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MPlayer advanced audio usage guide by Corey Hickey < bugfood-ml AH fatooh POUM org>

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Date: Sep 4, 2005 1:26 AM
Subject: [MPlayer-DOCS] [PATCH] mplayer advanced audio usage guide


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AUTHORS
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@ -331,6 +331,7 @@ Hess, Andreas <jaska@gmx.net>
Hickey, Corey <bugfood-ml@fatooh.org>
* telecine/interlacing HOWTO for the MEncoder documentation
* advanced audio usage guide for the MPlayer documentation
Hidvégi, Zoltán (Zoli) <mplayer@hzoli.com>
* filmdint video filter

1
ChangeLog
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@ -8,6 +8,7 @@ MPlayer (1.0)
which is to be merged into the existing guide
* encoding tips for x264 and XviD
* how to set up MEncoder for x264 support
* new advanced audio usage guide
* Hungarian XML documentation translation finished
* Czech documentation translation finished
* Italian man page synced

635
DOCS/xml/en/usage.xml
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@ -400,4 +400,639 @@ that time. You can then go back and fine-tune the generated EDL file.
</sect1>
<sect1 id="advaudio" xreflabel="Advanced Audio">
<title>Advanced Audio</title>
<sect2 id="advaudio-surround">
<title>Surround/Multichannel Playback</title>
<sect3 id="advaudio-surround-DVD">
<title>DVDs</title>
<para>
Most DVDs and many other files include surround sound.
<application>MPlayer</application> 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 <option>-channels</option>.
For example, to play a DVD with 5.1 audio:
<screen>mplayer dvd://1 -channels 6</screen>
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
<option>channels</option> option in your <application>MPlayer</application>
configuration file <filename>~/.mplayer/config</filename>. For example, to make
quadraphonic playback the default, add this line:
<programlisting>channels=4</programlisting>
<application>MPlayer</application> will then output audio in four channels when
all four channels are available.
</para>
</sect3>
<sect3 id="advaudio-surround-stereoinfour">
<title>Playing Stereo Files to Four Speakers</title>
<para>
<application>MPlayer</application> does not duplicate any channels by default,
and neither do most audio drivers. If you want to do that manually:
<screen>mplayer <replaceable>filename</replaceable> -af channels=2:2:0:1:0:0</screen>
See the section on
<link linkend="advaudio-channels-copying">channel copying</link> for an
explanation.
</para>
</sect3>
<sect3 id="advaudio-surround-passthrough">
<title>AC3/DTS Passthrough</title>
<para>
DVDs usually have surround audio encoded in AC3 (Dolby Digital) or DTS
(Digital Theater System) format. Some modern audio equipment is capable of
decoding these formats internally. <application>MPlayer</application> 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.
</para>
<para>
If your audio equipment can decode both AC3 and DTS, you can safely enable
passthrough for both formats. Otherwise, enable passthrough for only the format
your equipment supports.
</para>
<itemizedlist>
<title>To enable passthrough on the command line:</title>
<listitem><para>
For AC3 only, use <option>-ac hwac3</option>
</para></listitem>
<listitem><para>
For DTS only, use <option>-ac hwdts</option>
</para></listitem>
<listitem><para>
For both AC3 and DTS, use <option>-afm hwac3</option>
</para></listitem>
</itemizedlist>
<itemizedlist>
<title>To enable passthrough in the <application>MPlayer</application>
configuration file: </title>
<listitem><para>
For AC3 only, use <option>ac=hwac3,</option>
</para></listitem>
<listitem><para>
For DTS only, use <option>ac=hwdts,</option>
</para></listitem>
<listitem><para>
For both AC3 and DTS, use <option>-afm hwac3</option>
</para></listitem>
</itemizedlist>
<para>
Note that there is a comma ("<keycap>,</keycap>") at the end of
<option>ac=hwac3,</option> and <option>ac=hwdts,</option>. This will make
<application>MPlayer</application> fall back to the codecs it normally uses when
playing a file that doesn't have AC3 or DTS audio. <option>afm=hwac3</option>
does not need a comma; <application>MPlayer</application> will fall back anyway
when an audio family is specified.
</para>
</sect3>
<sect3 id="advaudio-surround-matrix">
<title>Matrix-encoded Audio</title>
<para>
***TODO***
This section has not be written yet as we would need some matrix-encoded
samples to test with.
I someone has such files, or would know how to handle them in detail,
please contact us though the mailling list adding [matrix-encoded samples]
to the subject of your mail.
If no such file can be found, this section will just be dropped.
