IP电话回声消除器的设计及其DSP实现

介绍了一种用于IP电话中的自适应回声消除器，采用归一化最小二乘（NLMS）自适应滤波器实现，包括语音模式检测器和粗略时延估计器。最后以TI公司的TMS320C5402 DSP芯片为平台，实现了该回声消除器，学对关键代码进行了分析。     

IP电话回声消除器中自适应滤波器的研究

介绍了一种用于IP电话的自适应回声消除器，采用LMS自适应滤波器实现。首先通过Matlab进行LMS算法的步长选择，并用生成的符合G．168要求的带限CSS测试信号对回声消除器的性能进行了测试。     

VoIP回声消除器设计及算法研究

本文提出了一种与线性预测编解码器相结合的新声学回声消除器,由去相关可变步长的NLMS自适应算法和基于回声路径失配方差的双端通话检测算法所组成.Matlab仿真结果表明,与Gordy所提出的回声消除算法相比,本文提出的算法在双端通话和回声路径改变时判别更准确,收敛速度更快;在收敛状态时,ERLE值平均提高了15dB,失调误差平均降低了10dB,具备更好的回声消除性能.     

与VoIP声码器结合的回声消除器

NLMS算法是回声消除器中最常用的算法之一,然而语音信号的强相关性使NLMS(归一化最小均方)算法的收敛速度变慢。鉴于VoIP(网络电话)常用的低速率声码器大多基于LP(线性预测)编码,给出了一种与声码器结合的回声消除器,通过利用从解码端获得的LP系数和激励信号,省去了对去相关滤波器系数的计算,并且不再需要专门的前置滤波器为远端信号去相关,同时又改善了回声消除器性能。仿真结果表现出更高的收敛速度和回声抵消量。     

VoIP声学回声消除算法研究

对回声消除技术原理进行了阐述,介绍自适应回声消除器基本结构,并分析了speex中的频域回声消除算法MDF,针对MDF算法提出了一种使估计回声和录音输入之间进行的同步方法;最后,以Speex开源项目为基础进行了测试.结果表明,该方法比现有的双段讲话检测(DTD)算法效果更好,实现也更为简单.     

VoIP电话中的回声消除算法

声学回声是降低VoIP通话通信质量的重要问题之一,自适应回声抵消是抑制回声的最有效方法之一,其采用自适应滤波器评估回声路径。常用的NLMS(Normalized Least Mean Square)算法计算复杂度高,实用性差,本文利用FFT技术,实现了NLMS频域快速算法FDNLMS(Frequency Domain Normalized Least Mean Square),将自适应更新变换到频域,逐块进行累加更新,保证收敛性能的同时,极大的降低了运算复杂度。实验表明,在滤波器系数为1024阶时,FDNLMS算法的处理速度比NLMS快12倍。     

回声消除基于NLMS改进算法的研究

为了提高VoIP的通信质量,减少回声干扰,对LMS算法、NLMS算法进行阐述,基于NLMS提出了一种运算量小并且提高收敛性能的改进的自适应滤波算法。通过在Matlab下的仿真研究和对误差曲线的分析,证明了该改进算法的收敛速度快,均方误差小。用改进的算法对语音回声信号进行消除,仿真得到消除回声后的信号效果明显,为IP电话中回声消除的自适应滤波问题提供了一个较好的算法。     

基于嵌入式的助听器回波算法研究与实现

回声会导致助听器产生啸叫,损坏助听器设备,破坏患者的残余听力。为此,本文在助听器回声抵消模型的基础上,针对输入信号的能量变化,研究了基于归一化最小均方（Normalized Least Mean Squares, NLMS）的自适应助听器回声抵消算法。通过对比LMS和NLMS两种算法的MSE,ERLE等性能,研究发现NLMS算法在数字助听器模型中有更好的回声抵消性能。此外,将NLMS算法移植到嵌入式平台中,并通过实验对比算法的性能。因此,本研究对于助听器的回声抵消算法设计具有很强的实用价值。     

自适应回声消除算法的仿真研究

介绍几种自适应回声消除算法:NLMS,PNLMS,PNLMSDU,IPNLMS,MPNLMS,并在Matlab环境下进行了仿真比较。采用高斯噪声作为输入,分别在回声路径稀疏、回声路径中等稀疏、回声路径完全非稀疏的条件下进行仿真。仿真结果表明,PNLMS算法是针对稀疏回声路径的有效快速算法,MPNLMS算法做了进一步的优化。IPN-LMS算法和PNLMSDU算法融合了NLMS算法和PNLMS算法的优点,既保证有较快的初始收敛速度,又保证后阶段收敛速度不明显下降。     

α-稳定分布噪声环境下的非线性回声消除研究

针对广泛存在的非线性回声,以及在非高斯噪声环境下,传统回声消除器中自适应算法性能衰退,继而导致回声消除效果下降的情况,本文提出了一种基于最小离差准则的协同函数链接型自适应滤波回声消除方法.该方法使用归一化最小l_p范数算法更新线性和非线性函数扩展的自适应滤波器的权值,并将输出信号协同组合,以消除非线性回声.仿真实验结果表明,该方法在α-稳定分布噪声且非线性回声存在的情况下比传统回声消除方法具有更高的回声衰减增益.     

