<!-- Converted to DocBook/SGML by Kevin Conder -->
<refentry id="tone">
  <refmeta>
    <refentrytitle>tone</refentrytitle>
  </refmeta>
 
  <refnamediv>
    <refname>tone</refname>
    <refpurpose>
      A first-order recursive low-pass with variable frequency response.
      <indexterm id="IndexTone"><primary>tone</primary></indexterm>
    </refpurpose>
  </refnamediv>
 
  <refsect1>
    <title>Description</title>
    <para>
      A first-order recursive low-pass with variable frequency response.
    </para>

    <para>
      Tone is a 1 term IIR filter. Its formula is:
        <informalexample>
          <para>
            y<subscript>n</subscript> = c1 * x<subscript>n</subscript> + c2 * y<subscript>n-1</subscript>
          </para>
        </informalexample>

      where

        <informalexample>
          <para>
            <itemizedlist>
              <listitem>
                <para>b  = 2 - cos(2 &Pi; hp/sr);</para>
              </listitem>
              <listitem>
                <para>c2 = b - sqrt(b<superscript>2</superscript> - 1.0)</para>
              </listitem>
              <listitem>
                <para>c1 = 1 - c2</para>
              </listitem>
            </itemizedlist>
          </para>
        </informalexample>
    </para>
  </refsect1>
 
  <refsect1>
    <title>Syntax</title>
    <synopsis>ar <command>tone</command> asig, khp [, iskip]</synopsis>
  </refsect1>
 
  <refsect1>
    <title>Initialization</title>
    <para>
      <emphasis>iskip</emphasis> (optional, default=0) -- initial disposition of internal data space. Since filtering incorporates a feedback loop of previous output, the initial status of the storage space used is significant. 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>ar</emphasis> -- the output audio signal.
    </para>

    <para>
      <emphasis>asig</emphasis> -- the input audio signal.
    </para>

    <para>
      <emphasis>khp</emphasis> --  the response curve's half-power point, in Hertz. Half power is defined as peak power / root 2. 
    </para>

    <para>
      <emphasis>tone</emphasis> implements a first-order recursive low-pass filter in which the variable <emphasis>khp</emphasis> (in Hz) determines the response curve's half-power point. Half power is defined as peak power / root 2.
    </para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <link linkend="areson"><citetitle>areson</citetitle></link>,
      <link linkend="aresonk"><citetitle>aresonk</citetitle></link>,
      <link linkend="atone"><citetitle>atone</citetitle></link>,
      <link linkend="atonek"><citetitle>atonek</citetitle></link>,
      <link linkend="port"><citetitle>port</citetitle></link>,
      <link linkend="portk"><citetitle>portk</citetitle></link>,
      <link linkend="reson"><citetitle>reson</citetitle></link>,
      <link linkend="resonk"><citetitle>resonk</citetitle></link>,
      <link linkend="tonek"><citetitle>tonek</citetitle></link>
    </para>
  </refsect1>
</refentry>
