Showing papers on "Noise measurement published in 1978"

Method and means for adaptively filtering near-stationary noise from an information bearing signal

Abstract: An input signal containing information such as speech or music as well as near-stationary noise is applied in parallel to a noise-analysis circuit and a noise-reduction circuit, each of which comprises a plurality of bandpass filters covering the range of frequencies associated with the information. The absolute value, or a function thereof, of the output of each bandpass filter in the noise-analysis circuit is produced and smoothed. The presence of near-stationary noise in the input signal is determined by examining the nature of the smoothed signal in each band assuming noise has a frequency spectrum which does not vary with time or varies only within a narrow range over a predetermined period of time with respect to the spectral parameters of the information signal. If noise is detected, the noise-analysis circuit identifies spectral parameters of the information and/or noise in each band using the smoothed signal therein. In the preferred embodiment of the invention, the bandpass filters of the noise-reduction circuit have gain elements that are adjusted in accordance with the identified parameters to minimize, under some continuous minimization criterion, the effect of the noise in the input signal thus enhancing intelligibility of the information therein. Minimization can be such that the gain-to-parameter relationships are similar to those in Weiner or Kalman filtering theory with a-priori knowledge of the noise, or of the noise and information, except that in this case, a-priori knowledge of the noise is acquired via identification and is not preassumed.

Computer-Aided Determination of Microwave Two-Port Noise Parameters

Abstract: The least-squares fit of measured noise figures as a function of source admittance is an accurate and rapid method also convenient from the experimental point of view, to determine linear two-port noise parameters. However, to avoid the erroneous results often obtained by experimenters, this paper presents some criteria to be followed in choosing the proper source admittances. In order to apply the method to microwave two-ports, a relationship relating noise parameters in a linearized form is introduced. The analytical developments are in terms of effective input noise temperature of the two-port. Experimental. results for a microwave transistor are also reported as a function of experimental redundancy.

Estimation of images degraded by film-grain noise

Abstract: Film-grain noise describes the intrinsic noise produced by a photographic emulsion during the process of image recording and reproduction. In this paper we consider the restoration of images degraded by film-grain noise. First a detailed model for the over-all photographic imaging system is presented. The model includes linear blurring effects and the signal-dependent effect of film-grain noise. The accuracy of this model is tested by simulating images according to it and comparing the results to images of similar targets that were actually recorded on film. The restoration of images degraded by film-grain noise is then considered in the context of estimation theory. A discrete Wiener filer is developed which explicitly allows for the signal dependence of the noise. The filter adaptively alters its characteristics based on the nonstationary first order statistics of an image and is shown to have advantages over the conventional Wiener filter. Experimental results for modeling and the adaptive estimation filter are presented.

On the choice of noise for the analysis of the peripheral auditory system

Abstract: The cross-correlation between output and input of a system containing nonlinearities, when that system is stimulated with Gaussian white noise, is a good estimate of the linear properties of the system. In practice, however, when sequences of pseudonoise are used, great errors may be introduced in the estimate of the linear part depending on the properties of the noise. This consideration assumes special importance in the analysis of the linear properties of the peripheral auditory system, where the rectifying properties of the haircells constitute a second order nonlinearity. To explore this problem, a simple model has been designed, consisting of a second order nonlinearity without memory and sandwiched between two bandpass filters. Different types of pseudonoise are used as input whereupon it is shown that noise based on binary m-sequences, which is commonly used in noise generators, will yield totally incorrect information about this system. Somewhat better results are achieved with other types of noise. By using inverse-repeat sequences the results are greatly improved. Furthermore, certain anomalies obtained in the analysis of responses from single fibers in the auditory nerve are viewed in the light of the present results. The theoretical analysis of these anomalies reveals some information about the organization of the peripheral auditory system. For example, the possibility of the existence of a second bandpass filter in the auditory periphery seems to be excluded.

