University of Canberra
Advanced Communications Topics
Television Broadcasting
into the Digital Era
1
Lecture 4
DTTB Types
Digital
Modulation Systems
by: Neil Pickford
Digital Terrestrial TV - Layers
. . . provide clean interface points. . . .
1920 x 1080
1280 x 720
50,25, 24 Hz
Picture
Layer
Video
Compression
Layer
Data
Headers
Motion
Vectors
Multiple Picture Formats
and Frame Rates
MPEG-2
compression
syntax
ML@MP
or
HL@MP
Chroma and Luma
DCT Coefficients
Variable Length Codes
Flexible delivery of data
Packet Headers
Transport
Layer
Transmission
Layer
2
Video packet
Audio packet
Video packet
VHF/UHF TV Channel
7 MHz
Aux data
MPEG-2
packets
COFDM / 8-VSB
Digital Television Encode Layers
Control
Data
Video
Picture
Coding
Control Data
MPEG-2
Data
Data
Coding
Sound
Audio
Coding
MPEG Transport
Stream Mux
Program 1 Multiplexer
Program 2
Other Data
Control Data
MPEG-2
or AC-3
Program 3
Service
Mux
Bouquet Multiplexer
MPEG Transport Data Stream 188 byte packets
Control Data
Modulator & Transmitter
Delivery
3
System
Error
Protection
Digital Television Decode Layers
MPEG-2
Transport
Stream
Mon
Data
Picture
Decoder
Data
Decoder
MPEG Transport Stream
De-Multiplexer
Demodulator & Receiver
Delivery
4
System
Speakers
Audio
Decoder
MPEG
DeMux
Error
Control
MPEG
or AC-3
Set top Box (STB) - Interfacing
Domestic and Professional interfaces
still to be defined
 Most probably Transport Stream via IEEE 1394
(Firewire)
 Baseband Audio & RGB/YUV Video signals.
 STB can convert between line standards
so you do not have to have a HD display.
 Display and transmitted information must be at
same Frame/Field rate. (25/50)

5
DTTB - Content & Services
DTTB was designed to carry video, audio and
program data for television
 DTTB can carry much more than just TV

 Electronic
program guide, teletext
 Broadband multimedia data, news, weather
 Best of internet service
 Interactive services
 Software updates, games

6
Services can be dynamically reconfigured
DVB Data Containers

MPEG Transport Stream is used to provide DVB
“data containers” which may contain a flexible
mixture of:
 Video
 Audio
 Data

services
Streams with variable data rate requirements can
be Statistically Multiplexed together.
 Allows
channel
7
Six 2 Mb/s programs to be placed in a 8 Mb/s
Examples of DVB Data Containers
Channel bandwidth can be used in different ways:
SDTV 1
SDTV 2
SDTV 3
HDTV 1
SDTV 4
SDTV 5
Multiple
SDTV
programs
8
HDTV 1
SDTV 1
Single
HDTV
program
Simulcast
HDTV &
SDTV
Video Program Capacity
For a payload of around 19 Mb/s
1
HDTV service - sport & high action
 2 HDTV services - both film material
 1 HDTV + 1 or 2 SDTV non action/sport
 3 SDTV for high action & sport video
 6 SDTV for film, news & soap operas
However you do not get more for nothing.
 More services means less quality
9
Spare Data Capacity



10
Spare data capacity is
available even on a fully
loaded channel.
Opportunistic use of
spare data capacity when
available can provide
other non real time data
services.
Example: 51 second
BMW commercial
The Commercial was
shown using 1080 Lines
Interlaced. 60 Mb of data
was transferred during it.
In the Final 3 seconds the
BMW Logo was displayed
allowing 3 Phone Books
of data to be transmitted.
Enabling Technologies
 Source
digitisation (Rec 601 digital studio)
 Compression technology (MPEG, AC-3)
 Data multiplexing (MPEG)
 Transmission technology (modulation)
 Display technology (large wide screens)
 Production
11
Digital Television - Types

