FIR and IIR low/high pass digital filter designer tool

• Low and high pass filters
• Finite and infinite impulse responses
• Various window types
• Frequency and phase analysis
• Pass and transition bands indication
• Free and open-source

Using window method for FIR filter design. Filter cofficients are generated by applying an inverse FFT on the ideal frequency response and then multiplying with a selected window function.

High pass filter is calculated from a low pass filter by shifting (multiplying) result coefficients with a sine wave of pi/2 frequency sampled at samplingRate/2.

Number of filter coefficients is either calculated based on the target Attenuation (dB) and Transition Length (Hz) or entered manually. Using odd number of coefficients allows for a linear phase response.

Frequency and phase responses are calculated with a direct FFT of the filter coefficients.

Filter coefficients c[k] must be convolved with a sample buffer to apply a FIR filter:

Vout[i] = c[k] * Vin[i-k] + c[k-1] * Vin[i-(k-1)] + ... + c[0] * Vin[i]

Where Vin[] is a input circle sample buffer of size n, k is a total number of filter coefficients, Vin[i] is a last input sample, Vout[i] is a current output filtered sample.

Basic analog filter replication. Using RC circuit for a low-pass and CR circuit for a high-pass filters.

Frequency and phase responses are calculated by simulating sine waves of various frequencies and applying IIR filters directly.

IIR filter with coefficients c[3] must be applied the following way:

Vout[i] = c[0] * Vin[i] + c[1] * Vin[i-1] + c[2] * Vout[i-1]

Where Vin[i] is a current input sample, Vout[i] current output filtered sample, Vin[i-1] and Vout[i-1] are previous input and output samples used as a feedback for the IIR filter.

The project depends on Qt, FFTW3, Welle and Boost libraries.

Qt path is passed to the CMake build via -DQT_PATH option, since you may wish to install it via Qt Maintenance Tool to any directory.

FFTW3, Welle and Boost must be available in you /usr/local/include for CMake to resolve the paths.

mkdir build; cd build
cmake ../ -DQT_PATH=/Users/frolovilya/Dev/tools/QT/6.7.0/macos

make