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标 题: FAQs of MPEG-2 Technical
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MPEG-2 Technical (and sometimes political)
Frequently Asked Questions
Copyright 1994 by Chad Fogg (cfogg@netcom.com)
Draft 3.3 (May 10, 1994)
1. MPEG is a DCT based scheme, right?
The DCT and Huffman algorithms receive the most press coverage (e.g. MPEG is
a
DCT based scheme with Huffman coding ), but are in fact fairly insignificant.
The variety of coding modes signaled to the decoder as context-dependent side
information are chiefly responsible for the efficiency of the MPEG syntax.
2. What does the MPEG video syntax feature that codes video efficiently?
A. Here are some of the statistical conditions and their syntax
counterparts.
B. Occlusion: forward, backwards, or bi-directional temporal prediction
in B pictures.
Smooth optical flow fields: variable length coding of 1-D prediction errors
for motion vectors.
Spatial correlation beyond 8x8 sample block boundaries: 1-D prediction of DC
coefficients in consecutive group intra-coded macroblocks.
High temporal correlation: variable on/off coding of prediction error at th
e
macroblock (no-coding) or individual block (coded block pattern) level.
Temporal de-correlation: forward, backwards, or bidirectional prediction.
Content dependent quality: locally adaptive quantization Temporal prediction
accuracy: "half-pel" sample accuracy.
High locally correlated signal refresh pictures (I picture) and prediction
errors: DCT Subjective coding: location-dependent quantization of DCT
coefficients.
3. What does the syntax provide for error robustness?
1. Byte-aligned start codes in the coded bitstream.
2. End of block codes in coded blocks.
3. Slices.
4. slice_vertical_position embedded as sub-field within slice start codes.
5. slices commencing at regular locations in picture (MPEG-2)
4. What is the significance of each layer in MPEG video ?
Sequence:
Set of pictures sharing same sampling dimensions, bit rate, chromaticy
(MPEG-1), quantization matrices (MPEG-1 only).
Group of Pictures:
Random access point giving SMPTE time code within sequence.
Guaranteed to start with an I picture.
Picture:
Samples of a common plane -- "captured" from the same time instant.
Slice:
Error resynchronization unit of macroblocks.
At the commencement of a slice, all inter-macroblock coding dependenci
es
are reset. Likewise, all macroblocks within a common slice can be dependently
coded.
Macroblock:
Least common multiple of Y, Cb, Cr 8x8 blocks in 4:2:0 sampling
structure.
For MPEG-1, the smallest granularity of temporal prediction.
Block:
Smallest granularity of spatial decorrelation.
5. How does the syntax facilitate parallelism?
A. For MPEG-1, slices may consist of an arbitrary number of macroblocks. The
coded bitstream must first be mapped into fixed-length elements before true
parallelism in a decoder application can be exploited. Further, since
macroblocks have coding dependencies on previous macroblocks within the same
slice, the data hierarchy must be pre-processed down to the layer of DC DCT
coefficients. After this, blocks may be independently inverse transformed and
quantized, temporally predicted, and reconstructed to buffer memory.
Parallelism is usually more of a concern for encoders. Macroblock motion
estimation and some rate control stages can be processed independently. An
encoder also has the freedom to choose the slice structure.
6. I hear the encoder is not part of the standard?
A. The encoder rests just outside the normative scope of the standard, as long
as the bitstreams it produces are compliant. The decoder, however, is almost
deterministic: a given bitstream should reconstruct to a unique set of picture
s.
Statistically speaking, an occasional error of a Least Significant Bit is
permitted as a result of the fact that the IDCT function is the only
non-normative stage in the decoder (the designer is free to choose among many
DCT algorithms and implementations). The IEEE 1180 test referenced in Annex A
of the MPEG-1 and MPEG-2 specifications spells out the statistical mismatch
tolerance between the Reference IDCT, which uses 64-bit floating point accurac
y,
and the Test IDCT.
7. Are some encoders better than others?
A. Yes. For example, the range over which a compensated prediction macroblock
is searched for has a great influence over final picture quality. At a certai
n
point a very large range can actually become detrimental (it may encourage lar
ge
differential motion vectors). Practical ranges are usually between +/- 15 and
+/- 32. As the range doubles, for instance, the search area quadruples.
8. Can MPEG-1 encode higher sample rates than 352 x 240 x 30 Hz ?
A. Yes. The MPEG-1 syntax permits sampling dimensions as high as 4095 x 4095 x
60 frames per second. The MPEG most people think of as "MPEG-1" is actually
a
kind of subset known as Constrained Parameters
bitstream (CPB).
9. What are Constrained Parameters Bitstreams (CPB) for video?
A. MPEG-1 CPB are a limited set of sampling and bitrate parameters designed to
normalize decoder computational complexity, buffer size, and memory bandwidth
while still addressing the widest possible range of applications. The paramete
r
limits were intentionally designed so that a decoder implementation would need
only 4 Megabits of DRAM.
Parameter Limit
-------------- ---------------------------
pixels/line 704
lines/picture 480 or 576
pixels*lines 352*240 or 352*288
picture rate 30 Hz
bit rate 1.862million bits/sec
buffer size 40 Kilobytes (327,680 bits)
The sampling limits of CPB are bounded at the ever popular SIF rate: 396
macroblocks (101,376 pixels) per picture if the picture rate is less than or
equal to 25 Hz, and 330 macroblocks (84,480 pixels) per picture if the picture
rate is 30 Hz. The MPEG nomenclature loosely defines a "pixel" or "pel" as a
unit vector containing a complete luminance sample and one fractional (0.25 in
4:2:0 format) sample from each of the two chrominance (Cb and Cr) channels.
Thus, the corresponding bandwidth figure can be computed as: 352 samples/line
x
240 lines/picture x 30 pictures/sec x 1.5 samples/pixel or 3.8 Ms/s (million
samples/sec) including chroma, but not including blanking intervals. Since mo
st
decoders are capable of sustaining VLC decoding at a faster rate than 1.8
Mbit/sec, the coded video bitratehas become the most often waived parameter of
CPB. An encoder which intelligently employs the syntax tools should achieve SI
F
quality saturation at about 2 Mbit/sec, whereas an encoder producing streams
containing only I (Intra) pictures might require as much as 4 Mbit/sec to
achieve the same video quality.
10. Why is Constrained Parameters so important?
A. It is an optimum point that allows (just barely) cost effective VLSI
implementations in 1992 technology (0.8 microns). It also implies a nominal
guarantee of interoperability for decoders and encoders. Since
CPB is a canonical conformance point, MPEG devices which are not capable of
meeting SIF rates are usually not considered to be true MPEG.
11. Who uses constrained parameters bitstreams?
A. Applications which are focused on CPB are Compact Disc (White Book or CD-I)
and computer video applications. Set-top TV decoders fall into a higher
sampling rate category known as CCIR 601 or Broadcast rate.
12. Are there ways of circumventing constrained parameters bitstreams for SIF
class applications and decoders ?
A. Yes, some. Remember that CPB limits pictures by macroblock count. 416 x 24
0
x 24 Hz sampling rates are still within the constraints, but this would only b
e
of benefit in NTSC (240 lines/field) displays. Deviating from 352 samples/line
could throw off many decoder implementations which possess limited horizontal
sample rate conversion abilities. Some decoders do in fact include a few rate
conversion modes, with a filter usually implemented via binary taps (shifts an
d
adds). Likewise, the target sample rates are usually limited or ratios (e.g.
640, 540, 480 pixels/line, etc.). Future MPEG decoders will likely include
on-chip arbitrary sample rate converters, perhaps capable of operating in the
vertical direction (although there is little need of this in applications usin
g
standard TV monitors, with the possible exception of windowing in cable box
graphical user interfaces).
13. Are there any other conformance points like CPB for MPEG-1?
A. Undocumented ones, yes. A second generation of decoder chips emerged on th
e
market about 1 year after the first wave of SIF-class decoders. Both LSI
Logic and SGS-Thomson introduced CCIR 601 class MPEG-1 decodersto fill in the
gap between canonical MPEG-1 and the emergence of MPEG-2. Under non-
disclosure agreement, C-Cube had the CL-950.
14. What frame rates are permitted in MPEG?
A. A limited set is available for the choosing in MPEG-1, although "tricks"
could be played with Systems-layer Time Stamps to convey non-standard rates. T
he
set is: 23.976 Hz (3-2 pulldown NTSC), 24 Hz (Film), 25 Hz (PAL/SECAM or 625/6
0
video), 29.97 (NTSC), 30 Hz (drop-frame NTSC or component 525/60), 50 Hz
(double-rate PAL), 59.97 Hz (double rate NTSC), and 60 Hz (double-rate
drop-frame NTSC/component 525/60 video).
