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<title>FFTW 3.3.5: The 1d Discrete Fourier Transform (DFT)</title>

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Next: <a href="The-1d-Real_002ddata-DFT.html#The-1d-Real_002ddata-DFT" accesskey="n" rel="next">The 1d Real-data DFT</a>, Previous: <a href="What-FFTW-Really-Computes.html#What-FFTW-Really-Computes" accesskey="p" rel="prev">What FFTW Really Computes</a>, Up: <a href="What-FFTW-Really-Computes.html#What-FFTW-Really-Computes" accesskey="u" rel="up">What FFTW Really Computes</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="The-1d-Discrete-Fourier-Transform-_0028DFT_0029-1"></a>
<h4 class="subsection">4.8.1 The 1d Discrete Fourier Transform (DFT)</h4>

<a name="index-discrete-Fourier-transform-1"></a>
<a name="index-DFT-2"></a>
<p>The forward (<code>FFTW_FORWARD</code>) discrete Fourier transform (DFT) of a
1d complex array <em>X</em> of size <em>n</em> computes an array <em>Y</em>,
where:
<center><img src="equation-dft.png" align="top">.</center>The backward (<code>FFTW_BACKWARD</code>) DFT computes:
<center><img src="equation-idft.png" align="top">.</center></p>
<a name="index-normalization-8"></a>
<p>FFTW computes an unnormalized transform, in that there is no coefficient
in front of the summation in the DFT.  In other words, applying the
forward and then the backward transform will multiply the input by
<em>n</em>.
</p>
<a name="index-frequency-1"></a>
<p>From above, an <code>FFTW_FORWARD</code> transform corresponds to a sign of
<em>-1</em> in the exponent of the DFT.  Note also that we use the
standard &ldquo;in-order&rdquo; output ordering&mdash;the <em>k</em>-th output
corresponds to the frequency <em>k/n</em> (or <em>k/T</em>, where <em>T</em>
is your total sampling period).  For those who like to think in terms of
positive and negative frequencies, this means that the positive
frequencies are stored in the first half of the output and the negative
frequencies are stored in backwards order in the second half of the
output.  (The frequency <em>-k/n</em> is the same as the frequency
<em>(n-k)/n</em>.)
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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Mon, 02 Mar 2020 14:03:47 +0000
parents	7867fa7e1b6b
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