Contains description for one research project

Polley, Michael O.

Schreiber, William F.

Wee, Susie J.

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Chapter 3. Combined Source and Channel 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. Wee3.1 Project DescriptionTerrestrial broadcasting of High-Definition Tele-vision (HDTV) in the channels currently beingused for today's televison system presents thesystem designer with many difficult problems.One difficulty is spectrum efficiency, i.e., to get themost service-the largest number of programs toeach viewer and the highest quality image andsound-within a given overall spectrum assign-ment. This is complicated by the fact that channelcapacity to the various receivers within theviewing area of each station varies widely. Thesecond most important problem relates to analogchannel impairments-ghosts, noise, interference,and frequency distortion. These problems actuallyset the limit for received image quality in mosthomes.In addition, there are many characteristics whichare desirable in any new television system. Theseinclude: (1) an undiminished service area for eachstation, (2) a practical transition scenario, (3)affordable costs to broadcasters and viewers, and(4) interoperability among systems with variousapplications and resolutions. The FCC has alsoexpressed a strong desire for nondisruptiveimprovement over time, a property that could bereadily achieved with the appropriate kind of inter-operability.The purpose of this project is to devise solutionsto these problems and to achieve the character-istics listed above. To this end, we are now simu-lating a two-stage spread-spectrum system. Inthis scheme, the signal is divided into a number,N, of frequency components that become the sep-arate signals in a code-division multiplex system.Each sample of each ac-component is multipliedby one of a number, N, of orthogonal pseudo-random (PN) sequences, thus being spread to thefull channel bandwidth. The product signals areadded together and transmitted. At the receiver,the sum-of-products signal is multiplied, in turn,by each of the PN sequences and integrated overthe sequence length. The components can beseparated perfectly since there are just Northogonal sequences of length N. The mainadvantage of this technique is that the relativeSNR of the various frequency components can beassigned arbitrarily while maintaining a signalformat that appears to be white random noise.This scheme can be used equally well with a widevariety of source-coding methods, analog anddigital, and intraframe or interframe. In the methodbeing used, which relies on intraframe processingonly, both digital and analog data are transmitted.The analog data comprises the adaptively selectedhigh-frequency subband samples. (They could beDCT coefficients.) The digital data comprises dcand low-frequency subband information, audio,data required to interpret the analog information,and ancillary data. The 6-MHz channel hascapacity for 10 Mbits/second plus 10 M analogsamples/second using a hybrid QAM format inwhich the analog information, processed as dis-cussed for code-division (spread-spectrum) multi-plexing, rides on top of the digital data.The hybrid signal samples are subjected to asecond spread-spectrum operation. They aredivided into a large number of parallel datastreams. A number of streams is used such thatthe symbol length is substantially longer than thedifferential time delay of the multipath signals.Each symbol is multiplied by a PN sequence as forthe analog information, added together, anddecoded at the receiver as described above. Thishas the effect of suppressing multipath, since alldelayed replicas of the signal arrive within onesymbol period and the autocorrelation functions ofthe sequences are nearly zero for all delays otherthan zero.An additional advantage of the second spread-spectrum operation is that the several streams ofhybrid data can be assigned different power levels.This extends the range at which some kind ofpicture can be received, since the highest-powercomponent can be detected at lower CNR than thelower-power component(s). For example, if the10-Mb/s digital data stream is divided into two5-Mb/s streams, then at the lowest CNR, a low-resolution image can be reconstructed. At a 6 dB285Chapter 3. Combined Source and Channel Coding for High-Definition Televisionhigher CNR, all 10 Mb/s plus 5 Msamples/secondare recoverable, making a better picture, whileanother 6 dB gives all 10 Msamples/second. Asoft threshold of this type is essential for highspectrum efficiency when transmission of eachsignal is from a single centrally located transmitter,since this arrangement results in widely varyingSNR at the various receivers.Code-division multiplex permits the transmissionof components whose total bandwidth is largerthan the channel bandwidth, at the cost of lowerSNR in the recovered image. Thus, more than 10Msamples/second can be transmitted, giving a softthreshold and a graceful degradation of qualitywith distance. By extension, it is also possible tohave coders of different resolutions "talk to"decoders of different resolutions, giving goodinteroperability.Software simulation of the system is proceeding.The first monochrome still images have been pro-duced and confirm our expectations for theviability of this method.286 RLE Progress Report Number 134

Combined Source and Channel Coding for High-Definition Television

http://dspace.mit.edu/bitstream/1721.1/57204/1/RLE_PR_134_03_02s_03.pdf

Combined Source and Channel Coding for High-Definition Television

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