicative, frequency domain filter which changes with the time varying spectral characteristics of the speech. Using this filter a speech activity detector is defined and used to allow the algorithm to adapt automatically to changing ambient noise environments. Also the bandwidth information of this filter is used to further reduce the residual narrowband noise components which remain after spectral subtraction.

Design of Microwave GaAs MESFET's for Broad-Band Low-Noise Amplifiers

Abstract: As a basis for designing GaAs MESFET's for broad-band low-noise amplifiers, the fundamental relationships between basic device parameters, and two-port noise parameters are investigated in a semiempirical manner. A set of four noise parameters are shown as simple functions of equivalent circuit elements of a GaAs MESFET. Each element is then expressed in a simple analytical form with the geometrical and material parameters of this device. Thus practical expressions for the four noise parameters are developed in terms of the geometrical and material parameters. Among the four noise parameters, the minimum noise figure F/sub min/, and equivalent noise resistance R/sub n/, are considered crucial for broad-band Iow-noise amplifiers. A low R/sub n/ corresponds to less sensitivity to input rnismatch, and can be obtained with a short heavily doped thin active channel. Such a high channel doping-to-thickness (N/a) ratio has a potential of producing high power gain, but is contradictory to obtaining a low F/min/. Therefore, a compromise in choosing N and a is necessary for best overall amplifier performance. Four numerical examples are given to show optimization processes.

Video noise reduction

Abstract: A video noise reduction system including a coefficient modifier for modifying at least a portion of the incoming video information Video storage is provided to store the modified information and an adder is provided for adding at least a portion of incoming picture information with at least a portion from a corresponding stored picture point from an earlier field The noise reduced picture information is provided at the store output The degree of noise reduction may be varied in dependence on movement detected between fields and whether luminance or chrominance components are present

Detection of hidden periodicities by adaptive extrapolation

Abstract: A method is presented for determining the harmonic components of a noisy signal by nonlinear extrapolation beyond the data interval. The method is based on an algorithm that adaptively reduces the spectral components due to noise.

System for reducing the visibility of the noise in television pictures

Abstract: A television signal noise reduction system employs a movement detector for automatically selecting one of three parallel processing paths, the first of which passes the video signal without modification and the other two of which calculate different weighted averages of the current pixel and surrounding pixels from the current frame and the previous frame.

Design of quiet efficient propellers

Abstract: A numerical computation scheme has been developed to determine the sound generated by propellers A comparison of these calculations to the noise data taken in the flight test of a propeller driven aircraft shows good agreement The method is then applied in a parametric study of fixed pitch propellers designed to reduce noise All these techniques reduce noise while maintaining shaft speed so that the method presented here may be used in a retrofit option for the general aviation fleet

Noise reduction system

Abstract: A process and apparatus (a) for reducing the perceived noise without perceptible loss of fidelity upon aural reproduction, of an audio input signal in electrical form by means of an electronic circuit wherein the upper limit of the bandwidth of the signal is automatically and continuously adjusted in accordance with the relationship of (i) the peak amplitude of the rate of change of the signal with respect to time to (ii) the instantaneous amplitude within a selected frequency band of the signal, and (b) a system for detecting and discriminating between noise and audio transients in the input signal and retaining the audio transients in the output signal.

Audible Noise Performance of Regular and Asymmetric Bundles and effect of Conductor Aging on Project UHV's Three-Phase Test Line

Abstract: Audible noise produced by corona during foul weather is an important consideration in the design of high-voltage transmission lines. The measurement, analysis and prediction of audible noise and the assessment of noise reduction techniques have constituted a primary area of research at Project UHV. Results of earlier short-term testing utilizing single-phase test facilities have, since 1975, been greatly augmented by longer-term field data obtained on a three-phase test line. This paper presents the results of a portion of this work The effect of the weathering or aging of conductors on noise levels is examined and the effectiveness of the concept of asymmetric conductor bundling for noise reduction is assessed.

