Showing papers on "Noise reduction published in 1986"

Speckle suppression and analysis for synthetic aperture radar images

Abstract: Speckle appearing in synthetic aperture radar (SAR) images is generated by coherent interference of radar echoes from target scatters. Basically, speckle noise has the nature of a multiplicative noise. In this paper procedures for defining and verifying a statistical noise model are developed, and two multiplicative noise-smoothing algorithms are pre-sented. These two algorithms are computationally efficient and have the potential of achieving real-time or near-real-time processing. Several SEASAT SAR and SIR-B (Shuttle Imaging Radar) images are used for illustration.

Nonlinear space-variant postprocessing of block coded images

Abstract: An important application of spatial filtering techniques is in the postprocessing of images degraded by coding. Linear, space-invariant filters are inadequate to reduce the noise produced by block coders. The noise in block coded images is correlated with the local characteristics of the signal, and such filters are unable to exploit this correlation to reduce the noise. We propose a new nonlinear, space-variant filtering algorithm which smooths jagged edges without blurring them, and smooths out abrupt intensity changes in monotone areas. Edge sharpness is preserved because near edges the filtering of the signal is negligible. Consequently, in-band noise is not reduced, but the well-known masking effect reduces the visibility of this in-band noise. The algorithm is only slightly more complex to implement than simple linear filtering. We present examples of processed images and SNR figures to demonstrate that a significant improvement in subjective and objective quality is achieved.

Adaptive microphone-array system for noise reduction

Abstract: This paper introduces a new adaptive microphone-array system for noise reduction (AMNOR system). It is first shown that there exists a tradeoff relationship between reducing the output noise power and reducing the frequency response degradation of a microphone-array to a desired signal. It is then shown that this tradeoff can be controlled by the introduction of a fictitious desired signal. A new optimization criterion is presented which minimizes the output noise power while maintaining the frequency response degradation below some pre-determined value (AMNOR criterion). AMNOR determines an optimal noise reduction filter based on this criterion by controlling the tradeoff utilizing the fictitious desired signal. Experiments on noise reduction processing were carried out in a room with a 0.4-s reverberation time. The superiority of the AMNOR criterion over conventional LMS and constrained LMS criteria for reducing noise in speech signals was confirmed in subjective preference tests. The AMNOR system improved the SNR by more than 15 dB in the 300-3200 Hz range.

A new application of adaptive noise cancellation

Abstract: A new application of Widrow's adaptive noise cancellation (ANC) is presented in this paper. Specifically, the method is applied to the case where an acoustic barrier exists between the primary and reference microphones. By updating the coefficients of the noise estimation filter only during silence, it is shown that ANC can provide substantial noise reduction with little speech distortion even when the acoustic barrier provides only moderate attenuation of acoustic signals. The use of the modified ANC method is evaluated using an oxygen facemask worn by fighter aircraft pilots. Experiments demonstrate that if a noise field is created using a single source, 11 dB signal-to-noise ratio improvements can be achieved by attaching a reference microphone to the exterior of the facemask. The length of the ANC filter required for this particular environment is only 50 points.

Roundoff noise minimization and pole-zero sensitivity in fixed-point digital filters using residue feedback

Abstract: The problem of designing a finite wordlength fixed-point realization of an Nth-order digital filter, which uses residue feedback to minimize the output roundoff noise subject to l 2 -scaling, is considered. The new structures require N extra additions, but no more multiplications than the earlier low noise structures of Mullis and Roberts, and have lower roundoff noise for sufficiently narrow bandwidth filters. A new set of filter invariants, called the residue modes, are defined which characterize the new low noise structures and determine the output noise variance. If the sum of the residue modes is less than the sum of the second-order modes of Mullis and Roberts, then lower roundoff noise is achieved. Every filter structure is shown to define a unique (symmetrizing) matrix Q. Pole-zero sensitivities and a new noise measure are defined in terms of Q. Numerical results are included which compare the noise and pole-zero sensitivity characteristics of different filter structures.

