The MPEG standard
http://bmrc.berkeley.edu/frame/research/mpeg/toc.html
This is a collection of frequently asked question about the MPEG compression standard.
It is organized as an hypertext in HTML format to be easily extensible and up-gradable. Many
sources contributed to this list. If you wish to contribute, correct any mistake or just send your
comments and impressions please contact: Luigi.Filippini@crs4.it
What is MPEG?
MPEG (Moving Pictures Experts Group) is a group of people that meet under ISO (the
International Standards Organization) to generate standards for digital video (sequences of
images in time) and audio compression. In particular, they define a compressed bit stream,
which implicitly defines a decompressor. However, the compression algorithms are up to the
individual manufacturers, and that is where proprietary advantage is obtained within the scope
of a publicly available international standard. MPEG meets roughly four times a year for
roughly a week each time. In between those meetings, a great deal of works are done by the
members, so it doesn't all happen at the meetings. The work is organized and planned at the
meetings. MPEG itself is a nickname. The official name is: ISO/IEC JTC1 SC29 WG11.
ISO: International Organization for Standardization IEC: International Electro-technical
Commission
JTC1: Joint Technical Committee 1
SC29: Sub-committee 29
WG11: Work Group 11 (moving pictures with... uh, audio)
Does it have anything to do with JPEG?
Well, it sounds the same, and they are part of the same subcommittee of ISO along with
JBIG and MHEG, and they usually meet at the same place at the same time. However, they
are different sets of people with few or no common individual members, and they have
different charters and requirements.
JPEG is for still image compression. JBIG is for binary image compression (like faxes),
and MHEG is for multi-media data standards (like integrating stills, video, audio, text, etc.).
The most fundamental difference between MPEG and JPEG is MPEG's use of
block-based motion compensated prediction (MCP), a general method falling into the
temporal DPCM, category.
The second most fundamental difference is in the target application. JPEG adopts a
general purpose philosophy: independence from color space (up to 255 components per frame)
and quantization tables for each component. Extended modes in JPEG include two sample
precisions (8 and 12 bit sample accuracy), combinations of frequency progressive, spatially
progressive, and amplitude progressive scanning modes. Color independence is made possible
thanks to down-loadable Huffman tables.
Since MPEG is targeted for a set of specific applications, there is only one color space
(4:2:0 Y Cb Cr), one sample precision (8 bits), and one scanning mode (sequential).
Luminance and chrominance share quantization tables. The range of sampling dimensions are
more limited as well. MPEG 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. Adaptive quantization is part of the JPEG-2
charter. MPEG variable length coding tables are non-downloadable, and 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 orthonormal 8x8 DCT. The resulting 63
AC transform coefficients are mapped in a zigzag pattern to statistically increase the runs of
zeros. Coefficients of 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 DPCM, of the block
DC coefficients, followed by quantization and variable length entropy coding.
MCP DCT ZZ Q Frame -> 8x8 spatial block -> 8x8 frequency block -> Zigzag scan ->
RLC VLC quantization -> run-length coding -> variable length coding.
The similarities have made it possible for the development of hard-wired silicon that can
code both standards. Even microcoded architectures can better optimize through hardwired
instruction primitives or functional blocks. There are many additional minor differences. They
include:
1. DCT and quantization precision in MPEG is 9-bits since the macroblock difference
operation expands the 8-bit signal precision by one bit.
2. Quantization in MPEG-1 forces quantized coefficients to become odd values
(oddification).
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 step size of 8), whereas
JPEG DC precision can occupy all possible 11-bits. MPEG-2, however, re-introduced
extra DC precision.
How do MPEG and H.261 differ?
H.261 was targeted for teleconferencing applications where motion is naturally more
limited. Motion vectors are restricted to a range of +/- 15 pixels. Accuracy is reduced since
H.261 motion vectors are restricted to integer-pel accuracy. Other syntactic differences
include: no B-pictures, different quantization method.
H.261 is also known as P*64. "P" is an integer number meant to represent multiples of
64kbit/sec. In the end, this nomenclature probably won't be used as many services other than
video will adopt the philosophy of arbitrary B channel (64kbit) bitrate scalability.
Is H.261 the de facto teleconferencing standard?
Not exactly. To date, about seventy percent of the industrial teleconferencing hardware
market is controlled by PictureTel of Mass. The second 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. Long
distance ISDN ain't cheap. In each direction, video and audio are coded at an aggregate 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.
Currently, ITU-TS (International Telecommunications Union--Teleconferencing Sector),
formerly CCITT, is quietly defining an improvement to H.261 with the participation of
industry vendors.
What is the reasoning behind MPEG syntax symbols?
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 coded
bitstreams for commencement of various layers of video without having to actually parse
or decode. 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. Inter coded blocks (prediction error or DFD)
naturally do not since the prediction error is the first derivative of the video signal. With
coded block patterns signalling all possible non-coded block patterns, 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 few 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-bit 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 normalized
complexity measures and buffer management performed on a a priori (pre-frame, pre-slice,
and pre-macroblock) basis 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 algorithm. Anyway,
macroblock stuffing is now illegal in MPEG-2.
 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 bit rates (sweet spots). With the
acception 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.
How would you explain MPEG to the data compression expert?
 MPEG video is a block-based video scheme
 Local decorrelations via DCT-Q-VLC hybrid
 Dead-zone quantizer
 DFD: quantized prediction error
What are the implementation requirements ?
MPEG pushes the limit of economical VLSI technology (but you get what you pay for in
terms of picture quality or compaction efficiency)
Video
Typical decoder
Profile transistor count
Dram @ speed
MPEG-1
CPB
0.4 to .75 million
4 Mbit
16 bits @ 80 ns
MPEG-1
601
0.8 to 1.1 million
16 Mbit 64 bits @ 80 ns
MPEG-2
MP@ML
0.9 to 1.5 million
16 Mbit
MPEG-2
MP@High1440
2 to 3 million
64 Mbit
64 bits @ 80 ns
N/A
70 or 80ns DRAM speed is a measure of the shortest period in which words can be
transferred across the bus. In the case of MPEG-1 SIF, 80ns implies (1/80ns)(16bits) or about
25 MBytes/sec of bandwidth. Lack of cheap memory (DRAM) utilization is where the
original DVI algorithm made a costly mistake. PAL required expensive VRAM/SRAM chips
(a static RAM transistor requires 6 transistors compared to 1 transistor for DRAM). Fast page
mode DRAM (which has slower throughput than SRAM and requires near-contiguous address
mapping) is viable for MPEG due almost exclusively to the block nature of the algorithm and
syntax (DRAM memory locations are broken into rows and columns).
How do I join MPEG ?
You don't join MPEG. You have to participate in ISO as part of a national delegation.
