Technology of the

     High power transmitter (Continuation 2)

     Single sideband broadcastingSince 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, and by so doing had pioneered the way. It is significant that VGA, the BBC World Service and Radio Moscow made no move to Implement SSB broadcasting. These three are the political broadcasters who command big audiences and wanted to keep them. Since the targets of their broadcasts are listeners with cheap AM receivers in Eastern Europe, Asia and the Middle East, there would be nothing to gain and a lot to lose.For manufactrurers of radio transmitters, far-sightedness is an essential survival trait. It takes ten years to develop a new generation of high-power transmitters, and when a transmitter goes into service the broadcaster, reckons on a service life of anything from 25 to 50 years. In 1982 AEG-Telefunken introduced its Pantel range of transmitters , and in anticipation of SSB broadcasting the company made sure that it could produce transimitteers for either DSB or SSB operation.Within a few years, Brown Bovire (now ABB *) and Thomson-CSF produced their versions, all suitable for SSB operation. Marconi , however, produced a transmitter that, because of its unique parallel design of pulsedivision modniator, would be very difficult to adapt. In 1989 it changed the design to a series modulator which, the company claims, is now suitable for SSB. About this time , Dallas based continental Electronics Produced its time, Dallas based continental Electronics pro duced its 420B transmitter, which has SSB facility fitted.Thus there are now five major manufacturers that have responded to the demand for SSB transmitters. It is reassuring that many intermational broadcasters have taken the opportunity to re-equip with high-power broadcast transmitters that are not only capable of high efficiency PDM operation (or PSM, in the case of ABB), but are also capable of providing conventional DSB operation, compatible AM and full SSB, in the next five years, many more will have re-equipped in preparation for SSB broadcasting. The next stage in the migration to SSB must be the development of AM receivers suitable for SBB at a reasonable price. At present about 20 such models are on the market ,all of which are professional or semi-professional devices selling at prices which are far too expensive for the domestic market. Although there is a growing market for such high ?performance radio receivers, if SSB listening is going to become popular then it is up to the receiver manufacturers to develop domestic versions.There are number of technical hurdles : SSB receivers must have a much better performance, and frequency stability must be very much higher than in a cheap domestic receiver. This means that such radios will always cost more than the ordinary AM receiver, eventually the problem will be solved by mass production the market is potentially huge. In many areas of the world, the ubiquitous MW/SW receiver is the main means of keeping in touch with current affairs.There is one other problem: the increased skill that is needed to tune into an SSB signal. Any child or uneducated peasant can tune an AMreceiver, but it takes care and experience to tune to an SSB signal, and if the receiver is not tuned accurately then the signal may become unintelligble. At present, listeners to compatible AM broadcasts (SSB signals that can be demodulated On an AM receiver) tend to be keen listeners, with sophisticated SW receivers, Thus even the lowest-end mass-market SBB receiver will need sophisticated Cmoputer-aided push-button tuning.Until these problems of easy-to-tune,cheap SW receivers are solved, without sacrificing performance ,the broadcasting authorities will continue to delay the switch to SSB broadcasting as long as possible.

     大功率发射机技术（续2）单边带广播

     自从无线电广播诞生以来，调幅广播（有时被称为双边带广播）就扮演了支配的角色。双边带广播产生两个边带，理论上一个就已足够了。双边带广播不仅在功率上是浪费的，它也浪费了频谱；如果能省掉一个边带，一个指定区段的频谱可以提供分配给两倍之多的电台使用。

     更好的发射效率是历代工程师们追求的目标。作为减轻广播波段频谱过分拥挤的手段，应用单边带进行无线电广播的最终可能性过去曾被推荐。1979年世界无线电行政大会（WARC）通过了一个决议以将来应用单边带广播来改善频谱的利用。在一部分广播主管当局中在一定程度上勉强接受了这一决议。然而，自从它一提出，不可避免地发射机制造商要在将来某个时候开始为此问题开展工作。

     在1987年的世界无线电行政大会上，在认可广播公司面临的这个问题上，情况有了进一步的发展。有少数发射机已适应单边带工作。然而，国际电联坚持它希望考虑单边带广播并号召制造商们在1990年着手开发。对此给出限定范围，一般认为在2015年以前普遍应用单边带广播是不现实的。这使广播公司松了一口气。

     这个问题是技术和政治性质的。单边带广播不是不可能的一自从1947年它已用于短波通信，甚至从1935年有些邮电部门，例如英国邮政部曾将它用于穿越大西洋的无线电话电路。通信的发展是迅速的，在六十年代所有的商业主管部门，包括民航，都已转向单边带。