Noise reduction system

Abstract: A noise reduction system and method for reducing noise occurring during pauses in information bearing audio signals has an audio input connected to one input of a differential output circuit. A variable gain circuit has an input connected to the audio input and an output connected to the other input of the differential circuit. A dc control signal is derived from the incoming signals and controls the variable gain circuit to cause cancellation of a noise only signal during periods of no information bearing audio signals.

Parametric projection filter for image and signal restoration

Abstract: A class of linear algebraic restoration filters is derived for the linear degradation model with additive noise. The filters are based on an optimization criterion involving two effects: first, the error in the restored noiseless image compared to the original image, and second, the energy of additive noise passed through the restoration. These two effects can be balanced with a scalar parameter. For both error types, explicit expressions are derived in terms of the parameter. It is shown that by allowing the first error norm to grow slightly, the noise energy may be considerably reduced. This also has a bearing on the analysis of the behavior of other linear parametric restoration filters.

Optimum PWM Waveforms of an Inverter for Decreasing Acoustic Noise of an Induction Motor

Abstract: Pulsewidth modulation (PWM) waveforms for decreasing the acoustic noise of an induction motor are presented and relate to earlier work concerning the loss of minimum PWM waveforms. The proposed control scheme combines merits of PWM control with those of pulse amplitude modulation (PAM) control to achieve silent operation of the induction motor. Calculation of the optimum PWM waveforms is complicated because the function representing the PWM wave contains many variables to converge and many modes in initial values. For example, when the pulse number of the PWM wave is 225, the number of variables is 112 and the initial values would be about 4 x 10 23 . Therefore, to get the optimum PWM wave, the convergence calculation of 112 variables using steepest-hill-gradient procedure must be tried with every mode of the waveforms. It is shown how to solve such difficult problems, and examples are presented of solved PWM waveforms. Through use of experimental means, acoustic noise is measured and better noise reduction of 10-15 dB can be achieved, compared with the noise of PAM operation.

Phase-noise reduction in coherence-multiplexed interferometric fibre sensors

Abstract: A method for the reduction of excess phase noise in coherence-multiplexed fibre sensors is described. The technique utilises diode laser frequency modulation to effect a frequency translation of the excess noise power. An improvement close to 40 dB in minimum detectable phase shift sensitivity is demonstrated using the technique.

Optimum conditions for the high frequency noise reduction method in optical videodisc players.

Abstract: In a previous paper, the authors demonstrated the usefulness of the high frequency (HF) current superposition noise reduction method. This paper discusses optimum conditions for the HF method. The main results obtained are that modulation current frequency must be higher than 600 MHz and modulation depth must be >120%. The optical path length between laser and disk and the removal for interference in the signal detectors must also be taken into consideration.

Controlling image signals in an imaging apparatus

Abstract: Thermal imaging apparatus is provided with a comparator and a digital counter for counting the number of pixels in the scene within certain defined brightness bands. Utilizing this data, a microprocessor controls gain, level, gamma correction and integration for noise reduction.

Model-Based Motion Estimation and Its Application to Restoration and Interpolation of Motion Pictures.

Abstract: : This report is concerned with the problem of motion estimation from digitally sampled motion pictures. Several models are developed that describe object motion with velocity fields. Given an image sequence, the velocity field is underconstrained and therefore cannot be determined uniquely. However, by imposing structural constraints on the velocity field in the form of a parametric model, it is possible to determine the model parameters uniquely. This report describes parametric models which form the basis for two motion estimation algorithms. Experimental results demonstrate that these algorithms determine velocity fields more accurately than conventional region matching methods. One of the algorithms, based on the least squares error criterion, also has the desirable property of being computationally efficient. To demonstrate the performance of the least square motion estimation algorithm, a motion-compensated noise reduction system was implemented. Experiments demonstrate that the motion-compensated noise reduction system can yield better results than conventional restoration methods. A motion-compensated frame interpolation system, also implemented, permits frame rate conversion by arbitrary rates.

