Transcript Digital Television Talk v3

University of Canberra
Advanced Communications Topics
Television Broadcasting
into the Digital Era
1
Lecture 4
Error Correction,
DTTB Planning &
System Information
by: Neil Pickford
64-QAM - Perfect & Failure
2
COFDM DTTB Block Diagram
Error Correction
3
Forward Error Correction (FEC)

Broadcast transmission
 One
way process - Tx to Rx
 Not possible to repeat any errored data
Forward Error Correction is a technique used to
improve the accuracy of data transmission
 Extra redundant bits are added to the data stream
 Error correction algorithms in the demodulator
use the extra FEC bits to correct data errors
 C OFDM uses a Convolutional FEC code

N bits
4
Encode
N+Code
Tx/Rx
Decode
N+Code+Error
N bits
Convolutional Coder
1111001
Data
Input
6
1-Bit
Delay
1-Bit
Delay
5
1-Bit
Delay
4
1-Bit
Delay
3
X Output
1-Bit
Delay
2
1-Bit
Delay
1
0
Y Output
1011011
5
Puncturing Codes (FEC)

6
The X and Y outputs of the Convolutional coder
are selected in a Puncturing pattern
Inner Coding
Convolutional coder generates the X & Y codes
 Puncturing operation selects X & Y in sequence
 Result then scrambled with an interleaver

X
Data
7
Convolutional
Y Puncturing
Encoder
Interleaver
Coded
Data
Viterbi Decoder
A special type of data decoder designed to work
with convolutional FEC codes
 Uses the past history of the data to identify valid
future data values
 Element in the Receiver Only

8
Reed Solomon (RS)
RS is a Block data correcting Code
 Hamming type cyclic Polynomial sequence

 Code
Generator Polynomial:
g(x) = (x+l0)(x+l1)(x+l2)...(x+l15), l=02 Hex
 Field Generator Polynomial:
p(x) = x8 + x4 + x3 + x2 + 1
Has special ability to correct multiple bursts of
errors in a code block
 DVB-T uses 204 bytes for each 188 byte Packet

(ATSC uses 207 bytes for each 187 byte Packet)

