Digital Television Talk Lecture 4 - Happy Emu Id Au

actuallyabandonedElectronics - Devices

Nov 15, 2013 (3 years and 10 months ago)

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1

University of Canberra

Advanced Communications Topics

Television Broadcasting
into the Digital Era

by: Neil Pickford

Lecture 4

DTTB Types

Digital

Modulation Systems

2

Digital Terrestrial TV
-

Layers

. . . provide clean interface points. . . .

Picture

Layer

Multiple Picture Formats

and Frame Rates

1920 x 1080

1280 x 720

50,25, 24 Hz

Transmission

Layer

7 MHz

COFDM / 8
-
VSB

VHF/UHF TV Channel

Video
Compression

Layer

MPEG
-
2

compression
syntax
ML@MP

or

HL@MP

Data

Headers

Motion

Vectors

Chroma and Luma

DCT Coefficients

Variable Length Codes

Transport

Layer

MPEG
-
2

packets

Video packet

Video packet

Audio packet


Aux data

Packet Headers

Flexible delivery of data

3

Digital Television Encode Layers

Delivery System

Bouquet Multiplexer

Program 2

Program 3

Service

Mux

Other Data

Control Data

Program 1 Multiplexer

MPEG Transport

Stream Mux

Control Data

Picture

Coding

Audio

Coding

Data

Coding

MPEG
-
2

or AC
-
3

MPEG
-
2

Control

Data

Video

Data

Sound

Modulator & Transmitter

Error
Protection

Control Data

188 byte packets

MPEG Transport Data Stream

4

Digital Television Decode Layers

Audio

Decoder

Data

Decoder

Picture

Decoder

MPEG

or AC
-
3

MPEG
-
2

Demodulator & Receiver

Error
Control

Delivery System

Data

Mon

Speakers

MPEG Transport Stream

De
-
Multiplexer

MPEG
DeMux

Transport

Stream

5

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)

6

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


Services can be dynamically reconfigured

7

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 Six 2 Mb/s programs to be placed in a 8 Mb/s
channel


8

Examples of DVB Data Containers

Single
HDTV
program

HDTV 1

SDTV 1

SDTV 2

SDTV 3

SDTV 4

SDTV 5

Multiple
SDTV
programs

SDTV 1


HDTV 1

Simulcast
HDTV &
SDTV

Channel bandwidth can be used in different ways:

9

Video Program Capacity


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

For a payload of around 19 Mb/s

10

Spare Data Capacity


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.

11

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

12

Digital Television
-

Types


Terrestrial (DTTB)


DVB
-
T / 8
-
VSB


Free to air TV (broadcasting)


Narrowcasting/value added services


Untethered
-

portable reception

13

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

14

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

15

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

16

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)

17

8
-
VSB
-

USA



Developed by the advance television systems committee
-

ATSC


Developed for use in a 6 MHz channel


A 7 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

18

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


A 7 & 6 MHz variants have been produced and tested.


Can use single frequency networks
-

SFNs


New technology with scope for continued
improvement & development

19

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

20

ISDB
-

Concept


Proposed to use band segmented transmission
-

orthogonal frequency division multiplex

(BST
-
OFDM)

21

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

22

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

Digital Modulation

Two techniques:


Conventional Single Carrier



8VSB


Multicarrier/Spread Spectrum



OFDM


24

8
-
VSB & COFDM
-

Spectrum

8
-
VSB

COFDM

25

Sin(x)/x

Digital Modulation

Fc

Fc
-

Fs

Fc + Fs

Sin X/X shaping

Spectrum of Conventional
Multi
-
Phase Keyed Carrier Fc
at Symbol Rate Fs

Frequency

Amplitude, dB

Digital Modulation

Frequency

Amplitude, dB

Low Symbol Rate

Medium Symbol Rate

High Symbol Rate

28

PSK


Digital Information
Phase Shift Keying Modulation
29

BPSK Modulation

I AXIS

0

1

180 Deg

Phase

Change

30

QPSK Modulation

I AXIS

Q AXIS

QPSK

Distance

0 0

1 1

0 1

1 0

31

16QAM Modulation

I AXIS

Q AXIS

16
-
QAM

Distance

1 1 0 0

1 1 0 1

1 1 1 0

0 0

1 1

0 1

1 0

1 1 1 1

0 0 0 0

0 0 1 0

0 0 1 1

0 0 0 1

32

8VSB Modulation

I AXIS

Q AXIS

16
-
QAM

Distance

1 1 0 0

1 1 0 1

1 1 1 0

0 0

1 1

0 1

1 0

1 1 1 1

0 0 0 0

0 0 1 0

0 0 1 1

0 0 0 1

33

1 1

0 0

Hierarchical Modulation

I AXIS

Q AXIS


QPSK

Distance

Hierarchical

Distance

1 1 0 0

1 1 0 1

1 1 1 0

1 1 1 1

0 0 0 0

0 0 1 1

0 0 0 1

0 1

1 0

0 0 1 0

Digital Modulation

Frequency

Amplitude

Fc

Fc
-

Fs

Fc + Fs

Occupied Channel
Bandwidth

Typical Filtered
Spectrum to give
about half original
bandwidth

8
-
VSB Digital Modulation

Frequency

Amplitude

Fc

Fc
-

Fs

Fc + Fs

Occupied Channel
Bandwidth


eg 6 MHz in US

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

8VSB uses symbol Rate with period 93 nanoseconds

Normal FDM

Amplitude, dB

Frequency

Carrier 1

Carrier 2

Guard
Band

37

Traditional SCPC Modulation

Frequency

Minimum

Carrier Spacing

Frequency

Amplitude, dB

Orthogonal Modulation

Frequency

Amplitude, dB

Orthogonal Modulation

40

COFDM
-

Orthogonal Carriers

Frequency

41

Spectrum of COFDM DTTB

6.67 MHz in 7 MHz Channel

Almost

Rectangular

Shape

1705 or 6817 Carriers

Carrier Spacing

2k Mode 3.91 kHz

8k Mode 0.98 kHz


OFDM

Amplitude, dB

Frequency

Frequency

Fcentre

Occupied bandwidth is: No. of
Carriers x Spectral Width.
Create with FFT

Spectral Width

2k is 4x wider than 8k

DIGITAL TERRESTRIAL
BROADCASTING

The Terrestrial Broadcasting has to cope with multipath propagation and Doppler
effects: COFDM is the response for these impairments !

