Contains description of one research project, reports on technology, final status and future of research projects, and a list of publications.Research Laboratory of ElectronicsDepartment of Electrical Engineering and Computer Scienc

Manning, Deborah S.

Polley, Michael O.

Schreiber, William F.

Wee, Susie J.

DSpace@MIT

Chapter 3. Combined Coding for High-Definition TelevisionChapter 3. Combined Source and Channel Coding forHigh-Definition TelevisionAcademic and Research StaffProfessor Emeritus William F. SchreiberGraduate StudentsMichael O. Polley, Susie J. WeeTechnical and Support StaffDeborah S. Manning3.1 Project DescriptionSponsorsResearch Laboratory of ElectronicsDepartment of Electrical Engineering andComputer ScienceThe purpose of this project was to develop a high-definition television (HDTV) system for terrestrialbroadcasting. Under decisions previously made bythe Federal Communications Commission, eachcurrent broadcaster will be given a second channel.Originally, this channel was to be used for HDTVwhile transmitting the same program on thestandard NTSC channel in a procedure calledsimulcasting. Fifteen years after the commence-ment of high-definition broadcasting, NTSC was togo off the air, and the original channels would bereturned to the FCC for reassignment. In manycases, the new assignments would have beentaboo channels not usable for NTSC because ofinterference. This obviously would require that theHDTV transmissions had excellent interference per-formance. Coverage must be comparable to whatis now achieved with NTSC.A great deal of controversy developed over thisplan. In part, the problem was related to what wasseen by some as a giant giveaway, since thesecond channels are to be given (actually loanedfor the transition period) only to currentbroadcasters at no charge. In part, the controversywas due to a dispute between the television andcomputer industries over certain aspects of thestandards. Finally, the Clinton administrationexpressed its desire to shorten the transition periodto only five years so that the original NTSC chan-nels could be auctioned off to help balance thebudget.The TV-computer issue was "solved" by an agree-ment between the two contending industries bro-kered by one of the FCC commissioners in whichthe table of scanning formats was eliminated fromthe proposed standard. The modified standard wasapproved in December 1996, and, in principle,permits any scanning format to be used. The effectthat this bizarre decision will have on the develop-ment of digital broadcasting remains to be seen.The FCC must still issue rules governing the use ofthe new channels. A major question is how much,if any, of the broadcasting will be HDTV. Cableand satellite operators have chosen to use com-pression technology to transmit multiple standard-definition programs in each channel, rather thanone or two HDTV programs. In fact, the newsystem is now more often referred to as "digitaltelevision" (DTV), rather than high-definition tele-vision (HDTV). It is even likely that some non-TVservices will be permitted in the new channels.The system developed in this project is intended tobe more useful than the all-digital scheme that hasevolved from the FCC process (the Grand Alliance(GA) system) because its features were chosenspecifically to be acceptable to the variousstakeholders, including broadcasters, equipmentmanufacturers, program producers, regulatoryauthorities, and viewers. The most important per-formance factors are efficient use of spectrum, cov-erage versus quality, cost, and interoperability.This system must also have a feasible transitionscenario. We concluded that all receivers do notneed to have the same picture quality and that low-cost receivers must be available for less-criticalapplications. We therefore designed a multiresolu-tion (MR) system, in which a hierarchy of encodersof different price/performance can communicatewith a hierarchy of decoders, also of varying cost325Chapter 3. Combined Coding for High-Definition Televisionand quality. As compared with the Grand Alliancesystem, the new scheme features:1. Extended coverage, albeit at lower quality inthe extra area,2. Higher quality, albeit only in the central portionof the service area,3. Comparable quality in a large portion of theservice area. The price for these features issomewhat lower quality in a part of the servicearea.4. Support for single-frequency networks, whichmake the most efficient use of spectrum,15. Support for less expensive receivers for lessdemanding applications,6. Automatic optimization of image quality as afunction of signal quality and receiver com-plexity.The multiresolution (MR) feature makes possiblethe design of cheaper receivers for small sizes andother noncritical applications. It also permitshigher-performance receivers to optimize theirimage quality as a function of received signalquality, thus improving spectrum utilization.The MR feature is achieved by use of combinedsource and channel coding employing a pyramidtechnique. The coded video and audio data isdivided into a number (typically three or four) ofdata streams. The quality of the reconstructedvideo depends on the number of streams recoveredand their signal-to-noise ratio. Thus the loweststream provides a picture somewhat better thanstudio-quality NTSC to the least-expensive recei-vers and/or under poor reception conditions. Thehigher streams are used to enhance the imagequality. The streams are packaged by the channelcoder so that the number recovered depends onthe signal strength and receiver performance. Thelowest quality level features standard MPEG-2coding so that the cheapest receivers can usesingle-chip MPEG decoders. 