Mello Roger G T, Oliveira Liliam F, Nadal Jurandir
Biomedical Engineering Program-COPPE, Federal University of Rio de Janeiro, P.O. Box 68510, 21941-972 Rio de Janeiro, RJ, Brazil.
Comput Methods Programs Biomed. 2007 Jul;87(1):28-35. doi: 10.1016/j.cmpb.2007.04.004. Epub 2007 Jun 4.
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 10 Hz, an eighth order low-pass filter, with cutoff at 400 Hz and six stop-band filters, second order, centered at the 60 Hz mains noise and its harmonics until 360 Hz. 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 60 Hz. 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 10 Hz to 1 kHz, 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 15 Hz and at 60 Hz caused only a small reduction of total power, preserving the original spectrum. Thus, the filter resulted suitable to the proposed application.
这项工作提出了一种数字滤波器，旨在界定表面肌电图（EMG）的频段，去除市电噪声及其谐波，以便在肌肉活动减弱期间聚焦信号分析。设计了一种巴特沃斯滤波器，它是一个截止频率为10 Hz的二阶高通滤波器、一个截止频率为400 Hz的八阶低通滤波器以及六个中心频率分别为60 Hz市电噪声及其谐波直至360 Hz的二阶阻带滤波器在频域的乘积。所得滤波器在信号的正向和反向都进行了应用，以避免相位失真。使用带有60 Hz倍数的加性噪声的模拟EMG信号对性能进行了评估。利用从16名年龄在20至32岁之间的受试者采集的真实EMG数据进行了定性评估。受试者站立21秒，仅对最后一秒进行分析以确保平稳性。通过Ag/AgCl电极在右外侧腓肠肌采集EMG，增益为5000进行放大，在10 Hz至1 kHz频段进行滤波，然后以2k样本/秒进行数字化。该滤波器有效地去除了市电噪声成分，衰减大于96.6%。模拟信号在低于15 Hz和60 Hz频率处的衰减仅导致总功率略有降低，保留了原始频谱。因此，该滤波器适用于所提出的应用。
