THE WIRELESS WORLD

Otago Daily Times, Issue 22675, 13 September 1935, Page 3

THE WIRELESS WORLD

By Magna Vox,

NEWS AND NOTES

Items of local interest are invited by " Magna Vox" for publication in this column. It is necessary that such matter should reach this office by Tuesday of each week for insertion on the following Friday. 2BL, Sydney.—74o K.C., 405 metres. 2FC, Sydney.—6lo K.C., 492 metres. 3AR, Melbourne. —630 K.C., 476 metres. 3LO, Melbourne.—77o K.C., 300 metres. SCL, Adelaide. —730 K.C., 411 metres. 4QG, Brisbane. —800 K.C., 311 metres. IYA, Auckland.—6so K.C., 461.3 metres. 2YA, Wellington.—s7o K.C., 526 metres. 3YA, Christclmrch.—72o K.C., 416.4 metres. 4YA, Dunedtn.—79o K.C., 379.5 metres. 4YO, Dunedln.—l*l4o K.C., 261 metres. ANSWERS TO CORRESPONDENTS. "Superhet," Kurow —(1) I have never heard of it. It might be quite all right, but why not stick to a standard make, which can be bought for practically the same cost. You will find that it pays to use valves of tested and known efficiency. There are several brands on the market, all of which can be relied on, and as there is a difference of only a shilling or two between the price of any of them and the type about which you are inquiring, you might consider whether it is really worth your while going in for the cheaper article. (2) Yes, your trouble is undoubtedly caused by electrical interference. Is your aerial paralleling a transmission line, or is there a leak in the house wiring? (3) Why not get an experienced serviceman to do the job? I have known of a good few sets that have been operated on by what might be termed " amateur experts. With very few exceptions, they have not been improved. See reply to "Dual, Mornington.

"Short Wave," Hampden.—Yes, you must be able to get oscillation on short waves before you can receive signals. Try using more turns on the reaction coil, or even reversing the reaction coil, which now may be connected the wrong way round. Is your detector O.K? And what about your aerial loading—have you made the connection through a condenser? If these points are all carefully watched, the circuit is so simple that there is not much to go wrong. On the broadcast band, the single coil is evidently not enough to cover the full tuning range. I would suggest that you make up two coils, one having fewer turns on the secondary and one having more turns than at present. The larger of the two will then allow you to tune up to the higher stations and the other down to the low.

