p x MPEG Video Compression Technology and Testing Page 1 MPEG Video Compression Technology and Testing CONVERGENCE SYMPOSIUM TEKTRONIX Video Services Telecommunications Mobile Communications
p x MPEG Video Compression
Technology and Testing Page 1
MPEG Video Compression Technology and Testing
CONVERGENCE SYMPOSIUM
TEKTRONIX
Video Services Telecommunications Mobile Communications
p x MPEG Video Compression
Technology and Testing Page 2
Seminar Topics
• Modern Television System – Video and Compression Standards
• MPEG-2 Compression • MPEG-2 System • Testing in Compressed Systems
– Traditional test methods – Picture quality assessment – MPEG-2 protocol analysis
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Other Programs
Program Demultiplexing Transmission
Program Decompression
Program Display or
Reuse
Program Production
Program Compression
Multi-Program Multiplexing
Modern Television System
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Video Production Formats
R G B
Component Analog Video (RGB)
4xFsc Sample (PAL=17.7 MHz)
(NTSC=14.4 MHz)
270 MHz clock
Component Analog Video (R-Y, B-Y, Y)
Y B-Y R-Y
10 bits
6.75 MHz Sample
13.5 MHz Sample
Multiplexed 27 Mwords/Sec
...Y / R-Y / Y / B-Y / Y...
Clock X10
Parallel Digital Component
Serial Digital Component 270 Mb/Sec “Rec 601”
Serial Digital Composite (144 Mb/Sec) (177 Mb/Sec)
Parallel Digital Composite
Y
B-Y
R-Y
C a m e r a
M a t r i x
Composite Encoder
A to D Conv Serializer
Analog Composite
Video
10 bits Parallel
A to D Conv
A to D Conv
A to D Conv
Serializer
AES/EBU Digital Audio
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Video Compression Standards • JPEG, still images (Joint Photographics Experts Group)
– M-JPEG; motion JPEG, not a standard, generally proprietary
• H.261 (px64), video conferencing – px64 kb/s (p=1, 2,…. 32)
• H.263, video conferencing, emphasis on low bitrates • MPEG-1, CD-ROM and multimedia (Motion Picture Experts Group) • ETSI 300 174, Broadcast distribution and contribution • MPEG-2, Broadcast entertainment/contribution and DVD • Non-DCT methods
– Wavelets, Fractal, DPCM – Lossless (e.g., special JPEG mode)
• MPEG-4, very low bitrate coding (possibly wavelets)
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Typical Video Data Rates
• 10-bit Rec 601 270 Mbps • 8-bit Rec 601 216 Mbps • 8-bit Rec 601 (active only) 167 Mbps • Digital Betacam ~90 Mbps • MPEG-2 4:2:2P@ML 15-50 Mbps • MPEG-2 MP@ML 1.5-15 Mbps • MPEG-1 constrain. param. 0.5-1.8 Mbps • H.261 videoconferencing 64 kbps - 1.5 Mbps • H.263 videoconferencing 4 kbps - 0.5 Mbps
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MPEG-2 Applications • RF Transmission
– DVB-S Digital Video Broadcasting - Satellite – DVB-C Cable, DVB-T Terrestrial broadcast – ENG Electronic (satellite) news gathering
• Broadband Network – Contribution quality programs – Video on demand
• Storage Media – DVD Digital Versatile Disk – Video servers
• Intra-studio – Point-to-point (being developed by SMPTE) – Networking (being specified by EBU/SMPTE)
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MPEG-2 Standards Documents
MPEG-2 ISO/IEC 13818 • Part 1 Systems • Part 2 Video • Part 3 Audio • Part 4 Conformance testing (for 1, 2 and 3) • Part 5 Software simulation • Part 6 System extensions - DSM-CC
(Digital Storage Media - Command & Control) • Part 7 Audio extension - NBC (non backward compatible) • Part 9 System extension RTI (real time interface) • Part 10 Conformance extension - DSM-CC
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Standards Organizations • ITU
Place des Nations 1211 Geneve 20, Switzerland Ph: 41 22 730 6003 http://www.itu.ch
• SMPTE 595 West Hartsdale Ave White Plains, NY 10607 Ph: 914-761-1100 http://www.smpte.org
• ISO Case Postale 56 1 rue de Varembe 1211 Geneve 20, Switzerland Phone: +41 22 749 01 11 http://www.iso.ch
• Documents for sale Global Engineering 15 Inverness Way East Englewood, CO 80112 Ph: 800-854-7179
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Seminar Topics
• Modern Television System – Video and Compression Standards
• MPEG-2 Compression • MPEG-2 System • Testing in Compressed Systems
– Traditional test methods – Picture quality assessment – MPEG-2 protocol analysis
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MPEG-2 Video Compression
• Pre-processing – Clean-up pictures and prepare video samples
• Temporal Compression (IntER-frame) – Compresses the data from multiple frames
• Spatial Compression (IntRA-frame) – Compresses the data within one frame – (Similar to JPEG)
• Rate Control – Constant bitrate – Constant (or nearly so) quality
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Important Points about MPEG
• Only specifics bitstream syntax and decoding • Encoding algorithms are not defined
– Open to invention and generally proprietary – Future improvements are compatible with all decoders
