Audio compression - The MPEG - page 125

Figure 4.37: The Layer III coder. Note the connection between the buffer and the quantizer which allows different
frames to contain different amounts of data.
Figure 4.38 shows that the buffer occupancy is fed back to the quantizer. During stationary program material, the
buffer contents are deliberately run down by slight coarsening of the quantizing. The buffer empties because the
output rate is fixed but the input rate has been reduced. When a transient arrives, the large coefficients which result
can be handled by filling the buffer, avoiding raising the output bit rate whilst also avoiding the pre-echo which
would result if the coefficients were heavily quantized.
Figure 4.38: The variable rate coding of Layer III. An approaching transient via the perceptual entropy signal
causes the coder to quantize more heavily in order to empty the buffer. When the transient arrives, the quantizing
can be made more accurate and the increased data can be accepted by the buffer.
In order to maintain synchronism between encoder and decoder in the presence of buffering, headers and side
information are sent synchronously at frame rate. However, the position of boundaries between the main data
blocks which carry the coefficients can vary with respect to the position of the headers in order to allow a variable
frame size. Figure 4.39 shows that the frame begins with an unique sync pattern which is followed by the side
information. The side information contains a parameter called main data begin which specifies where the main data
for the present frame began in the transmission. This parameter allows the decoder to find the coefficient block in
the decoder buffer. As the frame headers are at fixed locations, the main data blocks may be interrupted by the
headers.
Figure 4.39: In Layer III, the logical frame rate is constant and is transmitted by equally spaced sync patterns. The
data blocks do not need to coincide with sync. A pointer after each sync pattern specifies where the data block
starts. In this example block 2 is smaller whereas 1 and 3 have enlarged.
4.21 MPEG-2 AAC - advanced audio coding
The MPEG standards system subsequently developed an enhanced system known as advanced audio coding
(AAC). [ 26 ][ 27 ] This was intended to be a standard which delivered the highest possible performance using newly
developed tools that could not be used in any backward- compatible standard. AAC also forms the core of the
audio coding of MPEG-4.
AAC supports up to 48 audio channels with default support of monophonic, stereo and 5.1 channel (3/2) audio. The
AAC concept is based on a number of coding tools known as modules which can be combined in different ways to
produce bitstreams at three different profiles.