15. Special prediction switches for MPEG-2
MPEG-2 sequence
/ \
progressive interlaced sequence
sequence / \
Field picture Frame picture
/ \
Frame or field pred. Frame MB prediction onl
y
/ \
Field dct Frame dct
16. What is MPEG-2 Video Main Profile and Main Level?
A. MPEG-2 Video Main Profile and Main Level is analogous to MPEG-1's CPB, with
sampling limits at CCIR 601 parameters (720 x 480 x 30 Hz). Profiles limit
syntax (i.e. algorithms), whereas Levels place limits on coding parameters
(sample rates, frame dimensions, coded bitrates, etc.). Together, Video Main
Profile and Main Level (abbreviated as MP@ML) normalize complexity within
feasible limits of 1994 VLSI technology (0.5 micron), yet still meet the needs
of the majority of application users.MP@ML is the conformance point for most
cable and satellite systems.
Profiles
======
Simple: I and P pictures only. 4:2:0 sampling ratio. 8,9, or 10 bits DC
precision.
Main: I, P, and B pictures. Dual Prime with no B-pictures only. 4:2:0 sampli
ng
ratio. 8, 9, or 10 bits sample precision.
SNR profile:
Spatial profile:
High: 8,9,10, or 11 bits sample precision. 4:2:2 and 4:4:4 sampling ratio.
Level
====
Simple: SIF video rate (3.041280 Mhz), 4 Mbit/sec, 0.489472 Mbit VBV buffer
,
64 vertical in frame, 32Vertical in field, 1:7 fcode hor.
Main: CCIR 601 video rate (10.368 Mhz), 15 Mbit/sec, 1.835008 Mbit VBV buffer
,
128 V in frame, 64 V in field, 1:8 f_code Hor.
High 1440: 1440 x 1152 x 30 Hz (47.0016 Mhz), 60 Mbit/sec. 7.340032 Mbit VBV
buffer, 128 V in Fe, 1:9 fcode H.
High: 1920 x 1152 x 30 Hz (62.6688 Mhz), 80 Mbit/sec. 9.787392 Mbit VBV buffer
.
1:9 fcode H
17. Does anybody actually use the scalability modes?
A. At this time, scalability has found itself a limited number of applications
,
although research is definitely underway for its use in HDTV. Experiments have
been demonstrated in Europe where, for example, PAL-rate
video (720 x 576 x 25 fps) is embedded in the same stream as HDTV rate video
(1440 x 1152 x 25 fps). The Nov. 1992 VADIS experiment divided the base layer
(PAL) and enhancement into 4 and 16 Mbit/sec channels, respectively. The U.S.
Grand Alliance favors HDTV simulcasting (separate NTSC analog and digital HDTV
broadcasts). Temporal scalability is the pet scalability mode as the possible
future solution for coding 60 Hz progressive sequences while maintaining
backwards compatibility with early-wave equipment (e.g. 1920 x 1080 x 30 Hz
displays) . To elaborate, the first wave receivers of the late 1990's would be
limited to 62at 0 Hz interlaced/30 Hz progressive HDTV decoders. Essentially,
60 interlaced fields would be coded in a, for
example, 16 Mbit/sec stream in 1996, and when VLSI processes shift another
thousand or so angstroms down the wavelength scale, an 8 Mbit/sec enhancement
layer containing the coded "high pass" between 60 Hz progressive and 60 Hz
interlaced would be simulcasted or multiplexed. Several corporate mouths have
been known to water at the mention of charging the quality conscious subscribe
r
an extra fee for the enhancement layer.
18. What's the difference between Field and Frame pictures?
A. A frame-coded picture consists of samples from both even and odd fields.
A frame picture is coded in progressive order (an even line, then an odd line,
etc.) and in the case of MPEG-2, may optionally switch between field and fram
e
order on a macroblock basis. The Display Process, which is *almost* completely
outside the scope of the MPEG specification, can chose to re-interlace the
picture by displaying the odd and even lines at different times (16 millisecon
ds
apart for 60 Hz displays). In fact, most pictures, regardless of whether they
were coded as a Field or Frame, end up being displayed interlaced due to the
fact that most TV sets are interlaced.
19. What do B-pictures buy you?
A. Since bi-directional macroblock predictions are an average of two macrobloc
k
areas, noise is reduced at low bit rates (like a 3-D filter, if you will). At
nominal MPEG-1 video (352 x 240 x 30, 1.15 Mbit/sec) rates, it is said that
B-frames improves SNR by as much as 2 dB. (0.5 dB gain is usually considered
worth-while in MPEG). However, at higher bit rates, B-frames become less usefu
l
since they inherently do not contribute to the progressive refinement of an
image sequence (i.e. not used as prediction by subsequent coded frames).
Regardless, B-frames are still politically controversial.
B pictures are interpolative in two ways: 1. predictions in the bi-
directional macroblocks are an average from block areas of two pictures 2. B
pictures fill in or interpolate the 3-D video signal over a 33 or 25
millisecond picture period without contributing to the overall signal quality
beyond that immediate point in time. In other words, a B pictures, regardless
of its internal make-up of macroblock types, has a life limited to its immedia
te
self. As mentioned before, its energy does not propagate into other frames.
In
a sense, bits spent on B pictures are wasted.
20. Why do some people hate B-frames?
A. Computational complexity, bandwidth, delay, and picture buffer size are the
four B-frame Pet Peeves. Computational complexity in the decoder is increased
since a some macroblock modes require averaging between two macroblocks.
Worst case, memory bandwidth is increased an extra 15.2 MByte/s (4:2:0 601
rates, not including any half pel or page-mode overhead) for this extra
prediction. An extra picture buffer is needed to store the future prediction
reference (bi-directionality). Finally, extra delay is introduced in encoding
since the frame used for backwards prediction needs to be transmitted to the
decoder before the intermediate B-pictures can be decoded and displayed.
Cable television (e.g. -- more like i.e.-- General Instruments) have been
particularly adverse to B-frames since, for CCIR 601 rate video, the extra
picture buffer pushes the decoder DRAM memory requirements past the magic 8-Mb
it
(1 Mbyte) threshold into the evil realm of 16 Mbits (2 Mbyte).... although
8-Mbits is fine for 352 x 480 B picture sequence. However, cable often forgets
that DRAM does not come in convenient high-volume (low cost) 8-Mbit packages a
s
does the friendly 4-Mbit and 16-Mbit. In a few years, the
cost difference between 16 Mbit and 8 Mbit will become insignificant compared
to
the bandwidth savings gain through higher compression. For the time being, so
me
cable boxes will start with 8-Mbit and allow future drop-in upgrades to the fu
ll
16-Mbit.
21. Why was the 16x16 area chosen?
A. The 16x16 area corresponds to the Least Common Multiple (LCM) of 8x8 block
s,
given the normative 4:2:0 chroma ratio. Starting with medium size images, the
16x16 area provides a good balance between side
information overhead & complexity and motion compensated prediction accuracy.
In gist, 16x16 seemed like a good trade-off.
22. Why was the 8x8 DCT size chosen?
A. Experiments showed little improvements with larger sizes vs. the increased
complexity. A fast DCT algorithm will require roughly double the arithmetic
operations per sample when the transform point size is
doubled. Naturally, the best compaction efficiency has been demonstrated using
locally adaptive block sizes (e.g. 16x16, 16x8, 8x8, 8x4, and 4x4) [See Baker
and Sullivan]. Naturally, this introduces additional side information overhead
and forces the decoder to implement programmable or hardwired recursive DCT
algorithms. If the DCT size becomes too large, then more edges (local
discontinuities) and the like become absorbed into the transform block,
resulting in wider propagation of Gibbs (ringing) and other phenomena. Finally
,
with larger transform sizes, the DC term is even more critically sensitive to
quantization noise.
23. What is motion compensated prediction, and why is it a pain?
A. MCP in the decoder can be thought of as having four stages:
1. Motion vector computation
2. Prediction retrievalvarious predictions are 16x16, 16x8, 8x4, 8x8 plus any
half-pel overhead (e.g. 17x16, 17x17, etc).
3. Filtering
3.1 Forming half-pel predictions through bi-linear interpolation.
3.2 Averaging two predictions together (B macroblocks, Dual Prime)
4. Combination and ordering
4.1 combining 1 or 2 predictions from stage three into upper and lower
halves (16 x 8, field in frame)
4.2 interleaving or grouping together odd and even lines in frame
picture predictions.
The final, combined prediction is always a 16x16 block of luminance and 8x8
block of chrominance, just like we experience in MPEG-1.
A single motion vector can be associated with each source, hence a macroblock
can have as many as 4 motion vectors.
24. What are the various prediction modes in MPEG-2?
24.1 Frame:
Predictions are formed from a 16 x 16 pixel area in a previously reconstructed
frame. Identical to MPEG-1. There can be only one source in forward or backwar
d
predicted macroblocks, and two sources in bi-directional macroblocks. The
prediction frame itself may have been coded as either a frame or two fields,
however once a frame is reconstructed, it is simply a frame as far as future
predictions are concerned.
24.2 Field predictions in frame-coded pictures:
Separate predictions are formed for the top (8 lines from field 1)and bottom (
8
lines from field 2) portions of the macroblock. A total of two motion vectors
in forward or backward predictions, four in bi-directional.
