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doc/encoders: add doc for AAC encoder

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Thanks-to: Kostya Shishkov <kostya.shishkov@gmail.com>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
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Timothy Gu 2013-09-08 16:32:22 -07:00 committed by Stefano Sabatini
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doc/encoders.texi
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@ -25,6 +25,95 @@ enabled encoders.
A description of some of the currently available audio encoders
follows.
@anchor{aacenc}
@section aac
Advanced Audio Coding (AAC) encoder.
This encoder is an experimental FFmpeg-native AAC encoder. Currently only the
low complexity (AAC-LC) profile is supported. To use this encoder, you must set
@option{strict} option to @samp{experimental} or lower.
As this encoder is experimental, unexpected behavior may exist from time to
time. For a more stable AAC encoder, see @ref{libvo-aacenc}. However, be warned
that it has a worse quality reported by some users.
@c Comment this out until somebody writes the respective documentation.
@c See also @ref{libfaac}, @ref{libaacplus}, and @ref{libfdk-aac-enc}.
@subsection Options
@table @option
@item b
Set bit rate in bits/s. Setting this automatically activates constant bit rate
(CBR) mode.
@item q
Set quality for variable bit rate (VBR) mode. This option is valid only using
the @command{ffmpeg} command-line tool. For library interface users, use
@option{global_quality}.
@item stereo_mode
Set stereo encoding mode. Possible values:
@table @samp
@item auto
Automatically selected by the encoder.
@item ms_off
Disable middle/side encoding. This is the default.
@item ms_force
Force middle/side encoding.
@end table
@item aac_coder
Set AAC encoder coding method. Possible values:
@table @samp
@item faac
FAAC-inspired method.
This method is a simplified reimplementation of the method used in FAAC, which
sets thresholds proportional to the band energies, and then decreases all the
thresholds with quantizer steps to find the appropriate quantization with
distortion below threshold band by band.
The quality of this method is comparable to the two loop searching method
descibed below, but somewhat a little better and slower.
@item anmr
Average noise to mask ratio (ANMR) trellis-based solution.
This has a theoretic best quality out of all the coding methods, but at the
cost of the slowest speed.
@item twoloop
Two loop searching (TLS) method.
This method first sets quantizers depending on band thresholds and then tries
to find an optimal combination by adding or subtracting a specific value from
all quantizers and adjusting some individual quantizer a little.
This method produces similar quality with the FAAC method and is the default.
@item fast
Constant quantizer method.
This method sets a constant quantizer for all bands. This is the fastest of all
the methods, yet produces the worst quality.
@end table
@end table
@subsection Tips and Tricks
According to some reports
(e.g. @url{http://d.hatena.ne.jp/kamedo2/20120729/1343545890}), setting the
@option{cutoff} option to 15000 Hz greatly improves the quality of the output
quality. As a result, we encourage you to do the same.
@section ac3 and ac3_fixed
AC-3 audio encoders.
@ -553,6 +642,7 @@ Set MPEG audio original flag when set to 1. The default value is 0
@end table
@anchor{libvo-aacenc}
@section libvo-aacenc
VisualOn AAC encoder.