Advanced Audio Coding (AAC) is a lossy audio codec, which implements
a number of audio encoding tools and provides good audio quality
at relatively low bitrate. It is standardized as ISO/IEC 13818-7 (a.k.a.
MPEG-2 Part 7) and ISO/IEC 14496-3 (a.k.a. MPEG-4 Part 3) specifications.
It has gained wide adoption, including in Bluetooth A2DP devices.

Bluetooth A2DP specification standardizes the following AAC profiles:

 - MPEG-2 AAC Low Complexity (AAC LC)
 - MPEG-4 AAC LC
 - MPEG-4 AAC Long Term Prediction (AAC LTP)
 - MPEG-4 AAC Scalable

The former 3 are supported by this new PulseAudio codec for both decoding
and encoding.

AAC decoder is implemented in ffmpeg/libavcodec as a built-in decoder.

AAC encoder is implemented in ffmpeg/libavcodec in two variants:

 - built-in encoder, which is available since ffmpeg 3.0 (we require
   ffmpeg 3.1 because it introduces the modern API for working with
   codecs)
 - optional encoder backed by libfdk-aac

The A2DP codec is able to use either of these two encoders, with the
preference of libfdk-aac as it provides better audio quality. The choice
of the encoder to use is made at runtime and only depends on whether ffmpeg
was built with libfdk-aac or not. It does not add a build dependency on
libfdk-aac to PulseAudio.

Due to license incompatibility, the PulseAudio+ffmpeg+libfdk-aac combination
cannot be distributed in binary form in some jurisdictions but it can be built
by users locally from source. For binary-only distributions the built-in
encoder can be used.

Note that MPEG-4 AAC Scalable is supported by libfdk-aac when the library
is used through its API directly, but this is not implemented in this
PulseAudio codec at this point.

The A2DP encoder supports dynamic bitrate adaptation, which currently means
the encoder is able to reduce the output bitrate based on a callback
indicating transport congestion. Up to 4 bitrate reduction steps are
available, each step is 32 kbit/s. The default initial bitrate depends
on input sampling rate and channel count. For 48 kHz stereo and 44.1 kHz
stereo it amounts to 320 kbit/s.