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Digital Television Talk Lecture 3

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Digital Television Talk Lecture 3 Powered By Docstoc
					    University of Canberra
Advanced Communications Topics

Television Broadcasting
   into the Digital Era
        Lecture 3
       Audio/Video        by: Neil Pickford
       Compression
1
    MPEG-2 Multiplexing
        Enabling Technologies
 Source digitisation (Rec 601 digital studio)
 Compression technology (MPEG, AC-3)
 Data multiplexing (MPEG)
 Display technology (large wide screens)
 Transmission technology (modulation)
 Production




2
           Video Bitrate - HDTV
   2 M pixels * 25 pictures * 3 colours * 8 bits

    = 1.24416 G bits / sec for Interlace Scan
                        or
      = 2.4833 G bits / sec for Progressive

        We need to Compress this a bit!


3
       Compression Technology
 When low bandwidth analog information is
  digitised the result is high amounts of digital
  information.
        5 MHz bandwidth analog TV picture
                             
           170 - 270 Mb/s digital data stream.
 270 Mb/s would require a bandwidth of
  at least 140 MHz to transport
 Compression of the information is required


4
          Compression - Types
 Two   types of compression available

    Loss-less compression
      2 to 5 times

    Lossy  compression
      5 to 250 times


5
     Compression - Loss-less Types
 Picture differences - temporal
 Run length data coding - GIF
     101000100010001001101      = 1 + 4x0100 + 1101
     21 bits source = 12 bits compressed

   Huffman coding - PKZIP
     Shortcodes for common blocks
     Longer codes for uncommon blocks

   Lookup tables


6
    Compression - Lossy Types
 Quantisation - rounding
 Motion vectors
 Prediction & interpolation
 Fractal coding
 Discrete cosine transform (DCT)




7
Compression - DCT




                    8x8 Pixels

8
         MPEG-2 - I, P & B Frames
      Uncompressed SDTV Digital Video Stream - 170 Mb/s
Picture 830kBytes   Picture 830kBytes   Picture 830kBytes   Picture 830kBytes



       I Frame      B Frame             B Frame               P Frame
100 kBytes          12 kBytes           12 kBytes           33 kBytes
  MPEG-2 Compressed SDTV Digital Video Stream - 3.9 Mb/s
 I - intra picture coded without reference to other pictures.
       Compressed using spatial redundancy only
   P - predictive picture coded using motion compensated
        prediction from past I or P frames
   B - bidirectionally-predictive picture using both past and
        future I or P frames
9
      MPEG-2 - Formats ML & HL
    MPEG-2 defines profiles & levels
      They   describe sets of compression tools
 DTTB uses main profile.
 Choice of levels
 Higher levels include lower levels

 Level                 resolution
Low level (LL)         360 by 288        SIF
Main level (ML)        720 by 576        SDTV
High level (HL)       1920 by 1152       HDTV


10
                 MPEG Profiles and Levels
                                                                                422P@HL
 MAX.
 BIT-
 RATE
 300 Mbit/s




                                                                      HP@HL




 100 Mbit/s                      MP@HL                                        HP@H14L
  80 Mbit/s
  60 Mbit/s                                                    SSP@H14L
  40 Mbit/s
                                         MP@H14L                                                  422P@ML
  20 Mbit/s
                                                                                        HP@ML
     HIGH
                                                              SNRP@ML
                                                   MP@ML
              HIGH-1440
                                       SP@ML
                                                                                                              4:2:2
                                                                      SNRP@LL
LEVELS                    MAIN
                                                           MP@LL                          SPATIALLY
                                                                                                      HIGH
                                                                          SNR             SCALABLE
                                 LOW
            11                                                     MAIN   SCALABLE                       PROFILES
                                                     SIMPLE
     MP@ML            MP@HL

All decoders sold in Australia will
 be MP@HL capable allowing all
viewers access to HD resolution
    when it becomes available



12
       Digital Audio - Multichannel
    Two sound coding systems exist for Digital TV
      MPEG  1&2
      Dolby AC-3

    Cover a wide variety of Audio Applications
      DVB
      VCD  and S-VCD
      DAB, DBS, DVD
      Cinema (Film)
      Computer Operating Systems (Windows)
      Professional (ISDN codecs, tapeless studio, ….)

