Early Mechanical TV Systems Public Television Came Late to This Part of the World
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Early mechanical TV systems Public television came late to this part of the world. Australian transmissions did not commence until 1957, and New Zealand had to wait a further five years. Therefore, unlike the Americans and the British with their heritage of old and half-forgotten technologies, like pre-NTSC colour sets or old monochrome 405-line sets, we have little more than 625-line monochrome and PAL models. Although we do not have a legacy of working system was not available. In pioneer monochrome receivers, it is 1843, Alexander Bain published his How Many Lives has a quite possible that in time there will be ideas on a facsimile system, laying TELEVISION collectors delighted to get their hands on down the principles of horizontal and TU11:31E? — for example — a classic Philips K9 vertical scanning and synchronisation colour receiver. For the present though, between transmitter and receiver. The we will look at some of the early systems photo sensitivity of selenium was dis- developed long before there was Austral- covered in 1873. asian television. In Germany in 1884, only eight years In Britain and America, old mono- after the invention of the telephone, Paul chrome TV receivers, many now well Nipkow suggested that the serial trans- over 50 years old, do have an important mission of video data could be achieved place in collections. Most have small cir- by scanning the picture with a spirally cular picture tubes, some so long that perforated disc with one hole for each they had to be mounted vertically and line. In 1911 the English scientist Camp- viewed in a mirror. But there are even bell Swinton, with remarkable insight, older sets, with mechanical scanning stated that successful television would systems — including the 1928 Baird 30- never be accomplished by mechanical $7.50 line `Televisors' with perforated scan- methods. He also described an intricate ning discs, and there are also a few sets electronic system, but no working exam- Raytheonl with some very sophisticated mirror ple was then possible. It was only during Kin9-14nw The name. Raytheon on any technology — which, but for the inter- the 1920's, with the availability of high type of tube assures a service- life so long that it is often ruption by World War II, might today be speed sensitive photo-electric materials, equivalent to the lives of two or three ordinary tubes. far better known. that various workers were able to The principles of sturdiness and strength developed in the The fundamental principles of televi- demonstrate the transmission of images. production of Raytheon sound- reception tubes have been ap- sion were worked out in the 19th cen- Scotsman John Logie Baird, having plied to the manufacture of Raytheon television tubes. tury, but the technology to make a first failed in a venture to manufacture a The Raytheon Kino-Lamp is the long-life television reed,- ing tube—adapted to all sys- tems, and made in numerous types. tRaytheonl Fot9-WI The long-life television sending tube., in either hard-vacuum or iss-idled OP., and in two sines of each. Information and Prices upon application Correspondence is invited frost all interested in television RAYTHEON MFG. CO. CAMBRIDGE • MASS• Fig.1: Best known of the early TV systems was the Nipkow disc. This diagram Fig.2: Essential parts for disc TV were illustrates the size limitations and the wedge-shaped picture with its curved a photocell for transmitting and a scanning lines. With only 24 or so scanning lines, resolution was quite poor. neon-filled `kino lamp' for the receiver. 98 ELECTRONICS Australia, April 1994 1,792,683. TELEVISION APPARATUS. PAUL R. EGGER, Davenport, Iowa. Filed Mar. 22, 1930. Serial No. 438,004. 5 Claims. ,• • 1. In a television set, a scanning element comprising a revolvable shaft, and a plurality of narrow mirrors formed integral therewith extending transversely of the shaft parallel to and in line with the axis of the shaft. Fig.3: The mirror screw represented a major advance on the Nipkow disc This diagram is taken from the original 1930 patent. Fig.4: A selection of production model mirror screws (probably 180-line), mounted directly on their motor shafts. It is likely that the multi-turn models were patented undersock and then in an to simplify setting critical mirror angles. equally unsuccessful jam-making enter- prise, was determined to make his for- the loudspeaker in a standard receiver. from the same mains grid, synchronous tune from television. In October 1925, Only head-and-shoulders shots of people motors could be kept in step. If however after two years of experimenting, he had any chance of being recognisable, this was not the case, a line synchronis- demonstrated a crude working system scenery was out and furthermore the ing signal could be derived