standards video mpeg english
TRANSCRIPT
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Video coding standardsVideo coding standards -- MPEGMPEG Introduction
ISO video coding standards MPEG-x
I, P, B pictures
MPEG structure
MPEG syntax
MPEG bit allocation and rate control
Introduction to DVB
Thanks for material provided
Inald Lagendijk, Delft University of Technology
Thomas Wiegand, Heinrich-Hertz-Institut
Video coding standardsVideo coding standards -- historyhistory
1984 1986 1988 1990 1992 1994
H.261 H.263H.263+, H.26L
MPEG- 2
MPEG- 1ISO
ITU
MPEG- 4
1996 1998
Frame- based
Object- based
2000
H.264/AVC
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General video coding standardGeneral video coding standardControl
DCT Q VLC MUX Buffer
Overhead information
Q-1
IDCT
+
+
+-
PredictorMemory
Motion
estimationVLC
H.261: the basis of videoH.261: the basis of videocompressioncompression
ITU-T (ex-CCITT) Rec. H.261: The first widespread practical success
First design (late 80s) embodying typical structure that dominatestoday: 16x16 macroblock motion compensation, 8x8 DCT, scalar
quantization, and variable-length coding
Other key aspects: loop filter, integer-pel motion compensationaccuracy, 2-D VLC for coefficients
Operated at 64 - 2048 kbits/s ( n x 64 kbits/s, n = 1,,30)352 x 288 x 29.97 4:2:0
QCIF Progressive
176 x 144 x 29.97 4:2:0
Still in use, although mostly as a backward-compatibility feature overtaken by H.263
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The big improvementThe big improvement
IIPP -- B picturesB pictures
Time
I BB B P B BPB
I: Intraframe pictures (random access)
P: Prediction pictures
B: Bidireccional prediction pictures
I
Bidireccional predictionBidireccional prediction -- B picturesB pictures
B = A; Forward prediction
B = C; Backward prediction
B = (A + C)/2; Bidireccional prediction
A
B
C
Frame I or P
Frame I or P
Frame B
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I, P, B picturesI, P, B pictures
I - Picturescoded independent of any other pictureallows decoder for random accessused as reference for coding of other pictures
P - Picturescoded like in hybrid coderused as reference for coding of other pictures
B - Picturesextension of hybrid codernever used as reference for coding other pictures
GOP structuresGOP structures Typical GOP structures:
I I I I I I I I I I I I Only intra frames Much like Motion-JPEG
I P P P P P P P P P P
Pure hybrid coder Problem to start decoding process
(B B) I B B P B B P B B P Common practice Has I picture twice per second Presence/way of coding the first 2 B pictures determines open or
closed GOP
GOP structure not prescribed and can be changed on-the-fly:Scene changes, rapid transition, slow motion, freeze frame, ...
