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Spectral Doppler ultrasound
Published in Peter R Hoskins, Kevin Martin, Abigail Thrush, Diagnostic Ultrasound, 2019
The Doppler signal will contain not only the low-amplitude higher Doppler frequencies backscattered from the blood but also high-amplitude lower Doppler frequencies from the slow-moving tissue such as the vessel walls. These unwanted frequencies can be removed by a high-pass filter. As the name suggests, a high-pass filter removes the low-frequency signals while maintaining the high frequencies. There is, however, a compromise in the selection of the cut-off frequency to be used, as it is important not to remove the frequencies detected from the lower-velocity arterial or venous blood flow. Figure 9.6 shows the high-pass filter set at three different levels. The first (a) shows that the filter is set too low and the wall thump, generated by the slow-moving vessel wall, has not been removed. In the second situation (b), the filter has been correctly set to remove the wall thump and in the third situation (c) the filter has been set too high and the diastolic flow has been removed, giving a false impression of the waveform shape, which could lead to a misdiagnosis. The PRF and wall filter are often linked, so that increasing the PRF may automatically increase the wall filter.
Basic Instrumentation
Published in Vinayak Bairagi, Mousami V. Munot, Research Methodology, 2019
Pradeep B. Mane, Shobha S. Nikam
As filter settings impact the interpretation of the ECG and furthermore decrease electrical interference of ECG machine. Provision for both low pass filter and high pass filter settings is made in the ECG machine. A high-pass filter is mainly used to decrease low frequency noise. It diminishes base line drift in the ECG waveform whereas a low-pass filter is mainly used to decrease high frequency noise (delivered by chest, extremity muscles where electrical interference is produced by the power grid). In monitor mode the high-pass filter which is principally set at 0.5–1.0 Hz and the low-pass filter on the 40 Hz frequency. This is the most attainable filter setting and it allows only narrow frequency signals to pass through these filters. This setting is helpful for rhythm monitoring in that noise can divert ST segment, which is not that important. In this mode, pacemaker spikes seem to be invisible which further gets filtered on.
Electrophysiological Amplifier
Published in Mesut Sahin, Howard Fidel, Raquel Perez-Castillejos, Instrumentation Handbook for Biomedical Engineers, 2020
Mesut Sahin, Howard Fidel, Raquel Perez-Castillejos
One high-pass filter is located at each of the inputs of the circuit (Figure 2.6). As the name indicates, high-pass filters allow for high frequencies to pass whereas the low frequencies are attenuated. The frequency that separates attenuated from non-attenuated frequencies is the corner or cutoff frequency (fC). The high-pass filters used in this studio, known as first-order passive high-pass filters, have a cutoff frequency of fC=12πRC
A robust machine learning structure for driving events recognition using smartphone motion sensors
Published in Journal of Intelligent Transportation Systems, 2022
Mahdi Zarei Yazd, Iman Taheri Sarteshnizi, Amir Samimi, Majid Sarvi
The Fourier transform plays a crucial role in high and low pass filters which is formulated as in (1): where is the Fourier transform of the time series p is the specified frequency (p = 0, …, n-1) and n is the number of observations in In a low-pass filter, the frequency components of a time series are first decomposed by the Fourier transform. Then, the components with lower frequencies than the cutoff frequency are passed, and the other ones are neglected. With the usage of this filter, only the low-frequency patterns of the time series remain and the noises contributing to the high-frequency components are smoothed. The performance of the high-pass filters is similar to the low-pass types except that, reversely, in the high-pass filters, components with higher frequencies than the cutoff are passed.
Comparative statistical analysis of the quality of image enhancement techniques
Published in International Journal of Image and Data Fusion, 2018
Shivangi S. Somvanshi, Phool Kunwar, Sewata Tomar, Madhulika Singh
High pass filter is a filter that allows high frequencies to pass well, however, obstructs frequencies less than that of cut-off frequency. The high pass filter accentuates comparative difference between cell values and its neighbours. It has the effects of highlighting boundaries between features (e.g. where a water body meets the forests). A simple procedure to implement high pass filtering is to subtract the low pass filter image from original image. Though high pass filtering sharpens the edges, it also enhances the noise and produces a ‘rough’ appearance. A high pass filter (Hhp) is often represented by its relationship to the low pass filter (Hlp):
Interpreting geology from geophysics in poly-deformed and mineralised terranes; the Otago Schist and the Hyde-Macraes Shear Zone
Published in New Zealand Journal of Geology and Geophysics, 2019
Casey C. Blundell, Robin Armit, Laurent Ailleres, Steven Micklethwaite, Adam Martin, Peter Betts
Band pass filters are used to isolate wavelengths between selected frequencies. Low-pass filters (i.e. upward continuation) result in the attenuation of high-frequency wavelengths associated with shallow sources and enhancement of low-frequency, long wavelength features. A high-pass (downward continuation) suppresses long-wavelengths and enhances shallow short wavelength features. High-pass filters cannot be practically applied very far, as short-wavelength (high frequency) anomalies have characteristically low amplitudes, and the transform quickly becomes unstable.