86 lines
3.3 KiB
ReStructuredText
86 lines
3.3 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
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.. _tuner:
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*********************
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Tuners and Modulators
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*********************
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Tuners
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======
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Video input devices can have one or more tuners demodulating a RF
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signal. Each tuner is associated with one or more video inputs,
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depending on the number of RF connectors on the tuner. The ``type``
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field of the respective struct :c:type:`v4l2_input`
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returned by the :ref:`VIDIOC_ENUMINPUT` ioctl is
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set to ``V4L2_INPUT_TYPE_TUNER`` and its ``tuner`` field contains the
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index number of the tuner.
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Radio input devices have exactly one tuner with index zero, no video
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inputs.
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To query and change tuner properties applications use the
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:ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>` and
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:ref:`VIDIOC_S_TUNER <VIDIOC_G_TUNER>` ioctls, respectively. The
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struct :c:type:`v4l2_tuner` returned by :ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>`
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also contains signal status information applicable when the tuner of the
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current video or radio input is queried.
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.. note::
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:ref:`VIDIOC_S_TUNER <VIDIOC_G_TUNER>` does not switch the
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current tuner, when there is more than one at all. The tuner is solely
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determined by the current video input. Drivers must support both ioctls
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and set the ``V4L2_CAP_TUNER`` flag in the struct :c:type:`v4l2_capability`
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returned by the :ref:`VIDIOC_QUERYCAP` ioctl when the
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device has one or more tuners.
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Modulators
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==========
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Video output devices can have one or more modulators, uh, modulating a
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video signal for radiation or connection to the antenna input of a TV
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set or video recorder. Each modulator is associated with one or more
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video outputs, depending on the number of RF connectors on the
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modulator. The ``type`` field of the respective struct
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:c:type:`v4l2_output` returned by the
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:ref:`VIDIOC_ENUMOUTPUT` ioctl is set to
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``V4L2_OUTPUT_TYPE_MODULATOR`` and its ``modulator`` field contains the
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index number of the modulator.
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Radio output devices have exactly one modulator with index zero, no
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video outputs.
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A video or radio device cannot support both a tuner and a modulator. Two
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separate device nodes will have to be used for such hardware, one that
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supports the tuner functionality and one that supports the modulator
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functionality. The reason is a limitation with the
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:ref:`VIDIOC_S_FREQUENCY <VIDIOC_G_FREQUENCY>` ioctl where you
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cannot specify whether the frequency is for a tuner or a modulator.
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To query and change modulator properties applications use the
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:ref:`VIDIOC_G_MODULATOR <VIDIOC_G_MODULATOR>` and
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:ref:`VIDIOC_S_MODULATOR <VIDIOC_G_MODULATOR>` ioctl. Note that
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:ref:`VIDIOC_S_MODULATOR <VIDIOC_G_MODULATOR>` does not switch the current modulator, when there
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is more than one at all. The modulator is solely determined by the
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current video output. Drivers must support both ioctls and set the
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``V4L2_CAP_MODULATOR`` flag in the struct
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:c:type:`v4l2_capability` returned by the
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:ref:`VIDIOC_QUERYCAP` ioctl when the device has
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one or more modulators.
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Radio Frequency
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===============
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To get and set the tuner or modulator radio frequency applications use
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the :ref:`VIDIOC_G_FREQUENCY <VIDIOC_G_FREQUENCY>` and
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:ref:`VIDIOC_S_FREQUENCY <VIDIOC_G_FREQUENCY>` ioctl which both take
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a pointer to a struct :c:type:`v4l2_frequency`. These
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ioctls are used for TV and radio devices alike. Drivers must support
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both ioctls when the tuner or modulator ioctls are supported, or when
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the device is a radio device.
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