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http://www.dvb.org

technicaloverview


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Digital vs Analogue TV

1 1

0 1

1 0

0 0

1 0

1 1

1 1

1 1 0 1 1 0 0 0 1 0 1 1 1 1

1 1

0 1

1 0

0 0

1 0

1 11 1

MPEG-2

DVB

Encoder

Transmission medium

MPEG-2

DVB

Decoder

SourceTelevision

Television

Source

Transmitter


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Why is Digital TV different?

Digital TV squeezes the information that the viewer cant

see or hear out of a standard analogue TV

DVB uses:

Video compression - MPEG-2

Audio compression - MPEG Layer II

That means you can fit much more TV into the same

channel capacity as analogue TV

A satellite transponder using DVB can contain 6-8 times

more TV programmes than analogue TV

DVB is also completely digital, opening up the world of

EPGs, Internet, data broadcasting, advanced interactive

TV ..


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Source video compressed

Source audio compressed

Data information prepared

Video, Audio and Data

streams multiplexed

Transmission by cable / satellite /

MMDS / terrestrial

Demultiplexing

Decoding

View on TV

The DVB System

1 10 1

1 0

0 0

1 0

1 1

1 1

1 1 0 1 1 0 0 0 1 0 1 1 1 1

1 1

0 1

1 0

0 0

1 0

1 1

1 1

MPEG-2

DVB

Encoder

Transmission medium

MPEG-2

DVB

Decoder

Source

Television


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DVB-S Satellite Transmission

Designed to cope witha range of transponder

bandwidths (26MHz - 72 MHz)

Single carrier system

DVB-S is like an onion: centre: payload

series of layers to ensure error protection

adapt the payload for broadcasting implemented

satellite chain

MPEG-2

Transport

Stream

Q

I

IF

Inner

Convolu

-

tional

Coder

Baseband

Shaping

Modulator

MPEG-2

TS MUX

Adaption

& Energy

Dispersal

Outer

Reed -

Solomon

Coder

Convolu-

tional

Interleaver

RF

Up

Converter


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DVB-C Cable Transmission

Same core as satellite system to facilitate

interworking

No inner convolutional code

System based around 64 QAM, but higher andlower order systems possible

8MHz channel, with 64 QAM can accommodate

about 38.5 Mbits/s

MPEG-2Transport

StreamTuner

RF

ModulatorBaseband

Shaping

Convolu-

tional

Inter-

leaver

Outer

Reed -

Solomon

Coder

MPEG-2

TS MUX

Adaption

& Energy

Dispersal Q

I

IF


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DVB-T terrestrial transmission

Adaptation

of the trans-

mission

frame

Inner

FEC

Encoder

Sync

Inversion,

Energy

dispersal

(without hierarchical modulation)

Baseband

Interface,

Sync

Separator

Outer

FEC

Encoder

l =12

Inner

Inter-

leaver

Symbol

mapping,

Modulator

COFDMIntroduction

of guardinterval

Digital to

AnalogueTransmitter

To

Aerial

Data Clock

Generation of reference

symbols, pilots and TPS data

MPEG-2

Transport

Stream


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Common circuitry in the DVB

Receiver

RF &

QAM

demod

Differential

Decoder

Convolutional

de-interleaver

Reed-

Solomon

Decoder

Carrier & clock sync recovery

Matched

Filter &

equaliser

Sync inversion &

Energy dispersal

removal

Output

interface

Cable

Signal

Data

Clock

RF &

OFDM

demod

RF &

QPSK

demod

Epuncturing

& inner

DecoderMatched

Filter

Terrestrial

Signal

Satellite

Signal

Inner

de-interleaver

Sync

Decoder

OUTPUT


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The COFDM signal


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Separation of data streams


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Multi-resolution QAM

When reception conditions and C/N degrade, 16 points

of the constellation of the 64 QAM can be used as one point

in a QPSK constellation


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DVB-T 8k non-hierarchical

modulation

Programme 1

Useful data rate: 24.88 Mbit/s in a data container

defined by 8MHz channel B/W and 64-QAM modulation

code rates for inner FEC : 5/6

C/N needed at edge of coverage

typically 20 dB (Ricean channel)