How you get to be part of the national delegation is up to each nation. I only know the U.S.,
where you have to attend the corresponding ANSI meetings to be able to attend the ISO
meetings. Your company or institution has to be willing to sink some bucks into travel since,
naturally, these meetings are held all over the world. (For example, Paris, Santa Clara,
Kurihama Japan, Singapore, Haifa Israel, Rio de Janeiro, London, etc.)
What is the evolution of standard documents?
In chronological order:
 The Proposal Stage
Voting members ballot on the creation of a new standards project.
 The Preparatory Stage
Project Leader manages the development of a Working Draft.
 The Committee Stag


Consensus is achieved on a Committee Draft.
The Approval Stage
National bodies vote on a Draft International Standard.
The Publication Stage
ISO publishes the International Standard.
How do I get the documents?
MPEG is a draft ISO standard. It's exact name is ISO CD 11172. The draft consists of
three parts: System, Video, and Audio. The System part (11172-1) deals with synchronization
and multiplexing of audio-visual information, while the Video (11172-2) and Audio part
(11172-3) address the video and the audio compression techniques respectively. Part 4,
Conformance Testing, is currently a CD. You may order it from your national standards body
(e.g. ANSI in the USA) or buy it from other companies like :
ISO Sales Case Postale 56 CH-1211 Geneve 20 Switzerland ANSI
Attn: Sales 11 West 42nd Street New York, NY 10036 phone 212-642-4900
Phillips Business Information
7811 Montrose Rd Potomac, MD 20854.
phone +1 301 424-3338 (800) OMNICOM
fax +1 301 309-3847 Global Engineering Documents
For inquiries within the US: 1990 M Street NW, Suite 400 Washington, DC 20036
800-854-7179 (Voice) 202-331-0960 (Fax)
For inquiries from outside the US: 2805 McGaw Avenue Irvine, CA 92714
+1-714-261-1455 Beuth Verlag Postfach
1145 W-1000 Berlin 30 Germany
What are the important themes of MPEG?
1. Application specific. MPEG does not solve everybody's application needs, but offers
a syntax that is a good solution for most. MPEG does not, for example, decorrelate
energies situated 1/256th of a pixel between a non-linear combination of 1000 frames.
The syntax was designed to occupy an optimum between cost and quality ... in other
words, between computational complexity (VLSI area, memory size and bandwidth)
and compaction (compression) efficiency.
2. The DCT and Huffman algorithms are some of the least significant aspects of the
standard, and yet somehow receive the most press coverage.
3. In the encoding algorithm, you can do what you want as long as the bitstreams
produced are compliant. There is a huge difference in picture quality between, for
example, the test model and real-world propriety implementions of encoding.
How do you tell a MPEG-1 bitstream from a MPEG-2 bitstream?
All MPEG-2 bitstreams must have certain extension headers that *immediately* follow
MPEG-1 headers. At the highest layer, for example, the MPEG-1 style sequence_header() is
followed by sequence_extension() which is exclusive to MPEG-2. Some extension headers
are specific to MPEG-2 profiles. For example, sequence_scalable_extension() is not allowed
in Main Profile.
A simple program need only scan the coded bitstream for byte-aligned start codes to
determine whether the stream is MPEG-1 or MPEG-2.
What is the precision of MPEG samples?
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-610 recommendation
chromaticy, levels 0 through 14 and 236 through 255 are reserved for blanking signal
excursions. MPEG currently has no such clipped excursion restrictions.
What is the best compression ratio for MPEG ?
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 about an average of 1.2 bits/sample or about 6:1
compression ratio. Below this point, artifacts become noticeable.
What about MPEG artifacts?
If the encoder did its job properly, and the user specified a proper balance 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-filtering)
 Blockiness (not considering your neighbors before quantizing)
 Posterization (too few DC bits)
 Checkerboards (DCT eigenimages as a result of too few AC coefficients)
 Color bleeding (not considering color in encoder cost model)
Are there single chip MPEG encoder?
Yes, the C-Cube CL-4000 is the only single-chip, real-time encoder that can process true
MPEG-1 SIF rate video.
Single chip for +/- 15 pel motion estimation at SIF rates (352x240x30 Hz) Two chips for
+/- 32 pel at SIF rates (hierarchical) 5 or 6 chips for MPEG-2 at CCIR-610 rates (704 x 480 x
30 Hz) Highly microcoded architecture. Can code both H.261 and JPEG. Implements high
picture quality microcode programs. [more details from CICC'93 and HotChips '93
conference to be included]
IBM and SGS-Thomson plan to introduce more hard-wired, multi-chip solutions in
1994.
What about MPEG-1 decoder chips?
By implication of MPEG-2 Conformance requirements, all MPEG-2 decoders are
required to decode MPEG-1 bitstreams as well. These chips, however, are strictly MPEG-1:
C-Cube
CL-450 SIF rates. Single-chip. Has on-board CPU.
SGS-Thomson 3400 SIF rates. Single-chip. Hardwired.
Motorola MCD250 SIF rates. Single-chip.
LSI 641172
CCIR-601 rates. Single-chip. Systems packet decoder on-chip.
What about audio chips?
To date, only Layer I and Layer II have been implemented in dedicated (ASIC) silicon:
Motorola MCD260 Texas Instruments TI 320AV110 hardwired with systems parsing)
operates in free format (arbitrary sample rate) 120 pin PQFP package Serial data port
Part of technology exchange with C-Cube
LSI Logic L64111
hardwired w/CPU with on-chip systems parsing. Serial data port 100-pin PQFP
GCA/ASCII ?
Crystal Semiconductor CS4920 on-chip, 2 channel 16-bit digital-to-analog converter (DAC)
16 MIPS, 24-bit DSP programmable clock manager 44-pin PLCC package
Programmable architecture. For example, can download Layer II MPEG-1 audio or
Dolby AC-2 $38 each in large quantities Dolby AC-3 MPEG NY disclosure claimed
to be less computationally intensive Zoran, GI working on own DSP-like dedicated
chips.
Will there be an MPEG video tape format?
There is a consortium of companies (Philips, JVC, Sony, Matsushita, et al) developing a
metal particle based 6 millimeter consumer digital video tape format. It will initially use more
JPEG-like independent frame compression for cheap encoding of source analog (NTSC, PAL)
video. The consequence of course is less efficient use of bandwidth (25 Mbit/sec for the same
quality achieved at 6 Mbit/sec with MPEG). Pre-compressed video from broadcast sources
will be directly recorded to tape and "passed-through" as a coded bitstream to the video
decompression "box" upon playback.
Is so-and-so really MPEG compliant?
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-frame sequence is simply a subset of the full syntax. Compliant bitstreams must obey 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 prediction must meet the specification requirements which are fairly
rigid (e.g. no more than 1 least significant bit of error between reference and test decoders).
Real-time conformance is more complicated to measure than arithmetic precision, but it is
reasonable to expect that decoders that skip frames on reasonable bitstreams are not likely to
be considered compliant.
What are some journals on related MPEG topics?
