求各位大神帮看下这段s逆变换matlab code !!!!
最近刚开始学习s变换，这是从网上下载的一段Stockwell写的逆变换matlab代码，里面注释较多，实际其实有10行左右。想请教各位大神为什么要分情况讨论有奇数个点和偶数个点？if rem(ntimes ,2) ~= 0 ？非常感谢！
function = inverse_st(st) % Returns the inverse of the Stockwell Transform of the REAL_VALUED timeseries. % Code by Robert Glenn Stockwell. % Reference is "Localization of the Complex Spectrum: The S Transform" % from IEEE Transactions on Signal Processing, vol. 44., number 4, April 1996, pages 998-1001. % %-------Inputs Needed------------------------------------------------ % %S-Transform matrix created by the st.m function %-------Optional Inputs ------------------------------------------------ % none %-------Outputs Returned------------------------------------------------ % % ts -a REAL-VALUED time series %--------Additional details----------------------- % Copyright (c) by Bob Stockwell % $Revision: 1.0 $$Date: 2004/10/10$
% sum over time to create the FFT spectrum for the positive frequencies stspe = sum(st,2);
% get st matrix dimensions = size(st); if rem(ntimes ,2) ~= 0 % odd number of points, so nyquist point is not aliased, so concatenate % the reversed spectrum to create the negative frequencies % drop the DC value negspe = fliplr(stspe(2:nfreq)'); else % even number of points % therefore drop the first point (DC) and the last point (aliased nyqusit freq) negspe = fliplr(stspe(2:nfreq-1)'); end
% using symmetry of FFT spectrum of a real signal, recreate the negative frequencies from the positie frequencies fullstspe = ;
% the time series is the inverse fft of this ts = ifft(fullstspe); % and take the real part, the imaginary part will be zero. ts = real(ts);
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