Aircraft propeller noise reduction

Abstract: In one form of the invention, a counterrotating pair of aircraft propellers produces noise in the form of a frequency-modulated carrier wave The frequency modulation allows a designer to manipulate the energy spectrum of the noise in order to, for example, place much of the acoustic energy into inaudible frequency ranges

Coding apparatus for moving object image

Abstract: A coding apparatus for a moving object image input signal such as a television signal detects an uncovered background area appearing upon movement of a moving object in the moving object image input signal and encodes an image signal corresponding to the uncovered background area according to intraframe correlation instead of interframe correlation. The coding apparatus includes a noise reduction circuit for decreasing a noise reduction level for the signal corresponding to the uncovered background area.

Noise control characteristics of synchrophasing. II - Experimental investigation

Abstract: A simplified cylindrical model of an aircraft fuselage is used to investigate the mechanisms of interior noise suppression of the synchrophasing technique. This investigation allows isolation of important parameters to define the characteristics of synchrophasing. The optimum synchrophase angle for maximum noise reduction is found for several interior microphone positions with pure tone source conditions. Noise reductions of up to 30dB are shown for some microphone positions, however, overall reductions are less. A computer algorithm is developed to decompose the modal composition of the cylinder vibration over a wide range of synchrophase angles. The circumferential modal response of the shell vibration is shown to govern the transmission of sound into the cylinder rather than localized transmission.

Transmission Profile Filtering for Positron Emission Tomography

Abstract: Attenuation correction in PET performed by the transmission measurement method provides accurate compensation but increases image noise. Transmission profile filtering is evaluated in terms of image noise reduction as a trade-off against systematic error introduced by the filter itself. Phantom measurements performed on the UBC/TRIUMF PETT VI tomograph are used to evaluate filter performance. It is found that filtering with a Gaussian of 12.5 mm FWHM reduces the image variance due to noise in transmission profiles by a factor of 15.

Noise controlled immunoassays

Abstract: A method for reducing false positives and to assay background noise in solid phase immunoassays utilizes a matrix coated on a solid support, which matrix contains an effective amount of a noise reduction component and an effective amount of noise balancing component. Optimization of the matrix composition is obtained by measuring parameters for its effectiveness which include sensitivity ratio, noise balance ratio, and signal to noise ratio.

Improvements relating to noise reduction arrangements

Abstract: An active noise reduction (ANR) arrangement for reducing acoustic noise in an earphone structure (1) includes an automatic gain control arrangement (14-20) providing a variable loop gain dependent upon the variable noise reduction which is produced in the earphone (1) by the active noise reduction arrangement and which is effectively measured by noise pick-up microphones (5 and 13) located, respectively, in earphone structure front and rear internal cavities (11 and 12) positioned in front of and at the rear of a noise-cancelling transducer diaphragm (8,9) and providing a signal related to the measured noise reduction.

Radiator scanning with image enhancement and noise reduction

Abstract: An imaging system incorporating a scanned array of infrared detectors for presentation of scene data on a display includes gain normalization circuitry coupled to individual ones of the detectors of the detector array. The normalization circuitry is structured as a feedback loop introducing euality between the magnitude of an average value of a detector signal and a reference signal. The imaging system comprises a set of channels connecting with respective ones of the detectors, each channel having adaptive gain control circuitry. A common value of the reference signal is applied to each channel for uniformity in a displayed image. The averaging time is several times greater than the duration of a single scan in the scanning of the detector array so as the display is relatively free of nonuniformities associated with differing responsivities of the detectors to infrared radiation. A nonlinear gain function is produced by connection of multipliers and/or logarithmic circuitry in a forward branch of the loop. A feed forward branch includes a filter coupled to the multipliers for adaptive accentuation of higher frequencies of the signal spectrum to enhance subject edge lines. A noise reduction circuit reduces a maximum value of gain for low incremental values of detector signal to exclude noise where signal data is substantially absent.