9
Can correct 8 bytes in each 204 byte packet
Error Protection - Order
188
Bytes
Data
Input
204
Bytes
10
204
Bytes
Outer Code
RS
(204,188)
Inner Code
FEC
(2/3)
Interleaver
Interleaver
306
Bytes
2448
Bits
Error
Mapper
Protected
Data
6 bits x 1512 Carriers
64 QAM
6 bits x 6048 Carriers
DVB-T - Bit Rates [2k]
7 MHz
D/Tu = 1/4
64 us
D/Tu = 1/8
32 us
D/Tu = 1/32
8 us
64 - QPSK
QAM
16 QAM
64 - QPSK
QAM
16 QAM
64 QAM
Code
Rate
QPSK
16 QAM
1/2
4.35
8.71
13.06
4.84
9.68
14.51
5.28
10.56
15.83
2/3
5.81
11.61
17.42
6.45
12.90
19.35
7.04
14.07
21.11
3/4
6.53
13.06
19.59
7.26
14.51
21.77
7.92
15.83
23.75
5/6
7.26
14.51
21.77
8.06
16.13
24.19
8.80
17.59
26.39
7/8
7.62
15.24
22.86
8.47
16.93
25.40
9.24
18.47
27.71
11
Page 21 Table A1 - AS4599-1999
DVB-T - C/N Values
GAUSSIAN
RICEAN
RAYLEIGH
16 -
16 -
16 -
64 -
QAM
QAM
Code
Rate
QPSK
1/2
3.10
2/3
4.90
3/4
QAM
8.80
64 -
QAM QPSK
64 -
QAM
QAM QPSK
14.4
3.60
9.60
14.70
5.40
11.20
16.00
11.1
16.5
5.70
11.60
17.10
8.40
14.20
19.30
5.90
12.5
18.00
6.80
13.00
18.60 10.70
16.70
21.70
5/6
6.90
13.5
19.30
8.00
14.40
20.00
13.10 19.30
25.30
7/8
7.70
13.9
20.10
8.70
15.00
21.00
16.30 22.80
27.90
Simulated Theoretical Thresholds (bandwidth independent)
12
C/N - Signal Level Performance
F ig ure 3.19.1 - C /N T hresho ld vs M inim um R eceiver L evel fo r C O F D M 1/8 G uard Interval & 8-VSB
30
2828
26
2424
2020
C /N T h r e s h o ld ( d B )
C/N Threshold (dB)
22
18
1616
14
64Q A M 7/8 F E C
64Q A M 5/6 F E C
64Q A M 3/4 F E C
64Q A M 2/3 F E C
64Q A M 1/2 F E C
16Q A M 7/8 F E C
16Q A M 5/6 F E C
16Q A M 3/4 F E C
16Q A M 2/3 F E C
16Q A M 1/2 F E C
Q P S K 7/8 F E C
Q P S K 5/6 F E C
Q P S K 3/4 F E C
Q P S K 2/3 F E C
Q P S K 1/2 F E C
8V S B T est R ig
12
12
10
88
6
44
2
00
10
10
13
15
15
20
25
30
35
40
45
R eceiver
S ig n al L evel
(d B u V )
20
25
30
35
40
45
Receiver Signal Level (dBuV)
5050
5555
6060
General Parameters - Aust Tests
Parameter
DVB-T
ATSC
Data Payload
19.35 Mb/s 19.39 Mb/s
Carriers
1705
1
Symbol Time
256 us
93 ns
Time Interleaving
1 Symbol 4 ms
Reed Solomon code rate
188/204
IF Bandwidth (3 dB)
6.67 MHz 5.38 MHz
187/207
19
14
8VSB vs COFDM Latest
15
7 MHz COFDM Modulator Spectrum
00
-10
-10
Power spectrum density (dB)
Power Spectrum Density (dB)
7 MHz Theoretical DVB Transmission signal spectrum
-20
-20
-30
-30
-40
-40
-50
-50
8k 1/32 Guard
2k 1/32 Guard
-60
-8
-7
-6
-5
-4
-3
-2
-1
-8
-7
-6
-5
-4
-3
-2
-1
00 11 22 33
Frequency relative to centre frequency fc (MHz)
Frequency Offset (MHz)
16
44
55
66
77
88
Frequency Planning
 Fundamental
Matter - Scarce Resource
 Analogue Rules set limit to more Services
 No NEW TV Spectrum is Available
 Digital Transmission changes Rules
 Signals
 Digital
have different behaviour
Signals can occupy unused space -
“Taboos”
 Digital
Needs to fit in with Existing PAL
 Eventually Digital Only - but long wait??
17
Digital Has to Fit In With PAL

World TV channel bandwidths vary
 USA /
28
Japan 6 MHz
29
30
 Australian
28
29
 Europeans
28

18
29
31
32
33
34
35
7 MHz
30
31
32
33
34
35
8 MHz
30
31
32
33
Affects:- tuning, filtering, interference
& system performance
34
35
Channel Spacing


Existing analog TV channels are spaced so they do not
interfere with each other.
Gap between PAL TV services
 VHF
1 channel
 UHF 2 channels

Digital TV can make use of these gaps
Ch 6
Taboo
Ch 7
Ch 8
Taboo
VHF Television Spectrum
19
Ch 9
Ch 9A
Taboo
Digital Challenges

Digital TV must co-exist
with existing PAL services
 DTV
operates at lower power
 DTV copes higher interference levels
 Share transmission infra-structure
 DTV needs different planning methods
Ch 6
8-VSB
Ch 7
Ch 8
COFDM
VHF Television Spectrum
20
Ch 9
Ch 9A
DTTB & PAL
21
UHF Channels: London
Photograph by courtesy and © BBC R&D
Planning Issues

Channel Disturbances:




Antenna Pattern?




Static Roof Top? Directional? Wideband?
CCIR Antenna Rec BT-419-3
Portable Receivers? No Antenna?
Frequency Re-Use Distances


23
Noise, at edge of area with NO interference
Interference, Co Channel Interference and Adjacent
Channel Interference
Multipath, Echoes:How Many, How Large, Moving?

