DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

An MPEG bitstream must be constructed such that the
decoder’s buffer doesn’t overflow: If it did, bits would
be lost. The bitstream must also be constructed so that
the buffer doesn’t underflow: If it did, a picture to be
displayed would not be available at the required time.
Buffer management in MPEG-2 is based upon an idealized
model of the decoder’s buffer: All of the bits associated
with each picture are deemed to be extracted
from the decoder’s buffer at a certain precisely defined
instant in time with respect to the bitstream. Every
encoder implements a video buffering verifier (VBV) that
tracks the state of this idealized buffer. Each picture
header contains a VBV delay field that declares the fullness
of the buffer at the start of that picture. After
channel acquisition, a decoder waits for the specified
amount of time before starting decoding. (If the
decoder did not wait, buffer underflow could result.)
Bitstream syntax
The end product of MPEG-2 video compression is a bitstream
partitioned into what MPEG calls a syntactic
hierarchy having six layers: sequence, GOP, picture, slice,
macroblock, and block. Except for the video sequence
layer, which has a sequence end element, each syntactic
element has a header and no trailer. The sequence,
GOP, picture, and slice elements each begin with
a 24-bit start code prefix of 23 zero-bits followed by
a one-bit. A start code establishes byte alignment, and
may be preceded by an arbitrary number of zerostuffing
bits. All other datastream elements are
constructed so as to avoid the possibility of 23 or more
consecutive zero bits.
Video sequence layer The top layer of the MPEG syntax is the video sequence.
The sequence header specifies high-level parameters
such as bit rate, picture rate, picture size, picture aspect
ratio, profile, level, progressive/interlace, and chroma
format. The VBV buffer size parameter declares the
maximum buffer size required within the sequence. The
sequence header may specify quantizer matrices. At the
encoder’s discretion, the sequence header may be
retransmitted intermittently or periodically throughout
CHAPTER 40 MPEG-2 VIDEO COMPRESSION 493