Terrestrial (DTTB)
 DVB-T
/ 8-VSB
 Free to air TV (broadcasting)
 Narrowcasting/value added services
 Untethered - portable reception
12
Digital Terrestrial Television
Broadcasting - DTTB
Regional free to air television
 Replacement of current analog PAL broadcast
television services
 Operating in adjacent unused “taboo” channels
to analog PAL service
 Carries a range of services
HDTV, SDTV, audio, teletext, data
 Providing an un-tethered portable service

13
Transmission Technology
The transmission system is used to transport the
information to the consumer.
 The system protects the information being carried
from the transmission environment
 Current Australian analog television uses the
PAL-B AM modulation system

14
Digital TV Transmission Technology
The transmission system is a
“data pipe”
 Transports data rates
of around 20 Mb/s
 Transports data in
individual containers
called packets

4
15
Digital TV Transmission Systems
Australia has been following Digital TV & HDTV
 Europeans
- Digital SDTV
- 8 MHz on UHF
- DVB-T (COFDM)
 Americans
- Digital HDTV
- 6 MHz VHF/UHF
- ATSC (8-VSB)
 Japanese
- Integrated Broadcasting
- ISDB (BST-OFDM)
16
8-VSB - USA


Developed by the advance television systems committee
- ATSC
Developed for use in a 6 MHz channel
A7





17
MHz variant is possible but has not been produced.
Uses a single carrier with pilot tone
8 level amplitude modulation system
Single Payload data rate of 19.39 Mb/s
Relies on adaptive equalisation
Existing AM technology highly developed
COFDM - Europe





Developed by the digital video
broadcasting project group - DVB
Uses similar technology to DRB
Uses 1705 or 6817 carriers
Variable carrier modulation types are defined allowing
Payload data rates of 5-27 Mb/s in 7 MHz
Developed for 8 MHz channels
A7


18
& 6 MHz variants have been produced and tested.
Can use single frequency networks - SFNs
New technology with scope for continued
improvement & development
ISDB - Japan
Japanese are developing integrated services
digital broadcasting (ISDB)
 System integrates all forms of broadcasting
services into one common data channel which
can be passed by satellite, cable or terrestrial
delivery systems
 Video services

 Sound
services
 Bulk data services
 Interactive data services
19
ISDB - Concept

20
Proposed to use band segmented transmission orthogonal frequency division multiplex
(BST-OFDM)
Terrestrial Transmission Problems
 Multipath
interference - ghosts
 Noise interference - snow
 Variable path attenuation - fading
 Interference to existing services
 Interference from other services
 Channel frequency assignment where to place the signal
21
Digital Modulation - Functions
 Spreads
the data evenly across the channel
 Distributes the data in time
 Maintains synchronisation well below data
threshold
 Employs sophisticated error correction.
 Equalises the channel for best performance
22
Digital Modulation
Two techniques:
Conventional Single Carrier

