Technology of the
High power transmitter (Continuation 2)
Single sideband broadcasting

Since the birth of radio broadcasting, the dominant role has been played by amplitude-modulated broadcasting-sometimes called double-sideband broadcasting. DSB broadcasting produces two sidebands, when in theory one would suffice. DSB broadcasting is not only wasteful in power, it also wastes frequency spectrum; a given portion of the frequency specturm colud accommodate twice as many radio stations if one sideband could be dispensed with.
Better transmission efficiency has been the goal of Successive generations of engineers. TO this end the possibility of applying SSB broadcasting to radio has been suggested in the psat as a means of alleviating the overcrowede broacast bands of the frequency spectrum. In 1979 the World Administrative Radio Conference (WARC) passed a resolution to improve the untilisation of the spectrum by the future application of SSB broadcasting. There was a degree of reluctance on the part of broadcasting authorities to welcome this resolution. Nevertheless, since its introduction was inevitable at some time in the future transmitter manufacturers began working on the problem.

At the 1987 WARC, things moved a stage further in the recognition of the problems that confronted the broadcasters. Few of the transmitters already in service could be adapted for SSB. Never-theless, the International Telecommunications Union insisted that in wished to see SSB broadcasting, and called upon manufactruers to commence development by 1990. Given the extent of the problems, it accepted that widespread SSB broadcasting could not be a reality before the year 2015, at which the broadcasters heaved a sigh of relief.

The problems are of a technical and political nature. SSB broadcasting is not impossible-it has been in use on HF communications since 1947, and even from l93Ssome PTTs, such as the British Post Office, had been using it cross the Atlantic on radoio telephony circuits. progress in communications was swift, and by the 1960s All commercial authorities, including civil aviation, had made the swich to SSB.

Radio broadcasting ,on the other hand ,had developed Its transmitter technology on different lines. Sound broadcasting is performed by high-level modulation, because of its higher overall efficiency compared with low-level modulation followed by a linear amplifier (as used in communications transmitters). This created a situation worldwide where nearly all radio stations were incapable of being adapted for SSB broadcasting. Power transmitters already in service throughout the World, of which it is estimated that there are nearly 2000.

Then there was the political problem. The ordinary AM receiver is quite useless for receiving an SBB signal Any broadcasting station switching to SSB would therefore lose a large propotion of its listening audience at a stroke. Thus the introduction of SSB broadcasting on a wordwide basis would mean that most existing radio receivers would become useless. Since international propage and a broadcasting competes for its listening audience, it is easy ot see why broadcasters should be reiuctant to switch to SBB broadcasting. Many international broadcasting authorities who command huge listening audiences (the BBC World Service is one, with 120 millon listeners) would no doubt like SSB to be Shelved altogether.

To overcome this problem of alienating listening audiences, WARC in 1989 moved to adopting compatible'SBB. This wolud allow the most progressive broadcasters to make the switch to SSB, in such a way that they would remain capable of being heard on a conventional AM receiver. This solution seemed to solve all the problems in that it would allow those forward looding broadcasting authorities to adopt SSB as soon as was practial.

By 1989 Germany, Austria and Norway had begun to implement compatible SSB