The following artical appeared in the publication "Murphy News" for the 17th October 1936.
|The front of the Murphy television set chassis,
which is constructed in three tiers.
BY E. J. POWER
THE Alexandra Palace Television Station has now entered upon its last phase preparatory to the introduction of the public service. At the present time experimental transmissions of one hour's duration are being radiated each morning and afternoon. The morning transmission consists of stock films and is provided for the benefit of set manufacturers, but during the afternoon the B.B.C. lets itself go, doing a considerable amount of experimental work of its own, and, in the spontaneity of some of its efforts, recaptures a little of the spirit of the early days of sound broadcasting.
At present, the Marconi-E.M.I. and Baird systems are being used alternately under competitive conditions as laid down in the Selsdon Report, and, in these circumstances, it would be unfair at this early stage to make any statement which might be interpreted as criticism unfavourable to either of these systems as compared with the other. Both are capable of giving pictures of excellent definition, and since a considerable amount of misunderstanding appears to exist, it may be as well to point out that within the sideband width of 2 megacycles to which transmitter and receiver are now limited, a 405 line picture can only yield a slight increase in definition over one of 240 lines. The technical reasons for this will emerge later in this series of articles.
At Welwyn Garden City, some 15 miles from the Alexandra Palace, we are well within the range of the London transmitter, and good use is being made of this experimental period. Some of our dealers who visited the Works during the period of the Radio Show ["Radiolympia"] were able to see one of our experimental receivers in operation, reproducing the special programmes which were broadcast at that time for the Olympia demonstrations, and from this they were able to obtain some idea of the results we have achieved after years of work in the field of television. For television is a subject upon which a close watch has been kept ever since the inception of Murphy Radio, and three and half years ago it was decided that it had become of sufficient importance to necessitate a separate television department apart from the radio development staff. The necessity of some source of test signals was early apparent, and for this purpose, a film transmitter was constructed, which at first operated at 120 line definition.
With the opening of the German television service in 1934, I paid a visit to Berlin, accompanied by K. S. Davies, who is in charge of our television development (and incidentally by C. R. Casson, who apparently has since regarded it as the standard by which to judge all continental "visits" !) in order to obtain first hand knowledge of the conditions under which the German service was being introduced. One of the main conclusions reached was that the 180 line standard was insufficient for a permanent service, and our own development work was directed towards higher standards. Our experimental transmitter was modified to 240 line working, and immediately following the publication of the Selsdon Committee's report, in which 240 lines and 25 pictures per second was fixed as the minimum standard for the proposed service, a series of demonstrations were given to representative members of the inside and outside staffs of the Company, and to a number of persons outside the organisation who might be considered as representative of the general public.
Our object in this was to obtain an idea of the probable reaction of the public to television of this degree of definition. This appeared, on the whole, to be very favourable, and from that time onwards, effort has been concentrated on the development of a receiver suitable for general use.
Reliability and simplicity of operation have, of course, been the chief aims, and throughout it has been our endeavour to keep to a minimum the number of controls which require operation by the user. It is, unfortunately, necessary in the present state of the television art to incorporate several controls in a television receiver, but many of these can be located inside the receiver, being pre-set, just as the trimming condensers of the intermediate-frequency transformers of an A30 receiver are adjusted in the factory and do not require further attention.
Our present experimental sets have three knobs on the control panel, viz. (a) a tuning control which tunes sound and vision simultaneously, (b) a sound volume control, and (c) a vision contrast control. The last is, of course, analogous to the volume control of a sound receiver. In addition to these, a double switch at the back of the set enables the receiver to be changed over to receive either of the two types of signals at present transmitted while the set is switched on and off by raising and lowering the lid, which carries the mirror in which the picture is viewed.
In future issues of THE NEWS ["Murphy News"], details of these sets will be discussed, so that it is unnecessary for me to go further into them in this article. The photograph of the complete receiver in its present form, withdrawn from its cabinet, will, however, give some idea of the general layout of the set.
At the present time, we are obtaining valuable experience in the behaviour of these experimental receivers under service conditions. Their performance has been very satisfactory and ordinary wireless users with no previous experience of television have been found to be quite capable of handling the television receiver after a very small amount of instruction. The present experimental programmes transmitted by the B.B.C. are sufficient to remove any doubts with which one might be left as to the entertainment value of television, and it is perhaps significant that more critical observers pick upon such points as studio lighting, camera angles and so on, going to show that the television picture at least bears comparison with the cinema, even though it must necessarily be somewhat inferior in definition at the present time. Naturally, one cannot expect the staff at Alexandra Palace to acquire in a few months a technique which the film industry has taken twenty years to learn, but their skill improves rapidly from day to day, and some of their outdoor work is exceedingly good.
As far as the manufacture of a television receiver is concerned, we have as yet arrived at no definite decision. It is fairly certain that we shall produce a television set in next year's programme, and when a decision is reached, the dealers and others concerned will, of course, be given notice of the fact. Technically the set will follow closely the experimental receivers we are at present using, though it will differ considerably in layout and cabinet design. It will incorporate a broadcast radio receiver (probably of the A28 class) and gramophone equipment, but in view of the unavoidably high cost, it will, naturally, appeal to only a limited market.
This brief review of the work carried out up to now in the television laboratory will serve as an introduction to a series of articles by Mr. K. S. Davies. I hope that the personel both in Murphy Radio and among the dealers who will be affected by television, will take serious notice of what is said, and endeavour to equip themselves technically to handle this additional phase of wireless entertainment. If anyone feels that additional or different help is desirable, I shall be glad if they will let us know, so that we can consider meeting their requirements.
|AN "EXPLODED" VIEW OF THE
EXPERIMENTAL TELEVISION RECEIVER
|THE TOP CHASSIS carries the radio receiving equipment, the cathode ray tube with its magnetic shield passing through a hole in the centre. The vision-receiving equipment is grouped at the left of the chassis with the sound equipment at the right. The three sockets in the centre are for aerial connections, tappings enabling feeder lines of various impedances to be employed.|
|THE SECOND CHASSIS carries the time base equipment together with the filter circuits which separate the synchronising impulses from the picture signals. The cathode ray tube, whose lower portion is protected by a magnetic shield, is mounted on this chassis, while at the rear are the pre-set controls.|
THE BOTTOM CHASSIS carries the power supply equipment and the loudspeaker. The three rectifying valves are shown and these deliver current at 4000, 800 and 250 volts for the operation of the cathode ray tube, time bases, and radio receiver respectively. The cable which inter-connects the three chassis is also shown.
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