Interior noise control prediction study for high-speed propeller-driven aircraft

Abstract: An analytical model was developed to predict the noise levels inside propeller-driven aircraft during cruise at M = 0.8. The model was applied to three study aircraft with fuselages of different size (wide body, narrow body and small diameter) in order to determine the noise reductions required to achieve the goal of an A-weighted sound level which does not exceed 80 dB. The model was then used to determine noise control methods which could achieve the required noise reductions. Two classes of noise control treatments were investigated: add-on treatments which can be added to existing structures, and advanced concepts which would require changes to the fuselage primary structure. Only one treatment, a double wall with limp panel, provided the required noise reductions. Weight penalties associated with the treatment were estimated for the three study aircraft.

A Broad-Band Noise Technique for Fast-Scanning Radar Observations of Clouds and Clutter Targets

Abstract: This paper describes a relatively simple technique for suppressing interference fluctuations from precipitation and other cluttertype targets. The technique utilizes noise as a transmitted signal and receives the returns in a simple radiometer-type receiver. For noise of bandwidth B and an averaging time equal to transmitted pulselength T, the root-mean-square (rms) fluctuation in the return from a given volume in range is a fraction, ??/?BT of the mean return, down from a fractional value of unity for unaveraged single-frequency returns. Using this technique, a fast-scanning radar has been constructed which scans the hemisphere overhead every 20 s with a 2.2° beam. For this system, B = 30-40 MHz and T = 1 ?s, and the error in the reflectivity estimate per transmitted pulse is ?1-dB rms. Observations of precipitation returns using 300-MHz bandwidth noise are compared to simultaneous single-frequency observations to demonstrate the clutter reducing ability of the noise technique. For 300-MHz bandwidth noise transmissions and an averaging time of 1 ?s, interference fluctuations are reduced by 25 dB from the single-frequency case, and the mean reflected signal is determined to within 0.3-dB rms in a single transmitted pulse.

Noise performance of an achromatic coherent optical system.

Abstract: The noise-suppression characteristics of an achromatic optical processing system when operated with a broad-spectrum light source are analyzed. It is shown that such systems produce considerable noise improvement. Both signal-dependent and signal-independent noise are considered. In each case, we find that the achromatic coherent system behaves much like an incoherent imaging system.

Airframe noise component interaction studies

Abstract: Acoustic wind tunnel tests were conducted to examine the noise-generating processes of an airframe during approach flight. The airframe model was a two-dimensional wing section, to which highlift leading and trailing edge devices and landing gear could be added. Far field conventional microphones were utilized to determine component spectrum levels. An acoustic mirror directional microphone was utilized to examine noise source distributions on airframe components extended separately and in combination. Measured quantities are compared with predictions inferred from aircraft flyover data. Aeroacoustic mechanisms for each airframe component are identified. Component interaction effects on total radiated noise generally were small (within about 2 dB). However, some interactions significantly redistributed the local noise source strengths by changing local flow velocities and turbulence levels. Possibilities for noise reduction exist if trailing edge flaps could be modified to decrease their noise radiation caused by incident turbulent flow.

Noise reduction in electrical signals

Abstract: Reduction of noise in an electrical, e.g. television, signal is achieved by comparing signals from successive scans to produce a difference signal. Low amplitude differences are assumed to represent noise and are attenuated. Differences above a predetermined level are assumed to represent movement and are not so attenuated. The attenuation is achieved by a multiplier (26) which receives as multiplier the output of a non-linear transfer characteristic element (24). The non-linear element receives the differences after rectification (100) and after passing through a variable-gain element (102). The non-linear element (24) is such that above a predetermined value a constant predetermined minimum attenuation factor applies. A noise measurement circuit (X) measures the noise present in the difference signal and controls the gain of the variable-gain element (102) accordingly. Several noise-measurement circuits may be provided. a selected one of which is used in dependence upon the magnitude of the input signal to the apparatus.