8VSB
Multicarrier/Spread

OFDM
Spectrum
8-VSB & COFDM - Spectrum
8-VSB
COFDM
24
Sin(x)/x
25
Digital Modulation
Spectrum of Conventional
Multi-Phase Keyed Carrier Fc
at Symbol Rate Fs
Amplitude, dB
Sin X/X shaping
Frequency
Fc - Fs
Fc
Fc + Fs
Digital Modulation
Low Symbol Rate
Amplitude, dB
Medium Symbol Rate
High Symbol Rate
Frequency
PSK
Digital Information
Phase Shift Keying Modulation
28
BPSK Modulation
0
1
180 Deg
Phase
Change
29
I AXIS
QPSK Modulation
Q AXIS
10
11
QPSK
Distance
00
30
I AXIS
01
16QAM Modulation
Q AXIS
16-QAM
Distance
1110
10
1111
11
1100
1101
I AXIS
0010
0011
01
00
0000
31
0001
8VSB Modulation
Q AXIS
16-QAM
Distance
1110
10
1111
11
1100
1101
I AXIS
0010
0011
01
00
0000
32
0001
Hierarchical Modulation
Hierarchical
Distance
Q AXIS
1110
10
1111
11
1100
1101
QPSK
Distance
0010
I AXIS
0011
01
00
0000
33
0001
Digital Modulation
Amplitude
Typical Filtered
Spectrum to give
about half original
bandwidth
Occupied Channel
Bandwidth
Frequency
Fc - Fs
Fc
Fc + Fs
8-VSB Digital Modulation
Application of Vestigial
Sideband Filter to give
reduced spectral
occupancy BUT with
destruction of pure
Amplitude modulation
causing incidental Phase
modulation but some power
in a small carrier
Amplitude
Occupied Channel
Bandwidth
eg 6 MHz in US
Frequency
Fc - Fs
Fc
Fc + Fs
8VSB uses symbol Rate with period 93 nanoseconds
Normal FDM
Guard
Band
Carrier 1
Amplitude, dB
Carrier 2
Frequency
Traditional SCPC Modulation
Minimum
Carrier Spacing
Frequency
37
Orthogonal Modulation
Amplitude, dB
Frequency
Orthogonal Modulation
Amplitude, dB
Frequency
COFDM - Orthogonal Carriers
Frequency
40
Spectrum of COFDM DTTB
Carrier Spacing
2k Mode 3.91 kHz
8k Mode 0.98 kHz
Almost
Rectangular
Shape
1705 or 6817 Carriers
6.67 MHz in 7 MHz Channel
41
OFDM
Occupied bandwidth is: No. of
Carriers x Spectral Width.
Create with FFT
Spectral Width
2k is 4x wider than 8k
Amplitude, dB
Fcentre
Frequency
DIGITAL TERRESTRIAL
BROADCASTING
Among the four Digital Broadcasting standards available, three are based on
the Coded Orthogonal Frequency Division Multiplex modulation.... Why ?
Distant transmitter
Nearest transmitter
The Terrestrial Broadcasting has to cope with multipath propagation and Doppler
effects: COFDM is the response for these impairments !
COFDM : HOW ?

1 - Organize time & frequency partitions in the RF channel
time
RF
Channel
bandwidth
frequency
sub-band
time
segment
frequency
44
COFDM : HOW ?

2 - Spread sub-carriers over “time vs frequency” cells
time
OFDM
symbol
Make sub-carriers orthogonal to avoid “inter-carriers” interference
frequency
45
COFDM : HOW ?

3 - Insert Guard Interval to avoid “inter-symbol” interference
Guard Interval
duration
Useful symbol
duration
time
OFDM
symbol
frequency
Guard interval introduces a first loss in transport capacity
46
COFDM : HOW ?

4 - Insert “Synchronization Pilots”
Helps Receivers to lock onto the signal
OFDM Frame
(68 OFDM symbols)
time
frequency
47
FFT
time windows
for receivers
Synchronization markers introduce the second loss
in transport capacity
COFDM : HOW ?

5 - Prepare data to be carried on OFDM symbols
DATA to broadcast
Protected DATA (convolutionnal error protection codes)
time
frequency
48
Protection codes introduce the third loss in transport capacity
COFDM : HOW ?

6 - Map bits onto OFDM:
Spread contiguous data bits over distant sub-carriers
DATA to broadcast
Protected DATA
0
1
0
0
1
time
frequency
49
Create frequency diversity to improve robustness against fading
DTTB - Channel Estimation
The Terrestrial transmission channel is
continuously varying (position & time)
 Variations occur in Amplitude, Phase &
Frequency
 To correct for this variation Information needs
to be added to the transmission to quantify the
channels response at any instant
 Equalisers in the Digital receiver use this
information to remove these transmission
impairments