</para>
<para>
Good links:
<itemizedlist>
<listitem><para>
<ulink url="http://electronics.howstuffworks.com/surround-sound5.htm">http://electronics.howstuffworks.com/surround-sound5.htm</ulink>
</para></listitem>
<listitem><para>
<ulink url="http://www.extremetech.com/article2/0,1697,1016875,00.asp">http://www.extremetech.com/article2/0,1697,1016875,00.asp</ulink>
</para></listitem>
</itemizedlist>
</para>
</sect3>
<sect3 id="advaudio-surround-hrtf">
<title>Surround Emulation in Headphones</title>
<para>
<application>MPlayer</application> includes an HRTF (Head Related Transfer
Function) filter based on an MIT project wherein measurements were taken from
microphones mounted on a dummy human head. See the
<ulink url="http://sound.media.mit.edu/KEMAR.html">project page</ulink> for more
information.
</para>
<para>
Although it is not possible to exactly imitate a surround system,
<application>MPlayer</application>'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, <option>hrtf</option>
generates subtle echoes, increases the stereo separation slightly, and alters
the volume of some frequencies. Whether HRTF sounds better may be dependant on
the source audio and a matter of personal taste, but it is definitely worth
trying out.
</para>
<para>
To play a DVD with HRTF:
<screen>mplayer dvd://1 -channels 6 -af hrtf</screen>
</para>
<para>
<option>hrtf</option> only works well with 5 or 6 channels. Also,
<option>hrtf</option> 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
<option>hrtf</option> you must resample it:
<screen>mplayer <replaceable>filename</replaceable> -channels 6 -af resample=48000,hrtf</screen>
</para>
</sect3>
<sect3 id="advaudio-surround-troubleshooting">
<title>Troubleshooting</title>
<para>
If you don't hear any sound out of your surround channels, check your mixer
settings with a mixer program such as <application>alsamixer</application>;
audio outputs are often muted and set to zero volume by default.
</para>
</sect3>
</sect2>
<sect2 id="advaudio-channels">
<title>Channel Manipulation</title>
<sect3 id="advaudio-channels-general">
<title>General Information</title>
<para>
Unfortunately, there is no standard for how channels are ordered. The orders
listed below are those of AC3 and are fairly typical; try them and see if your
source matches. Channels are numbered starting with 0.
<orderedlist spacing="compact">
<title>Mono</title>
<listitem override="0"><simpara>center</simpara></listitem>
</orderedlist>
<orderedlist spacing="compact">
<title>Stereo</title>
<listitem override="0"><simpara>left</simpara></listitem>
<listitem><simpara>right</simpara></listitem>
</orderedlist>
<orderedlist spacing="compact">
<title>Quadraphonic</title>
<listitem override="0"><simpara>left front</simpara></listitem>
<listitem><simpara>right front</simpara></listitem>
<listitem><simpara>left rear</simpara></listitem>
<listitem><simpara>right rear</simpara></listitem>
</orderedlist>
<orderedlist spacing="compact">
<title>Surround 4.0</title>
<listitem override="0"><simpara>left front</simpara></listitem>
<listitem><simpara>right front</simpara></listitem>
<listitem><simpara>center rear</simpara></listitem>
<listitem><simpara>center front</simpara></listitem>
</orderedlist>
<orderedlist spacing="compact">
<title>Surround 5.0</title>
<listitem override="0"><simpara>left front</simpara></listitem>
<listitem><simpara>right front</simpara></listitem>
<listitem><simpara>left rear</simpara></listitem>
<listitem><simpara>right rear</simpara></listitem>
<listitem><simpara>center front</simpara></listitem>
</orderedlist>
<orderedlist spacing="compact">
<title>Surround 5.1</title>
<listitem override="0"><simpara>left front</simpara></listitem>
<listitem><simpara>right front</simpara></listitem>
<listitem><simpara>left rear</simpara></listitem>
<listitem><simpara>right rear</simpara></listitem>
<listitem><simpara>center front</simpara></listitem>
<listitem><simpara>subwoofer</simpara></listitem>
</orderedlist>
</para>
<para>
The <option>-channels</option> 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
<option>-channels 4</option> to play a stereo MP3 file will still result in
2-channel output since the MP3 codec will not produce the extra channels.
</para>
<para>
The <option>channels</option> 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.
</para>
</sect3>
<sect3 id="advaudio-channels-mono">
<title>Playing Mono in Two Speakers</title>
<para>
Mono sounds a lot better when played through two speakers -- especially when
using headphones. Audio files that truly have 1 channel are automatically played
in 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 with the <option>extrastereo</option>
filter:
<screen>mplayer <replaceable>filename</replaceable> -af extrastereo=0</screen>
</para>
<para>
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 <option>volume</option> filter and find
the right value. For example:
<screen>mplayer <replaceable>filename</replaceable> -af extrastereo=0,volume=5</screen>
</para>
</sect3>
<sect3 id="advaudio-channels-copying">
<title>Channel Copying/Moving</title>
<para>
The <option>channels</option> filter can move any or all channels. Setting up all
the suboptions to channels can be complicated and takes a little care.
