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<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>3.6.聽Advanced audio</title><link rel="stylesheet" href="default.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.73.2"><link rel="start" href="index.html" title="MPlayer - The Movie Player"><link rel="up" href="usage.html" title="Chapter聽3.聽Usage"><link rel="prev" href="edl.html" title="3.5.聽Edit Decision Lists (EDL)"><link rel="next" href="cd-dvd.html" title="Chapter聽4.聽CD/DVD usage"><link rel="preface" href="howtoread.html" title="How to read this documentation"><link rel="chapter" href="intro.html" title="Chapter聽1.聽Introduction"><link rel="chapter" href="install.html" title="Chapter聽2.聽Installation"><link rel="chapter" href="usage.html" title="Chapter聽3.聽Usage"><link rel="chapter" href="cd-dvd.html" title="Chapter聽4.聽CD/DVD usage"><link rel="chapter" href="faq.html" title="Chapter聽5.聽Frequently Asked Questions"><link rel="chapter" href="containers.html" title="Chapter聽6.聽Containers"><link rel="chapter" href="codecs.html" title="Chapter聽7.聽Codecs"><link rel="chapter" href="video.html" title="Chapter聽8.聽Video output devices"><link rel="chapter" href="audio.html" title="Chapter聽9.聽Audio output devices"><link rel="chapter" href="tv.html" title="Chapter聽10.聽TV"><link rel="chapter" href="radio.html" title="Chapter聽11.聽Radio"><link rel="chapter" href="ports.html" title="Chapter聽12.聽Ports"><link rel="chapter" href="mencoder.html" title="Chapter聽13.聽Basic usage of MEncoder"><link rel="chapter" href="encoding-guide.html" title="Chapter聽14.聽Encoding with MEncoder"><link rel="appendix" href="bugreports.html" title="Appendix聽A.聽How to report bugs"><link rel="appendix" href="bugs.html" title="Appendix聽B.聽Known bugs"><link rel="appendix" href="skin.html" title="Appendix聽C.聽MPlayer skin format"><link rel="appendix" href="history.html" title="Appendix聽D.聽History"><link rel="subsection" href="advaudio.html#advaudio-surround" title="3.6.1.聽Surround/Multichannel playback"><link rel="subsection" href="advaudio.html#advaudio-channels" title="3.6.2.聽Channel manipulation"><link rel="subsection" href="advaudio.html#advaudio-volume" title="3.6.3.聽Software Volume adjustment"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">3.6.聽Advanced audio</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="edl.html">Prev</a>聽</td><th width="60%" align="center">Chapter聽3.聽Usage</th><td width="20%" align="right">聽<a accesskey="n" href="cd-dvd.html">Next</a></td></tr></table><hr></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="advaudio"></a>3.6.聽Advanced audio</h2></div></div></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="advaudio-surround"></a>3.6.1.聽Surround/Multichannel playback</h3></div></div></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="advaudio-surround-DVD"></a>3.6.1.1.聽DVDs</h4></div></div></div><p>
Most DVDs and many other files include surround sound.
<span class="application">MPlayer</span> supports surround playback but does not
enable it by default because stereo equipment is by far more common. To play a
file that has more than two channels of audio use <tt class="option">-channels</tt>.
For example, to play a DVD with 5.1 audio:
</p><pre class="screen">mplayer dvd://1 -channels 6</pre><p>
Note that despite the name "5.1" there are actually six discrete channels.
If you have surround sound equipment it is safe to put the
<tt class="option">channels</tt> option in your <span class="application">MPlayer</span>
configuration file <tt class="filename">~/.mplayer/config</tt>. For example, to make
quadraphonic playback the default, add this line:
</p><pre class="programlisting">channels=4</pre><p>
<span class="application">MPlayer</span> will then output audio in four channels when
all four channels are available.
</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="advaudio-surround-stereoinfour"></a>3.6.1.2.聽Playing stereo files to four speakers</h4></div></div></div><p>
<span class="application">MPlayer</span> does not duplicate any channels by default,
and neither do most audio drivers. If you want to do that manually:
</p><pre class="screen">mplayer <em class="replaceable"><code>filename</code></em> -af channels=2:2:0:1:0:0</pre><p>
See the section on
<a class="link" href="advaudio.html#advaudio-channels-copying" title="3.6.2.3.聽Channel copying/moving">channel copying</a> for an
explanation.
</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="advaudio-surround-passthrough"></a>3.6.1.3.聽AC-3/DTS Passthrough</h4></div></div></div><p>
DVDs usually have surround audio encoded in AC-3 (Dolby Digital) or DTS
(Digital Theater System) format. Some modern audio equipment is capable of
decoding these formats internally. <span class="application">MPlayer</span> can be
configured to relay the audio data without decoding it. This will only work if
you have a S/PDIF (Sony/Philips Digital Interface) jack in your sound card.
</p><p>
If your audio equipment can decode both AC-3 and DTS, you can safely enable
passthrough for both formats. Otherwise, enable passthrough for only the format
your equipment supports.
