Presentation on theme: "Communications Laboratory Lecture Series"— Presentation transcript:
1 Communications Laboratory Lecture Series Digital television broadcastingPresentation by: Neil Pickford
2 Digital Television Broadcasting DTB What is digital TVHow was it developedWhat are the systemsEnabling technologiesTransmission technologyCompression technologyContent & servicesWhat is happening in australiaThe futureThis lecture will cover the following areas
3 Broad Objectives of DTB Overcome limitations of the existing analog television systemsImproved pictureHigh quality (no interference)Resolution (HDTV)Format (16:9)Enhanced service related featuresAdditional data capacity available for other value added servicesAspect ratio will change from 4:3 to 16:94
4 Digital Media First media systems were analog Most media are converting to digitalComputer storageMusic (LP-CD)TelecommunicationsMultimediaRadio (DAB)TelevisionNo-one is talkingabout or developing analog systems any more.
5 Standard Definition Television SDTV The current television display system4:3 aspect ratio picture, interlace scanAustralia/Europe625 lines pixels x 576 lines displayed50 frames/sec 25 pictures/secpixels totalUSA/Japan525 lines pixels x 480 lines displayed60 frames/sec 30 pictures/secpixels totalSDTV - The television system we have at the moment
6 Enhanced Definition Television EDTV Intermediate step to HDTVDoubled scan rate - reduce flickerDouble lines on picture - calculatedImage processing - ghost cancellingWider aspect ratio - 16:9Multi-channel soundNext Step up towards HDTVLine doubling is done by interpolating the lines inbetween the transmitted lines.Ghost Cancelling was one of the last advances in analog television technology to be incorporated in Australia
7 High Definition Television HDTV Not exactly defined - number of systemsSystem with a higher picture resolutionGreater than 1000 lines resolutionPicture with less artefacts or distortionsBigger picture to give a viewing experienceWider aspect ratio to use peripheral visionProgressive instead of interlaced picturesHDTVMany people are talking about it but it is not exactly defined.Wider aspect ratio so it starts to engrosse your peripheral vision, unlike normal TV which primarily occupies the central vision.
8 HDTV - Have We Heard This Before? The first TV system had just 32 linesWhen the 405 line system was introduced it was called HDTV!When 625 line black & white came along it was called HDTV!When the PAL colour system was introduced it was called HDTV by some people.Now we have line systems and digital television - guess what? Its called HDTV!HDTV - This term has been used over and over.Some people are saying we are not going to make any money out of HDTV, however in the press clipping of 1930s & 1950s the same type of people were saying the same things about the HDTV improvements then. Just talk to the TV companies and see if they are making any money?
9 Interlaced Vs Progressive Scan Interlaced pictures. - 1/2 the lines presented each scan 1,3,5,7,9,11, ,625 field 1 2,4,6,8,10,12, ,624 field 2Because the fields are recorded at separate times this leads to picture twitter & judderProgressive pictures - all the lines sent in the one scan. 1,2,3,4,5,6,7, ,624,625 pictureNo twitter or judder.But twice the information rate.The current interlace system uses 2 interleaved fields of lines to make up every picture. Differences between the temporal sampling of the two fields lead to effects such as twitter and judder on moving objects.With progressive scanning adjacent lines come from adjacent temporal moments and so do not exhibit these effects.
10 Have you seen HDTV pictures? Question - HDTVHave you seen HDTV pictures?I think more people may have seen HDTV than they think.
11 All Current Generation PCs use Progressive Do You Use A PC?All Current Generation PCs use ProgressiveScan and displayPictures which matchor exceed HDTVresolutions although the pixel pitch, aspect ratio and colorimetry are not correct.HDTVSome PC monitors have RGB bandwidths exceeding 120 MHz. HDTV only requires 30 MHz.
12 Digital Television Why digital? Noise free pictures Higher resolution images Widescreen / HDTVNo ghostingMulti-channel soundOther services.The average domestic TV in Australia has all sorts of distortions. Digital TV will remove those distortions. Just like a CD, you never hear a scratched CD. It’s either perfect or it’s nothing.
13 Digital Television - Types Satellite (DBS)DVB-SProgram interchangeDirect view / pay TVSMATVDownlinkWhat are the type of Digital Television?Satellite uses a central uplink to provide regional or national coverage.You need to have a satellite dish and down converter. Because accuracy in pointing is required this is a fixed service.Uplink
14 Digital Television - Types CableHFC - pay TVMATVDVB-C / 16-VSBFibreMain CoaxTethered cable systems coaxial or fibre. You need to be connected to a permanent cable. Again this is a relatively fixed system.SpurTeeTap
15 Digital Television - Types Terrestrial (DTTB)DVB-T / 8-VSBFree to air TV (broadcasting)Narrowcasting/value added servicesUntethered - portable receptionDTTB allows both fixed or portable operation.
16 Digital Terrestrial Television Broadcasting - DTTB Regional free to air televisionReplacement of current analog PAL broadcast television servicesOperating in adjacent unused “taboo” channels to analog PAL serviceCarries a range of services HDTV, SDTV, audio, teletext, dataProviding portable serviceWhat is DTTB?
