ON ALL WAVELENGTHS

Evening Star, Issue 20328, 9 November 1929, Page 8

ON ALL WAVELENGTHS

NOTES AND COMMENTS By

A FILTER FEED.

Received music when spoken or written ■of often has the word “frequency” connected to it, low notes being low,frequency and highmotes being : high frequency. This generally suggests an association with A.C., and the novice may therefore be misled. The presence of' A.O. would mean a flow of current taking place from plate to filament, for A.C. changes : its direction and magnitude constantly. In reality what is , in the loud-speaker windings when speech or music is being received is a pulsating' current, rising and falling, not necessarily regularly, but never changing direction, all its rising and falling being done on one side of a zero line, ./that is when signals are actually being received during the time the ’ microphone' at the transmitting station is in use,and the speech or music is being impressed on to the earlier wave. When the studio is silent the ■ pulsating current stops, and then wo have left the steady, direct current passing through the output valve and the loudspeaker. This steady D.C. is as it were the carrier of the receiving set upon which the pulsating currents representing the music and speech are superimposed. The magnetic field set up by the coil of wire in the loud speaker when current is flowing through it will either bo in phase with, or opposing, the field of the natural magnet. This is the point underlying the importance of joining an L.S. the right way round, for otherwise the field of the natural magnet is opposed, and eventually destroyed, and the speaker then becomes useless.

The object of tho natural magnet is to keep a tension on the diaphragm, so that the alterations of the strength of the magnetic field due to tho strengt h sating current flowing through the coil may be turned into sound.

The steady D.C. flow, on. which the pulsating currents are carried, also flows through -the coil, however, and acts as a magnetising current in addition to the pulsating current. With such an instrument (electro magnet) there is a definite limit to tho amount of current that may flow through tho coil before the soft iron is completely saturated magnetically. Afor this point is reached any further increase of current does not make a proportional increase in tho strength of the magnetic field, so that the steady D.C. flow through the speaker windings may be so considerable that before any signals are received the magnet is completely saturated. When the pulsating current of the signals does arrive it cannot make the correctfluctuations in the strength of theifiagnetio field, and hence distortion is produced. In the case of a modest set. no harm will result from using the speaker connected directly, but with a power valve in tho output stage some protection must he provided. The use of some form of filter circuit which only permits the fluctuations due to the incoming signals to flow through the speaker windings, and deflects the steady flow along a bypath, where it will not harm the delicate magnets of the speaker, will give the necessary protection. If anyone considers lie is experiencing this trouble, it may be easily detected by rigging up - a filter circuit and switch, so that the L.S. may be quickly changed from filter to direct, feed.

The diagram shows how.this is done. Although the filter,is not the sort that is generally used nowadays it will suffice for test, after which the filter feed to L.T. negative, now standard practice, may b© fitted with tho same components.

“MAKES LIFE WORTH LIVING.”

A. B. Creagh, tho only Australian member of the Byrd Antarctic Expedition who is at present on a visit to Australia picking up supplies for the expedition, recently made a visit to the Australian Broadcasting Company, and said that wireless is one of the few modern. luxuries that makes life _ worth living in tlfe Antarctic. Occasionally 2FGFwas heard on : the air, and the programmes were very much enjoyed. Tho wireless sets in Antarctica - are going all day, and. America is much easier to get owing to its great shortwave system. The Radio 1 League of America, with the co-operation of the R.C.A., and the different radio corporations, have given the Byrd expedition privilege to have messages transmitted and delivered to the homes of relatives and friends of members, free of charge. Before leaving for New Zealand to join the City of New York at Port Chalmers Mr Creagh gave a most interesting address through 2FC on the expedition. On tho last Byrd Antarctic broadcast seven months ago, through 2FC, an American Boy Scout, Hunter Mann Junior, who presented a Virginian flag to Commander Byrd to be flown at the South Pole, spoke to Paul Siple, the Scout who is now in Antarctica with Commander Byrd, and conveyed to him a message from Australian Scouts. . , . ■ , • The broadcast was picked up by the Byrd Expedition down on the Ross Barrier by A. B. Creagh, who was returning with the City of New York for supplies, and who promised Siple that, if possible, ho would locate Hunter Mann Junior and thank him for the messages broadcast. Hunter Mann Junior, who incidentally is the most travelled' Boy Scout in the world, since that time has been hack to America and across to the jamboiee in England, and is now back in Virginia prior -to his probable return to" China.