24.3 Field predictions in field-coded pictures:
Predictions are formed from the two most recently decoded fields. Prediction
sizes are 16x16, however the 16 lines have a corresponding projection onto a
16x32 pixel area of a frame. One motion vector for forward or backward
predictions, and two for bi-directional.
24.4 16x8 predictions in field-coded pictures:
Like field macroblocks in frame-coded pictures, the upper and lower 8 lines in
this macroblock mode can have different predictions (hence two motion vectors)
.
This mode compensates for the reduced temporal prediction precision of field
picture macroblocks (a result of the fact that fields inherently possess half
the number of lines that frames do). The field prediction area projected onto
a
frame is restored to 16 lines. 2 motion vectors for backwards or forwards, 4
for bi-directional.
24.5 Dual Prime prediction in frame and field-coded pictures
Predictions for the current macroblock are formed from the average of two 16 x
8
line areas from the two most recently decoded fields. Dual Prime was devised a
s
an alternative for B pictures in low delay applications, but still offers many
of the signal quality benefits of B-pictures. Dual Prime requires one less
prediction picture buffer, but still retains the same instantaneous prediction
bandwidth of a B picture system. As an alternative to coding separate motion
vectors for each of the upper and lower 16x8 areas, a full motion vector is se
nt
for the first area, and a +1, 0, or -1 differential vector (variable length
coded) is specified for the second prediction area. A macroblock will have
total of two full motion vectors and two differential vectors in frame-coded
pictures. Due to the prediction bandwidth overhead, Main Profile restricts th
e
use of Dual Prime prediction to P picture sequences only. High Profile permi
ts
use of Dual Prime in B pictures.
24.6 Field and frame organized macroblocks:
Originally intended as a cheaper means of achieving field-decorrelation in
frame-coded pictures without the fussy overhead of separate field prediction
estimates, the dct coefficients (quantized prediction error for a given
macroblock) may be organized into either a field or frame pattern. Essentially
this means that the prediction error for the combined 16x16 macroblock may be
grouped into field or frame blocks. A bit in the macroblock header (dct_type)
indicates whether the upper and lower portions of the macroblock are to be
interleaved (frame organized) or remain separated (field organized).
25. How do you tell a MPEG-1 bitstream from a MPEG-2 bitstream?
A. All MPEG-2 bitstreams must contain specific extension headers
that*immediately* follow MPEG-1 headers. At the highest layer, for example, t
he
MPEG-1 style sequence_header() is followed by sequence_extension() exclusive t
o
MPEG-2. Some extension headers are specific to MPEG-2 profiles. For example,
sequence_scalable_extension() is not allowed in Main Profile bitstreams.
A simple program need only scan the coded bitstream for byte-aligned start cod
es
to determine whether the stream is MPEG-1 or MPEG-2.
26. What is the reasoning behind MPEG syntax symbols?
A. Here are some of the Whys and Wherefores of MPEG symbols:
Start codes
These 32-bit byte-aligned codes provide a mechanism for cheaply searching code
d
bitstreams for commencement of various layers of video without having to
actually parse variable-length codes or perform any decoder arithmetic. Start
codes also provide a mechanism for resynchronization in the presence of bit
errors.
Coded block pattern (CBP --not to be confused with Constrained Parameters!)
When the frame prediction is particularly good, the displaced frame difference
(DFD, or prediction error) tends to be small, often with entire block energy
being reduced to zero after quantization. This usually happens only at low bit
rates. Coded block patterns prevent the need for transmitting EOB symbols in
those zero coded blocks.
DCT_coefficient_first
Each intra coded block has a DC coefficient. With coded block patterns
signaling all possible combinations of all-zero valued blocks, the
dct_coef_first mechanism assigns a different meaning to the VLC codeword
that would otherwise represent EOB as the first coefficient.
End of Block:
Saves unnecessary run-length codes. At optimal bitrates, there tends to be fe
w
AC coefficients concentrated in the early stages of the zig-zag vector. In
MPEG-1, the 2-bit length of EOB implies that there is an average of only 3 or
4
non-zero AC coefficients per block. In MPEG-2 Intra (I) pictures, with a 4-bi
t
EOB code, this number is between 9 and 16 coefficients. Since EOB is required
for all coded blocks, its absence can signal that a syntax error has occurred
in
the bitstream.
Macroblock stuffing
A genuine pain for VLSI implementations, macroblock stuffing was introduced
to
maintain smoother, constant bitrate control in MPEG-1. However, with normalize
d
complexity measures and buffer management
performed a priori (pre-frame, pre-slice, and pre-macroblock) in the MPEG-2
encoder test model, the need for such localized smoothing evaporated. Stuffing
can be achieved through virtually unlimited slice start code padding if
required. A good rule of thumb: if you find yourself often using stuffing more
than once per slice, you probably don't have a very good rate control algorith
m.
Anyway, macroblock stuffing is now illegal in MPEG-2, so don t start using it
if
you already haven t.
MPEG's modified Huffman VLC tables
The VLC tables in MPEG are not Huffman tables in the true sense of Huffman
coding, but are more like the tables used in Group 3 fax. They are entropy
constrained, that is, non-downloadable and optimized for a limited range of bi
t
rates (sweet spots). With the exception of a few codewords, the larger tables
were carried over from the H.261 standard of 1990. MPEG-2 added an "Intra
table". Note that the dct_coefficient tables assume positive/negative
coefficient pmf symmetry.
27. Why bother to research compressed video when there is a standard?
A. Despite the worldwide standard, many areas remain open for research:
advanced encoding and pre-processing, motion estimation, macroblock decision
models, rate control and buffer management in editing environments, etc. There
's
practically no end to it.
28. Where can I get a copy of the latest MPEG-2 draft?
A. Contact your national standards body (e.g. ANSI Sales in NYC for the U.S.,
British Standards Institute in the UK, etc.). A number of private organizatio
ns
offer ISO documents.
29. What are the latest working drafts of MPEG-2 ?
A. MPEG-2 has reached voting document of the Draft International Standard for
Information Technology -- Generic Coding of Moving Pictures and
Associated Audio. Recommendation H.262, ISO/IEC Draft International Standard
13818-2. [produced March 25, 1994, not yet approved by voting process].
Audio is Part 1, Video Part 2, and Systems is Part 3. A committee draft for
Conformance (Part 4) is expected in Novemeber 1994, as well as the Technical
Report on Software Simulation (Part 5).
30. What is the latest version of the MPEG-1 documents?
A. Systems (ISO/IEC IS 11172-1), Video (ISO/IEC IS 11172-2), and Audio (ISO/IE
C
IS 11172-3) have reached the final document stage. Part 4, Conformance Testin
g,
is currently DIS
31. What is the evolution of ISO standard documents?
A. In chronological order:
ISO/Committee notation Author's notation
--------------------------------------- -------------------------
Problem (unofficial first stage) Barroom Witticism
New work Item (NI) Napkin Item
New Proposal (NP) Need Permission
Working Draft (WD) We're Drunk
Committee Draft (CD) Calendar Deadlock
Draft International Standard (DIS) Doesn't Include Substance
International Standard (IS) Induced patent Statements
32. Where is a good introductory paper to MPEG?
A. Didier Le Gall, "MPEG: A Video Compression Standard for Multimedia
Applications," Communications of the ACM, April 1991, Vol.34, No.4, pp. 47-58
33. What are some journals on related MPEG topics ?
A.
IEEE Transactions on Consumer Electronics
IEEE Transactions on Broadcasting
IEEE Transactions on Circuits and Systems for Video Technology
Advanced Electronic Imaging
Electronic Engineering Times (EE Times -- more tabloid coverage. Unfortunate
columns by Richard Doherty)
IEEE Int'l Conference on Acoustics, Speech, and Signal Processing (ICASSP)
International Broadcasting Convention (IBC)
Society of Motion Pictures and Television Engineers (SMPTE)
SPIE conference on Visual Communications and Image Processing
SPIE conference on Video Compression for Personal Computers
IEEE Multimedia [first edition Spring 1994]
34. Is there a book on MPEG video?
A. Yes, there will be a book published sometime in 1994 by the same authors wh
o
brought you the JPEG book (Bill Pennebaker, Joan Mitchell). Didier Le Gall wil
l
be an additional co-author, and will insure digressions into, e.g. arithmetic
coding aspects, be kept to a minimum :-)
35. Is it MPEG-2 (Arabic numbers) or MPEG-II (roman)?
A. Committee insiders most often use the Arabic notation with the hyphen, e.g.
MPEG-2. Only the most retentive use the official designation: Phase 2. In fac
t,
M.P.E.G. itself is a nickname. The official title is: ISO/IEC JTC1 SC29 WG11.
The militaristic lingo has so far managed to keep the enemy (DVI) confused an
d
out of the picture.