<orderedlist spacing="compact">
<listitem><para>
Decide how many output channels you need. This is the first suboption.
</para></listitem>
<listitem><para>
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:
<programlisting>
channel 2 --> channel 3
channel 2 --> channel 2
</programlisting>
</para></listitem>
<listitem><para>
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
<replaceable>source:destination</replaceable> pairs.
</para></listitem>
</orderedlist>
</para>
<bridgehead>Example: one channel in two speakers</bridgehead>
<para>
Here's 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:
<orderedlist>
<listitem><para>
In order to provide an output channel for each of the two speakers, the first
suboption must be "2".
</para></listitem>
<listitem><para>
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.
</para></listitem>
<listitem><para>
To move the left channel (channel 0) into the right channel (channel 1), the
suboption pair is "0:1". To move the left channel into itself is "0:0".
</para></listitem>
</orderedlist>
Putting that all together gives:
<screen>mplayer <replaceable>filename</replaceable> -af channels=2:2:0:1:0:0</screen>
</para>
<para>
The advantage this example has over <option>extrastereo</option> 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's more difficult to remember and type.
</para>
<bridgehead>Example: left channel in two speakers shortcut</bridgehead>
<para>
There is actually a much easier way to use the <option>channels</option> filter
for playing the left channel in both speakers:
<screen>mplayer <replaceable>filename</replaceable> -af channels=1</screen>
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.
</para>
<bridgehead>Example: duplicate front channels to the rear</bridgehead>
<para>
Another common operation is to duplicate the front channels and play them back
on the rear speakers of a quadraphonic setup.
<orderedlist>
<listitem><para>
There should be four output channels. The first suboption is "4".
</para></listitem>
<listitem><para>
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".
</para></listitem>
<listitem><para>
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".
</para></listitem>
</orderedlist>
Combine all the suboptions to get:
<screen>mplayer <replaceable>filename</replaceable> -af channels=4:4:0:2:0:0:1:3:1:1</screen>
</para>
</sect3>
<sect3 id="advaudio-channels-mixing">
<title>Channel Mixing</title>
<para>
The <option>pan</option> filter can mix channels in user-specified proportions.
This allows for everything the <option>channels</option> filter can do and
more. Unfortunately, the suboptions are much more complicated.
<orderedlist>
<listitem><para>
Decide how many channels to work with. You may need to specify this with
<option>-channels</option> and/or <option>-af channels</option>. Later examples
will show when to use which.
</para></listitem>
<listitem><para>
Decide how many channels to feed into <option>pan</option> (further decoded
channels are discarded). This is the first suboption, and it also controls how
many channels are output.
</para></listitem>
<listitem><para>
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 output channel. Each suboption
within a set corresponds to an input channel. The number you specify will be the
percentage of the input channel that gets mixed into the output channel.
</para>
<para>
<option>pan</option> 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
<option>pan</option> with <option>,volume</option> to enable clipping, but it's
best to keep the values of <option>pan</option> low enough that clipping is not
necessary.
</para></listitem>
</orderedlist>
</para>
<bridgehead>Example: one channel in two speakers</bridgehead>
<para>
Here is yet another example for playing the left channel in two speakers. Follow
the steps above:
<orderedlist>
<listitem><para>
The number of output channels is 2. Since this is the same as the number of
channels in the input file, neither <option>-channels</option> nor
<option>-af channels</option> needs to be specified.
</para></listitem>
<listitem><para>
<option>pan</option> should receive and output two channels, so the first
suboption is "2".
</para></listitem>
<listitem><para>
Since we have two output channels, there will be two sets of suboptions. Since
there are also two input channels, there will be two suboptions per set. The
left channel should consist of 100% of itself and 0% of the right channel. So,
the first set of suboptions is "1:0". The right channel should be 100% of the
left channel and 0% of itself. This is "1:0" as well.
</para></listitem>
</orderedlist>
Putting those options together gives:
<screen>mplayer <replaceable>filename</replaceable> -af pan=2:1:0:1:0</screen>
If the right channel is desired instead of the left, the suboptions to
<option>pan</option> will be "2:0:1:0:1".
</para>
<bridgehead>Example: left channel in two speakers shortcut</bridgehead>
<para>
As with <option>channels</option>, there is a shortcut that only works with the
left channel:
<screen>mplayer <replaceable>filename</replaceable> -af pan=1:1</screen>
Since <option>pan</option> 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.
</para>
<bridgehead>Example: downmixing 6-channel PCM</bridgehead>
<para>
<application>MPlayer</application>'s decoder for 6-channel PCM is not capable of
downmixing. Here's a way to downmix PCM using <option>pan</option>:
<orderedlist>
<listitem><para>
The number of output channels is 2. Since <option>pan</option> will be handling
6 channels, it may be necessary to put <option>,channels=2</option> after pan to
get rid of the extra channels. If your sound card is capable of 6-channel
playback you can skip this, as the discarded channels will be empty by then anyway.
