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<refentry id="butterbr">
<indexterm id="IndexButterbr"><primary>butterbr</primary></indexterm>
<refentryinfo><title>Signal Modifiers:Standard Filters</title></refentryinfo>
<refmeta>
<refentrytitle>butterbr</refentrytitle>
</refmeta>
<refnamediv>
<refname>butterbr</refname>
<refpurpose>
A band-reject Butterworth filter.
</refpurpose>
</refnamediv>
<refsect1>
<title>Description</title>
<para>
Implementation of a second-order band-reject Butterworth filter. This opcode can also be written as <link linkend="butbr"><citetitle>butbr</citetitle></link>.
</para>
</refsect1>
<refsect1>
<title>Syntax</title>
<synopsis>ares <command>butterbr</command> asig, xfreq, xband [, iskip]</synopsis>
</refsect1>
<refsect1>
<title>Initialization</title>
<para>
<emphasis>iskip</emphasis> (optional, default=0) -- Skip initialization if present and non-zero.
</para>
</refsect1>
<refsect1>
<title>Performance</title>
<para>
These filters are Butterworth second-order IIR filters. They are slightly slower than the original filters in Csound, but they offer an almost flat passband and very good precision and stopband attenuation.
</para>
<para>
<emphasis>asig</emphasis> -- Input signal to be filtered.
</para>
<para>
<emphasis>xfreq</emphasis> -- Cutoff or center frequency for each of the filters.
</para>
<para>
<emphasis>xband</emphasis> -- Bandwidth of the bandpass and bandreject filters.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
Here is an example of the butterbr opcode. It uses the file <ulink url="examples/butterbr.csd"><citetitle>butterbr.csd</citetitle></ulink>.
<example>
<title>Example of the butterbr 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/butterbr.csd.xml" xmlns:xi="http://www.w3.org/2001/XInclude"/>
</example>
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<link linkend="butterbp"><citetitle>butterbp</citetitle></link>,
<link linkend="butterhp"><citetitle>butterhp</citetitle></link>,
<link linkend="butterlp"><citetitle>butterlp</citetitle></link>
</para>
</refsect1>
<refsect1>
<title>Credits</title>
<para>
<simplelist>
<member>Author: &nameparis;</member>
<member>MIT, Cambridge</member>
<member>1995</member>
</simplelist>
</para>
<para>Existed in 3.30</para>
<para>Audio rate parameters introduced in version 6.02</para>
<para>October 2013.</para>
</refsect1>
</refentry>
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