Interaural time sensitivity of high-frequency neurons in the inferior colliculus

Recent psychoacoustic experiments have shown that interaural time differences provide adequate cues for lateralizing high-frequency sounds, provided the stimuli are complex and not pure tones. We present here physiological evidence in support of these findings. Neurons of high best frequency in the cat inferior colliculus respond to interaural phase differences of amplitude modulated waveforms, and this response depends upon preservation of phase information of the modulating signal. Interaural phase differences were introduced in two ways: by interaural delays of the entire waveform and by binaural beats in which there was an interaural frequency difference in the modulating waveform. Results obtained with these two methods are similar. Our results show that high-frequency cells can respond to interaural time differences of amplitude modulated signals and that they do so by a sensitivity to interaural phase differences of the modulating waveform.     

Rapid updating of optical arbitrary waveforms via time-domain multiplexing

We demonstrate high-fidelity optical arbitrary waveform generation with 5 GHz waveform switching via time-domain multiplexing. Compact, integrated waveform shapers based on silica arrayed-waveguide grating pairs with 10 GHz channel spacing are used to shape (line-by-line) two different waveforms from the output of a 10-mode x 10 GHz optical frequency comb generator. Characterization of the time multiplexer's complex transfer function (amplitude and phase) by frequency-resolved optical gating permits compensation of its impact on the switched waveforms and matching of the measured and target waveforms to better than G'=5%.     

基于LabVIEW的特殊复杂信号源研究与设计

围绕导航通信系统的抗干扰测试需求以及对现役无线电导航系统的性能评估任务,研究并设计了一种复杂环境下的信号源并给出了数学模型。该信号源设计要求能够产生包括理想、多径、干扰以及信道噪声在内的、具有天线效应的实际复杂信号。在多径延时的设置上,给出了一种精度更高,延时任意的信号生成方法。最后以PXIe-5672为硬件测试平台,由计算机软件LabVIEW产生波形数据,并通过控制仪器驱动实现信号的存储和发生,完成了整个信号源的设计。仿真结果表明了该设计方案可行有效。     

Estimation of coherent detection performance for spread scattering in reverberation-noise mixtures

Narrow-band matched filter processing gain is estimated for medium- and high-frequency active sonars for which the random backscattering processes are assumed to be wide sense stationary in time frequency and uncorrelated in delay-Doppler wide sense stationary and uncorrelated scattering [WSSUS conditions]. Echo and reverberation processes that are WSSUS are described by two-dimensional scattering functions defined in the delay-Doppler plane. The average receiver responses are estimated from the convolution of the appropriate scattering function with the waveform ambiguity function. Estimates of matched filter processing gain are derived for continuous wave (CW) linear frequency modulation (LFM), and discrete frequency shift keyed (FSK) (hop code) waveforms reflected from point and uniform delay spread scatterers masked by reverberation. These bound matched filter performance for a particular waveform and interference distribution since most delay spread scattering falls somewhere between these extremes. The scattering and ambiguity functions are modeled by bounded constant amplitude functions in delay-Doppler that permit the convolutions to be approximated by overlapping area calculations. The results are presented in tabular form as simple formulas that are functions of the reverberation, noise, and waveform parameters. The estimates are shown to be consistent with processing gain measurements made from multiple realizations of synthesized and in-water data.     

Auditory nerve representation of a complex communication sound in background noise.

A population study of auditory nerve responses in the bullfrog, Rana catesbeiana, analyzed the relative contributions of spectral and temporal coding in representing a complex, species-specific communication signal at different stimulus intensities and in the presence of background noise. At stimulus levels of 70 and 80 dB SPL, levels which approximate that received during communication in the natural environment, average rate profiles plotted over fiber characteristic frequency do not reflect the detailed spectral fine structure of the synthetic call. Rate profiles do not change significantly in the presence of background noise. In ambient (no noise) and low noise conditions, both amphibian papilla and basilar papilla fibers phase lock strongly to the waveform periodicity (fundamental frequency) of the synthetic advertisement call. The higher harmonic spectral fine structure of the synthetic call is not accurately reflected in the timing of fiber firing, because firing is "captured" by the fundamental frequency. Only a small number of fibers synchronize preferentially to any harmonic in the call other than the first, and none synchronize to any higher than the third, even when fiber characteristic frequency is close to one of these higher harmonics. Background noise affects fiber temporal responses in two ways: It can reduce synchronization to the fundamental frequency, until fiber responses are masked; or it can shift synchronization from the fundamental to the second or third harmonic of the call. This second effect results in a preservation of temporal coding at high noise levels. These data suggest that bullfrog eighth nerve fibers extract the waveform periodicity of multiple-harmonic stimuli primarily by a temporal code.     

