98 lines
3.5 KiB
ReStructuredText
98 lines
3.5 KiB
ReStructuredText
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.. -*- coding: utf-8; mode: rst -*-
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.. _audio:
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************************
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Audio Inputs and Outputs
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************************
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Audio inputs and outputs are physical connectors of a device. Video
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capture devices have inputs, output devices have outputs, zero or more
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each. Radio devices have no audio inputs or outputs. They have exactly
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one tuner which in fact *is* an audio source, but this API associates
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tuners with video inputs or outputs only, and radio devices have none of
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these. [#f1]_ A connector on a TV card to loop back the received audio
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signal to a sound card is not considered an audio output.
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Audio and video inputs and outputs are associated. Selecting a video
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source also selects an audio source. This is most evident when the video
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and audio source is a tuner. Further audio connectors can combine with
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more than one video input or output. Assumed two composite video inputs
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and two audio inputs exist, there may be up to four valid combinations.
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The relation of video and audio connectors is defined in the
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``audioset`` field of the respective struct
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:c:type:`v4l2_input` or struct
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:c:type:`v4l2_output`, where each bit represents the index
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number, starting at zero, of one audio input or output.
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To learn about the number and attributes of the available inputs and
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outputs applications can enumerate them with the
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:ref:`VIDIOC_ENUMAUDIO` and
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:ref:`VIDIOC_ENUMAUDOUT <VIDIOC_ENUMAUDOUT>` ioctl, respectively.
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The struct :c:type:`v4l2_audio` returned by the
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:ref:`VIDIOC_ENUMAUDIO` ioctl also contains signal
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:status information applicable when the current audio input is queried.
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The :ref:`VIDIOC_G_AUDIO <VIDIOC_G_AUDIO>` and
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:ref:`VIDIOC_G_AUDOUT <VIDIOC_G_AUDOUT>` ioctls report the current
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audio input and output, respectively.
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.. note::
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Note that, unlike :ref:`VIDIOC_G_INPUT <VIDIOC_G_INPUT>` and
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:ref:`VIDIOC_G_OUTPUT <VIDIOC_G_OUTPUT>` these ioctls return a
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structure as :ref:`VIDIOC_ENUMAUDIO` and
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:ref:`VIDIOC_ENUMAUDOUT <VIDIOC_ENUMAUDOUT>` do, not just an index.
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To select an audio input and change its properties applications call the
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:ref:`VIDIOC_S_AUDIO <VIDIOC_G_AUDIO>` ioctl. To select an audio
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output (which presently has no changeable properties) applications call
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the :ref:`VIDIOC_S_AUDOUT <VIDIOC_G_AUDOUT>` ioctl.
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Drivers must implement all audio input ioctls when the device has
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multiple selectable audio inputs, all audio output ioctls when the
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device has multiple selectable audio outputs. When the device has any
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audio inputs or outputs the driver must set the ``V4L2_CAP_AUDIO`` flag
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in the struct :c:type:`v4l2_capability` returned by
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the :ref:`VIDIOC_QUERYCAP` ioctl.
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Example: Information about the current audio input
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==================================================
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.. code-block:: c
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struct v4l2_audio audio;
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memset(&audio, 0, sizeof(audio));
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if (-1 == ioctl(fd, VIDIOC_G_AUDIO, &audio)) {
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perror("VIDIOC_G_AUDIO");
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exit(EXIT_FAILURE);
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}
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printf("Current input: %s\\n", audio.name);
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Example: Switching to the first audio input
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===========================================
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.. code-block:: c
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struct v4l2_audio audio;
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memset(&audio, 0, sizeof(audio)); /* clear audio.mode, audio.reserved */
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audio.index = 0;
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if (-1 == ioctl(fd, VIDIOC_S_AUDIO, &audio)) {
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perror("VIDIOC_S_AUDIO");
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exit(EXIT_FAILURE);
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}
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.. [#f1]
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Actually struct :c:type:`v4l2_audio` ought to have a
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``tuner`` field like struct :c:type:`v4l2_input`, not
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only making the API more consistent but also permitting radio devices
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with multiple tuners.
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