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Transmission orderTransmission order
I IB B B P BBB
1 42 3 65 987
1 42 3 65 987Temporal order:
1 35 2 94 876Transmission order:
There is a delay of B + 1 frames
ITU (H.26x)ITU (H.26x) -- ISO (MPEG) standardsISO (MPEG) standards
MPEG could be used for communicative services
Often round trip delays are important, which has not been
an issues of much importance in MPEG
ITU has standards similar to MPEG but with
less delay
less overhead in terms of selectable options
more error robustness
optimized for lower rates
H.261 (~1988) and H.263 (~1995)
but the basics are the same as MPEG
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MPEG standardizationMPEG standardization
Joint standardization effort of ISO and IECISO-IEC JTC1/SC29/WG11
Nickname: MPEG - Moving Pictures Expert Group
Standard for the compression of video and associatedaudio for digital video applications
MPEG-1 (~1992): ISO/IEC 11172
Digital storage media and PC; up to 1.5 Mbit/sec
MPEG-2 (~1994): ISO/IEC 13818
Digital TV and HDTV broadcasting; up to 15 Mbit/sec
MPEG-4 (~1998)
Multimedia/Internet standard; Compression is less important
MPEG-7 (~2001)
Multimedia meta-data standard; Not a compression standard
MPEG structureMPEG structure
MPEG standards deals with:Video: describes coded representation of digital video
Audio: similar for digital audio
System: describes multiplexing structure, means to
represent timing information, and possibilities toenclose other information streams
Conformance: describes procedures for testingcompliance with the system requirements
Simulation software
Digital storage media command and control
Multi-channel audio coding
Real-time interface
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MPEG syntax standardization (1)MPEG syntax standardization (1)
MPEG does not standardize the encoder nor the decoder implicitly the structure of the encoder and decoder is definednamely, the hybrid video coder
The MPEG standard definesthe bit stream syntaxthe way this syntax needs to be interpreted
Within the syntax, many degrees of freedom existMPEG encoders come in all kinds of qualities
MPEG syntax standardization (2)MPEG syntax standardization (2)
MPEG encoder
A
MPEG encoder
B
MPEG decoder
I
MPEG decoder
II
MPEG decoder
III
Bit stream A
Bit stream B
Encoded at same bit rate
Output video 2
Video input
Output video 1
Output video 3
identica
quality
(very)different
quality
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Standard MPEGStandard MPEG -- 1 (1993)1 (1993)
Formally ISO/IEC 11172-2 (93), developed by ISO/IEC JTC1SC29 WG11 (MPEG) use is fairly widespread, but mostly
overtaken by MPEG-2
Can provide approximately VHS quality between 1-2 Mbps
Superior quality to H.261 when operated at higher bit rates ( 1Mbps for CIF 352x288 resolution)
Other video formats and bit rates are allowed
Internet: QSIF (176 pixels per line x 144 lines per frame) Technical features: Adds bi-directional motion prediction and
half-pixel motion to H.261 design
MPEGMPEG -- 1 preprocessing1 preprocessing
Most video sources are not SIF: preprocessing needed
Spatial resolution conversion, e.g. 720x576 to352x288
Interlaced to progressive picture conversion to getrid of the fields
Color conversion, e.g. RGB to YUV 4:2:0 Noise filtering Low pass filtering (smoothing) Frame skipping
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Standard MPEGStandard MPEG -- 2 (1994)2 (1994)
Formally ISO/IEC 13818-2 & ITU-T H.262, developed (94)jointly by ISO/IEC SC29 WG11 (MPEG) and ITU-T
Now in wide use for DVD standard and DTV
(the most commonly used video coding standard)
Primary new technical features: support for interlaced-scanpictures and scalability
Essentially the same as MPEG-1 for progressive-scan
pictures, and MPEG-1 forward compatibility required Not especially useful below 2 Mbps (range of use normally
2-20 Mbps)
Standard MPEGStandard MPEG -- 2 (1994)2 (1994)
Generic coding of video for TV broadcast and HDTV
I, P, B pictures
Profiles and Levels
HDTV: 18 - 36 Mbits/s
PAL quality: 4 Mbits/s
Transparent quality: 9 Mbits/s
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MPEG-1 MPEG-2
Standard 1992 1994Main application Digital video for
CD-ROMDigital TV(HDTV)
Spatial resolution CIF (1/4 TV)apr. 288 x 360 pixels
TV(4 x TV)576 x 720 pixels(1152 x 1440 pixels)
Temporal resolution 25 - 30 frames/s 50-60 fields/s(100-120 field/s)
Bit Rate 1.5 Mbit/s 15 Mbit/s
(60 Mbit/s)Quality equivalent to VHS equivalent to
NTSC/PALCompressionover PCM
apr. 20 - 30 apr. 30-40(apr. 30-40)
MPEG 1MPEG 1 -- MPEG 2 parametersMPEG 2 parameters
MPEG progressive formatsMPEG progressive formats
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
4:2:2 progressive
X
X
X
4:2:0 progressive MPEG 1
X
X
X
X
X
X
X
X
4:2:0 progressive MPEG 2
X
X
X X X
X
X
Luminance sample Y
X Chrominance sample Cb, Cr 5061
502
..