13
     Multichannel Sound
               TV



                C
           L   LFE   R




      Ls                 Rs




14
                      Masking
    Both use perceptual audio coding that exploits a
     psychoacoustic effect known as masking
                              masker


                                       Inaudible (masked) tone
Level
(dB)
                                                              Masked
                                                             Threshold




              500      1000             2000          4000          8000

                                                          Frequency (Hz)
15
     Multichannel Sound - MPEG 1/2
        MPEG Audio Layer II was developed in
         conjunction with the European DVB technology
          Uses  Musicam Compression with 32 sub bands
          MPEG 1 is basic Stereo 2 channel mode
          MPEG 2 adds enhancement information to allow
           5.1 or 7.1 channels with full backwards
           compatibility with the simple MPEG 1 decoders
          MPEG 1 is compatible with Pro-Logic processing.
          Bitrate 224 kb/s MPEG 1
          Bitrate 480 - 512 kb/s MPEG 2 5.1

16
              MPEG Audio Encoder
                                                Audio
                                                  Bit
          32 Subbands                           Stream
                        Quantiser                 O/P
Audio      Subband                     Frame
                           &
  In        Filter                     Packer       2x
2 x 768                  Coder
                                                   32-192
 kb/s                                               kb/s
                                       Coding
           Psycho-         Bit
                                       of Side
           Acoustic     Allocation
                                     Information
            Model



     17
                   MPEG Audio Decoder


   Audio                                           Inverse
    Bit          Frame
                                    De-Quantiser   Subband
  Stream        Unpacker                                     Audio
                                                    Filter
2 x 32-192 kb/s                                               Out
                                                       2 x 768 kb/s
                            Decoding
                             of Side
                           Information




       18
 Multichannel Sound - Dolby AC-3
        Dolby AC-3 was developed as a 5.1 channel
         surround sound system from the beginning.
          Compression   Filter bank is 8 x greater
            than MPEG 2 (256)
          Must always send full 5.1 channel mix
           One bitstream serves everyone
          Decoder provides downmix for
           Mono, Stereo or Pro-Logic
          Listener controls the dynamic range,
           Audio is sent clean
          Bitrate 384 kb/s or 448 kb/s
          Dialogue level passed in bit-stream
19
               AC-3 Coding Block Diagram
                                              Quantized Samples                      Coded
                   Analysis                                                       Bit Stream
Source Audio                             Quantization                 Multiplex
                  Filterbank




                       Bit Allocator                 Core
                                                Bit Allocator
                                    Bit Allocation Side Information

                                                                                           Channel
                          Encode         Encoded Spectral Envelope
                         Envelope




                                                                                     Coded
                  Synthesis                                                       Bit Stream
Reconstructed                           De-quantization          Demultiplex
   Audio          Filterbank




                                              Core                     Decode
   20                                    Bit Allocator                Envelope
      AC-3 Multichannel Coder


       L                          L
       R                          R
       C                 5.1-ch   C
            5.1-ch                LS
      LS
      RS   Encoder      Decoder   RS

     LFE                          LFE



              Encoder   Decoder




21
                AC-3 Stereo Decoder


      L                           L
      R                           R
                                  C                Lo
      C    5.1-ch       5.1-ch            Matrix   Ro
     LS                           LS
     RS   Encoder      Decoder    RS

LFE                              LFE



             Encoder         2-channel Decoder




22
             Problem of AC-3
  An AC-3 stereo decoder is more complex than
   an AC-3 multichannel decoder
  With AC-3, end users with stereo receivers must
   pay extra so that others can enjoy multichannel
   sound




