Change LPF and HPF cut-off frequencies

[Raztou3D]

Dear OYMotion,

Thank you for the library and the EMG sensors. While all is working, I need more data from my sensors. To do so, I want to use your EMGFilter library, however, EMG signal are active between 5 and 450 Hz. So I feel that your 20 and 150 Hz are trimming too much of the signal.

Could you please tell me what the coefficients would be for my proposed cut-off frequencies? I am aware that with such request, I will have to use the 1000Hz sampling frequency and won't use the 500Hz one.

In short, how do I adapt the following lines to 450Hz LPF and 5Hz HPF ?

// coefficients of transfer function of LPF
// coef[sampleFreqInd][order]
static float lpf_numerator_coef[2][3] = {{0.3913, 0.7827, 0.3913},
                                         {0.1311, 0.2622, 0.1311}};
static float lpf_denominator_coef[2][3] = {{1.0000, 0.3695, 0.1958},
                                           {1.0000, -0.7478, 0.2722}};
// coefficients of transfer function of HPF
static float hpf_numerator_coef[2][3] = {{0.8371, -1.6742, 0.8371},
                                         {0.9150, -1.8299, 0.9150}};
static float hpf_denominator_coef[2][3] = {{1.0000, -1.6475, 0.7009},
                                           {1.0000, -1.8227, 0.8372}};

Thanks in advance for your help !

[BigBro2014]

Higher signal frequency needs higher sample rate, and higher sample rate needs more powerful MCU, so using 20-150 a trade off.
You may have a try with BluePill, which use STM32F103. The coefficients can be easily caculated in matlab for a fixed sample rate.

[Raztou3D]

Hello @BigBro2014,
Thank you for answering. The MCU power and consumptions are not an issue. I am running the filter on a STM32F4 tuned at 168MHz. My current maximum measured sampling frequency is 2450Hz, so no problem for a 450Hz Filter.

I would greatly appreciate some guidance on the Matlab code.
I tried to play with the online TFilter platform, but could never find the same coefficients as you have. I tried:

• 3rd order Butterworth with 20 and 150hz cutoffs
• 6th order Notch around 50hz.
• the coefficients I'm getting a e nowhere close to yours. Do you have your original Matlab code for your coeffs?

I am aware what I am asking is a little bit out of your library's scope, and will be very grateful for any help you might provide.

[BigBro2014]

Well, the matlab can generates BPF parameters according your sample rate & signal frequency to pass.

[allrobot]

Dear OYMotion,

Thank you for the library and the EMG sensors. While all is working, I need more data from my sensors. To do so, I >want to use your EMGFilter library, however, EMG signal are active between 5 and 450 Hz. So I feel that your 20 and >150 Hz are trimming too much of the signal.

Could you please tell me what the coefficients would be for my proposed cut-off frequencies? I am aware that with >such request, I will have to use the 1000Hz sampling frequency and won't use the 500Hz one.

In short, how do I adapt the following lines to 450Hz LPF and 5Hz HPF ?

// coefficients of transfer function of LPF
// coef[sampleFreqInd][order]
static float lpf_numerator_coef[2][3] = {{0.3913, 0.7827, 0.3913},
                                         {0.1311, 0.2622, 0.1311}};
static float lpf_denominator_coef[2][3] = {{1.0000, 0.3695, 0.1958},
                                           {1.0000, -0.7478, 0.2722}};
// coefficients of transfer function of HPF
static float hpf_numerator_coef[2][3] = {{0.8371, -1.6742, 0.8371},
                                         {0.9150, -1.8299, 0.9150}};
static float hpf_denominator_coef[2][3] = {{1.0000, -1.6475, 0.7009},
                                           {1.0000, -1.8227, 0.8372}};

Dear OYMotion and BigBro2014

I may have a bit bit of a faux pas...

Can you share the coefficient of 5HZ~450HZ? I have no experience with MATLAB...

@BigBro2014 My MCU sampling frequency is 350ksps and CPU is 40MHZ. Could please provide the coefficient of 0~ 500Hz about suggestions?

[N, wc] = buttord(wp, ws, Rp, As, 's');
[B, A] = butter(N, wc, 's');

May I ask what HPF and LPF number you set wp, ws, Rp,As？

Thanks in advance for your help !

[allrobot]

Oh, it took a while, but it's solved!

Matlab input filterDesigner, after designed filter, press ctrl+E export parameter, finally use [b,a]=sos2tf(Hd.sosMatrix,Hd.ScaleValues) normal output.

[QuinIMG]

Dear OYMotion,
Thank you for the library and the EMG sensors. While all is working, I need more data from my sensors. To do so, I >want to use your EMGFilter library, however, EMG signal are active between 5 and 450 Hz. So I feel that your 20 and >150 Hz are trimming too much of the signal.
Could you please tell me what the coefficients would be for my proposed cut-off frequencies? I am aware that with >such request, I will have to use the 1000Hz sampling frequency and won't use the 500Hz one.
In short, how do I adapt the following lines to 450Hz LPF and 5Hz HPF ?

// coefficients of transfer function of LPF
// coef[sampleFreqInd][order]
static float lpf_numerator_coef[2][3] = {{0.3913, 0.7827, 0.3913},
                                         {0.1311, 0.2622, 0.1311}};
static float lpf_denominator_coef[2][3] = {{1.0000, 0.3695, 0.1958},
                                           {1.0000, -0.7478, 0.2722}};
// coefficients of transfer function of HPF
static float hpf_numerator_coef[2][3] = {{0.8371, -1.6742, 0.8371},
                                         {0.9150, -1.8299, 0.9150}};
static float hpf_denominator_coef[2][3] = {{1.0000, -1.6475, 0.7009},
                                           {1.0000, -1.8227, 0.8372}};

Dear OYMotion and BigBro2014

I may have a bit bit of a faux pas...

Can you share the coefficient of 5HZ~450HZ? I have no experience with MATLAB...

@BigBro2014 My MCU sampling frequency is 350ksps and CPU is 40MHZ. Could please provide the coefficient of 0~ 500Hz about suggestions?

[N, wc] = buttord(wp, ws, Rp, As, 's');
[B, A] = butter(N, wc, 's');

May I ask what HPF and LPF number you set wp, ws, Rp,As？

Thanks in advance for your help !

HPF for 500Hz sampling:

N=2;    %degree
f_cutoff = 20; %Hz
f_sample = 500; %Hz
Wn = f_cutoff/(f_sample/2);
[num, den] = butter(N, Wn, 'high')

The LPF & HPF coefficients for other sampling and cutoff frequencies can be calculated similarly.

[QuinIMG]

Would anyone be able to let me know how the coefficients for the notch/anti-hum filter are calculated??

I would like to modify the library to accommodate different sampling rates and cutoff frequencies, but I'm kind of a dummy with filtering. I was able to recreate the HPF and LPF coefficients using the MATLAB butter() function, but I can't figure out how to generate the AHF coefficients or gains.