</para></listitem>
<listitem><para>
<option>Pan</option> must receive all six channels to downmix, so the first
suboption is "6".
</para></listitem>
<listitem><para>
With six input channels there will be six suboptions per set. 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 AC3 5.1:
<programlisting>
0 - front left
1 - front right
2 - rear left
3 - rear right
4 - center front
5 - subwoofer
</programlisting>
The first set of suboptions lists the percentages of the original volume, in
order, at which each channel listed above should be mixed into the front left
channel: "1:0:1:0:0.5:1". For the front right channel, this is "0:1:0:1:0.5:1".
</para></listitem>
</orderedlist>
Put all that together, for:
<screen>mplayer <replaceable>6-channel.wav</replaceable> -af pan=6:1:0:1:0:0.5:1:0:1:0:1:0.5:1,channels=2</screen>
The percentages listed above are only a rough example. Feel free to tweak them.
</para>
<bridgehead>Example: Playing 5.1 audio on big speakers without a subwoofer</bridgehead>
<para>
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
<option>-channels 5</option> 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
<option>pan</option>:
<orderedlist>
<listitem><para>
Since <option>pan</option> needs to examine all six channels, specify
<option>-channels 6</option> so liba52 decodes them all.
</para></listitem>
<listitem><para>
<option>pan</option> needs all the channels, so the first suboption is 6 as
well.
</para></listitem>
<listitem><para>
Since the subwoofer channel is the last of six, we can specify five sets of
suboptions and omit the set belonging to the subwoofer channel. Each of those
five sets needs six suboptions so the list will get pretty long.
<itemizedlist spacing="compact">
<listitem><simpara>
The left front channel should have itself and half the subwoofer channel:
"1:0:0:0:0:0.5"
</simpara></listitem>
<listitem><simpara>
The right front channel should have itself and half the subwoofer channel:
"0:1:0:0:0:0.5"
</simpara></listitem>
<listitem><simpara>
The left rear channel stays the same:
"0:0:1:0:0:0"
</simpara></listitem>
<listitem><simpara>
The right rear channel stays the same:
"0:0:0:1:0:0"
</simpara></listitem>
<listitem><simpara>
The center front channel stays the same:
"0:0:0:0:1:0"
</simpara></listitem>
<listitem><simpara>
The subwoofer channel is left out and it will be silent.
</simpara></listitem>
</itemizedlist>
</para></listitem>
</orderedlist>
Combine all those options to get:
<screen>mplayer <replaceable>dvd://1</replaceable> -channels 6 -af pan=6:1:0:0:0:0:0.5:0:1:0:0:0:0.5:0:0:1:0:0:0:0:0:0:1:0:0:0:0:0:0:1:0</screen>
</para>
</sect3>
</sect2>
<sect2 id="advaudio-volume">
<title>Software Volume Adjustment</title>
<para>
Some audio tracks are too quiet to be heard comfortably without amplification.
This becomes a problem when your audio equipment cannot amplify the signal for
you. The <option>-softvol</option> option directs
<application>MPlayer</application> to use an internal mixer. You can then use
the volume adjustment keys (by default <keycap>9</keycap> and
<keycap>0</keycap>) to reach much higher volume levels. Note that this does not
bypass your sound card's mixer; <application>MPlayer</application> only
amplifies the signal before sending it to your sound card.
The following example is a good start:
<screen>mplayer <replaceable>quiet-file</replaceable> -softvol -softvol-max 300</screen>
The <option>-softvol-max</option> option specifies the maximum percentage of the
original volume. For example, <option>-softvol-max 200</option> would allow the
file to be played up to twice as loud. It is safe to specify a large value with
<option>-softvol-max</option>; the higher volume will not be used until you
use the volume adjustment keys. The only disadvantage of a large value is that,
since <application>MPlayer</application> adjusts volume by percentage of the
maximum, you will not have as precise control when using the volume adjustment
keys.
</para>
<para>
The <option>-softvol</option> option works by controlling the
<option>volume</option> audio filter. If you want to play a file at a certain
volume from the beginning you can specify <option>volume</option> manually:
<screen>mplayer <replaceable>quiet-file</replaceable> -af volume=10</screen>
This will play the file with a ten decibel gain. Be careful when using the
<option>volume</option> filter - you could easily hurt your ears if you use
too high a value. Start low and work your way up gradually until you get a feel
for how much adjustment is required. Also, if you specify excessively high
values <option>volume</option> may need to clip the signal to avoid sending your
sound card data that is outside the allowable range. You will hear distortion
when this happens.
</para>
</sect2>
</sect1>
</chapter>