■ J. S. D., Port Chalmers.—-There are several points on the circuit to which I would raise an objection. The 57 detector is one—l should prefer to see a 56 here as a power grid detector. Also the choke manufactured from an audio transformer —this is rather on the rough side, and straight resistance coupling would give better tone with slightly less gain. The .1 across the speaker cannot be tolerated at all, as high notes must inevitably go right off the map. Anyhow, thanks very much for _ your interest, and for forwarding the circuit. "Dual," Morhington.— (1) It appears to me that your trouble is due to the stopniiu; and starting of some electrical mac Sine, such as a sewing machine motor. Are there any such machines in your neighbourhood? See if the interference stops when the aerial is disconnected. If it does not, then the trouble is entering,by way of the mains, and a line filter may help. If it does, the best thing you can do is to track down the motor and get the owner to fit some kind of suppressor. A complaint to the radio inspector could be made as a last resort, but, naturally, one does not wish to take this course unless absolutely necessary. (2) I am glad my diagnosis was correct and that everything is O.K. It is encouraging to know that I can be of assistance sometimes. A GIANT AERIAL. Readers may be interested to know that the mast for the new 2YA at Titahi Bay will be far and away the highest in New Zealand, if not in the southern hemisphere. It will be 700 feet high, some 200 feet taller than the similar type of mast used a Auckland and Dunedin. The mast is, of course, entirely self-supporting. It stands on an insulated base made of porcelain, and is designed to withstand winds rilany times stronger than those that make Wellington so notoriously healthy. This type of mast supports an umbrella type of aerial. This type of aerial has been selected with the express purpose of giving an increased signal strength in the service area. The meeting point of the ground wave and the eky wave is pushed further ;away. Districts that suffer from fading with the. present aerial may find that in some cases this trouble is greatly reduced. The mast will be erected several hundred feet above sea level at Titahi Bay, and should be a landmark visible for many miles, both from sea and inland. DAVENTRY TRANSMISSIONS. The British Broadcasting Corporation advises that the various Empire transmissions are now: —Transmission No 1, GSB (31.55 metres) and GSD (25.53), 5.45 p.m. to 7.45 p.m.; transmission No. 2, GSG (16.86) and GSF (19.82), 10.30 p.m. to 1.15 a.m.; transmission to GSG (16.86) and GSF (19.82), 1.30 a.m. to 3.30 a.m.; transmission No. 4. GSB (31.55) and GSD' (25.53), 4.45 a.m. to 8.30 a.m.; experimental only, GSI (19.66), 4.45 a.m. to 6.45 a.m.; experimental only. GSL (49.10), 7 a.m. to 8.30 A.m.; transmission No. 4 (part two), GSF (19.82) and GSB (31.55), 8.45 a.m. to 10.15 a.m.; transmission No. 5, GSD (25.53) and GSC <31.32), 10.30 a,m. to 12.30 p.m.; transmission No. 6, GSD (25.53) and GSC (31.32), 2.30 p.m. to 3.30 p.m. AUSTRALIAN SHORT WAVERS. In view of the fact that the three Australian short-wave stations have become so popular with owners of all-wave sets, the following schedules of transmissions should prove useful to listeners: — VK3ME, 31.55 metres, or 9.51 megacycles, Melbourne; daily broadcasts, except Sunday, from 10 p.m to midnight. VK3LR, 31.31 metres, or 9.58 megacycles, Melbourne; occasional brodcasts in the afternoons, and 7.30 p.m. to midnight. VK2ME, 31.28 metres, or 9.59 megacycles, Sydney; Sunday afternoons and evenings. VK2ME also works on 28.51 metres, or 10.92 megacycles, at irregular intervals. The times given are New Zealand standard times. A POPULAR COMMENTATOR. A welcome return to the microphone has been made by Mr Howard Marshall, who has resumed his talks " Under Big Ben" from Daventry. Mr Marshall- has become very well known to overseas listeners, and no talks in the BBC Empire programmes have proved more popular than this series. Letters from all over the world reach the BBC praising them. "He brings England to us," a listener wrote from Australia. Marshall himself receives a huge fan mail, while Tim and Andy, his two small song, are not forgotten. Marshall was an only child, led an ordinary small boys life, followed by brilliant periods at Hailoybury and Oxford. Having heard his sporting commentaries it is easy to imagine him captaining the Harlequins and Surrey at Rugby football, though more difficult to picture him trying to find out six funny stories every week that were new to the editor of the paper for which he worked. After a spell of London magazine work lie shipped as steward on a cargo boat running hetween London and Hamburg. Then back to journalism, this time as sports correspondent for this London paper and that. From there ho graduated to the BBC. but. it was not till he had resigned, to take up a post on the start' of a London daily, that he began to make his name on the air. lie can' do most things at the microphone, and all of thern well, whether he is reading in the children's hour or giving his fortnightly talk. " Under Big Ben.'' He lives in an old Tudor farm in Surrey. His boys, Tim and Andy, frequently come into his Empire talks. One post card sent to Marshall summarises his listeners' feelings more effectively than would be possible from a survey of the whole collection. It contains just two words, " Thank you."