.
+
=
+
=
YRC
YBC
r
b
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MPEG 2 interlaced formatMPEG 2 interlaced format
XX X
XX X
XX X
4:2:0 interlaced
top field
4:2:0 interlaced
bottom field
Luminance sample Y
X Chrominance sample Cb, Cr (defined in the top field)
Macroblock 4:2:0Macroblock 4:2:0
16 samples
Y0 Y1
Y3Y2
16 samples Cb
8 samples 8 samples
8 samples
Motion estimation: Macroblocks of Y
Interlaced systems: Field interpolation before motion estimation
DCT: On blocks of 8 x 8
Cr
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Macroblock 4:2:2Macroblock 4:2:2
16 samples
Y0 Y1
Y3Y2
16 samples
Cb1
8 samples 8 samples
8 samplesCr1
Cb1 Cr1 8 samples
Motion estimation: Macroblocks of Y
Interlaced systems: Field interpolation before motion estimation
DCT: On blocks of 8 x 8
Structure of layered syntaxStructure of layered syntaxVideo sequence
Group of pictures
Picture
Slice
Slice
Macroblock
Block
8 pixels
8 pixels
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Example of syntaxExample of syntax Each layer in the syntax contains
header with relevant parameters for lower layers picture size, bit rate, coding method, motion vectors, ...
unique start code for resynchronization (high layers only) hex start code: 00 00 01 xx ~ 23 zeroes+1+8 bit parameter
Pic tur e header CD #bits Mnic
picture_header(){picture_sc 32 bslbf temporal reference 10 uimsbf picture_coding_type 3 uimsbf vbv_delay 16 uimsbf if (picture_coding_type==2 ||
picture_coding_type==3){full_pel_forward_vector 1forward_f_code 3 uimsbf
}
if (picture_coding_type==3){full_pell_backward_vector 1backward_f_code 3 uimsbf
}while (next_bits()=="1"){
extra_bit_picture 1 "1"extra_information_picture 8
}extra_bit_picture 1 "0"next_start_code()
}
Scalable syntaxScalable syntax
Spatial scalability Hierarchy of compatible resolutions, e.g. HDTV, SDTV, SIF
SNR scalability Hierarchy of qualities, e.g. graceful degradations
Temporal scalability Hierarchy of compatible frame/field rates (120-100-60-50 Hz)
Data partitioning scalability Low complexity solution to getting hierarchical bit streams
with different importance for decoding process
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Example of spatial scalabilityExample of spatial scalability
DCT
Q
VLC
1:2
IDCT
Q
-1
VLCDCT
+-
2:1
Q
supplement bit rate stream
for normal resolution video
low bit rate stream
for low resolution video
(e.g. MPEG-1)
high-end receiver
low-end receiver
Example of PSNR scalabilityExample of PSNR scalability
DCT
Q
VLC
IDCT
Q-1
VLCDCT
+-
Q
supplement bit rate streamfor normal quality MPEG-2
video
low bit rate stream
for low quality MPEG-2 video
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Coding of IntraCoding of Intra--pictures (1)pictures (1)
$
( , ),
,
,
,
k l
k l
k l
k l
QWk l
=
= =
nint
nint other
1 1
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=
8369564638352927
6956463834292726
5848403532292726
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3734292724221616
34292726221916.8
,lkW
Quantization procedure
Coarseness of quantization is controlled and adapted by
the parameter Q
can change per slice or permacroblock
the intra quantization matrix
can change per sequence (MPEG-1)
or per picture (MPEG-2)
Coding of IntraCoding of Intra--pictures (2)pictures (2)
Quantizer has no dead zone
DC coefficient is encoded differentially with previousDCT block (take into account the macroblock structure)
VLC codes: 2-D Huffman based on zig-zag scan and (zero-runlength,
amplitude)
Only combinations of high probability are VLC encoded Others get escape code plus FLC to avoid large Huffman
tables
Macroblock types: Default Q-setting / New Q-setting
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Coding of P pictures (1)Coding of P pictures (1)
=
lk
lk
lkWQ ,
,, nint
16 16 16 16 16 16 16 16
16 16 16 16 16 16 16 16
16 16 16 16 16 16 16 16
16 16 16 16 16 16 16 1616 16 16 16 16 16 16 16
16 16 16 16 16 16 16 16
16 16 16 16 16 16 16 16
