L64105 LSI Logic Corporation, L64105 Datasheet - Page 401

• Manufacturer Part Number: L64105
• Description: Mpeg-2 Audio/video Decoder
• Manufacturer: LSI Logic Corporation

A.1.3 Video Decoding
A.2 Audio Compression and Decompression Concepts
Video decoding is the reverse of video encoding and is intended to
reconstruct a moving picture sequence from a compressed, encoded
bitstream. Decoding is simpler than encoding because there is no motion
estimation performed and there are far fewer options.
The data in the bitstream is decoded according to the syntax defined in
the MPEG-2 standard. The decoder must first 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 macro-
block based on past and future reference pictures that the encoder has
already decoded and stored. Coefficient 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.
Given an elementary stream of audio data, an MPEG encoder first
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-defined 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.
Audio Compression and Decompression Concepts
A-7