[MPlayer-dev-eng] [PATCH] Audio filter docs for sound.html
Anders Johansson
ajh at watri.uwa.edu.au
Thu Jan 2 04:57:25 CET 2003
Hi,
Attached is a patch documenting the audio filters.
BTW: The docs are a bit harder to read nowadays. I don't think it was
a good idea to put all docs in the man page, they are not detailed
enough (due to the space limitation), also it is hard to find a
specific switch unless one knows exactly what to look for. Wouldn't it
be better to use some other more advanced documentation tool, that can
produce may different types of documents like for example
http://docbook.sourceforge.net/.
//Anders
--
______________________________________________________________________
Anders Johansson Room G09B WATRI University of .-_|\
Visiting Research Associate Western Australia / \
telephone: +61 8 9380 8125 39 Stirling Highway P_.-._/
e-mail: ajh at watri.uwa.edu.au Crawley WA 6009 o
______________________________________________________________________
-------------- next part --------------
--- ../main.dev/DOCS/sound.html Sat Dec 28 22:04:48 2002
+++ DOCS/sound.html Thu Jan 2 11:39:40 2003
@@ -149,6 +149,314 @@
<P>Feedback to this document is welcome. Please tell us how MPlayer
and your sound card(s) worked together.</P>
+<H4><A NAME="af">2.3.2.3 Audio filters</A></H4>
+<P>The old audio plugins have been superseded by a new audio filter
+ layer. Audio filters are used for changing the properties of the
+ audio data before the sound reaches the sound card. The activation
+ and deactivation of the filters is normally automated but can be
+ overridden. The filters are activated when the the properties of the
+ audio data differs from those required by the sound card, and
+ deactivated if unnecessary. The <CODE>-af filter1,filter2,...</CODE>
+ switch is used to override the automatic activation of filters or to
+ insert filters that aren't automatically inserted. The filters will
+ be executed as they appear in the comma separated list.</P>
+
+<P> Example:<BR> <CODE>mplayer-af resample,pan media.avi </CODE></P>
+
+<P>would run the sound through the resampling filter followed by the
+ pan filter. Observe that the list mustn't contain any spaces, or it
+ will fail.</P>
+
+<P>The filters often have switches that change their behavior. These
+ switches are explained in detail in the sections below. A filter
+ will execute using default settings if its switches are omitted.
+ Here is an example of how to use filters in combination with filter
+ specific switches:</P>
+
+<P> <CODE>mplayer -af resample=11025,pan=1:0.5:0.5
+ -channels 1 -srate 11025 media.avi</CODE></P>
+
+<P>would set the output frequency of the resample filter to 11025z
+ and down mix the audio to 1 channel using the pan filter.</P>
+
+<P>Most filters respond to the <CODE>-v </CODE> switch and it makes
+ the filters print out status messages when given.</P>
+
+<P>The overall execution of the filter layer is controlled using the
+ <CODE>-af-adv</CODE> switch. This swithc has two sub options:</P>
+
+<UL>
+ <LI><CODE>force</CODE> is an integer between 0 and 3 that controls
+ how the filters are inserted and what speed/accuracy optimizations
+ they use:
+ <TABLE BORDER=0 WIDTH="100%">
+ <TR><TD>Optimization</TD><TD>Description</TD></TR>
+ <TR><TD>0</TD><TD>Use automatic insertion of filters and optimize
+ according to CPU speed. </TD></TR>
+ <TR><TD>1</TD><TD>Use automatic insertion of filters and optimize
+ for the highest speed. If this option is set the processing of the
+ audio data will be done using fix point aritmetics. Warning some
+ features in the audio filters will silently fail, and the sound
+ quality may drop. </TD></TR>
+ <TR><TD>2</TD><TD>Use automatic insertion of filters and optimize
+ for quality. If this option is set the processing of the
+ audio data will be done using floating point instructions and is
+ therfore quite CPU intensive, but gives a lot higher sound quality
+ than fix point processing. </TD></TR>
+ <TR><TD>0</TD><TD>Use no automatic insertion of filters and no
+ optimization. Warning it may be possibe to crash mplayer using
+ this setting. </TD></TR>
+ </TABLE>
+ </LI>
+ <LI><CODE>list</CODE> is an alias for the -af switch.</LI>
+</UL>
+
+
+
+<H5><A NAME="af_resample">2.3.2.3.1 Up/Down-sampling</A></H5>
+
+<P>MPlayer fully supports up/down-sampling of the sound. This filter
+ can be used if you have a fixed frequency sound card or if you are
+ stuck with an old sound card that is only capable of max 44.1kHz.
