1、Rec. ITU-R BT.1125 1RECOMMENDATION ITU-R BT.1125BASIC OBJECTIVES FOR THE PLANNING AND IMPLEMENTATION OF DIGITALTERRESTRIAL TELEVISION BROADCASTING SYSTEMS(Question ITU-R 121/11)(1994)Rec. ITU-R BT.1125The ITU Radiocommunication Assembly,consideringa) that a digital TV system may offer a number of ad
2、vantages when compared with the existing PAL, SECAMand NTSC systems in terms of spectrum efficiency, picture quality and flexibility of operation;b) the rapid development of digital methods for the delivery of television and other image services to the homeover terrestrial broadcast as well as over
3、cable and satellite channels;c) that the need for a number of digital terrestrial television broadcasting (DTTB) service quality levels, rangingfrom HDTV to limited quality, has been identified;d) that flexible multi-programming and multiple sound and data services are feasible and desirable feature
4、s ofDTTB;e) that digital broadcasting technology may make it possible to improve reception on portable receivers, and thatit may be desirable to identify the different reception assumptions, e.g. fixed, portable or mobile reception, and the use ofomnidirectional or directional antennas;f) that with
5、certain service planning philosophies it may be desirable to provide for “graceful degradation” at theedge of the service area in order to reduce the effect of local propagation variations;g) that the introduction of digital services may require the use of adjacent channels and related (taboo) chann
6、elsin the current frequency plans;h) that digital services introduced into the current frequency plans must not cause significant disturbance toexisting analogue services;j) that the use of single frequency networks*may provide considerable improvement in spectrum utilization andin service reliabili
7、ty up to the edge of the service area;k) that the planning constraints, market implementation strategies and time-scales may be different from countryto country;l) that consideration should be given to the following concepts: source coding based on common algorithms,hierarchical (scalable) coding, f
8、lexible dynamically-variable multiplex and header descriptors,notingthat progress in the standardization of source coding and multiplexing under the auspices of ISO/IEC JTCMPEG has achieved great momentum, and that the MPEG standards offer the possibility of convergence between thesatellite/cable an
9、d terrestrial broadcasting standards,recommends1. that the system standards adopted for DTTB should be able to support television services in a range of picturequality levels including, at the highest level, HDTV*;2. that where the practical implementation of the system cannot initially support the
10、highest quality level, itshould be structured so that compatible upgrades to achieve this level can be implemented at a later date;3. that the system standard should basically support the transmission of a single quality level and, optionally, thetransmission of a hierarchy of nested quality levels
11、i.e., scalability;_*For an explanation of certain key concepts, see Annex 1.2 Rec. ITU-R BT.11254. that the system standard should provide for the option of multi-programming and a range of service options byreconfiguration*;5. that the system standard should optionally allow single-frequency networ
12、k (SFN) operation. This should notpreclude transmissions which do not allow SFN operation,further recommends1. that the system standards for DTTB, digital satellite and cable broadcasting should be harmonized.ANNEX 1Explanation of certain key concepts in the Recommendation1. The single-frequency net
13、work (SFN) conceptAn SFN can be implemented by: on a local scale, by the implementation of coverage extenders and/or gap-fillers, for example being fed bya particular transmitter (either from the main transmitter or from a coverage extender or gap-filler). Thisfirst option may be termed a “local SFN
14、”; on a larger scale, with medium-power transmitters, fed with the same programme in “regional” or“national” networks, in order to optimize terrestrial spectrum resources. This second option may betermed a “wide-area SFN”.2. Hierarchical multi-level and reconfigurable systems2.1 IntroductionThe foll
15、owing outline is intended to illustrate the concepts of reconfigurablility and hierarchical coding andmodulation, where the term hierarchical refers to source coding which allows for nested quality levels.The following quality level definitions are used here as a basis for comparison without prejudi
16、ce for futuredefinitions.2.2 Quality level definitionsHDTV quality, where the potential exists for the delivery of a picture which is subjectively identical with theinterlaced HDTV studio standard. Quality shall remain consistent with this for a given proportion of televisionprogramme material (wher
17、e this is a percentage in the high nineties, but is yet to be identified).EDTV quality, where the potential exists for the delivery of a picture which is subjectively indistinguishablefrom the 4:2:2 level of Recommendation ITU-R BT.601. This quality shall be maintained for a given proportion oftelev
18、ision programme material (where this is a percentage in the high nineties, but is yet to be identified).SDTV quality, where the quality is approximately equivalent to that of current PAL or SECAM. Thisequivalent quality may be achieved from pictures sourced at the 4:2:2 level of Recommendation ITU-R
19、 BT.601 andsubjected to processing as part of the bit rate compression. The result should be such that, when judged across arepresentative sample of programme material, subjective equivalence with PAL, NTSC and SECAM is achieved.LDTV quality, where the quality is equivalent to that obtainable with t
20、he MPEG-1 system, which operates on asource resolution approximately 1/4 that of the 4:2:2 level of Recommendation ITU-R BT.601. This quality isconsidered by some to resemble that of VHS (albeit over a relatively small proportion of programme material)._*For an explanation of certain key concepts, s
21、ee Annex 1.Rec. ITU-R BT.1125 32.3 Hierarchical systems and receiving situationsThe term “multi-layer” refers to reception of sub-sets of a data stream under various receiving conditions.Three types of receiving conditions can be foreseen: reception using a fixed rooftop antenna, reception using apo
22、rtable but non-moving receiver, and reception in a moving vehicle. Examples of hierarchical systems suitable in theseconditions are: a three-level digital hierarchical terrestrial television system, allowing simultaneously, within oneterrestrial channel, reception at HDTV quality via fixed roof-top
23、antennas, reception at EDTV quality viafixed roof-top antennas and reception at SDTV quality via set-top (or built-in) antennas. In this last case,SDTV quality is designed for portable receivers; a two-level digital hierarchical system, allowing simultaneously reception at HDTV quality via fixed roo
24、f-top antennas and reception at SDTV quality via set-top (or built-in) antennas. In this last case, SDTVquality is designed for portable receivers.2.4 ReconfigurabilityA reconfigurable system allows alternative configurations to be carried in the same channel, at different times.For example, a chann
25、el carrying one HDTV signal could be reconfigured to carry two independent EDTV signals. Thisand various other examples of hierarchical and reconfigurable systems are illustrated in Fig. 1.HDTVEDTVSDTVLDTVSDTVEDTVEDTVSDTV SDTVEDTVSDTVEDTVSDTVEDTVExample 1: A hierarchical HDTVservice reconfigured to providetwo EDTV services with portableSDTV capability.Example 2: A hierarchical EDTVservice (with mobile LDTV)reconfigured to provide twoEDTV servcies with portableSDTV capability.D01FIGURE 1Examples illustrating the conceptsFIGURE 1 BT.1125.D01 = 10 CM_