• Asymmetric Compression – Encoder is very complex – Encoder contains a decoder model – Decoder definition emphasizes low complexity (cost)
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Pre-Processing
• Required – Decode from composite to component – Produce correct picture size – Reduce 10-bit samples to 8-bit samples – Convert to 4:2:0 sampling (entertainment quality)
• Optional – Noise reduction – Other picture clean up
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Picture Sizes
• Rec 601/656 525/30/2:1 720 x 486 • Rec 601/656 625/25/2:1 720 x 576 • MPEG-2 30 fps (quasi-std) 704 x 480 • MPEG-2 422P@ML 30 fps 720 x 512 • MPEG-2 422P@ML 25 fps 720 x 608 • SIF (30fps, 25 fps) 352 x 240,288 • CIF (always 30 fps) 352 x 240 • QSIF (30fps, 25 fps) 176 x 128,144 • QCIF (always 30 fps) 176 x 144
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Rec 601 Component Video Sampling
(313)
(314)
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4:2:2 Rec 601
1 Luminance sample Y
2 Chrominance samples Cb, Cr
4:2:0
4:1:1
4:2:0 Chroma Sub-Sampling
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Redundancies
• Spatial Redundancies – Redundant information in the horizontal and vertical picture
dimensions. Data that is similar or repeats itself in picture areas which are close to one another.
• Temporal Redundancies – Redundant data over a given time. Data that is similar or
repeats itself from moment to moment, even if its location in the picture area changes.
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Discrete Cosine Transform (Definition)
The NxN two dimensional DCT is defined as:
Encoder:
Decoder:
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DCT Function
• 8 X 8 pixel Blocks are converted from the spatial domain to the spatial frequency domain.
• Transformed blocks are numerically represented as 8 X 8 DCT coefficients.
• DCT coefficients are more suitable for bit rate reduction techniques.
The transform process does not result in bit rate reduction.
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DCT Example
Picture Sample Values DCT Coefficients
223 191 159 128 98 72 39 16
223 191 159 128 98 72 39 16
223 191 159 128 98 72 39 16
223 191 159 128 98 72 39 16
223 191 159 128 98 72 39 16
223 191 159 128 98 72 39 16
223 191 159 128 98 72 39 16
223 191 159 128 98 72 39 16
43.8 -40 0 -4.1 0 -1.1 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
720 Pixels
8x8 Pixels 480 Lines
(Pixels)
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“Picture” of the DCT Coefficients H
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Horizontal spatial frequency waveforms
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Quantization
• Divides each DCT coefficient by a frequency-dependent value and truncates the results to an integer.
• Many of the resulting integers are zero or small values (e.g., 1, 2, 3, … 12, 13, .)
• Quantization coefficients can be tailored to complement limitations of the human visual system
• Quantization causes information to be irretrievably lost. Reconstructed pixels usually differ in value from the original
7842 199 448 362 342 112 31 22
198 151 181 264 59 37 14 3
142 291 218 87 27 88 27 12
111 133 159 119 58 65 36 2
49 85 217 50 8 3 14 12
58 120 60 40 41 11 2 1
30 121 61 22 30 1 0 1
22 28 2 33 24 51 44 81
8 16 19 22 26 27 29 34
16 16 22 24 27 29 34 37
19 22 26 27 29 34 34 38
22 22 26 27 29 34 37 40
22 26 27 29 32 35 40 48
26 27 29 32 35 40 48 58
26 27 29 34 38 48 56 69
27 29 35 38 46 56 69 83
980 12 23 16 13 4 1 0
12 9 8 11 2 1 0 0
7 13 8 3 0 2 0 1
5 6 6 4 2 1 0 0
2 3 8 1 0 0 0 0
2 4 2 1 1 0 0 0
1 4 2 1 0 0 0 0
0 0 1 0 0 0 0 0
Code Linear Non-Linear Quant Scale Quant Scale
1 2 1
8 16 8
16 32 24
20 40 40
24 48 56
28 56 88
31 62 112
Input DCT Coefficients (a more complex block)
Quant Scale Values Not all code values are shown
One value used for complete 8x8 block
Output DCT Coefficients Value for display only
not actual results
Quant Matrix Values Value used corresponds
to the coefficient location
Divide by Quant Matrix
Divide by Quant Scale
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Processing Sequences
Zigzag or Classic (nominally for frames)
Alternate (nominally for fields)
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Entropy Coding
• Run length coding uses a special code for repeating values (e.g., 13 “0s”, 5 “1s”, 4 “2s”)
• Variable length coding uses shorter code words for more probable symbols (like Morse code)
Symbol A B C D E F
Probability 0.5 0.25 0.125 0.0625 0.03125 0.03125
Code Word 0 10 110 1110 11110 11111
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Convert 4:2:2 to
8-bit 4:2:0 DCT Quantize Entropy
Coding Buffer Full
Bitrate 10-bit Data
Compressed Data
Rate Control
Information lost Data reduced
No Loss No Data reduced
Data reduced (information lost)
Data reduced (no loss)
Quantizing Reduce the number of bits for each coefficient.