ISO: International Organization for Standardization
IEC: International Electrotechnical Commission
JTC1: Joint Technical Committee 1
SC29: Sub-committee 29
WG11: Work Group 11 (moving pictures with... uh, audio)
36. What happened to MPEG-3?
A. MPEG-3 was to have targeted HDTV applications with sampling dimensions
up to 1920 x 1080 x 30 Hz and coded bitrates between 20 and 40 Mbit/sec. It wa
s
later discovered that with some (compatible) fine tuning, MPEG-2 and MPEG-1
syntax worked very well for HDTV rate video. The key is to maintain an optimal
balance between sample rate and coded bit rate.Also, the standardization windo
w
for HDTV was rapidly closing. Europe and the United States were on the brink
of
committing to analog-digital subnyquist hybrid algorithms (D-MAC, MUSE, et al)
.
European all-digital projects such as HD-DIVINE and VADIS demonstrated better
picture quality with respect to bandwidth using the MPEG syntax. In the Unite
d
States, the Sarnoff/NBC/Philips/Thomson HDTV consortium had used MPEG-1 syntax
from the beginning of its all-digital proposal, and with the exception of
motion artifacts (due to limited search range in the encoder), was deemed to
have the best picture quality of all three digital proponents. HDTV is now par
t
of the MPEG-2 High-1440 Level and High Level toolkit.
37. What is MPEG-4?
MPEG-4 targets the Very Low Bitrate applications defined loosely as having
sampling dimensions up to 176 x 144 x 10 Hz and coded bit rates between 4800 a
nd
64,000 bits/sec. This new standard would be used, for example, in low bit ra
te
videophones over analog telephone lines.
This effort is in the very early stages. Morphology, fractals, model based, a
nd
anal retentive block transform coding are all in the offering. MPEG-4 is now i
n
the application identification phase. Scaleable modes of MPEG-2
38. What are the scaleable modes of MPEG-2?
A. Scaleable video is permitted only in the High Profiles.
Currently, there are four scaleable modes in the MPEG-2 toolkit. These modes
break MPEG-2 video into different layers (base, middle, and high layers) mostl
y
for purposes of prioritizing video data. For example, the high priority chann
el
(bitstream) can be coded with a combination of extra error correction
information and/or increased signal strength (i.e. higher Carrier-to-Noise rat
io
or lower Bit Error Rate) than the lower priority channel. For example, in HDTV
,
the high priority bitstream (720 x 480) can be decoded under noise conditions
were the lower priority (1440 x 960) cannot. This is part of the "graceful
degradation concept. Breaking a video signal into two streams (base and
enhancements) has a penalty, however. Usually less than 1.5 dB.
Another purpose of salability is complexity division. A standard TV set need
only decode the 720 x 480 channel, thus requiring a less expensive decoder
processor than a TV set wishing to display 1440 x 960. This is known as
simulcasting.
A brief summary of the MPEG-2 video scalability modes:
Spatial Scalablity-- Useful in simulcasting, and for feasible software decodin
g
of the lower resolution, base layer. This spatial domain method codes a base
layer at lower sampling dimensions (i.e. "resolution") than the upper layers.
The upsampled reconstructed lower (base) layers are then used as prediction fo
r
the higher layers.
Data Partitioning-- Similar to JPEG's frequency progressive mode, only the sli
ce
layer indicates the maximum number of block transform coefficients contained i
n
the particular bitstream (known as the
"priority break point"). Data partitioning is a frequency domain method that
breaks the block of 64 quantized transform coefficients into two bitstreams.
The first, higher priority bitstream contains the more critical lower frequenc
y
coefficients and side informations (such as DC values, motion vectors). The
second, lower priority bitstream carries higher frequency AC data.
SNR Scalability-- Similar to the point transform in JPEG, SNR scalability is a
spatial domain method where channels are coded at identical sample rates, but
with differing picture quality (achieved through quantization step sizes). The
higher priority bitstream contains base layer data that can be added to a lowe
r
priority refinement layer to construct a higher quality picture.
Temporal Scalability--- A temporal domain method useful in, e.g., stereoscopic
video. The first, higher priority bitstreams codes video at a lower frame rat
e,
and the intermediate frames can be coded in a second bitstream using the first
bitstream reconstruction as prediction. In stereoscopic vision, for example, t
he
left video channel can be prediction from the right channel.
Other scalability modes were experimented with in MPEG-2 video (such as
Frequency Scalability), but were eventually dropped in favor of methods that
demonstrated comparable or better picture quality with greater
simplicity.
39. Why MPEG-2? Wasn't MPEG-1 enough?
A. MPEG-1 was optimized for CD-ROM or applications at about 1.5 Mbit/sec. Vide
o
was strictly non-interlaced (i.e. progressive). The international cooperation
executed well enough for MPEG-1, that the committee began to address
applications at broadcast TV sample rates using the CCIR 601 recommendation (7
20
samples/line by 480 lines per frame by 30 frames per second or about 15.2
million samples/sec including chroma) as the reference.
Unfortunately, today's TV scanning pattern is interlaced. This introduces a
duality in block coding: do local redundancy areas (blocks) exist exclusively
in a field or a frame.(or a particle or wave) ? The answer of course is that
some blocks are one or the other at different times, depending on motion
activity. The additional man years of experimentation and implementation betwe
en
MPEG-1 and MPEG-2 improved the method of block-based transform coding.
40. What did MPEG-2 add to MPEG-1 in terms of syntax/algorithms ?
A. Here is a brief summary:
Sequence layer:
More aspect ratios. A minor, yet necessary part of the syntax. Horizontal and
vertical dimensions are now required to be a multiple of 16 in frame coded
pictures, and the vertical dimension must be a multiple of 32 in field coded
pictures.
4:2:2 and 4:4:4 macroblocks were added in the Next profiles.
Syntax can now signal frame sizes as large as 16383 x 16383. Syntax signals
source video type (NTSC, PAL, SECAM, MAC, component) to help post-processing a
nd
display.
Source video color primaries (609, 170M, 240M, D65, etc.) and opto-electronic
transfer characteristics (709, 624-4M, 170M etc.) can be indicated.
Four scaleable modes [see scalability discussion]
Picture layer: All MPEG-2 motion vectors are specified to a half-pel sample
grid.
DC precision can be user-selected as 8, 9, 10, or 11 bits.
New scalar quantization matrices may be downloaded once per picture. In High
profile, separate chrominance matrices now exist (Y and C no longer have to
share)
Concealment motion vectors were added to I-pictures in order to increase
robustness from bit errors. I pictures are the most critical and sensitive
picture in a group of pictures.
A non-linear macroblock quantization factor providing a wider dynamic range,
from 0.5 to 56, than the linear MPEG-1 (1 to 32) range. Both are sent as a
5-bit FLC side information in the macroblock and slice headers.
New Intra-VLC table for dct_coefficient_next (AC run-level events) that is a
better match for the histogram of Intra-coded pictures. EOB is 4 bits. The old
table, dct_coef_next, are reserved for use in non-intra pictures (P, B),
although they new table can be used for Intra-coded macroblocks in P and B
pictures as well.
Alternate scanning pattern that (supposedly) improves entropy coding performan
ce
over the original Zig-Zag scan used in H.261, JPEG, and MPEG-1.
The extra scanning pattern is geared towards interlaced video.
Syntax to signal an irregular 3:2 pulldown process (repeat_field_first flag)
Progressive and interlaced frame coding
Syntax to indicate source composite video characteristics useful in
post-processing operations. (v-axis, field sequence, sub_carrier, phase,
burst_amplitude, etc.)
Pan & scanning syntax that tells decoder how to, for example, window a 4:3 ima
ge
within a wider 16:9 aspect ratio coded image. Vertical pan offset has 1/16th
pixel accuracy.
Macroblock layer:
Macroblock stuffing is now illegal in MPEG-2 (hurray!!). If stuffing is really
needed, the encoder can pad slice start codes.
Two organizations for macroblock coefficients (interlaced and progressive)
signaled by dct_type flag.
Now only one run-level escape code code (24-bits) instead of the single
(20-bits) and double escape (28-bits) in MPEG-1.
Improved mismatch control in quantization over the original oddification metho
d
in MPEG-1. Now specifies adding or subtracting one to the 63rd AC coefficient
depending on parity of the summed coefficients. MPEG-2 mismatch control is
performed on the transform coefficients, whereas in MPEG-1, it is applied to t
he
quantized transform coefficients.
Many additional prediction modes (16x8 MC, field MC, Dual Prime)
and,correspondingly, macroblock modes.
Overall, MPEG-2's greatest compression improvements over MPEG-1 are: predictio
n
modes, Intra VLC table, DC precision, non-linear macroblock quantization.
Implementation improvements: macroblock stuffing was eliminated.
41. How do MPEG and JPEG differ?
A. The most fundamental difference is MPEG's use of block-based motion
compensated prediction (MCP)---a method falling into the general category of
temporal DPCM.
The second most fundamental difference is in the target application. JPEG adop
ts
a general purpose philosophy: independence from color space (up to 255
components per frame) and quantization tables for each component. Extended mod
es
in JPEG include two sample precision (8 and 12 bit sample accuracy),
combinations of frequency progressive, spatial hierarchically progressive, and
amplitude (point transform) progressive scanning modes. Further color
independence is made possible thanks to downloadable Huffman tables (up to one
for each component.)
