University of Canberra Advanced Communications Topics Television Broadcasting

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

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 2

Video Bitrate - HDTV n 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 º 170 - 270 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 n 4

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

Compression - Loss-less Types Picture differences - temporal n Run length data coding - GIF n u 1010001001101 = 1 + 4 x 0100 + 1101 u 21 bits source = 12 bits compressed n Huffman coding - PKZIP u Short codes for common blocks u Longer codes for uncommon blocks n 6 Lookup tables

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

Compression - DCT 8 x 8 Pixels 8

MPEG-2 - I, P & B Frames Uncompressed SDTV Digital Video Stream - 170 Mb/s Picture 830 k. Bytes I Frame 100 k. Bytes Picture 830 k. Bytes B Frame 12 k. Bytes Picture 830 k. Bytes P Frame 33 k. Bytes MPEG-2 Compressed SDTV Digital Video Stream - 3. 9 Mb/s 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 9

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

MPEG Profiles and Levels 422 P@HL MAX. BITRATE 300 Mbit/s HP@HL 100 Mbit/s 80 Mbit/s 60 Mbit/s 40 Mbit/s MP@HL HP@H 14 L SSP@H 14 L MP@H 14 L 422 P@ML 20 Mbit/s HP@ML HIGH SNRP@ML MP@ML HIGH-1440 LEVELS SP@ML 11 4: 2: 2 SNRP@LL MAIN MP@LL LOW SIMPLE MAIN SNR SCALABLE SPATIALLY SCALABLE HIGH PROFILES

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 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, …. ) 13

Multichannel Sound TV L Ls 14 C LFE R Rs

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

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 480 - 512 kb/s MPEG 2 5. 1 16

MPEG Audio Encoder 32 Subbands Audio In 2 x 768 kb/s Subband Filter Psycho. Acoustic Model 17 Audio Bit Stream O/P Quantiser & Coder Frame Packer Bit Allocation Coding of Side Information 2 x 32 -192 kb/s

MPEG Audio Decoder Audio Frame Bit Unpacker Stream 2 x 32 -192 kb/s De-Quantiser Decoding of Side Information 18 Inverse Subband Filter Audio Out 2 x 768 kb/s

Multichannel Sound - Dolby AC-3 n Dolby AC-3 was developed as a 5. 1 channel surround system from the beginning. u Compression Filter bank is 8 x greater 19 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

AC-3 Coding Block Diagram 20

AC-3 Multichannel Coder L R C LS RS 5. 1 -ch Encoder 5. 1 -ch Decoder LFE Encoder 21 L R C LS RS Decoder

AC-3 Stereo Decoder L R C LS RS 5. 1 -ch Encoder Matrix LFE Encoder 22 5. 1 -ch Decoder L R C LS RS 2 -channel Decoder Lo Ro

Problem of AC-3 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 n 23

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

Low cost 2 -channel decoder L R C LS RS LFE Down mix Lo Ro MPEG-1 Encoder T 2 T 3 T 4 LFE Extension Encoder MPEG-2 Encoder Low cost 2 -channel decoder 25 MPEG-1 Decoder 2 -channel Decoder Lo Ro

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

Compatibility with Pro Logic (2) L R C LS RS LFE Down mix Lot Rot MPEG-1 Encoder T 2 T 3 T 4 LFE Extension Encoder MPEG-2 Encoder 27 MPEG-1 Decoder Lot Rot Pro Logic Decoder L R C S

Widely Available 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 n 28

Studio Multichannel Sound Present AES 3 PCM Audio does not cater for 5. 1 channel surround. n Dolby has produced a system called Dolby E n u Handles 6 -8 audio inputs u Uses low compression 3 -4: 1 u Can be transported/stored on 2 ch PCM audio equipment u Incorporates time stamps and is segmented at the video frame rate allowing editing on video frame boundaries 29

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

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

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 32

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 33

MPEG-2 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 n 34

Digital Terrestrial TV - Layers. . . provide clean interface points. . 1920 x 1080 1280 x 720 50, 25, 24 Hz Picture Layer Video Compression Layer Data Headers Motion Vectors Multiple Picture Formats and Frame Rates MPEG-2 compression syntax ML@MP or HL@MP Chroma and Luma DCT Coefficients Variable Length Codes Transport Layer Transmission Layer Video packet Audio packet Video packet VHF/UHF TV Channel 7 MHz 35 Flexible delivery of data Packet Headers Aux data MPEG-2 packets COFDM / 8 -VSB

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

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

Set top Box (STB) - Interfacing 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) n 38

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

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 40

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

Video Program Capacity For a payload of around 19 Mb/s 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 42

Spare Data Capacity n n n 43 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 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.