DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

structure,” where slices cover the picture with no gaps
or overlaps. The slice header contains the quantizer scale
code. The slice serves several purposes. First, the slice is
the smallest unit of resynchronization in case of uncorrected
data transmission error. Second, the slice is the
unit of differential coding of intra-macroblock DC
terms. Third, the slice is the unit for differential coding
of nonintra motion vectors: The first macroblock of
a slice has motion vectors coded absolutely, and motion
vectors for subsequent macroblocks are coded in terms
of successive differences from that.
Macroblock layer The macroblock is MPEG’s unit of motion prediction.
A macroblock contains an indication of the macroblock
type (intra, forward predicted, backward-predicted, or
bidirectionally predicted); a quantizer scale code; 0, 1, or
2 forward motion vectors; and 0, 1, or 2 backward
motion vectors. The coded block pattern flags provide
a compact way to represent blocks that are not coded
(owing to being adequately predicted without the need
for prediction error).
Block layer Each block is represented in the bitstream by VLEcoded
DCT coefficients – a differentially encoded DC
coefficient, and some number of AC coefficients. Each
coded block’s data is terminated by a 4-bit end of block
(EOB).
Transport
The syntax elements of an MPEG video or audio
bitstream are serialized to form an elementary stream
(ES). MPEG-2 defines a mechanism to divide an ES into
packets, forming a packetized elementary stream (PES).
Each PES pack header contains system-level clock information,
packet priority, packet sequence numbering,
and (optionally) encryption information.
An MPEG-2 program stream (PS) is intended for storage
of a single program on relatively error-free media, such
as DVD. PS packets are variable-length; each packet can
be as long as 64 KB, though packets of 1 KB or 2 KB are
typical. Synchronization is achieved through a system
clock reference (SCR).
CHAPTER 40 MPEG-2 VIDEO COMPRESSION 495