<!-- Converted to DocBook/SGML by Kevin Conder -->
<refentry id="lowpass2">
  <refmeta>
    <refentrytitle>lowpass2</refentrytitle>
  </refmeta>
 
  <refnamediv>
    <refname>lowpass2</refname>
    <refpurpose>
      A resonant lowpass filter.
      <indexterm id="IndexLowpass2"><primary>lowpass2</primary></indexterm>
    </refpurpose>
  </refnamediv>
 
  <refsect1>
    <title>Description</title>
    <para>
      Implementation of a resonant second-order lowpass filter.
    </para>
  </refsect1>
 
  <refsect1>
    <title>Syntax</title>
    <synopsis>ar <command>lowpass2</command> asig, kcf, kq [, iskip]</synopsis>
  </refsect1>
 
  <refsect1>
    <title>Initialization</title>
    <para>
      <emphasis>iskip</emphasis> -- 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.
    </para>
  </refsect1>
 
  <refsect1>
    <title>Performance</title>
    <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, defined, for bandpass filters, as bandwidth/cutoff. <emphasis>kq</emphasis> should be between 1 and 500
    </para>

    <para>
      <emphasis>lowpass2</emphasis> is a second order IIR lowpass filter, with k-rate controls for cutoff frequency (<emphasis>kcf</emphasis>) and Q (<emphasis>kq</emphasis>). As <emphasis>kq</emphasis> 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 &quot;sharpness&quot; of the resonant peak. For high values of <emphasis>kq</emphasis>, a scaling function such as <emphasis>balance</emphasis> 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.
    </para>
  </refsect1>
 
  <refsect1>
    <title>Examples</title>
    <para>
      Here is an example of the lowpass2 opcode. It uses the files <ulink url="examples/lowpass2.orc"><citetitle>lowpass2.orc</citetitle></ulink> and <ulink url="examples/lowpass2.sco"><citetitle>lowpass2.sco</citetitle></ulink>.

      <example>
        <title>Example of the lowpass2 opcode.</title>

        <programlisting>
<inlinemediaobject>
            <imageobject>
              <imagedata fileref="examples/lowpass2.orc" format="linespecific">
            </imageobject>
          </inlinemediaobject>
        </programlisting>

        <programlisting>
<inlinemediaobject>
            <imageobject>
              <imagedata fileref="examples/lowpass2.sco" format="linespecific">
            </imageobject>
          </inlinemediaobject>
        </programlisting>
      </example>
    </para>
  </refsect1>
 
  <refsect1>
    <title>Credits</title>
    <para>
      <simplelist>
        <member>Author: &namesean;</member>
        <member>Seattle, Washington</member>
        <member>August 1999</member>
      </simplelist>
    </para>

    <para>New in Csound version 4.0</para>
  </refsect1>
</refentry>