Since MPEG is targeted for a set of specific applications, there is only one
color space (4:2:0 YCbCr), one sample precision (8 bits), and one scanning mod
e
(sequential). Luminance and chrominance share quantization and VLC tables. MPE
G
adds adaptive quantization at the macroblock (16 x 16 pixel area) layer. This
permits both smoother bit rate control and more perceptually uniform
quantization throughout the picture and image sequence. However, adaptive
quantization is part of the Enhanced JPEG charter (ISO/IEC 10918-3) currently
in
verification stage. MPEG variable length coding tables are non-downloadable, a
nd
are therefore optimized for a limited range of compression ratios appropriate
for the target applications.
The local spatial decorrelation methods in MPEG and JPEG are very similar.
Picture data is block transform coded with the two-dimensional orthanormal 8x8
DCT, with asymmetric basis vectors about time (aka DCT-II ). The resulting 63
AC transform coefficients are mapped in a zig-zag pattern (or alternative scan
pattern in MPEG-2) to statistically increase the runs of zeros. Coefficients o
f
the vector are then uniformly scalar quantized, run-length coded, and finally
the run-length symbols are variable length coded using a canonical (JPEG) or
modified Huffman (MPEG) scheme. Global frame redundancy is reduced by 1-D DPC
M
of the block DC coefficients, followed by quantization and variable length
entropy coding of the quantized DC coefficient.
MCP DCT ZZ
Q
Frame -> 8x8 spatial block -> 8x8 frequency block -> Zig-zag scan ->
RLC VLC
quanitzation -> run-length coding -> variable length coding.
The similarities have made it possible for the development of hard-wired silic
on
that can code both standards. Even some highly microcoded architectures
employing hardwired instruction primitives or functional blocks benefit from
JPEG/MPEG similarities. There are many additional yet minor differences. They
include:
1. In addition to the 8-bit mode, DCT and quantization precision in MP
EG
has a 9-bit and 12-bit mode, respectively, exclusively in non-intra coded
macroblocks. A 1-bit expansion takes place in the macroblock difference
operation.
2. Mismatch control in MPEG-1 forces quantized coefficients to become
odd values (oddification). JPEG does not employ any mismatch mechanism.
3. JPEG run-length coding produces run-size tokens (run of
zeros,non-zero coefficient magnitude) whereas MPEG produces fully concatenated
run-level tokens that do not require magnitude differential bits.
4. DC values in MPEG-1 are limited to 8-bit precision (a constant
stepsize of 8), whereas JPEG DC precision can occupy all possible 11-bits.
MPEG-2, however, re-introduced extra DC precision critical even
at high compression ratios.Difference between MPEG and H.261
42. How do MPEG and H.261 differ?
A. H.261, also known as Px64, was targeted for teleconferencing applications
where motion is naturally more limited. Motion vectors are restricted to a ran
ge
of +/- 15 pixel unit displacements. Prediction accuracy is reduced since H.261
motion vectors are specified to only integer-pel accuracy. Other quality
syntactic differences include: no B-pictures, inferior mismatch control.
43. Is H.261 the de facto teleconferencing standard?
A. Not exactly. To date, about seventy percent of the industrial
teleconferencing hardware market is controlled by PictureTel of Mass. The seco
nd
largest market controller is Compression Labs of Silicon Valley. PictureTel
hardware includes compatibility with H.261 as a lowest common denominator, but
when in communication with other PictureTel hardware, it can switch to a mode
superior at low bit rates (less than 300kbits/sec). In fact, over 2/3 of all
teleconferencing is done at two-times switched 56 channel (~P = 2) bandwidth.
ISDN is still expensive. In each direction, video and audio are coded at an
aggregate rate of 112 kbits/sec (2*56 kbits/sec). The PictureTel proprietary
compression algorithm is acknowledged to be a combination of spatial pyramid,
lattice vector quantizer, and an unidentified entropy coding method. Motion
compensation is considerably more refined and sophisticated than the 16x16
integer-pel block method specified in H.261.
The Compression Labs proprietary algorithm also offers significant improvement
over H.261 when linked to other CLI hardware. Local decorrelation is based on
a
DCT-VQ hybrid.
Currently, ITU-TS (International Telecommunications Union--teleconferencing
Sector), formerly CCITT, is quietly defining an improvement to H.261 with the
participation of industry vendors.
Rate control
44. What is the TM rate control and adaptive quantization technique ?
A. The Test model (MPEG-2) and Simulation Model (MPEG-1) were not, by any
stretch of the imagination, meant to epitomize state-of-the art encoding
quality. They were, however, designed to exercise the syntax,
verify proposals, and test the *relative* compression performance of proposals
in a timely manner that could be duplicated by co-experimenters. Without
simplicity, there would have been no doubt endless debates over model
interpretation. Regardless of all else, more advanced techniques would probab
ly
trespass into proprietary territory.
The final test model for MPEG-2 is TM version 5b, aka TM version 6. The final
MPEG-1 simulation model is version 3. The MPEG-2 TM rate control method offers
a
dramatic improvement over the SM method. TM adds more accurate estimation of
macroblock complexity through use of limited a priori information. Macroblock
quantization adjustments are computed on a macroblock basis, instead of
once-per-slice.
45. How does the TM work?
A. Rate control and adaptive quantization are divided into three steps:
Step One:Bit Allocation
In Complexity Estimation, the global complexity measures assign relative
weights to each picture type (I,P,B). These weights (Xi, Xp, Xb) are reflecte
d
by the typical coded frame size of I, P, and B pictures (see typical frame siz
e
discussion). I pictures are usually assigned the largest weight since they hav
e
the greatest stability factor in an image sequence. B pictures are assigned t
he
smallest weight since B energy do not propagate into other pictures and are
usually highly correlated with neighboring P and I pictures.
The bit target for a frame is based on the frame type, the remaining number o
f
bits left in the Group of Pictures (GOP) allocation, and the immediate
statistical history of previously coded pictures.
Step Two: Rate Control
Rate control attempts to adjust bit allocation if there is significant
difference between the target bits (anticipated bits) and actual coded bits fo
r
a block of data. If the virtual buffer begins to overflow, the macroblock
quantization step size is increased, resulting in a smaller yield of coded bit
s
in subsequent macroblocks. Likewise, if underflow begins, the step size is
decreased. The Test Model approximates that the target picture has spatially
uniform distribution of bits. This is a safe approximation since spatial
activity and perceived quantization noise are almost inversely proportional.
Of
course, the user is free to design a custom distribution, perhaps targeting
more bits in areas that contain text, for example.
Step Three: Adaptive Quantization
The final step modulates the macroblock quantization step size obtained in Ste
p
2 by a local activity measure. The activity measure itself is normalized again
st
the most recently coded picture of the same type (I, P, or B). The activity fo
r
a macroblock is chosen as the minimum among the four 8x8 block luminance
variances. Choosing the minimum block is part of the concept that a macrobloc
k
is no better than the block of highest visible distortion (weakest link in the
chain).
46. What is a good motion estimation method, then?
A. When shopping for motion vectors, the three basic characteristics are: Sear
ch
range, search pattern, and matching criteria. Search pattern has the greatest
impact on finding the best vector. Hierarchical search patterns first find the
best match between downsampled images of the reference and target pictures and
then refine the vector through progressively higher resolutions. When compared
to other fast methods, hierarchical patterns are less likely to be confused by
extremely local distortion minimums as being a best match. Also note that
subsampled search and hierarchical search are not synonymous.
Q. Is there a limit to the length of motion vectors?
The search area is unlimited, but the reconstructed motion vectors must not:
a. point beyond the picture boundaries (1 <= MV_x <= luminancewidth - 16) an
d
(1 <= MV_y <= luminanceheight - 16). The - 16 is due to the fact that the
motion vector origin is the upper left hand corner of a macroblock)
b. In Constrained Parameters MPEG-1, the motion vector is limited to a range o
f
[-64,+63.5] luminance samples with half-pel accuracy, and [-128,+127.5] with
integer pel accuracy. Break the constrained parameters rules and your video
sequence will not likely display on many hardware devices.
c. In MPEG-2 Video Main Profile at Main Level, the motion vectors are always
on
a half-pel co-ordinate grid, and the vertical range is restricted to [-64,
+63.5], and the horizontal limit is [-256,+255.5].
d. in MPEG-1, the syntactic limit of the motion vector is [-1024,+1023] intege
r
pel, horizontal and vertical.
e. in MPEG-2, the syntactic limit of the motion vector is [-2048,+2047.5]
horizontal, [-1024,+1023.5] vertical.
47. Is exhaustive search "optimal" ?
A. Definitely not in the context of block-based MCP video. Since one motion
vector represents the prediction of 256 pixels, divergent pixels within the
macroblock are misrepresented by the "global" vector. This leads back to the
general philosophy of block-based coding as an approximation technique. In the
ir
ICASSP'93 paper, Sullivan discusses ways in which block-based prediction schem
es
can solve part of this problem.
Exhaustive search may find blocks with the least distortion (displaced frame
difference) but will not produce motion vectors with the lowest entropy.
