Docstoc

Digital Television Talk Lecture 4

Document Sample
Digital Television Talk Lecture 4 Powered By Docstoc
					    University of Canberra
Advanced Communications Topics

Television Broadcasting
   into the Digital Era
        Lecture 4
      DTTB Types         by: Neil Pickford
         Digital
1
    Modulation Systems
       Digital Terrestrial TV - Layers
                . . . provide clean interface points. . . .
                                              1920 x 1080
     Picture                                   1280 x 720
                                                                    Multiple Picture Formats
      Layer                                                            and Frame Rates
                                              50,25, 24 Hz



                                                                                     MPEG-2
   Video                                                                           compression
                                                                                     syntax
Compression         Data                       Chroma and Luma                       ML@MP
                                   Motion
   Layer           Headers         Vectors      DCT Coefficients                       or
                                                                                     HL@MP
                               Variable Length Codes

                             Packet Headers                    Flexible delivery of data
    Transport
      Layer         Video packet      Audio packet   Video packet       Aux data      MPEG-2
                                                                                      packets



Transmission           VHF/UHF TV Channel                                   COFDM / 8-VSB

    Layer
                                     7 MHz
2
       Digital Television Encode Layers
               Video               Data            Sound
Control        Picture             Data            Audio         MPEG-2
 Data                    MPEG-2
               Coding             Coding           Coding        or AC-3



                                                         MPEG Transport
Control Data             Program 1 Multiplexer            Stream Mux
  Program 2                                                 Program 3
  Other Data                                            Service
Control Data
                          Bouquet Multiplexer            Mux

       MPEG Transport Data Stream 188 byte packets
                                                      Error
       Control Data      Modulator & Transmitter    Protection

                           Delivery   System
   3
     Digital Television Decode Layers
                                               Speakers
             Mon              Data



MPEG-2
            Picture          Data               Audio        MPEG
            Decoder         Decoder            Decoder       or AC-3



Transport             MPEG Transport Stream          MPEG
 Stream                  De-Multiplexer              DeMux




                      Demodulator & Receiver      Error
                                                 Control

                       Delivery   System
 4
        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

   Services can be dynamically reconfigured

6
              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

7
Examples of DVB Data Containers

    Channel bandwidth can be used in different ways:

       SDTV 1
       SDTV 2
       SDTV 3           HDTV 1           HDTV 1
       SDTV 4
       SDTV 5                            SDTV 1
        Multiple         Single         Simulcast
         SDTV            HDTV            HDTV &
       programs         program           SDTV

8
       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
    Spare data capacity is
     available even on a fully
     loaded channel.
    Opportunistic use of
     spare data capacity when
     available can provide           The Commercial was
     other non real time data      shown using 1080 Lines
                                 Interlaced. 60 Mb of data
     services.                   was transferred during it.
    Example: 51 second           In the Final 3 seconds the
     BMW commercial              BMW Logo was displayed
                                   allowing 3 Phone Books
                                  of data to be transmitted.

10
        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
     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

17
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
18
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




        Proposed to use band segmented transmission -
         orthogonal frequency division multiplex
         (BST-OFDM)
20
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   Spectrum
    OFDM
     8-VSB & COFDM - Spectrum



                           8-VSB
                           COFDM




24
     Sin(x)/x




25
                 Digital Modulation
                               Amplitude, dB
 Spectrum of Conventional
Multi-Phase Keyed Carrier Fc
      at Symbol Rate Fs
                                     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



                            I AXIS
     0                  1

              180 Deg
               Phase
              Change



29
            QPSK Modulation
                Q AXIS
       10                11

      QPSK
     Distance                 I AXIS




        00               01


30
             16QAM Modulation
                   Q AXIS   1110        1111

            10                     11
16-QAM
Distance
                            1100        1101

                                           I AXIS

     0010        0011



            00                     01
     0000        0001


    31
                 8VSB Modulation
                   Q AXIS   1110        1111

            10                     11
16-QAM
Distance
                            1100        1101

                                           I AXIS

     0010        0011



            00                     01
     0000        0001


     32
                Hierarchical Modulation
Hierarchical         Q AXIS   1110        1111
 Distance
               10                    11
                              1100        1101