WAVELENGTH CHANGES. A cruise round the dial quickly discloses that great improvement has been effected by the new scale of wavelengths in Australia. Hardly one station seems to have been adversely affected, while several are decidedly clearer than before. Clarity is not the only feature, as all stations are now dead on their allotted frequencies, instead of being in some cases anywhere up to 5 k.c. astray. The readjustment was long overdue, as the procrastination of the Australian authorities in carrying out their part of the agreement has caused a lot of unmerited abuse to be showered on the P. & T. Department of this country. When all the stations now authorised come into operation, it will be imperative that all stations should adhere accurately to the frequency or wavelength that is allotted to them. The opportunity given by the changes has been taken to ensure that in future no transmitters will deviate appreciably from the allotted point. The official standard will not allow a variation which is more that 50 cycles above or below the nominal frequency, which, according to the position of the waveband, is between 550,000 and 1.500,000 cycles, To obtain stability of this order requires control apparatus of a high standard, and, therefore, crystal oscillators arc now being installed in stations not already provided with them. These require to be kept at a constant temperature, which means that ovens and accurate temperature control gear are wanted, and duplicate sets of apparatus are, being "provided to guard against breakdown. Gear of this nature will probably cost each of the stations so affected £2OO or more. TELEVISION SYSTEMS. The fundamental jproblem of television resembles somewhat that of an individual who has a printed picture which he wishes to send to another place, but has no other means available than a narrow pipe or conduit along which it would be impossible to send the entire picture in otie piece. A method of overcoming this problem would be to cut the picture up into small pieces, each of which could proceed with relative ease through the conduit, pass each piece along the communicating passage, and at the receiving end reassemble them until the original was rebuilt. It would, be necessary to adopt some previously determined system of cutting, sending, and reassembling the parts so that the finished article would be a true replica of the' original. If the sender wished to create at the receiving end the illusion of motion he would have to make arrangements whereby the complete pictures could be taken' to pieces, sent, and reassembled a sufficient number of times per second to' provide the necessary effect. The designer of a television system finds himself in a somewhat similar predicament, for the reason that the transmission channel, whether it be a wire or a radio circuit, can only handle small parts of a picture at a time. But he has the further necessity of changing the varying amount of light and shade in the individual parts of the scene which is to be sent into their electrical equivalents. He, too, is forced to make provision for ensuring that all the small picture parts are put together in the right order and at the right speed and in the right position at the receiver. There is this difference, however, between the problem of the television operator and the other individual, viz., that whereas the man who is restricted to the physical cutting up of the picture and sending it alone a definite physical conduit would prefer to have his sections as _ large as possible so that in th e rebuilt picture the number of joints between pieces would be relatively small, the television operator wishes to have his scene divided into as great a number of elements as he can in order to preserve the clarity and detail of the original scene. This difference is accounted for by the fact that, while in th e first instance the light or shade in each small part of the picture is unaffected by the transmission process, in the second instance the transferring of the light and shade into fluctuating electrical currents tends to smooth out the sharpness of contrast that may exist in the small divisions of the picture. For example, suppose that the picture were divided up into small fragments of one-tenth of an inch along each side, and that one of these fragments happen to be equally divided into a black part and white part. In the first case this equal division and sharp contrast would always be preserved, but in the second case tte arrangements for converting the light and shade of this small area into electrical currents would be unable to maintain the sharp differentiation, and would provide a current corresponding to the average amount of light and shade in this small part of the scene, a greyish colour. The desired sharp contrast and equal division between black and white would, therefore, be lost. The result could be considerably improved by dividing the picture area up into smaller parte, for example, one-hundredth of an inch or one-thousandth of an inch on each side, or even smaller. Then all the smaller portions .which were in the black part of the original piece would produce a current equivalent to black, and all the portions which were in the white part of the original piece would produce a current equal to white, and only the parts immediately on the border line between black and white would average out into some kind of a greyish tone. Thus, the smaller the elements into which the picture is divided, the more clear will be the line of demarcation in the final result. The above may explain why, in references to television systems, such terms as 30, or 180, 240, 405 line transmissions are used. These figures, which are an indication of the number of vertical or horizontal lines into which the scene is cut up in the transmission process are some measure of the detail of fidelity which may be expected from the system. It is also necessary to take into account the number of elements, each line being considered as broken up into a number of small parts. The number of these is a better measure of the ultimate detail. Approximately, the number of elements can be determined by multiplying the square of the number of lines" by the width of the picture in inches and dividing this result by the depth of the picture. The figure so obtained by this means is used for comparing the fineness of detail provided by the various systems.