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MPEG-2 Video Encoding
MPEG-2 coding offers high compression and high quality for
DVD-Video and SVCD. |
The MPEG-2 specification (ISO 13818) was designed
for broadcast TV and other applications using interlaced images. It provides higher
picture quality than MPEG-1 but using a higher data rate. At lower bit rates, MPEG-2
provides no advantage over MPEG-1. At higher bit rates (above about 4
Mbits/s) MPEG-2
should be used in preference to MPEG-1. Unlike MPEG-1, MPEG-2 supports interlaced TV
systems and Vertical Blanking Interval (VBI) signals.
MPEG-2 includes higher quality audio for full surround
sound with 5.1 channels, ie five full bandwidth channels and one sub-woofer channel. The
channels comprise left, centre and right front and left and right rear. A 7.1 channel
version (with additional left and right centre channels) is also possible.
MPEG-2 also allows variable video bit rate so that the
overall bandwidth can be limited to 4 Mb/s or less without loss of picture quality.
Broadcast applications, currently using MPEG-2, generally use 6 to 8 Mb/s fixed data rate.
MPEG-2 uses the same frame types as MPEG-1 except that each
frame comprises two interlaced fields. Also each group of pictures (GOP) is 15 (PAL) or 18
(NTSC) frames in length maximum and comprises the following sequence:
I B P B P B P ....
ie there is only one B-frame between P-frames instead of
two for MPEG-1.
MPEG-2 Example
The following diagram illustrates the
sequence of frames in an MPEG-2 video sequence. Note that the display order
is different from the order in which they are read from disc.
Each macroblock of a B-frame is coded as a change from the
previous or next P-frame or I-frame. The I-frames are repeated at approximately 0.5
second intervals, otherwise the PBPBP.. pattern is repeated indefinitely.
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