17 Mobile Digital TV Onboard a Tram in Cologne - Germany A trial in Germany has DTTB on a Tram showing portable operation.
18 How Was Digital TV Developed Japanese & Europeans wanted to improve analog TV - bigger pictures, more resolutionJapanese developed muse 1125 lines 60 HzEuropeans worked on HD-MAC 1250 linesAmericans broadcasters wanted to protect spare unused TV channels from the land mobile service and told FCC they required the channels for a future analog compatible HDTV system.Americans used the term HDTV as an ambit claim to protect their broadcasting spectrum from land mobile interests. This caused a problem because they did not even know what it was, so they had to do something about developing it before the FCC started asking hard questions.
19 Development RaceAmericans embarked on a HDTV race to develop an analog HDTV systemDigital television was seen as impossible.General instruments developed first digital TV system for satellite pay TV from experience with NASA deep space probesHDTV race became a digital HDTV race.Race outcome - poor performance but demonstrated digital could be done.Initial Race was analog, people agreed that digital television was impossible.
20 DTTB Transmission Systems 3 systems are being developed at present.USA ATSC 8-VSBEurope DVB COFDMJapan ISDB band segmented OFDMWhat are the Digtal Television systems?
21 8-VSB - USADeveloped by the advance television systems committee - ATSCDeveloped for use in a 6 MHz channelA 7 MHz variant is possible.Uses a single carrier with pilot tone8 level amplitude modulation systemPayload data rate of 19.3 Mb/sRelies on adaptive equalisationExisting technology developed to near limitATSC developed out of the Grand Aliance which resulted at the end of the initial digital television race in America.
22 COFDM - EuropeDeveloped by the digital video broadcasting project group - DVBUses similar technology to DRBUses 1705 or 6817 carriersVariable carrier modulation types are defined allowing data rates of 5-27 Mb/s in 7 MHzDeveloped for 8 MHz channelsA 7 MHz variant has been produced and testedCan use single frequency networks - SFNsNew technology with scope for continued improvement & development
23 ISDB - JapanJapanese 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 systemsVideo servicesSound servicesBulk data servicesInteractive data services
24 ISDB - ConceptMain TV menuNewspaper - Categories - Headlines Downloaded overnightTelevision ScheduleWeatherPreview of other stationsTime, ,Interactive services16:9 Display.Proposed to use band segmented transmission - orthogonal frequency division multiplex (BST-OFDM)
25 BST-OFDM - JapanBST-OFDM is a variant of the European COFDM system which allows segmenting of the data spectrum into 100 kHz blocks.2 receiver bandwidths proposed.500 kHz portable / mobile for sound and data5.6 MHz fixed / mobile for SDTV and LDTV5.6 MHz fixed for HDTVIndividual band segments can be allocated to separate services which can use different modulation systemsTakes a wide band system and segmented it into 100 kHz blocks. Data can be reallocated in these blocks.Radio/portable services on small blocks of spectrum, Television services on the larger blocks of spectrum.The ruggedness of the system can be tailored to the intended use by varying the modulation parameters within each of the 100 kHz blocks.
26 BST-OFDM - JapanAllows separate services to be replaced for local area broadcastingAllows for variable ruggedness for fixed / mobile / portable receptionCould straddle other existing services.Primarily being developed for japan as a solution to cluttered broadcasting spectrum.In early stages of developmentNo hardware available at this stageSolely being developed within Japan for Japanese use. It is not seen as a contender for the Australian environment.
27 8-VSB & COFDM - Spectrum 8-VSB COFDM This is a spectrum analyser plot of the two digital systems being considered in Australia. The little hump on the left side of the 8-VSB spectrum (yellow) is the pilot carrier.These are averaged spectrums and have shoulder levels of around dBThe spectrums are basically rectangular in shape. The COFDM signal is wider since it is a 7 MHz system in a 7 MHz channel while 8-VSB is 6 MHz wide.
