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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="content-type" content="text/html;
charset=windows-1252">
<meta name="GENERATOR" content="Microsoft FrontPage 3.0">
<title>Wind Tutorial</title>
<style>body {max-width: 40em}</style>
</head>
<body>
<h1>Wind Tutorial</h1>
<p>This page describes a "wind" effect created by Alex. The basic
function is as follows.<br>
</p>
<p><i>Note: all examples use SAL syntax followed by Lisp syntax in a
smaller font. Since SAL does not support optional parameters,
keyword parameters are used instead, so these implementations
are not exactly equivalent.</i><br>
</p>
<pre>function wind(dur: 3, scal: 3, cps: 590, bw: 40)
return (env(0.07, 0.08, 0.1, 1, 0.6, 0.8, 1) *
(reson(scal * noise(), cps, bw, 2) +
reson(scal * 1.13 * noise(),
cps * pwl(0, 0.74, 0.2, 0.96, 0.5, 0.8, <br> 0.75, 1.16, 0.9, 0.97, 1, 0.74, 1),
bw * 1.042, 2))) ~ dur
<small><small>(defun wind (&optional (dur 3) (scal 3) (cps 590) (bw 40))
(stretch dur
(mult (env 0.07 0.08 0.1 1 0.6 0.8 1)
(sum
(reson (scale scal (noise)) cps bw 2)
(reson (scale (mult scal 1.13) (noise))
(mult cps (pwl 0 0.74 0.2 0.96 0.5 0.8 0.75 1.16 0.9 0.97 1 0.74 1))
(mult bw 1.042)
2)))))</small></small></pre>
<p>The basic idea is to use bandpassed noise to create the sound of
wind. In this example,
two bandpass filters are added together. The first uses a fixed
center frequency and
bandwidth, and the second uses a piece-wise linear function to
control the center
frequency. The entire sound is multiplied by an envelope and
stretched to the desired
duration. Note how several optional parameters can be used to
change the default behavior.</p>
<p>A slight elaboration of the wind function uses several copies of
wind in sequence with
slight overlap:</p><pre>function multiwind()
return simrep(i, 4, wind() @ (i * 2.9))</pre>
<pre><small><small>(defun multiwind ()
(simrep (i 4) (at (* i 2.9) (wind))))</small></small></pre><small><small>
</small></small>One problem with this approach is that the wind sound becomes
periodic. This could be
solved by using noise or random numbers to generate the center
frequency variation rather
than a fixed envelope.<br><br>
</body></html>
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