Give preference to certain coefficients. Reduction can differ for each coefficient.
Variable Length Coding Use short words for
most frequent values (like Morse Code)
Run Length Coding Send a unique code
word instead of strings of zeros
Entropy Coding
Quantizing Data
INTRA-Frame Coding
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Redundancies
• Spatial Redundancies – Redundant information in the horizontal and vertical picture
dimensions. Data that is similar or repeats itself in picture areas which are close to one another.
• Temporal Redundancies – Redundant data over a given time. Data that is similar or
repeats itself from moment to moment, even if its location in the picture area changes.
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Temporal Redundancies • Frame to Frame redundancies • New location same data
• New data uncovered
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Motion Estimation
Frame N Frame N + 1
Macro Block 16x16 Pixels
Search Range
Motion Vector
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INTER-Frame Coding (Forward Prediction)
Motion Vectors
Predicted Present Frame
Video in
Present Frame
Motion Vectors
Difference Frame Much less information if
the prediction is good
Previous Frame or Reconstructed Present Frame to use in the next Prediction
Motion Estimation
Motion Compensation
Fixed Store (previous frame)
Subtract/Pass
SUM
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MPEG Video Compression P-pictures only
Motion Vectors
Motion Vectors
Predicted Present Frame
Video in
Rate Control
Quantizing Data
Group of Pictures Control
Motion Estimation
Motion Compensation
Fixed Store
Subtract DCT Q RLC VLC MUX
Buffer
Q-1
DCT-1
SUM
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N = Distance between anchors
Group of Pictures
I pictures: Inter-coding only P pictures: Contain forward motion compensation B pictures: Contain forward, backward & bi-directional motion compensation
I M = Distance between I pictures
I P B P B B B B
Bi-directional Prediction
Forward Prediction
0 1 2 3 4 5 6 7
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15/3 Frame Sequence I
B B
P B
B P
B B
P B
B P
B B
I
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MPEG-1
• Design focused on non-interlaced SIF (352x240) • Application was media storage e.g., CD-ROM • Uses most of the H.261 techniques • Introduced the concept of B-frames • Trick modes are supported
– Fast search – Reverse, etc
• Used in early DTV testing
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MPEG-2
• MPEG-2 = MPEG-1 + interlace tools + Profiles & Levels • New field/frame prediction and DCT modes for interlace • Quantization with greater range and adaptivity • New intra-frame VLCs (variable length codes) • New adaptive coefficient VLCs • Scalability extensions; Spatial, SNR, Temporal • System layer for multiple program transport streams • Audio extended to 5-channel sound
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HIGH 4:2:0 4:2:0. 4:2:2 1920 x 1152 1920 x 1152 80 Mb/s 100 Mb/s I, P, B I, P, B
HIGH-1440 4:2:0 4:2:0 4:2:0, 4:2:2 1440 x 1152 1440 x 1152 1440 x 1152 60 Mb/s 60 Mb/s 80 Mb/s I, P, B I, P, B I, P, B
MAIN 4:2:0 4:2:0 4:2:0 4:2:0, 4:2:2 760 x 576 720 x 576 720 x 576 720 x 576 15 Mb/s 15 Mb/s 15 Mb/s 20 Mb/s I, P I, P, B I, P, B I, P, B
LOW 4:2:0 4:2:0 352 x 288 352 x 288 4 Mb/s 4 Mb/s I, P, B I, P, B
LEVEL
PROFILE SIMPLE MAIN SNR SPATIAL HIGH
MPEG-2 Profiles and Levels
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4:2:0 352 x 288
4 Mb/s I, P, B
HIGH
HIGH-1440
MAIN
LOW
LEVEL PROFILE
4:2:0 720 x 576 15 Mb/s
I, P
SIMPLE
4:2:0 1920 x 1152
80 Mb/s I, P, B
4:2:0 1440 x 1152
60 Mb/s I, P, B
4:2:0 720 x 576 15 Mb/s I, P, B
MAIN
4:2:0 720 x 576 15 Mb/s I, P, B
4:2:0 352 x 288
4 Mb/s I, P, B
SNR
4:2:0 1440 x 1152
60 Mb/s I, P, B
SPATIAL
4:2:0, 4:2:2 1920 x 1152
100 Mb/s I, P, B
4:2:0, 4:2:2 1440 x 1152
80 Mb/s I, P, B
4:2:0, 4:2:2 720 x 576 20 Mb/s I, P, B
HIGH
MPEG-2 4:2:2 Profile
4:2:2 720 x 608 50 Mb/s I, P, B
4:2:2 PROFILE
4:2:2 1920 x 1088 300 Mb/s I, P, B
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• Quality – Better Chroma Resolution than MP@ML – Higher quality (bit rate) than MP@ML – Good multi-generation performance
• Flexibility – Short GOPs for editability – Capability to pass all active video, some vertical info
• Economy – Storage costs – Transmission costs – Compatibility