Characteristics of propeller noise on an aircraft fuselage related to interior noise transmission

Abstract: Exterior noise was measured on the fuselage of a twin-engine, light aircraft at four values of engine rpm in ground static tests and at forward speeds up to 36 m/s in taxi tests. Propeller noise levels, spectra, and correlations were determined using a horizontal array of seven flush-mounted microphones and a vertical array of four flush-mounted microphones in the propeller plane. The measured levels and spectra are compared with predictions based on empirical and analytical methods for static and taxi conditions. Trace wavelengths of the propeller noise field, obtained from point-to-point correlations, are compared with the aircraft sidewall structural dimensions, and some analytical results are presented that suggest the sensitivity of interior noise transmission to variations of the propeller noise characteristics.

Constrained minimization of roundoff noise in fixed-point digital filters

Abstract: A method for the minimization of roundoff noise in fixed-point digital filters is presented. In this method, a given filter structure is transformed successively by small increments while roundoff noise is reduced at each step. For 1-D filter structures describable by a state-space model, the results obtained are similar to those of Mullis and Roberts (1) and Hwang (2). However, the new method has the important advantage that arbitrary portions of the topology of a structure can be held fixed while the structure is transformed. Thus, the number of new nonzero coefficients created by the transformation can be constrained. A simple example is presented showing a structure which has nearly as low roundoff noise as the "optimum" according to Mullis and Roberts (1) and Hwang (2), yet has much fewer non-zero coefficients than the latter.

Effects of simulated forward flight on jet noise, shock noise and internal noise

Abstract: Inflight simulation experiments are conducted in an anechoic free-jet facility to examine the flight effects on various combinations of jet noise, shock noise and internal noise. The jet mixing noise component reduces with forward velocity at all angles and frequencies. When jet mixing noise is contaminated with internal noise, forward motion provides a noise reduction in the rear arc and a noise increase in the forward arc, with little change at 90 deg. The results are similar for shock-containing jets. It is found that the existing anomalies between full-scale flight data and model-scale flight simulation data could well be due to the contamination of the flight data by internal noise.

Engine-induced structural-borne noise in a general aviation aircraft

Abstract: Structural borne interior noise in a single engine general aviation aircraft was studied to determine the importance of engine induced structural borne noise and to determine the necessary modeling requirements for the prediction of structural borne interior noise. Engine attached/detached ground test data show that engine induced structural borne noise is a primary interior noise source for the single engine test aircraft, cabin noise is highly influenced by responses at the propeller tone, and cabin acoustic resonances can influence overall noise levels. Results from structural and acoustic finite element coupled models of the test aircraft show that wall flexibility has a strong influence on fundamental cabin acoustic resonances, the lightweight fuselage structure has a high modal density, and finite element analysis procedures are appropriate for the prediction of structural borne noise.

Reduction of nonstationary acoustic noise in speech using LMS adaptive noise cancelling

Abstract: Nonstationary acoustic noise with energy possibly equal to or greater than the speech is suppressed using a two microphone implementation of adaptive noise cancellation. The primary noise added to the speech is reduced by subtracting a filtered version of the second microphone reference noise. The reference noise filter is adaptively up dated using the Widrow-Hoff LMS algorithm [1]. The effectiveness of noise suppression depends directly on the ability of the filter to estimate the transfer function relating the primary and reference noise channels. A study of the filter length required to achieve a desired noise reduction level in a hard-walled room is presented. Results demonstrating noise reduction in excess 10dB in an environment with 0dB signal noise ratio are presented.

A practical approach to truck noise reduction

Abstract: An approach is described for reducing the drive-by noise of a 14-liter diesel-engined truck which involves a combination of conventional noise reduction techniques. The combustion and mechanical exciting forces within the engine are respectively lowered by turbocharging and low-noise pistons, coupled with a small reduction in the rated speed. The resulting noise is largely attenuated by employing simple shielding attached with isolation mounts to form a tunnel around the engine. A reduction of drive-by noise from 92 1/2 dBA to 82 dBA has been demonstrated with this approach. A level of 80 dBA was achieved with additional silencing and shielding of the non-engine noise sources. A method is appended for calculating the radiated combustion and mechanical noise levels from an engine.

