ruohoruotsi / butterworth-filter-design Goto Github PK

In Butterworth.cpp , in this piece of code (line 201):

    // correct the overall gain
    if(filter == kLoPass || filter == kBandPass){ // pre-blt is okay for S-plane
        ba[0] = preBLTgain * (preBLTgain / gain); // 2nd term is how much BLT boosts,
    }
    else if(filter == kHiPass || kBandStop){ // HF gain != DC gain
        ba[0] = 1 / ba[0];
    }

My compiler complains about the line: else if(filter == kHiPass || kBandStop){ // HF gain != DC gain

error: converting the enum constant to a boolean [-Werror,-Wint-in-bool-context]
    else if(filter == kHiPass || kBandStop){ // HF gain != DC gain

I am using: icpx -Wall -Wextra -pedantic -Werror -Wno-unused-parameter

Add audio processing examples using Biquad and Biquad Chain, to make it explicit how to integrate the toolkit into an external signal processing chain.

A fully working pd or Max/MSP external would be ideal.

Hi ruohoruotsi, Thanks for your work on this library.

I found the Butterworth class's output a1 and a2 of SOS are awayls change sign compare to matlab results.
Such as if c++ output a1=0.1,a2=-0.2 then matlab's a1=-0.1, a2=0.2.

Is there something that I missing? If the output of Butterworth.bandPass() gives digital filter Biquad coefficients then is it should be the
same as matlab's results?

Thanks for your help.

C++ Test code like below:

int filterOrder = 4;
double gain = 1.0;
double Fs = 100;
double fp1 = 0.5;
double fs1 = 10;

vector <Biquad> coeffs;  // second-order sections (sos)
Butterworth butterworth;

bool designedCorrectly = butterworth.bandPass(Fs,             // fs
                                            fp1,            // freq1
                                            fs1,            // freq2. N/A for lowpass
                                            filterOrder,
                                            coeffs,
                                            gain);

Matlab test code like:

  order = 4; 
  Fs=100;     
  fp1=0.5;    
  fs1=10;     
  % design digital Butter-Worth-Filter in SOS form
  [z,p,k]=butter(order,[fp1*2/Fs,fs1*2/Fs],'bandpass');
  [bsos,bk] = zp2sos(z,p,k);

Then you can see coeffs[i].a1 == -bsos[i+1, 5] and coeffs[i].a2 == -bsos[i+1, 6]. i from 0 to length of coeffs.

Hi, thanks for your very useful code. I found that the gain of the filter is unpredictable. I'd like the gain at the center frequency to be 1, i.e., no change between input and output. But in my code below, a gain gets applied that varies depending on the parameters chosen. How do I normalize things so that no gain gets applied?

// Design the filter
double overallGain = 1.0;
bool designedCorrectly = butterworth.bandPass(
    args.dSamplerate,               // sample rate (Hz)
    args.dLow,                      // lo cutoff (Hz)
    args.dHigh,                     // hi cutoff (Hz)
    args.nOrder,                    // filter order
    coeffs,                         // vector to hold coefficients
    overallGain);                   // gain (how to choose?)
if (!designedCorrectly) {
    throw std::runtime_error("Filter design failed");
}

// Implement the filter
int stages = (int) coeffs.size();
BiquadChain chain(stages);
chain.processBiquad(input, output, 1, (int)nItems, &coeffs[0]);

I'm newb in matlab and c++.

%% High-pass filter linear velocity to remove drift

order = 1;
filtCutOff = 0.1;
[b, a] = butter(order, (2*filtCutOff)/(1/samplePeriod), 'high');
linVelHP = filtfilt(b, a, linVel);

a,b are vectors
linVel linVelHP are vectors

Hi bro
Um working on an android app to do DSP in real-time from mic to speaker , but um using a superpowered SDK and unfortunately it doesn't allow to increase the order of filter so can i import your library in my project and replace the old filter , so Can i use your library in real-time ?? and is there an example on how to use low-pass filter and band-pass ?
Thanks in advance

your project In BiquadChain::reset()function，can't init variable xn3 and xn4 to zero,the can generate error audio data at first a certain time.

Migrate build system away from an XCode-based project (MacOS only) to something more platform independent like SCONS or CMake.

For odd filter orders a pair of real poles get added rather than a single pole, don't think that is right. In Butterworth::prototypeAnalogLowPass(int filterOrder) rather than

for(uint32_t k = 0; k < (filterOrder + 1) / 2; k++){
double theta = (double)(2 * k + 1) * M_PI / (2 * filterOrder);
double real = -sin(theta);
double imag = cos(theta);
poles.push_back(complex_double(real, imag));
poles.push_back(complex_double(real, -imag)); // conjugate
}

might need something like

for(uint32_t k = 0; k < filterOrder / 2; k++){
double theta = (double)(2 * k + 1) * M_PI / (2 * filterOrder);
double real = -sin(theta);
double imag = cos(theta);
poles.push_back(complex_double(real, imag));
poles.push_back(complex_double(real, -imag)); // conjugate
}
if (filterOrder % 2 == 1){
poles.push_back(complex_double(-1, 0);
}
`