23.2 TechnologyIn the GA system, digital source coding is used toget very high compression, which is needed, inpart, because all-digital channel coding does notmake best use of the analog channel capacity. Oursystem uses multiresolution hybrid analog/digitalcoding, which permits both high compression andefficient use of spectrum.The source coder uses some MPEG features, butproduces data in a form suitable for hybrid channelcoding. This data is not all of one kind; somerequires essentially perfect transmission and somecan tolerate a certain amount of noise and dis-tortion.The channel coder uses spread spectrum andorthogonal frequency-division multiplex (OFDM), 3which, in combination, give good resistance toanalog channel impairments and facilitate multireso-lution operation. It transmits more sensitive datadigitally and less sensitive data in analog form. Thereceived signal "looks" simple to a low-performancereceiver, while a high-performance receiver, with itsmore powerful decoder, "knows" where to look inthe signal for the higher streams of data. Theseare recoverable at locations where the signalstrength is higher and/or where the antenna andreceiver are of better quality. Digital performance isenhanced by extensive forward error correction.The simpler receivers are expected to be substan-tially cheaper than those used in the GA system,while the higher-performance receivers will be ofcomparable complexity and cost.SFNs make the signal level more uniform across the population of receivers by utilizing a cellular network of low-power transmitters,all operating in the same channel. Several signals sensed at each receiver appear to be ghosts, so very good ghost suppression isrequired.The Motion Picture Experts Group has promulgated an international all-digital standard for TV source coding, which has beenadopted by many groups. Inexpensive coder and decoder chips are becoming available, which makes the use of MPEG coding veryattractive.OFDM was invented in the U.S., but in recent years has been developed in Europe for digital audio broadcasting. It is also intendedfor use in digital TV terrestrial broadcasting in Europe. The coded form is referred to as COFDM. The Japanese announced in 1996that their forthcoming digital broadcasting system would also use COFDM. COFDM was briefly examined in the FCC inquiry andrejected as not ready for practical use.326 RLE Progress Report Number 1393.3 Final StatusBy the end of 1995, the system simulation wasnearly completed. Output sequences could bedemonstrated after transmission through a simu-lated terrestrial channel showing the effect of noiseand echoes on the digital and analog componentsof the signal. Three levels of resolution wereattained at three different receiver CNRs. Fordetails, see the 1995 RLE Progress Report. Asmall amount of work remained to be done on thesource coder.The channel coder work was complete by January1996. It fulfills all the expectations for the systemthat were held at the beginning of the project. Oneof the most important of these is the performance inthe presence of echoes. In contrast to the perfor-mance of conventional systems such as the GrandAlliance system, the performance of the newsystem actually improves with large echoes.The echo performance of this system makes it suit-able for use in single-frequency networks (SFNs)without the use of directional antennas. In thesesystems, the conventional centralized transmitter isreplaced by a cellular network of low-power trans-mitters, all emitting the same signal. Receivers,particularly with omnidirectional antennas, see anumber of very strong "echoes." The channelcoder that we have developed is the first, in myopinion, that is both practical in terms of computa-tion and has good enough performance to be usedin this way. SFNs permit much more efficient useof spectrum that conventional terrestrial broadcastsystems because they effectively eliminate theeffect of co-channel interference. The service areaof each station is delimited by the location of thecellular transmitters, and it is only in a narrowregion around the edge of the service area that twostations must compete for the same receivers.The source coder work was finished in May 1996.Like the channel coder, the performance achievedmet all the expectations at the beginning of theproject. Some work was done on dealing with theimage degradation produced by the use of MPEG-2Chapter 3. Combined Coding for High-Definition Televisionfor the base-level signal. Although the base-levelimages look quite good at normal frame rate, indi-vidual frames have errors that are automaticallyeliminated at the higher levels. While this improvesthe higher-level image quality, it uses up a substan-tial proportion of the channel capacity that might beused, instead, to increase the resolution. It was notpossible to devote enough time to this issue in thecourse of the project.3.4 The FutureThe December 1996 FCC decision, if not over-turned, means that the United States will not useeither COFDM or hybrid transmission. However, atlong last, some attention is being given to the effectof the cost of the first receivers on the speed of thetransition from NTSC to digital broadcasting, espe-cially now that a very short transition period beforethe shut-down of NTSC is being discussed. Thelikelihood that the initial broadcasting will be almostentirely standard definition has also reopened thesubject of "nondisruptive improvement over time," along-standing FCC desideratum for the new system.As a result, the Advanced Television Systems Com-mittee (ATSC), which is the official body for docu-menting DTV and HDTV standards, is reconsideringthe use of multiresolution systems (now often calledlayered systems) such as the one developed forthis project. Both the source-coding scheme andthe nonuniform constellations of the channel-codingscheme that have been developed may have somerelevance to the proposals of the ATSC.3.5 PublicationsSchreiber, W.F. An Advanced Multiresolutionvision System. RLE Technical ReportCambridge, Massachusetts: MIT Researchoratory of Electronics, January 1997.Schreiber, W.F.,TelevisionImprovementcoming.M.O. Polley, andBroadcasting:over Time." JTele-601.Lab-S.J.Wee. "DigitalNondisruptiveSSMPTE. Forth-327328 RLE Progress Report Number 139