Adaptive noise cancelling receiver

Abstract: A receiving system is described which has the capability of substantially cancelling broadband noise, such as impulse noise, atmospheric noise, electrical line noise and receiver front end noise, from a selected radio frequency passband. In a first embodiment, a desired radio frequency passband is selected and converted to a broadband intermediate frequency (IF) signal, which is applied in parallel to the inputs of first and second frequency channels. When broadband noise is present in the broadband IF signal, the first frequency channel develops a desired audio signal in the presence of an undesired first broadband noise signal, while the second frequency channel develops an undesired second broadband noise signal which is correlated with the first broadband noise signal. An adaptive transversal filter is responsive to the second broadband noise signal and to an output signal for adaptively developing an estimate of the first broadband noise. The estimate of the first broadband noise signal is subtracted from the combination of the desired audio signal and the first broadband noise signal in order to cancel out the first and second broadband noise signals and substantially develop only the desired audio signal as the output signal. When no broadband noise is present in the broadband IF signal, the adaptive filter automatically shuts itself off and the first frequency channel only develops the desired audio signal. Thus, regardless of whether or not broadband noise is present, the output signal is substantially comprised of only the desired audio signal.

Investigation of Interior Noise in a Twin-Engine Light Aircraft

Abstract: This paper describes experimental studies of interior noise in a twin-engine, propeller-driven, light aircraft. An analytical model for this type of aircraft is also discussed. Results indicate that interior noise levels in this aircraft due to propeller noise can be reduced by reducing engine rpm at constant airspeed (about 3 dB), and by synchrophasing the twin engines/propellers (perhaps up to 12 dB). Ground tests show that the exterior noise pressure imposed on the fuselage consists of a complex combination of narrow-band harmonics due to propeller and engine exhaust sources. This noise is reduced by about 20-40 dB (depending on the frequency) by transmission through the sidewall to the cabin interior. The analytical model described uses modal methods and incorporates the flat-side geometrical and skin-stringer structural features of this light aircraft.

Error rate predictions and measurements in the mobile radio data channel

Abstract: A method of predicting the loss in error performance in mobile radio data transmission is given in which the variation in local mean signal level (shadowing) is included as well as fading. Also, the density function of the received signal envelope is found for the case of fading and shadowing. Field measurements are reported of error rates at VHF and UHF due to vehicle ignition noise, and the distribution of errors is plotted.

Estimation-detection of object boundaries in noisy images

Abstract: Estimation of boundaries of objects in noisy images is considered when the objects and the background are statistically characterized. The noise is assumed white, additive, and Gaussian. Optimal recursive estimators in a joint estimation-detection context are derived. Applications to binary pictures are illustrated.

Application of the Bernoulli enthalpy concept to the study of vortex noise and jet impingement noise

Abstract: A complete theory of aeroacoustics of homentropic fluid media is developed and compared with previous theories. The theory is applied to study the interaction of sound with vortex flows, for the DC-9 in a standard take-off configuration. The maximum engine-wake interference noise is estimated to be 3 or 4 db in the ground plane. It is shown that the noise produced by a corotating vortex pair departs significantly from the compact M scaling law for eddy Mach numbers (M) greater than 0.1. An estimate of jet impingement noise is given that is in qualitative agreement with experimental results. The increased noise results primarily from the nonuniform acceleration of turbulent eddies through the stagnation point flow. It is shown that the corotating vortex pair can be excited or de-excited by an externally applied sound field. The model is used to qualitatively explain experimental results on excited jets.

Measurements of noise produced by flow past lifting surfaces

Abstract: Wind tunnel studies have been conducted to determine the specific locations of aerodynamic noise production within the flow field about various lifting-surface configurations. The models tested included low aspect ratio shapes intended to represent aircraft flaps, a finite aspect ratio NACA 0012 wing, and a multi-element wing section consisting of a main section, a leading edge flap, and dual trailing edge flaps. Turbulence was induced on the models by surface roughness. Lift and drag were measured for the flap models. Hot-wire anemometry was used for study of the flap-model vortex roll-up. Apparent noise source distributions were measured by use of a directional microphone system, located outside the tunnel, which was scanned about the flow region to be analyzed under computer control. These distributions exhibited a diversity of pattern, suggesting that several flow processes are important to lifting-surface noise production. Speculation concerning these processes is offered.

Low-noise microwave bipolar transistor with sub-half-micrometer emitter width

Abstract: This paper presents details of the fabrication process and performance of an n-p-n silicon microwave bipolar transistor with emitter opening widths as small as 0.3 µm. The fabrication process involves local oxidation, ion implantation, and lateral etching techniques for emitter definition. Noise figure as low as 1.0 dB at 1.5 GHz, 2.0 dB at 4 GHz, and 3.3 d B at 6 GHz were achieved. Measured noise figures and S -parameters are shown to be in approximate agreement with modeled performance based on device structure and process parameters. Prospects for further reductions in bipolar transistor noise figures are discussed.