神经网络自适应有源噪声对消器

本文提出一种新颖的非线性自适应滤波器-递归神经网络(TNN)自适应滤波器．讨论RNN滤波器的构成及算法,实现了RNN自适应有源噪声对消器．RNN自适应有源噪声对消器性能优于由线性滤波器组成的线性对消器,能适应各种噪声环境．仿真结果充分证明了这一点．     

Combinations of an adaptive array antenna and a canceller ofinterference for direct-sequence spread-spectrum multiple-access system

In the realization of code-division multiple access based on a spread-spectrum communication system, i.e. spread-spectrum multiple access (SSMA), reduction of cochannel interference is an important problem. An adaptive array antenna system is proposed that includes a cancellor of cochannel interference, which can improve performance by a combination of temporal and spatial filtering. While the adaptive array suppresses interference sources with arrival angles different from those of the desired user, the adaptive digital filter-canceller rejects those whose arrival angles are the same as those of the desired user. The proposed system can achieve stable acquisition and low error rate of demodulated data even in a heavy-interference channel where a conventional array antenna system cannot achieve satisfactory acquisition     

Far-echo cancellation in the presence of frequency offset [fullduplex modem]

A design is presented for a full-duplex echo-cancelling data modem based on a combined adaptive reference echo canceller and adaptive channel equalizer. The adaptive reference algorithm has the advantage that interference to the echo canceller caused by the far-end signal can be eliminated by subtracting an estimate of the far-end signal based on receiver decisions. This technique provides a novel approach for full-duplex far-echo cancellation in which the far echo can be cancelled in spite of carrier-frequency offset. To estimate the frequency offset, the system uses a separate receiver structure for the far echo which provides equalization of the far echo channel and tracks the frequency offset in the far echo. The feasibility of the echo-cancelling algorithms is demonstrated by computer simulation with realistic channel distortions and with 4800-b/s data transmission, at which rate the frequency offset in the far echo becomes important     

空时二维自适应处理的目标污染样本挑选方法

在空时二维自适应处理(STAP)中,杂波样本数据不满足独立同分布(iid)条件是影响STAP处理的一个重要因素.这种非iid特性不仅包含地面的不均匀性和散射体的内部运动,也包括了目标和一些类似目标的干扰造成的样本数据污染.在某些情况下,被目标污染的样本会对权系数矢量的估计造成明显的影响,使输出信杂比大大下降.本文首先分析了目标信号对广义旁瓣相消结构(GSC)处理方式的影响,指出采用阻塞矩阵和不包含检测单元的辅助数据不能有效消除目标信号的影响.而现有的非均匀检测(NHD)方法不能有效挑出污染样本.据此,本文提出杂波剩余功率下降的目标污染样本选择准则和两种新的非均匀样本检测方法,可以将对自适应有影响的污染样本数据剔除.试验结果证实了新方法的有效性,而且其中一种方法的运算量很小,便于工程实现,具有较高的应用价值.     

Performance analysis of an echo-cancellation arrangement thatcompensates for frequency offset in the far echo

Analytical and experimental results are presented for the performance of one echo canceller arrangement. It consists of a data-driven echo canceller having a so-called cross-coupled structure, which is followed by a rotator and a phase-locked loop (PLL). A cross-coupled echo canceller structure without a PLL is analyzed first. Expressions for speed of convergence and achievable echo-return-loss enhancement (ERLE) in the presence of frequency offset are derived. These results are compared in previously published results for a noncross-coupling echo canceller structure. Specifically, it is shown that the cross-coupled structure converges twice as fast as the noncross-coupled structure and provide an achievable ERLE that is about 6 dB better. The joint adaptation of the echo canceller and the PLL is then studied. It is shown that it is always possible to choose design parameters for the echo canceller which are consistent with adaptation requirements under double-talking conditions, provided that the PLL is properly engineered. The sensitivity of the performance of PLL to the power level of the far echo, as well as possible solutions to this problem, are discussed     

Echo cancellation in the telephone network

Most long-distance telephone connections generate echoes, which must be heavily attenuated in order to obtain satisfactory transmission quality. Voice-actuated switches (echo suppressors) are widely used to eliminate echoes but have an unfortunate tendency also to cut out part of the desired signal from the other end of the line. Because the distortion caused by echo suppressors is particularly noticeable on satellite-routed connections, the advent of telephone communication via satellite, including the recent introduction of satellite circuits into the U.S. domestic network, has motivated the search for a better way to eliminate echoes. The answer may be the echo canceler, an adaptive filter which selectively eliminates echoes. Advanced echo canceler designs have been undergoing field trials in recent years. This article explains why echo cancelers are advantageous and how they work.     

全尺寸目标激光脉冲后向散射回波功率测定和建模

运用粗糙面散射、激光脉冲散射理论,结合复杂目标粗糙表面相关建模参数,建立用于计算全尺寸复杂目标激光脉冲后向散射功率的理论模型.实验测量一种空中缩比目标模型后向散射激光脉冲回波功率,获取该缩比目标激光脉冲后向散射功率随接受天顶角的变化曲线.比较理论建模与实验测量数据,分析实验误差,证明了该理论模型的正确性.将建立的理论模型进一步应用于计算非合作空间全尺寸目标的激光脉冲回波功率.计算结果能够预估空间目标的激光散射特征,解决一些关于空间目标激光脉冲光学特征工程的应用问题.     

机载混合模式SAR波束指向控制对图像质量的影响

针对机载混合模式SAR系统,该文给出了步进式波束控制下雷达波束对地面点目标照射强度的数学模型;通过分析方位向回波压缩信号,结合数学推导,提出波束控制对图像质量的影响主要体现在:波束指向被控时间导致出现无穷多对成对回波;混合度造成的主瓣展宽导致方位向分辨率下降;混合度造成的旁瓣压低导致成对回波对图像质量影响加大;混合度和波束指向被控时间相互作用使成对回波存在畸变。并利用计算机仿真验证了论证分析的正确性。最后对机载混合模式SAR波控方案的制定提出了建议。