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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>