Securing Optimum Volume

Radio Record, Volume III, Issue 25, 3 January 1930, Page 11

Securing Optimum Volume

Reasons why Signals vary in Intensity

OST listeners know that at every broadcasting station there is a control room, with an-engineer on duty all the time broadcasting is in progress. His duty is to control the volume, toning down the too loud passages or giving extra amplification when the sounds which reach the microphone appear to be too weak. : . So skilfully is the work of the control engineer done that seldom is it realised how carefully he must be on the watch against underloading and overloading. If you doubt this you can ‘easily make .a test for yourself. The next time a strange speaker is announced to give a talk listen to his opening words and probably you will hear the control engineer at work. After the pleasant voicé of the announcer introducing him ceases it is probable that the opening words of the new speaker are spoken either too softly or too loudly. ' But before half a dozen words have been uttered he has been toned up or toned down to about the same strength as the announcer’s voice, and at that strength the .control engineer will hold him to the end of his talk, neither too soft nor too loud. ; It is not often realised that many other yarying forces are at work beside’ the control engineer, and that all day and all night the broadcasting which comes to your set is being carried across channels that are constantly tending to vary the volume of the reproduction. Night and Day Differences. ‘YHE words "all day and all night" were used advisedly, for here we have a very important volume control which affects all broadcasting. | If one lives quite close to a station he may not realise the difference between day and night transmission heard on the same apparatus, but every country listener knows that when the sun is up the volume goes down, daylight, reducing the range of every transmitting station to-about half its night strength. It is only during the hours.of darkness that the distant stations start to climb in on a small receiver, and many a long-distance enthusiast sits up till two or-three in the morning beeause he has found that the later the hour the better is his range of reception. This day-and-night effect operates in-the general as well as in the particular sense, so during the dark and cloudy winter days reception is far superior to that obtainable in the ' pright summer weather. Although the effect of better range ' during darkness is now so well known, it escaped notice for years after wirea al we less was introduced, and was first appreciated by Marconi in a long-distance test voyage on his yacht the Hlettra. This voyage was undertaken after his successful attempt to span the Atlantic by wireless from Poldhu to Newfoundland, and it is a remarkable fact that this astounding trans-Atlantic feat was carried out during daylight,

because those responsible were not aware of the far greater likelihood of success after the sun had set! (The real cause of day and night variations is the Heaviside Layer, which has received attention in a previous issue.) "he Effect of Rain. ANY. listeners have noticed that their strength of reception seems to fall off during wet weather, but this is not a true volume variation in the same class.as light and darkness. Generally when signals fall-off in wet weather it will be found that the insulators are either inadequate in size or number; and that the rain is forming a conductive:path across them to earth. The effect ‘would be to leak a certain proportion of the signals away, so it is hardly fair to class this among nature’s volume variations, as the provision of insulators which remain dry will completely remove these symptoms. Out of Tune. LL the foregoing affect the signal before it reaches the set. But in the set itself we have many volume controls in addition to the particular components which are so_ labelled. Most of these apply particularly to the valve sets, but even the crystal types have the effect of detuning. Everyone has noticed that throwing the set out of tune results in a marked drop in signal strength, The reason for this is that although broadcasting will always reach the aerial (provided that it is within range), t™e effect of it will only be at a maximum when the tuned circuits are adjusted

to exactly the same frequency as the distant station’s. At short distances there is a breakthrough effect which may lead the listener there to suppose that tuning is not very important, but all listeners who live a long way from the nearest station will have noticed how greatly the results improye when the tuning is set exactly right ‘ : In every valve set-there are many other factors besides tuning" which af‘fect the strength of reproduction. The basis of the operation of every valve set is the provjsion of suitable "A" and "B" batteries or other source. Everything that goes on in the receiver depends upon these, and obviously the ideal plan for even output would be an absolutely steady source of supply. The . Voltage Drop. N practice both battery voltages vary, and to a surprising degree in both cases. Everybody knows that a 4-volt valve must not be supplied dixect from a 6-volt accumulator or the extra voltage will ruin it. But not every listener realises that his ‘"6-volt" accumulator does not remain steadily at 6 volts, being more than this figure when newly charged and considerably less before it is recharged. A, good voltmeter will show that the voltage when a newly-charged cell is first in se is 2.2, and this falls slowly all the time that the accumulator is supplying current until it drops to 1.8, when results fall off quickly, and it becomes incapable of supplying the current demanded of it. One is accustomed to think of accumulator

volfhges being very steady* but these . figures show a drop of something like 15 per cent. between the newly-charged and’ the run-down accumulator. AS the results from many valves vary appreciably with the "A" supply, it is obvious that here we have an important reason for volume variations. Fortunately modern valves are less critieal than their predecessors. Magnified Alterations DVANTAGE is often taken of this very fact that filament voltage affects output by connecting a variable rheostat in the filament circuit of an R.F. valve, for the specific purpose of controlling the volume of the output. Here the slightest touch of the rheostat will affect volume to. a surprising degree, one reason for this being that’ the volume control in this case is. followed by aimplifying valves which magnify the effect of any input variations. A powerful receiver amplies the input many thousands of times, so that any changes in volume which take place in the earlier stages of the input are. enormously magnified before they are reproduced by the loudspeaker. Variations in "B’" voltage supplied to the receiver affect volume in the same way as variations in ‘A" volt- age, but generally not to the same} degree. It will thus be'seen that there are many factors working simultaneously to vary volume, and the marvel is not that the strength of the programme is sometimes better than at others but that the cumulative effects of these variations is not too great to spoil our enjoyment of the fare provided by the R.B.C.