from the using Nipkow's disc; his dogged but ulti- scanning lines were curved with a wedge video waveform. Fig.6 shows an Ameri- mately unsuccessful efforts to develop a shaped picture. The electronics were can system using a 'phonic wheel'. commercial TV system have given him a simple, but about the only other merit of There was no frame synchronisation. place in history. the disc system is that it clearly The physical constraints of the disc Meanwhile, although details of Camp- demonstrates the fundamentals of scan- system presented the really insurmount- bell Swinton's work were freely avail- ning and synchronisation. able problems. In line with today's TV, a able in publications like Harmsworth's Receivers were effectively standard scanning speed of 25 frames per second popular 1923 Radio Encyclopedia, other broadcast types, but with a `Kino' lamp or 1500 per minute was commonly used, workers too, in Britain, Germany and in place of the loudspeaker. In fact, the requiring a rotational speed of 150Orpm America were persisting with disc scan- early Baird `Televisors' were intended to — safe for small discs only. ning experiments. be driven from the family radio. As line To provide a sharp image, the holes In America, two farsighted pioneer in- rates increased, the frequency response needed to be small, but these attenuated ventors, Philo Farnsworth and the Rus- of receiver audio amplifiers became the already low intensity light. Increas- sian immigrant Vladimir Zworykin, more important, and resistance coupling ing the size of the scanning holes re- realised that although mechanical televi- became essential. duced the picture detail. With the dim sion scanning might give early results, it Synchronisation of mechanical sys- light produced from neon lamps, only a would ultimately be incapable of suffi- tems was not very sophisticated. If the very limited reduction in disc size was cient development. As a result they were transmitter and receiver were supplied made possible by using magnifying working independently on electronic viewing lenses. methods of scanning. By 1926 Farnsworth had solved most Supersonic speeds of the problems of a workable all-elec- An example will demonstrate the utter tronic system. In April 1930 he was impracticality of using a disc to provide a awarded patents for electronic transmit- reasonable image size. An acceptable ting and receiving technology funda- picture would be 30cm square, and mental to electronic television. would use 240 lines at a frame rate of 25 Zworykin invented the Iconoscope, and per second. The scanning disc would it is the work of these two that provided therefore require 240 holes, spaced at much of the foundation for today's tele- 30cm intervals in a one turn spiral. Sim- vision systems. ple arithmetic shows that the disc cir- cumference would need to be 240 times Poor disc images 30 centimetres, or 72 metres — giving a The images produced by the disc sys- diameter of about 24 metres! tem were impractically small, often not With a speed of 25 revolutions per sec- much more than postage stamp size, with ond, the rim of the disc would also have a a resolution of between 24 and 60 lines. velocity of nearly 6500 kilometres per One limitation to the number of lines was hour, over five times the speed of sound... the use of broadcast band transmitters There is one story of Baird nearly kill- with restricted bandwidth. ing himself with an experimental disc Initially, the only available source of 100.000010********Milpr. only one tenth this size, fitted with 30 light that could respond quickly enough Fig.& A mirror screw receiver in large lenses. A disc of even this size with to video signals was a neon tube produc- operation. The light source was lumps of glass around the rim rotating at ing a dim pinkish light — substituted for probably modulated by a Kerr cell. 150Orpm is not the sort of device to de- ELECTRONICS Australia, April 1994 99 of several hundred volts is applied to the dard receiver until the end of the low VINTAGE RADIO liquid, polarisation is set up in the liquid definition service in early 1937. itself. This polarisation is proportional to Although a considerable improve- light a safety inspector — especially the applied voltage, permitting modula- ment over the disc, the mirror drum was when, as happened in this instance, the tion of the amount of light transmitted not suitable for more than 60 lines. Set- lenses became dislodged at 650 kph! through the assembly. ting up the mirror angles was very criti- Clearly, the spinning disc was com- Baird experimented with various liq- cal and centrifugal force threw them out pletely impractical for a viable commer- uids in the Kerr cell, finally settling on of alignment. However, as we shall see, cial system and better scanning systems nitrobenzine. A major improvement was development of the mirror drum was were essential for further progress.