ESP32 FFT

This provides a vanilla radix-2 FFT out-of-place implementation and a test example.

Author

This code was written by Robin Scheibler during rainy days in October 2017.

Example

#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)

Documentation

1. 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.
   

2. Fill data in the input buffer

3. Call fft_execute to run the FFT

4. Use the transformed data located in the output buffer

5. Possibly free up memory by calling fft_destroy on the configuration structure

Note about Inverse Real FFT

When doing an inverse real FFT, the data in the input buffer is destroyed.

Buffer sizes and data organization

• For FFT_REAL FFTs (forward as well as backward) of size NFFT, the buffer is of size NFFT. Then, the input data is organized as

    Input  : [ 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_COMPLEX of size NFFT, the buffer is of size 2 * NFFT as 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]) ]
  

Arduino

You can download the library as zip and call include Library -> zip library. Or you can git clone this project into the Arduino libraries folder e.g. with

cd  ~/Documents/Arduino/libraries
git clone pschatzmann/esp32-fft.git

I recommend to use git because you can easily update to the latest version just by executing the git pull command in the project folder.

License

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
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.