28 Traditional SCPC Modulation Minimum Carrier SpacingWhat is this COFDM modulation?A Traditional Single Carrier Per Channel (SCPC) digital system there is a minimum spacing that you can put two individual carriers apart. It tends to be around 3 times the bessle frequency, where the modulation sidebands dimminish sufficiently to cause minimal interference to the adjacent carrier. SCPC is a type of Frequency Division Multiplex (FDM)Frequency
29 COFDM - Orthogonal Carriers COFDM is Coded Orthogonal FDMIf you observe the carrier spacing they are much closer together. This thing called “Orthogonality” is the key. It means that the peak of the yellow carrier (and all others) coincides with a null on every other carrier. Each peak sits directly above a null. You cannot generate these carriers by having individual oscillators mixed together. They are all generated at the same time using an Inverse Fast Fourier Transform (IFFT) using the same clock.Frequency
30 Almost Rectangular Shape Spectrum of COFDM DTTBCarrier Spacing2k Mode 3.91 kHz8k Mode 0.98 kHzAlmost Rectangular ShapeThese 1000s of carriers form the observed rectangular spectrum.There are two modes for the system 2K & 8K. This referes to the size of the FFT used to generate and demodulate them. The 2k system has 1705 carriers and the 8k system uses 6817 carriers. The extra locations in the FFT are used to ensure that the signals have a sharp roll-off and good out of channel performance.1705 or 6817 Carriers6.67 MHz in 7 MHz Channel
31 Digital Modulation - 8-AM 7654321What are the modulation types used.These are 8-VSB 8-AM eye diagrams taken of an analog CRO. Both diagrams are a direct coaxial feed with no path impairments. The left diagram is the waveform which comes out of the Tuner IF, while the right diagram is the same signal after equalisation.The left diagram has almost no data eye. The section in the middle is the segment sync which uses 2 level data for robust clock timing recovery.Before EqualiserAfter Equaliser8-VSB - Coaxial Direct Feed through Tuner on Channel 8 VHF3 Bits/Symbol
32 QPSKQIThe European System uses QPSK & QAM on the thousands of carriers within the COFDM signal.QPSK modulation uses 4 different phase angles 90 degrees apart, all at the same amplitude. The decoder only has to decide which quadrant the data point lies in. Each QPSK symbol carries 2 data bits.2 Bits/Symbol
33 16-QAMQIThe next modulation level is 16-QAM. Note that the original quadrant point for QPSK is in the middle of the four points in each quadrant. The insert constellation shows what a real modulated 16-QAM system looks like. The points are more fuzzy than the concept diagram, this is due to phase and amplitude noise which is encoutered in any communication system. Close examination reveals that there are a few extra points out in nowhere land between the points which may be interpreted as incorrect data values. To decode this signal the system has to set a boundry around around each point as a decision threshold.4 Bits/Symbol
34 64-QAMQIThe more points you have in the modulation constellation the less robust the system is. 64-QAM has 4 times as many points as 16-QAM. This is the modulation which is applied to each carrier in the COFDM Digital TV system we have been testing.6 Bits/Symbol
35 64-QAM - Perfect & Failure These figures are real 64-QAM system constelations for both a no noise “perfect” condition and at the failure threshold due to white noise. In the system on the right the data eye is no longer visible but the system is still just working. Again notice in the perfect case that there are some errant points inbetween the main constellation points.
36 64-QAM and QPSKQIHere is the 64-QAM diagram again. I have drawn in the positions of the QPSK points if this was only a QPSK system. They occupy the centre of each of the 4 quadrants. The QPSK system can take a noise hit which can move the amplitude and phase half a quadrant to anywhere else in the same quadrant, and still be decoded correctly. Thus it is much more robust than 64-QAM. QPSK however only allows 2 data bits per symbol to be transmitted. What if we could combine these two modes?6 or 2 Bits/Symbol
37 Non Uniform 64-QAMQIIf we modify the system and make it non uniform, we can encode 6 bits of data per symbol with two levels of robustness in the data. 2 bits encoded as QPSK and a further 4 bits at the normal 64-QAM spacing. The data can then be allocated to these data bits on the basis of service type, with a Robust low data rate mobile service using the QPSK data and the normal higher rate Fixed service using the remaining delicate 64-QAM data mode.2 + 4 Bits/Symbol
38 Non Uniform 16-QAMQIThe same technique can be applied to 16-QAM however you end up with two services each having 2 bits per symbol at different protection levels. This is known generally as Non-Uniform or Heirachial QAM and is covered within the COFDM system specification.2 + 2 Bits/Symbol
39 8-VSB - DTV - Development 1987 FCC inquiry into future TV systems and advisory committee on ATV service was established - ACATS1990 digital TV systems developedCompetitive testing race undertaken1993 poor results announced grand alliance (GA) formed by the contestants. - Extra development1994 re-testing of GA systemHow was the development of 8-VSB handled?Grand alliance took the best components of each of the condending digital systems and carried out extra development to come up with a single combined system.
40 8-VSB - DTV - Development 1996 FCC adopted ATSC standard1997 each full-power broadcaster loaned a second 6 MHz TV channel for simalcasting DTV.1997 FCC announced DTV service and mandated 8 year transition schedule1997 demonstration, laboratory testing and field trials of 8-VSB in Australia
41 8-VSB - Transition Schedule 1/5/99 coverage of 10 largest markets1/11/99 coverage top 30 markets1/5/02 all other commercials on air1/5/03 all non commercial stations2006 switch off analog service and recover 138 MHz of spectrumAn aggressive transition schedule has been laid out by the FCC.
42 What does this 8-VSB equipment look like? This is the 8-VSB receiver we had here in Australia during the Laboratory testing. The equipment is commonly refered to as the “Blue racks”. It is a half height 19 inch rack and would be better described as a set bottom decoder rather than a set top, since it takes at least 2 people to lift it. This prototype equipment from Zenith, operates on 110 volts and the right hand photos show a close up of the card rack and the technology used on an average card, of which there are 14. This particular card is a phase equaliser.