IEEE Multimedia [first edition Spring 1994]
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)
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
(to be held Feb 1994 in San Jose)
Which performances should I expect from MPEG boards?
The OptiVision, along with products from Optibase and Scientific Atlanta do real time
compression and storage to disk. The cheap video boards, at best, can only do 30 fps with
about 160 x 120 windows. Nobody can do 352 x 240 in real time without the right hardware.
The SA product is about $30K list and the Optibase somewhere around $20K for the board
set.
A board from Optivision that can do the MPEG conversion off line. Even this is costly
(about $2,000) to get it done in any decent time frame.
If you believe that $20,000 is high, AT&T at the Western Cable Show 1993 demonstrated
a real time MPEG-2 compression system at $90,000.
The market for these real time systems is very real; it is the satellite uplink and cable
television market. Nominal compression ratios are running about 200:1 for MPEG-1 in the
Optibase product. For broadcast quality, compression ratios are lower. Even here, you have to
be careful. 200:1 really means "take a 640 x 480 image, sub-sample it to 320x240 (throwing
out data to get 4:1 compression), then compress it 50:1 doing MPEG".
FrameRate Labs is about ready to release a board that does 640 x 240 real time capture
and storage to disk without any compression or dropped frames; it will compress offline. This
is brute force but far cheaper than a $20,000 solution. If you need real-time all day long, talk
to Scientific-Atlanta, Optibase or OptiVision. If you need real-time for a brief-time with
dropped frames, use the low-end boards like Video Spigot, etc. If you need real-time for a
brief-time without loss of data, Frame Rate Labs might have a solution.
The low end board manufacturers label their products real-time 30 FPS and then, in the
next sentence, they claim to be able to capture an image 640 x 480. But, they never say these
things in the same sentence.
Are there any MPEG FTP or WWW sites?
There are now many anonymous FTP site with MPEG programs or movies. A site
archiving most of the public domain programs and documents about the MPEG standard (and
also other compression techniques) may be found at ftp.crs4.it
MPEG-1 VIDEO
How does MPEG-1 VIDEO work ?
First off, it starts with a relatively low resolution video sequence (possibly decimated
from the original) of about 352 by 240 frames by 30 frames/s (US--different numbers for
Europe), but original high (CD) quality audio. The images are in color, but converted to YUV
space, and the two chrominance channels (U and V) are decimated further to 176 by 120
pixels. It turns out that you can get away with a lot less resolution in those channels and not
notice it, at least in "natural" (not computer generated) images.
The basic scheme is to predict motion from frame to frame in the temporal direction, and
then to use DCT's (discrete cosine transforms) to organize the redundancy in the spatial
directions. The DCT's are done on 8x8 blocks, and the motion prediction is done in the
luminance (Y) channel on 16x16 blocks. In other words, given the 16x16 block in the current
frame that you are trying to code, you look for a close match to that block in a previous or
future frame (there are backward prediction modes where later frames are sent first to allow
interpolating between frames). The DCT coefficients (of either the actual data, or the
difference between this block and the close match) are quantized, which means that you
divide them by some value to drop bits off the bottom end. Hopefully, many of the
coefficients will then end up being zero. The quantization can change for every "macroblock"
(a macroblock is 16x16 of Y and the corresponding 8x8's in both U and V). The results of all
of this, which include the DCT coefficients, the motion vectors, and the quantization
parameters (and other stuff) is Huffman coded using fixed tables. The DCT coefficients have
a special Huffman table that is two-dimensional in that one code specifies a run-length of
zeros and the non-zero value that ended the run. Also, the motion vectors and the DC DCT
components are DPCM, (subtracted from the last one) coded.
There are three types of coded frames. There are I or intra frames. They are simply a
frame coded as a still image, not using any past history. You have to start somewhere. Then
there are P or predicted frames. They are predicted from the most recently reconstructed I or P
frame. (I'm describing this from the point of view of the decompressor.) Each macroblock in a
P frame can either come with a vector and difference DCT coefficients for a close match in
the last I or P, or it can just be "intra" coded (like in the I frames) if there was no good match.
Lastly, there are B or bidirectional frames. They are predicted from the closest two I or P
frames, one in the past and one in the future. You search for matching blocks in those frames,
and try three different things to see which works best. (Now I have the point of view of the
compressor, just to confuse you.) You try using the forward vector, the backward vector, and
you try averaging the two blocks from the future and past frames, and subtracting that from
the block being coded. If none of those work well, you can intra- code the block.
The sequence of decoded frames usually goes like:
IBBPBBPBBPBBIBBPBBPB...
Where there are 12 frames from I to I (for US and Japan anyway.) This is based on a random
access requirement that you need a starting point at least once every 0.4 seconds or so. The
ratio of P's to B's is based on experience. Of course, for the decoder to work, you have to send
that first P *before* the first two B's, so the compressed data stream ends up looking like:
0xx312645...
where those are frame numbers. xx might be nothing (if this is the true starting point), or
it might be the B's of frames -2 and -1 if we're in the middle of the stream somewhere.
You have to decode the I, then decode the P, keep both of those in memory, and then decode
the two B's. You probably display the I while you're decoding the P, and display the B's as
you're decoding them, and then display the P as you're decoding the next P, and so on.
What do B-frames buy you?
Since bi-directional macroblock predictions are an average of two macroblocks blocks,
noise is reduced at low bit rates. 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 useful
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.
Why do some people hate B-frames ?
Computational complexity, bandwidth, delay, and picture buffer size are the four
B-frame Pet Peeves. Computational complexity is increased since a some macroblock modes
require averaging between two macroblocks. Worst case, memory bandwidth is increased an
extra 16 MByte/s (601 rate) 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. General Instruments) have been particularly adverse to B-frames
since the extra picture buffer pushes the decoder DRAM memory requirements past the magic
8-Mbit (1 Mbyte) threshold into the realm of 16 Mbits (2 MByte) for CCIR-610 frames (704
x 480), yet not for lowly 352 x 480. However, cable does not realize that DRAM does not
come in convenient high-volume (low cost) 8-Mbit packages as 16-Mbit does. In a few years,
the cost differences between 16 Mbit and 8 Mbit will become insignificant compared to the
gain in compression. For the time being, cable boxes will start with 8-Mbit and allow future
drop-in upgrades to 16-Mbit. The early market success of B-frames seem to have been
determined by a fire at a Japanese chemical plant.
Can motion vectors be used to measure object velocity?
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 form vectors that typically minimize prediction error and,
consequentially, the vectors often do not represent true object translation. Standards
converters that re-sample one frame rate to another (as in NTSC to PAL) use different
methods (field coding, edge detection, et al) that are not concerned with optimizing SNR vs
bitrate. Secondly, motion vectors are not transmitted for all macroblocks anyway.
How do you code interlaced video with MPEG-1 syntax?
Two methods can be applied to interlaced video that maintain syntactic compatibility
with MPEG-1 (which was originally designed for progressive frames 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 be 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 separately. Picture types are
keyed to motion activity to aid efficiency of prediction.