Combined Source and Channel Coding for High-Definition Television

Contains a description of one research projects, reports on technology and current research status and a list of publications.Research Laboratory of ElectronicsDepartment of Electrical Engineering and Computer Scienc

Chapter 3. Combined Coding for High-Definition TelevisionChapter 3. Combined Source and Channel Coding forHigh-Definition TelevisionAcademic and Research StaffProfessor Emeritus William F. SchreiberGraduate StudentsMichael O. Polley, Susie J. WeeTechnical and Support StaffDeborah S. Manning3.1 Project DescriptionSponsorsResearch Laboratory of ElectronicsDepartment of Electrical Engineering andComputer ScienceThe purpose of this project is to develop a high-definition television (HDTV) system for terrestrialbroadcasting. Under decisions previously made bythe Federal Communications Commission, eachcurrent broadcaster will be given a second channel.This chnnel is to be used for HDTV, transmittingthe same program as the standard NTSC channelin a procedure called simulcasting. Fifteen yearsafter the commencement of high-definition broad-casting, NTSC is to go off the air, the original chan-nels being returned to the FCC for reassignment.In many cases, the new assigments will be taboochannels not usable for NTSC because of interfer-ence. This obviously requires that the HDTV trans-missions have excellent interference performance.Coverage must be comparable to what is nowachieved with NTSC.The system being developed in this project isintended to be more useful than the all-digitalscheme that has evolved from the FCC process[the Grand Alliance (GA) system]. Its features havebeen chosen specifically to be acceptable to thevarious stakeholders including broadcasters, equip-ment manufacturers, program producers, regulatoryauthorities, and viewers. The most important per-formance factors are efficient use of spectrum, cov-erage versus quality, cost, and interoperability. Inaddition, it must have a feasible transition scenario.We have concluded that all receivers need not havethe same picture quality and that low-cost receiversmust be available for less-critical applications. Wehave therefore designed a multiresolution (MR)system, in which a hierarchy of encoders of dif-ferent price/performance can communicate with ahierarchy of decoders, also of varying cost andquality. As compared with the Grand Alliancesystem, the new scheme features:1. Extended coverage, albeit at lower quality inthe extra area,2. Higher quality, albeit only in the central portionof the service area,3. Comparable quality in a large portion of theservice area,4. Support for single-frequency networks (SFNs),which make the most efficient use ofspectrum, 1'5. Support for less expensive receivers for lessdemanding applications, and6. Automatic optimization of image quality as afunction of signal quality and receiver com-plexity.(The cost for features 1-3 is somewhat lower qualityin a part of the service area.)The multirepolution (MR) feature makes possiblethe design of cheaper receivers for small sizes andother noncritical applications. It also permits higherperformance receivers to optimize their image1 SFNs make the signal level more uniform across the population of receivers by utilizing a cellular network of low-power transmitters,all operating in the same channel. The several signals sensed at each receiver appear to be ghosts, so very good ghost sup-pression is required.357Chapter 3. Combined Coding for High-Definition Televisionquality as a function of received signal quality, thusimproving spectrum utilization.The MR feature is achieved using combined sourceand channel coding employing a pyramid technique.The coded video and audio data is divided into anumber (typically 3 or 4) of data streams. Thequality of the reconstructed video depends on thenumber of streams recovered and their signal-to-noise ratio. Thus the lowest stream provides apicture somewhat better than studio-quality NTSCto the least-expensive receivers and/or under poorreception conditions. The higher streams are usedto enhance image quality. The streams are pack-aged by the channel coder so that the numberrecovered depends on the signal strength andreceiver performance. The lowest quality level fea-tures standard MPEG-2 coding so that the cheapestreceivers can use single-chip MPEG decoders.23.2 TechnologyIn the GA system, digital source coding is used toget very high compression, which is needed, inpart, because all-digital channel coding does notmake the best use of the analog channel capacity.Our system uses multiresolution hybridanalog/digital coding, which permits both high com-pression and efficient use of spectrum.The source coder uses some