Databases Reference
In-Depth Information
By using some or all of these tools, the standard describes three profiles. These are the
main
profile, the
low complexity
profile, and the
sampling-rate-scalable
profile. The AAC
approach used inMPEG-2was later enhanced andmodified to provide an audio coding option in
MPEG-4. In the following section we first describe the MPEG-2 AAC algorithm, followed by
the MPEG-4 AAC algorithm.
17.4.1 MPEG-2 AAC
A block diagram of an MPEG-2 AAC encoder is shown in Figure
17.13
. Each block represents
a tool. The psychoacoustic model used in the AAC encoder is the same as the model used in
the MPEG Layer III encoder. As in the Layer III algorithm, the psychoacoustic model is used
Audio signal
Gain control
Psychoacou
s
tic
model
Window length
decision
Block switch
MDCT
Threshold
calculation
TNS
Intensity
coupling
Interblock
prediction
A
AC
bitstream
Data
M/S
Control
Scaling
Quantization
Huffman
coding
F I GU R E 17 . 13
An MPEG-2 AAC encoder [
221
].
