82 How does MPEG achieve compression? Block:
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
82 How does MPEG achieve compression? Block:
alternate scan introduced a new run-length entropy scanning pattern
generally more efficient for the statistics of interlaced video
signals. Zig-zag scan is the appropriate choice for progressive
pictures.
intra_dc_precision: the MPEG-1 DC value is mandatory quantized to a
precision of 8 bits. MPEG-2 introduced 9, 10, and 11 bit precision set
on a picture basis to increase the accuracy of the DC component, which
by very nature, has the most significant contribution towards picture
quality. Particularly useful at high bit rates to reduce
posterization. Main and Simple Profiles are limited to 8, 9, or 10 bits
of precision. The 4:2:2 High Profile, which is geared towards higher
bitrate applications (up to 50 Mbits/sec), permits all values (up to 11
bits).
separate quantization matrices for Y and C: luminance (Y) and
chrominance (Cb,Cr) share a common intra and non-intra DCT coefficient
quantization 8x8 matrix in MPEG-1 and MPEG-2 Main and Simple Profiles.
The 4:2:2 Profile permits separate quantization matrices to be
downloaded for the luminance and chrominance blocks. Cb and Cr still
share a common matrix.
intra_vlc_format: one of two tables may now be selected at the picture
layer for variable length codes (VLCs) of AC run-length symbols in
Intra blocks. The first table is identical to that specified for
MPEG-1 (dc_coef_next). The newer second table is more suited to the
statistics of Intra coded blocks, especially in I- frames. The best
illustration between Table 0 and Table 1is the length of the symbol
which represents End of Block (EOB). In Table zero, EOB is 2 bits. In
Table one, it is 4 bits. The implication is that the EOB symbol is
2^-n probable within the block, or from an alternative perspective,
there are an average of 3 to 4 non-zero AC coefficients in Non-intra
blocks, and 9 to 16 coefficients in Intra blocks. The VLC tree of
Table 1 was intended to be a subset of Table 0, to aid hardware
implementations. Both tables have 113 VLC entries (or events).
escape: When no entry in the VLC exists for a AC Run-Level symbol, an
escape code can be used to represent the symbol. Since there are only
63 positions within an 8x8 block following the first coefficient, and
the dynamic range of the quantized DCT coefficients is [-2047,+2048],
there are (63*2047), or 128,961 possible combinations of Run and Level
(the sign bit of the Level follows the VLC). Only the 113 most common
Run-Level symbols are represented in Table 0 or Table 1. The length of
the escape symbol (which is always 6 bits) plus the Run and Level
values in MPEG-1 could be 20 or 28 bits in length. The 20 bit escape
describes levels in the range [-127,+127]. The 28 bit double escape
has a range of [-255, +255]. MPEG-2 increased the span to the full
dynamic range of quantized IDCT coefficients, [-2047, +2047] and
simplified the escape mechanism with a single representation for this
event. The total length of the MPEG-2 escape codeword is 24 bits (6
bit VLC followed by a 6-bit Run value, and 12 bit Level value). It was
an assumption by MPEG-1 designers that no quantized DCT coefficient
would need greater representation than 10 bits [-255,+255]. Note:
MPEG-2 escape mechanism does not permit the value -2048 to be
represented.
mismatch control: The arithmetic results of all stages are defined
exactly by the normative MPEG decoding process, with the single
exception of the Inverse Discrete Cosine Transform (IDCT). This stage
can be implemented with a wide variety of IDCT implementations. Some
are more suited for software, others for programmable hardware, and
others still for hardwired hardware designs. The IDCT reference formula
in the MPEG specification would, if directly implemented, consume at
least 1024 multiply and 1024 addition operations for every block. A
wide variety of fast algorithms exist which can reduce the count to
less than 200 multiplies and 500 adds per block by exploiting the
innate symmetry of the cosine basis functions. A typical fast IDCT
algorithm would be dwarfed by the cost of the other decoder stages
combined. Each fast IDCT algorithm has different quantization error
statistics (fingerprint), although subtle when the precision of the
arithmetic is, for example, at least 16-bits for the transform
coefficients and 24-bits for intermediate dot product values.
Therefore, MPEG cannot standardize a single fast IDCT algorithm. The
accuracy can be defined only statistically. The IEEE 1180
recommendation (December 1990) defines the error tolerance between an
ideal direct-matrix floating point implementation (a direct
implementation of the MPEG reference formula) and the test IDCT.
Mismatch control attempts to reduce the drift between different IDCT
algorithms by eliminating bit patterns which statistically have the
greatest contribution towards mismatches between the variety of
methods. The reconstructions of two decoders will begin to diverge over
time since their respective IDCT designs will reconstruct occasional,
slightly different 8x8 blocks.
MPEG-1s mismatch control method is known canonicially as Oddification,
since it forces all quantized DCT coefficients to negative values. It
is a slight improvement over its predecessor in H.261. MPEG-2 adopted
a different method called, again canonically, LSB Toggling, further
reducing the likelihood of mismatch. Toggling affects only the Least
Significant Bit (LSB) of the 63rd AC DCT coefficient (the highest
frequency in the DCT matrix). Another significant difference between
MPEG-1 and MPEG-2 mismatch control is, in MPEG-1, oddification is
performed on the quantized DCT coefficients, whereas in MPEG-2,
toggling is performed on the DCT coefficients after inverse
quantization. MPEG-1s mismatch control method favors programmable
implementation since a block of DCT coefficients when quantized.
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