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<h2>
<span class="refentrytitle">lowpass2</span>
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<p>lowpass2 —
A resonant lowpass filter.
</p>
</div>
<div class="refsect1" title="Description">
<a id="idp29393760"></a>
<h2>Description</h2>
<p>
Implementation of a resonant second-order lowpass filter.
</p>
</div>
<div class="refsect1" title="Syntax">
<a id="idp29394512"></a>
<h2>Syntax</h2>
<pre class="synopsis">ares <span class="command"><strong>lowpass2</strong></span> asig, kcf, kq [, iskip]</pre>
</div>
<div class="refsect1" title="Initialization">
<a id="idp29414256"></a>
<h2>Initialization</h2>
<p>
<span class="emphasis"><em>iskip</em></span> -- initial disposition of internal data space. A zero value will clear the space; a non-zero value will allow previous information to remain. The default value is 0.
</p>
</div>
<div class="refsect1" title="Performance">
<a id="idp29415408"></a>
<h2>Performance</h2>
<p>
<span class="emphasis"><em>asig</em></span> -- input signal to be filtered
</p>
<p>
<span class="emphasis"><em>kcf</em></span> -- cutoff or resonant frequency of the filter, measured in Hz
</p>
<p>
<span class="emphasis"><em>kq</em></span> -- Q of the filter, defined, for bandpass filters, as bandwidth/cutoff. <span class="emphasis"><em>kq</em></span> should be between 1 and 500
</p>
<p>
<span class="emphasis"><em>lowpass2</em></span> is a second order IIR lowpass filter, with k-rate controls for cutoff frequency (<span class="emphasis"><em>kcf</em></span>) and Q (<span class="emphasis"><em>kq</em></span>). As <span class="emphasis"><em>kq</em></span> is increased, a resonant peak forms around the cutoff frequency, transforming the lowpass filter response into a response that is similar to a bandpass filter, but with more low frequency energy. This corresponds to an increase in the magnitude and "sharpness" of the resonant peak. For high values of <span class="emphasis"><em>kq</em></span>, a scaling function such as <span class="emphasis"><em>balance</em></span> may be required. In practice, this allows for the simulation of the voltage-controlled filters of analog synthesizers, or for the creation of a pitch of constant amplitude while filtering white noise.
</p>
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<div class="refsect1" title="Examples">
<a id="idp29435960"></a>
<h2>Examples</h2>
<p>
Here is an example of the lowpass2 opcode. It uses the file <a class="ulink" href="examples/lowpass2.csd" target="_top"><em class="citetitle">lowpass2.csd</em></a>.
</p>
<div class="example">
<a id="idp29436952"></a>
<p class="title">
<strong>Example 415. Example of the lowpass2 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 lowpass2.wav -W ;;; for file output any platform</span>
<span class="csdtag"></CsOptions></span>
<span class="csdtag"><CsInstruments></span>
<span class="op">/</span><span class="op">*</span> Written by Sean Costello <span class="op">*</span><span class="op">/</span>
<span class="comment">; Orchestra file for resonant filter sweep of a sawtooth-like waveform.</span>
<span class="ohdr">sr</span> <span class="op">=</span> 44100
<span class="ohdr">kr</span> <span class="op">=</span> 2205
<span class="ohdr">ksmps</span> <span class="op">=</span> 20
<span class="ohdr">nchnls</span> <span class="op">=</span> 1
<span class="oblock">instr</span> 1
idur <span class="op">=</span> p3
ifreq <span class="op">=</span> p4
iamp <span class="op">=</span> p5 <span class="op">*</span> .5
iharms <span class="op">=</span> (<span class="ohdr">sr</span><span class="op">*</span>.4) <span class="op">/</span> ifreq
<span class="comment">; Sawtooth-like waveform</span>
asig <span class="opc">gbuzz</span> 1, ifreq, iharms, 1, .9, 1
<span class="comment">; Envelope to control filter cutoff </span>
kfreq <span class="opc">linseg</span> 1, idur <span class="op">*</span> 0.5, 5000, idur <span class="op">*</span> 0.5, 1
afilt <span class="opc">lowpass2</span> asig,kfreq, 30
<span class="comment">; Simple amplitude envelope</span>
kenv <span class="opc">linseg</span> 0, .1, iamp, idur <span class="op">-</span>.2, iamp, .1, 0
<span class="opc">out</span> afilt <span class="op">*</span> kenv
<span class="oblock">endin</span>
<span class="csdtag"></CsInstruments></span>
<span class="csdtag"><CsScore></span>
/* Written by Sean Costello */
<span class="stamnt">f</span>1 0 8192 9 1 1 .25
<span class="stamnt">i</span>1 0 5 100 1000
<span class="stamnt">i</span>1 5 5 200 1000
<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="idp29439264"></a>
<h2>Credits</h2>
<p>
</p>
<table border="0" summary="Simple list" class="simplelist">
<tr>
<td>Author: Sean Costello</td>
</tr>
<tr>
<td>Seattle, Washington</td>
</tr>
<tr>
<td>August 1999</td>
</tr>
</table>
<p>
</p>
<p>New in Csound version 4.0</p>
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