1.. Permission is granted to copy, distribute and/or modify this
2.. document under the terms of the GNU Free Documentation License,
3.. Version 1.1 or any later version published by the Free Software
4.. Foundation, with no Invariant Sections, no Front-Cover Texts
5.. and no Back-Cover Texts. A copy of the license is included at
6.. Documentation/userspace-api/media/fdl-appendix.rst.
7..
8.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
9
10.. _audio:
11
12************************
13Audio Inputs and Outputs
14************************
15
16Audio inputs and outputs are physical connectors of a device. Video
17capture devices have inputs, output devices have outputs, zero or more
18each. Radio devices have no audio inputs or outputs. They have exactly
19one tuner which in fact *is* an audio source, but this API associates
20tuners with video inputs or outputs only, and radio devices have none of
21these. [#f1]_ A connector on a TV card to loop back the received audio
22signal to a sound card is not considered an audio output.
23
24Audio and video inputs and outputs are associated. Selecting a video
25source also selects an audio source. This is most evident when the video
26and audio source is a tuner. Further audio connectors can combine with
27more than one video input or output. Assumed two composite video inputs
28and two audio inputs exist, there may be up to four valid combinations.
29The relation of video and audio connectors is defined in the
30``audioset`` field of the respective struct
31:c:type:`v4l2_input` or struct
32:c:type:`v4l2_output`, where each bit represents the index
33number, starting at zero, of one audio input or output.
34
35To learn about the number and attributes of the available inputs and
36outputs applications can enumerate them with the
37:ref:`VIDIOC_ENUMAUDIO` and
38:ref:`VIDIOC_ENUMAUDOUT <VIDIOC_ENUMAUDOUT>` ioctl, respectively.
39The struct :c:type:`v4l2_audio` returned by the
40:ref:`VIDIOC_ENUMAUDIO` ioctl also contains signal
41status information applicable when the current audio input is queried.
42
43The :ref:`VIDIOC_G_AUDIO <VIDIOC_G_AUDIO>` and
44:ref:`VIDIOC_G_AUDOUT <VIDIOC_G_AUDOUT>` ioctls report the current
45audio input and output, respectively.
46
47.. note::
48
49   Note that, unlike :ref:`VIDIOC_G_INPUT <VIDIOC_G_INPUT>` and
50   :ref:`VIDIOC_G_OUTPUT <VIDIOC_G_OUTPUT>` these ioctls return a
51   structure as :ref:`VIDIOC_ENUMAUDIO` and
52   :ref:`VIDIOC_ENUMAUDOUT <VIDIOC_ENUMAUDOUT>` do, not just an index.
53
54To select an audio input and change its properties applications call the
55:ref:`VIDIOC_S_AUDIO <VIDIOC_G_AUDIO>` ioctl. To select an audio
56output (which presently has no changeable properties) applications call
57the :ref:`VIDIOC_S_AUDOUT <VIDIOC_G_AUDOUT>` ioctl.
58
59Drivers must implement all audio input ioctls when the device has
60multiple selectable audio inputs, all audio output ioctls when the
61device has multiple selectable audio outputs. When the device has any
62audio inputs or outputs the driver must set the ``V4L2_CAP_AUDIO`` flag
63in the struct :c:type:`v4l2_capability` returned by
64the :ref:`VIDIOC_QUERYCAP` ioctl.
65
66
67Example: Information about the current audio input
68==================================================
69
70.. code-block:: c
71
72    struct v4l2_audio audio;
73
74    memset(&audio, 0, sizeof(audio));
75
76    if (-1 == ioctl(fd, VIDIOC_G_AUDIO, &audio)) {
77	perror("VIDIOC_G_AUDIO");
78	exit(EXIT_FAILURE);
79    }
80
81    printf("Current input: %s\\n", audio.name);
82
83
84Example: Switching to the first audio input
85===========================================
86
87.. code-block:: c
88
89    struct v4l2_audio audio;
90
91    memset(&audio, 0, sizeof(audio)); /* clear audio.mode, audio.reserved */
92
93    audio.index = 0;
94
95    if (-1 == ioctl(fd, VIDIOC_S_AUDIO, &audio)) {
96	perror("VIDIOC_S_AUDIO");
97	exit(EXIT_FAILURE);
98    }
99
100.. [#f1]
101   Actually struct :c:type:`v4l2_audio` ought to have a
102   ``tuner`` field like struct :c:type:`v4l2_input`, not
103   only making the API more consistent but also permitting radio devices
104   with multiple tuners.
105