Audio Compression and Decompression Concepts
A-7
The data in the bitstream is decoded according to the syntax dened in
the MPEG-2 standard. The decoder must rst identify the beginning of a
coded picture, identify the type of picture, then decode each individual
macroblock of the picture. Motion vectors and macroblock types (each of
the picture types I, P, and B have their own macroblock types) present
in the bitstream are used to construct a prediction of the current
macroblock based on past and future reference pictures that the encoder
has already decoded and stored. Coefcient data is then inverse
quantized and operated on by an inverse DCT process that changes data
from the frequency domain to the time and space domain.
After the decoder processes all of the macroblocks, the picture
reconstruction is complete. If the picture just reconstructed is a reference
picture (I or P picture), it replaces the oldest stored reference picture and
is used as the new reference for subsequent pictures. The pictures may
need to be reordered before they are displayed.
A.2 Audio Compression and Decompression Concepts
Given an elementary stream of audio data, an MPEG encoder rst
digitally compresses and codes the data. The MPEG algorithm offers a
choice of levels of complexity and performance for this process.
To prepare a stream of compressed audio data for transmission, it is
formatted into audio frames. Each audio frame contains audio data,
error-correction data, and optional user-dened ancillary data. The audio
frames are then sent in packets grouped within packs in an ISO MPEG
System Stream.
The packs in system streams may contain a mix of audio packets and
video packets for one or more channels. Packs may contain packets from
separate elementary streams. Thus, MPEG can easily support multiple
channels of program material, and a decoder given access to a system
stream may access large numbers of channels.
A.2.1 MPEG Audio Encoding
MPEG audio encoding is intended to efciently represent a digitized
audio stream by removing redundant information. Because different
applications have different performance goals, MPEG uses different