SignalExpress
EMG analysis, median frequency, FFT
Dear Will,
The only output that i really need is the median frequency of the EMG signal. To get this value, we generally apply a FFT on the raw signal and after look for the median frequency. We got a FFT transformation (see below) but we're not able to get a median frequency of it.
Here is an example of the procedure applied by a specialised software (i.e. Biometrics) to calculate the median frequency. We can mimic this procedure or use any other one, doesn't matter.
From Biometrics: The median frequency is calculated for a block of samples whose length is defined by the filter constant. This calculation is repeated as necessary across the trace and the resulting frequencies plotted as a series of lines connecting the calculated median frequencies. The following steps are performed: - A block of samples defined by the filter constant is taken and zero padded to the nearest power of 2. - A Windowing function applied to the data as specified by the selection at the bottom left of the Settings Window. See Using FFT Windowing Functions for more information.- An FFT is performed and, if selected, FFT High Pass Filters are used to Remove DC and to Remove Very Low Frequencies from the calculations. Note that a large DC component can seriously degrade the accuracy of the median frequency calculation.- The amplitude magnitude of each FFT output frequency is squared.- The median frequency is determined such that the area of the amplitude-squared frequency graph below the median frequency is the same as above the median frequency i.e. there is equal power either side of the median frequency.- The process is repeated for the next block of samples.