48. What are some advanced encoding methods?
Quantizer feedback: determine the dependent quantization stepsize by modeling
quantization error propagating over multiple pictures. [Uz/et al ICASSP 93,
Ortega/Vetterli/et al ICASSP 93]
Smoothness constraint placed on local activity measures. immediate blocks
outside target macroblock are considered when selecting macroblock quantizatio
n
stepsize .[Thomson/Savitier patent]
Horizontal variance: measure variance between columns of pixels in addition to
the traditional measure of variance along rows (lines) when making field/frame
macroblock prediction decision.
DFD energy: examine DFD energy/variance when making Intra/Non-intra macroblock
decision.
Activity measures: use total bits from a first-pass encoding of a picture or
macroblock as a measure of the activity. Coded bits is a more accurate
reflection of local complexity than variance. [Thomson/Savitier patent] motion
vector cost: this is true for any syntax elements, really. Signaling a
macroblock quantization factor or a large motion vector differential can cost
more than making up the difference with extra quantized DFD (prediction error)
bits. The optimum can be found with, some Lagrangian operator. In summary,
any compression system with side information, there is a optimum point between
signaling overhead (e.g. prediction) and prediction error.
Liberal Interpretations of the Forward DCT: Borrowing from the concept that th
e
DCT is simply a filter bank, a technique that seems to be gaining popularity i
s
basis vector shaping. Usually this is combined with the quantization stage
since the two are tied closely together in a rate-distortion sense. The idea i
s
to use the basis vector shaping as a cheap alternative to pre-filtering by
combining the more desirable data adaptive properties of pre-filtering/
pre-processing into the transformation process... yet still reconstruct a
picture in the decoder using the standard IDCT that looks reasonably like the
source. Some more clever schemes will apply a form of windowing.
[Warning: watch out for eigenimage/basis vector orthoganality. ]
Frequency-domain enhancements:
Enhancements are applied after the DCT (and possibly quantization)stage to the
transform coefficients. This borrows from the concept: if you don't like the
(quantized) transformed results, simply reshape them into
something you do like. Suppressing isolated small amplitudes is popular.
Temporal spreading of quantization error: This method is similar to the origin
al
intent behind color subcarrier
phase alternation by field in the NTSC, PAL, and SECAM analog TV standards: fo
r
stationary areas, noise does not hang" in one location, but dances about the
image over time to give a more uniform effect.
Distribution makes it more difficult for the eye to "catch on" to trouble spot
s
(due to the latent temporal response curve of human vision). Simple encoder
models tend to do this naturally but will not solve all situations.
Look-ahead and adaptive frame cycle structures: analyze picture activity sever
al
pictures into the future, looking for scene changes or motion statistics.
It is easy to spot encoders that do not employ any advanced encoding technique
s:
reconstructed video usually contains ringing around edges, color bleeding, and
lots of noise.
49. Is so-and-so really MPEG compliant ?
A. At the very least, there are two areas of conformance/compliance in MPEG:
1.
Compliant bitstreams 2. compliant decoders. Technically speaking, video
bitstreams consisting entirely of I-frames (such as those generated by Xing
software) are syntactically compliant with the MPEG specification. The I-fram
e
sequence is simply a subset of the full syntax. Compliant bitstreams must obe
y
the range limits (e.g. motion
vectors limited to +/-128, frame sizes, frame rates, etc.)and syntax rules (e.
g.
all slices must commence and terminate with a non-skipped macroblock, no gaps
between slices, etc.).
Decoders, however, cannot escape true conformance. For example, a decoder that
cannot decode P or B frames are *not* legal MPEG. Likewise, full arithmetic
precision must be obeyed before any decoder can be called "MPEG compliant."
The IDCT, inverse quantizer, and motion compensated predictor must meet the
specification requirements... which are fairly rigid (e.g. no more than 1 leas
t
significant bit of error between reference and test decoders). Real-time
conformance is more complicated to measure than arithmetic precision, but it i
s
reasonable to expect that decoders that skip frames on reasonable bitstreams a
re
not likely to be considered compliant.Artifacts
50. What are the tell-tale MPEG artifacts?
A. If the encoder did its job properly, and the user specified a proper balanc
e
between sample rate and bitrate, there shouldn't be any visible artifacts.
However, in sub-optimal systems, you can look for:
Gibbs phenomenon/Ringing/Aliasing (too few AC bits, not enough
pre-processing)
Blockiness (not considering your neighbors before quantizing)
Posterization (too few DC bits)
Checkerboards (DCT eigenimages as a result of too few AC
coefficients)Colorbleeding (not considering color in encoder cost model, not
subtracting color at edges of objects, etc.)
51. Where are the weak points of MPEG video ?
A. Texture patterns (rapidly alternating lines) sharp edges (especially
text) [installment 3]
52. What are some myths about MPEG?
A. There are a few major myths that I am aware of:
1. Block displacements: macroblock predictions are formed out of arbitrary
16x16 (or 16x8/16x16 in MPEG-2) areas from previously reconstructed pictures.
Many people believe that the prediction macroblocks have boundaries that fall
on interchange boundaries (pixel 0, 15, 31, 53... line 0, 15, 31, 53... etc.).
In fact, motion vectors represent relative translations with respect to the
target reconstruction macroblock coordinates. The motion vectors can point to
half pixel coordinates, requiring that the prediction macroblock to be
formed via bi-linear interpolation of pixels.
2. Displaced frame (macroblock) difference construction: the prediction error
formed as the difference between the prediction macroblock and source macroblo
ck
is coded much like an Intra macroblock. The
prediction may come from different locations (as in bi-directional
prediction--or in MPEG-2--16x8, field-in-frame, and Dual Prime), but the DFD i
s
always coded as a 16x16 unit.
3. Compression ratios
You hear 200:1 and 100:1 in the media. Utter rubbish. The true range is
between 16:1 and 40:1. Spreading misinformation about compression ratios in
public will catch the attention of the infamous MPEG Police.
They say mild-mannered Michael Barnsley will snap, without warning, into viole
nt
rage if he doesn't get the upper bunk bed.
4. Picture coding types all consist of the same macroblocks
Macroblocks within I pictures are strictly intra-coded. Macroblocks within P
pictures can be either predicted or intra-coded, and B pictures they can be
bi-directional, forward, backward, or intra. Additional
macroblock modes switches include: predicted with no motion compensation,
modified macroblock quantization, coding of prediction error or not. The
switches are concatenated into the macroblock_type side information and variab
le
length coded in the macroblock header.
53. What is the color space of MPEG?
MPEG strictly specifies the YCbCr color space, not YUV or YIQ or YPbPr or YDrD
b
or any other color difference variations. Regardless of any bitstream
parameters, MPEG-1 and MPEG-2 Video Main Profile specify 4:2:0 chroma ratio,
where the color difference channels (Cb, Cr) have half the resolution or
sample grid density in both the horizontal and vertical direction with respect
to luminance.
MPEG-2 High Profile includes an option for 4:2:2 and 4:4:4 coding. Application
s
for this are likely to be broadcasting and contribution equipment.
54. Don't you mean 4:1:1 ?
A. No, here is a table of ratios:
CCIR 601 (60 Hz) image Chroma sub-sampling factors
format Y Cb, Cr Vertical Horizontal
----- --------- ---------- -------- ----------
4:4:4 720 x 480 720 x 480 none none
4:2:2 720 x 480 360 x 480 none 2:1
4:2:0 720 x 480 360 x 240 2:1 2:1
4:1:1 720 x 480 720 x 120 none 4:1
4:1:0 720 x 480 180 x 120 4:1 4:1
3:2:2, 3:1:1, and 3:1:0 are less common variations.
55. Why did MPEG choose 4:2:0 ? Isn't 4:2:2 the standard for TV?
A. At least three reasons I can think of:
1. 4:2:0 picture memory requirements are 33% less than the size of 4:2:2
pictures. MPEG-1 decoder are able to snugly fit all 3 SIF pictures (1
reconstruction & display, 2 prediction) into 512 KBytes of buffer space. CCIR
601 is a tighter fit into 2 Mbytes.
2. The subjective difference between 4:2:0 and 4:2:2 is minimal, when
considering consumer display equipment and distribution compression ratios.
3. Vertical decimation increases compression efficiency by reducing syntax
overhead posed in an 8 block (4:2:0) macroblock structure.
4. You re compressing the hell out of the video signal, so what possible
difference can the 0:0:2 high-pass make?
Interlacing and the 62 microsecond gap between successively scanned lines
introduces some discontinuities, but most of this can be alleviated through
pre-processing.
56. What is the precision of MPEG samples?
A. By definition, MPEG samples have no more and no less than 8-bits uniform
sample precision (256 quantization levels). For luminance (which is unsigned)
data, black corresponds to level 0, white is level 255. However, in CCIR
recommendation 601 chromaticy, levels 0 through 14 and 236 through 255 are
reserved for blanking signal excursions. MPEG currently has no such clipped
excursion restrictions, although decoder might take care to insure active
samples do not exceed these limits. With three color components per pixel, th
e
total combination is roughly 16.8 million colors (i.e. 24-bits).
