1. technical overview

2. Digital vs Analogue TV MPEG-2 DVB Encoder Decoder Source Television
1 1
0 1
1 0
0 0
MPEG-2
DVB
Encoder
Transmission medium
Decoder
Source
Television
Transmitter

3. Why is Digital TV different?
Digital TV squeezes the information that the viewer can’t 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 ……..

4. The DVB System MPEG-2 DVB Encoder Decoder Source Television
1 1
0 1
1 0
0 0
MPEG-2
DVB
Encoder
Transmission medium
Decoder
Source
Television
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

5. DVB-S Satellite Transmission
MPEG-2
Transport
Stream
MPEG-2
TS MUX
Adaption
& Energy
Dispersal
Outer
Reed -
Solomon
Coder I
Convolu-
tional
Interleaver
Inner
Convolu-
tional
Coder
Baseband
Shaping
Modulator Q IF
Designed to cope with a 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 RF Up
Converter

6. DVB-C Cable Transmission
MPEG-2
Transport
Stream
MPEG-2
TS MUX
Adaption
& Energy
Dispersal I
Outer
Reed -
Solomon
Coder
Convolu-
tional
Inter-
leaver
Baseband
Shaping
Modulator
Tuner Q IF RF
Same core as satellite system to facilitate interworking
No inner convolutional code
System based around 64 QAM, but higher and lower order systems possible
8MHz channel, with 64 QAM can accommodate about 38.5 Mbits/s

7. DVB-T terrestrial transmission
Inner
FEC
Encoder
Baseband
Interface,
Sync
Separator
Sync
Inversion,
Energy
dispersal
Outer
FEC
Encoder
l =12
MPEG-2
Transport
Stream
Inner
Inter-leaver
Symbol
mapping,
Modulator
Generation of reference symbols, pilots and TPS data
Data
Clock
Adaptation
of the trans-mission
frame
Transmitter
Digital to Analogue
Introduction
of guard
interval To
Aerial
COFDM
(without hierarchical modulation)

8. Common circuitry in the DVB Receiver
Terrestrial
Signal
RF &
OFDM
demod
Inner
de-interleaver
Epuncturing
& inner
Decoder
Sync
Decoder
Satellite
Signal
RF &
QPSK
demod
Matched
Filter
Cable
Signal
Matched
Filter &
equaliser
RF &
QAM
demod
Differential
Decoder
Carrier & clock sync recovery
OUTPUT
Reed- Solomon Decoder
Sync inversion &
Energy dispersal
removal
Data
Convolutional
de-interleaver
Output
interface
Clock

9. The COFDM signal

10. Separation of data streams

11. 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

12. DVB-T 8k non-hierarchical modulation
Useful data rate: Mbit/s in a data container defined by 8MHz channel B/W and 64-QAM modulation
Programme 1
Programme 2
code rates for inner FEC : 5/6
Programme 3
Programme 4
C/N needed at edge of coverage
typically 20 dB (Ricean channel)

13. DVB-T 8k (hierarchical modulation)
Useful data rate: Mbit/s within a data container of two levels of robustness
Programme 1
64 QAM for ‘fragile’ part
16.59 Mbit/s
Programme 2
QPSK for ‘robust’ part
4.98 Mbit/s
Programme 3
Baseline
Programme 4
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

14. The Guard Interval Guard Interval Main Symbol Period TG TS
e.g. for a national service Ts = 1ms, TG=0.25ms

15. Current European UHF Spectrum usage
Number of transmitters
per UHF channel
1400
1000
600
200 21 30 40 50 60 68
UHF channel No.

16. Implementation Scenario
OFDM DVB-T signal fits between existing analogue UHF carriers
sound
sound
sound
sound
vision
vision
vision
vision

17. 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.

18. Transmission frame for DVB-T
f MHz f1
Frequency
Carrier-position kmin (-0)
Carrier-position kmax
Symbol 67
Symbol 0
Symbol 1
Symbol 2
Symbol 3
Continual Pilot
TPS-Pilot
Continual Pilot
Data
Continual Pilot
2k-mode: kmax
Scattered Pilot
TPS-Pilot
8k-mode: kmax
Carrier spacing in 2k-mode Hz, in 8k-mode Hz

19. The DVB-T Guard Interval
Main signal t
Echo 1 t
Echo 2 t
Co-channel t
received signal after gain correction t Tg
Twant Ts

20. COFDM offers variable bit-rate
System thresholds for 8-VSB and DVB-T COFDM 25 20
8-VSB 15
COFDM modes
SNR (dB) 10 5 10 20 30
Bit Rate (Mbit/s)

21. 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

22. 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

23. 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

24. 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

25. 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 different needs of all countries

26. 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