6a49674239fdf07569d1b2167b0aef86.ppt
- Количество слайдов: 44
Digital TV ATV, Freeview, Freesat and all that… Noel Matthews G 8 GTZ
Agenda • Digital TV basics – – Why compress? Compression and modulation standards HD and SD Multiplexing • Services – SKY – Free. View – Free. Sat • Digital ATV – Why and How
Why digital? • Analogue signals have been around since day 1 of broadcasting • The signals are very fragile and degrade very easily – poor “signal to noise ratio” – – Scratchy records Medium and long wave radio FM radio hiss Ghosting on the TV pictures • So we digitise it • Once the signal is digital we can modify and transmit it without degrading it • We can also copy and store it like computer data • CD were first real consumer digital products • But the problem is audio and pictures are very BIG
Digital video – how big? 11001010 Analogue PAL A to D converter 270 Mbit/s SDI
HD – an even bigger problem! 11001010 High Definition A to D converter 1. 3 Gbit/s
Why compress video? • Serial Digital Interface (SDI) is the uncompressed digital studio standard • High quality loss-less digital video – 270 Mbit/sec is a lot of data! – approximately 33. 3 Mbytes per second • Not practical to transmit 270 Mbit/s - except on optical fibre • Even worse with High Definition – Studio standard is 1. 3 Gbit/s – Approximately 150 M Bytes per second
Compression • In the broadcast world we compress using industry standard techniques – MPEG-2 – MPEG-4 • MPEG-2 Standard definition compresses video at 270 Mbit/s down to 2 Mbit/s • MPEG-4 High definition compresses HD video at 1. 3 Gbit/s to 8 Mbit/s – In less than 1 second • 99% reduction in bit rate but still better than VHS quality? – watch the background and grass! – And you get delay • Both standard use similar techniques – Interframe – Intraframe
Inter frame compression Macro block
Intra frame compression I non-ref B P P
Inter frame compression – 1 Source Frame A
Inter frame compression – 2 Source Frame A
Inter frame compression – 3 Source Frame A
Inter frame compression – 4 Source Frame A
MPEG-2 and MPEG-4 • Standards are crucial to the success of the broadcasting industry – Ensures interoperability between the transmission head end and the set top box • 2 main standards affect what we do – Motion Pictures Expert Group (MPEG) • Video compression – Digital video broadcasting (DVB) • Transmission standards • MPEG-2 has been the major standard for the last 15 years – 100 million STBs! • • But we needed something better MPEG-4 is a completely new algorithm – 50% more efficient – Standard Definition used for “green field” • DTV and IPTV – High Definition used everywhere • SKY • Free. Sat
Bit Rate (Mbits per second) MPEG-2 - improvements Bit-rate for Broadcast Quality Television* First Broadcast MPEG-2 Encoder 8 7 Reflex. TM Statistical Multiplexer Enhancements 6 Enhanced Motion Estimation 5 4 TTV Noise Reduction 3 Advanced Preprocessing MPEG-2 Future 2 Advanced Encoding Future 1 05 20 04 20 03 20 02 20 01 20 00 20 99 19 98 19 97 19 96 19 95 19 94 19 Year
How good is MPEG-4?
Typical System Architecture
Don’t forget the audio! • As many channels as you like – it’s only bits • • Mono, dual Mono Stereo, joint Stereo Multiple languages 5 Channel surround sound – Dolby & DTS • Sample rates from 32 – 256 Kbit/s
Not just pictures - EPG and CA • The EPG is the user interface • Transmitted at same time as TV and radio services • Contains program info plus forward looking data • Also available as XML files on the internet • Condition Access prevents illegal viewing of pay TV channels • None encrypted channels are known as Free To Air (FTA)
Putting it all together - Multiplexing • Alongside pictures we have stereo audio, conditional access EPG data etc • Can even multiplex SD & HD together • Also many programs are combined in to one big bit stream • – Sky = 10 channels + – Freeview = 6 channels – to get many channels in a single radio channel Can multiplex SD & HD together • Even greater gains using statistical multiplexing! • – But that’s another story SPTS used by IPTV and amateurs
Modulators • Now we have a data stream (or multiplex) containing all our programs we transmit it – Using DVB standards • Over terrestrial channels – Using DVB-T (OFDM) – From transmitter towers • Over satellite – Using DVB-S (QPSK, 8 PSK or DVB-S 2) • Over cable – Using DVB-C = QAM – Modulator at edge for Vo. D – hence “edge QAM” • Over IPTV – No standard adopted! – Not web TV
