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author Chris Cannam <cannam@all-day-breakfast.com>
date Mon, 02 Mar 2020 14:03:47 +0000
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cannam@95 49 <a name="The-1d-Real-data-DFT"></a>
cannam@95 50 <a name="The-1d-Real_002ddata-DFT"></a>
cannam@95 51 <p>
cannam@95 52 Next:&nbsp;<a rel="next" accesskey="n" href="1d-Real_002deven-DFTs-_0028DCTs_0029.html#g_t1d-Real_002deven-DFTs-_0028DCTs_0029">1d Real-even DFTs (DCTs)</a>,
cannam@95 53 Previous:&nbsp;<a rel="previous" accesskey="p" href="The-1d-Discrete-Fourier-Transform-_0028DFT_0029.html#The-1d-Discrete-Fourier-Transform-_0028DFT_0029">The 1d Discrete Fourier Transform (DFT)</a>,
cannam@95 54 Up:&nbsp;<a rel="up" accesskey="u" href="What-FFTW-Really-Computes.html#What-FFTW-Really-Computes">What FFTW Really Computes</a>
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cannam@95 57
cannam@95 58 <h4 class="subsection">4.8.2 The 1d Real-data DFT</h4>
cannam@95 59
cannam@95 60 <p>The real-input (r2c) DFT in FFTW computes the <em>forward</em> transform
cannam@95 61 Y of the size <code>n</code> real array X, exactly as defined
cannam@95 62 above, i.e.
cannam@95 63 <center><img src="equation-dft.png" align="top">.</center>This output array Y can easily be shown to possess the
cannam@95 64 &ldquo;Hermitian&rdquo; symmetry
cannam@95 65 <a name="index-Hermitian-296"></a><i>Y<sub>k</sub> = Y<sub>n-k</sub></i><sup>*</sup>,where we take Y to be periodic so that
cannam@95 66 <i>Y<sub>n</sub> = Y</i><sub>0</sub>.
cannam@95 67
cannam@95 68 <p>As a result of this symmetry, half of the output Y is redundant
cannam@95 69 (being the complex conjugate of the other half), and so the 1d r2c
cannam@95 70 transforms only output elements 0<small class="dots">...</small>n/2 of Y
cannam@95 71 (n/2+1 complex numbers), where the division by 2 is
cannam@95 72 rounded down.
cannam@95 73
cannam@95 74 <p>Moreover, the Hermitian symmetry implies that
cannam@95 75 <i>Y</i><sub>0</sub>and, if n is even, the
cannam@95 76 <i>Y</i><sub><i>n</i>/2</sub>element, are purely real. So, for the <code>R2HC</code> r2r transform, these
cannam@95 77 elements are not stored in the halfcomplex output format.
cannam@95 78 <a name="index-r2r-297"></a><a name="index-R2HC-298"></a><a name="index-halfcomplex-format-299"></a>
cannam@95 79
cannam@95 80 <p>The c2r and <code>H2RC</code> r2r transforms compute the backward DFT of the
cannam@95 81 <em>complex</em> array X with Hermitian symmetry, stored in the
cannam@95 82 r2c/<code>R2HC</code> output formats, respectively, where the backward
cannam@95 83 transform is defined exactly as for the complex case:
cannam@95 84 <center><img src="equation-idft.png" align="top">.</center>The outputs <code>Y</code> of this transform can easily be seen to be purely
cannam@95 85 real, and are stored as an array of real numbers.
cannam@95 86
cannam@95 87 <p><a name="index-normalization-300"></a>Like FFTW's complex DFT, these transforms are unnormalized. In other
cannam@95 88 words, applying the real-to-complex (forward) and then the
cannam@95 89 complex-to-real (backward) transform will multiply the input by
cannam@95 90 n.
cannam@95 91
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