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

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

3 Video Bitrate - HDTV n 2 M pixels * 25 pictures * 3 colours * 8 bits = G bits / sec for Interlace Scan or = G bits / sec for Progressive We need to Compress this a bit!

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

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

6 Compression - Loss-less Types n Picture differences - temporal n Run length data coding - GIF u = 1 + 4x u 21 bits source = 12 bits compressed n Huffman coding - PKZIP u Short codes for common blocks u Longer codes for uncommon blocks n Lookup tables

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

8 Compression - DCT 8x8 Pixels

9 MPEG-2 - I, P & B Frames Uncompressed SDTV Digital Video Stream Mb/s Picture 830kBytes I FrameB FrameP FrameB Frame 100 kBytes12 kBytes33 kBytes12 kBytes n I - intra picture coded without reference to other pictures. Compressed using spatial redundancy only n P - predictive picture coded using motion compensated prediction from past I or P frames n B - bidirectionally-predictive picture using both past and future I or P frames MPEG-2 Compressed SDTV Digital Video Stream Mb/s

10 MPEG-2 - Formats ML & HL n MPEG-2 defines profiles & levels u They describe sets of compression tools n DTTB uses main profile. n Choice of levels n Higher levels include lower levels Level resolution Level resolution Low level (LL) 360 by 288SIF Main level (ML) 720 by 576SDTV High level (HL)1920 by 1152HDTV

11 MPEG Profiles and Levels Mbit/s 100 Mbit/s 80 Mbit/s 60 Mbit/s 40 Mbit/s 20 Mbit/s HIGH HIGH-1440 MAIN LOW SIMPLE MAIN SNR SCALABLE HIGH SPATIALLY SCALABLE 4:2:2 LEVELS MAX. BIT- RATE PROFILES

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

13 Digital Audio - Multichannel n Two sound coding systems exist for Digital TV u MPEG 1 & 2 u Dolby AC-3 n Cover a wide variety of Audio Applications u DVB u VCD and S-VCD u DAB, DBS, DVD u Cinema (Film) u Computer Operating Systems (Windows) u Professional (ISDN codecs, tapeless studio, ….)

14 Multichannel Sound TV C LFE L R RsLs

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

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

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

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

19 Multichannel Sound - Dolby AC-3 n Dolby AC-3 was developed as a 5.1 channel surround sound system from the beginning. u Compression Filter bank is 8 x greater than MPEG 2 (256) u Must always send full 5.1 channel mix One bitstream serves everyone u Decoder provides downmix for Mono, Stereo or Pro-Logic u Listener controls the dynamic range, Audio is sent clean u Bitrate 384 kb/s or 448 kb/s u Dialogue level passed in bit-stream

20 AC-3 Coding Block Diagram

21 AC-3 Multichannel Coder 5.1-ch Encoder R L C LS RS LFE 5.1-ch Decoder R L C LS RS LFE Encoder Decoder

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

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

24 MPEG-2 Multichannel Coder concept Down mix R L C LS RS LFE Ro Lo Ro Lo MPEG-1 Encoder MPEG-1 Decoder Extension Encoder Re matrix R L C LS RS LFE MPEG-2 Encoder MPEG-2 Decoder Extension Decoder

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

26 Compatibility with Pro Logic RoRoRoRo LoLoLoLo RoRoRoRo LoLoLoLo RtLtRtLt MPEG-1 Encoder MPEG-1 Decoder RLC S Surround Encoder RLC S Pro Logic Decoder

27 Compatibility with Pro Logic (2) RoRo LoLo RoRo LoLo R L C LS RS LFE LFE T2 T3 T4 MPEG-1 Encoder Extension Encoder Down mix MPEG-2 Encoder MPEG-1 Decoder RtRtRtRt LtLtLtLt RtRtRtRt LtLtLtLt Pro Logic Decoder RLC S

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

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

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

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

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

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

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

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

36 Digital Television Encode Layers Delivery System Bouquet Multiplexer Program 2Program 3 Service Mux Other Data Control Data (PAT) Picture Coding Audio Coding Data Coding MPEG-2 or AC-3 MPEG-2 Control Data VideoDataSound Modulator & Transmitter Error Protection Control Data 188 byte packetsMPEG Transport Data Stream Program 1 Multiplexer MPEG Transport Stream Mux Control Data (PMT) PESPESPES

37 Digital Television Decode Layers Audio Decoder Data Decoder Picture Decoder MPEG or AC-3 MPEG-2 Demodulator & Receiver Error Control Delivery System Data Mon Speakers MPEG Transport Stream De-Multiplexer MPEG DeMux Transport Stream

38 Set top Box (STB) - Interfacing n Domestic and Professional interfaces still to be defined n Most probably Transport Stream via IEEE 1394 (Firewire) n Baseband Audio & RGB/YUV Video signals. n STB can convert between line standards so you do not have to have a HD display. n Display and transmitted information must be at same Frame/Field rate. (25/50)

39 DTTB - Content & Services n DTTB was designed to carry video, audio and program data for television n DTTB can carry much more than just TV u Electronic program guide, teletext u Broadband multimedia data, news, weather u Best of internet service u Interactive services u Software updates, games n Services can be dynamically reconfigured

40 DVB Data Containers n MPEG Transport Stream is used to provide DVB “data containers” which may contain a flexible mixture of: u Video u Audio u Data services n Streams with variable data rate requirements can be Statistically Multiplexed together. u Allows Six 2 Mb/s programs to be placed in a 8 Mb/s channel

41 Examples of DVB Data Containers Single HDTV program HDTV 1 SDTV 1 SDTV 2 SDTV 3 SDTV 4 SDTV 5 Multiple SDTV programs SDTV 1 HDTV 1 Simulcast HDTV & SDTV Channel bandwidth can be used in different ways:

42 Video Program Capacity è 1 HDTV service - sport & high action è 2 HDTV services - both film material è 1 HDTV + 1 or 2 SDTV non action/sport è 3 SDTV for high action & sport video è 6 SDTV for film, news & soap operas However you do not get more for nothing. n More services means less quality For a payload of around 19 Mb/s

43 Spare Data Capacity n Spare data capacity is available even on a fully loaded channel. n Opportunistic use of spare data capacity when available can provide other non real time data services. n Example: 51 second BMW commercial The Commercial was shown 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.