Multi vs single carrier • All use error correction – Transmitted rate is not useable video rate • Single carrier QPSK – – – Bandwidth varies with Bit rate Gives DX mode Half the symbol rate = 3 d. B • Multi-carrier OFDM – Fixed bandwidth – Freeview = PAL replacement = 8 MHz – Uses “guard interval” to protect against frequency selective fading
Multi-carrier vs single QPSK O F D M
OFDM on air
QPSK on air
Typical System
Hampshire technology = 2, 000+ customers
Agenda • Digital TV basics – – Why compress? Compression and modulation standards HD and SD Multiplexing • Services – SKY – Free. View – Free. Sat • Digital ATV – Why and How
The UK market • Probably the most advanced market in the world – 22 million TV households • 5 major platforms providing a variety of services over different delivery systems: – – – Sky = DTH pay TV satellite Freeview = Terrestrial Virgin = cable Free. Sat = Free to air satellite BTVision = “Hybrid IPTV”
Free. View • Digital terrestrial service – Transmitted from TV masts around the country • Will become the main service after Analogue Switch Off in 2012 – Government funded through Free To View Ltd • Very tight on bandwidth – a channel costs £ 12 m pa • 6 frequencies transmitted from each station – 6 services per channel – Approximately 30 TV channels and 30 radio stations – 4 pay to view channels • Currently 16 m STBs (not homes!) – Available via a STB or integrated TV • Currently uses MPEG 2 Standard Definition and DVB-T – Experiments with MPEG 4 HD and DVB-T 2 • Large amount of TTV equipment – and growing; -0)
SKY • Satellite based pay to view service – 8. 5 m subs • Virtually unlimited bandwidth – 300 programs approximately • Mixture of pay to view, FTA and national TV services • SKY services are encrypted – Need a Sky Box to view – SKY boxes can view free to air – The SKY EPG joins it all up • Most channels are MPEG-2 Standard definition • 20+ MPEG-4 High definition channels • All TANDBERG TV technology ; -0)
BT Vision – 300 k subs • BTVision is a “hybrid” service • Uses Free. View to deliver TV - free • Uses phone line to deliver on demand assets – Hybrid PVR set top box with hard disk • Is NOT a true IPTV service! • Why Hybrid? – Needed to have a TV offering to stop customers leaving – BT network cannot support live TV – Movies are trickle fed to the STB • Uses Microsoft MSTV system • Great way to get a free Freeview PVR and powerline adaptors!
Virgin Media – 3 m subs • Consolidation of all UK cable companies (except IOW!) • Only available where the streets have been dug up! • Very controlled environment so high bit rates – 36 Mbit/s & 54 Mbit/s • MPEG 2 SD and HD – Plenty of bandwidth • USP is a 2 way system with return path – High speed internet – Vo. D services
Free. Sat – 100 k subs • • The BIG secret! Is just an EPG!! Uses same transmissions as SKY at 28 degrees east Free. Sat boxes have a new EPG to only list FTA channels • Will have MPEG 2 SD and MPEG 4 HD • Note ITV HD is encrypted at moment – Issues with SKY contract! • Designed to compliment Free. View – Gives 99. 5% coverage – But could end up being the winner!
Internet or Web TV • Internet or web TV is watching video from a website – Not IPTV as quality is not guaranteed • 28% of Europeans currently watch short or full-length videos on-line – an increase of 150% since 2006. • Biggest sites are “catch up TV” – BBC i. Player, Ch 4 on demand • BBC i. Player is the outstanding success – 271 m plays in 2008 – 41 m plays in December • AND BATC. TV!
Agenda • Digital TV basics – – Why compress? Compression and modulation standards HD and SD Multiplexing • Services – SKY – Free. View – Free. Sat • Digital ATV – Why and How
ATV • Most ATV is on 23 cms or 1300 MHz – Analogue FM • GB 3 IV = 1316 output • Some DATV activity using QPSK – 2 or 4 MHz bandwidth – Cheap receivers! – Expensive transmitters – Got gain but you can’t tweak!
Analogue P 2
Digital P 5
In the shack FM rxr Pre-amp Switch/ splitter/ amp Digital rxr
DATV transmit QPSK modulator SSB transverter MPEG encoder Power Amp
WWW. BATC. TV • Internet web site with loads of video clips! • The easy way to get on ATV • Most UK repeaters stream in real time – Including GB 3 IV from the IOW • Video & repeater audio plus 144. 750 MHz talkback – GB 3 IW streamed on IV site • Real time chat room for talkback
Summary • It’s a digital world – Every picture you watch has been digitzed at some point! • Pictures are not better – You start at 270 Mbit/s and watch 2 Mbit/s! • Digital will happen – Digital has gain over analogue – Spectrally efficient • We should start to experiment now – Digital can save 70 cms ATV • Have a look at www. batc. tv