Distant transmitter

Nearest transmitter

Among the four Digital Broadcasting standards available, three are based on
the Coded Orthogonal Frequency Division Multiplex modulation.... Why ?

44

COFDM : HOW ?


1
-

Organize time & frequency partitions in the RF channel

time

frequency

RF

Channel

bandwidth

frequency

sub
-
band

time

segment

45

Make sub
-
carriers orthogonal to avoid “inter
-
carriers” interference

COFDM : HOW ?


2
-

Spread sub
-
carriers over “time vs frequency” cells

time

frequency

OFDM

symbol

46

COFDM : HOW ?


3
-

Insert Guard Interval to avoid “inter
-
symbol” interference

time

frequency

Useful symbol

duration

Guard Interval

duration

OFDM

symbol

Guard interval introduces a first loss in transport capacity

47

COFDM : HOW ?


4
-

Insert “Synchronization Pilots”


Helps Receivers to lock onto the signal

FFT

time windows

for receivers

OFDM Frame

(68 OFDM symbols)

time

frequency

Synchronization markers introduce the second loss
in transport capacity

48

COFDM : HOW ?


5
-

Prepare data to be carried on OFDM symbols

time

frequency

DATA to broadcast

Protected DATA (convolutionnal error protection codes)

Protection codes introduce the third loss in transport capacity

49

COFDM : HOW ?


6
-

Map bits onto OFDM:


Spread contiguous data bits over distant sub
-
carriers

time

frequency

DATA to broadcast

Protected DATA


0


1


0


0

1

Create frequency diversity to improve robustness against fading

50

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

51

Data Multiplex
-

8
-
VSB

4
Field Sync #1
Field Sync #2
Test Segment
S
e
g
m
e
n
t
S
y
n
c
828 Symbols
313
Segments
Data + FEC
1 Segment
= 77.3 us
Data + FEC
313
Segments
24.2
ms
24.2
ms
52

8
-
VSB Segment Sync & Data

+7
+5
+3
+1
-1
-3
-5
-7
Levels Before
Pilot Addition
(Pilot=1.25)
828 Symbols
207 Bytes
4
Symbols
Data + FEC
Data
Segment
SYNC
Data Segment
832 Symbols
208 Bytes
4
Symbols
Data
Segment
SYNC
Symbol Duration 93 ns

53

Digital Modulation
-

8
-
AM

3 Bits/Symbol

7

1

2

3

4

5

6

0

8
-
VSB
-

Coaxial Direct Feed through Tuner on Channel 8 VHF

Before Equaliser

After Equaliser

54

8
-
VSB
-

Field Sync

+7
+5
+3
+1
-1
-3
-5
-7
Levels Before
Pilot Addition
(Pilot=1.25)
832 Symbols
4
Symbols
511
Symbols
63
Sym-
bols
63
Sym-
bols
24
Sym-
bols
63
Sym-
bols
104
Symbols
PN511
PN63
PN63
PN63
VSB
Mode
Reserved
12
Sym-
bols
* 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.
Sync
Precode*
Field Sync is Repeated Every 24 ms

55

8
-
VSB
-

Field Sync

7

1

2

3

4

5

6

0

Before Equaliser

After Equaliser

8
-
VSB Frame

832

Symbols
per Data
Segment
of 77.3 µs

Data

Data

Training Sequence

Training Sequence

S
y
n
c

The Training Sequence is only 0.3% of signal time

Total Sync is only 0.8% of time

313 Data

Segments

of 24.2 ms

57

DVB
-
T
-

Carriers + Pilots

Kmin

SYMBOLS IN SEQUENCE
-

68 PER BLOCK.

Kmax

1704 for 2K or

6816 for 8K

Kmax =

Modulated Carriers

0.977/3.906 kHz

Scattered Pilots

58

DVB
-
T Super Frame

Super

Frame

4 x 68

Symbols

=

N MPEG

Packets

45/177

TPS Carriers

Fixed Pilots

Frame 1

Frame 2

Frame 3

Frame 4

54

48

34

50

17/68

1512:

1705

in 2k

6048:

6817

in 8k

Co
-

Incident with Scattered pilots

59

DVB
-
T Transmission Frame

Data

Scattered Pilot

TPS
-

Pilot

Continuous Pilot

Kmin=0

Kmax

Carrier Spacing & Position

2k mode
-

3906 Hz
-

Kmax=1704

8k mode
-

977 Hz
-

Kmax=6816

Symbol Duration 256 us (2k) or 1024 us (8k)

60

DVB
-
T
-

Estimating the Channel

A

B

C

D

E

A

B=3/4A+1/4E

C=1/2(A+E)

E

D=1/4A+3/4E

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

61

DVB
-
T
-

Estimating the Channel

A B C D E F G

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

A B C D E F G

B=2/3A+1/3D

C=1/3A+2/3D

E=2/3D+1/3G

F=1/3D+2/3G

62

Channel Estimation &
Equalisation

Time

Time

ATSC

DVB
-
T

63

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

64

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
C
OFDM



Uses Fast Fourier Transform [FFT]



”2k” and “8k” versions




Single Frequency Networks [SFN]