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<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>4.2. Channel manipulation</title><link rel="stylesheet" href="default.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.75.2"><link rel="home" href="index.html" title="MPlayer - The Movie Player"><link rel="up" href="advaudio.html" title="Chapter 4. Advanced audio usage"><link rel="prev" href="advaudio-surround.html" title="4.1. Surround/Multichannel playback"><link rel="next" href="advaudio-volume.html" title="4.3. Software Volume adjustment"><link rel="preface" href="howtoread.html" title="How to read this documentation"><link rel="chapter" href="intro.html" title="Chapter 1. Introduction"><link rel="chapter" href="install.html" title="Chapter 2. Installation"><link rel="chapter" href="usage.html" title="Chapter 3. Usage"><link rel="chapter" href="advaudio.html" title="Chapter 4. Advanced audio usage"><link rel="chapter" href="cd-dvd.html" title="Chapter 5. CD/DVD usage"><link rel="chapter" href="tv.html" title="Chapter 6. TV"><link rel="chapter" href="radio.html" title="Chapter 7. Radio"><link rel="chapter" href="video.html" title="Chapter 8. Video output devices"><link rel="chapter" href="ports.html" title="Chapter 9. Ports"><link rel="chapter" href="mencoder.html" title="Chapter 10. Basic usage of MEncoder"><link rel="chapter" href="encoding-guide.html" title="Chapter 11. Encoding with MEncoder"><link rel="chapter" href="faq.html" title="Chapter 12. Frequently Asked Questions"><link rel="appendix" href="bugreports.html" title="Appendix A. How to report bugs"><link rel="appendix" href="skin.html" title="Appendix B. MPlayer skin format"><link rel="subsection" href="advaudio-channels.html#advaudio-channels-general" title="4.2.1. General information"><link rel="subsection" href="advaudio-channels.html#advaudio-channels-mono" title="4.2.2. Playing mono with two speakers"><link rel="subsection" href="advaudio-channels.html#advaudio-channels-copying" title="4.2.3. Channel copying/moving"><link rel="subsection" href="advaudio-channels.html#advaudio-channels-mixing" title="4.2.4. Channel mixing"></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">4.2. Channel manipulation</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="advaudio-surround.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Advanced audio usage</th><td width="20%" align="right"> <a accesskey="n" href="advaudio-volume.html">Next</a></td></tr></table><hr></div><div class="sect1" title="4.2. Channel manipulation"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="advaudio-channels"></a>4.2. Channel manipulation</h2></div></div></div><div class="sect2" title="4.2.1. General information"><div class="titlepage"><div><div><h3 class="title"><a name="advaudio-channels-general"></a>4.2.1. General information</h3></div></div></div><p>
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Unfortunately, there is no standard for how channels are ordered. The orders
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listed below are those of AC-3 and are fairly typical; try them and see if your
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source matches. Channels are numbered starting with 0.
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</p><div class="orderedlist" title="mono"><p class="title"><b>mono</b></p><ol class="orderedlist" type="1" compact><li class="listitem" value="0"><p>center</p></li></ol></div><p>
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</p><div class="orderedlist" title="stereo"><p class="title"><b>stereo</b></p><ol class="orderedlist" type="1" compact><li class="listitem" value="0"><p>left</p></li><li class="listitem"><p>right</p></li></ol></div><p>
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</p><div class="orderedlist" title="quadraphonic"><p class="title"><b>quadraphonic</b></p><ol class="orderedlist" type="1" compact><li class="listitem" value="0"><p>left front</p></li><li class="listitem"><p>right front</p></li><li class="listitem"><p>left rear</p></li><li class="listitem"><p>right rear</p></li></ol></div><p>
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</p><div class="orderedlist" title="surround 4.0"><p class="title"><b>surround 4.0</b></p><ol class="orderedlist" type="1" compact><li class="listitem" value="0"><p>left front</p></li><li class="listitem"><p>right front</p></li><li class="listitem"><p>center rear</p></li><li class="listitem"><p>center front</p></li></ol></div><p>
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</p><div class="orderedlist" title="surround 5.0"><p class="title"><b>surround 5.0</b></p><ol class="orderedlist" type="1" compact><li class="listitem" value="0"><p>left front</p></li><li class="listitem"><p>right front</p></li><li class="listitem"><p>left rear</p></li><li class="listitem"><p>right rear</p></li><li class="listitem"><p>center front</p></li></ol></div><p>
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</p><div class="orderedlist" title="surround 5.1"><p class="title"><b>surround 5.1</b></p><ol class="orderedlist" type="1" compact><li class="listitem" value="0"><p>left front</p></li><li class="listitem"><p>right front</p></li><li class="listitem"><p>left rear</p></li><li class="listitem"><p>right rear</p></li><li class="listitem"><p>center front</p></li><li class="listitem"><p>subwoofer</p></li></ol></div><p>
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The <tt class="option">-channels</tt> option is used to request the number of
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channels from the audio decoder. Some audio codecs use the number of specified
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channels to decide if downmixing the source is necessary. Note that this does
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not always affect the number of output channels. For example, using
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<tt class="option">-channels 4</tt> to play a stereo MP3 file will still result in
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2-channel output since the MP3 codec will not produce the extra channels.