Where did they get 352x240?
That derives from the CCIR-610 digital television standard which is used by professional
digital video equipment. It is (in the US) 720 by 243 by 60 fields (not frames) per second,
where the fields are interlaced when displayed. (It is important to note though that fields are
actually acquired and displayed a 60th of a second apart.) The chrominance channels are 360
by 243 by 60 fields a second, again interlaced. This degree of chrominance decimation (2:1 in
the horizontal direction) is called 4:2:2. The source input format for MPEG I, called SIF, is
CCIR-610 decimated by 2:1 in the horizontal direction, 2:1 in the time direction, and an
additional 2:1 in the chrominance vertical direction. And some lines are cut off to make sure
things divide by 8 or 16 where needed. For 50 Hz display standards (PAL, SECAM) change
the number of lines in a field from 243 or 240 to 288, and change the display rate to 50
fields/s or 25 frames/s. Similarly, change the 120 lines in the decimated chrominance channels
to 144 lines. Since 288*50 is exactly equal to 240*60, the two formats have the same source
data rate.
Can MPEG-1 encode higher sample rates than 352x240x30?
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).
What are Constrained Parameters Bitstreams?
CPB are a limited set of sampling and bitrate parameters designed to normalize
computational complexity, buffer size, and memory bandwidth while still addressing the
widest possible range of applications. CPB limits video to 396 macroblocks (101,376 pixels)
per frame if the frame rate is less than or equal to 25 fps (frames per second), and 330
macroblocks (84,480 pixels) per frame if the frame rate is less or equal to 30 fps. Therefore,
MPEG video is typically coded at SIF dimensions (352 x 240 x 30fps or 352 x 288 x 25 fps).
The total maximum sampling rate is 3.8 Ms/s (million samples/sec) including chroma.
The coded video rate is limited to 1.862 Mbit/sec. In industrial practice, the bitrate is the most
often waived parameter of CPB, with rates as high as 6 Mbit/sec in use.
Why are Constrained Parameters Bitstreams so important?
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. MPEG devices which are not capable of meeting SIF rates are not
canonically considered to be true MPEG.
Are there ways of getting around Constrained Parameters Bitstreams for SIF class
applications and decoder?
Yes, some. Remember that CPB limits frames to 396 macroblocks (as in 352 x 288 SIF
frames). 416 x 240 x 24 Hz sampling rates are still within the constraints, but this only aids
NTSC (240 lines/field) displays. Deviating from 352 samples/line could throw off many
decoder implementations that have limited horizontal sample rate conversion modes. Due to
chip die size constraints (most chips barely pack in the necessary features), many decoders
use simple doubling, e.g. 352 to 704 samples/line via binary taps which are simple
shift-and-add operations. Future MPEG decoders will have arbitrary sample rate converters
on-chip. Also remember that the 1.86 Mbit/sec limit is often ignored in real life.
How much does it compress?
As mentioned before, audio CD data rates are about 1.5 Mbits/s. You can compress the
same stereo program down to 256 Kbits/s with no loss in discernible quality. (So they say. For
the most part it's true, but every once in a while a weird thing might happen that you'll notice.
However the effect is very small, and it takes a listener trained to notice these particular types
of effects.) That's about 6:1 compression. So, a CD MPEG I stream would have about 1.25
MBits/s left for video. The number I usually see though is 1.15 MBits/s (maybe you need the
rest for the system data stream). You can then calculate the video compression ratio from the
numbers here to be about 26:1. If you step back and think about that, it's little short of a
miracle. Of course, it's lossy compression, but it can be pretty hard sometimes to see the loss,
if you're comparing the SIF original to the SIF decompressed. There is, however, a very
noticeable loss if you're coming from CCIR-610 and have to decimate to SIF, but that's
another matter. I'm not counting that in the 26:1.
The standard also provides for other bit rates ranging from 32Kbits/s for a single channel,
up to 448 Kbits/s for stereo.
MPEG-1 AUDIO
Is the same video compression applied to audio?
Definitely no. The eye and the ear, even if they are only a few centimeters apart, works
very differently. The ear has a much higher dynamic range and resolution. It can pick out
more details but it is slower than the eye.
The MPEG committee chose to recommend 3 compression methods and named them
Audio Level I, II and III. Level I is the simplest, a sub-band coder with a psycoacustic model
(You'll get the details of this stuff further on). Layer II adds more advanced bit allocation
techniques and greater accuracy. Layer III adds a hybrid filterbank and non- uniform
quantization. Layer I, II and III gives increasing quality/compression ratios with increasing
complexity and demands on processing power.
The reason for recommending 3 methods where partly that the testers felt that none of
the coders was 100% transparent to all material and partly that the best coder (Layer III) was
so computing heavy that it would seriously impact the acceptance of the standard.
The specs say that a valid Layer III decoder shall be able to decode any Layer I, II or III
MPEG Audio stream. A Layer II decoder shall be able to decode Layer I and Layer II streams.
I would not worry too much about Layer III. Layer II is where its happening and the info in
this FAQ is mainly about this coder.
How does MPEG-1 AUDIO work?
Well, first you need to know how sound is stored in a computer. Sound is pressure
differences in air. When picked up by a microphone and fed through an amplifier this
becomes voltage levels. The voltage is sampled by the computer at a number of times per
second. For CD-audio quality you need to sample 44100 times per second and each sample
has a resolution of 16 bits. In stereo this gives you 1,4Mbit per second and you can probably
see the need for compression.
To compress audio MPEG tries to remove the irrelevant parts of the signal and the
redundant parts of the signal. Parts of the sound that we do not hear can be thrown away. To
do this MPEG Audio uses psyco- acustic principles.
How good is MPEG-1 AUDIO compression?
MPEG can compress to a bitstream of 32kbit/s to 384kbit/s (Layer II). A raw PCM audio
bitstream is about 705kbit/s so this gives a max. compression ratio of about 22. Normal
compression ratio is more like 1:6 or 1:7. If you think that this is not much please remember
that unlike video we are talking about no perceivable quality loss here. 96kbit/s is considered
transparent for most practical purposes. This means that you will not notice any difference
between the original and the compressed signal for rock'n roll or popular music. For more
demanding stuff like piano concerts and such you will need to go up to 128kbit/s.
How does MPEG-1 AUDIO achieve this compression ratio?
Well, with audio you basically have two alternatives. Either you sample less often or you
sample with less resolution (less than 16 bit per sample). If you want quality you can't do
much with the sample frequency. Humans can hear sounds with frequencies from about 20Hz
to 20kHz. According to the Nyquist theorem you must sample at least two times the highest
frequency you want to reproduce. Allowing for imperfect filters, a 44.1 kHz sampling rate is a
fair minimum. So you either set out to prove the Nyquist theorem is wrong or go to work on
reducing the resolution. The MPEG committee chose the latter.