水声JANUS信号的分数低阶时频谱迁移学习识别方法

非合作第三方水下标准协议信号识别在水声通信信号识别中具有重要研究意义。针对浅海水声JANUS信号的特征提取因易受脉冲噪声和多径效应等复杂水声环境影响而导致识别率低下的问题,提出一种分数低阶时频谱和ResNet18 （Residual Network 18）相结合的迁移学习识别方法。首先,选取JANUS固定前导作为识别对象,设计分数低阶傅里叶同步压缩变换（FLOFSST）,以分数低阶操作抑制脉冲噪声,以时频重排特性增强时频集中性。其次,将基于ImageNet的ResNet18预训练模型微调,迁移至JANUS信号和常见水声信号时频图集。仿真表明所提算法在信噪比为-10 dB时JANUS信号的识别率为96.15%,能够有效抑制脉冲噪声并减小多径效应影响,比传统算法识别性能好。海试中JANUS信号识别率达90.00%,证明算法识别准确率和网络的泛化性较高。      

Mode separation of Lamb waves based on dispersion compensation method

Ultrasonic Lamb modes typically propagate as a combination of multiple dispersive wave packets. Frequency components of each mode distribute widely in time domain due to dispersion and it is very challenging to separate individual modes by traditional signal processing methods. In the present study, a method of dispersion compensation is proposed for the purpose of mode separation. This numerical method compensates, i.e., compresses, the individual dispersive waveforms into temporal pulses, which thereby become nearly un-overlapped in time and frequency and can thus be extracted individually by rectangular time windows. It was further illustrated that the dispersion compensation also provided a method for predicting the plate thickness. Finally, based on reversibility of the numerical compensation method, an artificial dispersion technique was used to restore the original waveform of each mode from the separated compensated pulse. Performances of the compensation separation techniques were evaluated by processing synthetic and experimental signals which consisted of multiple Lamb modes with high dispersion. Individual modes were extracted with good accordance with the original waveforms and theoretical predictions.     

基于时频波谱对易原理分析莫尔条纹谱

数值解析信号波形时频域对易演变过程,研究任意信号波形与频率组分的内在联系, 并将该对易原理应用于莫尔条纹相位分析,提取相位信息。采用矩形窗模拟冲激波形和直流波形的变换过程,通过控制矩形窗函数窗宽,获得各种宽度矩形脉冲,窗宽趋于零情况下获得冲激波形,趋于∞获得直流波形。自开发快速傅里叶变换(fast Fourier transform, FFT)系统,对矩形脉冲实施离散傅里叶变换,方便快捷获得相应频谱数值解析波形,分析波形与频谱对易关系。结果发现,矩形窗函数频谱是Sa函数,窗宽变化导致Sa函数波形变化。窗宽减小时,Sa函数波形展宽,振动舒缓,趋于零极限时,变成直流波形。窗宽增大时,Sa函数波形紧缩,振动加剧,趋于∞的极限时,演变成δ冲激波形,信号波形时频域是对易的。根据时频域波形与频谱对易关系,在分析莫尔条纹时,将莫尔条纹的一级谱滤出并归一,由波谱对易原理,时域信号将体现Sa函数,使条纹对比分明,便于提取相位信息。     

A photonic scheme for the generation of dual linear chirp microwave waveform based on the external modulation technique and its airborne application

