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  <title>Radio Interface</title>

  <para>This interface is intended for AM and FM (analog) radio
receivers and transmitters.</para>

  <para>Conventionally V4L2 radio devices are accessed through
character device special files named <filename>/dev/radio</filename>
and <filename>/dev/radio0</filename> to
<filename>/dev/radio63</filename> with major number 81 and minor
numbers 64 to 127.</para>

  <section>
    <title>Querying Capabilities</title>

    <para>Devices supporting the radio interface set the
<constant>V4L2_CAP_RADIO</constant> and
<constant>V4L2_CAP_TUNER</constant> or
<constant>V4L2_CAP_MODULATOR</constant> flag in the
<structfield>capabilities</structfield> field of &v4l2-capability;
returned by the &VIDIOC-QUERYCAP; ioctl. Other combinations of
capability flags are reserved for future extensions.</para>
  </section>

  <section>
    <title>Supplemental Functions</title>

    <para>Radio devices can support <link
linkend="control">controls</link>, and must support the <link
linkend="tuner">tuner or modulator</link> ioctls.</para>

    <para>They do not support the video input or output, audio input
or output, video standard, cropping and scaling, compression and
streaming parameter, or overlay ioctls. All other ioctls and I/O
methods are reserved for future extensions.</para>
  </section>

  <section>
    <title>Programming</title>

    <para>Radio devices may have a couple audio controls (as discussed
in <xref linkend="control" />) such as a volume control, possibly custom
controls. Further all radio devices have one tuner or modulator (these are
discussed in <xref linkend="tuner" />) with index number zero to select
the radio frequency and to determine if a monaural or FM stereo
program is received/emitted. Drivers switch automatically between AM and FM
depending on the selected frequency. The &VIDIOC-G-TUNER; or
&VIDIOC-G-MODULATOR; ioctl
reports the supported frequency range.</para>
  </section>