MPEG-2 4:2:2 Profile for Production
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How Good is MPEG-2 4:2:2?
• The MPEG committee has conducted subjective assessment tests to verify the performance of the MPEG-2 4:2:2 profile.
• Tests demonstrated that, with proper choices of data rate and GOP structure, MPEG-2 4:2:2 can meet professional requirements.
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Achieving Quality
I IB IBBP 20 Mb/s
30 Mb/s
50 Mb/s
Bit
Rat
e
GOP Structure
Higher Quality
Lower Quality
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Application Examples News and Acquisition 18 Mb/s IB GOP Structure
Archive 30 Mb/s IB GOP Structure
Post-Production 50 Mb/s
I only GOP Structure
Distribution 20 Mb/s
IBBP GOP Structure
Fully Compliant MPEG-2
4:2:2 Decoders
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• If you can’t hear it, don’t send it • Psychoacoustic Models
– Pre-Masking – Post-Masking – Simultaneous Masking
• Data Structures
MPEG Audio
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Temporal Masking
Post- masking
Pre- masking
time
sound pressure
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Simultaneous Masking
20 kHz
Masking threshold
1 kHz sinewave
Threshold in quiet
20 Hz 1 kHz
sound pressure
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Audio Frame input PCM samples 384 for Layer 1
3 * 384 = 1152 for Layer 2
MPEG Audio Encoder
Filterbank 32
Subbands
Scaler and
Quantizer
512 Point FFT
MUX
Masking Thresholds
Dynamic Bit and
Scale Factor Allocator
and Coder
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Filterbank 32 Subbands
Audio Time Frame
32 Samples
32 Samples
32 Samples
32 Samples
12 Sections of 32 Samples
32 Samples = 0.66 msec (@ 48 kHz)
1 MPEG audio layer 1 frame = 8 msec of audio
1 MPEG audio layer 2 frame = 24 msec
12 x 32 Samples
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Header
CR
C
Bit A
llocation
Scalefactors
Anc D
ata
20 Bit System
12 B
it Sync
Optional
4 bit linear
6 bit linear
Unspecified
Length
Subband Samples
GR0
0 1 2 31
GR1 GR2 GR11
Layer I Frame Structure
384 PCM Audio Input Samples Duration 8 msec @ 48 kHz
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Seminar Topics
• Modern Television System – Video and Compression Standards
• MPEG-2 Compression • MPEG-2 System • Testing in Compressed Systems
– Traditional test methods – Picture quality assessment – MPEG-2 protocol analysis
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System encoder
PES Syntax
System decoder
PES Syntax
Elementary Streams
Elementary Streams
MPEG-2 Standards Not Standardized
Transport Stream
MPEG System
Audio encoder
Video encoder
Audio decoder
Video decoder
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MPEG-2 System Mux
Audio Encoder
Video Encoder
Video Data
Audio Data
Packetizer
Packetizer Video PES
Audio PES
Program Stream (DVD)
Program Stream
MUX
Single Program Transport Stream
Transport Stream
MUX
Elementary Stream
Data
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Transport Packets
• The Transport Stream (TS) is a continuous data stream in 188 byte packets containing format (syntax) information and payload data
Header Payload
Packet Packet Packet Packet Packet Packet Packet Packet
188 Bytes
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Transport Packet Header
Sync Byte
8
Transport Error
Indicator 1
Start Indicator
1
Transport Priority
1
PID
13
Scrambling Control
2
Adaptation Field
Control 2
Continuity Counter
4
Adaption Field Payload
Header Payload
188 Bytes
Adaptation Field
Length 8
Discontinuity Indicator
1
Random Access
Indicator 1
Elem Stream Priority
Indicator 1
5 Flags
5
Optional Fields
Stuffing Bytes
OPCR
48
Splice Countdown
8
Transport Private
Data
Adaption Field
Extension PCR
48
Minimum 4-byte header
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Program Clock Model
variable delay = e(n) variable delay = d(n) constant trans delay = Ctrans
constant total delay = Ctotal
PCR clock phase
generator
PCR: encoder stamps
departure time of packet
PCR clock frequency generator
PCR: arrival time of packet
PES Syntax
System MUX
PES Syntax
Display
System DEMUX
PCR clock recovered
Audio Decoder
Video Decoder
Audio Encoder
Video Encoder
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Reference Clock Synchronization
Elementary Stream
_ _ _ _