+ This filter is automatically enabled if it is necessary, but it also
+ be explicitly enabled on the command line. This filter has three
+ switches:</P>
+
+<UL>
+ <LI><CODE>srate</CODE> is an integer used for setting the output
+ sample frequency in Hz. The valid range for this parameter is 8kHz
+ to 192kHz. If the input and output sample frequency are the same
+ or if this parameter is omitted the filter is automatically
+ unloaded. A high sample frequency normally improves the audio
+ quality, especially when used in combination with other
+ filters. </LI>
+ <LI><CODE>sloppy</CODE> is binary and optional and allows the output
+ frequency differ slightly from the frequency given by
+ <CODE>srate</CODE>. This switch can be used if the startup of the
+ playback is extremely slow.</LI>
+ <LI><CODE>fast</CODE> is binary and optional and enables linear
+ interpolation as resampling method. Linear interpolation is
+ extremely fast, but suffers from poor sound quality especially
+ when used for up-sampling.</LI>
+</UL>
+
+<P>Example:<BR>
+ <CODE>mplayer -af resample=44100:0:1</CODE></P>
+
+<P>would set the output frequency of the resample filter to 44100Hz
+ using exact output frequency scaling and linear interpolation.</P>
+
+<H5><A NAME="af_channels">2.3.2.3.2 Changing the number of channels</A></H5>
+
+<P>The <CODE>channels</CODE> filter can be used for adding and
+ removing channels, it can also be used for routing or copying
+ channels. It is automatically enabled when the output from
+ the audio filter layer differs from the input layer or when it is
+ requested by another filter. This filter unloads itself if it isn't
+ needed. The number of switches is dynamic: </P>
+
+<UL>
+ <LI><CODE>nch</CODE> is and integer between 1 and 6 and is used for
+ setting the number of output channels. This switch is required,
+ leaving it empty results in a runtime error. </LI>
+ <LI><CODE>nr</CODE> is an integer between 1 and 6 and is used for
+ specifying the number of routes. This parameter is optional. If it
+ is omitted the default routing is used.</LI>
+ <LI><CODE>from1:to1:from2:to2:from3:to3...</CODE> are pairs of
+ numbers between 0 and 5 that defies which channels should be routed
+ where. </LI>
+</UL>
+
+<P> If only <CODE>nch</CODE> is given the default routing is used, it
+ works as follows: If the number of output channels is bigger than
+ the number of input channels empty channels are inserted (except
+ mixing from mono to stereo, then the mono channel is repeated in
+ both of the output channels). If the number of output channels is
+ smaller than the number of input channels the exceeding channels are
+ truncated.</P>
+
+<P>Example 1:<BR>
+ <CODE>mplayer -af channels=4:4:0:1:1:0:2:2:3:3 media.avi </CODE></P>
+
+<P>would change the number of channels to 4 and set up 4 routes that
+ swaps channel 0 and channel 1 and leaves channel 2 and 3
+ intact. Observe that if media containing two channels was played
+ back channels 2 and 3 would contain silence but 0 and 1 would still
+ be swapped.</P>
+
+<P>Example 2:<BR>
+ <CODE>mplayer -af channels=6:4:0:0:0:1:0:2:0:3 media.avi </CODE></P>
+
+<P>would change the number of channels to 6 and set up 4 routes that
+ copies channel 0 to channels 0 to 3. Channel 4 and 5 will contain
+ silence.</P>
+
+<H5><A NAME="af_format">2.3.2.3.3 Sample format converter</A></H5>
+
+<P>This filter is a sample format converter. It is automatically
+ enabled when needed by the sound card or the by another filter.
