DSPL - Open source library for digital signal processing. DSPL includes follow algorithm groups:
Discrete and Fast Fourier Transform algorithms
Window functions collection
Convolution and digital filtration
IIR Filters Design
FIR Filters Design
Digital and analog filters analysis
Special math functions
Input - output functions
DSPL is dynamic link library (dspl.dll) under LGPL v3 license and can be used in an open source and commercial software.
DSPL is open source C - library.
You can get DSPL source code on GitHub.