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SERVICE MANUAL
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
prediction from the past I or P reference pictures.
- Bidirectionally
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.
S-00-005(96.03.27)
pictures (P-pictures) are coded using motion compensated
predictive-coded
REV.1
Date of Origin
pictures (B-pictures) are coded using motion
PAGE REV.
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Document No
98-10-21
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