Terrain Data
Propagation Models
Protection Ratios
Signal Strength
SIGNAL STRENGTH, MicroVolts
REGION OF SERVICE
FAILURES FOR
PERCENTAGE
OF TIME
MEAN
RECEIVER C/N LIMIT
TIME
24
Digital Service Area Planning

Analog TV has a slow gradual failure
 Existing
PAL service was planned for:
50 % availability at
50 % of locations

Digital TV has a “cliff edge” failure
 Digital
TV needs planning for:
90-99 % availability at
90-99 % of locations
25
TV System Failure Characteristic
Good
Analog
Digital 1
Quality
Edge
of
Service
Area
Rotten
Close
Far
Distance
26
TV System Failure Characteristic
Good
Analog
Digital 1
Quality
Edge
of
Service
Area
Rotten
Close
Far
Distance
27
TV System Failure Characteristic
Good
HDTV
Analog
Digital 1
Digital 2
PAL
Quality
Edge
of
Service
Area
Rotten
Close
Far
Distance
28
SDTV
Service Area Planning
PICTURE
QUALITY
DIGITAL
5
4
Modulation
Dependent
Variation
ANALOGUE
3
THRESHOLD OF
ACCEPTABILITY
Typical
Choice of
C/N
2
1
NO
10
SERVICE
29
15
ANALOGUE
FAILURE
C/N
20
25
30
35
40
45
Service Areas - Current
50 - 100 KILOMETRES
TRANSMITTER A
30
TRANSMITTER B
Service Areas - SFN
50 - 100 KILOMETRES
TRANSMITTER B
31
Digital Provides New Concepts

Single frequency networks (SFNs) can help
solve difficult coverage situations
 SFNs
allow the reuse of a transmission frequency
many times in the same area so long as
exactly the same program is carried
 Allows lower power operation
 Better shaping of coverage
 Improved service availability
 Better spectrum efficiency
32
MPEG Packet
PACKET ST ART CO DE PREF IX
ST REAM
ID
24
8
O PT I O NAL
PES
HEADER
PES
PACKET
LENG T H
16
10
2 11111111111
DAT A BYT ES
BYT ES
OOC
PT SDT SF
ESCRF
ESRF
DSMT MF
ACIF
PESCRCF
PESHDL
PES
HEADER
DAT A
LENG T H
DAI
PESP CY
PSC
2
ST UF F I NG
8
O PT I O NAL
F I ELDS
8
DSM
T RICK
MO DE
PT S
DT S
33
33
ESCR
42
PESSC = Packet Elementary Stream Scrambling Control
PESP = Packet Elementary Stream Priority
DAI = Data Alignment Indicator
CY = Copyright
OOC = Original or Copy
PTSDTSF = PTS & DTS Flags
ESCRF = ESCR Flag
ESRFES = Rate Flag
DSMTMF = Trick Mode Flag
PES
PRIVAT E
ACIF = Additional Copy Info. Flag
DAT A
PESCRCF = PES Extension Flag
PESHDL = PES Header Data Length
PTS = 33
Presentation Time Stamp
DTS = Date Time Stamp
ADDI T I O NAL
CO PY
I NF O
ES
RAT E
22
8
O PT I O NAL
PES
CRC
F I ELDS
16
5 F LAG S
PES
EXT ENSI O N
11111
PACK
HEADER
F I ELD
PRO G RAM
PACKET
SEQ UENCE
CO UNT ER
8
P-ST D
BUF F ER
PES
F I ELD
EXT ENSI O N
System Level Multiplexing Approaches
V id e o
D a ta
V id e o
Encoder
A u d io
D a ta
A u d io
Encoder
34
V id e o P E S
P a c k e t iz e r
PS
M ux
P ro g ra m S t r
P a c k e t iz e r
A u d io P E ST S
M ux
T ra n s p o rt S
Packetisation Approaches
Fixed Length
A u d io
V id e o
A u d io
A u d io
V id e o
V id e o
V id e o
A u d ioA u d io
V id e o
V id e o
A u d ioV id e o
A u d io
A u d io
T ra n s p o rt S t re a m
A u d io
V id e o
A u d io
P ro g ra m S t re a m
Variable Length
35
V id e o
Transport Stream
1 8 8 B yte s
A d a p ta tio n H e a d e r (V a ria b le L e n g th )
4 B yte s
P a ylo a d
n o t t o s c a le
L in k H e a d e r
Link Header Format
s y n c _ b y te
(0 x 4 7 )
1 3 b it P I D
o r A d a p t a t io n h e a d e r
p a c k e t p a y lo a d
1 b it : t ra n s p o rt _ p rio rit y 4 b it : c o n t in u it y _ c o u n t e r
1 b it : p a y lo a d _ u n it _ s t a rt _2in bd itic: aa tdo ar p t a t io n _ f ie ld _ c o n t ro l
1 b it : t ra n s p o rt _ p a c k e t _ e rro r_ in
2 db ic
it :a ttra
o rn s p o rt _ s c ra m b lin g _ c o n t ro l
36
System Information (SI)