Now, the real reason for using 16 bits is to get a good signal-to- noise (s/n) ratio. The
noise we're talking about here is quantization noise from the digitizing process. For each bit
you add, you get 6dB better s/n. (To the ear, 6dBu corresponds to a doubling of the sound
level.) CD-audio achieves about 90dB s/n. This matches the dynamic range of the ear fairly
well. That is, you will not hear any noise coming from the system itself (well, there is still
some people arguing about that, but lets not worry about them for the moment). So what
happens when you sample to 8 bit resolution? You get a very noticeable noise floor in your
recording. You can easily hear this in silent moments in the music or between words or
sentences if your recording is a human voice. Wait a minute. You don't notice any noise in
loud passages, right? This is the masking effect and is the key to MPEG Audio coding. Stuff
like the masking effect belongs to a science called psyco- acoustics that deals with the way
the human brain perceives sound. And MPEG uses psycoacustic principles when it does its
thing.
Explain the masking effect
Say you have a strong tone with a frequency of 1000Hz. You also have a tone nearby of
say 1100Hz. This second tone is 18 dB lower. You are not going to hear this second tone. It is
completely masked by the first 1000Hz tone. As a matter of fact, any relatively weak sounds
near a strong sound is masked. If you introduce another tone at 2000Hz also 18 dB below the
first 1000Hz tone, you will hear this. You will have to turn down the 2000Hz tone to
something like 45 dB below the 1000Hz tone before it will be masked by the first tone. So the
further you get from a sound the less masking effect it has. The masking effect means that you
can raise the noise floor around a strong sound because the noise will be masked anyway. And
raising the noise floor is the same as using less bits and using less bits is the same as
compression.
Let's now try to explain how the MPEG Audio coder goes about its thing. It divides the
frequency spectrum (20Hz to 20kHz) into 32 sub-bands. Each sub-band holds a little slice of
the audio spectrum. Say, in the upper region of sub-band 8, a 1000Hz tone with a level of
60dB is present. OK, the coder calculates the masking effect of this sound and finds that there
is a masking threshold for the entire 8th sub-band (all sounds w. a frequency...) 35dB below
this tone. The acceptable s/n ratio is thus 60 - 35 = 25 dB. The equals 4 bit resolution. In
addition there are masking effects on band 9-13 and on band 5-7, the effect decreasing with
the distance from band 8. I a real-life situation you have sounds in most bands and the
masking effects are additive. In addition the coder considers the sensitivity of the ear for
various frequencies. The ear is a lot less sensitive in the high and low frequencies. Peak
sensitivity is around 2-4kHz, the same region that the human voice occupies.
The sub-bands should match the ear, that is each sub-band should consist of frequencies
that have the same psycoacoustic properties. In MPEG layer II, each subband is 625Hz wide.
It would be better if the sub-bands where narrower in the low frequency range and wider in
the high frequency range. To do this you need complex filters. To keep the filters simple they
chose to add FFT in parallel with the filtering and use the spectral components from the FFT
as additional information to the coder. This way you get higher resolution in the low
frequencies where the ear is more sensitive.
But there is more to it. We have explained concurrent masking, but the masking effect
also occurs before and after a strong sound (pre- and postmasking)
If there is a significant (30 - 40dB) shift in level. The reason is believed to be that the
brain needs some processing time. Premasking is only about 2 to 5 ms. The postmasking can
be up till 100ms. Other bit-reduction techniques involve considering tonal and non- tonal
components of the sound. For a stereo signal you have a lot of redundancy between channels.
The last step before formatting is Huffman coding.
The coder calculates masking effects by an iterative process until it runs out of time. It is
up to the implementor to spend bits in the least obtrusive fashion. For layer II the coder works
on 23 ms of sound (1152 samples) at a time. For some material the 23 ms time-window can be
a problem. This is normally in a situation with transients where there are large differences in
sound level over the 23 ms. The masking is calculated on the strongest sound and the weak
parts will drown in quantization noise. This is perceived as a noise-echo by the ear. Layer III
addresses this problem specifically.
What is the hardware demand?
According to my information Layer III needs about 20MIPS per channel for real-time
coding. This means a real fast DSP. Layer II on the other hand needs only a simple DSP like
for example the AD2015 that can be had for a few dollars. The process is asymmetrical, much
more processing is needed on the coding side. A decoder could be made to work without
hardware assistance on a decent computer.
Who is using MPEG-1 AUDIO?
Philips uses MPEG for their new digital video CD's. They say they will start shipping
movies and music videos on CD's for their CD-I player by the end of this year. MPEG is
accepted by Eureka-147. That means that when digital radio broadcasts starts in Europe a
couple of years from now, you will receive MPEG coded audio.
Which sampling frequencies are used?
You can have 48kHz, (used in professional sound equipment), 44,1kHz (used in
consumer equipment like CD-audio) or 32kHz (used in some communications equipment).
How many audio channels?
MPEG I allows for two audio channels. These can be either single (mono) dual (two
mono channels), stereo or joint stereo (intensity stereo or m/s-stereo). In normal (l/r) stereo
one channel carries the left audio signal and one channel carries the right audio signal. In m/s
stereo one channel carries the sum signal (l+r) and the other the difference (l-r) signal. In
intensity stereo the high frequency part of the signal (above 2kHz) is combined. The stereo
image is preserved but only the temporal envelope is transmitted. In addition MPEG allows
for pre-emphasis, copyright marks and original/copy marks. MPEG II allows for several
channels in the same stream.
Where can I get more details about MPEG audio?
There is no description of the coder in the specs. The specs describe in great detail the
bitstream and suggests psycoacustic models.
MPEG-1 SYSTEMS
What about MPEG-1 SYSTEMS?
The MPEG system committee completed and approved for release the technical
specification for combining a plurality of coded audio and video streams into a single data
stream. The specification provides a fully synchronized audio and video and facilitates the
storage in and the possible further transmission of the combined information through a variety
of digital media.
This systems coding includes necessary and sufficient information in the bit stream to
provide the system-level functions of synchronization of decoded audio and video, initial and
continuous management of coded data buffers to prevent overflow and underflow, random
access start-up, and absolute time identification. The coding layer specifies a multiplex data
format that allows multiplexing of multiple simultaneous audio and video streams as well as
privately defined data streams.
The basic principle of MPEG System coding is the use of time stamps which specify the
decoding and display time of audio and video and the time of reception of the multiplexed
coded data at the decoder, all in terms of a single 90kHz system clock. This method allows a
great deal of flexibility in such areas as decoder design, the number of streams, multiplex
packet lengths, video picture rates, audio sample rates, coded data rates, digital storage
medium or network performance. It also provides flexibility in selecting which entity is the
master time base, while guaranteeing that synchronization and buffer management are
maintained. Variable data rate operation is supported. A reference model of a decoder system
is specified which provides limits for the ranges of parameters available to encoders and
provides requirements for decoders.
