信噪比后滤波与特征空间融合的最小方差超声成像算法

为了提高超声成像空间分辨率和对比度,提出了一种信噪比后滤波与特征空间融合的最小方差波束形成算法。首先,利用信号子空间划分将最小方差算法得到的权矢量投影到信号子空间中提高成像对比度,然后基于信号相干性设计滤波系数,并引入基于信噪比的噪声加权系数,最终得到融合信噪比后滤波与特征空间的最小方差算法。为验证本算法的有效性,使用FieldⅡ对点目标和吸声斑目标进行了仿真实验验证,并采用密歇根大学geabr_0实验数据进行成像。实验结果表明:所提算法在对比度和分辨率上均有所提高,明显优于传统延时叠加算法,最小方差算法和ESBMV_wiener算法,且对噪声具有较强鲁棒性。      

矢量泰勒级数特征补偿的说话人识别

将矢量泰勒级数(Vector Taylor Series,VTS)特征补偿算法应用于说话人识别,给出了卷积噪声方差的近似闭式解,构建了联合快速估计卷积噪声和加性噪声均值和方差的框架。该算法可在无需失配环境先验信息的前提下,直接从失配语音中估计出卷积噪声和加性噪声的均值和方差,实现对环境失配的补偿。实验结果表明,在信道变化较大的无线信道下,卷积噪声方差的补偿最高可降低误识率3.24%.提升了系统的识别性能。在存在加性噪声的无线信道下,与基于线性失真模型的特征映射算法和倒谱均值减算法相比,本文算法可分别最大降低49.65%和68.06%的误识率,适合于信道变化较大的失配环境补偿。      

Investigation of noise properties in grating-based x-ray phase tomography with reverse projection method

The relationship between noise variance and spatial resolution in grating-based x-ray phase computed tomography(PCT) imaging is investigated with reverse projection extraction method, and the noise variances of the reconstructed absorption coefficient and refractive index decrement are compared. For the differential phase contrast method, the noise variance in the differential projection images follows the same inverse-square law with spatial resolution as in conventional absorption-based x-ray imaging projections. However, both theoretical analysis and simulations demonstrate that in PCT the noise variance of the reconstructed refractive index decrement scales with spatial resolution follows an inverse linear relationship at fixed slice thickness, while the noise variance of the reconstructed absorption coefficient conforms with the inverse cubic law. The results indicate that, for the same noise variance level, PCT imaging may enable higher spatial resolution than conventional absorption computed tomography(ACT), while ACT benefits more from degraded spatial resolution. This could be a useful guidance in imaging the inner structure of the sample in higher spatial resolution.     

乘性二次噪声驱动的线性过阻尼振子的随机共振

针对乘性二次噪声和加性周期调制噪声联合驱动的线性过阻尼振子, 利用随机平均法推导了系统响应的一阶、 二阶稳态矩以及稳态响应振幅和方差的解析表达式. 理论分析和仿真实验均表明这类系统具有比传统的由线性噪声驱动的线性系统更丰富的动力学特性; 当二次噪声的系数满足一定条件时, 系统稳态响应的振幅及方差均存在广义随机共振现象.     

Resonance effects in the nonadiabatic nonlinear quantum dimer

In the presence of 1/f ^β noise, we investigate the logical stochastic resonance (LSR) in an asymmetric bistable model driven by various cycling combinations of two logic inputs. The probability of correct logic outputs is calculated according to true table of logic relationships. Two major results are presented. Firstly, it is shown that the LSR effect can be obtained by changing noise strength. Over entire range of noise variance, white noise can be considered to be better than 1/f noise or 1/f ² noise to obtain clean logic operation. At a smaller noise level, 1/f noise can realize higher output probability than white noise or 1/f ² noise. In the sense, 1/f noise can be considered to be better than white noise or 1/f ². On the other hand, the correct probability can evolves nonmonotonically as noise exponent β increases, and a kind of SR-like effect can be obtained as a result of β. At certain intermediate noise variance, the output probability is able to attain its minimum at β = 1. It is also shown that actually some finite β sometime can be better than β = 0 at small range of noise variance. The study might provide some potential complement to LSR effect in the presence of 1/f ^β noise.     

光纤陀螺随机游走分析方法研究

 光纤陀螺测试中,随机游走系数的分析对陀螺整体性能的评价起着很重要的作用。分别介绍了分析光纤陀螺随机游走噪声的两种方法：Allan方差法和改进的Allan方差法。分别应用这两种方法,在对某型光纤陀螺(FOG)实测静态数据计算的基础上进一步分析了该光纤陀螺中的噪声分量,并对分析结果进行比较。最终计算结果表明：在增加计算时间为代价的前提下,改进Allan方差法能方便地区别出各种噪声,并明显提高了光纤陀螺各随机噪声游走系数的计算精度。     

Prebifurcational noise rise in nonlinear systems

The phenomenon of prebifurcational noise increase in nonlinear systems in the process of period-doubling bifurcation is investigated. The study is conducted for a discrete system (quadratic mapping); how-ever, many of the laws discovered apply to more general systems. Estimates of the fluctuation variance are obtained both for the linear (away from the bifurcation threshold) and for the nonlinear mode (in the vicinity of the bifurcation threshold). It is shown that the variance of forced fluctuations in the strongly nonlinear mode is proportional to the root-mean-square of the noise intensity rather than to the variance. The possibility of measuring the noise in nonlinear systems on the basis of the prebifurcational noise amplification factor is demonstrated.     

