<!-- Converted to DocBook/SGML by Kevin Conder -->
<refentry id="butterlp">
  <refmeta>
    <refentrytitle>butterlp</refentrytitle>
  </refmeta>
 
  <refnamediv>
    <refname>butterlp</refname>
    <refpurpose>
      A low-pass Butterworth filter.
      <indexterm id="IndexButterlp"><primary>butterlp</primary></indexterm>
    </refpurpose>
  </refnamediv>
 
  <refsect1>
    <title>Description</title>
    <para>
      Implementation of a second-order low-pass Butterworth filter. This opcode can also be written as <link linkend="butlp"><citetitle>butlp</citetitle></link>.
    </para>
  </refsect1>
 
  <refsect1>
    <title>Syntax</title>
    <synopsis>ar <command>butterlp</command> asig, kfreq [, 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>kfreq</emphasis> -- Cutoff or center frequency for each of the filters.
    </para>
  </refsect1>
 
  <refsect1>
    <title>Examples</title>
    <para>
      Here is an example of the butterlp opcode. It uses the files <ulink url="examples/butterlp.orc"><citetitle>butterlp.orc</citetitle></ulink> and <ulink url="examples/butterlp.sco"><citetitle>butterlp.sco</citetitle></ulink>.

      <example>
        <title>Example of the butterlp opcode.</title>

        <programlisting>
<inlinemediaobject>
            <imageobject>
              <imagedata fileref="examples/butterlp.orc" format="linespecific">
            </imageobject>
          </inlinemediaobject>
        </programlisting>

        <programlisting>
<inlinemediaobject>
            <imageobject>
              <imagedata fileref="examples/butterlp.sco" format="linespecific">
            </imageobject>
          </inlinemediaobject>
        </programlisting>
      </example>
    </para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <link linkend="butterbp"><citetitle>butterbp</citetitle></link>,
      <link linkend="butterbr"><citetitle>butterbr</citetitle></link>,
      <link linkend="butterhp"><citetitle>butterhp</citetitle></link>
    </para>
  </refsect1>
 
  <refsect1>
    <title>Credits</title>
    <para>
      <simplelist>
        <member>Author: &nameparis;</member>
        <member>MIT, Cambridge</member>
        <member>1995</member>
      </simplelist>
    </para>
  </refsect1>
</refentry>
