L64105 Datasheet, PDF (396/454 Pages) List of Unclassifed Manufacturers – AUDIO/VIDEO DECODER

A.1.1 Video Encoding
For a video signal to be compressed, it must be sampled, digitized, and
converted to luminance and chrominance signals (Y, Cr, Cb). The MPEG
standard stipulates that the brightness or luminance component (Y) be
sampled with respect to the color difference or chrominance signals (Cr
and Cb) by a ratio of 4:1. That is, for every four samples of Y, there is to
be one subsample each of Cr and Cb. This is because the human eye
is much more sensitive to luminance than to chrominance. 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 two fields for each frame, one with
all of the odd lines and the other with all of the even lines.
The encoder must also choose which picture type to use. A picture
corresponds 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 predictions from a previous and a future I or P
picture.
A typical coding scheme contains a mixture of I, P, and B pictures. An I
picture may occur every half of a second, with two B pictures inserted
between each pair of I or P pictures.
Once the picture types have been defined, the encoder must estimate
motion vectors for each macroblock in the picture. A macroblock (see
Figure A.1) consists of a 16-pixel by 16-line section of luminance and two
spatially corresponding 8-pixel by 8-line sections, one for each
chrominance component. Each macroblock then consists of a total of six
8 x 8 blocks (four 8 x 8 luminance blocks, one 8 x 8 Cr block, and one
8 x 8 Cb block). The spatial picture area covered by the four 8 x 8 blocks
of luminance is the same area covered by each of the 8 x 8 chrominance
blocks.
A-2
Video/Audio Compression and Decompression Concepts