PCR = X PCR = X plus the time of exactly n bits n bits 188 byte packets
PCR
Load
Local PCR
Receiver 27 MHz clock
Video In
Transport Stream
Decoder Compare
Low Pass Filter
27 MHz Xtal VCO
27 MHz Clock
27 MHz Clock
Transport Stream
Formation Video
Encoder
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Decoding the Transport Stream Program Specific Information (PSI)
• Program Association Table (PAT) – PID = 0, must be present in every transport stream
• Program Map Table (PMT) – PID values assigned by transmission system (DVB, ATSC, etc.)
• Conditional Access Table (CAT) – PID = 1
• Network Information Table (NIT) – PID values assigned by transmission system – DVB considers this part of System Information (SI)
• Null Packets – PID = 8191 (1FFFhex = 13 “1”s binary)
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PSI Example Program Association Table (PID 0)
Program Map Tables
Private Network
Data
Network Information Table
Transport Stream
Conditional Access Table (PID 1)
Prog 1 MAP
33 22 48 82 19 19 54 19 0
Program 0 16 Program 1 22 Program 3 33
... ...
Program k 55 ... ...
Stream 1 Video 54 Stream 2 Audio 48 Stream 3 Audio 49
... ... ...
Stream k Data 66 ... ... ...
Stream 1 Video 19 Stream 2 Audio 81 Stream 3 Audio 82
... ... ...
Stream k Data 88 ... ... ...
PAT Prog 3 MAP EMM Prog 1
Audio 1 Prog 3
Audio 2 Prog 3 Video 1
Prog 3 Video 1
Prog 1 Video 1
Prog 3 Audio 1
Prog 3 Video 1
1 81
Conditional Access Data
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DVB System • MPEG-2 Transport Streams
– Service Information (SI) in addition to MPEG-2 (PSI)
• SI includes – NIT - Network Information – SDT - Service Description – EIT - Event Information – TDT - Time and Date – BAT - Bouquet Association – RST - Running Status – ST - Stuffing Tables
• DVB Systems provide: – Common Scrambling systems – A common Conditional Access Interface – Facilities for reverse channel operation
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DVB Channel Coding
• Provides error correction over the channel – ( 1 in 10-4 on channel to 1 in 10-11 on Transport Stream )
• Outer Coding - energy dispersal and RS • Inner Coding - interleaving and viterbi
– Not used for cable transmission
Reed Solomon Coding 16 Bytes
204 Bytes per packet
Transport Stream Packet 188 Bytes
Reed Solomon Coding 16 Bytes
204 Bytes per packet
Transport Stream Packet 188 Bytes
Reed Solomon Coding 16 Bytes
204 Bytes per packet
Transport Stream Packet 188 Bytes
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Seminar Topics
• Modern Television System – Video and Compression Standards
• MPEG-2 Compression • MPEG-2 System • Testing in Compressed Systems
– Traditional test methods – Picture quality assessment – MPEG-2 protocol analysis
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Test Function
Video Testing Layers Signal
Standardized Video Signals Video Quality
MPEG-2 Transport
Stream Protocol Analysis
INTRA-Facility Connections
INTER-Facility Connections SDH/ATM or
Modulated RF Transmission
Channel Analysis
Studio Connections
Program Compression
Transmission Channel
Formatting
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Signal vs Picture Quality
• Analog and digital video systems are linear – Superposition applies – Results are time invariant and signal independent – Test signals can be substituted for program material – Testing in the vertical interval is equivalent to full-field tests – Static test signals are sufficient (Indirect measurement)
• Compression video systems are non-linear – Test signals are easily/accurately compressed – Picture quality is a function of; data rate, picture complexity and
encoding algorithm capabilities – Test with complex motion sequences (Direct measurement)
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Seminar Topics
• Modern Television System – Video and Compression Standards
• MPEG-2 Compression • MPEG-2 System • Testing in Compressed Systems
– Traditional test methods – Picture quality assessment – MPEG-2 protocol analysis
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Traditional Test Methods
A significant portion of the modern television system is analog or full bandwidth digital video
These signals will continue to be used in the foreseeable future
Input picture quality to the compression system must be maintained
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Analog Video Testing