43 8-VSB Equipment Still at the prototype stage First chips are being tested nowThis year domestic receiverThey have now compressed all of the logic in the “Blue Racks” into three VLSI chips which are being tested and integrated into receivers at present..
44 European Development - DVB 1991 European launching group (ELG)1992 ELG developed MoU for cooperation1993 ELG became digital video broadcasting (DVB) project - a forum for all interested in digital TV to participate in research and development as a unified group.DVB is a consortium of over 200 network operators, broadcasters, manufacturers and regulators in 30 countries working together.How did the Europeans do it?Until late 1990, DTB to the home was thought to be impractical and costly During 1991, broadcasters, consumer equipment manufacturers and regulatory bodies formed a group that would oversee the development of digital television in Europe - the European Launching Group (ELG).The ELG expanded and drafted a MoU establishing the rules by which this new and challenging game of collective action would be played. This meant that commercial competitors needed to appreciate their common requirements and agendas. In September 1993, and the Launching Group became DVB(Digital Video Broadcasting). Development work in digital television, already underway in Europe, moved into top gear.Around this time, the Working Group on Digital Television prepared a study of the prospects and possibilities for digital terrestrial television in Europe and introduced concepts, such as allowing several different consumer markets to be served at the same time (eg. Portable television and HDTV).
45 DVB ProjectThe DVB philosophy - open, interoperable, flexible, market-led, global standards for digital TV1980s MAC systems under development gave way to all digital technologyBased on common MPEG-2 coding systemIntegrated set of standards allowing flexible operation across cable, microwave, satellite and terrestrial distributionThe DVB project want everyone to participate, and everything to be open, and not heavilly locked up in restrictive patents.In the 1980s it was becoming clear that the once state-of-the-art MAC systems would have to give way to all-digital technology. DVB provided the forum for gathering all the major European television interests into one group. It promised to develop a complete digital television system based on a unified approach.
46 DVB - COFDM - Development Easier satellite (DVB-S) & cable (DVB-C) systems were developed first.DVB-T is the terrestrial member of the DVB family of standards.OFDM transmission originally developed for cable systems, adapted to digital radio broadcasting, extended by DVB to digital TVDVB-T based on COFDM technologySatellite and cable were developed and delivered the first broadcast digitaltelevision services. Fewer technical problems and a simpler regulatory climate meant that they could develop more rapidly than terrestrial systems. Market priorities meant that digital satellite and cable broadcasting systems would have to be developedrapidly. DVB-T Terrestrial broadcasting followed since it has more interference problems than satellite or cable.By 1997 the development of the DVB Project had successfully followed the initial plans, and the project had entered its next phase, promoting its open standards globally, and making digital television a reality.
47 COFDM - Transition Schedule DTTB test transmission programs are currently occurring in Denmark, Holland, Finland, France, Germany & Italy1998 Britain & Sweden on air with SDTV DTTB system using UHF band.2001 Spain plans DTTB to be operational, achieving 100% coverage by 2010.Simulcasting is expected to be around 20 years in Europe. Focus is SDTV to EDTVWhat is the transition schedule in Europe?The European way is not as aggressive as the US FCC system with an anticipated 20 year change-over period for simualcasting Analog and Digital TV. Europeans are focusing on SDTV at this stage as they had their fingers burnt with HD-MAC. They are looking to improve normal trelevision.
48 COFDM - Commercial Receiver News data systems - system 3000What does the European equipment we tested look like? It is a 2 rack unit, 19 inch wide rack box as shown in the picture above with two receivers on top of each other showing the front and back panels. It is a commercial receiver with an infr-red remote control utilising on screen menus. It has a single RF input, comonent & composite video outputs along with a digital tansport stream output. It has RS232 interfaces to allow remote control and diagnostics.
49 COFDM DTTB Equipment System 3000 - NDS Project mummy bear - NDS zenith Dvbird - Thomson SGS Philips3 chip receiver - PhilipsTest receiver - ITIS HarrisChip set - Hokia SiemensOver 20 manufacturers showing hardwareOther equipment that is available or under construction.Quite a few different companies are involved in producing hardware for the COFDM system.
50 COFDM - Current Hardware What is inside the box?There are three main boards in this receiver. On the far right is the tuner mixer A/D converter and synthesizer, in the cenre is the COFDM demodulator, and the final board is the MPEG decoder which is normally located at the front on top of the COFDM demodulator. In fact the MPEG decoder is a standard PACE decoder with some parts unused. This is the same board which is in the Galaxy pay TV satellite set top decoders. This is because the components of Digital TV are modular with common interface points.
