WIRELESS NEWS

Press, Volume LXVII, Issue 20168, 21 February 1931, Page 9

WIRELESS NEWS

(By MAX.) It will probably surprise many of our readers to know that nearly all the electrical'energy in the output valve of an audio amplifier iB wasted. Different writers place the losses as somewhere between 96 and 99$ per cent. Even with the former figure there is a tremendous loss in converting electrical energy into sound energy. »In other words, the efficiency of an average loudspeaker is less than two per cent. * * Special gramophone records are now on[the British market and these records are what is termed "sound-test" records. The Parlophone makers have recently put out two and each contains eight sine-wave constant frequencies distributed at octave intervals throughout the useful musical range. The first gives frequencies of 32, 64, 128, 256, 512, 1024, 2048, and 4096 cycles, and thp second 50, 100, and so on up to 6400 cycles. Each separate frequency lasts for 40 seconds, and a plain groove iB cut in the spaces between each band of frequencies to locate the nccdlo point at the end of each band. i * # Records of this type are useful for testing out different amplifiers and also loudspeakers. It enables tests to show how the ' speaker and/or amplifier responds to or reproduces the high and low tones. Mod-, ern transformer coupled audio amplifiers can be made which give a fairly faithful response to the useful range of musical frequencies. The LoftinWhite resistance coupled outfit is noted ' for its even amplification and a recently ! developed: battery type gives perfect results from 60 cycles to 5000, and with .ljttle fal,ling-off of frequencies up to 10,000. * # ' The annual of the lladio Society of Christohurch is to be held next Thursday evening at 8 p.m., and it. is' hoped that there will be a large attendance ;There hag .been a good increase in membership lately, the new classes for intending amateurs being exceptionally well. attended. After .the annual meeting a new series "of .tulles on alternating current will given, .and special attention is to be <giren to the practical methods of demonstra* ting the various points. A few weeks ago the writer was asked for advice as to the best system to eliminate a howl or whistle which was .set up when the hands of the listener' were plaoed against the headphones of a short-wave receiver. It was suggested that the enquirer try wrapping wire-or metal foil around the leads ana 'earthing them.'An alternative -.mentioned was;.,to 1 tise high frequency chokes -, in hqtji 'phone leads. The use of wire was tried and proved quite successful >so who are troubled the same way. or witty capacity effqet. should favour the same remedy. * * . A, description was seen a short tiind ago of" a way to get over bad electri,_bal pickjup which evidently came-vi&> the antenna.,. The idea "was to coVer the lead-in with; a? metal sheating and • earth-4t. Of course it would-be essential to Have an ■ insulated covering on metal; shield Shbttld extend for some fifteen feet of so from the building, and also be. con» tinuouuJijjside the .house afeffat! titf the Static- reduced to a conAidefabie extent, "mi' it U- claimed, and but little, Idas >* of ,> signal 'Strati gthfo. > 1 \ ' ♦'* - * The members of the New Zealand Amdteur!' rAs&oMptipo lire organising to make a/suitable' record „ of, the excellent communication jrecpntearthquake. One'of the ideas suggested » to /isjpue certificates of jnaerit whew w is ; (Considered 1 they they have been earned'. ! ' j " , •. , 1 T * * During the last three months of 1930 two 'Australian stations, 3DB tuid BU9 of Melboui;n< beejl putting /ove* "stilt pictures. " These special transmit-, sions' have taken place outside of the .regular, broadcast hours of these ©lass B*' stations.,Jßafore permission to pujt still pictured on the air «ould be obtained, 'a test was made between the well-known Fnltograph system and the t.E.L. Company's. As a result of these te&ta, ithe'latter system was chosen as the' better. - It is the one developed by -Mr Donald v. Macdonald, M.1.8.E., an Australian. Now there has been an amalgamation between the Fultograph interests in Australia and th®i T.R.L. (Television and Radio Labora* 'tories Pty., Ltd.). * # ' 1 It does not require an expensive apparatus to receive and record -still pictures. The writer obtained prices from a British firm twelve months ngo, and the special apparatus would be about £ls. Any radio receiver capable ■ of giving normal signals «from a 1 is quite sufficient to operate the machine. In fact, "Wireless World" gave complete directions, profusely illustrated, for making most of the parts. * * \ Briefly, the receiver feeds the,signals from the transmitter to a platinum stylus- which rests upon a piece - of paper, which is sensitised with iodised starch and is wrapped around a revolving cylinder for all the world > similar to the old style Edison phonograph. The arm carrying.thp-platinum point tiavels. alpng a threaded..spindle and thp varying ele?tri6 current makes, the image by its effect ,UPA» <fhe B§nejitised paper. A Btrong , momentary current gives greats depth' !6f , colotir at that point and' vice,, VerSa. f % jli ' # WheW{t}ie'>meeha2Usm differa'from the old phonograph is in t)ie weights-.of Thesd must be light because''the cylinder, ifr., stopped automatically? by a catch at' evory revoluana is again released by a "starting*/ sigrtal from the transmitter at every'fresh-revolution. The receiver's ' cylinder Jttiuat therefore be spinning a fraction faster than that at the transmittiiig pnd.,' Special delicate relays are included in the receiving apparatus for the continual tripping by the starting signalj Which is simply a stronger impulse at every revolution, sent at the transmitting end. * ' * * One type of mechanism that wan illustrated'some time, ago utilised-'a light spring wJUch ivas placed under tension by the stopping unit, and when the catch'was released the storcd-up energy at the small spring gave' the' cylinder a'slight impetus which,enabled it to 'reach-normal'running speed almost immediately, thereby giving a much -clearer picturo. It is obvious that a lag in the*speeding up of the cylinder would 'distort the photograph .if the transmitting cylinder were running' at 1 a constant speedj controlled by a'special method; as Jt does. . » < The quartz crystal has many uses jirhexe an accurate jfreg.uency; has to be, l ' j" 1 t > •*" 1 / V) '

Notes From Far and Near

maintained, « To overcome a change of frequency due to alteration of temperature, special means are taken to. ensure a .constant temperature* •. The. crystal is enclosed within special BeT3.es, of' boxes, a most ingenious sys*' 'tem, simple withal, and with a thermo--stat and heating "resistances to keep a .Biettdy temperature. >■ The particular crystal outfit referred to / is.used to give a fundamental frequency of one million cycles/ This 'controls 1 different lower 'frequency circuits using hsjtmonlca 'of tlje fundamental, ' ten - thousand cycles operating one special vibrator whilst the tenth harmonic of this latter gives one thousand cycles to- be used on motor which' rotates at thisnumber \Ofv revolutions per ' second. A step-down cog arrangementon the motor .operates the second hand of a standard e]ock. n With this ap-, par&ius snd suitable checking operations, it is considered thai the'amount of error is not more than one second in ten million, About one second in 115 days. This machinery is in use $t the American Badio Belay League 'a key station for 'standard, frequencies^'