An investigation of several frequency-domain processing methods for enhancing the intelligibility of speech in wideband random noise

Abstract: This paper describes results of a study of several frequency-domain processing methods for enhancing the intelligibility of speech in wideband random noise. Five categories of processing methods are explored. These include the INTEL technique, a technique based upon minimum mean square filtering, several techniques based upon subtraction of the estimated spectrum of the noise from the spectrum of the speech plus noise, spectrum squaring, and techniques based upon pitch frequency analysis. The results of this study have provided considerable insight into the individual processing methods and into the use of frequency-domain processing methods in general. A major conclusion of this work is that all successful techniques investigated are similar in that they are an attempt to emphasize spectral components as a function of the amount by which they exceed the noise. A second conclusion is that unless the spectral weighting within a time-window is relatively smooth, it will introduce conspicuous background distortion.

FM-noise measurements on an optically pumped FIR laser

Abstract: We report the noise measurements performed on a synthesized signal at 761 GHz starting from a 5 MHz quartz oscillator, as well as the investigation of the frequency noise of an optically pumped FIR laser and the possible origin of this noise. Bandwidth requirements for phase-locking an FIR laser are obtained.

Single-threshold detection of a random signal in noise with multiple independent observations. 1: Discrete case with application to optical communications.

Abstract: A single-threshold processor is derived for a wide class of classical binary decision problems involving the likelihood-ratio detection of a signal embedded in noise. The class of problems we consider encompasses the case of multiple independent (but not necessarily identically distributed) observations of a nonnegative (nonpositive) signal, embedded in additive, independent, and noninterfering noise, where the range of the signal and noise is discrete. We show that a comparison of the sum of the observations with a unique threshold comprises optimum processing, if a weak condition on the noise is satisfied, independent of the signal. Examples of noise densities that satisfy and violate our condition are presented. The results are applied to a generalized photocounting optical communication system, and it is shown that most components of the system can be incorporated into our model. The continuous case is treated elsewhere [ IEEE Trans. Inf. TheoryIT-25, (March, 1979)].

Suppression of noise in speech using the saber method

Abstract: A stand alone noise suppression algorithm is described for reducing the spectral effects of acoustically added noise in speech. A fundamental result is developed which shows that the spectral magnitude of speech plus noise can be effectively approximated as the sum of magnitudes of speech and noise. Using this simple phase independent additive model, the noise bias present in the short time spectrum is reduced by subtracting off the expected noise spectrum calculated during nonspeech activity. After bias removal, the time waveform is recalculated from the modified magnitude and saved phase. This Spectral Averaging for Bias Estimation and Removal, or SABER method requires only one FFT per time window for analysis and synthesis.

Identification of Power-Law Type Oscillator Phase Noise Spectra from Measurements

Abstract: In this paper we demonstrate how structure functions can be used to identify power-law type spectral density components 1/fv associated with oscillator phase noise measurements when polynomial type systematic drifts are present. The technique discussed is new and novel in that it avoids the usual errors that are introduced in the measurement due to removing systematic trends in the observed data by the usual estimation detrending techniques. Several applications of the theory are given and a convenient and expedient look-up table is provided for those who perform measurements. The paper begins by summarizing accepted frequency stability measures and places these into perspective with structure functions of oscillator instability. The paper ends by showing how the rms fractional frequency deviation differs from the Allan variance; numerical examples are given.

Nonlinearity Measurements Using Alternating Current

Abstract: The nonlinearity measurement technique described uses an adapted conventional lock-in-amplifier. This technique enables us to measure small nonlinearities over a wide frequency band and is more sensitive than the 1/f noise measurement used to detect non-homogeneous structures in conductors, resistors and semiconductor components. Results illustrating uses of this method are presented for different types of resistor and semiconductor structures.

Speech intelligibility enhancement in a power generating noise environment

Abstract: A previous paper described a method for processing speech which resulted in the enhancement of speech intelligibility in the presence of high noise levels at the listener. This processing technqiue consists of high-pass filtering followed by rapid amplitude compression. A significant intelligibility gain was achieved over unprocessed speech for signal-to-noise ratios from -10 to +10 dB (noise at 90 dB). In this previous work, only white noise was considered. In the present paper, results are presented for this processing technique for a listener in the environment of power generating noise.