Voice communications in the cockpit noise environment : the role of active noise reduction

Abstract: This thesis addresses the topic of voice communications in the cockpit noise environment of modern `fast-jet' aircraft and helicopters, and in particular, describes research undertaken in support of the development of a system for reducing the noise level at the operators' ear by acoustic cancellation within the ear defender, known as Active Noise Reduction or ANR. The internal noise spectra of today's high performance `fast-jet' aircraft and military helicopters is described, and the complex interaction of acoustic noise transmission, speech, and microphone noise pick-up, which produces the total acoustic environment at the aircrews' ears, is discussed. Means of mathematically modelling the audio channel, quantifying the components identified above, and identifying areas of shortfall in performance are derived, leading to a procedure for the development of attenuation requirements, described as the communications audit. A model of the electroacoustic characteristics of the ANR ear defender assembly is presented and the sound field distribution within the ear defender/ear cavity, and its effect upon cancellation performance, is discussed. The extensive laboratory and flight testing of the ANR system that has been undertaken is reviewed, paying particular attention to the measurement and analysis techniques employed in such testing. Finally, the performance characteristics of ANR are discussed and compared with the requirements previously established. Design limitations placed upon the system by the constraints of its area of application are described, and the scope for future improvements is considered.

Noise reduction circuit for video tape recording and playback apparatus

Abstract: Disclosed is a noise reduction circuit for reducing noise contained in a luminance signal, wherein a first subtractor has a first input terminal connected to be responsive to the luminance signal. A low-pass filter passes a low-frequency component of the output of the first subtractor to a second subtractor which combines it with the luminance signal to extract high-frequency noise contained therein. The extracted noise is limited in amplitude by a limiter and applied through a feedback circuit to the second input terminal of the first subtractor. The noise reduction circuit operates as if it were a low-pass filter having a cutoff frequency which increases as a function of the level of the luminance signal to reduce the length of time in which smear and noise tend to occur in response to high amplitude luminance signals.

A switched differential op-amp with low offset and reduced 1/f noise

Abstract: Fully differential techniques are becoming a popular method to alleviate the problem of low dynamic range in an analog VLSI environment. Low-frequency (1/f) noise contributions and input offset voltage remain, however, as two inherent difficulties in MOS analog designs. In this paper, a dynamic technique for implementing low-noise low-offset differential op-amps by time-sharing conventional single-ended output op-amps is introduced. A differential op-amp (DOA) implemented using this technique requires relatively little redesign effort, provides both positive and negative differential inputs, and demonstrates significant reduction in 1/f noise and input offset voltage over its single-ended counterpart. The op-amp is applicable in many sampled-data networks, such as in the design of SC filters, and can be easily interfaced with existing single-ended or differential networks.

Noise reduction system for video signal

Abstract: A noise reduction system for reducing noise in a video signal comprises a pre-emphasis circuit (21) for subjecting the video signal which is supplied to a transmission system to a pre-emphasis in a vertical direction of a picture and in a direction oblique to the vertical direction, and a de-emphasis circuit (17) for subjecting the pre-emphasized video signal obtained through the transmission system to a de-emphasis complementary to the pre-emphasis.

Error feedback in a class of orthogonal polynomial digital filter structures

Abstract: A method of applying error feedback in three digital ladder and lattice filter structures derived from the theory of orthogonal polynomials is proposed. It is shown that error feedback leads to significant improvement in both the roundoff noise characteristics and limit cycle behavior. In particular, the error-compensated one-multiplier structure is superior to the (uncompensated) normalized ladder. This results in significant savings in both hardware and Speed of implementation.

Two Integrated Progressive Scan Converters

Abstract: Before explaining the application and implementation of the two scan converters in detail, a brief description will be given of the digital environment in which these converters can be applied. Some general aspects of progressive scan and its impact on vertical bandwidth will also be discussed.

Method and circuit arrangement for reducing noise in digitised television signals

Abstract: A noise reduction of digitised television signals should become effective even with movements in the image content and not perceptibly impair the image sharpness. An estimated-value signal for the noise component is calculated (6) from the difference (3) of a frame-to-frame comparison (1, 5) and the difference (4) between the noisy input signal (1) and the calculated estimated-value signal (6) is formed which supplies the reduced-noise output signal (2) which is stored for subsequent calculations (5).