Some optional sets of constraints provide a framework for common industry acceptance
of certain key parameters for use by decoder designs and information providers. While the
MPEG Systems specification is included in the current work item of MPEG, it is designed for
compatibility with future extensions to audio, video and hypermedia coding, and a wide
variety of bitrates.
MPEG-2 FAQ
What is MPEG-2?
At a meeting hosted in New York by Columbia University, the Moving Picture Experts
Group (MPEG) completed definition of MPEG-2 Video, MPEG-2 Audio, and MPEG-2
Systems. MPEG therefore confirmed that it is on schedule to produce, by November 1993,
Committee Drafts of all three parts of the MPEG-2 Standard, for balloting by its member
countries.
To ensure that a harmonized solution to the widest range of applications is achieved,
MPEG, an ISO/IEC working group designated ISO/IEC JTC1/SC29/WG11, is working jointly
with the ITU-TS Study Group 15 Experts Group for ATM Video Coding. MPEG also
collaborates with representatives from other parts of ITU-TS, and from EBU, ITU-RS,
SMPTE, and the North American HDTV community.
Why MPEG-2?
MPEG-1 was optimized for CD-ROM or applications at about 1.5 Mbit/sec. Video was
strictly non-interlaced (i.e. progressive). The international co-operation had executed so well
for MPEG-1, that the committee began to address applications at broadcast TV sample rates
using the CCIR-610 recommendation (720 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 between MPEG-1 and
MPEG-2 improved the method of block-based transform coding.
What are the typical MPEG-2 bitrates and picture quality?
Here are some examples of typical frame sizes in bits :
Picture type
I
P
B
Average
MPEG-1 SIF @ 1.15 Mbit/sec
150,000
50,000
20,000
38,000
MPEG-2 601 @ 4.00 Mbit/sec
400,000
200,000 80,000
130,000
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 with scene changes and more advanced encoder models found in any
real-world implementation, these numbers can be very different.
When will an MPEG-2 decoder chip be available?
Several chips will be sampling in late 1993. For reasons of economy and scale in the
cable TV application, all are single-chip (not including DRAM and host CPU/controller)
implementations. They are:
SGS-Thomson STi-3500 first MPEG-2 chip on market multi-tap binary horizontal
sample rate converter. pan & scanning support for 16:9 requires external, dedicated
microcontroller (8 bit) 8-bit data bus, no serial data bus.
LSI Logic L64112 successor (pin compatible) serial bus, 15 Mbit coded throughput.
smaller pin-count version due soon.
C-Cube CL-950 successor (?)
In 1994, we can look forward to:
Pioneer single-chip MPEG-2 successor to CD-1100 MPEG-1 chip set.
IBM single-chip decoder.
Where will we see MPEG in everyday life?
Just about wherever you see video today.
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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. No doubt Thomson's STi-3500 MPEG-2 video
decoder chip will be featured.
Hughes/USSB DBS will begin service in North America in April 1994. Two
satellites at 101 degrees West will share the power requirements of 120 Watts per 27
MHz transponder. Multi-source channel rate control methods will be employed to
optimally allocate bits between several programs on one data carrier. An average of
150 channels are planned.
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 (the largest U.S. consumer cable set-top box manufacturer) have
announced the planned use of the Dolby AC-3 audio algorithm.
The General Instruments DigiCipher I video syntax is similar to MPEG-2 syntax
but uses smaller macroblock predictions and no B-frames. The DigiCipher II
specification will include modes to support both the GI and full MPEG-2 Video
Main Profile syntax. Services such as HBO will upgrade to DigiCipher II in 1994.
HDTV. The U.S. Grand Alliance, a consortium of companies that formerly
competed for the U.S. terrestrial HDTV standard, have already agreed to use the
MPEG-2 Video and Systems syntax---including B-pictures. Both interlaced (1440 x
960 x 30 Hz) and progressive (1280 x 720 x 60 Hz) modes will be supported. The
Alliance must then settle upon a modulation (QAM, VSB, OFDM), convolution
(MS or Viterbi), and error correction (RSPC, RSFC) 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 nail 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-analog hybrid system left in the world is NHK's MUSE (which will probably be axed
in a few years).
What did MPEG-2 add to MPEG-1 in terms of syntax/algorithm?
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 and display.
Source video color primaries (609, 170M, 240M, D65, etc.) and opto- electronic transfer
characteristics (709, 624-4M, 170M etc.) can be indicated.
Four scalable modes [see scalable section below]
 Picture layer:
All MPEG-2 motion vectors are half-pel accuracy.
DC precision can be user-selected as 8, 9, 10, or 11 bits.
Concealment motion vectors were added to I-pictures in order to increase robustness
from bit errors since I pictures are the most critical and sensitive in a group of pictures.
A non-linear macroblock quantization factor that results in a more dynamic step size
range, from 0.5 to 56, than in MPEG-1 (1 to 32).
New Intra-VLC table for dct_next_coefficient (AC run-level events) that is more geared
towards I-frame probability distribution. EOB is 4 bits. The old tables are still included.
Alternate scanning pattern that (supposedly) improves entropy coding performance 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 3:2 pull down process (repeat_field_first flag)
Syntax flag to signal chrominance post processing type (4:2:0 to 4:2:2 up-sampling
conversion)
Progressive and interlaced frame coding
Syntax to signal 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 image
within a wider 16:9 aspect ratio image. Vertical pan offset has 1/16th pixel accuracy.
 Macroblock layer:
Macroblock stuffing is now illegal in MPEG-2 (hurray!!)
Two line modes (interlaced and progressive) for DCT operation.
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 method in
MPEG-1. Now specifies adding or subtracting one to the 63rd AC coefficient depending on
parity of summed quantized 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: prediction
modes, Intra VLC table, DC precision, non-linear macroblock quant. Implementation
improvements, well,.. uh... macroblock stuffing was eliminated.
What are the scalable modes of MPEG-2?
Scalable video is permitted only in the Main+ and Next profiles. Currently, there are four
scalable modes in the MPEG-2 toolkit. These modes break MPEG-2 video into different
layers (base, middle, and high layers) mostly for purposes of prioritizing video data. For
example, the high priority channel (bitstream) can be coded with a combination of extra error
correction information and decreased bit error (i.e. higher Carrier-to-Noise ratio or signal
strength) than the lower priority channel.
Another purpose of scalability is complexity division. 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 "graceful" degradation. By the same division however, a standard
TV set need only decode the 720 x 480 channel, thus requiring a less expensive decoder than
a TV set wishing to display 1440 x 960. This is simulcasting.
A brief summary of the MPEG-2 video scalability modes:
 Spatial Scalability
Useful in simulcasting, and for feasible software decoding 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 for the higher layers.
 Data Partitioning
Similar to JPEG's frequency progressive mode, only the slice layer indicates the
maximum number of block transform coefficients contained in 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 frequency 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 (through
quantization step sizes). The higher priority bitstream contains base layer data that can be
added to a lower 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 rate, and the intermediate frames can be coded in a
second bitstream using the first bitstream reconstruction as prediction. In stereoscopic vision,
for example, the 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 similar
quality and greater simplicity.