Due to high frequency and large time bandwidth product; photonic generation and processing of arbitrary microwave waveforms has been an interesting topic in recent time. Here, a relatively new photonic technique has been proposed for the generation of a dual linear chirp microwave waveform in Ku-band. In this method two single drive Mach–Zehnder Modulators are cascaded at minimum transmission point and in push–pull mode. Theoretical analysis and simulation are developed by giving a complete mathematical model. As the result of this methodology, a dual linear chirp microwave waveform in Ku-band with relatively large bandwidth is generated. Comparative analysis is done in the present cascading technique with dual parallel Mach–Zehnder Modulator (DPMZM) technique. Range-Doppler coupling of the radar system has been investigated with the help of an ambiguity function diagram of the generated waveform. Results have analyzed through MATLAB simulation and verified by experimental results.     

Experimental study of an acoustic communication system in shallow-water conditions

The paper presents the results of experimental studies of an acoustic communication system containing parallel transmission digital information channels, which came about from applying the principles of orthogonal frequency division of individual information channels, resulting in an increased information rate. Received signals were processed by two separate receivers differing in the method by which the pulse response parameters of the medium were estimated during signal propagation. It is shown that to increase the reliability of received digital information for coherent separated reception of a multipath signal, it is necessary to estimate the parameters of this signal’s components in frequency bands corresponding to individual information channels. When such estimates were taken into account, the acoustic communication system provided error-less reception of digital information in all experiments conducted under conditions of the shallow Volgograd Reservoir and Lake Ladoga.     

利用等离子体声源测量浅海低频段水声信道特性

浅海水声信道的结构特征及其变化特性是浅海环境水下远程探测和通讯应用的基础.为了有效测量低频水声信道特性,我们在实验中采用了拖曳等离子体声源和垂直阵接收系统。通过实验测量与信道模型仿真输出的比较,对等离子体声源特性,特别是脉冲波形、空间指向性和电声转换效率,以及在信道中的传播特性等进行测量分析。结果表明:实验测量与信道模型仿真符合良好,实验中采用的等离子体声源的发射源级和拖曳姿态稳定,波形一致性好,可以满足走航式连续水下信道测量要求。      

Switchable down-, up- and dual-chirped microwave waveform generation with improved time-bandwidth product based on polarization modulation and phase encoding

A switchable down-, up- and dual-chirped microwave waveform generation technique with improved time-bandwidth product (TBWP) is proposed and demonstrated based on a dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM) cascaded with a polarization modulator (PolM). By properly controlling the phase shifts of the radio frequency signals applied to the DP-DPMZM, switchable down-, up- and dual-chirped waveforms with simultaneous frequency and bandwidth doubling can be generated. To enlarge the TBWP further, splitting parabolic signal and phase-encoding splitting parabolic signal are used to drive the PolM for the enhancement of bandwidth and time duration. Numerical results demonstrate the generation of down-, up- and dual-chirped microwave waveform with TBWP of 8, 160 and 10240. The proposed method may find applications in future multifunction radar systems due to the high performance and flexibility.     

Effects of modulation wave shape on modulation frequency discrimination with electrical hearing

Amplitude modulations of pulsitile stimulation can be used to convey pitch information to cochlear implant users. One variable in designing cochlear implant speech processors is the choice of modulation waveform used to convey pitch information. Modulation frequency discrimination thresholds were measured for 100 Hz modulations with four waveforms (sine, sawtooth, a sharpened sawtooth, and square). Just-noticeable differences (JNDs) were similar for all but the square waveform, which often produced larger JNDs. The results suggest that a sine, sawtooth, and sharpened sawtooth waveforms are likely to provide similar pitch discrimination within a speech processing strategy.     

On driving a transducer to produce pulses shorter than the natural period of the transducer

When an acoustical transducer is given a short electrical impulse, the pressure wave produced is usually some sort of damped sine wave whose exact shape is determined by the physical properties of the transducer and the medium to which it is coupled. In order to produce pressure pulses shorter than this, it is necessary to drive the transducer with a more complex waveform. This paper describes how to calculate and produce the voltage waveform for any desired pressure waveform and shows some examples of the voltage waveforms necessary to drive a loudspeaker to produce pulses shorter than the natural resonant period of the loudspeaker.     