+</P>
+
+<UL>
+ <LI><CODE>bps</CODE> is and integer 1, 2 or 4 and denotes the number
+ of bytes per sample. This switch is required, leaving it empty
+ results in a runtime error. </LI>
+ <LI><CODE>f</CODE> is a text string describing the sample
+ format. The string is a concatenated mix of: alaw, mulaw or
+ imaadpcm float or int unsigned or signed le or be (little or big
+ endian). This switch is required, leaving it empty results in a
+ runtime error.</LI>
+</UL>
+
+<P>Example:<BR>
+ <CODE>mplayer media.avi -af format=4:float</CODE></P>
+
+<P>would set the output output format to 4 bytes per sample floating
+ point data.</P>
+
+<H5><A NAME="af_delay">2.3.2.3.4 Delay</A></H5>
+
+<P>This filter delays the sound to the loudspeakers differently in
+ order to make the sound in the different channels arrive at the same
+ time to the listening position. This filter is only useful if you
+ have more than 2 loudspeakers. This filter has a variable number of
+ parameters:</P>
+
+<UL>
+ <LI><CODE>d1:d2:d3...</CODE> are floating point numbers representing
+ the delays in ms that should be imposed on the different
+ channels. The minimum delay is 0ms and the maximum is 1000ms. </LI>
+</UL>
+
+ <P>To calculate the required delay for the different channels do as
+ follows:</P>
+<OL>
+ <LI> Measure the distance to the loudspeakers in meters in relation to
+ your listening position, giving you the distances s1 to s5 (for a
+ 5.1 system). There is no point compensating for the sub-woofer (you
+ won't hear the difference anyway). </LI>
+ <LI>Subtract the distances s1 to s5 from the maximum distance
+ i.e. <BR>
+ s[i] = max(s) - s[i]; i = 1...5</LI>
+ <LI>Calculated the required delays in ms as <BR>
+ d[i] = 1000*s[i]/342; i = 1...5 </LI>
+</OL>
+
+<P>Example:<BR>
+ <CODE>mplayer -af delay=10.5:10.5:0:0:7:0 media.avi</CODE></P>
+
+<P>would delay front left and right by 10.5ms, the two rear channels
+ and the sub by 0ms and the center channel by 7ms.</P>
+
+<H5><A NAME="af_volume">2.3.2.3.5 Software volume control</A></H5>
+
+<P>This filter is a software volume control. Use this filter with
+ caution since it can reduce the signal to noise ratio of the
+ sound. In most cases it is best to set the level for the PCM sound
+ to max, leave this filter out and control the output level to your
+ speakers with the master volume control of the mixer. If there is an
+ external amplifier connected to the computer (this is almost always
+ the case), the noise level can be minimized by adjusting the master
+ level and the volume knob on the amplifier until the hissing noise
+ in the background is gone. This filter has two switches:</P>
+
+<UL>
+ <LI><CODE>v</CODE> is a floating point number between -200 and +60
+ who represents the volume level in dB. The default level is -10
+ dB. </LI>
+ <LI><CODE>c</CODE> is a binary control that turns on and off soft
+ clipping. Soft-clipping can make the sound more smooth if very high
+ volume levels are used. Enable this switch if the dynamic range of
+ the loudspeakers is very low. Be aware that this feature creates
+ distortion and should be considered a last resort.</LI>
+</UL>
+
+<P>Example:<BR>
+ <CODE>mplayer -af volume=10.1:0 media.avi</CODE></P>
+
+<P>would amplify the sound by 10.1dB and hard-clip if the sound level is
