74 What is statistical multiplexing ? (MPEG-2)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
74 What is statistical multiplexing ? (MPEG-2)
Progressive explanation:
In the simplest coded bitstream, a PCM (Pulse Coded Modulated) digital
signal, all samples have an equal number of bits. Bit distribution in a
PCM image sequence is therefore not only uniform within a picture,
(bits distributed along zero dimensions), but is also uniform across
the full sequence of pictures.
Audio coding algorithms such as MPEG-1s Layer I and II are capable of
distributing bits over a one dimensional space, spanned by a frame. In
layer II, for example, an audio channel coded at a bitrate of 128
bits/sec and sample rate of 44.1 Khz will have frames (which consist of
1152 subband coefficients each) coded with approximately 334 bits.
Some subbands will receive more bits than others.
In block-based still image compression methods which employ 2-D
transform coding methods, bits are distributed over a 2 dimensional
space (horizontal and vertical) within the block. Further, blocks
throughout the picture may contain a varying number of bits as a
result, for example, of adaptive quantization. For example, background
sky may contain an average of only 50 bits per block, whereas complex
areas containing flowers or text may contain more than 200 bits per
block. In the typical adaptive quantization scheme, more bits are
allocated to perceptually more complex areas in the picture. The
quantization stepsizes can be selected against an overall picture
normalization constant, to achieve a target bit rate for the whole
picture. An encoder which generates coded image sequences comprised of
independently coded still pictures, such as JPEG Motion video or MPEG
Intra picture sequences, will typically generate coded pictures of
equal bit size.
MPEG non-intra coding introduces the concept of the distribution of
bits across multiple pictures, augmenting the distribution space to 3
dimensions. Bits are now allocated to more complex pictures in the
image sequence, normalized by the target bit size of the group of
pictures, while at a lower layer, bits within a picture are still
distributed according to more complex areas within the picture. Yet in
most applications, especially those of the Constant Bitrate class, a
restriction is placed in the encoder which guarantees that after a
period of time, e.g. 0.25 seconds, the coded bitstream achieves a
constant rate (in MPEG, the Video Buffer Verifier regulates the
variable-to-constant rate mapping). The mapping of an inherently
variable bitrate coded signal to a constant rate allows consistent
delivery of the program over a fixed-rate communications channel.
Statistical multiplexing takes the bit distribution model to 4
dimensions: horizontal, vertical, temporal, and program axis. The 4th
dimension is enabled by the practice of mulitplexing multiple programs
(each, for example, with respective video and audio bitstreams) on a
common data carrier. In the Hughes' DSS system, a single data carrier
is modulated with a payload capacity of 23 Mbits/sec, but a typical
program will be transported at average bit rate of 6 Mbit/sec each. In
the 4-D model, bits may be distributed according the relative
complexity of each program against the complexities of the other
programs of the common data carrier. For example, a program undergoing
a rapid scene change will be assigned the highest bit allocation
priority, whereas the program with a near-motionless scene will receive
the lowest priority, or fewest bits.
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