        QPSK
       Distance
                                             I AXIS



       0010         0011

               00                    01
       0000         0001


      33
                  Digital Modulation
                          Amplitude
 Typical Filtered                 Occupied Channel
Spectrum to give                     Bandwidth
about half original
   bandwidth




                                        Frequency
        Fc - Fs           Fc      Fc + Fs
           8-VSB Digital Modulation
                             Amplitude
  Application of Vestigial
  Sideband Filter to give                Occupied Channel
     reduced spectral                       Bandwidth
   occupancy BUT with                     eg 6 MHz in US
    destruction of pure
  Amplitude modulation
 causing incidental Phase
modulation but some power
     in a small carrier

                                              Frequency
       Fc - Fs               Fc       Fc + Fs
    8VSB uses symbol Rate with period 93 nanoseconds
             Normal FDM
            Guard
                    Amplitude, dB
            Band




Carrier 1                           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.    Amplitude, dB
       Create with FFT




    Spectral Width                              Frequency
                          Fcentre
 2k is 4x wider than 8k
                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

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

            45
                          COFDM : HOW ?
      3 - Insert Guard Interval to avoid “inter-symbol” interference
                                  Guard Interval   Useful symbol
                                    duration         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




                                       FFT
                                 time windows
                                  for receivers
frequency
             Synchronization markers introduce the second loss
        47                  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    Protection codes introduce the third loss in transport capacity

        48
                                        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
                          Create frequency diversity to improve robustness against fading

                     49
     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




                S
       313                          24.2
     Segments
                e   Data + FEC
                g                    ms
                m
                e
                n
                t
                    Field Sync #2
                S
                y
                n
                c
       313          Data + FEC      24.2
     Segments                        ms




                    Test Segment

                     1 Segment
51                    = 77.3 us
                          8-VSB Segment Sync & Data
                        Data                              Data
                      Segment            Data + FEC     Segment
+7                     SYNC                              SYNC
+5
+3
+1
-1
-3
-5
-7

     Levels Before       4
                                        828 Symbols        4
     Pilot Addition   Symbols            207 Bytes      Symbols
     (Pilot=1.25)
                                        Data Segment
                                        832 Symbols
                                          208 Bytes



                                Symbol Duration 93 ns
               52
        Digital Modulation - 8-AM

                              7
                              6
                              5
                              4
                              3
                              2
                              1
     Before Equaliser         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                                          PN511                    PN63 PN63 PN63 VSB            Reserved
-3                                                                                  Mode
-5
-7                 Sync
                                                                                                               12
                                                                                                              Sym-
                                                                                                              bols
 Levels Before       4                       511                      63   63   63   24              104
 Pilot Addition   Symbols                  Symbols                   Sym- Sym- Sym- Sym-           Symbols
 (Pilot=1.25)                                                        bols bols bols 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.



                              Field Sync is Repeated Every 24 ms
             54
               8-VSB - Field Sync

                        7
                        6
                        5
                        4
                        3
                        2
                        1
     Before Equaliser   0    After Equaliser



55
                    8-VSB Frame
     832
                     Training Sequence
  Symbols                                    313 Data
  per Data                                   Segments
  Segment S               Data
  of 77.3 µs                                 of 24.2 ms
              y
                     Training Sequence
              n
              c
                          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.
          Modulated Carriers
   57     Scattered Pilots
            DVB-T Super Frame
            34        50    TPS Carriers               17/68
 Super
                                                        1512:
Frame                         Frame 1
                                                         1705
 4 x 68                       Frame 2                    in 2k
Symbols
                              Frame 3                   6048:
   =
                                                         6817
N MPEG
                              Frame 4                    in 8k
Packets
                 48    54    Fixed Pilots              45/177
                  Co- Incident with Scattered pilots
       58
              DVB-T Transmission Frame
Kmin=0                                   2k mode - 3906 Hz - Kmax=1704 Kmax
              Carrier Spacing & Position 8k mode - 977 Hz - Kmax=6816




       Data               TPS - Pilot
       Scattered Pilot    Continuous Pilot
  59    Symbol Duration 256 us (2k) or 1024 us (8k)
     DVB-T - Estimating the Channel

     A                                    A
     B                          B=3/4A+1/4E
     C                           C=1/2(A+E)
     D                          D=1/4A+3/4E
     E                                    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 D E FG                         AB C D E 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

    Single Frequency Networks [SFN]
64

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:20
posted:9/10/2011
language:English
pages:64