</p><div class="itemizedlist"><p class="title"><b>To enable passthrough on the command line:</b></p><ul type="disc"><li><p>
For AC-3 only, use <tt class="option">-ac hwac3</tt>
</p></li><li><p>
For DTS only, use <tt class="option">-ac hwdts</tt>
</p></li><li><p>
For both AC-3 and DTS, use <tt class="option">-afm hwac3</tt>
</p></li></ul></div><div class="itemizedlist"><p class="title"><b>To enable passthrough in the <span class="application">MPlayer</span>
configuration file: </b></p><ul type="disc"><li><p>
For AC-3 only, use <tt class="option">ac=hwac3,</tt>
</p></li><li><p>
For DTS only, use <tt class="option">ac=hwdts,</tt>
</p></li><li><p>
For both AC-3 and DTS, use <tt class="option">afm=hwac3</tt>
</p></li></ul></div><p>
Note that there is a comma (",") at the end of
<tt class="option">ac=hwac3,</tt> and <tt class="option">ac=hwdts,</tt>. This will make
<span class="application">MPlayer</span> fall back on the codecs it normally uses when
playing a file that does not have AC-3 or DTS audio.
<tt class="option">afm=hwac3</tt> does not need a comma;
<span class="application">MPlayer</span> will fall back anyway when an audio family
is specified.
</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="hwmpa-surround-passthrough"></a>3.6.1.4.聽MPEG audio Passthrough</h4></div></div></div><p>
Digital TV transmissions (such as DVB and ATSC) and some DVDs usually have
MPEG audio streams (in particular MP2).
Some MPEG hardware decoders such as full-featured DVB cards and DXR2
adapters can natively decode this format.
<span class="application">MPlayer</span> can be configured to relay the audio data
without decoding it.
</p><p>
To use this codec:
</p><pre class="screen"> mplayer -ac hwmpa </pre><p>
</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="advaudio-surround-matrix"></a>3.6.1.5.聽Matrix-encoded audio</h4></div></div></div><p>
<span class="emphasis"><em>***TODO***</em></span>
</p><p>
This section has yet to be written and cannot be completed until somebody
provides sample files for us to test. If you have any matrix-encoded audio
files, know where to find some, or have any information that could be helpful,
please send a message to the
<a class="ulink" href="http://lists.mplayerhq.hu/mailman/listinfo/mplayer-docs" target="_top">MPlayer-DOCS</a>
mailing list. Put "[matrix-encoded audio]" in the subject line.
</p><p>
If no files or further information are forthcoming this section will be dropped.
</p><p>
Good links:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<a class="ulink" href="http://electronics.howstuffworks.com/surround-sound5.htm" target="_top">http://electronics.howstuffworks.com/surround-sound5.htm</a>
</p></li><li><p>
<a class="ulink" href="http://www.extremetech.com/article2/0,1697,1016875,00.asp" target="_top">http://www.extremetech.com/article2/0,1697,1016875,00.asp</a>
</p></li></ul></div><p>
</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="advaudio-surround-hrtf"></a>3.6.1.6.聽Surround emulation in headphones</h4></div></div></div><p>
<span class="application">MPlayer</span> includes an HRTF (Head Related Transfer
Function) filter based on an
<a class="ulink" href="http://sound.media.mit.edu/KEMAR.html" target="_top">MIT project</a>
wherein measurements were taken from microphones mounted on a dummy human head.
</p><p>
Although it is not possible to exactly imitate a surround system,
<span class="application">MPlayer</span>'s HRTF filter does provide more spatially
immersive audio in 2-channel headphones. Regular downmixing simply combines all
the channels into two; besides combining the channels, <tt class="option">hrtf</tt>
generates subtle echoes, increases the stereo separation slightly, and alters
the volume of some frequencies. Whether HRTF sounds better may be dependent on
the source audio and a matter of personal taste, but it is definitely worth
trying out.
</p><p>
To play a DVD with HRTF:
</p><pre class="screen">mplayer dvd://1 -channels 6 -af hrtf</pre><p>
</p><p>
<tt class="option">hrtf</tt> only works well with 5 or 6 channels. Also,
<tt class="option">hrtf</tt> requires 48 kHz audio. DVD audio is already 48 kHz, but if
you have a file with a different sampling rate that you want to play using
<tt class="option">hrtf</tt> you must resample it:
</p><pre class="screen">
mplayer <em class="replaceable"><code>filename</code></em> -channels 6 -af resample=48000,hrtf
</pre><p>
</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="advaudio-surround-troubleshooting"></a>3.6.1.7.聽Troubleshooting</h4></div></div></div><p>
If you do not hear any sound out of your surround channels, check your mixer
settings with a mixer program such as <span class="application">alsamixer</span>;
audio outputs are often muted and set to zero volume by default.
</p></div></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="advaudio-channels"></a>3.6.2.聽Channel manipulation</h3></div></div></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="advaudio-channels-general"></a>3.6.2.1.聽General information</h4></div></div></div><p>
Unfortunately, there is no standard for how channels are ordered. The orders
listed below are those of AC-3 and are fairly typical; try them and see if your
source matches. Channels are numbered starting with 0.
