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<refentry id="svfilter">
<indexterm id="IndexSvfilter"><primary>svfilter</primary></indexterm>
<refentryinfo><title>Signal Modifiers:Standard Filters:Resonant</title></refentryinfo>
<refmeta>
<refentrytitle>svfilter</refentrytitle>
</refmeta>
<refnamediv>
<refname>svfilter</refname>
<refpurpose>
A resonant second order filter, with simultaneous lowpass, highpass and bandpass outputs.
</refpurpose>
</refnamediv>
<refsect1>
<title>Description</title>
<para>
Implementation of a resonant second order filter, with simultaneous lowpass, highpass and bandpass outputs.
</para>
</refsect1>
<refsect1>
<title>Syntax</title>
<synopsis>alow, ahigh, aband <command>svfilter</command> asig, kcf, kq [, iscl]</synopsis>
</refsect1>
<refsect1>
<title>Initialization</title>
<para>
<emphasis>iscl</emphasis> -- coded scaling factor, similar to that in <link linkend="reson"><citetitle>reson</citetitle></link>. A non-zero value signifies a peak response factor of 1, i.e. all frequencies other than <emphasis>kcf</emphasis> are attenuated in accordance with the (normalized) response curve. A zero value signifies no scaling of the signal, leaving that to some later adjustment (see <link linkend="balance"><citetitle>balance</citetitle></link>). The default value is 0.
</para>
</refsect1>
<refsect1>
<title>Performance</title>
<para>
<emphasis>svfilter</emphasis> is a second order state-variable filter, with k-rate controls for cutoff frequency and Q. As Q is increased, a resonant peak forms around the cutoff frequency. <emphasis>svfilter</emphasis> has simultaneous lowpass, highpass, and bandpass filter outputs; by mixing the outputs together, a variety of frequency responses can be generated. The state-variable filter, or "multimode" filter was a common feature in early analog synthesizers, due to the wide variety of sounds available from the interaction between cutoff, resonance, and output mix ratios. <emphasis>svfilter</emphasis> is well suited to the emulation of "analog" sounds, as well as other applications where resonant filters are called for.
</para>
<para>
<emphasis>asig</emphasis> -- Input signal to be filtered.
</para>
<para>
<emphasis>kcf</emphasis> -- Cutoff or resonant frequency of the filter, measured in Hz.
</para>
<para>
<emphasis>kq</emphasis> -- Q of the filter, which is defined (for bandpass filters) as bandwidth/cutoff. <emphasis>kq</emphasis> should be in a range between 1 and 500. As <emphasis>kq</emphasis> is increased, the resonance of the filter increases, which corresponds to an increase in the magnitude and "sharpness" of the resonant peak. When using <emphasis>svfilter</emphasis> without any scaling of the signal (where <emphasis>iscl</emphasis> is either absent or 0), the volume of the resonant peak increases as Q increases. For high values of Q, it is recommended that <emphasis>iscl</emphasis> be set to a non-zero value, or that an external scaling function such as <link linkend="balance"><citetitle>balance</citetitle></link> is used.
</para>
<para>
<emphasis>svfilter</emphasis> is based upon an algorithm in Hal Chamberlin's <emphasis>Musical Applications of Microprocessor</emphasis>s (Hayden Books, 1985).
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
Here is an example of the svfilter opcode. It uses the file <ulink url="examples/svfilter.csd"><citetitle>svfilter.csd</citetitle></ulink>.
<example>
<title>Example of the svfilter opcode.</title>
<para>See the sections <link linkend="UsingRealTime"><citetitle>Real-time Audio</citetitle></link> and <link linkend="CommandFlags"><citetitle>Command Line Flags</citetitle></link> for more information on using command line flags.</para>
<xi:include href="examples-xml/svfilter.csd.xml" xmlns:xi="http://www.w3.org/2001/XInclude"/>
</example>
</para>
</refsect1>
<refsect1>
<title>Credits</title>
<para>
<simplelist>
<member>Author: &namesean;</member>
<member>Seattle, Washington</member>
<member>1999</member>
</simplelist>
</para>
<para>New in Csound version 3.55</para>
</refsect1>
</refentry>
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