To improve quality and reduce spectrum congestion in HF bands, there is a move away from conventional DSB towards more efficient modulation techniques. In line with other systems, drive is towards digital mod. In this respect, ITU-R promoting a single world-wide standard for digital AM sound b’casting below 30 MHz. The achievement of a single standard would lead to the production of a single economic RX. System will use existing channels; will have a better interference potential than analogue; far improved quality than analogue; allow greater coverage areas to be served by several TXs using the same channel. New ITU-R Rec recently agreed containing characteristics of such a digital sound b’casting system for < 30 MHz.
Referred in earlier talk to WRCs producing frequency plans. WRC-2000 adopted revised b’sat [and associated feeder link] plan (in R1 and 3). (3 pictures). Such a plan based on technical studies and criteria developed in SG 6. Questions remain regarding sharing between BSS, FSS and terrestrial stations, e.g. pfd limits set for BSS to protect FSS don’t vary as function of orbital separation between the respective space stations and therefore don’t provide adequate protection to FSS networks at small orbital separations.
DTTB being introduced into the VHF/UHF bands. Unfortunately, several different systems have evolved, with various advantages and disadvantages. An ITU-R Recommendation (BT.1306) gives the details of such systems but further work is underway to give guidance to the user to help choose a system according to requirements, (e.g. ruggedness against multipath interference, mobile reception, need for single frequency network).