Noise estimation in parallel MRI: GRAPPA and SENSE

Parallel imaging methods allow to increase the acquisition rate via subsampled acquisitions of the k-space. SENSE and GRAPPA are the most popular reconstruction methods proposed in order to suppress the artifacts created by this subsampling. The reconstruction process carried out by both methods yields to a variance of noise value which is dependent on the position within the final image. Hence, the traditional noise estimation methods – based on a single noise level for the whole image – fail. In this paper we propose a novel methodology to estimate the spatial dependent pattern of the variance of noise in SENSE and GRAPPA reconstructed images. In both cases, some additional information must be known beforehand: the sensitivity maps of each receiver coil in the SENSE case and the reconstruction coefficients for GRAPPA.     

Transient Properties of One-Dimensional Stochastic Processes: Unified-Colored-Noise Approximation

By virtue of the unified-colored-noise approximation the formulas of mean, variance and skewness of first-passage-time distributions (FPTD) for the systems driven by colored noise are derived. The mean and variance of FPTD of bistable system driven by colored noise are calculated. The approximation expressions of the mean, variance and skewness of FPTD for the colored cubic model of a dye laser are obtained. The role of the "color" of pump noise in the dye laser transient is analyzed.     

Wave propagation in a one-dimensional randomly perturbed periodic medium

We study the variance of the solution of a periodic randomly perturbed one-dimensional Schrödinger operator after propagation through N periods. It is shown that if the frequency of propagation lies inside the band, then the total variance is proportional to Nσ², where σ is the intensity of the white noise. However, if the wave frequency is close to the band edge (where the transfer matrix has a Jordan block structure), the resulting variance is proportional to Nσ^2/3. Thus, propagation becomes highly sensitive to random perturbations.

Numerical simulations reveal that even low noise in a periodic potential can suppress transmission near the band edges and make it strongly irregular inside the band. Further increase of the noise amplitude leads to intermittent behaviour of the transmission coefficient, and makes transmission possible only for a few random frequencies in the band.     

Features of temporal noise spatial localization while indexing in accordance with the 2D hadamard transform

It is shown that the 2D indexation of a communication channel 1D noise with the help of the 2D Hadamard transform makes it possible to represent the distribution of this noise in the restored image space in the form of a 2D distribution of its variance. The width of noise variance distribution depends on the extent of preservation of correlation between elements of communication channel 1D noise. It is shown that the degree of 2D spatial localization of noise in this case exceeds the degree of noise localization in the case when 1D Hadamard transform is used.     

A subpixel motion estimation algorithm based on digital correlation for illumination variant and noise image sequences

One of the challenges in practical subpixel motion estimation is how to obtain high accuracy with sufficient robustness to both illumination variations and additive noise. Motivated by the fact that the normalized spatial cross-correlation is invariant to illumination, we introduce a gradient-based subpixel registration method by maximizing the digital correlation (DC) function between the reference and target frames. Such DC function is remodeled with the presence of image noise, yielding that the correlation coefficient is only sensitive to noise standard variance. To fairly suppress the noise corruption, not only the target frame but also the reference one is reformulated into Taylor gradient expression with half but opposite motion vector. The final solution to motion estimates can be approximated into a closed form by reserving first-order coefficient terms of unregistered motion variables. The error trend of approximated solution is discussed. Computer simulations and actual experiments’ results demonstrate the superiority of the proposed method to the LMSE-based method and ordinary DC method when illumination variations and noise exist. Among the experiments, the influences of real subpixel translation value and noise variance degree on accuracy are studied; correspondingly, an optimized iterative idea for big translations and the recommended noise level adaptive to our method are introduced.     

单模激光系统线性化近似适用范围的分析

采用线性化近似方法计算了具有实虚部关联的量子噪声和泵噪声驱动的单模激光损失模型的光强相对涨落,为了对线性化近似的适用范围进行详细的分析,分别讨论了量子噪声强度、泵噪声强度、量子噪声实虚部间关联系数对光强相对涨落的影响,得出在小噪声、远离阈值时,线性化近似适用范围扩大;小噪声、远离阈值且当量子噪声实虚部无关联时,线性化近似适用范围极大的结论.     

