SERVICE MANUAL
Document No
Date of Origin
Date of REV
51-L-DSR004
98-10-21
PAGE REV.
0
PAGE
16 /102
S-00-005(96.03.27)
REV.1
4-7. Video
Enc o d i n g
For a video signal to be compressed,
it must be sampled,
digitized,
and
converted
to luminance
and color difference
signals
(Y, Cr, Cb ).
The MPEG standard
stipulates
that the luminance
component(Y)
be sampled
with
respect
to the color difference
signal ( Cr and Cb ) by the ratio of 4:1.
That is, for every four samples
of Y, there is to be one sub-sample
each of Cr
and Cb, because
the human eye is much more sensitive
to luminance
components
than to color components.
Video sampling
takes place in both the vertical
and horizontal
directions.
Once video is sampled,
it is reformatted,
if necessary,
into a non-interlaced
signal.
An interlaced
signal contains
only part of the picture
content(
every other
horizontal
line, for example)
for each complete
display
scan.
The encoder
must also choose which picture
type is used. A picture
correspond
to
a single frame of motion video, or to a movie frame. There are three picture
types:
- Intracoded
pictures
(I-pictures)
are coded without
reference
to any other
pictures.
- Predictive-coded
pictures
(P-pictures)
are coded using motion compensated
prediction
from the past I or P reference
pictures.
- Bidirectionally
predictive-coded
pictures
(B-pictures)
are coded using motion
compensation
from a previous
and future I or P-pictures.
A typical
coding scheme contains
a mixture
of I, P, and B-pictures.
Typically,
an I-picture
may occur every half a second,
to give reasonably
fast random
access,
with two B-pictures
inserted
between
each pair of I or B-pictures.