What is the TM rate control and adaptive quantization technique?
Test model was not by any stretch of the imagination meant to be the show-stopping,
best set of algorithm. It was designed to exercise the syntax, verify proposals, and test the
*relative* performance of proposals in a way that could be duplicated by co-experimentors in
a timely fashion. Otherwise there would be more endless debates about model interpretation
than actual time spent in verification. [MPEG-2 Test model is frozen as v5b] The MPEG-2
Test Model (TM) rate control method offers a dramatic improvement to the Simulation Model
(SM) method used for MPEG-1. TM's improvements are due to more sophistication
pre-analysis and post-analysis routines. Rate control and adaptive quantization are divided
into three steps:
1. Bit Allocation
In Complexity Estimation, the global complexity measures assign relative weights to
each picture type. These weights (Xi, Xp, Xb) are reflected by the typical coded frame size of
I, P, and B pictures (see typical frame size section). I pictures are assigned the largest weight
since they have the greatest stability factor in an image sequence. B pictures are assigned the
smallest weight since B data does not propagate into other frames through the prediction
process.
Picture Target Setting allocates target bits for a frame based on the frame type and the
remaining number of frames of that same type in the Group of Pictures (GOP).
2. Rate Control
Rate control attempts to adjust bit allocation if there is significant difference between the
target bits (anticipated bits) and actual coded bits for a block of data.
3. Adaptive Quantization
Recomputes macroblock quantization factor according to activity of block against the
normalized activity of the frame.
The effect of this step is to roughly assign a constant number of bits per macroblock (this
results in more perceptually uniform picture quality).
What is MPEG-2 VIDEO?
MPEG is developing the MPEG-2 Video Standard, which specifies the coded bit stream
for high-quality digital video. As a compatible extension, MPEG-2 Video builds on the
completed MPEG-1 Video Standard (ISO/IEC IS 11172-2), by supporting interlaced video
formats and a number of other advanced features, including features to support HDTV.
As a generic International Standard, MPEG-2 Video is being defined in terms of
extensible profiles, each of which will support the features needed by an important class of
applications. At the March MPEG meeting in Sydney, the MPEG-2 Main Profile was defined
to support digital video transmission in the range of about 2 to 15 Mbits/sec over cable,
satellite, and other broadcast channels, as well as for Digital Storage Media (DSM) and other
communications applications. Building on this success at the New York meeting, MPEG
experts from participating countries in Asia, Australia, Europe, and North America further
defined parameters of the Main Profile and Simple Profile suitable for supporting HDTV
formats.
MPEG experts also extended the features of the Main Profile by defining a
hierarchical/scalable profile. This profile aims to support applications such as compatible
terrestrial TV/HDTV, packet-network video systems, backward-compatibility with existing
standards (MPEG-1 and H.261), and other applications for which multi-level coding is
required. For example, such a system could give the consumer the option of using either a
small portable receiver to decode standard definition TV, or a larger fixed receiver to decode
HDTV from the same broadcast signal.
The technical definition of MPEG-2 Video has been completed. This was a critical
milestone, and shows that MPEG-2 Video is on schedule for a Committee Draft in November
1993.
What are MPEG-2 VIDEO Main Profile and Main Level?
MPEG-2 Video Main Level is analogous to MPEG-1's CPB, with sampling limits at
CCIR-610 parameters (720 x 480 x 30 Hz). Profiles limit syntax (i.e. algorithms), whereas
Levels limit 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.
Level/ Max. sampling pixels/ Max. significance dimensions/ fps sec /bitrate –
Low 352 x 240 x 30
3.05 M
4 Mb/s CIF, consumer tape equiv.
Main
720 x 480 x 30
10.40 M
15 Mb/s CCIR 601, studio TV
High 1440 x 1152 x 30
47.00 M
60 Mb/s 4x 601, consumer HDTV
High 1920 x 1080 x 30
62.70 M
80 Mb/s production SMPTE 240M std
Note 1: pixel rate and luminance (Y) sample rate are equivalent.
2: Low Level is similar MPEG-1's Constrained Parameters Bitstreams.
Profile Comments ------- ----------------------------------------------------------Simple Same as Main, only without B-pictures. Intended for software applications,
perhaps CATV. Main Most decoder chips, CATV, satellite. 95% of users. Main+
Main with Spatial and SNR scalability Next Main+ with 4:2:2 macroblocks Profile
Level Simple Main Main+ Next ------------ -------------- -------------- ------------------------- High illegal illegal 4:2:2 chroma High-1440 illegal With spatial
4:2:2 chroma Scalablity Main 90% of users Main with SNR 4:2:2 chroma scalability
Low illegal Main with SNR illegal scalabiliy [Subject to change at whim of MPEG
Requirements sub-group]
At what bitrates is MPEG-2 video optimal?
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]
How does MPEG video really compare to TV, VHS, LD?
VHS picture quality can be achieved for source film video at about 1 million 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). VHS 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 lines
compared to 240)... but when taking into account interfield magnetic tape crosstalk and the
TV monitor Kell factor, not by all that much. VHS is prone to timing errors (which can be
improved with time base correctors), whereas digital video is fully discretized. Pre-recorded
VHS is typically recorded at very high duplication speeds (5 to 15 times real time playback),
which leads to further shortfalls for the format that has been with us since 1977.
Broadcast NTSC quality can be approximated at about 3 Mbit/sec, and PAL quality at
about 4 Mbit/sec. Of course, sports sequences with complex spatial-temporal activity need
more like 5 and 6 Mbit/sec, respectively.
Laserdisc is a tough one to compare. Disc is composite video (NTSC or PAL) with up to
425 TVL (or 567 samples/line) response. Thus it could be said laserdisc has 567 x 480 x 30
Hz "resolution". The carrier-to-noise ratio is typically better than 48 dB. Timing is excellent.
Yet some of the clean characteristics of laserdisc can be achieved at 1.15 Mbit/sec (SIF rates),
especially for those areas of medium detail (low spatial activity) in the presence of uniform
motion. This is why some people say MPEG-1 video at 1.15 Mbit/sec looks almost as good as
Laserdisc or Super VHS.
Regardless of the above figures, those clever proprietary encoding algorithms can push
these bitrates even lower.
Why film does so well with MPEG?
Several reasons, really:
 The frame rate is 24 Hz (instead of 30 Hz) which is a savings of some 20%.
 The film source video is inherently progressive. Hence no fussy interlaced spectral
frequencies.
 The pre-digital source was severely oversampled (compare 352 x 240 SIF to 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.
Finally, the spatial and temporal modulation transfer function (MTF) characteristics
(motion blur, etc) of film are more amendable to the transform and quantization
methods of MPEG.