NEW LONDON STATION.

Mention was made in these notes last week of the British Broadcasting Corporation’s new London station at Brockman’s Park, and interest has been taken; in- the station, two or three people asking for further details. Tho following article from an English publication may satisfy tho wants of tho inquirers:— “The London regional transmitting station at Brookman’s Park, which commenced -test transmissions on a wave length of 356.3 metres recently, stands about ,400 ft above sea level. Power is generated by means of four Diesel.engines coupled to a 220 V d.c. generator, while a 2,000 Ah floating battery .used for light loads can, if. necessary, take full load. “The main motor generator room contains six large machines and a number of smaller types. in the xtre of the room are three 160 k\v. machines for applying potentials hetwetne 7,000 and 12,000 V d.c. to tho anodes of the main transmitting valves. The sets consist of three machines on one bedplate, the driving motor being coupled to two generators, each with two commutators capable of producing a voltage of 3,000, making 12,000 V maximum in all. The station is capable of transmitting two programmes simultaneously -on two wave lengths, when two generators will be in use—one for each transmitter—with the third as a spare for either transmitter. The remaining three largo machines are lor filament heating, and give 1,300 A at

voltages between 15' and 30. The arrangement is the same as in the h.t. machines —that is, one machine for each transmitter and one spare. The small motor generator sets supply grid negative voltages and separate anode supplies for the drive and modulated, amplifier stages of the transmitters. . “ The transmitter hall contains -the two transmitters and main switchboard. In-the .middle of tho room arc two control tables, one for each transmitter. Directly underneath each transmitter and the switchboard is a vault 7ib deep which contains all wiring between the switchboard and tho transmitter and various accessories, such as the pumps for the circulating water for tho transmitting valves. There are also two small storage batteries for beating the filaments ot tho drive valves, for which a steady voltage is necessary to prevent any risk of slight frequency changes. “ The transmitters normally deliver 30 kxv. to each aerial and employ modulation at low power. “ The first operation in the transmitters is a drive stage, for which special precautions are taken to keep the Ircquency constant to within about 200 cycles. Tho output from the valve is applied to the separator stage, followed by the modulated amplifier, where tho power level is approximately i kw. The output from tho modulated amplifier operates the first power stage, consisting of two water-cooled valves, tho output of which operates the final stage of a maximum of eight valves on cither side of a neutrodyned push-pull system. “ The output circuit, is coupled to a feeder system which runs straight out from the building and is carried oxerhead on steel standards to small buildings containing tho aerial transformers and tuning circuits. ■ “The circulating/ system tor the xvater-cooled valves in the transmitters is of tho enclosed type, and is arranged so that water stored in two large closed tanks circulates through the valve jackets to txvo larger tanks in the vault. “ Leading out of the main transmitter hall are txvo control rooms, where the programme is received from the underground telephone lines running to Savoy Hill. These rooms—one for each transmitter—contain the necessary amplifiers, controlling and monitoring gear. In addition the front portion of the building is occupied by offices, mess rooms, kitchen, etc., txvo sound-proof listening rooms (one for each transmitter) for quality checking by loud speaker, and a test room for taking measurements of the performance of either transmitter. A small studio for local speech or music tests is also provided.

