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University of Canberra Advanced Communications Topics

University of Canberra Advanced Communications Topics. Television Broadcasting into the Digital Era. Lecture 3 Audio/Video Compression MPEG-2 Multiplexing. by: Neil Pickford. Enabling Technologies. Source digitisation (Rec 601 digital studio) Compression technology (MPEG, AC-3)

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University of Canberra Advanced Communications Topics

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  1. University of Canberra Advanced Communications Topics Television Broadcasting into the Digital Era Lecture 3 Audio/Video Compression MPEG-2 Multiplexing by: Neil Pickford

  2. 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

  3. 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!

  4. Compression Technology • When low bandwidth analog information is digitised the result is high amounts of digital information. 5 MHz bandwidth analog TV pictureº170 - 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

  5. Compression - Types • Two types of compression available • Loss-less compression 2 to 5 times • Lossy compression 5 to 250 times

  6. 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 • Short codes for common blocks • Longer codes for uncommon blocks • Lookup tables

  7. Compression - Lossy Types • Quantisation - rounding • Motion vectors • Prediction & interpolation • Fractal coding • Discrete cosine transform (DCT)

  8. Compression - DCT 8x8 Pixels

  9. 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

  10. 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

  11. 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 LEVELS SNRP@LL MAIN HIGH MP@LL SPATIALLY SCALABLE SNR SCALABLE LOW PROFILES MAIN SIMPLE

  12. 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

  13. 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, ….)

  14. Multichannel Sound TV CLFE R L Ls Rs

  15. Masking • Both use perceptual audio coding that exploits a psychoacoustic effect known as masking

  16. 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

  17. MPEG Audio Encoder AudioBitStream 32 Subbands O/P SubbandFilter Quantiser&Coder FramePacker AudioIn 2 x 32-192kb/s 2 x 768kb/s BitAllocation Codingof SideInformation Psycho-AcousticModel

  18. MPEG Audio Decoder AudioBitStream FrameUnpacker De-Quantiser InverseSubbandFilter AudioOut 2 x 32-192 kb/s 2 x 768 kb/s Decodingof SideInformation

  19. 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

  20. AC-3 Coding Block Diagram

  21. AC-3 Multichannel Coder L L R R 5.1-chDecoder C C 5.1-chEncoder LS LS RS RS LFE LFE Decoder Encoder

  22. AC-3 Stereo Decoder L L R R Lo 5.1-chDecoder C C 5.1-chEncoder Matrix Ro LS LS RS RS LFE LFE Encoder 2-channel Decoder

  23. 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

  24. MPEG-2 Multichannel Coder concept MPEG-1Encoder MPEG-1Decoder Lo Lo L Ro Ro L R R Rematrix Downmix C C LS LS RS RS ExtensionDecoder ExtensionEncoder LFE LFE MPEG-2 Encoder MPEG-2 Decoder

  25. Low cost 2-channel decoder MPEG-1Encoder MPEG-1Decoder Lo Lo L Ro Ro R Downmix C LS T2 RS ExtensionEncoder T3 T4 LFE LFE 2-channel Decoder MPEG-2 Encoder  Low cost 2-channel decoder

  26. Compatibility with Pro Logic L L MPEG-1Encoder MPEG-1Decoder SurroundEncoder Pro LogicDecoder Lo Lo R R Lt Ro Ro C C Rt S S Lt Rt

  27. Compatibility with Pro Logic (2) Lt Lt Rt Rt L Pro LogicDecoder MPEG-1Encoder MPEG-1Decoder R Lo Lo C L Ro Ro S R Downmix C LS T2 RS ExtensionEncoder T3 T4 LFE LFE MPEG-2 Encoder

  28. 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 Widely Available

  29. 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

  30. Satellite (DBS) DVB-S Program interchange Direct view / pay TV SMATV Digital Television - Types Downlink Uplink

  31. Cable HFC - pay TV MATV DVB-C / 16-VSB Digital Television - Types Fibre Main Coax Spur Tee Tap

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

  33. 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

  34. 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

  35. . . . 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

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

  37. 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

  38. 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)

  39. 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

  40. 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

  41. 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:

  42. 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

  43. 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.

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