VIDEO Operational Monitoring
Technical Measurements
SYNCHRONIZING WAVEFORMS Technical Measurements
PHYSICAL LAYER (COAX) Technical Measurements
Functional Layers
Waveform Monitors
and Measurement
Sets
TDRs
RGB PAL NTSC
PAL NTSC
Composite Encoder
Transmission or
Operation
Record or
Display Camera
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Analog Tests
Amplitude Rise/fall times Bandwidth Group delay Sig/Noise ratio Non-linearities Color gamut Diff Phase/Gain
No one test will do it all
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Digital Video Testing
VIDEO SIGNAL CODING (Rec 601)
DIGITAL FORMATTING (Rec 656) DIGITAL WAVEFORM (Rec 656)
PHYSICAL LAYER (COAX/FIBER)
Functional Layers
Waveform Monitors, Measurement Sets
with Analog or Digital
Capabilities
TDRs, OTDRs
PAL NTSC
PAL NTSC
Rec 601/656 Rec 601/656
Composite Encoder
Transmission or
Operation
Record or
Display Camera
Studio Interconnect,
Operation
Decode A/D
Encode D/A
RGB
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More Analog Tests
• A/D and D/A Converters – Converters add distortions to the signal – Measure SNR on a shallow ramp – Differential gain/phase
– Full ramp for “analog” measurements – Shallow ramp for “digital” measurements
• Ringing on digitally generated signals
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Rec 601 Digital Measurements
• Digital coding – Levels, excluded values
• Formatting – Synchronization data, Embedded audio
• Error Detection • Waveform (eye pattern)
– Amplitude, risetime, overshoot – Jitter by eye pattern or demodulation – Headroom (cable length)
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Seminar Topics
• Modern Television System – Video and Compression Standards
• MPEG-2 Compression • MPEG-2 System • Testing in Compressed Systems
– Traditional test methods – Picture quality assessment – MPEG-2 protocol analysis
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Factors Affecting Video Quality in a Compression System
• Quality of the input video – Amplitude, dc level, bandwidth, ringing, jitter – Noise, composite/component decoding artifacts – Prefiltering to eliminate the above problems
• Nature of the input video – Picture spatial and temporal complexity
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Factors Affecting Video Quality
• Encoding parameters used – Profile/level, field/frame, output data rate, GOP
• Encoding algorithm – Speed required, hardware vs software – Multiple pass (iteration of parameters) – Algorithm design
– Quantizing table selection – Use of motion vectors, search range
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Compression Impairments Blocking: appearance of underlying block structure Error Blocks: a form of block distortion
– One or more blocks bear no resemblance to the current or previous scene and often contrast greatly with the adjacent blocks
Edge busyness: distortion concentrated at edges of objects – Characterized by temporal and spatial features
Mosquito noise: edge busyness associated with movement – Characterized by moving artifacts or blotchy noise patterns superimposed
over the objects Quantization noise: snow or salt & pepper
– Similar to random noise but not uniform over the image Blurring: distortion of the entire image,
– Characterized by reduced sharpness of edges and spatial details Jerkiness: smooth, continuous motion now perceived as a series of distinct images
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Picture Quality Measurements
• We can not actually measure “picture quality” – We can measure picture degradation comparison to reference
• Subjective measurements – ITU-R BT.500 has been updated – Further work to extend subjective methods
• Objective measurements are most useful if they have good correlation with subjective results
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Subjective Tests (Human Viewer Trials)
Strengths • Produces valid results in
conventional and digital television system applications
• Provides a scalar result – Mean Objective Score
• Works well over a wide range of video (and still image) applications
Weaknesses • Requires meticulous setup and
control • Needs lots of participants • Is time-consuming
Subjective tests are only applicable for development purposes. They do not lend themselves to operational monitoring,
production line testing or troubleshooting.
