No. 971. THURSDAY, APRIL 7TH, 1938. VoL. XLII. No. 14. Proprietors : ILIFFE & SONS LTD. Editor : EDITORIAL COMMENT HUGH S. POCOCK. Editorial, relayed by wire fairly satisfactorily, Advertising and Publishing Offices : Television in the Home and it is for this very reason that so DORSET HOUSE, STAMFORD STREET, concern has been felt by listeners LONDON, S.E.I. Relay Reception Impracticable ? much and by the radio industry over the Telephone: Waterloo 3333 (5o lines). Telegrams: "Ethaworld, Sedist, London." HEN the television service attitude of the Post Office in proposing was started the fact that to establish a nation-wide relay system CovENTRv : Hertford Street. Telegrams: Telephone: high ground had to be which would gradually mean the dis- "Autocar, Coventry." 5210 Coventry. chosen for the transmitter, appearance of manufactured wireless BIRMINGHAM: and that at that time it was necessary sets, except in comparatively small Guildhall Buildings, Navigation Street, 2. for the cameras to be located very near numbers and, therefore, at vastly en- Telegrams : Telephone : itself, meant hanced to the public. "Autopress, Birmingham." 2971 Midland (4 lines). the transmitting apparatus prices that the B.B.C. had no alternative but At the same time that the proposals MANCHESTER : 260, Deansgate, 3. to accommodate the television trans- for a sound -broadcasting relay organisa- Telegrams: Telephone: Biffe, Manchester." Blackfriars 4412 (4 lines). mitter and studios on the same site tion on a national scale are being and at a spot remote from other B.B.C. pushed, statements have been made GLASGOW : 26s, Renfield Street, C.2. Telegrams: "Biffe, Glasgow." Telephone: Central 4857. centres. that the ultimate distribution of tele- As improvements have been made vision may also be by wire to individual PUBLISHED WEEKLY. ENTERED AS SECOND CLASS MATTER AT NEW YORK, N.Y. and it has been found possible to homes. televise from a considerable distance Subscription Rates : by means of special television cables Technical Considerations Home, LI is. 8d. ; Canada, LI Is. 8d. ; other countries, LI 3s. Iod. per annum. or a short-wave wireless link, the posi- tion is gradually developing where it An article in this issue discusses the As many of the circuits and apparatus described in these possibilities of distributing television pages are covered by patents, readers are advised, before is no longer so essential for television making use Of them, to satisfy themselves that they would to be regarded as a separate unit. by means of a_ relay system, and a not be infringing patents. study of facts there revealed leads us Merging Television and Sound to the conclusion that the chances of a television service by wire to individual Broadcasting homes are extremely remote. If this CONTENTS We think there is no doubt that the is so, and the article leaves very little doubt on the matter, then to encourage Page technical directors of the B.B.C. will be anxious to merge sound and tele- a sound -relay system will not contri- Editorial Comment .. 301 vision broadcasting more and more as bute in the least to popularising tele- Testing PA Equipment .. 302 time goes on. Television must ulti- vision, as television receivers will still Return from Malaya .. 306 mately become the means of illustrating have to be designed for reception by wireless, and once we are faced with Unbiased .. .. 308 the sound -broadcasting programme. this necessity, the addition of the small Television Relay Systems 309 If we accept this view, a first step made towards planning the amount of further apparatus for sound Broadcast Programmes .. 310 has been future of television on a satisfactory reception as well will add very little Timing London's Buses . 311 basis and, as far as wireless transmission to the cost. If, then, for television Practical Aspects of Design 312 is concerned, we anticipate that pro- we must continue to depend on wireless Ambassador Superheterodyne Re- gress in this direction will be rapid. transmission, and if we agree that viewed .. .. , 314 television and sound -broadcasting must ultimately merge as one service, it News of the Week 316 The Relay Position is only logical to enquire what benefit Television Topics 318 Let us now turn to a consideration it can be to listeners to pay for a relay Random Radiations .. .. 320 of relay possibilities in relation to service for sound when a wireless re- Principal Broadcasting Stations 322 sound and television. Experience has ceiver for television remains a Recent Inventions.. 324 shown that sound-broadcasting can be necessity ? 