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algorithm, the quantizer step sizes are increased. If this is not sufficient, then the higher-order
frequency coefficients are dropped until the need for rate reduction is past.
The format for MPEG is very flexible. However, the MPEG committee has provided some
suggested values for the various parameters. For MPEG-1 these suggested values are called
the
constrained parameter bitstream
(CPB). The horizontal picture size is constrained to be
less than or equal to 768 pixels, and the vertical size is constrained to be less than or equal to
576 pixels. More importantly, the pixel rate is constrained to be less than 396 macroblocks per
frame if the frame rate is 25 frames per second or less, and 330 macroblocks per frame if the
frame rate is 30 frames per second or less. The definition of a macroblock is the same as in
the ITU-T H.261 recommendations. Therefore, this corresponds to a frame size of 352
×
288
pixels at the 25-frames-per-second rate, or a frame size of 352
240 pixels at the 30-frames-
per-second rate. Keeping the frame at this size allows the algorithm to achieve bit rates of
between 1 and 1.5Mbits per second. When referring to MPEG-1 parameters, most people are
actually referring to the CPB.
The MPEG-1 algorithm provides reconstructed images of VHS quality for moderate- to
low-motion video sequences, and worse than VHS quality for high-motion sequences at rates
of around 1.2Mbits per second. As the algorithm was targeted to applications such as CD-
ROM, there is no consideration of interlaced video. In order to expand the applicability of the
basic MPEG algorithm to interlaced video, the MPEG committee provided some additional
recommendations, the MPEG-2 recommendations.
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19.9 The MPEG-2 Video Standard
__
H.262
While MPEG-1 was specifically proposed for digital storage media, the idea behind MPEG-2
was to provide a generic, application-independent standard. To this end, MPEG-2 takes a “tool
kit” approach, providing a number of subsets, each containing different options from the set of
all possible options contained in the standard. For a particular application, the user can select
from a set of
profiles
and
levels
. The profiles define the algorithms to be used, while the levels
define the constraints on the parameters. There are five profiles:
simple, main, snr-scalable
(where
snr
stands for signal-to-noise ratio),
spatially scalable
, and
high
. There is an ordering
of the profiles; each higher profile is capable of decoding video encoded using all profiles up
to and including that profile. For example, a decoder designed for profile
snr-scalable
could
decode video that was encoded using profiles
simple, main
, and
snr-scalable
.The
simple
profile eschews the use of
B
frames. Recall that the
B
frames require the most computation
to generate (forward and backward prediction), require memory to store the coded frames
needed for prediction, and increase the coding delay because of the need to wait for “future”
frames for both generation and reconstruction. Therefore, removal of the
B
frames makes
the requirements simpler. The
main
profile is very much the algorithm we have discussed in
the previous section. The
snr-scalable, spatially scalable
, and
high
profiles may use more
than one bitstream to encode the video. The base bitstream is a lower-rate encoding of the
video sequence. This bitstream could be decoded by itself to provide a reconstruction of the
video sequence. The other bitstream is used to enhance the quality of the reconstruction. This
layered approach is useful when transmitting video over a network, where some connections