23
                      MPEG-2
              Multichannel Coder concept


                    Lo    MPEG-1      MPEG-1      Lo
  L                 Ro    Encoder     Decoder     Ro             L
  R                                                              R
  C        Down                                          Re      C
 LS         mix                                         matrix   LS
 RS                                                              RS
                          Extension   Extension
LFE                        Encoder     Decoder                   LFE



                  MPEG-2 Encoder            MPEG-2 Decoder




      24
       Low cost 2-channel decoder


                    Lo    MPEG-1          MPEG-1      Lo
      L             Ro    Encoder         Decoder     Ro
      R
      C    Down
     LS     mix
     RS             T2
                    T3    Extension
     LFE            T4     Encoder
                    LFE
                                          2-channel
                  MPEG-2 Encoder           Decoder


            Low cost 2-channel decoder

25
      Compatibility with Pro Logic


L                                                         L
R    Surround   Lt
                Lo   MPEG-1    MPEG-1    Lo
                                         Lt   Pro Logic   R
C               Rt
                Ro   Encoder   Decoder   Ro
                                         Rt               C
     Encoder                                   Decoder
S                                                         S




26
      Compatibility with Pro Logic (2)


                                                             L
                Lo
                 t    MPEG-1      MPEG-1    Lo
                                             t   Pro Logic   R
 L              Ro
                 t    Encoder     Decoder   Ro
                                             t    Decoder    C
 R                                                           S
 C     Down
 LS     mix
 RS              T2
                 T3   Extension
LFE              T4    Encoder
                LFE


              MPEG-2 Encoder




 27
               Widely Available
      All major MPEG-2 Video decoders incorporate
       2-channel or 5.1 channel MPEG-2 Audio
      Several dedicated MPEG-2 multichannel
       decoders
      More than 100 Million decoders world-wide




28
          Studio Multichannel Sound
      Present AES3 PCM Audio
       does not cater for 5.1 channel surround.
      Dolby has produced a system called Dolby E
          Handles  6-8 audio inputs
          Uses low compression 3-4:1
          Can be transported/stored on 2ch PCM audio
           equipment
          Incorporates time stamps and is segmented at the
           video frame rate allowing editing on video frame
           boundaries

29
          Digital Television - Types
    Satellite (DBS)
      DVB-S
      Program  interchange   Downlink
      Direct view / pay TV
      SMATV




          Uplink




30
         Digital Television - Types
    Cable
      HFC- pay TV
      MATV
      DVB-C / 16-VSB
                                     Fibre

                   Main Coax




             Tap   Spur        Tee


31
          Digital Television - Types
    Terrestrial (DTTB)
      DVB-T    / 8-VSB
      Free to air TV (broadcasting)
      Narrowcasting/value added services
      Untethered - portable reception




32
        Enabling Technologies
 Source digitisation (Rec 601 digital studio)
 Compression technology (MPEG, AC-3)
 Data multiplexing (MPEG)
 Display technology (large wide screens)
 Transmission technology (modulation)
 Production




33
                  MPEG-2
 Compresses source video, audio & data
 Segments video into I, P & B frames
 Generates system control data
 Packetises elements into data stream
 Multiplexes program elements - services
 Multiplexes services - transport stream
 Organises transport stream data
  into 188 byte packets


34
        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
35
       Digital Television Encode Layers
                Video               Data              Sound
Control         Picture             Data              Audio            MPEG-2
 Data                     MPEG-2
                Coding             Coding             Coding           or AC-3

                          PES          PES          PES

  Control Data                                                 MPEG Transport
    (PMT)
                          Program 1 Multiplexer                 Stream Mux
  Program 2                                                       Program 3
  Other Data                                                  Service
 Control Data              Bouquet Multiplexer                 Mux
    (PAT)
       MPEG Transport Data Stream 188 byte packets
                                                            Error
       Control Data       Modulator & Transmitter         Protection

                            Delivery   System
  36
     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
37
         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)

38
        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

39
               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

40
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

41
        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
42
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.

43

				
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