Hybrid Estimators for Discrete-Time Stochastic Systems

Abstract: The problem of estimating the state of a linear discrete-time system from input-output data in the presence of input disturbance noise when some of the outputs are not obscured by output measurement noise is considered. The proposed hybrid estimator gives the linear least squares state estimate and has order less than the order of the system whose state is being estimated by the number of outputs which are not obscured by noise.

Characterization of Charge-Coupled Analog Memories for Nuclear Data Acquisition

Abstract: Analog memory charge-coupled devices (CCDs) have potential use in instrumentation for nuclear data acquisition, particularly for large detectors for high-energy physics experiments. Some measurements aimed at characterizing the Fairchild 321 for this application have been made. This includes measurements on linearity, noise, dark current and transfer inefficiency. The results are given together with the measurement techniques used.

A method for calculating externally blown flap noise

Abstract: Several basic noise components were described. These components are: (1) compact lift dipoles associated with the wing and flaps; (2) trailing edge noise associated with the last trailing edge; and (3) quadrupole noise associated with the undeflected exhaust jet and the free jet located downstream of the trailing edge. These noise components were combined to allow prediction of directivity and spectra for under the wing (UTW) slotted flaps with conventional or mixer nozzles, UTW slotless flaps, upper surface blowing (USB) slotless flaps, and engine in front of the wing slotted flaps. A digital computer program listing was given for this calculation method. Directivities and spectra calculated by this method were compared with free field data for UTW and USB configurations. The UTRC method best predicted the details of the measured noise emission, but the ANOP method best estimated the noise levels directly below these configurations.

Modal content of noise generated by a coaxial jet in a pipe

Abstract: Noise generated by air flow through a coaxial obstruction in a long, straight pipe was investigated with concentration on the modal characteristics of the noise field inside the pipe and downstream of the restriction. Two measurement techniques were developed for separation of the noise into the acoustic duct modes. The instantaneous mode separation technique uses four microphones, equally spaced in the circumferential direction, at the same axial location. The time-averaged mode separation technique uses three microphones mounted at the same axial location. A matrix operation on time-averaged data produces the modal pressure levels. This technique requires the restrictive assumption that the acoustic modes are uncorrelated with each other. The measured modal pressure spectra were converted to modal power spectra and integrated over the frequency range 200-6000 Hz. The acoustic efficiency levels (acoustic power normalized by jet kinetic energy flow), when plotted vs. jet Mach number, showed a strong dependence on the ratio of restriction diameter to pipe diameter. The acoustic energy flow analyses based on the thermodynamic energy equation and on the results of Mohring both resulted in orthogonality properties for the eigenfunctions of the radial mode shape equation. These orthogonality relationships involve the eigenvalues and derivatives of the radial mode shape functions.

Improvement of autoregressive spectral estimates in the presence of noise

Abstract: The autoregressive (maximum entropy, linear prediction) power spectral density estimator has been shown to possess excellent resolution properties. However, the addition of noise to the time series under analysis may drastically alter the spectral estimate. To reduce the effects of noise, an adaptive filtering algorithm is proposed that is directly applicable to sinusoidal signals in white noise. Its use for more general signals is discussed.

Linear predictive digital filtering of narrowband processes in additive broadband noise

Abstract: : Linear prediction filters (LPF) have recently found a large number of applications in such areas as speech analysis and power spectral estimation. An important application of these filters is to aid the cancellation or additive broadband noise from narrowband signal components. Provided the signal bandwidth is significantly less than the noise bandwidth, the LPF may be used to suppress the additive noise without requiring an external reference noise input (as is required in many noise cancelling applications). The purpose of this report is to provide an analytical basis for bounding the performance of a digital LPF when applied to the problem of cancelling broadband additive noise from narrowband signals. Experimental results obtained with a hardware implementation of an adaptive Wiener filter are shown to verify the analytical results. (Author)

Hydrophone Preamplifier Optimization. Prediction of Hydrophone Self- Noise by a Noise Model

Abstract: : This report presents analysis of the self-noise which determines the threshold pressure level in hydrophone designs. The noise present in the acoustic sensor, coupling network, and preamplifier input stage are represented by an equivalent circuit noise model. A model provides an effective method to predict in advance of sensor-preamplifier construction the noise levels expected for a specific design.