Required for :
Automatic Tuning of receiver upon selection

Program location

EPG (Electronic Program Guide)

API (Application Programming Interface)

CA (Conditional Access)

37
DVB SI Model
Networks
Satellite
Terrestrial
Cable
Transport Streams
Transponder
1
Transponder
2
Transponder
3
Services
Service
1
Transponder
T
Channel
1
Channel
C
Channel
1
Channel
2
Service
1
Service
2
Channel
C
Bouquet
Service
2
Service
3
Service
S
Audio 1
Audio 2
Service
1
Components
Video
38
Channel
2
Data
Service
2
Service
S
Service
S
System Information
The DVB SI structure has it’s derivation in
MPEG ISO/IEC 13818-1 and is defined in a
set of tables.
 The primary link between DVB SI and MPEG
is the“PSI” (Program Specific Information)
in MPEG and is contained primarily in the
“PAT”, “PMT” and “CAT” set of tables

39
What is SI?
 SI
data provides information for:
Automatic
tuning to transport stream
User Information for:
Service selection
 “Event” selection
 “Component” selection

 PSI
data provides information for:
Configuration
of decoder for selected Service
DVB extensions for non-MPEG components
40
PSI and DVB SI Tables
DVB
OPTIONAL
MPEG
DVB
MANDATORY
PID=0x0000
PID=P
PID=Ox0001
PID=0X0002
41
PAT
NIT
PID=0x0010
OTHER
Delivery Sys.
ACTUAL
Delivery Sys.
PMT
CAT
TSDT
NIT
PID=0x0010
BAT
PID=Ox0011
SDT
PID=Ox0011
ACTUAL
TS
EIT
PCR
TDT
PROGRAMME
CLOCK REF.
TIME AND
DATE.
PID=0X0012
SERVICE
DESCRIPTION.
OTHER
TS
EIT
EIT
ACTUAL
TS
OTHER
TS
PID=0X0012
ACTUAL
TS
BOUQUET
INFORMATION.
SDT
PID=Ox0011
NETWORK
INFORMATION.
RST
TOT
RUNNING
STATUS.
TIME
OFFSET.
EVENT
INFORMATION.
ST
STUFFING
TABLE.
MPEG Program PIDs

What is a program ?
 MPEG
has a definition which is different to that
normally understood.
 A “program” in broadcasting is a collection of
elements with a common time base and the same
start and stop times.
 A program in MPEG is a collection of elements
with a common time base only. That is a
collection of elementary streams with same
PCR_PID and referenced to the same
program_number
42
Virtual Channels & PCR Timing
 A conventional
Broadcaster of a TV channel or
service having one program would be composed of a
series of “broadcaster programs” or events with the
same program_number and a common PCR_PID.
 In other words the PCR time base effectively creates a
virtual channel which may be associated with a single
or multiple program_numbers.
 A TV channel having multiple programs would have
multiple program_numbers with either single or
multiple PCR_PID between program streams.
NOTE : Services with different program_numbers may draw upon the same video as
with the case of multilingual services.
43
Decoding the Program

Decoding the correct program (ie “channel”) ?
 Where
there are several Transport Streams available
to a decoder, in order to successfully demultiplex a
program, the decoder must be notified of both the
transport_stream_id (to find the correct multiplex) and
the program_number of the service (to find the correct
program within the multiplex).
Note again the program here refers to the channel not
the event or actual broadcast program.

44
Now to the various main table purposes :
PAT, PMT & CAT Tables

PAT (Program Association Table)
 provides
the link between the transport_stream_id, the
program_number and the program_map_id (PMT).

PMT (Program Map Table)
 when
pointed to from the PAT, the PMT provides the
associated group of elements (video, audio etc) with
the program_number.

