77 How does MPEG achieve compression? Sequence layer:
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
77 How does MPEG achieve compression? Sequence layer:
MPEG-2 can represent interlaced or progressive video sequences,
whereas MPEG-1 is strictly meant for progressive sequences since the
target application was Compact Disc video coded at 1.2 Mbit/sec.
MPEG-2 changed the meaning behind the aspect_ratio_information
variable, while significantly reducing the number of defined aspect
ratios in the table. In MPEG-2, aspect_ratio_information refers to the
overall display aspect ratio (e.g. 4:3, 16:9), whereas in MPEG-2, the
ratio refers to the particular pixel. The reduction in the entries of
the aspect ratio table also helps interoperability by limiting the
number of possible modes to a practical set, much like frame_rate_code
limits the number of display frame rates that can be represented.
Optional picture header variables called display_horizontal_size and
display_vertical_size can be used to code unusual display sizes.
frame_rate_code in MPEG-2 refers to the intended display rate, whereas
in MPEG-1 it referred to the coded frame rate. In film source video,
there are often 24 coded frames per second. Prior to bitstream
coding, a good encoder will eliminate the redundant 6 frames or 12
fields from a 30 frame/sec video signal which encapsulates an
inherently 24 frame/sec video source. The MPEG decoder or display
device will then repeat frames or fields to recreate or synthesize the
30 frame/sec display rate. In MPEG-1, the decoder could only infer the
intended frame rate, or derive it based on the Systems layer time
stamps. MPEG-2 provides specific picture header variables called
repeat_first_field and top_field_first which explicitly signal which
frames or fields are to be repeated, and how many times.
To address the concern of software decoders which may operate at rates
lower or different than the common television rates, two new variables
in MPEG-2 called frame_rate_extension_d and frame_rate_extension_n can
be combined with frame_rate_code to specify a much wider variety of
display frame rates. However, in the current set of define profiles
and levels, these two variables are not allowed to change the value
specified by frame_rate_code. Future extensions or Profiles of MPEG
may enable them.
In interlaced sequences, the coded macroblock height (mb_height) of a
picture must be a multiple of 32 pixels, while the width, like MPEG-1,
is a coded multiple of 16 pixels. A discrepancy between the coded
width and height of a picture and the variables horizontal_size and
vertical_size, respectively, occurs when either variable is not an
integer multiple of macroblocks. All pixels must be coded within
macroblocks, since there cannot be such a thing as fractional
macroblocks. Never intended for display, these overhang pixels or
lines exist along the left and bottom edges of the coded picture. The
sample values within these trims can be arbitrary, but they can affect
the values of samples within the current picture, and especially future
coded pictures. In the current pictures, pixels which reside within
the same 8x8 block as the overhang pixels are affect by the ripples of
DCT quantization error. In future coded pictures, their energy can
propagate anywhere within an image sequence as a result of motion
compensated prediction. An encoder should fill in values which are
easy to code, and should probably avoid creating motion vectors which
would cause the Motion Compensated Prediction stage to extract samples
from these areas. The application should probably select
horizontal_size and vertical_size that are already multiples of 16 (or
32 in the vertical case of interlaced sequences) to begin with.
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