57. What is all the fuss with cositing of chroma components?
A. It is moderately important to properly co-site chroma samples, otherwise a
sort of chroma shifting effect (exhibited as a halo ) may result when the
reconstructed video is displayed. In MPEG-1 video, the chroma samples are
exactly centered between the 4 luminance samples (Fig 1.) To maintain
compatibility with the CCIR 601 horizontal chroma locations and simplify
implementation (eliminate need for phase shift), MPEG-2 chroma samples are
arranged as per Fig.2.
Y Y Y Y Y Y Y Y YC Y YC Y
C C C C
Y Y X Y Y Y Y Y YC Y YC Y
Y Y Y Y Y Y Y Y YC Y YC Y
C C C C
Y Y Y Y Y Y Y Y YC Y YC Y
Fig.1 MPEG-1 Fig.2 MPEG-2 Fig.3 MPEG-2 and
4:2:0 organization 4:2:0 organization CCIR Rec. 601
4:2:2 organization
MPEG for the data compression expert
58. How would you explain MPEG to the data compression expert?
A. MPEG video is a block-based video scheme.
59. How does MPEG video really compare to TV, VHS, laserdisc ?
A. VHS picture quality can be achieved for source film video at about 1 millio
n
bits per second (with proprietary encoding methods). It is very difficult to
objectively compare MPEG to VHS. The response curve of VHS places -3 dB at
around 2 MHz of analog luminance bandwidth (equivalent to 200 samples/line). V
HS
chroma is considerably less dense in the horizontal direction than MPEG source
video (compare 80
samples/line to 176!). From a sampling density perspective, VHS is superior
only in the vertical direction (480 luminance lines compared to 240)... but wh
en
taking into account (supposedly such things as) interfield magnetic tape
crosstalk and the TV monitor Kell factor, the perceptual vertical advantage is
not all that significant. VHS is prone to such inconveniences as timing error
s
(an annoyance addressed by time base correctors), whereas digital video is ful
ly
discretized. Pre-recorded VHS is typically recorded at very high duplication
speeds (5 to 15 times real time playback speed), opening up additional avenues
for artifacts. In gist, MPEG-1 at its nominal parameters can match VHS's sexy
low-pass-filtered look.
With careful coding schemes, broadcast NTSC quality can be approximated at abo
ut
3 Mbit/sec, and PAL quality at about 4 Mbit/sec. Of course, sports sequences
with complex spatial-temporal activity should be treated with bit rates more
like 5 and 6 Mbit/sec, respectively. Laserdisc is a tough one to compare.
Laserdisc's are encoded with composite video (NTSC or PAL). Manufacturers of
laser disc players make claims of up to 425 TVL (or 567 samples/line) respons
e.
Thus it could be said the laserdisc has a 567 x 480 x 30 Hz "potential
resolution". The carrier-to-noise ratio is typically better than 48 dB. Timin
g
is excellent. Yet some of the clean characteristics of laserdisc can be achiev
ed
with MPEG-1 at 1.15 Mbit/sec (SIF rates),
especially for those areas of medium detail (low spatial activity) in the
presence of uniform motion. This may be why some people say MPEG-1 video at 1.
15
Mbit/sec looks almost as good as Laserdisc or Super VHS at times.
60. What are the typical MPEG-2 bitrates and picture quality?
Picture type
I P B Average
MPEG-1 SIF
@ 1.15 Mbit/sec 150,000 50,000 20,000
38,000
MPEG-2 601 400,000 200,000 80,000
130,000
@ 4.00 Mbit/sec
Note: parameters assume Test Model for encoding, I frame distance of 15 (N =
15), and a P frame distance of 3 (M = 3).
Of course, among differing source material, scene changes, and use of
advanced encoder models... these numbers can be significantly different.
61. At what bitrates is MPEG-2 video optimal?
A. The Test subgroup has defined a few examples:
"Sweet spot" sampling dimensions and bit rates for MPEG-2:
Dimensions Coded rate Comments
------------- ---------- ----------------------------------------
352x480x24 Hz 2 Mbit/sec Half horizontal 601. Looks almost NTSC
(progressive) broadcast quality, and is a good (better)
substitute for VHS. Intended for film src.
544x480x30 Hz 4 Mbit/sec PAL broadcast quality (nearly full capture
(interlaced) of 5.4 MHz luminance carrier). Also
4:3 image dimensions windowed within 720
sample/line 16:9 aspect ratio via pan&scan.
704x480x30 Hz 6 Mbit/sec Full CCIR 601 sampling dimensions.(interlaced)
[these numbers subject to change at whim of MPEG Test subgroup]
62. Why does film perform so well with MPEG ?
A. Several reasons, really:
1) The frame rate is 24 Hz (instead of 30 Hz) which is a savings of some 20
%.
2) the film source video is inherently progressive. Hence no fussy
interlaced spectral frequencies.
3) the pre-digital source was severely oversampled (compare 352 x 240 SIF t
o
35 millimeter film at, say, 3000 x 2000 samples). This can result in a very
high quality signal, whereas most video cameras do not oversample,
especially in the vertical direction.
4) Finally, the spatial and temporal modulation transfer function (MTF)
characteristics (motion blur, etc) of film are more amenable to the
transform and quantization methods of MPEG.
63. What is the best compression ratio for MPEG ?
A. The MPEG sweet spot is about 1.2 bits/pel Intra and .35 bits/pel inter
.
Experimentation has shown that intra frame coding with the familiar
DCT-Quantization-Huffman hybrid algorithm achieves optimal performance at abou
t
an average of 1.2 bits/sample or about 6:1 compression ratio. Below this point
,
artifacts become noticeable.
64. Can MPEG be used to code still frames?
A. Yes. There are, of course, advantages and disadvantages to using MPEG over
JPEG:
Disadvantages:
1. MPEG has only one color space
2. MPEG-1 and MPEG-2 Main Profile luma and chroma share quanitzation and VLC
tables
3. MPEG-1 is syntactically limited to 4k x 4k images, and 16k x 16k for MPEG-2
.
Advantages:
1. MPEG possesses adaptive quantization
2. With its limited still image syntax, MPEG averts any temptation to use
unnecessary, expensive, and academic encoding methods that have little impact
on the overall picture quality (you know who you are). Philips' CD-I spec. has
a
requirement for a MPEG still frame mode, with double SIF image resolution. Th
is
is technically feasible mostly thanks to the fact that only one picture buffer
is needed to decode a still image
instead of three buffers.
65. Is there an MPEG file format?
A. Not exactly. The necessary signal elements that indicate image size, pictu
re
rate, aspect ratio, etc. are already contained within the sequence layer of th
e
MPEG video stream. The Whitebook format for Karoke and CD-I movies specify a
range of (time-division) multiplexing strategies for audio and video bitstream
s.
A directory format listing scenes and their locations on the disc is associate
d
with the White Book specification.
66. What are some pre-processing enhancements ?
Adaptive de-interlacing:
This method maps interlaced video from a higher sampling rate (e.g 720 x 480)
into a lower rate, progressive format (352 x 240). The most basic algorithm
measures the correlation between two immediate macroblock fields, and if the
correlation is high enough, uses an average of both fields to form a frame
macroblock. Otherwise, a field area from one field (usually of the same parity
)
is selected. More clever algorithms are much more complex than this, and may
involve median filtering, and multirate/multidimensional tools.
Pre-anti-aliasing and Pre-blockiness reduction:A common method in still image
coding is to pre-smooth the image before encoding. For example, if pre-analys
is
of a frame indicates that serious artifacts will arise if the picture were to
be
coded in the current condition (i.e. below the sweet spot), a pre-anti-aliasin
g
filter can be applied. This can be as simple as having a smoothing severity
proportional to the image activity. The pre-filter can be global (same
smoothing factor for whole image or sequence) or locally adaptive. More comple
x
methods will again use multirate/multidimensional methods.
One straightforward concept from multidimensional/multirate e-processing is to
apply source video whose resolution (sampling density) is greater than the
target source and reconstruction sample rates. This follows the basic principl
es
of oversampling, as found in A/D converters.
These filters emphasize the fact that most information content is contained in
the lower harmonics of a picture anyway. VHS is hardly considered to be a
sharp cut-off medium, tragically implying that "320 x 480 potential" of VHS
is
never truly realized.
67. Why use these "advanced" pre-filtering techniques?
A. Think of the DCT and quantizer as an A/D converter. Think of the DCT/Q
pre-filter as the required anti-alias prefilter found before every A/D. The b
ig
difference of course is that the DCT quantizer assigns a varying number of bit
s
per transform coefficient. Judging on the normalized activity measured in the
pre-analysis stage of video encoding (assuming you even have a pre-analysis
stage), and the target buffer size status, you have a fairly good idea of how
many bits can be spared for the target macroblock, for example.
Other pre-filtering techniques mostly take into account: texture patterns,
masking, edges, and motion activity. Many additional advanced techniques can
be
applied at different immediate layers of video encoding (picture, slice,
macroblock, block, etc.).