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Quest for Objective Measurements
• Intuition has led many developers to the same starting point:
– Picture quality is related to the differences between the original and impaired scenes.
– A measurement of the magnitude of these differences is somehow related to “picture quality”
– Therefore, construct a device which indicates the magnitude of these differences.
• There are two approaches to objective measurements – Feature Extraction – Picture Differencing
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Feature Extraction
Low Bandwidth Data
Results
Reference Picture
Degraded Picture Processing System
Feature Extraction
Feature Extraction
Feature Difference
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Picture Differencing
Picture Data
Results
Picture Data
Reference Picture
Degraded Picture Processing System
Image Processing
Image Processing
Picture Data
Difference
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Mean Squared Error
• Prediction of quality derived from the result of computing the mean of the squares of the differences
– As the result approaches zero, the more identical are the original and copy.
– Conversely, as the result grows, the more different is the copy from the original.
• Peak Signal to Noise Ratio is a variation of MSE:
PSNR=10 log10 MSE2 2552
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But MSE (and PSNR) is Easily Fooled!
MSE = 27.10 MSE = 21.26
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Objective Picture Measurements
• ANSI T1.801.03 – Peak Sig/noise Average gain Offset level – Spatial shift Spatial info Temporal info – Added/lost; spatial frequencies, motion/edge energy – Radial average of spatial frequencies
• Insufficient for comparison of systems • Useful for yesterday/today comparisons
– Unfortunately most systems are not constant – Bit rate changes – Concatenation of different coding systems
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Human Vision System
• A number of picture assessment methods have been proposed based on the human vision model
• A robust metric of image quality – Independent of nature of the video material – Independent of the type of impairments – Independent of the compression system
• Principles of HVS models – Contrast sensitivity – Spatio-temporal response – Color perception
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The Tektronix/Sarnoff Method
• In a review of 32 human visual models – “All but one of the models can be regarded as simplified
versions of the Lubin model.”
• Dr. Lubin at Sarnoff Labs – Extended the work to cover picture quality – Temporal chroma models have been added
• “JND Image Quality Metric” – Tektronix/Sarnoff cooperative product development
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Just Noticeable Differences
• Central features – Approximates the optics of the eye and retinal structure – Filters that decompose image into subbands – Directional filters – Psycho-visually valid calibration curves – Pooling process to combine local results
• JND scale – At a value of 1 JND, 3 out of 4 can detect a difference – Values above 1 have more noticeable differences – Values below 1 have less noticeable differences
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JND Algorithm
...
contrast pyramid
impaired image
transducer
JND map
oriented responses
gain control
JND value
sampling
optics
Identical Process
...
reference image
distance
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Reference Image JND Map
Model Output: JND Maps
• Local magnitude of JND Map indicates probability of seeing a difference between two images at that point
• Average of JND Map indicates overall magnitude of visible differences
Degraded Image
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JND vs. MSE
MSE = 27.10 Average JNDs = 0.75
MSE = 21.26 Average JNDs = 2.52
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Objective Measurement Operation
• Close matching of “presentation” – Chroma/luma gain and dc level – Spatial and temporal alignment
• Test motion sequences – Difficult but not killer – Several different program types
• Program material not okay (for now) – Compute power/time for matching – Material often not difficult
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Basic Concept
Transport Encoder Decoder
Test Sequence Source
Picture Quality Meter Reference
Sequence Source
Play test sequences 5+ seconds
Perform automatic measurements
2 seconds
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Use of Calibration Stripes
Special markings used to derive gain/level/shift.
