Filter library for C

This repostitory contains a C-library that can be used to both design digital filters, as well as use them. The library also provides bindings to various other programming languages, like C++ and Javascript via emscripten.

You can try out the library online on github pages.

The following filter types are supported:

Name	Type	Characteristics	Examples
Moving average	FIR	lowpass, highpass	n/a
exponential	FIR	lowpass	n/a
exponential	IIR	lowpass	n/a
butterworth	IIR	lowpass, highpass, bandpass, bandstop	n/a
chebyshev (type 1)	IIR	lowpass, highpass, bandpass, bandstop	n/a
chebyshev (type 2)	IIR	lowpass, highpass, bandpass, bandstop	n/a
brickwall	FIR	lowpass, highpass, bandpass, bandstop	n/a

The filters are designed using the bilinear z-transform. In broad terms, you start with the frequency response of the analog filter and compute its poles and zeros. These values are transformed via a bilinear z-transform and expanded into a polynomial. The polynomial is normalized for gain 1 at 0 or the middle of the pass band. Here is a Presentation explaining the process.

MIT Signal processing continuous and discrete course

Build and run

Here is a quick start on how to build the code in this repo. On linux systems, you should be able to copy these command into bash.

git clone https://github.com/domohuhn/filter.git
mkdir -p filter/build
cd filter/build
cmake -DCMAKE_BUILD_TYPE='Release' ..
cmake --build .
# run the unit tests
ctest
# use the example program to design a chebyshev bandstop
# -o: order 4
# -c: cutoff 15 hz and 35 Hz
# -s: sampling rate 100 Hz
# -r: ripple 3 dB
# -g: generate graphs
./design-filter -p chebyshev -t bandstop -o 4 -c 15,35 -s 100 -r -3 -g 

Documentation

The API Documentation is generated with doxygen and can read online on github pages.

License

All code in this repository is licensed with the MIT license, see file "LICENSE".

Dependencies

The library itself has no dependencies. However the tests, examples and documentation have the following dependencies:

• Catch 2 is used for the test cases.
• cxxopts parses the command line options in the examples.
• Doxygen Awesome for the css styles of the api documentation.
• Chart.js to render the graphs in the documentation.

Issues

Please report issues and bugs using the issue tracker.