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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>Audio Inputs and Outputs</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1" /><link rel="home" href="index.html" title="LINUX MEDIA INFRASTRUCTURE API" /><link rel="up" href="common.html" title="Chapter 1. Common API Elements" /><link rel="prev" href="video.html" title="Video Inputs and Outputs" /><link rel="next" href="tuner.html" title="Tuners and Modulators" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Audio Inputs and Outputs</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="video.html">Prev</a> </td><th width="60%" align="center">Chapter 1. Common API Elements</th><td width="20%" align="right"> <a accesskey="n" href="tuner.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="audio"></a>Audio Inputs and Outputs</h2></div></div></div><p>Audio inputs and outputs are physical connectors of a
device. Video capture devices have inputs, output devices have
outputs, zero or more each. Radio devices have no audio inputs or
outputs. They have exactly one tuner which in fact
<span class="emphasis"><em>is</em></span> an audio source, but this API associates
tuners with video inputs or outputs only, and radio devices have
none of these.<a href="#ftn.idm140470036184416" class="footnote" id="idm140470036184416"><sup class="footnote">[5]</sup></a> A connector on a TV card to loop back the received
audio signal to a sound card is not considered an audio output.</p><p>Audio and video inputs and outputs are associated. Selecting
a video source also selects an audio source. This is most evident when
the video and audio source is a tuner. Further audio connectors can
combine with more than one video input or output. Assumed two
composite video inputs and two audio inputs exist, there may be up to
four valid combinations. The relation of video and audio connectors
is defined in the <em class="structfield"><code>audioset</code></em> field of the
respective struct <a class="link" href="vidioc-enuminput.html#v4l2-input" title="Table A.35. struct v4l2_input">v4l2_input</a> or struct <a class="link" href="vidioc-enumoutput.html#v4l2-output" title="Table A.39. struct v4l2_output">v4l2_output</a>, where each bit represents
the index number, starting at zero, of one audio input or output.</p><p>To learn about the number and attributes of the
available inputs and outputs applications can enumerate them with the
<a class="link" href="vidioc-enumaudio.html" title="ioctl VIDIOC_ENUMAUDIO"><code class="constant">VIDIOC_ENUMAUDIO</code></a> and <a class="link" href="vidioc-enumaudioout.html" title="ioctl VIDIOC_ENUMAUDOUT"><code class="constant">VIDIOC_ENUMAUDOUT</code></a> ioctl, respectively. The
struct <a class="link" href="vidioc-g-audio.html#v4l2-audio" title="Table A.47. struct v4l2_audio">v4l2_audio</a> returned by the <code class="constant">VIDIOC_ENUMAUDIO</code> ioctl
also contains signal status information applicable when the current
audio input is queried.</p><p>The <a class="link" href="vidioc-g-audio.html" title="ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO"><code class="constant">VIDIOC_G_AUDIO</code></a> and <a class="link" href="vidioc-g-audioout.html" title="ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT"><code class="constant">VIDIOC_G_AUDOUT</code></a> ioctl report
the current audio input and output, respectively. Note that, unlike
<a class="link" href="vidioc-g-input.html" title="ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT"><code class="constant">VIDIOC_G_INPUT</code></a> and <a class="link" href="vidioc-g-output.html" title="ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT"><code class="constant">VIDIOC_G_OUTPUT</code></a> these ioctls return a structure
as <code class="constant">VIDIOC_ENUMAUDIO</code> and
<code class="constant">VIDIOC_ENUMAUDOUT</code> do, not just an index.</p><p>To select an audio input and change its properties
applications call the <a class="link" href="vidioc-g-audio.html" title="ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO"><code class="constant">VIDIOC_S_AUDIO</code></a> ioctl. To select an audio
output (which presently has no changeable properties) applications
call the <a class="link" href="vidioc-g-audioout.html" title="ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT"><code class="constant">VIDIOC_S_AUDOUT</code></a> ioctl.</p><p>Drivers must implement all input ioctls when the device
has one or more inputs, all output ioctls when the device has one
or more outputs. When the device has any audio inputs or outputs the
driver must set the <code class="constant">V4L2_CAP_AUDIO</code> flag in the
struct <a class="link" href="vidioc-querycap.html#v4l2-capability" title="Table A.88. struct v4l2_capability">v4l2_capability</a> returned by the <a class="link" href="vidioc-querycap.html" title="ioctl VIDIOC_QUERYCAP"><code class="constant">VIDIOC_QUERYCAP</code></a> ioctl.</p><div class="example"><a id="idm140470036161136"></a><p class="title"><strong>Example 1.3. Information about the current audio input</strong></p><div class="example-contents"><pre class="programlisting">
struct <a class="link" href="vidioc-g-audio.html#v4l2-audio" title="Table A.47. struct v4l2_audio">v4l2_audio</a> audio;
memset (&audio, 0, sizeof (audio));
if (-1 == ioctl (fd, <a class="link" href="vidioc-g-audio.html" title="ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO"><code class="constant">VIDIOC_G_AUDIO</code></a>, &audio)) {
perror ("VIDIOC_G_AUDIO");
exit (EXIT_FAILURE);
}
printf ("Current input: %s\n", audio.name);
</pre></div></div><br class="example-break" /><div class="example"><a id="idm140470036155664"></a><p class="title"><strong>Example 1.4. Switching to the first audio input</strong></p><div class="example-contents"><pre class="programlisting">
struct <a class="link" href="vidioc-g-audio.html#v4l2-audio" title="Table A.47. struct v4l2_audio">v4l2_audio</a> audio;
memset (&audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */
audio.index = 0;
if (-1 == ioctl (fd, <a class="link" href="vidioc-g-audio.html" title="ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO"><code class="constant">VIDIOC_S_AUDIO</code></a>, &audio)) {
perror ("VIDIOC_S_AUDIO");
exit (EXIT_FAILURE);
}
</pre></div></div><br class="example-break" /><div class="footnotes"><br /><hr style="width:100; text-align:left;margin-left: 0" /><div id="ftn.idm140470036184416" class="footnote"><p><a href="#idm140470036184416" class="para"><sup class="para">[5] </sup></a>Actually struct <a class="link" href="vidioc-g-audio.html#v4l2-audio" title="Table A.47. struct v4l2_audio">v4l2_audio</a> ought to have a
<em class="structfield"><code>tuner</code></em> field like struct <a class="link" href="vidioc-enuminput.html#v4l2-input" title="Table A.35. struct v4l2_input">v4l2_input</a>, not only
making the API more consistent but also permitting radio devices with
multiple tuners.</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="video.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="common.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="tuner.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Video Inputs and Outputs </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Tuners and Modulators</td></tr></table></div></body></html>
distrib
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Fedora
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x86_64
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media
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updates
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1a595394b241504ff370a8d12ebfcea7
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files
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