This provides a vanilla radix-2 FFT out-of-place implementation and a test example.
This code was written by Robin Scheibler during rainy days in October 2017.
#include "fft.h"
...
// Create the FFT config structure
fft_config_t *real_fft_plan = fft_init(NFFT, FFT_REAL, FFT_FORWARD, NULL, NULL);
// Fill array with some data
for (k = 0 ; k < fft_analysis->size ; k++)
real_fft_plan->input[k] = (float)k;
// Execute transformation
fft_execute(real_fft_plan);
// Now do something with the output
printf("DC component : %f\n", real_fft_plan->output[0]); // DC is at [0]
for (k = 1 ; k < real_fft_plan->size / 2 ; k++)
printf("%d-th freq : %f+j%f\n", k, real_fft_plan->output[2*k], real_fft_plan->output[2*k+1]);
printf("Middle component : %f\n", real_fft_plan->output[1]); // N/2 is real and stored at [1]
// Don't forget to clean up at the end to free all the memory that was allocated
fft_destroy(real_fft_plan)
-
Create the FFT configuration by running
fft_init.fft_config_t *fft_init(int size, fft_type_t type, fft_direction_t direction, float *input, float *output) Parameters ---------- size : int The FFT size (should be a power of two), if not, returns NULL. type : fft_type_t The type of FFT, FFT_REAL or FFT_COMPLEX direction : fft_direction_t The direction, FFT_FORWARD or FFT_BACKWARD (inverse transformation) input : float * A pointer to a buffer of the correct size. If NULL, a buffer is allocated dynamically output : float * A pointer to a buffer of the correct size. If NULL, a buffer is allocated dynamically. Returns ------- A pointer to an `fft_config_t` structure that holds pointers to the buffers and all the necessary configuration options. -
Fill data in the
inputbuffer -
Call
fft_executeto run the FFT -
Use the transformed data located in the
outputbuffer -
Possibly free up memory by calling
fft_destroyon the configuration structure
When doing an inverse real FFT, the data in the input buffer is destroyed.
-
For
FFT_REALFFTs (forward as well as backward) of sizeNFFT, the buffer is of sizeNFFT. Then, the input data is organized asInput : [ x[0], x[1], x[2], ..., x[NFFT-1] ] Output : [ X[0], X[NFFT/2], Re(X[1]), Im(X[1]), ..., Re(X[NFFT/2-1]), Im(X[NFFT/2-1]) ] -
For
FFT_COMPLEXof sizeNFFT, the buffer is of size2 * NFFTas both real and imaginary parts should be saved.Input : [ Re(x[0]), Im(x[0]), ..., Re(x[NFFT-1]), Im(x[NFFT-1]) ] Output : [ Re(X[0]), Im(X[0]), ..., Re(X[NFFT-1]), Im(X[NFFT-1]) ]
This software is released under the MIT license.
Copyright (c) 2017 Robin Scheibler
Permission is hereby granted, free of charge, to any person obtaining a copy
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