If you mis-spent your teenage years fishing broken televisions from dumpsters and either robbing them for parts or fixing them for the ability to watch The A Team upstairs rather than in the living room as I did, then it’s possible that you too will have developed a keen interest in analogue television technology. You’ll know your front porch from your blanking interval and your colour burst, you might say.
There was one piece of television technology that evaded a 1980s dumpster-diver, no 625-line PAL set from the 1970s was ever going to come close to the fascination of the earliest TV sets. Because instead of a CRT and its associated electronics, they featured a spinning disk with a spiral pattern of holes. These mechanical TV systems were quickly superseded in the 1930s by all-electronic systems, so of the very few sets manufactured only a fraction have survived the intervening decades.
The spinning disk in a mechanical TV is referred to as a Nipkow disk, after its inventor, [Paul Gottlieb Nipkow]. [Nipkow] conceived and patented the idea of a spinning disk with a spiral of holes to dissect an image sequentially into a series of lines in the 1880s, but without the benefit of the electronic amplification that would come a few decades later was unable to produce a viable system to demonstrate it. It would be in the 1920s before [John Logie Baird] would develop the first working television system using [Nipkow]’s invention.
The operation of a Nipkow disk is simple enough. An image is projected onto its surface across the region through which the spiral of holes pass. As the disk rotates, each of its holes traverses its own arc across the image that is immediately adjacent to that traversed by the hole before it. As each of the holes performs the traverse they gradually scan the image line by line, and when the last hole in the spiral has passed it is immediately followed by the first one at the other end of the spiral and the process is repeated. If a light-sensitive detector is placed behind the disk then it receives a light intensity that corresponds to a voltage output representing the picture as video scan lines.
If the process is reversed and a lamp is placed behind the disk and fed an amplified video signal, as each hole passes in front of it there will be displayed a new line of the picture, and due to persistence of vision in the eye of the viewer the resulting fast-moving dot of light is built up into an image.
It is a given that a Hackaday reader is unlikely to stumble upon a Baird Televisor or other mechanical TV set. But the beauty of this technology is that a Nipkow disk is straightforward to make. The elementary school method involves marking a piece of card or similar flat material at the appropriate angles for the position of the holes and then measuring their position from the centre at each angle with a ruler, but a contemporary suggestion was to draw a spiral with the aid of a piece of piano wire wrapped round a central shaft. Alternatively Hackaday readers may wish to try creating a pattern for one programatically, this is the solution I opted for back when I was experimenting with Nipkow disks. My code – VBScript, but it was the 1990s! – has been lost in the mists of time, but it involved first drawing a ring of sync holes following a clock face demonstration script, then drawing another ring with appropriate decrease in distance from the centre for each hole.
Lest you imagine the Nipkow disks are an antiquated technology found only in museums and on the benches of mechanical TV enthusiasts, there is one field at the cutting edge of science in which they still play a part. Confocal microscopy is a technique in which a sample is scanned with a pinpoint of tightly focused light, to produce an extremely narrow depth of field and to reduce or eliminate reflections from out-of-focus parts of the sample. The Nipkow disk has been joined by laser scanning in this task, but retains an edge when a very low light intensity is required for a photosensitive sample.
An outdated method for producing low-resolution television is probably not something that will set every Hackaday reader’s pulse racing. But I’m sure I’ll not be the only member of our community with an interest in this direction. If I’ve just described you, then maybe it’s time to cut yourself a Nipkow disk, and post your mechanical TV set on hackaday.io.
Header image: H. G. Cisin [Public domain].