MPEG features, butproduces data in a form suitable for hybrid channelcoding. This data is not all of one kind; somerequire essentially perfect transmission, and somecan tolerate a certain amount of noise and dis-tortion.The channel coder uses spread spectrum andorthogonal frequency-division multiplex (OFDM), 3which, in combination, give good resistance toanalog channel impairments and facilitate multires-olution operation. The coder transmits the moresensitive data digitally and the less sensitive data inanalog form. The received signal "looks" simple toa low-performance receiver, while a high-performance receiver, with its more powerfuldecoder, "knows" where to look in the signal for thehigher streams of data. These are recoverable atlocations where the signal strength is higher and/orwhere the receiver is of better quality. Digital per-formance is enhanced by extensive forward errorcorrection. The simpler receivers are expected tobe substantially cheaper than those used in the GAsystem, while the higher-performance receivers willbe of comparable complexity and cost.3.3 Current StatusBy the end of 1995, the system simulation wasnearly complete. Output sequences can be demon-strated after transmission through a simulated ter-restrial channel showing the effect of noise andechoes on the digital and analog components of thesignal. Three levels of resolution are attained atthree different receiver CNRs. For details, see ourchapter in RLE Progress Report No. 137 (1994),inclduing the first paper referenced at the end of thechapter. A small amount of work needs to be doneon the source coder.The channel coder work is complete and fulfills allour expectations for the system. One of the mostimportant of these is the performance in the pres-ence of echoes. Figure 1 shows the BER withseveral levels of echo as a function of CNR for avery simplified version of the channel coder. Ascan be seen, in distinction to the performance ofconventional systems such as the Grand AllianceSystem, the performance of the new system actu-ally improves with large echoes.The echo performance of this system makes it suit-able for use in SFNs without the use of directionalantennas. In these systems, the conventional cen-tralized transmitter is replaced by a cellular networkof low-power transmitters, all emitting the samesignal. Receivers, particularly with omnidirectionalantennas, see a number of very strong "echoes."The channel coder that we have developed is thefirst, in my opinion, that is both practical in terms ofcomputation and that performs well enough to beused in this way. SFNs permit much more efficientuse of spectrum than conventional terrestrial broad-cast systems because they effectively eliminate theeffect of co-channel interference. The service areaof each station is delimited by the location of thecellular transmitters, and it is only in a narrowregion around the edge of the service area that twostations must compete for the same receivers.2 The Motion Picture Experts Group has promulgated an international all-digital standard for TV source coding, which has beenadopted by many groups. Inexpensive coder and decoder chips are becoming available, which makes the use of MPEG coding veryattractive.3 OFDM was invented in the United States, but in recent years has been developed in Europe for digital audio broadcasting. It is alsointended for use in digital TV terrestrial broadcasting in Europe. The coded form is referred to as COFDM.358 RLE Progress Report Number 138Chapter 3. Combined Coding for High-Definition Television-10 __ dBh echo10 . " , ..i.. ....... . ... ......... \ .......   .... , a. o.o-0.5 0 0.5 1 1.5 2- - - - Coded single carrier SNR (dB)Coded OFDMFigure 1. Comparison of single-carrier and OFDM concatenated coder performance for a single-echo channel as theecho power increases.-2......................   ...  . ..-1'.''. ,- " .. *'' :....... ............. .............. ...1 0 ... . ... .. A : :.. .. . ........ echo power increases.3.4 PublicationsPolley, M.O., W.F. Schreiber, and S.J. Wee. "Com-ments on Transmission Techniques for DigitalTerrestrial TV Broadcasting." IEEE Commun.Mag. 33(11): 22-25 (1995).Schreiber, W.F. "A Unified Approach to MovingImage Coding for Most Media and Most Applica-tions." Sig. Proc. 43: 305-321 (1995).Schreiber, W.F. "Advanced Television Systems forTerrestrial Broadcasting: Some Problems andSome Proposed Solutions." Proc. IEEE. 83(6):958-981 (1995).Wee, S.J., M.O. Polley, and W.F. Schreiber. "AScalable Source Coder for a Hybrid Trans-mission HDTV System." Paper published in theProceedings of the International Symposium onMultimedia Communications and Video Coding,Brooklyn, New York, October 1995.359360 RLE Progress Report Number 138

