<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>
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" 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>
</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>
</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>
</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>
</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>
</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>
</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="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>
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="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>
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 class="orderedlist" type="1" compact><li class="listitem"><p>
Decide how many output channels you need. This is the first suboption.
</p></li><li class="listitem"><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 class="listitem"><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><h4><a name="id3059325"></a>Example: one channel in two speakers</h4><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 class="orderedlist" type="1"><li class="listitem"><p>
In order to provide an output channel for each of the two speakers, the first
suboption must be "2".
</p></li><li class="listitem"><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 class="listitem"><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
</pre><p>
</p><p>
The advantage this example has over <tt class="option">extrastereo</tt> is that the
volume of each output channel is the same as the input channel. The disadvantage
is that the suboptions must be changed to "2:2:1:0:1:1" when the desired audio
is in the right channel. Also, it is more difficult to remember and type.
</p><h4><a name="id3059384"></a>Example: left channel in two speakers shortcut</h4><p>
There is actually a much easier way to use the <tt class="option">channels</tt> filter
for playing the left channel in both speakers:
</p><pre class="screen">mplayer <em class="replaceable"><code>filename</code></em> -af channels=1</pre><p>
The second channel is discarded and, with no further suboptions, the single
remaining channel is left alone. Sound card drivers automatically play
single-channel audio in both speakers. This only works when the desired channel
is on the left.
</p><h4><a name="id3059410"></a>Example: duplicate front channels to the rear</h4><p>
Another common operation is to duplicate the front channels and play them back
on the rear speakers of a quadraphonic setup.
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
There should be four output channels. The first suboption is "4".
</p></li><li class="listitem"><p>
Each of the two front channels needs to be moved to the corresponding rear
channel and also to itself. This is four moves, so the second suboption is "4".
</p></li><li class="listitem"><p>
The left front (channel 0) needs to moved to the left rear (channel 2):
"0:2". The left front also needs to be moved to itself: "0:0". The right
front (channel 1) is moved to the right rear (channel 3): "1:3", and also to
itself: "1:1".
</p></li></ol></div><p>
Combine all the suboptions to get:
</p><pre class="screen">
mplayer <em class="replaceable"><code>filename</code></em> -af channels=4:4:0:2:0:0:1:3:1:1
</pre><p>
</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>
The <tt class="option">pan</tt> filter can mix channels in user-specified proportions.
This allows for everything the <tt class="option">channels</tt> filter can do and
more. Unfortunately, the suboptions are much more complicated.
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
Decide how many channels to work with. You may need to specify this with
<tt class="option">-channels</tt> and/or <tt class="option">-af channels</tt>.
Later examples will show when to use which.
</p></li><li class="listitem"><p>
Decide how many channels to feed into <tt class="option">pan</tt> (further decoded
channels are discarded). This is the first suboption, and it also controls how
many channels to employ for output.
</p></li><li class="listitem"><p>
The remaining suboptions specify how much of each channel gets mixed into each
other channel. This is the complicated part. To break the task down, split the
suboptions into several sets, one set for each input channel. Each suboption
within a set corresponds to an output channel. The number you specify will be
the percentage of the input channel that gets mixed into the output channel.
</p><p>
<tt class="option">pan</tt> accepts values from 0 to 512, yielding 0% to 51200% of
the original volume. Be careful when using values greater than 1. Not only
can this give you very high volume, but if you exceed the sample range of
your sound card you may hear painful pops and clicks. If you want you can
follow <tt class="option">pan</tt> with <tt class="option">,volume</tt> to enable clipping,
but it is best to keep the values of <tt class="option">pan</tt> low enough that
clipping is not necessary.
</p></li></ol></div><p>
</p><h4><a name="id3059546"></a>Example: one channel in two speakers</h4><p>
Here is yet another example for playing the left channel in two speakers. Follow
the steps above:
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
<tt class="option">pan</tt> should output two channels, so the first
suboption is "2".
</p></li><li class="listitem"><p>
Since we have two input channels, there will be two sets of suboptions.
