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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+.. _tuner:
+
+*********************
+Tuners and Modulators
+*********************
+
+
+Tuners
+======
+
+Video input devices can have one or more tuners demodulating a RF
+signal. Each tuner is associated with one or more video inputs,
+depending on the number of RF connectors on the tuner. The ``type``
+field of the respective struct :c:type:`v4l2_input`
+returned by the :ref:`VIDIOC_ENUMINPUT` ioctl is
+set to ``V4L2_INPUT_TYPE_TUNER`` and its ``tuner`` field contains the
+index number of the tuner.
+
+Radio input devices have exactly one tuner with index zero, no video
+inputs.
+
+To query and change tuner properties applications use the
+:ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>` and
+:ref:`VIDIOC_S_TUNER <VIDIOC_G_TUNER>` ioctls, respectively. The
+struct :c:type:`v4l2_tuner` returned by :ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>`
+also contains signal status information applicable when the tuner of the
+current video or radio input is queried.
+
+.. note::
+
+   :ref:`VIDIOC_S_TUNER <VIDIOC_G_TUNER>` does not switch the
+   current tuner, when there is more than one. The tuner is solely
+   determined by the current video input. Drivers must support both ioctls
+   and set the ``V4L2_CAP_TUNER`` flag in the struct :c:type:`v4l2_capability`
+   returned by the :ref:`VIDIOC_QUERYCAP` ioctl when the
+   device has one or more tuners.
+
+
+Modulators
+==========
+
+Video output devices can have one or more modulators, that modulate a
+video signal for radiation or connection to the antenna input of a TV
+set or video recorder. Each modulator is associated with one or more
+video outputs, depending on the number of RF connectors on the
+modulator. The ``type`` field of the respective struct
+:c:type:`v4l2_output` returned by the
+:ref:`VIDIOC_ENUMOUTPUT` ioctl is set to
+``V4L2_OUTPUT_TYPE_MODULATOR`` and its ``modulator`` field contains the
+index number of the modulator.
+
+Radio output devices have exactly one modulator with index zero, no
+video outputs.
+
+A video or radio device cannot support both a tuner and a modulator. Two
+separate device nodes will have to be used for such hardware, one that
+supports the tuner functionality and one that supports the modulator
+functionality. The reason is a limitation with the
+:ref:`VIDIOC_S_FREQUENCY <VIDIOC_G_FREQUENCY>` ioctl where you
+cannot specify whether the frequency is for a tuner or a modulator.
+
+To query and change modulator properties applications use the
+:ref:`VIDIOC_G_MODULATOR <VIDIOC_G_MODULATOR>` and
+:ref:`VIDIOC_S_MODULATOR <VIDIOC_G_MODULATOR>` ioctl. Note that
+:ref:`VIDIOC_S_MODULATOR <VIDIOC_G_MODULATOR>` does not switch the current modulator, when there
+is more than one at all. The modulator is solely determined by the
+current video output. Drivers must support both ioctls and set the
+``V4L2_CAP_MODULATOR`` flag in the struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl when the device has
+one or more modulators.
+
+
+Radio Frequency
+===============
+
+To get and set the tuner or modulator radio frequency applications use
+the :ref:`VIDIOC_G_FREQUENCY <VIDIOC_G_FREQUENCY>` and
+:ref:`VIDIOC_S_FREQUENCY <VIDIOC_G_FREQUENCY>` ioctl which both take
+a pointer to a struct :c:type:`v4l2_frequency`. These
+ioctls are used for TV and radio devices alike. Drivers must support
+both ioctls when the tuner or modulator ioctls are supported, or when
+the device is a radio device.