</p><div class="orderedlist"><p class="title"><b>mono</b></p><ol type="1" compact><li value="0"><p>center</p></li></ol></div><p>
</p><div class="orderedlist"><p class="title"><b>stereo</b></p><ol type="1" compact><li value="0"><p>left</p></li><li><p>right</p></li></ol></div><p>
</p><div class="orderedlist"><p class="title"><b>quadraphonic</b></p><ol type="1" compact><li value="0"><p>left front</p></li><li><p>right front</p></li><li><p>left rear</p></li><li><p>right rear</p></li></ol></div><p>
</p><div class="orderedlist"><p class="title"><b>surround 4.0</b></p><ol type="1" compact><li value="0"><p>left front</p></li><li><p>right front</p></li><li><p>center rear</p></li><li><p>center front</p></li></ol></div><p>
</p><div class="orderedlist"><p class="title"><b>surround 5.0</b></p><ol type="1" compact><li value="0"><p>left front</p></li><li><p>right front</p></li><li><p>left rear</p></li><li><p>right rear</p></li><li><p>center front</p></li></ol></div><p>
</p><div class="orderedlist"><p class="title"><b>surround 5.1</b></p><ol type="1" compact><li value="0"><p>left front</p></li><li><p>right front</p></li><li><p>left rear</p></li><li><p>right rear</p></li><li><p>center front</p></li><li><p>subwoofer</p></li></ol></div><p>
</p><p>
The <tt class="option">-channels</tt> option is used to request the number of
channels from the audio decoder. Some audio codecs use the number of specified
channels to decide if downmixing the source is necessary. Note that this does
not always affect the number of output channels. For example, using
<tt class="option">-channels 4</tt> to play a stereo MP3 file will still result in
2-channel output since the MP3 codec will not produce the extra channels.
</p><p>
The <tt class="option">channels</tt> audio filter can be used to create or remove
channels and is useful for controlling the number of channels sent to the sound
card. See the following sections for more information on channel manipulation.
</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="advaudio-channels-mono"></a>3.6.2.2.聽Playing mono with two speakers</h4></div></div></div><p>
Mono sounds a lot better when played through two speakers - especially when
using headphones. Audio files that truly have one channel are automatically
played through two speakers; unfortunately, most files with mono sound are
actually encoded as stereo with one channel silent. The easiest and most
foolproof way to make both speakers output the same audio is the
<tt class="option">extrastereo</tt> filter:
</p><pre class="screen">mplayer <em class="replaceable"><code>filename</code></em> -af extrastereo=0</pre><p>
</p><p>
This averages both channels, resulting in both channels being half as loud as
the original. The next sections have examples of other ways to do this without a
volume decrease, but they are more complex and require different options
depending on which channel to keep. If you really need to maintain the volume,
it may be easier to experiment with the <tt class="option">volume</tt> filter and find
the right value. For example:
</p><pre class="screen">
mplayer <em class="replaceable"><code>filename</code></em> -af extrastereo=0,volume=5
</pre><p>
</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="advaudio-channels-copying"></a>3.6.2.3.聽Channel copying/moving</h4></div></div></div><p>
The <tt class="option">channels</tt> filter can move any or all channels.
Setting up all the suboptions for the <tt class="option">channels</tt>
filter can be complicated and takes a little care.
</p><div class="orderedlist"><ol type="1" compact><li><p>
Decide how many output channels you need. This is the first suboption.
</p></li><li><p>
Count how many channel moves you will do. This is the second suboption. Each
channel can be moved to several different channels at the same time, but keep
in mind that when a channel is moved (even if to only one destination) the
source channel will be empty unless another channel is moved into it. To copy
a channel, keeping the source the same, simply move the channel into both the
destination and the source. For example:
</p><pre class="programlisting">
channel 2 --> channel 3
channel 2 --> channel 2</pre><p>
</p></li><li><p>
Write out the channel copies as pairs of suboptions. Note that the first
channel is 0, the second is 1, etc. The order of these suboptions does not
matter as long as they are properly grouped into
<em class="replaceable"><code>source:destination</code></em> pairs.
</p></li></ol></div><p>
</p><h5><a name="id2574337"></a>Example: one channel in two speakers</h5><p>
Here is an example of another way to play one channel in both speakers. Suppose
for this example that the left channel should be played and the right channel
discarded. Following the steps above:
</p><div class="orderedlist"><ol type="1"><li><p>
In order to provide an output channel for each of the two speakers, the first
suboption must be "2".
</p></li><li><p>
The left channel needs to be moved to the right channel, and also must be
moved to itself so it won't be empty. This is a total of two moves, making
the second suboption "2" as well.
</p></li><li><p>
To move the left channel (channel 0) into the right channel (channel 1), the
suboption pair is "0:1", "0:0" moves the left channel onto itself.
</p></li></ol></div><p>
Putting that all together gives:
</p><pre class="screen">
mplayer <em class="replaceable"><code>filename</code></em> -af channels=2:2:0:1:0:0
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