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1 <html>
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2 <body>
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3 <p>If 2150 iterations of real-to-complex FFT of size 2048 takes less than 10 seconds, then we may be able to make a high quality real-time phase vocoder (just).</p>
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4
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5 <p>A phase-vocoder of course must use overlapped
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6 windowed FFT (although you can choose the size, within limits), IFFT,
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7 and cartesian-polar conversion to calculate the phase for the
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8 instantaneous frequency.</p>
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9
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10 <p>A reasonable estimate of CPU cost for the whole thing is somewhere
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11 around 10x the cost of simple non-overlapping short-time forward
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12 Fourier transforms across the signal. </p>
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13
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14 <p>2150 iterations corresponds to 100 seconds of audio non-overlapped at
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15 44.1kHz, so if that takes less than 10 second, then in theory we might
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16 be OK.</p>
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17 <pre>
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18 <script src="nayuki/fft.js"></script>
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19 <script>
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20
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21 /* for a phase vocoder, we probably want 2048-point real-to-complex
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22 * FFTs (if available) */
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23
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24 function inputReals(size) {
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25 var result = new Array(size);
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26 for (var i = 0; i < result.length; i++)
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27 result[i] = (i % 20) / 10.0 - 1.0;
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28 return result;
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29 }
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30
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31 function zeroReals(size) {
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32 var result = new Array(size);
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33 for (var i = 0; i < result.length; i++)
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34 result[i] = 0.0;
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35 return result;
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36 }
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37
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38 function inputReal64s(size) {
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39 var result = new Float64Array(size);
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40 for (var i = 0; i < result.length; i++)
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41 result[i] = (i % 20) / 10.0 - 1.0;
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42 return result;
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43 }
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44
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45 var iterations = 2150;
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46 var size = 2048;
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47
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48 var start = performance.now();
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49
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50 var total = 0.0;
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51
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52 for (var i = 0; i < iterations; ++i) {
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53 var real = inputReals(size);
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54 var imag = zeroReals(size);
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55 transform(real, imag);
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56 for (var j = 0; j < size; ++j) {
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57 total += real[j] + imag[j];
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58 }
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59 }
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60
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61 document.write("total = " + total + "<br>");
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62
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63 var end = performance.now();
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64
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65 document.write("nayuki fft.js: " + iterations + " iterations at size " + size + " took " + (end - start) + "ms (" + (1000.0 / ((end - start) / iterations)) + " iterations/sec)<br>");
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66
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67 </script>
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68 <script src="fft.js/lib/complex.js"></script>
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69 <script>
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70
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71 var fft = new FFT.complex(size, false);
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72
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73 start = performance.now();
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74
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75 total = 0.0;
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76
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77 for (var i = 0; i < iterations; ++i) {
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78 var ri = inputReal64s(size);
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79 var co = new Float64Array(2 * size);
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80 fft.simple(co, ri, 'real');
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81 for (var j = 0; j < 2 * size; ++j) {
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82 total += co[j];
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83 }
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84 }
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85
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86 document.write("total = " + total + "<br>");
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87
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88 var end = performance.now();
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89
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90 document.write("nockert fft.js: " + iterations + " iterations at size " + size + " took " + (end - start) + "ms (" + (1000.0 / ((end - start) / iterations)) + " iterations/sec)<br>");
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91
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92 </script>
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93 </pre>
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94 </body>
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95
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