Amplitude Of High-frequency Component Research Articles

Pointing stabilization of 140 mJ, 10 Hz UV laser for Laser-Assisted Charge Exchange

This paper details a laser beam delivery and pointing stabilization system designed for the Laser-Assisted Charge Exchange (LACE) experiment at the Spallation Neutron Source (SNS) accelerator. The UV (355 nm) laser beam of 140 mJ pulses at 10 Hz must be transported over 65 m through a retrofitted laser transport line in a high-radiation area of the accelerator, causing the alignment of the beam to jitter and drift significantly on timescales of 0.1–1 s and >1 min, respectively, due to thermal effects and other disturbances. The laser pointing stabilization system is based on a novel technique which uses active feedback between CMOS cameras and a piezoelectric steering mirror controlled by feedback software to make corrections to the horizontal angle and vertical position of the laser beam at 10 Hz. Low frequency drift is easily controlled, and the long-term stability is limited by the amplitude of high frequency components of the spectrum near the Nyquist limit (5 Hz). With feedback turned on, the root-mean-square (RMS) deviations of the horizontal angle and vertical position over 20 min fell from 314 to 157 μrad and 309 to 167μm, respectively, which is below the pulse-to-pulse jitter at 10 Hz of 189 μrad RMS and 180μm RMS.

Acoustic performance of effervescent sprays by time–frequency method with different atomizer structures under different operating conditions

The acoustic performance of effervescent sprays was experimentally investigated at various liquid flow rates and gas to liquid ratios (GLRs) by mass under pressure<0.5MPa. A transparent effervescent atomizer with different mixing chambers and exit orifices was used in the study. An unconventional description of spray characteristics under various internal flow regimes was obtained using the adaptive optimal kernel (AOK) spectrogram of the spray acoustic in the time–frequency domain. Acoustic energy was calculated using the Hilbert Huang transform (HHT) method, which provides a quantitative comparison of vibration intensity under different structures and operating conditions. A new evaluation of spray unsteadiness was established based on acoustic energy analysis. Results showed that gas flowing out of the orifice leads to an increase in the amplitude of high frequency components. The spray characteristics under different internal flow regimes could be reflected from the AOK spectrograms, through which a new perspective on effervescent sprays in the time domain is presented. Acoustic energy was mostly influenced by both the air flow rate and geometry of the exit orifice and showed reduced sensitivity to the structure of the mixing chamber. The structure of the atomizer affected the internal gas–liquid two-phase flow structures and further influenced spray steadiness. A study of the present condition revealed that a structure with a farther location of aerator holes and a small exit orifice diameter strengthens the stability of pray performance.

HIDING VOICE DATA IN CENTER DENSITY OF SPEECH SPECTRUM FOR SECURE TRANSMISSION

Speech data is becoming an effective and indispensable way for fast information transmission but associated with it are number of unprecedented threats. The idea of this paper is to present a robust speech watermarking method to realize secure voice data transmission. In this method carrier is transformed into frequency domain. Pre-processed normalized covert voice data undergoes exponential transformation which is then substituted in center density of high frequency, high energy subband of carrier. High frequency components are chosen for embedding watermark for two reasons. First, during transmission carrier might get contaminated with noise and filteration generally suppress low frequency components so embedding in high frequency component will keep watermark intact. Second reason is human ears are less sensitive to high frequencies so slight change in amplitude of high frequency components is imperceptible.. Technique uses frequency masking, invisible, and blind approach also. By applying reverse approach sensitive message can be extracted from the watermarked carrier. For embedding and retrieving watermark secret key have been used. Experimental results have shown that proposed method does not change the size of the cover signal even after embedding, does not degrade the quality of carrier and exhibit vigorous voice data hiding performance. Proposed work can be used in those applications where maintaining integrity and secrecy about the information against intentional or unintentional access is given utmost importance.

Analyzing possible pitfalls of cross-frequency analysis

One of the central questions in neuroscience is how neural activity is organized across different spatial and temporal scales. As larger populations oscillate and synchronize at lower frequencies and smaller ensembles are active at higher frequencies, a cross-frequency coupling would facilitate flexible coordination of neural activity simultaneously in time and space. Although various experiments have revealed amplitude-to-amplitude and phase-to-phase coupling, the most common and most celebrated result is that the phase of the lower frequency component modulates the amplitude of the higher frequency component. Over the recent 5 years the amount of experimental works finding such phase-amplitude coupling in LFP, ECoG, EEG and MEG has been tremendous (summarized in [1]). We suggest that although the mechanism of cross-frequency-coupling (CFC) is theoretically very tempting, the current analysis methods might overestimate any physiological CFC actually evident in the signals of LFP, ECoG, EEG and MEG. In particular, we point out three conceptual problems in assessing the components and their correlations of a time series. Although we focus on phase-amplitude coupling, most of our argument is relevant for any type of coupling. 1) The first conceptual problem is related to isolating physiological frequency components of the recorded signal. The key point is to notice that there are many different mathematical representations for a time series but the physical interpretation we make out of them is dependent on the choice of the components to be analyzed. In particular, when one isolates the components by Fourier-representation based filtering, it is the width of the filtering bands what defines what we consider as our components and how their power or group phase change in time. We will discuss clear cut examples where the interpretation of the existence of CFC depends on the width of the filtering process. 2) A second problem deals with the origin of spectral correlations as detected by current cross-frequency analysis. It is known that non-stationarities are associated with spectral correlations in the Fourier space. Therefore, there are two possibilities regarding the interpretation of any observed CFC. One scenario is that basic neuronal mechanisms indeed generate an interaction across different time scales (or frequencies) resulting in processes with non-stationary features. The other and problematic possibility is that unspecific non-stationarities can also be associated with spectral correlations which in turn will be detected by cross frequency measures even if physiologically there is no causal interaction between the frequencies. 3) We discuss on the role of non-linearities as generators of cross frequency interactions. As an example we performed a phase-amplitude coupling analysis of two nonlinearly related signals: atmospheric noise and the square of it (Figure ​(Figure1)1) observing an enhancement of phase-amplitude coupling in the second signal while no pattern is observed in the first. Figure 1 Time-evolution of the amplitudes of high frequency components locked to the trough of a low frequency component (from 4 to 8 Hz) for random noise from an atmospheric source (left) and its square (right). Finally, we discuss some minimal conditions need to be tested to solve some of the ambiguities here noted. In summary, we simply want to point out that finding a significant cross frequency pattern does not always have to imply that there indeed is physiological cross frequency interaction in the brain.