ANALYSIS OF WAVEFORM ERRORS IN MILLIMETER-WAVE LFMCW SYNTHETIC APERTURE RADAR

In remote sensing applications, there is a special interest in the lightweight, cost effective, and high resolution imaging sensors. The combination of linearly frequency modulated continuous wave (LFMCW) technology and synthetic aperture radar (SAR) technique can lead to such a sensor. This paper concentrates on the analysis of waveform errors in millimeter-wave (MMW) LFMCW SAR. The generating scheme of millimeter-wave LFMCW waveforms with phase locked loop (PLL) and direct digital synthesizer (DDS) combined frequency synthesizer is investigated. The impacts of quantization errors, spurs, and frequency nonlinearities are analyzed. Simulation results show that the quality of LFMCW waveforms has a direct influence on the SAR images. Hence a scheme of frequency synthesizer to achieve high performance MMW LFMCW waveform is proposed. This synthesizer driven by a DDS array can adaptive suppress the spurious level without degradation of excellent frequency linearity and fast switching speed.     

Source-filter Comparison of Measurements of Fundamental Frequency Perturbation and Amplitude Perturbation for Synthesized Voice Signals

SUMMARY: An investigation of the effect of glottal source aperiodicities (jitter, shimmer, and aspiration noise) on the estimation of fundamental frequency (f0) perturbation and amplitude perturbation, of synthesized, glottal source and voiced speech waveforms, is considered. Firstly, 4, cycle-event f0 estimators are examined: (1) waveform matching of the low-pass filtered waveform, (2) positive peaks (PPs) from the speech waveform, (3) PPs from the low-pass filtered waveform, and (4) positive zero crossings from the low-pass filtered waveform. The analysis shows that f0 perturbation measures taken from the low-pass filtered waveform are affected by both amplitude perturbation and random glottal noise, whereas, f0 perturbation measures taken from the PPs of the original waveform are affected by noise but not by amplitude perturbation. It is shown for the low-pass filter methods that the effects of amplitude perturbation and noise lead to increased errors in the measurement of f0 perturbation for the synthesized speech waveforms when compared with the synthesized glottal waveforms. Shimmer of the synthesized speech waveform is approximately equal to shimmer of the synthesized glottal source. However, noise and jitter affect measures of amplitude perturbation. The estimation of f0 perturbation from the synthesized speech waveform is shown to be nonlinearly related to f0 perturbation estimation from the synthesized glottal waveform as a consequence of the filtering action of the vocal tract. Low-pass filtering the voiced speech waveform is shown to provide a partial solution to this problem.     

Spin-lock MRI with amplitude- and phase-modulated adiabatic waveforms: an MR simulation study

INTRODUCTION: Image contrast between tissue types can be generated based on their T1/T2 ratio using spin-lock MRI techniques. An interesting application of such a concept would be to generate contrast in tissue with tissue relaxation times modified using exogenous contrast agents. An amplitude-modulated adiabatic waveform has been shown in the past to perform spin-lock MRI. However, implementation of this waveform may not prove to be efficient and practical in research or a clinical setup due to high radiofrequency power deposition. Recent advancement in software and hardware MR technology allows implementation of amplitude- and phase-modulated adiabatic waveforms on MR systems. The aim of this work was to explore role of adiabatic waveforms in performing rho imaging and demonstrate that amplitude- and phase-modulated waveforms [e.g., hyperbolic secant, B1 independent rotation-4 (BIR-4) waveforms] can be used to distinguish materials that differ in T1/T2 ratio. METHODS AND RESULTS: MR simulation was performed using computer routines implemented in MATLAB environment (Mathworks, Natick, MA). Modified Bloch equations with trapezoidal, hyperbolic secant and BIR-4 waveforms were used to perform MR simulation. Trapezoidal waveforms were only used for comparison to other waveforms. Gadolinium DTPA (Gad-DTPA) (T1/T2 approximately 1) and manganese chloride (MnCl(2)) (T1/T2 approximately 10) were used as examples of contrast agents due to their routine use in clinical and research setups and more importantly because they provide good examples of materials differing in T1/T2 ratios. Results of spin locking using trapezoidal waveform agree very well with the previously published results, thereby validating the computer routines used in this MR simulation. Plots of M(rho) (magnetization vector in rho domain) vs. offset frequency show distinct curves for these materials differing in T1/T2 for the three waveforms. BIR-4 waveform demonstrated a 40% difference in M(rho) ( approximately 150 Hz) for the materials. Rate of spin lock with hyperbolic secant waveform was rapid compared to other waveforms. DISCUSSION: MR simulation using contrast agents Gad-DTPA and MnCl(2) provided a useful way to demonstrate that amplitude- and phase-modulated adiabatic waveforms can be used to perform spin-lock imaging. Future work involves implementation of these waveforms on MR scanners and performing in vivo imaging to generate tissue contrast based on relaxation times ratio.     

