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Developing Digital Television Broadcasting: Understanding DTTB and Transmission Systems

Explore digital modulation systems, MPEG-2 compression, and DTTB types for advanced television broadcasting. Learn about DVB data containers, HDTV, source digitization, and transmission technologies. Discover the evolution from analog to digital terrestrial television.

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Developing Digital Television Broadcasting: Understanding DTTB and Transmission Systems

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  1. University of Canberra Advanced Communications Topics Television Broadcasting into the Digital Era Lecture 4 DTTB Types Digital Modulation Systems by: Neil Pickford

  2. . . . provide clean interface points. . . . 1920 x 1080 1280 x 720 50,25, 24 Hz Picture Layer Multiple Picture Formats and Frame Rates MPEG-2 compression syntax ML@MPorHL@MP Video Compression Layer Data Headers Chroma and Luma DCT Coefficients Motion Vectors Variable Length Codes Flexible delivery of data Packet Headers Transport Layer MPEG-2packets Video packet Audio packet Video packet Aux data VHF/UHF TV Channel COFDM / 8-VSB Transmission Layer 7 MHz Digital Terrestrial TV - Layers

  3. Digital Television Encode Layers Video Data Sound Control Data Picture Coding Data Coding AudioCoding MPEG-2or AC-3 MPEG-2 MPEG TransportStream Mux Program 1 Multiplexer Control Data Program 2 Program 3 Other Data ServiceMux Bouquet Multiplexer Control Data MPEG Transport Data Stream 188 byte packets Error Protection Modulator & Transmitter Control Data Delivery System

  4. Digital Television Decode Layers Speakers Data Mon Picture Decoder Data Decoder AudioDecoder MPEGor AC-3 MPEG-2 TransportStream MPEG Transport Stream De-Multiplexer MPEG DeMux Error Control Demodulator & Receiver Delivery System

  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 standardsso 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 SDTV 1 HDTV 1 SDTV 2 HDTV 1 SDTV 3 SDTV 4 SDTV 5 SDTV 1 Multiple SDTV programs Single HDTV program 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 secondBMW commercial The Commercial wasshown 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

  23. Digital Modulation Two techniques: • Conventional Single Carrier • 8VSB • Multicarrier/Spread Spectrum • OFDM

  24. 8-VSB & COFDM - Spectrum 8-VSB COFDM

  25. Sin(x)/x

  26. Digital Modulation Amplitude, dB Spectrum of Conventional Multi-Phase Keyed Carrier Fc at Symbol Rate Fs Sin X/X shaping Frequency Fc - Fs Fc + Fs Fc

  27. Digital Modulation Low Symbol Rate Medium Symbol Rate High Symbol Rate Amplitude, dB Frequency

  28. PSK

  29. BPSK Modulation I AXIS 0 1 180 Deg Phase Change

  30. QPSK Modulation Q AXIS 1 0 1 1 QPSK Distance I AXIS 0 1 0 0

  31. 16QAM Modulation Q AXIS 1 1 1 0 1 1 1 1 1 0 1 1 16-QAM Distance 1 1 0 0 1 1 0 1 I AXIS 0 0 1 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 0 1

  32. 8VSB Modulation Q AXIS 1 1 1 0 1 1 1 1 1 0 1 1 16-QAM Distance 1 1 0 0 1 1 0 1 I AXIS 0 0 1 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 0 1

  33. Hierarchical Modulation 0 0 Hierarchical Distance Q AXIS 1 1 1 0 1 1 1 1 1 0 1 1 1 1 0 0 1 1 0 1 QPSK Distance I AXIS 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 0 0 0

  34. Digital Modulation Amplitude Typical Filtered Spectrum to give about half original bandwidth Occupied Channel Bandwidth Frequency Fc - Fs Fc + Fs Fc

  35. 8-VSB Digital Modulation Amplitude 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 Occupied Channel Bandwidth eg 6 MHz in US Frequency Fc - Fs Fc + Fs Fc 8VSB uses symbol Rate with period 93 nanoseconds

  36. Normal FDM Guard Band Amplitude, dB Carrier 1 Carrier 2 Frequency

  37. Traditional SCPC Modulation MinimumCarrier Spacing Frequency

  38. Orthogonal Modulation Amplitude, dB Frequency

  39. Orthogonal Modulation Amplitude, dB Frequency

  40. COFDM - Orthogonal Carriers Frequency

  41. Spectrum of COFDM DTTB Carrier Spacing 2k Mode 3.91 kHz 8k Mode 0.98 kHz AlmostRectangularShape 1705 or 6817 Carriers 6.67 MHz in 7 MHz Channel

  42. OFDM Occupied bandwidth is: No. of Carriers x Spectral Width. Create with FFT Amplitude, dB Frequency Frequency Spectral Width 2k is 4x wider than 8k Fcentre

  43. DIGITAL TERRESTRIAL BROADCASTING Distant transmitter Nearest transmitter Among the four Digital Broadcasting standards available, three are based on the Coded Orthogonal Frequency Division Multiplex modulation.... Why ? The Terrestrial Broadcasting has to cope with multipath propagation and Doppler effects: COFDM is the response for these impairments !

  44. COFDM : HOW ? time RF Channel bandwidth frequency sub-band time segment frequency • 1 - Organize time & frequency partitions in the RF channel

  45. COFDM : HOW ? time OFDM symbol Make sub-carriers orthogonal to avoid “inter-carriers” interference frequency • 2 - Spread sub-carriers over “time vs frequency” cells

  46. COFDM : HOW ? Guard Interval duration Useful symbol duration time OFDM symbol frequency • 3 - Insert Guard Interval to avoid “inter-symbol” interference Guard interval introduces a first loss in transport capacity

  47. COFDM : HOW ? OFDM Frame (68 OFDM symbols) time FFT time windows for receivers frequency • 4 - Insert “Synchronization Pilots” Helps Receivers to lock onto the signal Synchronization markers introduce the second loss in transport capacity

  48. time frequency COFDM : HOW ? Protected DATA (convolutionnal error protection codes) • 5 - Prepare data to be carried on OFDM symbols DATA to broadcast Protection codes introduce the third loss in transport capacity

  49. time frequency COFDM : HOW ? DATA to broadcast Protected DATA 0 1 0 0 1 • 6 - Map bits onto OFDM: Spread contiguous data bits over distant sub-carriers 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

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