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review.gerrithub.io / DU-Final / external_kissfft / cfa95dc35972f97a9186bf8f33bb1a9f87a40d42 / . / README

KISS FFT - A mixed-radix Fast Fourier Transform based up on the principle, | |

"Keep It Simple, Stupid." | |

There are many great fft libraries already around. Kiss FFT is not trying | |

to be better than any of them. It only attempts to be a reasonably efficient, | |

moderately useful FFT that can use fixed or floating data types and can be | |

incorporated into someone's C program in a few minutes with trivial licensing. | |

USAGE: | |

The basic usage for 1-d complex FFT is: | |

#include "kiss_fft.h" | |

kiss_fft_cfg cfg = kiss_fft_alloc( nfft ,is_inverse_fft ,0,0 ); | |

while ... | |

... // put kth sample in cx_in[k].r and cx_in[k].i | |

kiss_fft( cfg , cx_in , cx_out ); | |

... // transformed. DC is in cx_out[0].r and cx_out[0].i | |

free(cfg); | |

Note: frequency-domain data is stored from dc up to 2pi. | |

so cx_out[0] is the dc bin of the FFT | |

and cx_out[nfft/2] is the Nyquist bin (if exists) | |

Declarations are in "kiss_fft.h", along with a brief description of the | |

functions you'll need to use. | |

Code definitions for 1d complex FFTs are in kiss_fft.c. | |

You can do other cool stuff with the extras you'll find in tools/ | |

* multi-dimensional FFTs | |

* real-optimized FFTs (returns the positive half-spectrum: (nfft/2+1) complex frequency bins) | |

* fast convolution FIR filtering (not available for fixed point) | |

* spectrum image creation | |

The core fft and most tools/ code can be compiled to use float, double | |

or Q15 short samples. The default is float. | |

BACKGROUND: | |

I started coding this because I couldn't find a fixed point FFT that didn't | |

use assembly code. I started with floating point numbers so I could get the | |

theory straight before working on fixed point issues. In the end, I had a | |

little bit of code that could be recompiled easily to do ffts with short, float | |

or double (other types should be easy too). | |

Once I got my FFT working, I was curious about the speed compared to | |

a well respected and highly optimized fft library. I don't want to criticize | |

this great library, so let's call it FFT_BRANDX. | |

During this process, I learned: | |

1. FFT_BRANDX has more than 100K lines of code. The core of kiss_fft is about 500 lines (cpx 1-d). | |

2. It took me an embarrassingly long time to get FFT_BRANDX working. | |

3. A simple program using FFT_BRANDX is 522KB. A similar program using kiss_fft is 18KB (without optimizing for size). | |

4. FFT_BRANDX is roughly twice as fast as KISS FFT in default mode. | |

It is wonderful that free, highly optimized libraries like FFT_BRANDX exist. | |

But such libraries carry a huge burden of complexity necessary to extract every | |

last bit of performance. | |

Sometimes simpler is better, even if it's not better. | |

FREQUENTLY ASKED QUESTIONS: | |

Q: Can I use kissfft in a project with a ___ license? | |

A: Yes. See LICENSE below. | |

Q: Why don't I get the output I expect? | |

A: The two most common causes of this are | |

1) scaling : is there a constant multiplier between what you got and what you want? | |

2) mixed build environment -- all code must be compiled with same preprocessor | |

definitions for FIXED_POINT and kiss_fft_scalar | |

Q: Will you write/debug my code for me? | |

A: Probably not unless you pay me. I am happy to answer pointed and topical questions, but | |

I may refer you to a book, a forum, or some other resource. | |

PERFORMANCE: | |

(on Athlon XP 2100+, with gcc 2.96, float data type) | |

Kiss performed 10000 1024-pt cpx ffts in .63 s of cpu time. | |

For comparison, it took md5sum twice as long to process the same amount of data. | |

Transforming 5 minutes of CD quality audio takes less than a second (nfft=1024). | |

DO NOT: | |

... use Kiss if you need the Fastest Fourier Transform in the World | |

... ask me to add features that will bloat the code | |

UNDER THE HOOD: | |

Kiss FFT uses a time decimation, mixed-radix, out-of-place FFT. If you give it an input buffer | |

and output buffer that are the same, a temporary buffer will be created to hold the data. | |

No static data is used. The core routines of kiss_fft are thread-safe (but not all of the tools directory). | |

No scaling is done for the floating point version (for speed). | |

Scaling is done both ways for the fixed-point version (for overflow prevention). | |

Optimized butterflies are used for factors 2,3,4, and 5. | |

The real (i.e. not complex) optimization code only works for even length ffts. It does two half-length | |

FFTs in parallel (packed into real&imag), and then combines them via twiddling. The result is | |

nfft/2+1 complex frequency bins from DC to Nyquist. If you don't know what this means, search the web. | |

The fast convolution filtering uses the overlap-scrap method, slightly | |

modified to put the scrap at the tail. | |

LICENSE: | |

Revised BSD License, see COPYING for verbiage. | |

Basically, "free to use&change, give credit where due, no guarantees" | |

Note this license is compatible with GPL at one end of the spectrum and closed, commercial software at | |

the other end. See http://www.fsf.org/licensing/licenses | |

A commercial license is available which removes the requirement for attribution. Contact me for details. | |

TODO: | |

*) Add real optimization for odd length FFTs | |

*) Document/revisit the input/output fft scaling | |

*) Make doc describing the overlap (tail) scrap fast convolution filtering in kiss_fastfir.c | |

*) Test all the ./tools/ code with fixed point (kiss_fastfir.c doesn't work, maybe others) | |

AUTHOR: | |

Mark Borgerding | |

Mark@Borgerding.net |