16 16 16 16 16 16 16 16
Quantization:
plus dead zone
Use fixed inter quantization matrix
Wk,l=
Coding of P pictures (2)Coding of P pictures (2)
Motion estimation: Not specified
Motion compensation: on macro blocks (16x16pixels) Half pixel accurate or finer
Encoding of motion vectors differentially encoded to last vector (of same time) VLC for base vector FLC for residues
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Coding of P pictures (3)Coding of P pictures (3)
Badly predicted macroblocks may yield a larger difference signal than
the original picture information
Can decide to use intra coding mode for such macroblock
Within P-pictures macro-blocks have the following coding types:
non-intra / intra
motion vectors / no-motion vectors
quantized prediction difference is zero / not zero default Q-setting / new Q-setting
Coding of B picturesCoding of B pictures
In B-pictures badly predicted macroblocks may occur Can decide to use intra coding mode for such macroblock
Within B-pictures macro-blocks have the following codingtypes:
intra / non-intra
forward predicted / backward predicted / interpolated
quantized prediction difference is zero / not zero
default Q-setting / new Q-setting
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Rate control and bit allocationRate control and bit allocation
Objectives of rate control: Make sure that output buffer at encoder side and input
buffer at decoder side do not overflow or underflow
Try to make the quality of the encoded frames asconstant as possible on frame level
Objectives of bit allocation: Given the budget allocated to a frame by the rate
control, try to make the quality of the picture as good as
possible Uses techniques similar to bit allocation for images Sometimes extended with temporal considerations
Rate control (1)Rate control (1)
Hybrid coders such as MPEG generate a variable number of bitsper frame
Constant bit rate channel - cbr requires that on the average the generated bit rate is fixed encoder uses output buffer for smoothing the produced bit rate decoder uses input buffer to collect enough bits to decode a frame example: digital video broadcasting
Variable bit rate channel - vbr allows for variation of the channel bit rate up to certain maximum encoder still uses buffer for smoothing short time variations decoder uses input buffer to collect enough bits to decode a frame example: digital versatile disk (DVD)
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Rate control (2)Rate control (2)
Hybrid coders such as MPEG generate a variable
number of bits per frame: Need buffer
0
1
2
3
4
5
6
7
8
9
0 1 2 3 4 5
T ime (sec)
Bitrate(Mbit/sec)
Rate control (3)Rate control (3)
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7 8 91011121314151617181920212223
Time (sec)
Bitrate(M
bit/sec)
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Constant bit rate codingConstant bit rate coding
Video encoder needs to know size of input buffer of decoder way the decoder consumer the bits from this input buffer
Model decoder describes the external behavior of any MPEG decoder encoder anticipates this behavior all decoders should internally compensate for deviations from this
model
VBR CBR VBR
Video
encoderVideo
decoder
Output
bufferInput
buffer
Constant bit rateConstant bit rateCBRCBR -- controlcontrol Bit budget is assigned to current picture on basis of
fullness of (decoder input) buffer number of bits used by previous picture of same type (I,P,B) complexity of current picture
Two-pass coding: quantization with fixed Q to get complexity
indication
Measure statistics from current image (variance, motion, ) anticipated video effects
slow motion freeze framescene change ahead
Rate control attempts to make the picture quality as constant aspossible over time
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Variable bit rateVariable bit rate -- VBRVBR -- controlcontrol
Within short time windows behaves similar as CBR Buffering needed to smooth for instance I,P,B effects
