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FIR1

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An efficient finite impulse response (FIR) filter class in C++ and Python wrapper.

The FIR filter class offers also adaptive filtering using the least mean square (LMS) or normalised least mean square (NLMS) algorithm.

Installation

Packages for Ubuntu LTS

Add this repository to your package manager:

sudo add-apt-repository ppa:berndporr/dsp
sudo apt-get update
sudo apt install fir1
sudo apt install fir1-dev

This adds fir1-dev and fir1 to your package list. The demo files are in /usr/share/doc/fir1-dev. Copy them into a working directory, type gunzip *.gz, cmake . and make.

Linux / Unix / MACOSX: compilation from source

The build system is cmake. Install the library with the standard sequence:

cmake .
make
sudo make install
sudo ldconfig

or for debugging run cmake with: cmake -DCMAKE_BUILD_TYPE="Debug" . By default optimised release libraries are generated.

Windows

Under windows only the static library is generated which should be used for your code development.

For example for Visual Studio 2019 you write:

cmake -G "Visual Studio 16 2019" -A x64 .

and then start Visual C++ and compile it. Usually you want to compile both the release and debug libraries because they are not compatible to each other under Windows.

Python

Installation from the python package index (PyPi)

Windows / Linux / Mac

pip install fir1

under Windows it might be just pip for python3.

Installation from source

Windows / Linux / Mac: make sure that you have swig and a C++ compiler installed. Then type:

python setup.py install

How to use it

cmake

Add to your CMakeLists.txt either

target_link_libraries(myexecutable fir)

for the dynamic library or

target_link_libraries(myexecutable fir_static)

for the statically linked library.

You can also use find_package(fir).

Generating the FIR filter coefficients

Set the coefficients either with a C floating point array or with a text file containing the coefficients. The text file or the floating point array with the coefficients can easily be generated by Python or OCTAVE/MATLAB:

Python

Use the firwin command to generate the coefficients:

# Sampling rate
fs = 1000
# bandstop between 45 and 55 Hz:
f1 = 45
f2 = 55
b = signal.firwin(999,[f1/fs*2,f2/fs*2])

octave/MATLAB:

octave:1> h=fir1(100,0.1);

which creates the coefficients of a lowpass filter with 100 taps and normalised cutoff 0.1 to Nyquist.

Initialisation

C++ floating point FIR filter:

Fir1 fir("h.dat");

or import the coefficients as a const double array:

Fir1 fir(coefficients)

there is also an option to import a non-const array (for example generated with the ifft) and using std::vector. You can also create a moving average filter by initialising all coefficients with a constant value:

Fir1 moving_average(100,1.0/100);

Python

f = fir1.Fir1(coeff)

Realtime filtering

C++ double:

double b = fir.filter(a);

Python

b = f.filter(a)

Utility methods

These functions are the same in C++ and Python:

  • getTaps() returns the length of the FIR filter kernel.
  • reset() sets all delay lines to zero.
  • zeroCoeff() sets all coefficients to zero.

Retreiving the coefficients/kernel from the FIR filter is different depending on the language used:

C++

  • void getCoeff(double* target, unsigned length) const copies the FIR kernel into the given C array of doubles with length length.

    If length exceeds the length of the filter kernel, the result is zero-padded to fill the given array.

    If length is smaller than the filter kernel, a std::out_of_range exception is thrown.

  • std::vector<double> getCoeffVector() const returns a copy of the filter kernel.

Python

  • getCoeff(n : int) -> numpy.array as per the C++ method, following the zero-padding and exception-throwing behaviour of the C++. The returned array will have n elements.
  • getCoeff() -> numpy.array additional to the C++ methods, this returns an numpy array which is a copy of the filter kernel. This is probably the default use case in Python.

LMS algorithm

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The least mean square algorithm adjusts the FIR coefficients h_m with the help of an error signal e(n):

h_m(n+1) = h_m(n) + learning_rate * h_m(n) * e(n)

using the function lms_update(e) while performing the filtering with filter().

How to use the LMS filter

  • Construct the Fir filter with all coefficients set to zero: Fir1(nCoeff)
  • Set the learning_rate with the method setLearningRate(learning_rate).
  • Provide the input signal x to the FIR filter and use its standard filter method to filter it.
  • Define your error which needs to be minimised: e = d - y
  • Feed the error back into the filter with the method lms_update(e).

The lmsdemo in the demo directory makes this concept much clearer how to remove artefacts with this method.

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The above plot shows the filter in action which removes 50Hz noise with the adaptive filter. Learning is very fast and the learning rate here is deliberately kept low to show how it works.

Stability

The FIR filter itself is stable but the error signal changes the filter coefficients which in turn change the error and so on. There is a rule of thumb that the learning rate should be less than the "tap power" of the input signal which is just the sum of all squared values held in the different taps:

learning_rate < 1/getTapInputPower()

That allows an adaptive learning rate which is called "normalised LMS". From my experiments that works in theory but in practise the realtime value of getTapInputPower() can make the algorithm easily unstable because it might suggest infinite learning rates and can fluctuate wildly. A better approach is to keep the learning rate constant and rather control the power of the input signal by, for example, normalising the input signal or limiting it.

See the demo below which removes 50Hz from an ECG which uses a normalised 50Hz signal which guarantees stability by design.

Python

The commands under JAVA and Python are identical to C++.

Demos

Demo programs are in the "demo" directory which show how to use the filter.

  1. firdemo sends an impulse into the filter and you should see the impulse response at its output.
  2. lmsdemo filters out 50Hz noise from an ECG with the help of adaptive filtering by using the 50Hz powerline frequency as the input to the filter. This can be replaced by any reference artefact signal or signal which is correlated with the artefact.
  3. filter_ecg.py performs the filtering of an ECG in python using the fir1 python module which in turn calls internally the C++ functions.

C++ documentation

The doxygen generated documentation can be found here:

Unit tests

Under C++ just run make test or ctest.

Credits

This library has been adapted form Graeme Hattan's original C code.

Enjoy!

Bernd Porr & Nick Bailey