51 Dvbird - Receiver 4 VLSI COFDM receiver Implements an 8K FFT (2K/8K mode)QPSK, 16QAM & 64QAM1/4,1/8 & 1/32 guard intervalsOnboard tunerThe Tuner and COFDM Demodulator in the previous photo will be replaced by this board which utilises 4 VLSI chips.Originally it was thought that the first integrated boards would only do 2048 point (2k) FFTs because of the complexity, however this is one of the first and it includes the full 8096 point (8k) FFT. It allows all the various QAM modulation modes
52 Enabling Technologies Source digitisation (Rec 601 digital studio)Compression technology (MPEG, AC-3)Data multiplexing (MPEG)Transmission technology (modulation)Display technology (large wide screens)Digital TV has Key Technologies that make it possible. Most production within the current TV stations already happens in the digital domain using standards such as Rec 601 digital video. It only becomes analog when it is transmitted over the air to the viewer.Display technology has not reached the level needed for HDTV to be fully implementable at present.
53 Digital Terrestrial TV - Layers . . . provide clean interface pointsPictureLayer1920 x 10801280 x 72060,30, 24 HzMultiple Picture Formatsand Frame RatesMPEG-2compression syntax orVideo CompressionLayerDataHeadersMotionVectorsChroma and LumaDCT CoefficientsVariable Length CodesPacket HeadersFlexible delivery of dataTransportLayerDTTB is about layers.PictureCompressionTransport/MultiplexingTransmissionVideo packetAudio packetVideo packetAux dataMPEG-2 packetsVHF/UHF TV ChannelCOFDM / 8-VSBTransmissionLayer7 MHz
54 Digital Television Encode Layers VideoDataSoundControlDataPictureCodingDataCodingAudio CodingMPEG-2 or AC-3MPEG-2MPEG Transport Stream MuxProgram 1 MultiplexerControl DataProgram 2Program 3Other DataService MuxBouquet MultiplexerControl DataWhat are the inputs to these layers?MPEG Transport Data Stream188 byte packetsError ProtectionModulator & TransmitterControl DataDelivery System
55 Digital Television Decode Layers SpeakersDataMonPictureDecoderDataDecoderAudio DecoderMPEG or AC-3MPEG-2MPEG Transport StreamDe-MultiplexerMPEG DeMuxAt the receiver we simply select the portions of the existing data stream we wish to decode and throw away the rest.Demodulator & ReceiverError ControlDelivery System
56 Transmission Technology The transmission system is used to transport the information to the consumer.The system protects the information being carried from the transmission environmentCurrent Australian analog television uses the PAL-B AM modulation systemThe transmission technology protects the data from the communications environment.In something like the satellite environment it is very easy since there are not many variable sources of transmission impairment, A terrestrial system has to cope with obstructions, reflections, multipath, and various types of man made interference
57 Digital TV Transmission Technology The transmission system is a “data pipe”Transports data rates of around 20 Mb/sTransports data in individual containers called packetsBeing Digital, it is just like any computer data. You put a byte of data into the pipe, you must receive the same byte out the end of the pipe. If it is different at the other end it simply does not work. The testing of these systems had primarilly been done (95%) looking at Bit Error Rates (BER), not the displayed pictures. The data is transported in 188 byte groups called packets.
58 Terrestrial Transmission Problems Multipath interference - ghostsNoise interference - snowVariable path attenuation - fadingInterference to existing servicesInterference from other servicesChannel frequency assignment - where to place the signalThe current Analog TV system has lots of problems which can arise in the transmission path and are reflected in a poor quality picture.We also have problems with planning where to place these services, because we have to avoid known interference mechanisms.
59 Digital Modulation - Functions Spreads the data evenly across the channelDistributes the data in timeMaintains synchronisation well below data thresholdEmploys sophisticated error correction.Equalises the channel for best performanceDigital modulation distributes the information across the channel bandwidth so the rectangular spectrum shown earlier is produced. This makes more efficient use of the spectrum.Highly rugged transmission techniques are used for the system synchronisation data such as QPSK and 2 level data. This is because if the system clock is lost, the whole system immediately stops working and may take some time to reaquire lock.Error correction techniques such as Reed Solomon and Viterbi Coding are used to correct errors in the data.
60 Digital Has to Fit In With PAL We need a digital system that can co-exist with the existing analog broadcast TV currently in use in AustraliaWe use the PAL-B with sound system GAustralian TV channels are 7 MHz wide on both VHF & UHFAustralia uses:VHF bands I, II & IIIUHF bands IV & IVWe have an existing service in the television brodcasting spectrum. To introduce a Digital TV service in Australia it must be able to co-exist with the current system for a simualcast period.Unlike most of the rest of the world we use 7 MHz TV channels for both the VHF and UHF TV bands.
61 Digital Has to Fit In With PAL World TV channel bandwidths varyUSA / japan 6 MHzAustralian 7 MHzEuropeans 8 MHzAffects:- tuning, filtering, interference & system performance28293031323334352829303132333435Why would the channel spacing matter?It means that we need a special 7 MHz channel varient for us. The diagram shows 6,7 & 8 MHz channel spacings at UHF starting at channel 28. You can see that by the time we get to channel 35 we are over a channel out. If nothing else this means that Digital TV receivers for Australia will need a flexible tuning system which allows for 7 MHz spacings instead of 6 or 8 MHz. This would mean a software change.2829303132333435
62 Digital Has to Fit In With PAL Digital television system development is focused in Europe & USAThe systems standards are designed to meet the needs of the developersThey focus on their countries needs firstAustralian input is through standards organisations such as the ITU-RDigital TV development has Focused on the needs of the countries that have developed it.