一种正交频分复用系统抗水下时变多径信道的频率偏移估计方法

提出了一种水下正交频分复用(OFDM)系统鲁棒的频率偏移估计方法,即通信帧由一个训练帧和多个数据帧组成。其中克服大的多普勒频移,也就是粗频移估计(Rough Frequency Offset Estimation,RFOE),是通过训练帧来实现的,而克服时变的和小的多普勒频移,也就是精细多普勒频移估计(Accurate Frequency Offset Estimation,AFOE),是通过每一个数据帧实现的。RFOE和AFOE都是使用OFDM数据或循环前缀(Cyclic Prefix,CP)中没有被多径干扰的那部分数据计算得到的。在海试中,采用这种算法实现了通信速率与作用距离的乘积为133km·kbps的无误码通信,并比较了不同算法的试验结果,试验结果表明该算法能克服水下信道多径和时变的影响。      

Basilar-membrane responses to multicomponent (Schroeder-phase) signals: understanding intensity effects

Harmonic complexes comprised of the same spectral components in either positive-Schroeder (+Schr) or negative-Schroeder (-Schr) phase [see Schroeder, IEEE Trans. Inf. Theory 16, 85-89 (1970)] have identical long-term spectra and similar waveform envelopes. However, localized patterns of basilar-membrane (BM) excitation can be quite different in response to these two stimuli. Measurements in chinchillas showed more modulated (peakier) BM excitation for +Schr than -Schr complexes [Recio and Rhode, J. Acoust. Soc. Am. 108, 2281-2298 (2000)]. In the current study, laser velocimetry was used to examine BM responses at a location tuned to approximately 17 kHz in the basal turn of the guinea-pig cochlea, for +Schr and -Schr complexes with a 203-Hz fundamental frequency and including 101 equal-amplitude components from 2031 to 22,344 Hz. At 35-dB SPL, +Schr response waveforms showed greater amplitude modulation than -Schr responses. With increasing stimulation level, internal modulation decreased for both complexes. To understand the observed phenomena quantitatively, responses were predicted on the basis of a linearized model of the cochlea. Prediction was based on an "indirect impulse response" measured in the same animal. Response waveforms for Schroeder-phase signals were accurately predicted, provided that the level of the indirect impulse used in prediction closely matched the level of the Schroeder-phase stimulus. This result confirms that the underlying model, which originally was developed for noise stimuli, is valid for stimuli that produce completely different response waveforms. Moreover, it justifies explanation of cochlear filtering (i.e., differential treatment of different frequencies) in terms of a linear system.     

Synthesis of arbitrary frequency domain transmitting pulses applicable to pulsed NMR instruments

The synthesis of specialized excitation waveforms for pulsed NMR spin systems is an integral part of many present-day NMR experiments. Methods in use include various types of constant amplitude rectangular rf pulse trains with controlled interpulse delay times, modulating amplitudes of the envelopes of a chain of rectangular pulses, special long pulse rectangular pulses, and recently, single sideband spectral selections for NMR imaging. This paper describes, in quantitative fashion with experimental results, analysis and implementation of a method to generate pulsed NMR excitation waveforms using AM and FM techniques which yield individual pulses with controllable frequency spectral distributions. Examples given are of simplified cases, but the extension to complex frequency distribution requirements is clear.