From 0d2811ff213aa88ec2584df458267d9dc1f0faa0 Mon Sep 17 00:00:00 2001
From: Sebastian Held <sebastian.held@uni-due.de>
Date: Wed, 14 Jul 2010 10:23:55 +0200
Subject: [PATCH] bugfix DFT_time2freq.m

The DFT used to operate on absolute time. This is now fixed.
---
 matlab/DFT_time2freq.m | 18 +++++++++++++++++-
 matlab/FFT_time2freq.m |  3 +++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/matlab/DFT_time2freq.m b/matlab/DFT_time2freq.m
index 79baac1..63a7011 100644
--- a/matlab/DFT_time2freq.m
+++ b/matlab/DFT_time2freq.m
@@ -1,12 +1,28 @@
 function f_val = DFT_time2freq( t, val, freq )
-% val = FFT_time2freq( t, val, freq )
+% f_val = DFT_time2freq( t, val, freq )
 %
 % computes the DFT at the given frequencies
+% f_val: single-sided spectrum
+%
+% example:
+%   t=linspace(0,1,100);
+%   t_val=0.9*sin(2*pi*3*t); % sine wave; amplitude 0.9; frequency 3 Hz
+%   f=linspace(1,5,101);
+%   f_val=DFT_time2freq( t, t_val, f );
+%   interp1(f,abs(f_val),3)
+% ans = 0.8910
+%   plot( t, t_val )
+%   plot( f, abs(f_val) )
+%
+% See also FFT_time2freq
 
 if numel(t) ~= numel(val)
     error 'numel(t) ~= numel(val)'
 end
 
+% convert absolute time into relative time
+t = t - t(1);
+
 f_val = zeros(1,numel(freq));
 for f_idx=1:numel(freq)
     f_val(f_idx) = sum( val .* exp( -1i * 2*pi*freq(f_idx) * t ) );
diff --git a/matlab/FFT_time2freq.m b/matlab/FFT_time2freq.m
index d62131a..885d4cb 100644
--- a/matlab/FFT_time2freq.m
+++ b/matlab/FFT_time2freq.m
@@ -1,4 +1,7 @@
 function [f,val] = FFT_time2freq( t, val )
+% [f,val] = FFT_time2freq( t, val )
+%
+% See also DFT_time2freq
 
 dt=t(2)-t(1); % timestep
 L=numel(val); % signal length