Digital Filtering of Speckle-photography Data

Abstract: Optical methods for nondestructive analysis of strain fields are continuing to generate a great deal of interest among experimentalists; the potential of these methods is considered to be great. One persistent obstacle to the accuracy of such methods is the contamination of the data by ‘optical noise’. This paper describes a digital method which allows extensive reduction of noise components from data records obtained in point-by-point interrogation of double-exposure speckle photographs. The computations prove to be inexpensive and effective. Sample results are given and further applications are suggested.

High‐accuracy magnetic field difference measurements and improved noise reduction techniques for use in tectonomagnetic studies

Abstract: Previous geomagnetic field measurements in Colorado have been extended using three self-calibrating rubidium magnetometers accurate to about 0.01 γ. The magnetometers were deployed either in an L-shaped array or in a collinear array and provided simultaneous measurements of the scalar field at sites separated by 12–16 km. Previous noise reduction work using two-station magnetic field difference data from Colorado removed variations from the east-west scalar difference which were correlated with variations in the field component D. The earlier analysis work has been extended to remove also noise correlated with the field components H and Z. Noise due to susceptibility and magnetization differences between sites or induced currents in conductivity anomalies is reduced by this technique. However, the main purpose of this paper is to demonstrate a new noise reduction technique that removes noise due to large-scale gradients in geomagnetic variations. In effect, the method removes variations in the magnetic field difference between two sites which are correlated with variations in the difference between two other sites in a collinear array. This approach, which does not require vector data, reduced gradiometer noise to 0.03 γ rms for 37 days of hourly averages.

Reduced multiplier, low roundoff noise digital filters

Abstract: Minimal roundoff noise IIR digital filters require (n+1)2multiplies per output sample for an nth order filter. By optimizing second-order sections which are then parelleled or cascaded, one can reduce this to approximately 4n multiplies per output. This compares to approximately 3n multiplies for the direct form. This paper presents explicit design equation and noise performance equations for low roundoff noise, second-order filters with fewer multipliers than optimal forms. These structures are a compromise between direct forms (minimal number of multipliers) and minimal roundoff noise structures.

Normal realizations of IIR digital filters

Abstract: Normal realizations of IIR digital filters possess many desirable properties including freedom from overflow oscillations, low coefficient sensitivity, and invariance of form and roundoff noise under frequency transformations of the filter. Normal realizations are closely related to minimal roundoff noise structures. Given a state variable model {A,b,c,d} of a filter, we define a normal structure as a filter whose A matrix satisfies AAT=ATA. This paper develops design equations and roundoff noise equations for normal and minimal roundoff noise filters in terms of the poles and zeros of the filter transfer function. The development has geometric interpretations which yield additional insights into low roundoff noise structures.

Method and apparatus for reduction of modal noise in fiber optic systems

Abstract: A dithering means is applied to a laser diode to provide dither modulation along with a signal input thereto. Modal noise, otherwise generated by the system, is reduced by the introduction of dither modulation.

Reduction of quantization noise in PCM speech coding

Abstract: A new technique to reduce the effect of quantization noise in PCM speech coding is proposed. The procedure consists of using dither noise to ensure that the quantization errors can be modeled as additive signal-independent noise, and then reducing this noise through the use of a noise reduction system. The procedure is illustrated with examples.

Noise transmission - Turboprop problem

Abstract: An analytical model is presented to predict low frequency (1000 Hz and below) noise transmission into aircraft. Noise transmission is determined by solving the acoustic wave equation for the interior noise field, the equivalent 'smeared' monocoque shell equation for overall fuselage vibrations and skin-stringer panel equation for local vibrations of panels and stringers. The solution to this system of equations is obtained by using a Galerkin-like procedure. The modes and frequencies for skin-stringer panels are determined by a transfer matrix method. Results include noise transmission through discretely stiffened panels and cabin noise due to turbulent boundary layer and propeller passage harmonics for a turboprop type aircraft.