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 variance
between two fields, and if the variance is small enough, uses an average of both fields to form
a frame macroblock. Otherwise, a field area from one field (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 compression
encoding. For example, if pre-analysis of a frame indicates that serious artifacts will arise if
the picture were to be coded in the current condition, a pre-anti-aliasing 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 locally adaptive. More
complex methods will use multirate/multidimensional tools again.
The basic idea of multidimensional/multirate pre-processing is to apply source video
whose resolution (sampling density) is greater than the target source and reconstruction
sample rates. This follows the basic principles of oversampling, as found in A/D converters.
Most detail is contained in the lower harmonics anyway. Sharp-cut off filters are not
widely practiced, so the "320 x 480 potential" of VHS is never truly realized.
Why use "advanced" pre-filtering techniques?
Think of the DCT and quantizer as an A/D converter. Think of the pre-filter as the
required anti-alias prefilter found before every A/D. The big difference of course is that the
DCT quantizer assigns a varying number of bits per sample (transform coefficient).
Judging on the normalized activity measured in the pre-analysis stage of video encoding,
and the target buffer size status, you have a fairly good idea of how many bits can be spared
for the target macroblock, for instance.
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.).
What are some advanced encoding methods?
 Quantizer feedback [Thomson patent]
 Horizontal variance
 Motion vector cost:
this is true for any syntax elements, really. Signalling 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, for example, a
Lagrangian process. In summary, any compression system with side information, there is a
optimum point between signalling overhead (e.g. prediction) and prediction error.
 Liberal Interpretations of the Forward DCT
Borrowing from the concept that the DCT is simply a filter bank, a technique that
seems to be gaining popularity is 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 is to use the basis vector shaping as a cheap alternative to pre-filtering by combining
the more desiderable 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
windowing. [Warning: watch out for eigenimage/basis vector orthogonality. ]
 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.

Temporal spreading of quantization error:
This method is similar to the original intent behind color subcarrier phase
alternation by field in the NTSC analog TV standard: for 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 spots (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: Scene changes
 Post-processing
(non-linear) Interpolation methods (Wu-Gersho) Convex hull projections Some
ICASSP '93 papers, etc.
 Conformance vs. post-processing:
Post-processing makes judging decoder output for conformance testing near impossible.
It is easy to spot encoders that do not employ any advanced encoding techniques:
reconstructed video usually contains ringing around edges, color bleeding, and lots of noise.
What is MPEG-2 AUDIO?
MPEG is developing the MPEG-2 Audio Standard for low bitrate coding of multichannel
audio. MPEG-2 Audio coding will supply up to five full bandwidth channels (left, right,
center, and two surround channels), plus an additional low frequency enhancement channel,
and/or up to seven commentary/multilingual channels. The MPEG-2 Audio Standard will also
extend the stereo and mono coding of the MPEG-1 Audio Standard (ISO/IEC IS 11172-3) to
half sampling-rates (16 kHz, 22.05 kHz, and 24 kHz), for improved quality for bitrates at or
below 64 kbits/s, per channel.
MPEG produced an updated version of the MPEG-2 Audio Working Draft, and is on
track for achieving a Committee Draft specification by the November MPEG meeting.
The MPEG-2 Audio multichannel coding Standard will provide backward-compatibility
with the existing MPEG-1 Audio Standard (ISO/IEC IS 11172-3). Together with ITU-RS,
MPEG is organizing formal subjective testing of the proposed MPEG-2 multichannel audio
codecs and up to three non-backward-compatible (NBC) codecs. The NBC codecs are
included in order to determine whether an NBC mode should be introduced as an addendum
to the standard. If the results show clear evidence that an NBC mode improves the
performance, a formal call for NBC proposals will be issued by MPEG, with a view to
incorporate these features in the audio syntax.
MPEG-2 audio attempts to maintain as much compatibility with MPEG-1 audio syntax
as possible, while adding discrete surround-sound channels to the original MPEG-1 limit of 2
channels (Left, Right or matrix center and difference). The main channels (Left, Right) in
MPEG-2 audio will remain backwards compatible, whereas new coding methods and syntax
will be used for the surround channels.
A total of 5.1 channels are included that consist of the two main channels (L, R), two
side/rear, center, and a 100 Hz special effects channel (hence the ".1" in "5.1").
At this time, non-backwards compatible (NBC) schemes are being considered as an
amendment to the MPEG-2 audio standard. One such popular system is Dolby AC-3.
What is MPEG-2 SYSTEMS?
MPEG is developing the MPEG-2 Systems Standard to specify coding formats for
multiplexing audio, video, and other data into a form suitable for transmission or storage.
There are two data stream formats defined: the Transport Stream, which can carry multiple
programs simultaneously, and which is optimized for use in applications where data loss may
be likely, and the Program stream, which is optimized for multimedia applications, for
performing systems processing in software, and for MPEG-1 compatibility.
Both streams are designed to support a large number of known and anticipated
applications, and they retain a significant amount of flexibility such as may be required for
such applications, while providing interoperability between different device implementations.
The Transport Stream is well suited for transmission of digital television and video telephony
over fiber, satellite, cable, ISDN, ATM, and other networks, and also for storage on digital
video tape and other devices. It is expected to find widespread use for such applications in the
very near future.
The Program Stream is similar to the MPEG-1 Systems standard (ISO/IEC 11172-1). It
includes extensions to support new and future applications. Both the Transport Stream and
Program Stream are built on a common Packetized Elementary Stream packet structure,
facilitating common video and audio decoder implementations and stream type conversions.
This is well-suited for use over a wide variety of networks with ATM/AAL and alternative
transports. In New York, MPEG completed definitions of the features, syntax, and semantics
of the Transport and Program Streams, enabling product designers to proceed. Among other
items, the Transport Stream packet length was fixed at 188 bytes, including the 4-byte header.
This length is suited for use with ATM networks, as well as a wide variety of other
transmission and storage systems.
What about the Grand Alliance?
The Grand Alliance was formed in May 1993 by seven organizations (AT&T, GI, MIT,
Philips, Sarnoff, Thomson, Zenith) to evaluate technologies and to decide on key elements
that will be at the heart of the best of the best HDTV system.
The video compression and transport technologies selected by the Grand Alliance are
based on the proposed MPEG-2 standards. The scanning formats selected are focused
primarily on computer-friendly progressive scanning, while offering and interlaced mode
important to some broadcasters.
They have already agreed to use the MPEG-2 Video and Systems syntax, including
B-pictures. Both interlaced (1440 x 960 x 30 Hz) and progressive (1280 x 720 x 60 Hz)
modes will be supported. The Alliance must then settle upon a modulation (QAM, VSB,
OFDM), convolution (MS or Viterbi), and error correction (RSPC, RSFC) specification.
The audio technology selected is a six-channel, compact-disc-quality digital surround
sound system. The last major technical decision, the broadcast and cable transmission
subsystem, is expected in early 1994 following testing of competing technologies.