Over longer time windows, the CBR rate changes
Objectives: Guarantees constant picture quality, according to quality measure
used
Quality degrades only if maximum rate is surpassed
Bitrate
time (minutes)
0 1 2 3 4 5 6 7 8
maximum
Bit allocationBit allocation
CBR bit allocationGiven the bit budget, determine the Q setting (per
slice/macroblock) such that the picture quality is spatially optimal
VBR quality allocationGiven the (spatial) quality of previous pictures, find the Q setting(per slice/macroblock) such that the picture quality is constant in
time
Most MPEG coders use bit allocation techniquesdiscussed with DCT compression
Two-pass encoding or multi-pass encoding
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MPEGMPEG--1 Constrained Parameter Set1 Constrained Parameter Set
Decoders are advised to be able to handle at least MPEG-1 streams that satisfy the following parameters:
Parameter Maximum value
Horizontal size 768 pixelsVertical size 576 pixels#Macroblocks/picture 396#Macroblocks/sec 396*25 = 330*30 = 9900Picture rate 30
Bit rate 1.856 Mbit/secEncoder buffer size 372 680 bitsMotion vector range +/- 64 pixels
MPEGMPEG--2 levels and profiles (1)2 levels and profiles (1) Given the big amount of possible applications,
MPEG-2 defines levels and profiles to facilitate
design and implementation.
Low
Main
High 1440
High
Level Characteristics
Picture of 352x288
Picture of 720x576x25Hz, 720x480x29,97Hz
HDTV picture up to 1440x1152
HDTV Picture up to 1920x1152
Level: Defines a set of constraints imposed
on parameters in the bit stream of a profile
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MPEGMPEG--2 levels and profiles (2)2 levels and profiles (2) Profiles refer to a subset of tools of MPEG-2
Profiles support different classes of applications
Simple
Profile
Main
SNR
Spatial
High
Characteristics
No B pictures
Very similar to MPEG-1
Main profile with PSNR scalability
Main profile with spatial scalability
Main profile, PSNR, spatial and other options
The most used is Main Level - Main Profile (ML @ MP)
MPEGMPEG--2 levels2 levels -- profiles (3)profiles (3)4:2:0 4:2:0,4:2:2
1920 x 1152 1920 x 1152
HIGH 80 Mb/s 100 Mb/sI, P, B I, P, B
4:2:0 4:2:0 4:2:0,4:2:2HIGH 1440 x 1152 1440 x 1152 1440 x 1152
1440 60 Mb/s 60 Mb/s 80 Mb/s
I, P, B I, P, B I, P, B
4:2:0 4:2:0 4:2:2 4:2:0 4:2:0,4:2:2720 x 576 720 x 576 720 x 608 720 x 576 720 x 576
MAIN 15 Mb/s 15 Mb/s 50 Mb/s 15 Mb/s 20 Mb/sI, P I, P, B I, P, B I, P, B I, P, B
4:2:0 4:2:0352 x 288 352 x 288
LOW 4 Mb/s 4 Mb/sI,P,B I, P, B
LEVEL SIMPLE MAIN 4:2:2 SNR SPATIAL HIGHPROFILE PROFILE
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Freedom within ML@MPFreedom within ML@MP
GOP structure Open/closedGOP size, position I framesFrame types used
Frame coding types I, P or B
Interlacing tools Frame/field picturesFrame/field prediction
Slice structure Fixed/adaptive
Slice quantization parameter Quantizer level for entire sliceMacroblock coding type Coded/Not-coded
Motion/No-motionIntra/Predicted
Macroblock quantization Adapt quantizer level within sliceMotion vectors Estimation algorithm used
Rate control Assignment of bits to a frameFeedforward (pre-analysis)/ FeedbackCBR versus VBR
Bit allocation Distribution of bits over macroblocks
MPEG system aspectsMPEG system aspects
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MPEG 2 multiplexingMPEG 2 multiplexing
Video
codingVideo
Auido
codingAudio
Packetizer
Packetizer
Elementary
Stream
Video
PES
Audio
PES
Program
Stream
Mux
TransportStream
Mux
Data
Program
Stream
Single Program
Transport Stream
PES: Packetized Elementary stream
MPEG 2: Decoding standard
DataDataMPEGMPEG--22
Single Program
Transport StreamTransport Stream Transport Stream Transport Stream
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Elementary video streamElementary video stream -- ESES
Block of
coefficients
Motion
vector
Macro
blockSlice Picture
Group
of
Pictures
Video
sequence