Contains an introduction and reports on three research projects.Scitex America Corporatio

Chapter 3. Combined Coding for High-Definition TelevisionChapter 3. Combined Source and Channel Coding forHigh-Definition TelevisionAcademic and Research StaffProfessor Emeritus William F. SchreiberGraduate StudentsMichael O. Polley, Susie J. WeeTechnical and Support StaffDeborah S. Manning3.1 IntroductionSponsorScitex America CorporationThe purpose of this project is to develop a high-definition television (HDTV) system for terrestrialbroadcasting. In accordance with ground rules setup by the Federal Communications System, eachTV licensee will be given a second channel withinthe current spectrum allocation. This new channel,to be awarded without competition, must be usedfor HDTV broadcasting. The existing channels willcontinue to be used for standard broadcasting. Formuch of the day, the same program must be senton both channels, a procedure called simulcasting.In many cases, the new channels will be so-calledtaboo channels-those that cannot be used todayfor standard broadcasting because of interference.However, coverage on the new channels must beabout the same as coverage on existing channels.Obviously, the HDTV format must have much betterinterference performance than the current format.Two additional requirements for this new systemare: (1) the picture must be the highest quality pos-sible and (2) the system must operate satisfactorilyin spite of noise, ghosts, interference, and fre-quency distortion. In addition, the system musthave good interoperability with other media, andnondisruptive improvement over a period of timemust be possible.3.2 System DevelopmentAs discussed in RLE Progress Reports Numbers134 and 135, in our initial plan we had intended touse a two-stage spread-spectrum (SS) system. Inthis scheme, the video bandwidth is divided into alarge number of narrow-band signals. Then eachsignal is spread to full (6 MHz) channel bandwidthby a unique spreading sequence with all the spreadsignals added. However, we found that this methoddid not have adequate ghost (multipath) sup-pression. As discussed in last year's ProgressReport, we combined the spread-spectrum methodwith the orthogonal frequency-division multiplex(OFDM) method to improve ghost performance.In both the OFDM and the SS methods, the signalis divided into many channels so that the symbollength in each channel is substantially longer thanthe temporal spread of the multipath. A guardinterval is added to each symbol in which a portionof the symbol is repeated. By integrating overexactly one symbol length at the receiver, multipathup to the length of the guard interval is entirelyeliminated. This performance has been verified inthe simulation exercise.3.2.1 Current StatusThe system shown in figure 1 is under simulation.As in our earlier system, this system uses hybridanalog/digital source coding and channel coding.This gives higher spectrum efficiency than all-digitalchannel coding while having just as good com-pression performance in the source coder.The source coder generates both analog and digitaldata. The analog data consists of selected Fouriercoefficients. The digital data comprises DC andvery low-frequency data, audio, ancillary data, andadaptive selection information for identifying theselected coefficients. As in our earlier system,analog data is subjected to a spread-spectrumoperation. After forward error correction (FEC) hasbeen added to the digital data, the analog data isadded to it.This procedure produces hybrid analog/digitalsymbols that are fed to the OFDM modulator. (Thecombination of coding with OFDM is called coded337Chapter 3. Combined Coding for High-Definition TelevisionFigure 1.OFDM or COFDM.) At the receiver, the signal isdemodulated into hybrid symbols and decoded,producing error-corrected data. This data isrecoded and subtracted from the hybrid symbols toproduce analog data, which is then subjected to aninverse SS operation. The source decoder pro-cesses analog and digital data streams to producean approximation to the original video.3.3 Error CorrectionThe digital data must be received nearly error-free.We accomplish this by using a concatenated coderconsisting of a Reed-Solomon coder (adding checkbits to locate errors) and a convolutional coder,which increases the number of levels in the signalto regain channel capacity normally lost by quanti-zation. This combination has proved extremelyeffective.3.4 Channel EqualizationOne benefit of OFDM is that, since the multipathspread is shorter than the symbol length, demodu-lation into the separate channels can be donebefore equalization, thus improving the SNR of therecovered signals. Multipath causes the gain to benonuniform across the channel. At frequencieswhere the gain is low, noise is amplified. In thiscase, there are fewer problems if equalization isdone after demodulation.Since there are so many subcarriers, the channelgain can be described by a single gain and phasefor each. These are easily measured by a trainingsignal. Correction is easily implemented since theamplitudes of the various carriers are separated outby the demodulation process.338 RLE Progress Report Number 1363.4.1 Current StatusWe have fully simulated the channel coder and arenow measuring its properties. We are designingthe source coder which is a modification of thepopular MPEG scheme but, in our scheme, theamplitude and identification of coefficients are trans-mitted independently. We are also developing aChapter 3. Combined Coding for High-Definition Televisionmultiresolution version of the system. Pictures ofdifferent quality are recovered, depending onreceiving conditions. Finally, we are investigating atransmission constellation that separates the digitaland analog data streams so that each is recoveredwithout interference from the other.339340 RLE Progress Report Number 136

