请教关于MATLAB实现任意点IFFT的算法

lb2003

MATLAB可以实现任意点的IFFT,且能保证输入输出是一样的长度,速度还很快,这是用的什么"高级"算法啊?他是怎么做的啊?我想用C语言做同样的事.
请知道的人指点下,谢谢!

风花雪月

The algorithm for ifft(X) is the same as the algorithm
for fft(X), except for a sign change and a scale factor
of n = length(X). As for fft, the execution
time for ifft depends on the length of the transform.
It is fastest for powers of two. It is almost as fast for lengths that have
only small prime factors. It is typically several times slower for lengths
that are prime or which have large prime factors.

以上是matlab帮助中的一段话
下面是我看到一个ifft.m的代码，不知道是否是matlab自带的

1. % IFFT.M
   
2. % IFFT        IFFT(X) is the inverse discrete Fourier transform of
   
3. %         vector X.   IFFT(X,N) is the N-point IFFT, where N must
   
4. %         be a power of 2.  See also FFT, FFT2, IFFT2.
   
5. %
   
6. % ---------------------------------------------------------------
   
7. %
   
8. % COPYRIGHT : (c) NUHAG, Dept.Math., University of Vienna, AUSTRIA
   
9. %             http://nuhag.eu/
   
10. %             Permission is granted to modify and re-distribute this
    
11. %             code in any manner as long as this notice is preserved.
    
12. %             All standard disclaimers apply.
    
13. %
    
14. function y = ifft(x,nfft)
    
15. %IFFT        IFFT(X) is the inverse discrete Fourier transform of
    
16. %        vector X.   IFFT(X,N) is the N-point IFFT, where N must
    
17. %        be a power of 2.  See also FFT, FFT2, IFFT2.
    
18. 
    
19. %        J.N. Little 4-21-85
    
20. %        Revised 6-11-88 JNL
    
21. %        Copyright (c) 1985-88 by the MathWorks, Inc.
    
22. 
    
23. if nargin == 2
    
24.         y = conj(fft(conj(x),nfft));
    
25. else
    
26.         y = conj(fft(conj(x)));
    
27. end
    
28. [m,n] = size(y);
    
29. if m == 1
    
30.         m = n;
    
31. end
    
32. y = y/m;
    

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