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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _radio:
+
+***************
+Radio Interface
+***************
+
+This interface is intended for AM and FM (analog) radio receivers and
+transmitters.
+
+Conventionally V4L2 radio devices are accessed through character device
+special files named ``/dev/radio`` and ``/dev/radio0`` to
+``/dev/radio63`` with major number 81 and minor numbers 64 to 127.
+
+
+Querying Capabilities
+=====================
+
+Devices supporting the radio interface set the ``V4L2_CAP_RADIO`` and
+``V4L2_CAP_TUNER`` or ``V4L2_CAP_MODULATOR`` flag in the
+``capabilities`` field of struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl. Other combinations of
+capability flags are reserved for future extensions.
+
+
+Supplemental Functions
+======================
+
+Radio devices can support :ref:`controls <control>`, and must support
+the :ref:`tuner or modulator <tuner>` ioctls.
+
+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.
+
+
+Programming
+===========
+
+Radio devices may have a couple audio controls (as discussed in
+:ref:`control`) such as a volume control, possibly custom controls.
+Further all radio devices have one tuner or modulator (these are
+discussed in :ref:`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
+:ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>` or
+:ref:`VIDIOC_G_MODULATOR <VIDIOC_G_MODULATOR>` ioctl reports the
+supported frequency range.