Contains description of one research project, reports on technology, current status and future plans of research and a list of publications.Scitex America Corporatio

Chapter 3. Combined Coding for High-Definition TelevisionChapter 3. Combined Source and Channel Coding forHigh-Definition TelevisionAcademic and Research StaffProfessor Emeritus William F. SchreiberGraduate StudentsMichael O. Polley, Susie J. WeeTechnical and Support StaffDeborah S. Manning3.1 Project DescriptionSponsorScitex America CorporationThe purpose of this project is to develop a high-definition television (HDTV) system for terrestrialbroadcasting. Under decisions previously made bythe Federal Communications Commission, eachcurrent broadcaster will be given a second channel.This channel is to be used for HDTV, transmittingthe same program as the standard NTSC channelin a procedure called simulcasting. Fifteen yearsafter the commencement of high-definition broad-casting, NTSC is to go off the air, the original chan-nels being returned to the FCC for reassignment.In many cases, the new assigments will be taboochannels not usable for NTSC because of interfer-ence. This obviously requires that the HDTV trans-missions have excellent interference performance.Coverage must be comparable to what is nowachieved with NTSC.The system being developed in this project isintended to be more useful than the all-digitalscheme that has evolved from the FCC process[the Grand Alliance (GA) system]. Its features havebeen chosen specifically to be acceptable to thevarious stakeholders including broadcasters, equip-ment manufacturers, program producers, regulatoryauthorities, and viewers. The most important per-formance factors are efficient use of spectrum, cov-erage versus quality, cost, and interoperability. Inaddition, it must have a feasible transition scenario.We have concluded that all receivers need not havethe same picture quality and that low-cost receiversmust be available for less-critical applications. Wehave therefore designed a multiresolution (MR)system, in which a hierarchy of encoders of dif-ferent price/performance can communicate with ahierarchy of decoders, also of varying cost andquality. As compared with the Grand Alliancesystem, the new scheme features:1. Extended coverage, albeit at lower quality inthe extra area,2. Higher quality, albeit only in the central portionof the service area,3. Comparable quality in a large portion of theservice area,4. Support for single-frequency networks (SFNs),which make the most efficient use ofspectrum,'5. Support for less expensive receivers for lessdemanding applications, and6. Automatic optimization of image quality as afunction of signal quality and receiver com-plexity.(The price for features 1-3 is somewhat lowerquality in a part of the service area.)The multiresolution (MR) feature makes possiblethe design of cheaper receivers for small sizes andother noncritical applications. It also permits higherperformance receivers to optimize their imagequality as a function of received signal quality, thusimproving spectrum utilization.1 SFNs make the signal level more uniform across the population of receivers by utilizing a cellular network of low-power transmitters,all operating in the same channel. The several signals sensed at each receiver appear to be ghosts, so very good ghost sup-pression is required.317Chapter 3. Combined Coding for High-Definition TelevisionThe MR feature is achieved using combined sourceand channel coding employing a pyramid technique.The coded video and audio data is divided into anumber (typically 3 or 4) of data streams. Thequality of the reconstructed video depends on thenumber of streams recovered and their signal-to-noise ratio. Thus the lowest stream provides apicture somewhat better than studio-quality NTSCto the least-expensive receivers and/or under poorreception conditions. The higher streams are usedto enhance image quality. The streams are pack-aged by the channel coder so that the numberrecovered depends on the signal strength andreceiver performance. The lowest quality level fea-tures standard MPEG-2 coding so that the cheapestreceivers can use single-chip MPEG decoders. 