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May 2019
Pictures on your radio

Pictures on your radio

01 mayo 2019

Keith Wilson - Electrical engineer 

Digital radio (HD Radio in the USA, DAB or DAB+ in most of the rest of the world) is steadily displacing traditional AM and FM broadcasting. Many benefits are claimed for the new technology, including the option for the broadcaster to attach still pictures, such as album artwork, to their transmissions. Suitably equipped receivers display the pictures automatically. Still pictures on your radio – wow, that’s a novel idea! Except that it isn’t. In the UK, the BBC was doing it in 1928!

To be fair there are quite a few differences between today’s digital systems and the Fultograph used by the BBC. For a start, with the Fultograph, pictures and audio couldn’t be transmitted simultaneously. Instead, pictures were transmitted during breaks in the programme. And they weren’t displayed on a screen, they were printed on a small piece of specially treated paper. Nevertheless, the system briefly created a lot of interest. So how did it work?

Essentially, it was an early form of fax machine. At the transmitter, the pictures were wrapped around a rotating drum and illuminated by a “fine pencil of light”. The light reflected from the picture was picked up by a photocell. The drum was mounted on a screw thread so that, as it rotated, it also traversed longitudinally. This meant that the pencil of light would progressively scan the whole picture, with the photocell producing a varying output that depended on the darkness or lightness of the area currently being scanned. 

In principle, all that was necessary at the receiving end was to have another drum, rotating and traversing in synchronism with the drum at the transmitter. The receiving drum would have a piece of paper wrapped round it and a means of producing marks on the paper that varied in density according to the amplitude of the received signal. In reality, as always, things were a little more complicated. 

One problem was that the signal from the photocell could not be used directly to modulate the radio transmitter because large areas of a single tone in the photograph – a white background, for example – would produce a very low frequency signal which neither the transmitter nor the radio receiver could easily handle. The solution was to use the signal from the photocell to amplitude modulate a 1,000 Hz tone. At the receiving end, the tone was rectified to produce a varying DC voltage that could be used to mark the paper. 

But how was the marking achieved? The receiving drum was made of a conductive material, and a metallic stylus, carrying the varying DC voltages corresponding to the picture information, was made to bear on the drum. The paper was impregnated with potassium iodide and was used damp. When a current passed through the paper from the stylus to the drum, the point on the paper that the stylus was touching darkened almost instantly, allowing the transmitted picture to be reproduced. 

Ensuring synchronisation between the transmitting and receiving drums was another crucial issue, especially as the receiving drum was driven by clockwork which meant that purely electrical methods couldn’t be used. Instead, the receiver used a system of cams and electromagnetically operated latches arranged so that the drum would stop at the end of each full rotation and wait for a synchronising signal before starting on its next rotation. 

The receiving set up is shown in the photograph, which is taken from a contemporary issue of Wireless World magazine. The picture receiver with its drum clearly visible is on the right. The other box is the extra valve (vacuum tube) needed to rectify the 1,000 Hz tone. 

Did it work? Apparently so. In October 1928, a press event was held at the Savoy Hotel in London to mark the first transmission of Fultograph pictures by the BBC. The results, as published in Amateur Wireless magazine, are reproduced here. It’s clear that someone didn’t start the receiver promptly when receiving the picture of King George VI because the left side of the picture is missing. The magazine also notes, “These reproductions do not do justice to the originals … [they] were very much more detailed and the streaky effect much less pronounced.” 

The BBC transmitted Fultograph pictures for a year, the service ceasing in October 1929. Take up of picture receivers was reportedly modest, probably because they cost almost £24. Adjusted for inflation that’s about £1,500 (US$ 2,000), which is a lot of money to pay for a handful of pictures each week, especially when a much more tantalising prospect is emerging – that of television. John Logie Baird had demonstrated television as early as 1926 and, on September 30, 1929 the BBC inaugurated an experimental television service using his 30-line system. Crude it may have been, but it nevertheless helped to end still picture broadcasting on the radio for almost 90 years.