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<section id="OrchUDO">
<title>User Defined Opcodes (UDO)</title>
<para>
Csound allows the definition of opcodes inside the orchestra header using the opcodes <link linkend="opcode"><citetitle>opcode</citetitle></link> and <link linkend="endop"><citetitle>endop</citetitle></link>. The defined opcode may run with a different number of control samples (<link linkend="ksmps"><citetitle>ksmps</citetitle></link>) using <link linkend="setksmps"><citetitle>setksmps</citetitle></link>.
</para>
<para>
To connect inputs and outputs for the UDO, use <link linkend="xin"><citetitle>xin</citetitle></link> and <link linkend="xout"><citetitle>xout</citetitle></link>.
</para>
<para>
An UDO looks like this:
</para>
<programlisting>
<emphasis role="oblock">opcode</emphasis> Lowpass, a, akk
<emphasis role="opc">setksmps</emphasis> 1 <emphasis role="comment">; need sr=kr</emphasis>
ain, ka1, ka2 <emphasis role="opc">xin</emphasis> <emphasis role="comment">; read input parameters</emphasis>
aout <emphasis role="opc">init</emphasis> 0 <emphasis role="comment">; initialize output</emphasis>
aout <emphasis role="op">=</emphasis> ain<emphasis role="op">*</emphasis>ka1 <emphasis role="op">+</emphasis> aout<emphasis role="op">*</emphasis>ka2 <emphasis role="comment">; simple tone-like filter</emphasis>
<emphasis role="opc">xout</emphasis> aout <emphasis role="comment">; write output</emphasis>
<emphasis role="oblock">endop</emphasis></programlisting>
<para>
This UDO called <emphasis>Lowpass</emphasis> takes 3 inputs (the first is a-rate, and the next two are k-rate), and delivers 1 a-rate output. Notice the use of <link linkend="xin"><citetitle>xin</citetitle></link> to receive inputs and <link linkend="xout"><citetitle>xout</citetitle></link> to deliver outputs. Also note the use of <link linkend="setksmps"><citetitle>setksmps</citetitle></link>, which is needed for the filter to work properly.
</para>
<para>To use this UDO within an instrument, you would do something like:</para>
<programlisting>
afiltered Lowpass asource, kvalue1, kvalue2</programlisting>
<para>See the entry for <link linkend="opcode"><citetitle>opcode</citetitle></link> for detailed information on UDO definition.
</para>
<para>
You can find many ready made UDO's (or contribute your own) at <ulink url="http://www.csounds.com/"><citetitle>Csounds.com</citetitle></ulink>'s <ulink url="http://www.csounds.com/udo/"><citetitle>User Defined Opcode Database</citetitle></ulink>.
</para>
</section>
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