OPTICODEC-PC
GLOSSARY
8-3
This does not stand for Apple Audio Coding, although Apple Computer brought
mainstream attention to AAC by supporting it in its iPod and iTunes products. It pro-
vides better and more stable quality than MP3 at equivalent or slightly lower bi-
trates. Like other MPEG codecs, the AAC family of codecs can be wrapped in a vari-
ety of rights management solutions. Apple Fairplay, Real Helix, SDC, and others have
all been used in commercial systems with AAC.
AAC (Advanced Audio Coding) Profiles
AAC has a modular approach to
encoding. Depending on the complexity of the bitstream to be encoded, the desired
performance and the acceptable output, implementers may create profiles to define
which of a specific set of tools to use for a particular application. The standard offers
six profiles:
AAC Main
(Main Profile) AAC Main is AAC LC coupled with a backwards
adaptive predictor. AAC Main profile is rarely used, as it requires a huge
increase in complexity for very little gain in efficiency.
AAC LC
(Low Complexity Profile) is the most widely used today, but is
starting to be replaced by HE AAC and aacPlus v2. It provides a good
trade off between complexity, quality, and bit rate.
AAC LTP
(Long Term Prediction) AAC LTP combines AAC LC with the LTP
tool. LTP has the same purpose as the backwards adaptive predictor in
the MPEG-2 Main Profile and has the same limitations. Because the bene-
fits in audio quality are still small compared to the increase in complexity,
AAC LTP is rarely, if ever used and cannot be recommended for broad-
casting.
AAC LD
(Low Delay) is the low delay variant of AAC. It specifically ad-
dresses the needs of two-way communication applications and sacrifices
compression efficiency. Since the typical delay of AAC or AAC+SBR is usu-
ally uncritical for broadcasting systems, it cannot be recommended for
broadcasting because of the loss in compression.
AAC Scalable
is for hierarchical audio coding. Used in IP based systems,
it can also be useful in special broadcasting systems with robust core lay-
ers and less robust enhancement layers. However, no known broadcast-
ing systems use AAC Scalable today because of the very special use case
and the loss of compression efficiency in the higher layers.
HE AAC,
also known as aacPlus, is the combination of AAC and SBR
Technology as specified in MPEG ISO/IEC 14496-3:2001/AMD-1: Band-
width Extension. It is the most efficient of the named AAC profiles, so it is
strongly recommended for broadcasting at bitrates of 128 K bps and be-
low. HE AAC is not intended to have a transparent rating; instead, it
minimizes audibly objectionable artifacts at low bitrates. Double-blind
MUSHRA testing by the European Broadcasting Union rates 48 kbit/s ste-
reo HE AAC in the “Excellent” category.
Depending on the AAC profile and the MP3 encoder, 96 kbit/s AAC or 48 kbit/s
aacPlus v2 can give nearly the same or better perceptional quality as 128 kbit/s MP3.
Different MP3 encoders perform differently and they produce output of sometimes
wildly varying quality.
