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<h2>
<span class="refentrytitle">lowpass2</span>
</h2>
<p>lowpass2 —
A resonant lowpass filter.
</p>
</div>
<div class="refsect1">
<a id="idm281472918074424"></a>
<h2>Description</h2>
<p>
Implementation of a resonant second-order lowpass filter.
</p>
</div>
<div class="refsect1">
<a id="idm281472918073112"></a>
<h2>Syntax</h2>
<pre class="synopsis">ares <span class="command"><strong>lowpass2</strong></span> asig, kcf, kq [, iskip]</pre>
</div>
<div class="refsect1">
<a id="idm281472918070856"></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">
<a id="idm281472918068872"></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>
</div>
<div class="refsect1">
<a id="idm281472918060952"></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="idm281472918059176"></a>
<p class="title">
<strong>Example 494. 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>
<div class="refsect1">
<a id="idm281472744144504"></a>
<pre class="programlisting">
<span class="nt"><CsoundSynthesizer></span>
<span class="nt"><CsOptions></span>
<span class="c1">; Select audio/midi flags here according to platform</span>
<span class="c1">; Audio out Audio in No messages</span>
-odac -iadc -d <span class="c1">;;;RT audio I/O</span>
<span class="c1">; For Non-realtime ouput leave only the line below:</span>
<span class="c1">; -o lowpass2.wav -W ;;; for file output any platform</span>
<span class="nt"></CsOptions></span>
<span class="nt"><CsInstruments></span>
<span class="cm">/* Written by Sean Costello */</span>
<span class="c1">; Orchestra file for resonant filter sweep of a sawtooth-like waveform.</span>
<span class="vg">sr</span> <span class="o">=</span> <span class="mi">44100</span>
<span class="vg">kr</span> <span class="o">=</span> <span class="mi">2205</span>
<span class="vg">ksmps</span> <span class="o">=</span> <span class="mi">20</span>
<span class="vg">nchnls</span> <span class="o">=</span> <span class="mi">1</span>
<span class="kd">instr</span> <span class="nf">1</span>
i<span class="n">dur</span> <span class="o">=</span> <span class="nb">p3</span>
i<span class="n">freq</span> <span class="o">=</span> <span class="nb">p4</span>
i<span class="n">amp</span> <span class="o">=</span> <span class="nb">p5</span> <span class="o">*</span> <span class="mf">.5</span>
i<span class="n">harms</span> <span class="o">=</span> <span class="p">(</span><span class="vg">sr</span><span class="o">*</span><span class="mf">.4</span><span class="p">)</span> <span class="o">/</span> i<span class="n">freq</span>
<span class="c1">; Sawtooth-like waveform</span>
a<span class="n">sig</span> <span class="nb">gbuzz</span> <span class="mi">1</span><span class="p">,</span> i<span class="n">freq</span><span class="p">,</span> i<span class="n">harms</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mf">.9</span><span class="p">,</span> <span class="mi">1</span>
<span class="c1">; Envelope to control filter cutoff </span>
k<span class="n">freq</span> <span class="nb">linseg</span> <span class="mi">1</span><span class="p">,</span> i<span class="n">dur</span> <span class="o">*</span> <span class="mf">0.5</span><span class="p">,</span> <span class="mi">5000</span><span class="p">,</span> i<span class="n">dur</span> <span class="o">*</span> <span class="mf">0.5</span><span class="p">,</span> <span class="mi">1</span>
a<span class="n">filt</span> <span class="nb">lowpass2</span> a<span class="n">sig</span><span class="p">,</span>k<span class="n">freq</span><span class="p">,</span> <span class="mi">30</span>
<span class="c1">; Simple amplitude envelope</span>
k<span class="n">env</span> <span class="nb">linseg</span> <span class="mi">0</span><span class="p">,</span> <span class="mf">.1</span><span class="p">,</span> i<span class="n">amp</span><span class="p">,</span> i<span class="n">dur</span> <span class="o">-</span><span class="mf">.2</span><span class="p">,</span> i<span class="n">amp</span><span class="p">,</span> <span class="mf">.1</span><span class="p">,</span> <span class="mi">0</span>
<span class="nb">out</span> a<span class="n">filt</span> <span class="o">*</span> k<span class="n">env</span>
<span class="kd">endin</span>
<span class="nt"></CsInstruments></span>
<span class="nt"><CsScore></span>
<span class="cm">/* Written by Sean Costello */</span>
<span class="nb">f</span><span class="mi">1</span> <span class="mi">0</span> <span class="mi">8192</span> <span class="mi">9</span> <span class="mi">1</span> <span class="mi">1</span> <span class="mf">.25</span>
<span class="nb">i</span><span class="mi">1</span> <span class="mi">0</span> <span class="mi">5</span> <span class="mi">100</span> <span class="mi">1000</span>
<span class="nb">i</span><span class="mi">1</span> <span class="mi">5</span> <span class="mi">5</span> <span class="mi">200</span> <span class="mi">1000</span>
<span class="nb">e</span>
<span class="nt"></CsScore></span>
<span class="nt"></CsoundSynthesizer></span>
</pre>
</div>
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</div>
<p><br class="example-break" />
</p>
</div>
<div class="refsect1">
<a id="idm281472918054856"></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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