Original
Transported
Compare original and transported
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Seminar Topics
• Modern Television System – Video and Compression Standards
• MPEG-2 Compression • MPEG-2 System • Testing in Compressed Systems
– Traditional test methods – Picture quality assessment – MPEG-2 protocol analysis
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MPEG Protocol Analysis
MPEG Transport Stream
Hardware I/O Interface
Real-time Firmware
Display key errors
Hard-disk Storage
Generation and Analysis Software
Display header and Timing Information
• Tektronix MTS 100 provides creation, generation and in-depth analysis of MPEG-2 transport streams
• Real-time analysis and display of key errors is planned
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MTS Analysis • Analyzes transport and/or PES
packets – Syntax (structural)
• Extracts – PES packets – Elementary streams
• Hierarchical view – PSI (program specific
information) – Programs – Channels
MPEG TS
MPEG TS Data Files on the NT
Disk
Multiple Hardware Output Types
Windows NT Application
Control
Windows NT Application
MPEG Transport Stream
Analysis Program
MTS Data Files in the Data
Store System
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MTS Generator • ES Data Files are supplied by
Tektronix or the user • The ES Data Files can
be on disk or CD ROM • MTS Data Files are built using
multiplexing software – Formatted ES – User defined configuration – Application of MPEG rules
• MTS outputs the transport stream to the device under test
•
MPEG TS
ES Data Files on the NT
Disk
Multiple Hardware Output Types
Windows NT Application
Control
Windows NT Application
Control MTS Formatter
and Multiplexer
MTS Data Files in the Data
Store System
CD ROM
User Defined
Configure File
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0.804640 0.985496 1.166352
PCR number 21 - 30
PCR Base : 79659 units PCR Extension : 93 units PCR Value (PCRV) : 0.885103444 sec Interpolated PCR (PCRI) : 0.885104096 sec PCRI - PCRV : 0.000000652 sec Previous PCR Arrived Since : 0.040156800 sec Packet Number : 5886 Packet Number of PID 50 : 2279
PCR number 23
PCR Data Analysis
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System Target Decoder
• An abstract model for deciding legality of streams and decoders (This is really control on encoder operation.)
• Based on passing of streams through cascading buffers • Overflow of any buffer implies illegality of stream • Sometimes underflow is allowed; sometimes it isn’t.
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Automatic Analysis
• Selectable functions • Error tables available for
quick access to data
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Service Information
• Selection of SI data
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• Decoded Service Description Table
Service Information Interpreted
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Sarnoff Compliance Bitstreams STRM100
• A visual inspection of Video Decoder Operation – No digital interface or capture equipment required – Ease of use: pass-fail testing, rapid fault isolation – Look for gray embossed “VERIFY” without artifacts
• One parameter or technique at a time • Loopable
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Tests without B-pictures
Tests with B-pictures
Sarnoff Compliance Bitstreams: How They Work
IPPP .... PP PP .... P PP .... PP “Verify” (60 to 90 frames) -- does not change image
Test section (1 or more frame)
Start Title (operator convenience only)
IPPP .... PP PBBBB .... BBBBP Test and Verify (60 to 90 frames) -- the test runs continuously, each picture should be “verify”
Start Title (operator convenience only)
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Sarnoff Compliance Bitstream Tests • Different picture types • Motion vector range • DC differential range • Macroblock pattern range • Macroblock type range • AC run/level test • Macroblock address range • Dynamic GOP structure • Dynamic slice size • Macroblock & zero stuffing • Postprocessing
– Zone plate – Color bars – Ramp
• Downloadable quant matrices • Different bitrates • Dynamic picture size • 2:3 pulldown insertion • VBV buffer sizes • Mixed MPEG-1 & MPEG-2 • Alternate scan pattern • Alternate AC run/level VLC • Frame/field coding modes • Frame/field prediction modes • Non-linear quantization scale • Pan and Scan
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Summary
• Modern television systems are much more than “DTV” – Cameras and displays will continue to be analog – Program production uses analog and full bandwidth digital
• MPEG-2 will be the dominant compression method – Entertainment quality video to the home
– Satellite, Terrestrial, Cable – Digital Versatile Disk (DVD) – Video servers – Contribution quality, reusable video for studios
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Summary
• MPEG only specifies bitstream syntax and decoding – A non-symmetrical system
– Complex encoders, Simple (inexpensive) decoders – Encoding can improve and be compatible with today’s decoders
– Increasing compute power for complex algorithms – Manufacturers can compete with proprietary algorithms
• 4:2:2 profile provides contribution quality – Decoders must be compliant for flexibility of application
• MPEG-2 System for multi-program transmission
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Summary
• System testing requires a layered approach – Signal/Picture quality – MPEG-2 system protocol – Transmission channel
• Signal quality testing uses traditional methods – Indirect measurement, Static test signals, vertical interval
• Picture quality testing is a complete new paradigm – Dynamic, complex test scenes utilize the complete channel – Reliable objective measurements use human visual model – Picture differencing methods provide best results
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Summary
• MPEG-2 system protocol testing – Generation of valid (and invalid) test signals is important – In-depth testing provides complete data analysis off-line – Real-time testing provides limited analysis of all packets
• Tektronix equipment for video and protocol testing – TSG422 Digital Video Test Signal Generator – SPG422 Digital Video Sync Pulse Generator – TG2000 Multi-format Test Signal Generator – 1700 series Composite/Component Waveform Monitors – WFM600 Series Digital Video Monitors – VM700T Video Signal Measurement Set – MTS100 MPEG Transport Stream Analyzer/Generator
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