2 30 The Wireless World, April 7th, 1938 Testing PA Equipment Checking Amplifier Performance By P. H. WALKER, B.Sc. (Eng.) THE best can be got out of an amplifier only by designing it specially to fulfil a particular purpose, and it is interesting, if not always necessary, that has given very satisfactory results. to know its technical performance in some detail. In this article it is Turning now to the methods of measur- ing the proposed (a) to mention briefly the details of design of one of the PA performance of the equipment, we shall deal first amplifiers used at University College, London ; (b) to describe with the frequency - at least one response from method of determining the most useful characteristics, the pick=up terminals of and lastly (c) to give the tone control stage actual results of tests to the secondary made upon the University College equipment. terminals of the output transformer. The input impedance is a volume control, or A pasratu inlled U pensive moving -iron milliammeter as the grid leak of 0.5 M0, say, and the connec- I. sityap College «wassta designed toniver-give output meter (with series resistance) or tions between the oscillator and ampli- really high -quality reproduction, anode current meter as required. A ten - fier are arranged as in Fig. i. Next, the and was built by Sound Sales, volt accumulator is Ltd., to our specification. This specifi- used for micro- cation was briefly as follows.: -Two I2 - phone polarisation, watt amplifiers to a design based on The mounted inside the Wireless World Quality Amplifier, micro- cabinet in a lead phone amplifier with independent treble box. The superio- o8C. and bass tone controls, local -station re- rity of this method ceiver, volume controls on, and mixing over dry batteries of, all inputs, headphone monitoring is very marked, jacks, output meter, output valve anode - particularly in re- MICROPHONE MICROPHONE current meter, twin turntables with record gard to cost. TRANSFORMER TRANSFORMER groove cue scales on the pick-up arms. The radio set is a (a) (b) The use of two small amplifiers made simple affair, con- the equipment much more flexible, and sisting of an RF complete breakdown Fig. 3:-When the input is applied to the microphone transformer almost impossible pentode feeding a primary the arrangement when compared with a of (a) should not be used. For correct single 24-watt am- power grid -detector results it is necessary to load the transformer properly as in (b). plifier. They were arranged so that two and thence to the completely independent ` programmes " tone control amplifier. The tone control output impedance into which the ampli- could be dealt with at the same time, such circuit is similar to that designed for The fier is designed to work must be known as stage - effects and a load resist- noises, independ- ance made up of ently of our stage - g a )'ilI this value and con- greenroom micro- / AMPLIFIE'--_ 2 ---- nected to the out - phone system.' 2'° ---- -- p u t transformer When required, a B AMPLIFIER \ - secondary in place switch parallels the I I 1 1 1 l t s of the speech coil. amplifiers to give a a There are two combined output of FREQUENCY IN CYCLES PER SECOND ways of measuring over 20 watts un- the input to the distorted, though Fig. overall response amplifier z.-The curves of two amplifiers are given here ; the differences are - with a each amplifier is quite small. valve voltmeter or capable of coping with a thermo- with most situations when operating alone. Wireless World i2 -watt PA Amplifier, ex- milliammeter. The first method is always Provision was made for headphone cept that bass attenuation is provided in preferable if it can be made to read low monitoring with the assistance of an out- addition to the other ranges. It is not voltages accurately and the instrument put meter : speech needs much more care- often required, but it becomes particularly should be connected as shown by VV in ful monitoring than music, and the rela- useful for improving the intelligibility of Fig. r. If, however, the load resistance tively low fundamental frequency of speech at very high volume and also safe- R is made up as a fixed potential divider, guards the loud the voltmeter may be connected as shown speakers from dotted and the voltage injected into the damage. amplifier will be a known fraction of the Marconi - Reisz VV reading. carbon micro- Measuring the output is much simpler, phones are used, and valve or metal -rectifier voltmeters while at the other may be úsed. end of the chain we have a number of Output Level for Measurements baffle speakers, also PM Standard Fig. r.-The connections between the AF oscillator and the input of Rola Gil practice with receiver mea- the amplifier are shown here. speakers which can surements asks us to show the frequency be bolted rapidly, response as the decibel ratio of the input speech made it possible to use an inex- either on to baffles or horns, and for our voltage. at all audio frequencies to the in- ' Described by the Prompter in the London more important work we are equipped put voltage at 40o c /s to give a constant Evening News of 16th December, 1936, and else- with a Voigt twin diaphragm double -power output of 5o milliwatts.