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The <tt class="option">channels</tt> audio filter can be used to create or remove
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channels and is useful for controlling the number of channels sent to the sound
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card. See the following sections for more information on channel manipulation.
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</p></div><div class="sect2" title="4.2.2. Playing mono with two speakers"><div class="titlepage"><div><div><h3 class="title"><a name="advaudio-channels-mono"></a>4.2.2. Playing mono with two speakers</h3></div></div></div><p>
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Mono sounds a lot better when played through two speakers - especially when
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using headphones. Audio files that truly have one channel are automatically
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played through two speakers; unfortunately, most files with mono sound are
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actually encoded as stereo with one channel silent. The easiest and most
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foolproof way to make both speakers output the same audio is the
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<tt class="option">extrastereo</tt> filter:
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</p><pre class="screen">mplayer <em class="replaceable"><code>filename</code></em> -af extrastereo=0</pre><p>
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This averages both channels, resulting in both channels being half as loud as
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the original. The next sections have examples of other ways to do this without a
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volume decrease, but they are more complex and require different options
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depending on which channel to keep. If you really need to maintain the volume,
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it may be easier to experiment with the <tt class="option">volume</tt> filter and find
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the right value. For example:
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</p><pre class="screen">
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mplayer <em class="replaceable"><code>filename</code></em> -af extrastereo=0,volume=5
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</p></div><div class="sect2" title="4.2.3. Channel copying/moving"><div class="titlepage"><div><div><h3 class="title"><a name="advaudio-channels-copying"></a>4.2.3. Channel copying/moving</h3></div></div></div><p>
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The <tt class="option">channels</tt> filter can move any or all channels.
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Setting up all the suboptions for the <tt class="option">channels</tt>
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filter can be complicated and takes a little care.
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</p><div class="orderedlist"><ol class="orderedlist" type="1" compact><li class="listitem"><p>
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Decide how many output channels you need. This is the first suboption.
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</p></li><li class="listitem"><p>
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Count how many channel moves you will do. This is the second suboption. Each
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channel can be moved to several different channels at the same time, but keep
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in mind that when a channel is moved (even if to only one destination) the
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source channel will be empty unless another channel is moved into it. To copy
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a channel, keeping the source the same, simply move the channel into both the
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destination and the source. For example:
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</p><pre class="programlisting">
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channel 2 --> channel 3
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channel 2 --> channel 2</pre><p>
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</p></li><li class="listitem"><p>
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Write out the channel copies as pairs of suboptions. Note that the first
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channel is 0, the second is 1, etc. The order of these suboptions does not
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matter as long as they are properly grouped into
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<em class="replaceable"><code>source:destination</code></em> pairs.
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</p></li></ol></div><p>
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</p><h4><a name="id464538"></a>Example: one channel in two speakers</h4><p>
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Here is an example of another way to play one channel in both speakers. Suppose
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for this example that the left channel should be played and the right channel
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discarded. Following the steps above:
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</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
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In order to provide an output channel for each of the two speakers, the first
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suboption must be "2".
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</p></li><li class="listitem"><p>
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The left channel needs to be moved to the right channel, and also must be
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moved to itself so it won't be empty. This is a total of two moves, making
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the second suboption "2" as well.
