How to design multirate filter system for EMG signals

Technical Source
2 min readJan 5, 2021

I am using Delsys Bagnoli EMG System to acquire sEMG from forearm muscles. When I use Delsys EMGworks application, output signals are clean as expected with minor interferences. However, when I use the following Matlab script to acquire the signal unusual signal pattern occured (spikes).

the image of frequency domain of the Data (Red color frequencies contains EMG information) acquired from Matlab script is attached herewith.

I want to know why this happens and how to remove thoses spikes

the Matlab script:

%% EMG initialize
raw = zeros(1,6);
% EMG setup
emg = daq.createSession('ni');
ch1 = addAnalogInputChannel(emg, 'Dev1', 0, 'Voltage');
ch2 = addAnalogInputChannel(emg, 'Dev1', 1, 'Voltage');
ch3 = addAnalogInputChannel(emg, 'Dev1', 2, 'Voltage');
ch4 = addAnalogInputChannel(emg, 'Dev1', 3, 'Voltage');
ch5 = addAnalogInputChannel(emg, 'Dev1', 4, 'Voltage');
ch6 = addAnalogInputChannel(emg, 'Dev1', 5, 'Voltage');
emg.Rate = 4000; % sampling rate
lh = emg.addlistener('DataAvailable',@stopWhenExceed);
emg.NotifyWhenDataAvailableExceeds = 400; % data per one feature set (total of all channels)
emg.IsContinuous = true;
while 1

% check the existing time.mat file
time(end + 1 : end + 1, 1) = str2double(datestr(now,'SS.FFF'));
pause(0.01); % this pause should be there to acquire EMG data
if (getGlobalSx==0) % if new set of data is available
data = getGlobalx; % aquire data
raw(end +1:end+400, : ) = data;


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I realized that the pause we provide to the code to acquire EMG data should be equal to the,

emg.NotifyWhenDataAvailableExceeds / emg.Rate

which means 400/ 4000 = 0.1 not 0.01

Otherwise this multirate signal sampling happens.

while 1

% check the existing time.mat file
time(end + 1 : end + 1, 1) = str2double(datestr(now,'SS.FFF'));




Technical Source

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