一种估计半导体激光器1/f噪声参数的新方法

对含白噪声的1/f分形信号小波变换系数的方差随尺度变化的关系进行适当的变换,提出了一种基于最小二乘法的估计半导体激光器1/f噪声参数的新方法.实验表明,该方法可以有效地提取出淹没在白噪声中的激光器1/f噪声,而且估计出的噪声信号的功率谱与对比仪器的测量结果有较好的一致性. 关键词： 半导体激光器 f噪声')" href="#">1/f噪声 参数估计 小波分析     

基于Allan方差的光纤陀螺随机漂移建模与仿真

随机漂移是影响光纤陀螺精度的主要因素之一,建立陀螺随机漂移模型以便在滤波时加以修正是提高系统精度的有效方法。针对传统随机漂移模型建模耗时长、过敏感等问题,提出基于Allan方差的光纤陀螺随机漂移模型。通过各噪声项的功率谱密度函数推导出随机微分方程,用Allan方差分析出光纤陀螺各噪声项量化参数,将量化参数代入以单位白噪声驱动的随机微分方程,得到随机漂移模型。实验结果表明,该模型拟合出的随机漂移单项噪声误差不超过8.6％,远低于传统模型产生的单项噪声误差58.3％,是一种有效的光纤陀螺随机漂移建模方法。     

Local polarization fluctuations in an aging glass

Polarization fluctuations were measured in nanoscale volumes of a polymer glass during aging following a temperature quench through the glass transition. Statistical properties of the noise were studied in equilibrium and during aging. The noise spectral density had a larger temporal variance during aging; i.e., the noise was more non-Gaussian, suggesting stronger correlations during aging.     

FTIR噪声修正方法用于提高挥发性有机物定量精度

经典最小二乘法(CLS)是傅里叶红外光谱定量分析软件中的一种常用算法。统计学中,当待测信号中的误差为正态分布零均值等方差时,CLS法可以得到最优无偏估计,但对异方差误差则不然。傅里叶红外光谱仪(FTIR)吸光度谱中的噪声相当于异方差误差,通过对噪声源的理论分析和实验统计计算获得了噪声的方差分布,并以此对CLS方法进行加权修正。计算表明,在分析气体中挥发性有机化合物(VOCs)污染浓度时,相比CLS方法, 采用仪器噪声方差分布修正后的加权最小二乘法(WLS)可以显著降低噪声对定量精度的影响,提高了分析结果的准确性和可靠性。     

Suppression of noise in a noisy optical trap

We have experimentally explored and clarified the concept of noise suppression in intrinsically noisy systems by adding noise at the input using a microscopic bead held in a moving and intensity-modulated optical trap. By coupling the stiffness of the optical trap to its position, we have explicitly constructed an experimental model system in which added fluctuations in the trap position result in reduced variance of the observed bead position as compared to a stationary trap. This reduction in variance and the spectral properties of the observed output noise agree with theoretical predictions. Our experiment demonstrates that the essential aspect of noise reduction in such a system is that the added fluctuations drive the system into states with a reduced intensity of intrinsic noise sufficiently often.     

Evolution of Robustness and Plasticity under Environmental Fluctuation: Formulation in Terms of Phenotypic Variances

The characterization of plasticity, robustness, and evolvability, an important issue in biology, is studied in terms of phenotypic fluctuations. By numerically evolving gene regulatory networks, the proportionality between the phenotypic variances of epigenetic and genetic origins is confirmed. The former is given by the variance of the phenotypic fluctuation due to noise in the developmental process; and the latter, by the variance of the phenotypic fluctuation due to genetic mutation. The relationship suggests a link between robustness to noise and to mutation, since robustness can be defined by the sharpness of the distribution of the phenotype. Next, the proportionality between the variances is demonstrated to also hold over expressions of different genes (phenotypic traits) when the system acquires robustness through the evolution. Then, evolution under environmental variation is numerically investigated and it is found that both the adaptability to a novel environment and the robustness are made compatible when a certain degree of phenotypic fluctuations exists due to noise. The highest adaptability is achieved at a certain noise level at which the gene expression dynamics are near the critical state to lose the robustness. Based on our results, we revisit Waddington’s canalization and genetic assimilation with regard to the two types of phenotypic fluctuations.     

Cross-correlation procedures for measuring noise and distortion in AGC hearing aids

The magnitude-squared coherence function (MSC) has been used to measure noise and distortion in linear and compression hearing aids. However, the MSC will overestimate the distortion in a linear time-varying system such as a compression amplifier. The reduction in coherence caused by varying the gain in an otherwise linear system can be substantial, and can lead to large errors in estimating the distortion present in a compression hearing aid. The effects of gain changes in a linear system can be reduced by measuring the normalized system input-output cross correlation, which emphasizes the variance in the system phase response and deemphasizes the system gain fluctuations. Estimates of the total noise and distortion produced using the MSC, phase variance, and notched-noise measurement techniques are compared for additive noise, clipping distortion, and compression amplification. The MSC is found to give the most accurate results for estimating the noise and distortion in a linear time-invariant system, and the notched noise measurements are the most accurate for a compression system. The phase variance is found to give reasonable measurements for a time-varying gain as long as the system variations are slow relative to the length of the analysis data segments.