23.2 TechnologyIn the GA system, digital source coding is used toget very high compression, which is needed, inpart, because all-digital channel coding does notmake best use of the analog channel capacity. Oursystem uses multiresolution hybrid analog/digitalcoding, which permits both high compression andefficient use of spectrum.The source coder uses some MPEG features, butproduces data in a form suitable for hybrid channelcoding. This data is not all of one kind; somerequire essentially perfect transmission, and somecan tolerate a certain amount of noise and dis-tortion.The channel coder uses spread spectrum andorthogonal frequency-division multiplex (OFDM), 3which, in combination, give good resistance toanalog channel impairments and facilitate multires-olution operation. The coder transmits the moresensitive data digitally and the less sensitive data inanalog form. The received signal "looks" simple toa low-performance receiver, while a high-performance receiver, with its more powerfuldecoder, "knows" where to look in the signal for thehigher streams of data. These are recoverable atlocations where the signal strength is higher and/orwhere the receiver is of better quality. Digital per-formance is enhanced by extensive forward errorcorrection. The simpler receivers are expected tobe substantially cheaper than those used in the GAsystem, while the higher-performance receivers willbe of comparable complexity and cost.3.3 Current StatusAll the novel elements of the system have beensimulated and picture quality at the first three levelshas been demonstrated. These have resolutions of384 x 640 (better than NTSC), 576 x 960, and 768x 1280, all 60 frames/sec progressively scannedcompared to 720 x 1280 for the GA system. It isbelieved that further system optimization will permitsubstantially higher resolution for all levels abovethe first.3.4 Future PlansOne small coding job remains, which is the codingof the coefficient identification data. Based on pre-vious work, we expect no problem with this. Weshall continue to improve the system. There is littledoubt that the quality at the middle and highestlevels will become better. (It is already very good.)These levels, at present, must cope with distortionsintroduced by MPEG at the lowest level. Althoughthe MPEG pictures are good when displayed at 60fps, there are significant frame-to-frame errors thatplace a burden on the motion compensation atlevels two and three. This is so much the case thatwe can get nearly as good results using intraframecoding only at these levels.If we retain motion compensation at levels two andthree, we plan to go to a scheme in which themotion vectors will be calculated for all three levelsfrom the original high-resolution sequence. Thismay well solve some of the problems we are nowhaving at the lowest level, and it also will facilitatethe coding of the vectors for levels two and three.2 The Motion Picture Experts Group has promulgated an international all-digital standard for TV source coding, which has beenadopted by many groups. Inexpensive coder and decoder chips are becoming available, which makes the use of MPEG coding veryattractive.3 OFDM was invented in the US, but in recent years has been developed in Europe for digital audio broadcasting. It is also intendedfor use in digital TV terrestrial broadcasting in Europe. The coded form is referred to as COFDM.318 RLE Progress Report Number 137Chapter 3. Combined Coding for High-Definition Television35'i 30o2  20=10I' - I 1 5S101' 51 ,~40150 5 10 15 20 25 30 35 40-CNR (dB)Figure 1. Noise Performance With and WithoutEchoes. This figure shows the performance of both thedigital and the analog data streams as a function of thereceiver CNR. (We use CNR to mean the signal/noiseratio of the radio-frequency carrier and SNR to apply tothe recovered baseband signal representing the analogdata.) The solid lines represent the no-echo situation,while the dotted lines represent the performance in thepresence of five echoes ranging from -10 to -20 dB.3.5 PublicationsPolley, M.O., S.J. Wee, and W.F. Schreiber."Hybrid Channel Coding for MultiresoultionHDTV Terrestrial Broadcasting." Proceedings ofthe IEEE International Conference on ImageProcessing. November 1994.Schreiber, W.F. "A Unified Approach to MovingImage Coding for Most Media and Most Applica-tions." Sig. Proc. Forthcoming.Schreiber, W.F. "Advanced Television Systems forTerrestrial Broadcasting: Some problems andsome proposed solutions." Proc. IEEE. Forth-coming.Wee, S.J., M.O. Polley, and W.F. Schreiber. "AScalable Source Coder for a Hybrid Trans-mission HDTV System." Proceedings of theIEEE International Conference on Image Proc-essing. November 1994.319Table 1. Composition, Rates, and Thresholds of the MR Data Streams. This table shows details ofthe three levels of performance. Note that it is possible to transmit only the lowest level, or only thelowest two levels, in which case the threshold is reduced and coverage increased.Incremental TotalClass Composition Incremental Total ThresholdRate Ratelow-res MPEG stream audio digital 4 Mb/s 4 Mb/s 6 dB CNR384 x 640 ancillary dataenhanced motionvectors, selection infor- digital 4 Mb/s 8 Mb/smedium-res mation, additional audio 17 dB CNR576 x 960selected residual 2.5coeffs, analog 2.5 Ms/s Ms/scoeffs. Ms/senhanced motion 12vectors, selection infor- digital 4 Mb/s Mb/shigh-res mation, additional audio 29 dB CNR768 x 1280selected residual analog 2.5 Ms/s 5 Ms/scoeffs.320 RLE Progress Report Number 137

