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Fir1

An efficient Finite Impulse Response (FIR) filter class written in C++ with python wrapper.

Adaptive filtering is also implemented using the Least Mean Square (LMS) or Normalised Least Mean Square (NLMS) algorithm.

Installation

Install the python package with pip:

pip install fir1

You can also install from source:

git clone https://github.com/berndporr/fir1
cd fir1
python setup.py install

Usage

Realtime filtering

The filter is a realtime filter which receives samples one by one so it can process data as it arrives from an ADC converter. This is simulated here with the for loop:

import fir1
b = signal.firwin(999,0.1)
f = fir1.Fir1(b)
for i in range(len(noisy_signal)):
    clean_signal[i] = f.filter(noisy_signal[i])

The constructor Fir1() receives the filter coefficients (= impulse response) and then filtering is performed with the method filter().

LMS adaptive filter

The file lms_50Hz_ecg_filter.py removes 50Hz from an ECG with the help of the lms filter. The filter learns its own frequency response from a reference 50Hz sine wave:

f = fir1.Fir1(NTAPS)
f.setLearningRate(LEARNING_RATE);

y= np.empty(len(ecg))
for i in range(len(ecg)):
    ref_noise = np.sin(2.0 * np.pi / 20.0 * i);
    canceller = f.filter(ref_noise)
    output_signal = ecg[i] - canceller
    f.lms_update(output_signal)
    y[i] = output_signal

You can query the filter coefficients using getCoeff. This is most useful to obtain the kernel of a trained adaptive filter:

>>> from fir1 import Fir1
>>> fir = Fir1([.25, -.5, 1, -.5, .25])
>>> fir.getCoeff()
array([ 0.25, -0.5 ,  1.  , -0.5 ,  0.25])

You may override the length of array to return and the result will be zero-padded. Specifying too small an array causes an exception to be raised:

>>> fir.getCoeff(8)
array([ 0.25, -0.5 ,  1.  , -0.5 ,  0.25,  0.  ,  0.  ,  0.  ])
>>> fir.getCoeff(3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File ".../fir1.py", line 97, in getCoeff
return _fir1.Fir1_getCoeff(self, *args)
RuntimeError: Fir1: target of getCoeff: too many weights to copy into target

Both the demo file and an explanation how the LMS filter works can be found on the homepage of the project:

https://github.com/berndporr/fir1