     另一方面，无线电广播从不同的方面发展了其发射机技术。由于与采用线性放大器（如用于通信发射机）的低电平调制相比，它有较高的整机效率，使声音广播实行了高电平调制。这形成了世界范围的几乎所有无线电台都适应单边带广播是不可能的情况。显而易见，唯一的解决方案是预计在接近2000年时更换遍及世界的所有在用的大功率发射机。

     而后还有政治问题，普通的调幅收音机对接收单边带信号是完全无用的。任何广播电台转到单边带将因此失去一大部分的听众。从而在全世界的主要部分引入单边带意味着将使大部分现有广播收音机成为无用。因为国际广播宣传大部分为其听众展开竞争，那就容易理解为什么各广播公司当然不愿转向单边带广播。拥有庞大听众的许多国际广播 当局（英国广播公司的环球广播是其中之一，有一亿二千万听众）它将毫不怀疑地愿意在一起对单边带给以缓议。

     为了克服脱离听众的问题，世界无线电行政大会于1989年鼓励采用兼容的单边带。这将容许多数先进的广播公司安排转向单边带，在这种情形下他们将保留用传统的调幅收音机收听的可能。这个解决方案似乎解决了容许那些有远见的广播当局尽早使单边带付诸实用的所有问题。

     1989年，德国、奥地利和挪威已经开始实施兼容的单边带，成为这一方面的先驱。重要的是美国之音、英国广播公司的环球广播和莫斯科电台都没有实现单边带广播。这三者是政治性的广播公司，他们拥有大量的听众并要继续留住他们。因为他们广播的对象是在东欧、亚洲和中东使用便宜的调幅收音机的听众，在那里，将得不到任何东西而损失巨大。

     对于广播发射机制造商，具有远见是一个基本的生存特点。它用十年的时间去开发新一代的大功率发射机，当一部发射机投产于广播电台时，估计其工作寿命怎么也有25年到50年。1982年AEG一德律风根提出了它的潘太尔系列发射机，在对单边带广播的预测上该公司将有把握和生产出既能工作于双边带也能工作于单边带的发射机。

     几年之内，勃朗一包弗利（现在的ABB* ）和汤姆逊-CSF生产了他们完 全适宜于单边带工作的机型。而马可尼提出了其独特的很难适应（单边带）的并馈脉冲分割调制设计。1989年公司声称，它的设计已改为现在的适应单边带的串馈调制器。大约就在同时，设于达拉斯的大陆电子公司生产了它的带有单边带设备的420B型发射机。

     从而现在已有五个主要的制造商对单边带发射机的需求作出响应。可以确信，许多国际广播电台已经获得了用不仅具有高效率的 PDM（或 PSM，在 ABB的情况下）方式工作，而且也具有传统的双边带、兼容的调幅和全单边带工作能力的大功率广播发射机换装的机会，在下一个五年内将有更多的单边带广播作换装的装备。在转移到单边带的下一步必须开发价格合理适应单边带的调幅收音机。现在市场上大约有20个这样的型号，全是专业或半专业的设备，对家庭市场来说价格过于昂贵。如果单边带接收普及了，尽管这种高性能的广播收音机有一个增长的市场，而收音机制造商会更快地开发民用产品。

     存在许多技术障碍：单边带收音机必须具有许多更佳的性能，与便宜的家用收音机相比，其频率稳定度须非常高。就意味着这种收音机的成本将总是高于普通的调幅收音机。这个问题最终将通过大规模生产来解决——一潜在的巨大市场。在世界的多数地区，普遍存在中、短波收音机是与时事保持联系的主要手段。

     还有另外一问题：需要增进调谐到一个单边带信号的技能。随便哪个小孩或未受过教育的农民都能调谐一个调幅收音机，而调谐一个单带信号则需要细心和经验，如果收音机没有精确调谐，信号将变成无法听懂的。目前，使用复杂的起码是大量销售的由电脑和按钮调谐的短波收音机的兼容调幅广播（能由调幅收音机解调的单边带信号）的听从将能成为热心的听众。直到在不牺牲性能的情况下这些易于调谐和便宜的短波收音机的问题得到解决。广播当局将继续尽可能长时间的推迟转向单边带。

     译注：ABB现已与汤姆逊-CSF合组为“THOMCAST”

     摘译自 JAMES WOOD：“The History of International Broadcastting”