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</p></li><li class="listitem"><p>
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To move the left channel (channel 0) into the right channel (channel 1), the
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suboption pair is "0:1", "0:0" moves the left channel onto itself.
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</p></li></ol></div><p>
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Putting that all together gives:
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</p><pre class="screen">
86
mplayer <em class="replaceable"><code>filename</code></em> -af channels=2:2:0:1:0:0
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The advantage this example has over <tt class="option">extrastereo</tt> is that the
90
volume of each output channel is the same as the input channel. The disadvantage
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is that the suboptions must be changed to "2:2:1:0:1:1" when the desired audio
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is in the right channel. Also, it is more difficult to remember and type.
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</p><h4><a name="id464587"></a>Example: left channel in two speakers shortcut</h4><p>
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There is actually a much easier way to use the <tt class="option">channels</tt> filter
95
for playing the left channel in both speakers:
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</p><pre class="screen">mplayer <em class="replaceable"><code>filename</code></em> -af channels=1</pre><p>
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The second channel is discarded and, with no further suboptions, the single
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remaining channel is left alone. Sound card drivers automatically play
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single-channel audio in both speakers. This only works when the desired channel
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</p><h4><a name="id464607"></a>Example: duplicate front channels to the rear</h4><p>
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Another common operation is to duplicate the front channels and play them back
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on the rear speakers of a quadraphonic setup.
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</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
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There should be four output channels. The first suboption is "4".
106
</p></li><li class="listitem"><p>
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Each of the two front channels needs to be moved to the corresponding rear
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channel and also to itself. This is four moves, so the second suboption is "4".
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</p></li><li class="listitem"><p>
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The left front (channel 0) needs to moved to the left rear (channel 2):
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"0:2". The left front also needs to be moved to itself: "0:0". The right
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front (channel 1) is moved to the right rear (channel 3): "1:3", and also to
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</p></li></ol></div><p>
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Combine all the suboptions to get:
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</p><pre class="screen">
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mplayer <em class="replaceable"><code>filename</code></em> -af channels=4:4:0:2:0:0:1:3:1:1
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</p></div><div class="sect2" title="4.2.4. Channel mixing"><div class="titlepage"><div><div><h3 class="title"><a name="advaudio-channels-mixing"></a>4.2.4. Channel mixing</h3></div></div></div><p>
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The <tt class="option">pan</tt> filter can mix channels in user-specified proportions.
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This allows for everything the <tt class="option">channels</tt> filter can do and
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more. Unfortunately, the suboptions are much more complicated.
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</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
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Decide how many channels to work with. You may need to specify this with
125
<tt class="option">-channels</tt> and/or <tt class="option">-af channels</tt>.
126
Later examples will show when to use which.
127
</p></li><li class="listitem"><p>
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Decide how many channels to feed into <tt class="option">pan</tt> (further decoded
129
channels are discarded). This is the first suboption, and it also controls how
130
many channels to employ for output.
131
</p></li><li class="listitem"><p>
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The remaining suboptions specify how much of each channel gets mixed into each
133
other channel. This is the complicated part. To break the task down, split the
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suboptions into several sets, one set for each output channel. Each suboption
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within a set corresponds to an input channel. The number you specify will be
136
the percentage of the input channel that gets mixed into the output channel.
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<tt class="option">pan</tt> accepts values from 0 to 512, yielding 0% to 51200% of
139
the original volume. Be careful when using values greater than 1. Not only
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can this give you very high volume, but if you exceed the sample range of
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your sound card you may hear painful pops and clicks. If you want you can
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follow <tt class="option">pan</tt> with <tt class="option">,volume</tt> to enable clipping,
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but it is best to keep the values of <tt class="option">pan</tt> low enough that
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clipping is not necessary.
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</p></li></ol></div><p>
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</p><h4><a name="id464724"></a>Example: one channel in two speakers</h4><p>
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Here is yet another example for playing the left channel in two speakers. Follow
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</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
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<tt class="option">pan</tt> should output two channels, so the first
152
</p></li><li class="listitem"><p>
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Since we have two input channels, there will be two sets of suboptions.
154
Since there are also two output channels,
155
there will be two suboptions per set.
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The left channel from the file should go with full volume to
157
the new left and the right channels.