63 Channel SpacingExisting analog TV channels are spaced so they do not interfere with each other.Gap between PAL TV servicesVHF 1 channelUHF 2 channelsDigital TV can make use of these gapsCh 6Ch 7Ch 8Ch 9Ch 9AI talked earlier about there being problems with channel spacing and taboo channels. Analog TV cannot cope with another analog service in the adjacent channel without some interference occuring. An analog service in channel 8 above would interfere with channel 7 & 9 in the same area. Digital has been designed to use these inbetween channels without interfering with the Analog service.VHF Television Spectrum
64 Digital Challenges Digital TV must co-exist with existing PAL services DTV operates at lower powerDTV copes higher interference levelsShare transmission infra-structureDTV needs different planning methodsCh 6Ch 7Ch 8Ch 9Ch 9AThe digram here shows the spectrums of both 8-VSB and COFDM in the adjacent channels 6 & 8. Note that the 8-VSB has a lot more room at the edge of the channel. This is because it is a 6 MHz system operating in a 7MHz channel. The COFDM signal gets very close to the sound subcarriers of channel 7.Because it is in the same area as current transmissions the antennas & towers can be shared by combining the signals. This avoids constructing new towers but does need some complex combiners to be built.8-VSBCOFDMVHF Television Spectrum
65 Digital Service Area Planning Analog TV has a slow gradual failureExisting PAL service was planned for: 50 % availability at 50 % of locationsDigital TV has a “cliff edge” failureDigital TV needs planning for: % availability at % of locationsAt the edge of the analog service area at 50% of the locations in that area you will have a “good” service at least 50% of the time. This is because analog TV has a slow and gradual failure mechanism.Digital has a cliff edge failure, Its either perfect or its nothing. So we have to change these planning numbers to achieve 90-99% availability at 90-99% of locations.
66 TV System Failure Characteristic GoodQualityEdgeofServiceAreaThe cliff edge failure is demonstrated in this diagram. As soon as you transmit the analog signal it starts degrading however digital remains perfect until it suddenly fails.RottenCloseFarDistance
67 TV System Failure Characteristic GoodQualityEdgeofServiceAreaThe real problem arises in the shaded area where people outside the analog service area have been receiving “fortuitos” reception of analog, usually with a poor picture, but will receive no digital service.This will be a difficult problem to deal with, as some of the people in these areas have gone to great lengths to get capital city reception.RottenCloseFarDistance
68 Digital Provides New Concepts Single frequency networks (SFNs) can help solve difficult coverage situationsSFNs allow the reuse of a transmission frequency many times in the same area so long as exactly the same program is carriedAllows lower power operationBetter shaping of coverageImproved service availabilityBetter spectrum efficiencyI mentioned SFNs earlier. This is a feature of the COFDM modulation technology which allows reuse of the same spectrum in the same coverage area, and thus greater spectrum efficiency. The circles represent the coverage in Canberra, Green being the main transmitter and the other circles being existing or future translators which all use separate channels with Analog TV. Using a SFN all these fill in transmitters could be on the same frequency. Unfortunately we cannot use the digital SFN technique in channels adjacent to existing analog services, so this will be a restriction on the use of this technique, until the analog services can be switched off at the end of the simualcast period.SFNs must have exactly the same data on each transmitter.
69 Compression Technology When low bandwidth analog information is digitised the result is high amounts of digital information.5 MHz bandwidth analog TV picture º Mb/s digital data stream.270 Mb/s would require a bandwidth of at least 140 MHz to transportCompression of the information is requiredCompression technology is also an enabling thechnology. When you digitise video you end up with massive amounts of real time data. In the above example you would need 20 channels to transmit one digital video signal. We have to compress the television signal so there is less data, allowing it to fit in a normal channel
70 Compression - Types Two types of compression available Loss-less compression 2 to 5 timesLossy compression 5 to 250 timesLoss-Less compression produces exactly the same data out as went into the process, like over a telephone modem.Lossy Compression allows changes to occur in the data such that subtle approximations are made to the images or sounds that the viewer will not be able to notice.
