Digital Terrestrial Broadcasting: Innovation and Development or a ...

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92 COMMUNICATIONS & STRATEGIESTable 1: Typical net bit-ratesTerrestrial pr. 8 MHz (*)Cable pr. 8 MHzSatellite – typical (**)18-24Mb/s38 Mb/s39 Mb/s(*) This corresponds to the capacity for an analogue program transmission, but the result fordigital depends on the chosen error correction mode.(**) The frequency bandwidth of satellite can vary considerably and different error correctionmodes can be deployed. This is an example for 36 MHz bandwidth with 3/4 of the total capacityused for data transmission.Source: DAMBACHER, 1998.The amount of capacity needed to transmit a TV service depends on thesignal quality requirements and the characteristics of the transmissionmedium. Among other things, this capacity is determined by (2) :• The number of multiplex blocks defined within given frequencyresources – E.g., Single Frequency Network (SFN) technology can be usedto increase this number in terrestrial networks.• Data capacity of a multiplex block – Here modulation technology, theamount of overhead for error correction and statistical multiplexing areamong the important factors.• Data capacity requirements of one TV service – This is determined bythe required signal quality (TV standards) and audio-video 'compression'(coding) technologies, for example.MPEG2, the coding technology used for digital TV, is able to compressthe digital TV signal at different levels resulting in different picture qualities.Generally, four different quality categories are identified for digital TV:• Low Definition TV (LDTV): requires approximately 2 Mbit/s and iscomparable with VHS format (regular home video quality).• Standard Definition TV (SDTV): requires approximately 5-6 Mbit/s andis comparable with PAL TV-format (the quality of current analogue TV inmany countries).• Enhanced Definition TV (EDTV): requires approximately 8 Mbit/s andis comparable with the TV camera quality• • High Definition TV (HDTV): requires approximately 20 Mbit/s and iscomparable with high-resolution TV cameras.(2) For a more detailed discussion please see (TADAYONI & SKOUBY, 1999).
R. TADAYONI 93For the sake of comparison with analogue broadcasting, let us assumethat digital TV services will continue to be transmitted in PAL quality (SDTV),which a number of European countries have opted for. It then becomesobvious that in the same spectrum of one analogue TV channel, a minimumof four digital TV services can be transmitted in the terrestrial network andabout seven services in cable and satellite networks. Digital TV's ability toincrease the number of services available in analogue broadcasting four orseven-fold eliminates (or at least substantially reduces) the problem ofresource scarcity and opens up the possibility of new players entering themarket.Digital broadcasting resources can be increased further through moreefficient frequency planning such as the use of taboo channels and thedeployment of technologies like statistical multiplexing (TADAYONI &SKOUBY, 1999).When a TV station receives a licence to operate in the digital terrestrialnetwork, the ratio of the total capacity in a multiplex block must be specifiedin addition to the details concerning geographical coverage, hours ofoperation, type of services, etc. that are commonly stated in analoguebroadcasting. The optimal allocation of this ‘ratio’ will have an influence onthe different types of organisation described hereafter.In the following analysis it is assumed that (TADAYONI & SKOUBY, 1999):• On the basis of technical considerations, data capacity in a multiplexblock will be determined at the political level, i.e. technological parameterssuch as the type of modulation technology, the FEC overhead, the choicebetween 2K and 8K COFDM modulation, the guard interval, etc. will all bespecified at techno-political level.• On the basis of these political and technological considerations, theregulatory body will then decide on the number of multiplex blocks that are tobe made available in a market, and this will also include details of thenumber of MFN and SFN multiplex blocks and the multiplex blocks plannedfor portable and mobile reception.Once this has been performed, digital TV's total capacity and the differentmultiplex blocks' characteristics will be known parameters. In addition,multiplex operators are required to facilitate the sharing of capacity. Themultiplex function can be a purely technical function, but the task ofmanaging the multiplex block's capacity can also be performed by thisfunction. The multiplex operator can also maintain functions like ConditionalAccess (CA) and/or an EPG service.
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