“ There are four masts, txvo carrying the aerial for each transmitter. These masts are of the self-supporting type, insulated from earth at the base, but they are only 200 ft high. This is lower than, would have been chosen from tho engineering point of viexv, but Government limitations would not alloxv of the height being exceeded. The span betxveen each mast is 600 ft, and there is 900 ft between the centre line of each aerial. The earth system,.-which radiates from the aerial transformer houses, consists of a number of xvires buried lit beloxv the surface of the grounds, tho ends of which form an oval extending about 200 ft to each side of tho aerial and 150ft,beyond each mast.”

STRAY SPARKS

What is known as a cathedral studio lias keen oi>ened in Chicago, measuring 75ft by 45ft, with a domed ceiling in the centre. The studio was designed to accommodate the largest orchestras to reproduce the music in full tonal form. Only two microphones are used to pick up the music, one being in the ceiling and the other standing on the floor. A huge _ pipe organ is set in one wall. The Chicago Svmphony Orchestra, known to gramophone enthusiasts in this country, broadcasts in the studio every Sunday evening. * * ♦ ♦ The famous coloured singer and actor, Paul Robeson, has been engaged by the 8.8. C. to give a series of broadcasts of negro spirituals. He lias signed a four-figure contract with Savoy Hill, but even at a high rate the 8.8. C. considers that it has secured a broadcasting “ scoop." # * ♦ •* It is learned that the Columbia Gramophone Company has decided to enter the radio field on a large scale in England. The company intends spending an enormous amount _of money on the manufacture of receiving sets, employing hundreds of workers at the factories. * ♦ * • A new offer for the facilities for television transmission has been made by the 8.8. C. to the Baird Television Development Company', Ltd. The offer has been accepted. The offer gives the companv the opportunity to make use of live-hour periods per week for experimental transmissions the times of these transmissions to fall outside the normal broadcasting hours.

The trend in American broadcasting technique is towards high power. Although the use of powers exceeding ten Kilowatts is subject to special permission from the Federal Radio Commission, there are now nearly twenty stations using from twentyfive to fifty kilowatts. The General Electric Company’s station, WJZ, occasionally operates for experimental purposes with an aerial power of 100 kilowatts. Broadcasting stations in England have not yet reached the fifty kilowatt class. The most powerful British station, SXX, at Coventry, operates with an aerial power of twenty-live kilowatts.

The absence of any orchestral combination through 4YA has been the subject of much comment of late. At one time we had two picture theatre relays a week, and these were most acceptable. Then the relays were confined to one orchestra through only one theatre in town keeping an orchestra after the introduction of the talkies. However, we lost that orchestra, and since then have had to make the best of a bad job with records.

The summer is now with us, and with it often comes poor reception. A set will be working normally, but it may have lost its “kick" on distant stations, proving that the weather conditions have had a bad effect on some part of the receiver. Listeners who cannot get good reception of outside stations should take their sets to a reliable dealer,- who will test them and remedy any defective component.

It has been suggested by several that a Dunedin Listeners’ League should lie formed to hear the grievances held by 'the listeners, and to put them, before the right authorities. Coming from an official body, the complaints should receive attention from the responsible parties. Have listener's here any desire to form a league? If so, send a line to the writer, and offer any suggestions in connection with its formation.

The very latest short-wave adaptors are now on the Dunedin market. These are for fitting to an all-electric set, and until qutc recently it was considered almost impossible to work the short waves from the light mains.

AYhafc is often thought to he natural atmospherics is sometimes man-made static. A common cause of manufactured stations when the wheels of a tramcar make bad contact with the rails, and this class of interference is particularly noticeable in a vicinity where sand has been thrown on the track between the rails, portion of the sand settling on the rails themselves. One dealer in town was recently called out to nearly a dozen sets which the owners thought had developed faults. In each case it was found that sand on the tram rails was the cause of the trouble. Then, again, a set will often pick up foreign noises from a defective electric light in the street. When listeners have located the light causing the disturbance they should notify the electric power and lighting department, which always attends to the troublesome lights immediately after notification.