The Human Side of Noise

Radio Record, Volume IV, Issue 12, 3 October 1930, Page 8

The Human Side of Noise

BRUNSTEN BRUNN Surveys the Interference Problem and draws conclusions as to how it may be ultimately overcome

ANY owners of super-sensitive H yadio. receivers are disappointed with the performance of the sets because of the amount of noise they pick up. They find that : their receivers are capable 4 to pick up regularly stations 1000 miles away, and frequently stations as far as 2000 miles away, but they also find that the sets bring in all kinds of electric noises originating for miles around their receivers. Naturally these disappointed listeners want to know the cure of the noise evil, What should be done? Engineers have looked into this problem thoroughly and are now closely conversant with its peculiarities. On the question of reducing the amount of noise they do not offer much encouragement. One famed engineer versed in mathematics has proved that there is practically no hope of improving the signal-to-noise’ ratio, the criterion which determines the usefulness of a sensitive receiver. Experimental engineers have corroborated these mathematical conclusions, and their assertions seem’ to give less hope than ever, since we have, as a rule, greater confidence in experiment than in mathematical deduction. The mathematical and experimental engineers both agree that there is little hope of clearing up the noise intéermingled with signals from distant stations. . Sources of Noise.

HE sources of noise are legion, and it is surprising that any clear signals ean be received from either local or distant stations. Every household electrical appliance, from a lamp to a cooking stove, is a potential or actual source of noise. Some of these are continuous, others intermittent. 'The continuous sources of noise cause the greater interference, since they give no respite. The intermittent sources of noise also may become continuous when there are many intermittent sources in one locality. This statement, no one living in a crowded apartment-house section of a city will deny. The noise from turning on of electric lamps in the gloaming is alone-sufficient to spoil most programmes, : Continuous sources of noise are those that continue all the time between starting and stopping. Thermostats, motors of all kinds having prushes, X-ray machines, vibrators, electric bells, sign-flashers, and the like are continuous sources, for as long as they are on they give out a steady interference. Most of these sources of noise are man-made and cannot be called static or atmospheric. Since they are manmade they can also be cured, provided that they are attacked at their origin. But the remedy is expensive and it seems that nobody bothers. Why, for example, should one housewife stop cleaning her house with a vacuumcleaner just because somebody else wants to listen to a radio programme? Why should she have the motor in the cleaner fixed as long as it runs witht trouble to her? Why should an-X-specialist stop plying his trade

just because somebody happens to be in no need of X-raying? There may be.a time when the law will say that only electrically silent vacuum-cleaners, X-ray machines, doorbells and the like shall operate. They may stop the operation of nearly all electrical machines and appliances, for it is an undertaking of the first magnitude to silence all the electrical devices. Nature’s ‘Noise. [t may be that a law prohibiting such devices would be unenforceable, for at this time at least nobody is really in favour of it. While everybody is in favour of having the noise stopped, nobody wants to stop the noise he makes himself. Nobody could operate a dis-turbance-creating device in secret if a general law were passed, for every noise-maker shouts loudly that it exists and where it exists. While man-made noise predominates in the average radio receiver on an amnheenabs

average day, there are plenty of natural noises which are entirely uncontrollable, and it may be that stopping the man-made noise would not be worth while in view of the situation. What would it avail, for example, if all the man-made noises were stopped, only to have 49 per cent. of the total noise remaining in the form of natural noise? It really makes no difference whether all the noise is present or only part of it on certain days, for just a small part of it is sufficient to force the listener to turn his set off.

‘It is the super-sensitive radio. receiver that brings in most of the noise: Why should this be? Because if the set is sensitive enough to pick up the signals from a station located 8000 miles away, it is also sensitive enough to pick up a larger proportion of the man-made and natural noises originating in an area having a 3000 miles radius, or 28,000,000 square miles. In that area there must always be many atmospheric discharges every few sec: onds and countless examples of manmade noises. It is not enough to say that only those atmospheric discharges occurring in the area of 3000 miles radius about the receiver will be picked up. Chances are that, as far as static crashes are concerned, the radius will be increased to 12,000 miles, and the area, that of the surface of the entire 2arth. If shortwave signals travel round the earth a time or two, why not the static erashes which are much stronger than a

w~wweve_veowveoveNTe,rlerl eae the shortwave intensity? And at least a portion of the crashes will be within the range of waves that travel long distances. Local Reception Clear. RECEPTION of local stations is usually clear except on certain days when thunderstorms are in the vicinity. And this clearness holds as well for the super-sensitive sets as for the relatively insensitive sets, or at least it holds nearly as well The reasons why the signals from the

local stations are clear is that the signals are strong as compared with the noise strength. That is, the ratio of the signal to the noise is high. The only practical method of combating noise that has yet been discovered is to inerease the strength of the signal until it drowns out the interference. This method of combating the noise, whether natural or man-made, has been advanced by radio engineers since the beginning of broadcasting. It was for this reason that the power of broadeasting stations was increased from" 500 watts to 50,000 watts, the limit now set by the Federal Radio Commission If the engineers and certain membex of the Federal Radio Commission who understand the problem could have their way, the limit would not. be 50,000 watts. Possibly it would be a million watts. If such powers were used. by stations in well-selected geographizal positions the " problem of noise would be solved in all but a few out-of-the-way places. , Since the signal strength ofa station varies approximately inversely as the distance, if the strength of a station were increased ten-fold, the service radius of the station would be increased in the same proportion, and the service-area would be ‘increased . 100-fold. . For equal distribution of listeners in that area the reliable service, in terms of listeners, would also be inereased a hundred-fold, and that by increasing the power of the station only ten-fold. That, obviously, would be economy both in power and channel space. " Reduction of Noise. HE noise would remain constant in so far as the power of the transmitter is concerned, but the signal at any given point in the service area of the station would be increased in proportion to the increase.in power of the station. Hence super-sensitive receivers would not be necessary, and there would be little complaint .of noises in signals, At least one of. these superpower transmitting stations could be received in any part of the country with a crystal or a one-valve set. Two or three stations could be. received everywhere with a three-valve set, and with much less noise. ©

BH said above that local stations eould be received with almost alittle interference with a very sensitive receiver as with the modest receiver. They could be received with as exactly as little noise were it not for the fact that receiving ‘valves introduce noises. Not a little of the noise that is heard . in multi-valve receivers is. introducod by. the valves themselves, and therefore the Jarger the number of valves in a set the more noisy that set will be. This is quite apart from the interference that’ the multi-valve set, when adjusted to high sensitivity, brings in from outside. Is there any wonder that the super-sensitive set mingles 4 great deal. of noise with the signals from the. distant stations? Quieter valves must be developed. to reduce valve noises, and this is a problem which may: find a solution in research.

Radio World