50
Data Multiplex - 8-VSB
4
828 Symbols
Field Sync #1
313
Segments
S
e
g
m
e
n
t
S
y
n
c
313
Segments
Data + FEC
Field Sync #2
Data + FEC
Test Segment
51
24.2
ms
1 Segment
= 77.3 us
24.2
ms
8-VSB Segment Sync & Data
+7
+5
+3
+1
-1
-3
-5
-7
Levels Before
Pilot Addition
(Pilot=1.25)
Data
Segment
SYNC
Data + FEC
Data
Segment
SYNC
4
Symbols
828 Symbols
207 Bytes
4
Symbols
Data Segment
832 Symbols
208 Bytes
Symbol Duration 93 ns
52
Digital Modulation - 8-AM
7
6
5
4
Before Equaliser
3
2
1
0
After Equaliser
8-VSB - Coaxial Direct Feed through Tuner on Channel 8 VHF
3 Bits/Symbol
53
8-VSB - Field Sync
Precode*
832 Symbols
+7
+5
+3
+1
-1
-3
-5
-7
PN511
PN63 PN63 PN63 VSB
Mode
Reserved
Sync
Levels Before
Pilot Addition
(Pilot=1.25)
4
Symbols
511
Symbols
63
63
24
63
Sym- Sym- Sym- Symbols bols bols bols
104
Symbols
* For trellis coded terrestrial 8 VSB the last 12 symbols of the previous segment
are duplicated in the last 12 reserved symbols of the field sync.
Field Sync is Repeated Every 24 ms
54
12
Symbols
8-VSB - Field Sync
7
6
5
4
Before Equaliser
55
3
2
1
0
After Equaliser
8-VSB Frame
832
Training Sequence
Symbols
per Data
Segment S
of 77.3 µs
y
n
c
Data
313 Data
Segments
of 24.2 ms
Training Sequence
Data
The Training Sequence is only 0.3% of signal time
Total Sync is only 0.8% of time
DVB-T - Carriers + Pilots
0.977/3.906 kHz
Kmin
Kmax = 1704 for 2K or 6816 for 8K
Kmax
SYMBOLS IN SEQUENCE - 68 PER BLOCK.
57
Modulated Carriers
Scattered Pilots
DVB-T Super Frame
34
50
TPS Carriers
17/68
Super
1512:
Frame
Frame 1
4 x 68
Frame 2
in 2k
Frame 3
6048:
1705
Symbols
=
6817
N MPEG
Frame 4
in 8k
Packets
48
58
54
Fixed Pilots
Co- Incident with Scattered pilots
45/177
DVB-T Transmission Frame
Kmin=0
59
2k mode - 3906 Hz - Kmax=1704
Carrier Spacing & Position 8k mode - 977 Hz - Kmax=6816 Kmax
Data
TPS - Pilot
Scattered Pilot
Continuous Pilot
Symbol Duration 256 us (2k) or 1024 us (8k)
DVB-T - Estimating the Channel
A
B
C
D
E
A
B=3/4A+1/4E
C=1/2(A+E)
D=1/4A+3/4E
E
A to E - 1.024 ms (2k)
- 4.096 ms (8k)
For a varying transmission channel
DVB-T estimation is 23.5 times faster than ATSC
60
DVB-T - Estimating the Channel
AB C DE FG
AB C DE FG
B=2/3A+1/3D
C=1/3A+2/3D
E=2/3D+1/3G
F=1/3D+2/3G
A to D - 11.724 kHz (2k)
- 2.931 kHz (8k)
For a varying transmission channel
DVB-T estimation is 23.5 times faster than ATSC
61
Channel Estimation &
Equalisation
ATSC
Time
DVB-T
Time
62
Estimation Rate Comparison
ATSC Equaliser is updated every 24 ms
(~260 000 symbols)
 DVB-T Equaliser is updated every symbol period
(256 us). 1/12 of data carriers are pilots
 DVB-T Full Channel estimate is available every 4
symbols (1.024 ms)
 For a varying transmission channel DVB-T
estimation is at least 23.5 times faster than ATSC

63
OFDM - Features




Multicarrier - many carriers sharing
Reduced C/N compared to Analogue
Resistant to echoes, Interference etc
Low symbol rate per carrier



~ 1 kBaud: Long Symbol Period, can Extend with Guard
Interval
With FEC becomes COFDM
Uses Fast Fourier Transform [FFT]
 ”2k” and “8k” versions

64
Single Frequency Networks [SFN]