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.

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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. the incorporation of twin prisms, split- not finished. ting the light into two rays which, after Meanwhile, vibratory scanning was New light sources passing through the liquid were recom- being tried. A suspended mirror was Major problems to be solved were the bined by another pair of prisms. How- rocked horizontally and vertically to need for more effective light generation, ever, the light transmission of even this scan the screen. To improve efficiency, better picture resolution and size. Al- double-image Kerr cell was still very in- the mirror mount was made resonant at though by 1931, teams from RCA in efficient, some workers even resorting to the two sweep frequencies, but unfortu- America and Marconi-EMI in Britain arc lamps in attempts to provide suffi- nately this produced a sinewave rather were at work on all-electronic TV sys- cient image intensity. than a sawtooth motion, making the trace tems, many experimenters including incompatible with the other systems. Baird were still spending much time and The mirror drum effort in persisting with the disc system. The first and simplest mirror scanning Mirror screws Within a year, however, the physical system was a series of mirrors mounted In 1930, Paul R. Egger, of Davenport, problems of the scanning disc were fi- around a drum. Baird Television, who Iowa USA filed a patent for a 'mirror nally acknowledged, and development had persuaded a reluctant BBC to permit screw' array, with a series of angled switched to the use of mirrors. a pair of their broadcast band transmit- mirrors, equal in number to the number To obtain a faster response and ters to be used for experimental trans- of lines. Much of the development of brighter illumination, a scientific curios- missions, was in 1932 able to sell an the mirror screw was done in Germany. ity, the Kerr cell, which had been a solu- improved 'Televisor' with a 9" by 4" pic- For a 180-line system, there would be tion waiting around for a problem, was ture, using this system. There were 30 180 long thin metal mirrors, each dis- dusted off and improved. mirrors, each carefully aligned so that in placed 2° (360°/180) from the previous Photographers will be familiar with one revolution of the drum, their reflec- one, making the array like a spiral stair- the ability of two polarising filters to tions successively scanned the frame way. The picture line thickness was the block the passage of light if the plane of (one line to a mirror). Much more com- same as the mirror thickness, and the one is rotated 90°. A Kerr cell consists of pact than the equivalent two-metre diam- mirror lengths became the picture two correctly oriented polarising prisms eter disc, and illustrated in Fig.7, this width. When the screw was rotated, per- separated by a suitable liquid. If a field mirror system remained the Baird stan- sistence of vision made the mirrors ap-

Fig.6 (above): Flywheel synchronisation, 1930's style. This unit was plugged into the output stage of a standard broadcast receiver. The scanning mechanism was driven by a synchronous motor in approximate synchrony. Attached to the drive shaft was a serrated 'phonic wheel' with a tooth for each line, running past the electromagnets in the valve anode circuit. The valve was overdriven so it clipped off the video and passed only the sync pulses.

Fig.7 (right): The 30-line mirror drum from a Baird receiver, as pictured in 'Television, a Guide for the Amateur' (1936).

100 ELECTRONICS Australia, April 1994 pear to run together, the image appear- celerations and retardations will occur ing to be as tall as the stack, and as wide in the light beam. This property can be as the mirror length. used to modulate the light beam, with Illuminated by a Kerr cell, the mirror minimal loss in transmission. screw was a major advance in light effi- The first Scophony receivers had mir- ciency and compactness. By the stan- ror screws, but eventually mirror drums dards of the time, the resolution was and rotating prisms were used. In 1938, high, and it had a wide viewing angle. Scophony demonstrated three mirror Some German 180-line mirror screws drum receivers for the new high defini- were 9" by 12" (about 23cm by 30cm), tion 405-line system. A domestic model which would produce an image equiva- with a 22" by 24" picture was never lent to that from a Nipkow disc about 16 sold, but theatre models with 5 x 6 foot metres in diameter. Mihaly-Traub system The Mihaly-Traub scanning system overcame the mirror drum problems in a very ingenious way. In the first develop- ment, the mirror drum was made station- ary, with the mirrors now on the inside and aimed at a double-sided mirror rotat- ing at half line speed at the centre of the circle. Not only were the mirrors in solid adjustment, but each was used twice each revolution, once for the upper half of the picture and once for the lower half. Later it was realised that with a suit- which, according to my calculations is able alignment of the mirrors, and by about A$16,000! using a multifaceted rotating prism, a full Admittedly, the AD65 with its circular ring of mirrors was not required. The Bakelite cabinet is a very collectable transmitting unit in Fig.8 used a 10-sided Fig.8: A Mihaly-Traub 240/360-line model and this was a rare coloured ver- scanner, with six mirrors and a sion. But even so, I do not know any prism and six adjustable stationary mir- 10-sided rotating prism, was a rors, for 240- or 360-line systems. enthusiasts who would be prepared to considerable advance on the simple pay even one tenth of this price. Scophony 30-line scanner of Fig.7. This report gives credibility to recent Scophony, a name derived from the American articles asserting that classic Greek for sight-sound, was the name of and even 9 x 12 foot pictures were oper- radios are a good investment, as they will the advanced system first conceived in ated successfully. The scanner operated increase in value in the same way that Britain by G.W. Walton. Although smoothly at 30,75Orpm and could be old cars did a generation ago. This is not based on mirror scanning, Scophony used on the American 441-line system a good trend from the point of view of came close to holding its own with all- at 39,600rpm! the enthusiast, who is interested in vin- electronic television. In an innovative With its brighter and larger pictures, tage equipment for its own sake, rather split-focus optical system used in the rugged reliability and absence of a le- than as an investment. ❖ scanning process, the picture was di- thal EHT supply, it is likely that Scoph- vided into horizontal strips, each one ony, had World War II not intervened, representing one line. Focusing of the would have provided electronic TV light beams was accomplished by with some real competition. As it was, crossed cylindrical lenses which con- by the time transmissions were resumed centrated the light in two planes. This after the war, experience gained with allowed the use of smaller lenses and radar had given electronic TV such an mirrors, thereby reducing the cost and advantage that mechanical systems size of the equipment. Scanning was by were never resurrected. a 20-sided stainless steel polygon 60mm in diameter. What's it worth? Although the Kerr cell had been a As a footnote this month, one ques- considerable improvement over the tion I am frequently asked is "What's neon lamp, a still brighter light source that radio worth?" There are no stan- was required. In 1934, J.H. Jeffree of dard values for vintage radio equipment the Scophony Company developed a and prices are very dependent on condi- modulator with a light transmitting abil- tion, age, fashion and demand. ity two hundred times greater than the One recent transaction has caused a Kerr cell. The Jeffree cell operates on few raised eyebrows. If a report, with the principle that when a supersonic photograph, in the English Daily Mail mechanical oscillation passes through a for September 15, 1993 is to be be- transparent liquid, through which is also lieved, a 1934 Ekco AD65 sold at a passing a beam of light, a series of ac- West London auction for £7500 —