Introduction to the Analysis and Processing of Signals
Introduction to the Analysis and Processing of Signals
Signals and Systems
Computer Methods and Programs in Biomedicine
Reducing cross terms effects in the Choi-Williams transform of mioelectric signals
Computer Methods and Programs in Biomedicine
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This work presents a digital filter designed to delimitate the frequency band of surface electromyograms (EMG) and remove the mains noise and its harmonics, focusing the signal analysis during reduced muscle activity. A Butterworth filter was designed as the frequency-domain product of a second order, high-pass filter with cutoff frequency 10Hz, an eighth order low-pass filter, with cutoff at 400Hz and six stop-band filters, second order, centered at the 60Hz mains noise and its harmonics until 360Hz. The resulting filter was applied in both direct and reverse directions of the signals to avoid phase distortions. The performance was evaluated with a simulated EMG signal with additive noise in multiples of 60Hz. A qualitative assessment was made with real EMG data, acquired from 16 subjects, with age from 20 to 32 years. Subjects were positioned in orthostatic position during 21s, being only the last second analyzed to assure stationarity. EMG were collected by Ag/AgCl electrodes on right lateral gastrocnemius, amplified with gain 5000, filtered in the band from 10Hz to 1kHz, and thus digitized with 2ksamples/s. The filter effectively removed the mains noise components, with attenuations greater than 96.6%. The attenuation of the simulated signal at frequencies below 15Hz and at 60Hz caused only a small reduction of total power, preserving the original spectrum. Thus, the filter resulted suitable to the proposed application.