71 Compression - Loss-less Types Picture differences - temporalRun length data coding - GIF= 1 + 4x21 bits source = 12 bits compressedHuffman coding - PKZIPShort codes for common blocksLonger codes for uncommon blocksLookup tablesTemporal Picture Differences rely on the premise that most of the background in pictures does not change from picture to picture, so why transmit it more than once. A difference is calculated by subtraction and only the information that has changed is transmitted.Run Length encoding exploits repeating sequences or data patterns
72 Compression - Lossy Types Quantisation - roundingMotion vectorsPrediction & interpolationFractal codingDiscrete cosine transform (DCT)Quantisation - A 8 or 16 bit signal may not need that level of resolution, 4 or 6 bits may suffice. It might, however make things a bit more fuzzy.Motion Vectors - is a technique where common pixel blocks are identified from picture to picture and their movement transmitted, instead or retransmitting all the information for the blocks. Eg a hand moving, the pixel blocks displaying the hand are identified and the information transmitted that it they moved X pixels in direction Y. The main problem with this technique is the high level of processor power needed to carry out a pixel block search and match.Prediction & Interpolation use averaging to determine data between known points without having to transmit it.DCT is used by MPEG-2 along with Differencing, Motion Vectors, Prediction and Interpolation.
73 Compression - DCTHere is an example of DCT compression. A simple 8x8 pixel area around the “1” on the calendar has been compressed using the DCT. The original and compressed data values in an 8x8 matrix are shown. Notice the 64 original values have been reduced in this particular example to 4 non zero numbers. The numbers in the DCT matrix represent a frequency distribution of the H & V pixel information in the original picture. The quantisation level of the final numbers can also be reduced with typically half of the matrix being zero or very close to zero not requiring transmission. The reverse process produces a pixel block which is a very close approximation to the original, even when some of the elements have been quantised.8x8 Pixels
74 MPEG-2 - I, P & B FramesUncompressed SDTV Digital Video Stream Mb/sPicture 830kBytesPicture 830kBytesPicture 830kBytesPicture 830kBytesI FrameB FrameB FrameP Frame100 kBytes12 kBytes12 kBytes33 kBytesMPEG-2 Compressed SDTV Digital Video Stream Mb/sI - intra picture coded without reference to other pictures. Compressed using spatial redundancy onlyP - predictive picture coded using motion compensated prediction from past I or P framesB - bidirectionally-predictive picture using both past and future I or P framesMPEG has different types of frames which allow interpolation and prediction to be used to reduce the amount of data that needs to be sent.Three types of frames I, P & B frames.I frames have around 9 times less data, P frames 25 times less and B frames 70 times less data than the original frame. In this example the total compression is 43 times.These frames are usually sequenced in a 12 frame Group Of Pictures (GOP) sequence. Typically structured I B B P B B P B B P B B I B B P B B P B B P B B I etc
75 MPEG-2 Compresses source video, audio & data Segments video into I, P & B framesGenerates system control dataPacketises elements into data streamMultiplexes program elements - servicesMultiplexes services - transport streamOrganises transport stream data into 188 byte packetsThe functions of MPEG-2 are then as shown in this table.
76 MPEG-2 - Formats ML & HL MPEG-2 defines profiles & levels They describe sets of compression toolsDTTB uses main profile.Choice of levelsHigher levels include lower levelsLevel resolutionLow level (LL) by 288 SIFMain level (ML) by 576 SDTVHigh level (HL) 1920 by 1152 HDTVProfiles and Levels within MPEG-2 define sets of tools or syntaxes for compressing the picture information. If you have a higher level toolbox than you can handle the lower level compression modes, however if you have a middle level tool set you are unable to process images built with the high level tool set.For SDTV you need Main Level (ML) where as for HDTV you need High Level (HL). If you want to process HDTV you must have a HL decoder. During HL only transmissions a ML decoder will stop decoding and go “Black”. Although HDTV may not be ready at the start of the Digital TV era, the decoders must have a HL decoder otherwise when HDTV is available those decoders will not work. A more inefficient solution which may need to be adopted in Britain, which is installing ML only decoders, is to always transmit a ML signal along with the HDTV. Unfortunately this reduces the data rate available to HDTV by 3-4 Mb/s.
77 Video Formats - SDTV - 50 Hz Here are some of the SDTV video formats, with the relevant number of pixels and bitrate.All these formats are Interlaced
78 Video Formats - HDTV - 50 Hz HDTV formats with pixels and bitrate.Yellow are interlaced formatsWhite are progressive formatsGreen can support both formats
79 Common Image Format CIF 1920 pixels x 1080 lines progressive scan is now being promoted as the world CIF.All HDTV systems will need to support this image format and then allow conversion to any other display formats that are supported by the equipment.Finally the International community have got together and defined a Common Image Format (CIF) for HDTV type production. This means that world wide people can produce material in the same HD format allowing easier interchange without standards conversion. All the future HDTV systems will support this format even if they are working with lower level display devices, they will be required to be able to up or down convert to the CIF.
80 DTTB - Content & Services DTTB was designed to carry video, audio and program data for televisionDTTB can carry much more than just TVElectronic program guide, teletextBest of internet serviceBroadband multimedia data, news, weatherInteractive servicesSoftware updates, gamesServices can be dynamically reconfiguredDTTB can carry many other things than just television.Interactive services need a back channel such as the telephone line or a cable/wireless modem.The analog television we are used to uses a very dumb device to display the pictures.Digital TV uses a smart box which can be dynamically re-configured. You can choose the change the channel/data structure mid program, upload new operating software with different funtions. No longer will the transmission be totally constrained by the dumb receiver at the other end.
