SERVICE MANUAL
Document No
Date of Origin
Date of REV
51-L-DSR004
98-10-21
PAGE REV.
0
PAGE
17 /102
S-00-005(96.03.27)
REV.1
4-8. Video
Dec o d i n g
Video decoding
is the reverse
of video encoding
and is intended
to reconstruct
a moving picture
sequence
from a compressed,
encoded
bitstream.
Decoding
is simple then encoding
because
there is no motion estimation
performed
and there are far fewer options.
The data in the stream is decoded
according
to the syntax defined
in the MPEG-2
standard.
The decoder
must first identify
the beginning
of a coded picture
and
identify
the type of picture,
then decode each individual
macroblock(
a 16 x 16
region of video, corresponding
to 16 horizontal
pixels and 16 vertical
display
lines.)
within a particular
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
stored.
Coefficient
data is then inversed
quantized
and operated
on by inverse
DCT
(Discrete
Cosine Transform)
process
that changes
data from the frequency
domain
to the time space domain.
After the decoder
processes
all of the macroblocks
the
picture
reconstruction
is completed.
If the picture
just reconstructed
is a reference
picture
(I-picture
or B-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,
in accordance
with the display
order instead
of the coding order. After the pictures
are
reordered,
they may be displayed
on an appropriate
output device.
4-9. System
Ove r v i e w
A/V decoding
is for use in digital
audio and video-decoding
systems
based on the
MPEG-2 algorithm.
It may consider
as a "black box" that receives
coded audio and
video data and produces
a decoded
audio and video data stream.