+ too high.</P>
+
+<P>This filter has a second feature: it measures the over all maximum
+ sound level and prints out that level when mplayer exits. This
+ volume estimate can be used for setting the sound level in mencoder
+ such that the maximum dynamic range is utilized.</P>
+
+<H5><A NAME="af_equalizer">2.3.2.3.6 Equalizer</A></H5>
+
+<P> This filter is a 10 octave band graphic equalizer, implemented
+ using 10 IIR band pass filters. This means that it works regardless
+ of what type of audio being played back. The center frequencies for
+ the 10 bands are:</P>
+
+<TABLE BORDER=0 WIDTH="100%">
+ <TR><TD>Band nr.</TD><TD>Center frequency</TD></TR>
+ <TR><TD>0</TD><TD>31.25 Hz</TD></TR>
+ <TR><TD>1</TD><TD>62.50 Hz</TD></TR>
+ <TR><TD>2</TD><TD>125.0 Hz</TD></TR>
+ <TR><TD>3</TD><TD>250.0 Hz</TD></TR>
+ <TR><TD>4</TD><TD>500.0 Hz</TD></TR>
+ <TR><TD>5</TD><TD>1.000 kHz</TD></TR>
+ <TR><TD>6</TD><TD>2.000 kHz</TD></TR>
+ <TR><TD>7</TD><TD>4.000 kHz</TD></TR>
+ <TR><TD>8</TD><TD>8.000 kHz</TD></TR>
+ <TR><TD>9</TD><TD>16.00 kHz</TD></TR>
+</TABLE>
+
+<P> If the sample rate of the sound being played back is lower than
+ the center frequency for a frequency band, then that band will be
+ disabled. A known bug with this filter is that the characteristics
+ for the upper most band isn't completely symmetric if the sample rate
+ is close too the center frequency of that band. This problem can be
+ worked around by up-sampling the sound using the resample filter
+ before it reaches this filter. </P>
+
+<P> This filter has 10 parameters:</P>
+<UL>
+ <LI><CODE>g1:g2:g3...g10</CODE> are floating point numbers between
+ -12 to +12 dB representing the gain in dB for each frequency
+ band.
+ </LI>
+</UL>
+
+<P>Example:<BR>
+ <CODE>mplayer -af equalizer=11:11:10:5:0:-12:0:5:12:12 media.avi</CODE></P>
+
+<P>would amplify the sound in the upper and lower frequency region
+ while canceling it almost completely around 1kHz.</P>
+
+<H5><A NAME="af_equalizer">2.3.2.3.7 Panning filter </A></H5>
+
+<P> This filter can be used for mixing the channels arbitrarily. It is
+ basically a combination of the volume control and the channels
+ filter. There are two major uses for this filter: </P>
+
+<OL>
+ <LI> Down-mixing many channels to only a few, stereo to mono for
+ example. </LI>
+ <LI> Varying the "width" of the center speaker in a surround sound
+ system. </LI>
+</OL>
+
+<P> This filter is hard to use, and will require some tinkering before
+ the desired result is obtained. The number of switches for this
+ filter depends on the number of output channels:</P>
+
+<UL>
+ <LI><CODE>nch</CODE> is and integer between 1 and 6 and is used for
+ setting the number of output channels. This switch is required,
+ leaving it empty results in a runtime error. </LI>
+ <LI><CODE>l00:l01:l02:..l10:l11:l12:...ln0:ln1:ln2:...</CODE> are
+ floating point values between 0 and 1 that determines the level
+ <CODE>l[i][j]</CODE> that the input channel j is mixed into output
+ channel i.</LI>
+</UL>
+
+<P>Example:<BR>
+ <CODE>mplayer -af pan=1:0.5:0.5 -channels 1 media.avi</CODE></P>
+
+<P>would down-mix from stereo to mono.</P>
+
<H4><A NAME="plugins">2.3.2.3 Audio plugins</A></H4>
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