Since there are also two output channels,
there will be two suboptions per set.
The left channel from the file should go with full volume to
the new left and the right channels.
Thus the first set of suboptions is "1:1".
The right channel should be discarded, so the second would be "0:0".
Any 0 values at the end can be left out, but for ease of
understanding we will keep them.
</p></li></ol></div><p>
Putting those options together gives:
</p><pre class="screen">mplayer <em class="replaceable"><code>filename</code></em> -af pan=2:1:1:0:0</pre><p>
If the right channel is desired instead of the left, the suboptions to
<tt class="option">pan</tt> will be "2:0:0:1:1".
</p><h4><a name="id3059597"></a>Example: left channel in two speakers shortcut</h4><p>
As with <tt class="option">channels</tt>, there is a shortcut that only works with the
left channel:
</p><pre class="screen">mplayer <em class="replaceable"><code>filename</code></em> -af pan=1:1</pre><p>
Since <tt class="option">pan</tt> has only one channel of input (the other channel is
discarded), there is only one set with one suboption, which specifies that the
only channel gets 100% of itself.
</p><h4><a name="id3059623"></a>Example: downmixing 6-channel PCM</h4><p>
<span class="application">MPlayer</span>'s decoder for 6-channel PCM is not capable of
downmixing. Here is a way to downmix PCM using <tt class="option">pan</tt>:
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
The number of output channels is 2, so the first suboption is "2".
</p></li><li class="listitem"><p>
With six input channels there will be six sets of options. Fortunately,
since we only care about the output of the first two channels, we only need to
make two sets; the remaining four sets can be omitted. Beware that not all
multichannel audio files have the same channel order! This example
demonstrates downmixing a file with the same channels as AC-3 5.1:
</p><pre class="programlisting">
0 - front left
1 - front right
2 - rear left
3 - rear right
4 - center front
5 - subwoofer</pre><p>
The first set of suboptions lists the percentages of the original volume, in
order, which each output channel should receive from the
front left channel: "1:0".
The front right channel should go into the right output: "0:1".
The same for the rear channels: "1:0" and "0:1".
The center channel goes into both output channels with half volume:
"0.5:0.5", and the subwoofer goes into both with full volume: "1:1".
</p></li></ol></div><p>
Put all that together, for:
</p><pre class="screen">
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
</pre><p>
The percentages listed above are only a rough example. Feel free to tweak them.
</p><h4><a name="id3059690"></a>Example: Playing 5.1 audio on big speakers without a subwoofer</h4><p>
If you have a huge pair of front speakers you may not want to waste any money on
buying a subwoofer for a complete 5.1 sound system. If you use
<tt class="option">-channels 5</tt> to request that liba52 decode 5.1 audio in 5.0,
the subwoofer channel is simply discarded. If you want to distribute the
subwoofer channel yourself you need to downmix manually with
<tt class="option">pan</tt>:
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
Since <tt class="option">pan</tt> needs to examine all six channels, specify
<tt class="option">-channels 6</tt> so liba52 decodes them all.
</p></li><li class="listitem"><p>
<tt class="option">pan</tt> outputs to only five channels, the first suboption is 5.
</p></li><li class="listitem"><p>
Six input channels and five output channels means six sets of five suboptions.
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc" compact><li class="listitem"><p>
The left front channel only replicates onto itself:
"1:0:0:0:0"
</p></li><li class="listitem"><p>
Same for the right front channel:
"0:1:0:0:0"
</p></li><li class="listitem"><p>
Same for the left rear channel:
"0:0:1:0:0"
</p></li><li class="listitem"><p>
And also the same for the right rear channel:
"0:0:0:1:0"
</p></li><li class="listitem"><p>
Center front, too:
"0:0:0:0:1"
</p></li><li class="listitem"><p>
And now we have to decide what to do with the subwoofer,
e.g. half into front right and front left:
"0.5:0.5:0:0:0"
</p></li></ul></div><p>
</p></li></ol></div><p>
Combine all those options to get:
</p><pre class="screen">
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
</pre><p>
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