Technology: Do not adjust your dish . . . yet

 作者:杭耙抠     |      日期:2019-02-28 04:04:04
By BARRY FOX People who own or buy TV satellite dishes could face a nasty surprise this autumn. They may find that the systems they bought to receive programmes from the Astra satellites need to be modified or replaced to receive signals from a new satellite which Astra plans to launch this autumn. In many cases, it will be necessary to modify the dish aerial mounted high on a roof or wall, safely accessible only to professional fitters. The problem could affect more than 3 million homes in Britain and 12 million elsewhere in Europe. To make matters worse, it will not be clear which countries and which channels will be affected until the new satellite is launched in the autumn. Societe Europeenne des Satellites, based in Luxembourg, launched its first satellite five years ago, and now has three, known as Astra 1A, 1B and 1C, in geostationary orbits at 19 degrees East. Each broadcasts 16 TV channels in the frequency range 10.95 to 11.7 gigahertz, so one dish aerial and one receiver can receive a total of 48 channels. SES charges broadcasters who transmit via its satellites up to £4.4 million per channel per year. SES will launch a fourth satellite, Astra 1D, this autumn. It was originally planned as a back-up transmitter to be used if one of the others failed. But broadcasters want more channels for new programmes, so Astra has agreed to let them use all of 1D’s 16 new channels. The fly in the ointment is that to avoid interference with the existing satellite TV signals, 1D will have to broadcast the new channels on a new band of frequencies – 10.7 to 10.95 gigahertz. SES estimates that 90 per cent of existing dish systems in Europe will not work properly in this band. Satellite dishes include a device called a low noise block converter, which cuts out incoming signals outside certain frequency limits, and then reduces the frequency of the signals it does let through before sending them down the cable to the set-top receiver. Converters in many existing dishes will not work at the lower frequencies to be used by 1D. The tuning circuits in the set-top receiver will not cope with the lower frequencies either. SES says it started warning receiver manufacturers about the need for a wider-frequency design early in 1992, and published a technical specification in December 1992. But manufacturers are only now starting to sell wideband receivers, labelled ‘1D compatible’. SES fears that some firms will try to sell off old stock, which cannot receive from 1D, before the public understands the problem. SES also fears that entrepreneurs will sell modifications for existing systems before anyone knows whether they are necessary and will work. Some existing roof aerials may work with some new channels at the top end of the new band, and only need a modification to the receiver. Others may be unable to receive anything at all from 1D. SES admits it now faces its most critical task: deciding which countries to upset most by putting their new programmes on the ‘difficult’ frequencies. The announcement will probably not be made before 1D’s launch. Robin Crossley, technical adviser to SES’s board, says: ‘If anyone is now buying a new receiver system, they should insist that it carries the 1D-compatible sticker, and anyone with an existing system should do and spend nothing until 1D is up and working,