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<div class="refentry" title="lpf18">
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<h2>
<span class="refentrytitle">lpf18</span>
</h2>
<p>lpf18 —
A 3-pole sweepable resonant lowpass filter.
</p>
</div>
<div class="refsect1" title="Description">
<a id="idp29480392"></a>
<h2>Description</h2>
<p>
Implementation of a 3 pole sweepable resonant lowpass filter.
</p>
</div>
<div class="refsect1" title="Syntax">
<a id="idp29481064"></a>
<h2>Syntax</h2>
<pre class="synopsis">ares <span class="command"><strong>lpf18</strong></span> asig, kfco, kres, kdist [, iskip]</pre>
</div>
<div class="refsect1" title="Initialization">
<a id="idp29491504"></a>
<h2>Initialization</h2>
<p>
<span class="emphasis"><em>iskip</em></span> (optional, default=0) -- Skip initialization if present and non-zero.
</p>
</div>
<div class="refsect1" title="Performance">
<a id="idp29492392"></a>
<h2>Performance</h2>
<p>
<span class="emphasis"><em>kfco</em></span> -- the filter cutoff frequency in Hz. Should be in the range 0 to <span class="emphasis"><em>sr</em></span>/2.
</p>
<p>
<span class="emphasis"><em>kres</em></span> -- the amount of resonance. Self-oscillation occurs when <span class="emphasis"><em>kres</em></span> is approximately 1. Should usually be in the range 0 to 1, however, values slightly greater than 1 are possible for more sustained oscillation and an <span class="quote">“<span class="quote">overdrive</span>”</span> effect.
</p>
<p>
<span class="emphasis"><em>kdist</em></span> -- amount of distortion. <span class="emphasis"><em>kdist</em></span> = 0 gives a clean output. <span class="emphasis"><em>kdist</em></span> > 0 adds <span class="emphasis"><em>tanh</em></span>() distortion controlled by the filter parameters, in such a way that both low cutoff and high resonance increase the distortion amount. Some experimentation is encouraged.
</p>
<p>
<span class="emphasis"><em>lpf18</em></span> is a digital emulation of a 3 pole (18 dB/oct.) lowpass filter capable of self-oscillation with a built-in distortion unit. It is really a 3-pole version of <a class="link" href="moogvcf.html" title="moogvcf"><em class="citetitle">moogvcf</em></a>, retuned, recalibrated and with some performance improvements. The tuning and feedback tables use no more than 6 adds and 6 multiplies per control rate. The distortion unit, itself, is based on a modified <a class="link" href="tanh.html" title="tanh"><em class="citetitle">tanh</em></a> function driven by the filter controls.
</p>
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<th align="left">Note</th>
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<td align="left" valign="top">
<p>
This filter requires that the input signal be normalized to one.
</p>
</td>
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</div>
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<div class="refsect1" title="Examples">
<a id="idp29529792"></a>
<h2>Examples</h2>
<p>
Here is an example of the lpf18 opcode. It uses the file <a class="ulink" href="examples/lpf18.csd" target="_top"><em class="citetitle">lpf18.csd</em></a>.
</p>
<div class="example">
<a id="idp29530760"></a>
<p class="title">
<strong>Example 418. Example of the lpf18 opcode.</strong>
</p>
<div class="example-contents">
<p>See the sections <a class="link" href="UsingRealTime.html" title="Real-Time Audio"><em class="citetitle">Real-time Audio</em></a> and <a class="link" href="CommandFlags.html" title="Csound command line"><em class="citetitle">Command Line Flags</em></a> for more information on using command line flags.</p>
<pre class="programlisting">
<span class="csdtag"><CsoundSynthesizer></span>
<span class="csdtag"><CsOptions></span>
<span class="comment">; Select audio/midi flags here according to platform</span>
<span class="comment">; Audio out Audio in No messages</span>
-odac -iadc -d <span class="comment">;;;RT audio I/O</span>
<span class="comment">; For Non-realtime ouput leave only the line below:</span>
<span class="comment">; -o lpf18.wav -W ;;; for file output any platform</span>
<span class="csdtag"></CsOptions></span>
<span class="csdtag"><CsInstruments></span>
<span class="comment">; Initialize the global variables.</span>
<span class="ohdr">sr</span> <span class="op">=</span> 44100
<span class="ohdr">kr</span> <span class="op">=</span> 4410
<span class="ohdr">ksmps</span> <span class="op">=</span> 10
<span class="ohdr">nchnls</span> <span class="op">=</span> 1
<span class="comment">; Instrument #1.</span>
<span class="oblock">instr</span> 1
<span class="comment">; Generate a sine waveform.</span>
<span class="comment">; Note that its amplitude (kamp) ranges from 0 to 1.</span>
kamp <span class="opc">init</span> 1
kcps <span class="opc">init</span> 440
knh <span class="opc">init</span> 3
ifn <span class="op">=</span> 1
asine <span class="opc">buzz</span> kamp, kcps, knh, ifn
<span class="comment">; Filter the sine waveform.</span>
<span class="comment">; Vary the cutoff frequency (kfco) from 300 to 3,000 Hz.</span>
kfco <span class="opc">line</span> 300, p3, 3000
kres <span class="opc">init</span> 0.8
kdist <span class="op">=</span> p4
ivol <span class="op">=</span> p5
aout <span class="opc">lpf18</span> asine, kfco, kres, kdist
<span class="opc">out</span> aout <span class="op">*</span> ivol
<span class="oblock">endin</span>
<span class="csdtag"></CsInstruments></span>
<span class="csdtag"><CsScore></span>
<span class="comment">; Table #1, a sine wave.</span>
<span class="stamnt">f</span> 1 0 16384 10 1
<span class="comment">; different distortion and volumes to compensate</span>
<span class="stamnt">i</span> 1 0 4 0.2 30000
<span class="stamnt">i</span> 1 4.5 4 0.9 27000
<span class="stamnt">e</span>
<span class="csdtag"></CsScore></span>
<span class="csdtag"></CsoundSynthesizer></span>
</pre>
</div>
</div>
<p><br class="example-break" />
</p>
</div>
<div class="refsect1" title="Credits">
<a id="idp29533032"></a>
<h2>Credits</h2>
<p>
</p>
<table border="0" summary="Simple list" class="simplelist">
<tr>
<td>Author: Josep M Comajuncosas</td>
</tr>
<tr>
<td>Spain</td>
</tr>
<tr>
<td>December 2000</td>
</tr>
</table>
<p>
</p>
<p>Example written by Kevin Conder with help from Iain Duncan. Thanks goes to Iain for helping with the example.</p>
<p>New in Csound version 4.10</p>
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