#### Other Tools

```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.r and cx_out.i ```
``` ```
``` free(cfg); ```
``` ```
``` Note: frequency-domain data is stored from dc up to 2pi. ```
``` so cx_out 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, ```
``` Q15 short or Q31 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 ```