Contains description of one research project

Chapter 3. Combined Coding for High-Definition TelevisionChapter 3. Combined Source and Channel Coding forHigh-Definition TelevisionAcademic and Research StaffProfessor William F. SchreiberGraduate StudentsMichael O. Polley, Susie J. WeeTechnical and Support StaffDeborah S. Manning3.1 Project DescriptionThe purpose of this project is to develop a high-definition television system (HDTV) for terrestrialbroadcasting that has high-spectrum efficiency,effective suppression of analog channel impair-ments (ghosts, noise, interference, and frequencydistortion), a feasible transition scenario, reasonablecost, good interoperability with other media andapplications, and permits nondistruptive improve-ment over time. To this end, a two-stage spread-spectrum system has been simulated in accordancewith the plans discussed in RLE Progress ReportNumber 134.In spread-spectrum television, the video signal isdivided into a large number of narrow-band compo-nents so that the symbol length is larger than thetemporal spread of the echoes. All echoes arrivewithin one symbol time, eliminating intersymbolinterference. Each narrow-band component isspread to the full spectrum width (6 MHz) by multi-plying it with a different pseudorandom sequence.All the products are added and transmitted. At thereceiver, the components are separated by multipli-cation by the same sequences, whose orthogonalityensures elimination of crosstalk.Unfortunately, echoes reduce the orthogonality ofthe random sequences. While scrambling is helpfulbecause it causes interference to become randomnoise, the noise level is still excessive. It is not the-oretically possible to design sequences that haveadequate cross-correlation performance when time-shifted.We have found a possible solution to this problemby using orthogonal frequency-division multiplex(OFDM). This channel-coding method, invented atBell Laboratories in the 1960s, also divides thesignal into narrow-band components. Each signalis modulated on a separate carrier. The signals inadjacent channels are orthogonal, permitting theircomplete separation at the receiver. A guardinterval is inserted between symbols to ensure thatall ghosts arrive entirely within one integrationperiod. A very simple implementation is providedby the use of the Fourier transform. This generatesa baseband version of the transmitted signal in asingle operation.OFDM, which has been used in Europe for digitalaudio broadcasting, is being investigated now fordigital video broadcasting by a number of laborato-ries. At a meeting held at MIT in October 1992, allEuropean and Canadian OFDM projects wererepresented. A particularly attractive version ofOFDM, called single-frequency networks (SFN),was discussed at some length. In this method, thereceiving area of a television station is filled with acellular network of low-power transmitters, operatingon the same frequency. The multiple signals seenat each receiver, particularly if omnidirectionalreceiving antennas are used, constitute "activeechoes," which are suppressed by OFDM. Thisarrangement results in a much more uniform signallevel at the receivers, obviating the need for a softthreshold. It also permits much higher spectrumefficiency, since stations serving contiguous areascan use the same frequency. The "no-man's land"between stations, in which directional antennaswould have to be used, are about the same widthas the cells. Thus, to provide about 20 inde-pendent services in each locality, not much morethan 20 (instead of the 68 which are needed atpresent) channels must be allocated for televisionservice.We are now actively pursuing the goal of config-uring our system to support the SFN concept.There is some uncertainty about the precisemanner in which the echoes combine at thereceiver and we are considering a field test to eluci-date the phenomenon.269270 RLE Progress Report Number 135

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Combined Source and Channel Coding for High-Definition Television

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