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Thus the first set of suboptions is "1:1".
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The right channel should be discarded, so the second would be "0:0".
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Any 0 values at the end can be left out, but for ease of
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understanding we will keep them.
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</p></li></ol></div><p>
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Putting those options together gives:
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</p><pre class="screen">mplayer <em class="replaceable"><code>filename</code></em> -af pan=2:1:1:0:0</pre><p>
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If the right channel is desired instead of the left, the suboptions to
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<tt class="option">pan</tt> will be "2:0:0:1:1".
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</p><h4><a name="id464767"></a>Example: left channel in two speakers shortcut</h4><p>
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As with <tt class="option">channels</tt>, there is a shortcut that only works with the
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</p><pre class="screen">mplayer <em class="replaceable"><code>filename</code></em> -af pan=1:1</pre><p>
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Since <tt class="option">pan</tt> has only one channel of input (the other channel is
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discarded), there is only one set with one suboption, which specifies that the
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only channel gets 100% of itself.
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</p><h4><a name="id464790"></a>Example: downmixing 6-channel PCM</h4><p>
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<span class="application">MPlayer</span>'s decoder for 6-channel PCM is not capable of
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downmixing. Here is a way to downmix PCM using <tt class="option">pan</tt>:
177
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
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The number of output channels is 2, so the first suboption is "2".
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</p></li><li class="listitem"><p>
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With six input channels there will be six sets of options. Fortunately,
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since we only care about the output of the first two channels, we only need to
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make two sets; the remaining four sets can be omitted. Beware that not all
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multichannel audio files have the same channel order! This example
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demonstrates downmixing a file with the same channels as AC-3 5.1:
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</p><pre class="programlisting">
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5 - subwoofer</pre><p>
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The first set of suboptions lists the percentages of the original volume, in
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order, which each output channel should receive from the
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front left channel: "1:0".
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The front right channel should go into the right output: "0:1".
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The same for the rear channels: "1:0" and "0:1".
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The center channel goes into both output channels with half volume:
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"0.5:0.5", and the subwoofer goes into both with full volume: "1:1".
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</p></li></ol></div><p>
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Put all that together, for:
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</p><pre class="screen">
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mplayer <em class="replaceable"><code>6-channel.wav</code></em> -af pan=2:1:0:0:1:1:0:0:1:0.5:0.5:1:1
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The percentages listed above are only a rough example. Feel free to tweak them.
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</p><h4><a name="id464846"></a>Example: Playing 5.1 audio on big speakers without a subwoofer</h4><p>
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If you have a huge pair of front speakers you may not want to waste any money on
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buying a subwoofer for a complete 5.1 sound system. If you use
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<tt class="option">-channels 5</tt> to request that liba52 decode 5.1 audio in 5.0,
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the subwoofer channel is simply discarded. If you want to distribute the
210
subwoofer channel yourself you need to downmix manually with
211
<tt class="option">pan</tt>:
212
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
213
Since <tt class="option">pan</tt> needs to examine all six channels, specify
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<tt class="option">-channels 6</tt> so liba52 decodes them all.
215
</p></li><li class="listitem"><p>
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<tt class="option">pan</tt> outputs to only five channels, the first suboption is 5.
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</p></li><li class="listitem"><p>
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Six input channels and five output channels means six sets of five suboptions.
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</p><div class="itemizedlist"><ul class="itemizedlist" type="disc" compact><li class="listitem"><p>
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The left front channel only replicates onto itself:
222
</p></li><li class="listitem"><p>
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Same for the right front channel:
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</p></li><li class="listitem"><p>
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Same for the left rear channel:
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</p></li><li class="listitem"><p>
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And also the same for the right rear channel:
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</p></li><li class="listitem"><p>
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</p></li><li class="listitem"><p>
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And now we have to decide what to do with the subwoofer,
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e.g. half into front right and front left:
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</p></li></ul></div><p>
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</p></li></ol></div><p>
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Combine all those options to get:
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</p><pre class="screen">
242
mplayer <em class="replaceable"><code>dvd://1</code></em> -channels 6 -af pan=5:1:0:0:0:0:0:1:0:0:0:0:0:1:0:0:0:0:0:1:0:0:0:0:0:1:0.5:0.5:0:0:0
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