CAT (Conditional Access Table)
 provides
the association between CA system(s) and
their EMM (Entitlement Management Messages)
streams and any special parameters associated with
them.
45
DVB SI Features
Data structured as several “Tables”
 Structures use “fixed format” for essential data,
and descriptors for optional or variable-length
data (similar to PSI)

Efficient data transmission
 Extensible while maintaining compatibility
 Support for “private” extensions


Can provide standard EPG data-stream

“Look and Feel” determined by receiver software
• Resident or Downloaded
46
SI Features: NIT

Network Information Table
 Identification
of transmission as a member of a group
of multiplexes - “Network”
 Network Name
 Tuning parameters with support for various delivery
media

List of additional frequencies for terrestrial transmission
 Designed
47
for simple transcoding of transport streams
SI Features: SDT & BAT

Service Description Table
 Identifies
all Service names and Service types in TS
 Linked Services
 Pointer to MPEG Program in PSI


Service_id = MPEG Program Number
Bouquet Association Table (Optional)
 Groupings
of Services
 May convey “logical channel number”
48
SI Features: EIT

Present/Following


Schedule (optional)


Information on current and next events
Up to 64 days ahead - ordered by service and time
Event Information
Title, short description
 Start time & duration
 Content classification & parental rating
 Longer text description
 Information on components

49
SI Features: TDT,TOT, RST

Time and Date Table


Time Offset Table (optional)


Transmission of time offset by zone - both current, and next
offset values, with date at which next occurs
Running Status Table (optional)

50
Transmission of current time for automatic setting of
receiver clock
Mechanism for signalling status transitions with greater
timing precision
Electronic Program Guide - EPG

EPG (Electronic Program Guide)
 Combining
primarily the EIT and the SDT, both the
time and description is provided to the viewer via
some form of EPG ranging from “vanilla EPG’s”,
simple “eye plate style” displays to full blown EPG’s,
either from Receiver manufacturers designs or
downloaded EPG’s with GUI interfaces designed by
the Networks.
 This information so constructed and displayed can be
used to provide a Parental Guidance lock function
through PIN number access.
51
Example EPG using DVB SI
52
Example: Event Details
53
Example: Selection by Genre
54
Example: Selection by Genre
55
Application Program Interface API

API (Application Programming Interface)
 some
form of API must be used to allow the control by
the viewer or installer of the decoder / receiver. The
API software provides the connection between the
applications (eg. EPG) and the hardware.
 Some API’s
may employ MHEG-5 multimedia support
and Java programming language for EPG generation.
 CA in
56
practice is reliant upon EPG’s and the API’s.
DVB - Conditional Access

CA (Conditional Access)
 Access
to the EMM (Entitlement Management
Message) is provided by the CAT.
 The
EMM allows a single decoder to view the
program material which is scrambled via a DVB
“common scrambling algorithm” by providing the key
to the code word which is involved in the scrambling.
The code word is sent via the
ECM (Entitlement Control Message).
57
Typical Conditional Access System
58
A Future Digital System Concept
MMDS
Satellite
Hypermedia
Integrated Receiver
Decoder (IRD)
Terrestrial
Cable
Broadcast
Interactivity
B-ISDN
XDSL
59
CD, DVD
DVC
DTTB Systems Doppler
Performance Limits
for current
implementations
300
250
DOPPLER
SHIFT
(Hz)
UHF
200
VHF - Band III
COFDM 2K, 3dB degrade
140
COFDM 2K
100
50
0
ATSC
see separate curves
0
100
300
400
500
600
700
800
900
SPEED (Km/Hr)
Vehicles
60
200
AIRCRAFT
Over Cities
COFDM implementations will inherently handle post and
pre-ghosts equally within the selected guard interval.
1000
Main Results - Lab Tests
C/N ATSC 4 dB better than DVB-T.
This Advantage offset by Poor Noise Figure
 DVB-T is better than ATSC for Multipath
 ATSC is better than DVB-T for Impulse Noise
 ATSC cannot handle Flutter or Doppler Echoes
 ATSC is very sensitive to Transmission system
impairments and IF translation
 DVB-T is better at handling Co-channel PAL
 DVB-T is better rejecting on channel
13
interference (CW)

61