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Mechanical television was a television system that uses mechanical or electromechanical devices to capture and display images. However, the images themselves were usually transmitted electronically and via radio waves.
Mechanical television in history
The mechanical part usually consisted of a Nipkow disk, which had a series of holes arranged in a spiral. In the camera, the disk had a light-detecting device behind it and, in the reproducer (the display), it had a modulated light source behind. As each hole flies by, a scan line was produced and transmitted to the reproducing device.
Because only a limited number of holes could be made in the disks, image resolution on mechanical TV was typically very low, ranging from about one dozen lines up to 100 or so. A few systems ranging into the 200-line region were also attempted. Lines were usually oriented vertically rather than horizontally as in modern TVs, often resulting in a portrait image instead of the common landscape orientation viewers are familiar with today.
There are several other technologies that can be used instead of a Nipkow disk. Other arrangements often made use of a rotating drum, either with holes or with a series of mirrors mounted on it. A few of these systems were able to produce images several feet wide and of comparable quality to the CRT-based televisions that were to follow. CRT technology at that time was limited to small , low-brightness screens. Perhaps the best mechanical televisions ever devised used the Scophony system, which could produce images of more than 400 lines and display them on screens at least 9×12 feet (2.8×3.7 m) in size (at least a few models of this type were actually produced). The Scophony system used multiple drums rotating at fairly high speed to create the images. One using a 441-line American standard of the day had a small drum rotating at 39,690 rpm (a second slower drum moved at just a few hundred rpm).
The advancement of all-electronic television (including image dissectors and other camera tubes and cathode ray tubes for the reproducer) marked the beginning of the end for mechanical systems as the dominant form of television. Mechanical TV usually only produced small images. It was the main type of TV until the 1930s. By the end of the 1930s, electronic television was quickly advancing past this point, reaching 400 to more than 600 lines with fast field scan rates in the next few decades.
Recent uses of mechanical television
Since the 1970s, some amateur radio enthusiasts have experimented with mechanical systems. The early light source of a neon lamp has now been replaced with super-bright LEDs. There is some interest in creating these systems for narrow-bandwidth television, which would allow a small moving image to fit into a channel less than 40 kHz wide (modern TV systems usually have a channel about 6 MHz wide, 150 times larger). Also associated with this is slow-scan TV, although that typically uses electronic systems.
The re-emergence of mechanical TV techniques
Today, a mechanical system of a sort has seen moderate popularity. DLP (Digital Light Processing) projectors use an array of tiny (16 μm²) electrostatically-actuated mirrors selectively reflecting a light source to create an image. Many low-end DLP systems also use a color wheel to provide a sequential color image, a common feature of many early color television systems before the shadow mask CRT provided a practical method for producing a simultaneous color image.
Another place where high-quality imagery is produced by opto-mechanics is the laser printer, where a small rotating mirror is used to deflect a modulated laser beam in one axis while the motion of the photoconductor provides the motion in the other axis. A modification of such a system using high power lasers is used in laser video projectors, with resolutions as high as 1024 lines and each line containing >1500 points. Such systems produce, arguably, the best quality video images. They are used, for instance, in planetariums.
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"Mechanical television"