81 Video Program Capacity For a payload of around 19 Mb/s1 HDTV service - sport & high action2 HDTV services - both film material1 HDTV + 1 or 2 SDTV non action/sport3 SDTV for high action & sport video6 SDTV for film, news & soap operasHowever you do not get more for nothing.More services means less qualityDigital TV will have a data capacity around 20 Mb/s.For Sport or high action we can have relatively few services.Films have high levels of temporal redundancy because both fields are scanned from the same frame. This allows the compression systems to perform higher levels of compression allowing spare data capacity and the ability to have more services.Generally News & Soapies have lower data requirements so more channels are possible.You do not ge more channels for nothing.More Services Means Less Quality.
82 Spare Data CapacitySpare 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 commercialThe Commercial was shown using 1080 LinesInterlaced. 60 Mb of datawas transferred during it.In the Final 3 seconds theBMW Logo was displayedallowing 3 Phone Booksof data to be transmitted.An example from tests in America.Other non-real time services can use opportunistic use of the DTV data pipe to transmit data when the full bandwidth of the channel is not required for the main services. These services would be data specifically intended for broadcast application with no need for acknowledgement or a back channel. Teletext or Newspaper type information are good examples.
83 Navigation SystemsThe concept of channel numbers for networks may disappear with DTVTelevision becomes one of a number of services carried within the data pipe.Users will select what service or program they wish to decode. The box then finds it.Each digital transmission can carry program directories for all service providersThis area is still under heavy developmentHow will you drive this new Receiver?
84 Australian ActivityABA report on digital television in Australia recommended using HDTVFACTS have set up a specialists group to advise and direct commercial advanced television developmentRepresents commercial television (7,9,10)ABA and communications lab have been assisting this groupNTA, ABC and SBS are not representedNTA commenced own trialsABA ran a public enquiry.FACTS running commercial network tests on air in Sydney channels 6 & 8.NTA running their own COFDM trials on channels 12 & 29 in Canberra in mid 1998.
85 Laboratory Tests Tested both COFDM & 8-VSB systems Investigated operation within the existing Australian broadcasting infrastructureSystems evaluated as data pipesBoth systems operate satisfactorily with only small operational differences evidentReport on measurements was produced for the FACTS specialists groupLaboratory tests were conducted by the Communications Lab on the equipment shown earlier in this lecture.Many of the tests had been done overseas but they had focused on the 6 & 8 MHz channel plans.We looked at the systems as data pipes, however pictures were also run on the COFDM system. Unfortunately the 8-VSB equipment did not have any source coding/decoding equipment, so we were unable to assess pictures through the system.There was not enough difference between the COFDM and 8-VSB systems, for the tests performed, to clearly eliminate either system from contention.
86 Laboratory Tests - Test Rig C/N Set & AttenuatorsEUTPAL & CWThis is the main test rig which generated the wanted and unwanted signals for Digital and Analog television. Many of the tests were computer controlled. The receivers were located in a separate shielded area.Control ComputerDomestic Television ReceiverModulator Control ComputersSpectrum AnalysersPlot & Printing
87 Laboratory Tests - Test Rig Power MeterPAL & CW Interference GeneratorsRF LOCOFDM ModulatorMPEG MuxMPEG MuxThe Digital modulators and Pal generation equipment.All digital power measurements were made with a thermal type power meter.The MPEG Encoders are the boxes which compress the video and audio data.MPEG Encoder8-VSB ModulatorMPEG Encoder
88 Field TestsField tests conducted in Sydney on VHF channel 8 during oct-nov 1997Both COFDM & 8-VSB systems evaluated at over 150 sites using an ABA field vehicleComparison of the digital and existing PAL systems performance at each siteConcentrating on difficult reception sitesReport on field trials was produced for the FACTS specialists groupThe Field Tests used an adjacent channel combiner at TCN-9 in sydney and transmitted the channel 8 digital signal at around 400 watts next to the channel 9 signal which uses a vision sync power of watts. This configuration was also capable of transmitting channels 7 & 10 as well in an emergency standby situation.
89 A Future Digital System Concept MMDSHypermedia Integrated Receiver Decoder (IRD)SatelliteTerrestrialCableBroadcastInteractivityWhat will the future home digital system components be? The IRD will be the central hub of the home television and information system.Interactivity will be via Cable or the Telephone line. Inputs can be from Satellite, MMDS, Cable or Terrestrial.It will primarily be controlled by a IR remote control and on screen menus. It will most likely link not only to a display device and DVD/DVC but also your home computer, allowing data applications.B-ISDN XDSLCD, DVD DVC
90 Future - Things to Be Done Decide on digital transmission standardPolicy HDTV vs multiple SDTVMinimum data rates / quality ?Multiplex / content provider relationshipsPay vs free to airSort out service provider issuesConditional access systemsAncillary dataMajor issues to be thought about.
91 Thankyou for your attention The EndThankyou for your attentionAny questions?
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