68. What about post-processing enhancements?
Some research has been carried out in this area. Non-linear interpolation
methods have been published by Wu and Gersho (e.g. ICASSP 93), convex hull
projections for MAP (Severinson, ICASSP 93), and others. Post-processing
unfortunately defies the spirit of MPEG conformance. Decoders should produce
similar reconstructions. Enhancements should ideally be done during the
pre-processing and encoding stages.
69. Can motion vectors be used to measure object velocity?
A. Motion vector information cannot be reliably used as a means of determining
object velocity unless the encoder model specifically set out to do so. First
,
encoder models that optimize picture quality generate
vectors that typically minimize prediction error and, consequently, the vector
s
often do not represent true object translation. Standards converters that
resample one frame rate to another (as in NTSC to PAL)
use different methods (motion vector field estimation, edge detection, et al)
that are not concerned with optimizing ratios such as SNR vs bitrate. Secondly
,
motion vectors are not transmitted for all macroblocks anyway.
70. How do you code interlaced video with MPEG-1 syntax?
A. Two methods can be applied to interlaced video that maintain syntactic
compatibility with MPEG-1 (which was originally designed for progressive frame
s
only). In the field concatenation method, the
encoder model can carefully construct predictions and prediction errors that
realize good compression but maintain field integrity (distinction between
adjacent fields of opposite parity). Some pre-processing techniques can also b
e
applied to the interlaced source video that would, e.g., lessen sharp vertical
frequencies.
This technique is not efficient of course. On the other hand, if the original
source was progressive (e.g. film), then it is more trivial to convert the
interlaced source to a progressive format before encoding. (MPEG-2 would then
only offer superior performance through greater DC block precision, non-linear
mquant, intra VLC, etc.) Reconstructed frames are re-interlaced in the decoder
Display process.
The second syntactically compatible method codes fields as separate pictures.
This approach has been acknowledged not to work as well.
71. Is MPEG patented?
A. Yes and no. Many encoding methods are patented. Approximately 11 blocking
patents, that is, patents that are general enough to be unavoidable in any
implementation have been recently identified.
A patent pool is being formed within MPEG where a single royalty fee would be
split among the 31 patent-holding companies.
72. How many cable box alliances are there?
A. Many. To start with:
Scientific Atlanta (SA), Kaledia, and Motorola:
SA will build the box, Motorola the chips, and Kaleida the
O/S and user interface (using ScriptX of course).
Silicon Graphics (SGI), Scientific Atlanta, and Toshiba
For the Time Warner's Orlando trial, SGI will provide the
RISC (MIPS R4000) and software, SA will do the box again, and Toshiba will
provide the chips.
General Instruments (GI) and Microsoft:
GI will make the box and Intel will supply the special low-cost
386SL processor on which a 1MB flash EPROM executable core of Microsoft
windows and DOS will run. Microsoft will develop the user interface.
Hewlett Packard (HP):
HP will manufacture and/or design low cost, open architecture set-top decode
r
boxes (not a part of the Eon wireless deal). The CPU will explicitly not use
a
80x68 based processor.
CLI and Philips:
Compression Labs will provide the encoder technology and Philips will provid
e
the decoder techology for an ADSL system whose transport structure will be put
together by Broadband Technologies.
["These alliances subject to change at the whim of PR departments and market
forces."]
73. Will there be an MPEG video tape format?
A. Not exactly. A consortium of international companies are co-developing a
consumer digital video 6 millimeter wide, metal particle tape format. Due to
the initial high cost of MPEG encoders, a JPEG-like compression method will be
used for inexpensive encoding of typical consumer source video (broadcast PAL,
NTSC). The natural consequence of still image methods is less efficient use o
f
bandwidth: 25 Mbit/sec for the same subjective real-time playback quality
achieved at 6 Mbit/sec possible with MPEG-2. A second bit rate mode, 50
Mbit/sec, is designated for HDTV.
Pre-coded digital video from, e.g., broadcast sources will be directly recorde
d
to tape and "passed-through" as a coded bitstream to the video decompression b
ox
upon tape playback. Assuming if linear tape speed is
to be proportional to bit rate, the recording time of a pre-compressed MPEG-2
program at the upper limit of 5 Mbit/sec for broadcast quality video, the
recording time would be over 20 hours. Channel coding schemes (error
correction, convolution coding, etc.), however, will most likely be optimized
for the tape medium and therefore may differ from the channel methods for cabl
e,
terrestrial, and satellite. (A Zenith-Goldstar S-VHS based experiment did,
however, directly record the 4-VSB broadcast baseband signal of the old
Zenith/AT&T HDTV proposal).More specs: (Summarized from EE Times July 5, 1993
article)
tape width: 6.35 mm
Audio: two channel 48 KHz 16-bit audio, or 4 channel at 32 KHz at 12-bit
Tape format: metal evaporated tape, 13.5 microns thick
Cassette dimensions: (millimeters) Recording times:
Size Width Height Depth 525/625 (25Mb/sec) HDTV (50 Mb/s)
-------- ----- ------ ----- ------------------ --------------
Standard 125 78 14.6 4h30min 2h15min
Small 66 48 12.2 1 hour 30min
Linear tape speeds: 18.812 mm/s (60Hz), 18.831 mm/s (50 Hz)
Video compression: DCT based
Participants: Matsushita, Sony, Philips, Thomson, Hitachi, Mitsubishi,
Sanyo, Sharp, Toshiba, JVC.
MPEG in everyday life
74. Where will be see MPEG in everyday life?
A. Just about wherever you see video today.
DBS (Direct Broadcast Satellite)
The Hughes/USSB DBS service will use MPEG-2 video and audio. Thomson has
exclusive rights to manufacture the decoding boxes for the first 18 months of
operation. Hughes/USSB DBS will begin its U.S. service in April 1994. Two
satellites at 101 degrees West will share the power requirements of 120 Watts
per 27 MHz transponder over a total of 32 transponders. Multi source channel
rate control methods will be employed to optimally allocate bits between sever
al
programs normalized to one 22 Mbit/sec data carrier. Bit allocation adapts to
instantaneous co-channel spatial and co-channel temporal activity. An average
of
150 channels are planned with the addition of a second set of satellites
augmenting the power level of each transponder to 240 Watts. The coded
throughput of each transponder will increase to 30 Mbit/sec.
CATV (Cable Television)
Despite conflicting options, the cable industry has more or less settled on
MPEG-2 video. Audio is less than settled. For example, General Instruments (t
he
largest U.S. consumer cable set-top box manufacturer) have announced the plann
ed
exclusive use of Dolby AC-3. The General Instruments DigiCipher I video syntax
is similar to MPEG-2 syntax, but employs smaller macroblock predictions and n
o
B-frames. The DigiCipher II specification will include modes to support both
the GI and full MPEG-2 Video Main Profile syntax. Digicipher-I services such
as
HBO will upgrade to DigiCipher II in 1994.
HDTV
The U.S. Grand Alliance, a consortium of companies that formerly competed to w
in
the U.S. terrestrial HDTV standard, have already agreed to use the MPEG-2 Vid
eo
and Systems syntax---including B-pictures. Both interlaced(1920 x 1080 x 30 Hz
)
and progressive (1280 x 720 x 60 Hz) modes will be supported. The Alliance has
also settled upon a modulation method (VSB) convolution coding (Viterbi), and
error correction (Reed-Soloman) specification.
In September 1993, the consortium of 85 European companies signed an agreement
to fund a project known Digital Video Broadcasting (DVB) which will develop a
standard for cable and terrestrial transmission by the
end of 1994. The scheme will use MPEG-2. This consortium has put the final na
il
in the coffin of the D-MAC scheme for gradual migration towards an all-digital
,
HDTV consumer transmission standard. The only remaining analog or digital-anal
og
hybrid system left in the world is NHK's MUSE (which will probably be axed in
a
few years as soon as it appears to be politically secure thing to do).
75. What is the best compression ratio for MPEG ?
A. The MPEG sweet spot is about 1.2 bits/pel Intra and .35 bits/pel inter.
Experimentation has shown that intra frame coding with the familiar
DCT-Quantization-Entropy hybrid algorithm achieves optimal performance at abou
t
an average of 1.2 bits/sample or about 6:1 compression ratio. Below this point
,
artifacts become noticeable.
76. Is there a MPEG CD-ROM format?
A. Yes, a consortium of international companies (Matsushita, Philips, Sony, JV
C,
et al) have agreed upon a specification for MPEG video and audio. 2 hour long
movies are stored on two 650 MByte compact discs. The video rate is 1.15
Mbit/sec, the audio rate is either 128 kbit/sec or 192 kbit/sec Layer I or Lay
er
II.(this seems to contradict the Philips 224 kbit/s audio spec?). Although the
Video, Systems, and Audio syntax are identical, the CD-I movie format and the
White Book format are not compatible.
Researchers are busy experimenting with denser and faster rate CD formats,
perhaps using green or blue laser wavelengths. One demonstration stretched th
e
pit and track density to its limits, improving areal density by
almost 2 fold.
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