Programme 2

Programme 3

Programme 4


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DVB-T 8k (hierarchical

modulation)

64 QAM for fragile part

16.59 Mbit/s

QPSK for robust part

4.98 Mbit/s

Useful data rate: 21.57 Mbit/s within a data container

of two levels of robustness

With multiresolution QAM used

code rates for inner FEC : 5/6, 1/2

C/N needed at edge of coverage

(Ricean channel)

22.7 dB for fragile part

7.1 dB for robust part

Programme 1

Programme 2

Programme 3

Baseline

Programme 4


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The Guard Interval

Guard

Interval

TG

Main Symbol Period

TS

e.g. for a national service Ts = 1ms, TG=0.25ms


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CurrentEuropean UHF

Spectrum usage

Number of transmitters

per UHF channel

UHF channel No.

200

600

1000

1400

40 50 60 6821 30


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Implementation Scenario

OFDM DVB-T signal fits between existinganalogue UHF carriers

sound

vision

sound

vision

sound

vision

sound

vision


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DVB-T: key features

Same core as DVB-S and DVB-C systems

DVB-S punctured convolutional coding

OFDM based QPSK or QAM modulation very rugged

against multipath fading

DVB-T offers a 2k or and 8k OFDM option

Two level hierarchical channel coding and modulation

possible

Hierarchical MPEG-2 source coding not included

Possibility of national or regional signal frequency

networks.


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Transmission frame for DVB-T

Carrier spacing in 2k-mode - 4464 Hz, in 8k-mode - 1116 Hz

Continual Pilot

TPS-Pilot

Data

Scattered Pilot

Continual Pilot TPS-Pilot Continual Pilot

2k-mode: kmax - 1704

8k-mode: kmax - 6816

f1 + 7.61 MHz

Frequency

Carrier-position kmax

Carrier-position kmin (-0)

f1

Symbol 67Symbol 0Symbol 1Symbol 2Symbol 3


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The DVB-T Guard Interval

Main signal

Echo 2

Co-channel

Echo 1

received signal

after gain

correction

Tg TwantTs

t

t

t

t

t


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COFDM offers variable bit-rate

System thresholds for 8-VSB and DVB-T COFDM

25

20

15

10

5

0

0 10 20 30

SNR (dB)

Bit Rate (Mbit/s)

8-VSB

COFDM modes


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Digital TV is different!

Compared with analogue:

digital TV requires less C/N

digital TV is more tolerant of interference

BUT

digital systems fail more abruptly

Clearly, expect to use new strategies in

frequency planning


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DVB-T offers choice of

strategies

COFDM in DVB-T tolerates multipath (echoes)

COFDM can also tolerate artificial multipath of a

single-frequency network (SFN) new concept in spectrum planning

spectrally-efficient in right circumstances

multi-frequency networks (MFNs)

can interleave channels amongst analogue


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Choosing a strategy

Every country has its own scenario ...

different history of broadcasting TV

different aspirations for the new digital service

different existing spectrum usage

different future spectrum availability

different neighbours

... so each country must find its best solution


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Single-frequency networks

SFNs can offer improved spectrum efficiency

examples for DAB, DVB-T

SFN requires an RF channel free over whole

service area of network

reason why not chosen for UK DVB-T

best results with a dense network of lower-

power transmitters


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Conclusions (i)

planning digital TV services is different

from analogue TV

countries have different requirements

DVB-T permits choice of planning methods

single-frequency networks (SFNs)

multi-frequency networks (MFNs)

thus uniquely able to satisfy differentneeds of all countries


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Conclusions (ii)

SFNs can give greater spectrum efficiency

MFNs can be interleaved amongst analogue

European administrations have done a great deal of

work to enable DVB-T introduction

DVB-T is being introduced in Europe

firm plans in place in UK and Sweden for 1998

other countries will follow

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