doc/encoders: add doc for AAC encoder

Thanks-to: Kostya Shishkov <kostya.shishkov@gmail.com>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/encoders.texi

@@ -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.