<!-- Converted to DocBook/SGML by Kevin Conder -->
<refentry id="octpch">
  <refmeta>
    <refentrytitle>octpch</refentrytitle>
  </refmeta>
 
  <refnamediv>
    <refname>octpch</refname>

    <refpurpose>
      Converts a pitch-class value to octave-point-decimal.
      <indexterm id="IndexOctpch"><primary>octpch</primary></indexterm>
    </refpurpose>
  </refnamediv>

 
  <refsect1>
    <title>Description</title>
    <para>
      Converts a pitch-class value to octave-point-decimal.
    </para>
  </refsect1>
 
  <refsect1>
    <title>Syntax</title>
    <synopsis><command>octpch</command> (pch)  (init- or control-rate args only)</synopsis>

    <para>
      where the argument within the parentheses may be a further expression.
    </para>
  </refsect1>
 
  <refsect1>
    <title>Performance</title>
    <para>
      These are really <emphasis>value converters</emphasis> with a special function of manipulating pitch data.
    </para>

    <para>
      Data concerning pitch and frequency can exist in any of the following forms:
    </para>

    <table frame="none" colsep="0">
      <title>Pitch and Frequency Values</title>

      <tgroup cols="2">
        <thead>
          <row>
            <entry>Name</entry>
            <entry>Abbreviation</entry>
          </row>
        </thead>

        <tbody>
          <row>
            <entry>octave point pitch-class (8ve.pc)</entry>
            <entry>pch</entry>
          </row>

          <row>
            <entry>octave point decimal</entry>
            <entry>oct</entry>
          </row>

          <row>
            <entry>cycles per second</entry>
            <entry>cps</entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <para>
      The first two forms consist of a whole number, representing octave registration, followed by a specially interpreted fractional part. For <emphasis>pch</emphasis>, the fraction is read as two decimal digits representing the 12 equal-tempered pitch classes from .00 for C to.11 for B. For <emphasis>oct</emphasis>, the fraction is interpreted as a true decimal fractional part of an octave. The two fractional forms are thus related by the factor 100/12. In both forms, the fraction is preceded by a whole number octave index such that 8.00 represents Middle C, 9.00 the C above, etc. Thus A440 can be represented alternatively by 440 (<emphasis>cps</emphasis>),8.09 (<emphasis>pch</emphasis>), or 8.75 (<emphasis>oct</emphasis>). Microtonal divisions of the <emphasis>pch</emphasis> semitone can be encoded by using more than two decimal places.
    </para>

    <para>
      The mnemonics of the pitch conversion units are derived from morphemes of the forms involved, the second morpheme describing the source and the first morpheme the object (result). Thus <emphasis>cpspch</emphasis>(8.09) will convert the pitch argument 8.09 to its <emphasis>cps</emphasis> (or Hertz) equivalent, giving the value of 440. Since the argument is constant over the duration of the note, this conversion will take place at i-time, before any samples for the current note are produced. 
    </para>

    <para>
      By contrast, the conversion <emphasis>cpsoct</emphasis>(8.75 + k1) which gives the value of A440 transposed by the octave interval <emphasis>k1</emphasis>. The calculation will be repeated every k-period since that is the rate at which <emphasis>k1</emphasis> varies.
    </para>

    <note>
      <title>Note</title>
      <para>
        The conversion from <emphasis>pch</emphasis> or <emphasis>oct</emphasis> into <emphasis>cps</emphasis> is not a linear operation but involves an exponential process that could be time-consuming when executed repeatedly. Csound now uses a built-in table lookup to do this efficiently, even at audio rates.
      </para>
    </note>
  </refsect1>

  <refsect1>
    <title>Examples</title>
    <para>
      Here is an example of the octpch opcode. It uses the files <ulink url="examples/octpch.orc"><citetitle>octpch.orc</citetitle></ulink> and <ulink url="examples/octpch.sco"><citetitle>octpch.sco</citetitle></ulink>.

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

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

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

      Its output should include a line like this:
      <screen>
instr 1:  ioct = 8.750
      </screen>
    </para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <link linkend="cpsoct"><citetitle>cpsoct</citetitle></link>,
      <link linkend="cpspch"><citetitle>cpspch</citetitle></link>,
      <link linkend="octcps"><citetitle>octcps</citetitle></link>,
      <link linkend="pchoct"><citetitle>pchoct</citetitle></link>
    </para>
  </refsect1>

  <refsect1>
    <title>Credits</title>
    <para>Example written by &namekevin;.</para>
  </refsect1>
</refentry>