Syncronism I, P, B Time
Global vectors
Open /
closed
Quantization
matrices
Picture
rate
Level
Profile
Picture
sizeAspect
ratio
Progressive /
interlaced
Chroma
type
Elementary stream
Packetized elementaryPacketized elementarystreamsstreams -- PESPES
24 bits
Packet start
code prefixPES packet
length
Stream
identification
Video or audio
identification
Identificacin del comienzo
de paquete
PES packet header
PES packet
Packet
lengthElementary stream
in packets
MPEG defines only the transport stream
The designer can chose to convert ES in Transport Stream
without specifying PES
PES can be used for Programs Streams or Transport Streams
Length depends on
the application
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Program streamsProgram streams -- PSPS
DVD applications (Digital Video / Versatile Disc)
Possibilities
One video program + several audio programs
Only several audio programs
The video Elementary Stream is divided in Access Units (AU). Each AU
contains information of a compressed image (I, P, B)
AU are converted into PST (Program stream packets)
Each PST can be of variable lenght
Variable bit-rate
Transport streamsTransport streams -- TSTS Transmission application for different media
Several different programs of audio and video. Typical: 4 - 6 TV programs,
3 or 4 audio programs, data, teletext, captions, etc.
Eacg program can have a different compression factor
bit-rate can be changed dynamically although the total be constant
(statistical multiplexing). Typical: 45 Mbits/sec
Each PES of video can have a different number of PES of audio and associated
data
Some programs can be protected. The TS must have conditional access
The TS has to provide syncronization for each program individually
This syncronization is called Program Clock Reference - PCR
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Transport stream packetTransport stream packet
Header Payload
188 bytes
Sync
Byte
Adaptationfield
controlScrambling
control
Packet
identificationPID
Transport
priority
Start
indicator
Transport
errorindicator
Continuity
counterAdaptation
field
Discontinuity
indicator
Adaptation
field
length
Random
access
indicator
Elem stream
priority
indicator 5 Flags
Optional
fields
Stuffing
bytes
PCR OPCRSplice
countdown
Transport
private
data
Adaptation
fieldextension
Packet identificationPacket identification -- PIDPID
PID is used by the demultiplexer to distinguish packets which contain
different types of information
All packets which belong to the same ES have the same PID
The data of a program can be selected using the corresponding
PIDs of video, audio and teletext.
To know the PIDs associated to a program the Program Specific
Information PSI is used
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Digital video broadcastingDigital video broadcasting -- DVBDVB
DVB - S: satelite system which could be used by any transponder
DVB - C: cable system for any type of cable network
DVB - T: Terrestrial system
DVB - MC / S: Multipoint microwave distribution system
DVB - SI: System service information allowing the user to browse
in a DVB environment
DVB - CA: Common scrambling system
DVB - CI: Common interface for conditional access
Objetive (1993):
To provide a common format allowing the transmission of TV digital
DVB characteristicsDVB characteristics
MPEG 2 compression system for audio and video
DVB provides modulation techniques and error correction coding techniques
for satellite, cable and terrestrial systems
DVB provides format for data insertion in the tranmission channel
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DVB tranmission systemDVB tranmission system
Video coding
MultiplexerAudio coding
Video coding
Audio coding
Error
correctionModulation
Video270Mbits/seg
Audio6Mbits/seg
MPEG 2 Transport Stream Mbits/seg
DVB - C: QAM
DVB - S: QPSK
DVB - T: COFDM
Energy dispersion
Reed Solomon
InterleavingConvolutional codes: DVB - S, DVB - T
DVB integrated systemDVB integrated system