Television, Electromechanical Systems

Television is a form of telecommunication for the transmission of signals representing scenes—the images of the scenes being reproduced on a screen as they are received or recorded for subsequent use. In monochrome television the luminance, but not the color, of an object is reproduced; in color television the reproduced picture simulates both the color and the luminance of the object.

As with a cine film, television consists of a series of successive images that are manifested by the brain as a continuous picture, because of the persistence of vision. Unlike a cine film, where the light values of each picture element of a scene are simultaneously recorded on a film, the light values of a televised scene are scanned in portions that are transmitted sequentially to the recording or display device, since in practice, only a single transmission link is used for a given television camera-display system.

This restriction necessitates the utilization of a scanning device and a photosensitive cell at the transmitter to sample a two dimensional image and convert it into an electrical signal, and another scanner and a display device at the receiver to synthesize or reconstitute the reproduced image from the transmitted electrical signals. Usually, the scanning process follows a left-to-right, and a top-to-bottom sequence as in the reading of a printed page.

Electromechanical television scanners date from c.1880. Although very many suggestions were advanced during the period 1880 to 1930, only the apertured disk scanner of Paul Nipkow (1884), the mirror drum scanner of Lazare Weiller (1889), and the lensed disk scanner of Marcel Brillouin (1891) were subsequently extensively used by experimenters. Of these types, Nipkow’s disk was the simplest and the most versatile.

Figure 6 shows a scanning disk (for picture analysis) pierced by 24 apertures arranged at equal angular displacements along a spiral line. Each of the apertures has the shape of a picture element (pixel) and allows the light flux corresponding to the brightness of a picture element of the scene being televised to be incident, via a lens, onto a photoelectric cell. When the disk rotates, each aperture scans a given path (line) of the image field, and after one rotation of the disk the whole of the image has been scanned by 24 paths (lines).

Figure 6. A Nipkow disk scanner and the paths of the apertures across the image plane

The process is then repeated, the number of scans of the complete image (rotations of the disk) per second being the frame rate. With a Nipkow disk the commencement of the line scanning, and also the frame scanning, is carried out automatically. Despite his patent, Nipkow never built a working prototype, as he had no way of amplifying the weak signal from the photocell.

John Logie Baird (1888-1946) employed various types of Nipkow disk type scanners in his early work on television and on 26 January 1926 demonstrated in London, for the first time anywhere, a rudimentary television system. Subsequently his basic scheme was adopted and adapted by many workers.

Initially Baird used a ‘‘floodlight’’ method of illuminating his subjects (Figure 7(a)), but when this caused discomfort to his subjects he inverted the positions of the light sources and the photocells to produce a ‘‘spotlight’’ method of scanning (Figure 7(b)). Baird patented the method, as did Bell Telephone Laboratories (BTL) in New York, but unknown to Baird or BTL the method had been patented in 1910.

Figure 7. (a) Floodlight scanning system; (b) spotlight scanning system

Both Baird and BTL adapted their systems to demonstrate color television, stereoscopic television (Baird), large screen television, multichannel television, and two-way television. BTL’s experimental two-way system was in operation in New York for approximately one year from April 1930 and was used by around 17,000 persons. A novel application was observed when two deaf persons carried on a telephone conversation by reading each other’s lips.

From 1880 to 1930 progress in television proceeded in an empirical manner. However, in a 1930 issue of the periodical Fernsehen, Moller and Kirschtein, in two separate papers, showed how the optical efficiencies of scanners could be calculated according to the principles of optics and photometry.

Analyses of the relative efficiencies of the Nipkow disk and the Weiller mirror drum led to the conclusion that the aperture disk should be used at the transmitting end of a low-definition system of more than 40 lines or so. Below 40 lines it was advisable and more efficient to employ the mirror drum. Since Baird’s low-definition system was based on 30 lines per picture, at 12.5 pictures per second, the British Broadcasting Corporation (BBC) utilized mirror drum scanners in its lowdefinition service.

Further analysis showed that there was no optical advantage in using a Brillouin multilensed disk rather than a single lens-aperture disk combination for either floodlight or spotlight scanning. Again, analysis indicated that the Nipkow disk was unsuitable for image synthesis and from about 1932 its use declined. In some systems it was replaced by the mirror screw, as advocated by Hatzinger (1930). Essentially, this scanning element comprised a number of mirrors arranged like a spiral staircase on a central shaft so that the mirrors made one complete spiral.

The mirror length was the same as the picture width the number of mirrors was equal to the number of scanned lines. For the 84-line images, first displayed in 1931, the mirror screw used 84 mirrors, each 100 millimeters long and 1 millimeter deep to produce a picture 84 by 10.0 millimeters in size. This image size was a substantial improvement over the display size achievable with the Nipkow disk.

By the early 1930s, the Radio Corporation of America (RCA), Farnsworth Television Inc., and Electric and Musical Industries (EMI) were undertaking much research and development work on the evolution of an all-electronic, high-definition television system. Their electronic cameras were called the iconoscope, the image dissector, and the emitron respectively. Of these, the iconoscope and the emitron embraced the very important principle of charge storage. Philo Farnsworth’s image dissector, basically, was the electronic equivalent of the Nipkow disk-photocell combination and was relatively insensitive compared to tubes of the iconscope and emitron type.

On 2 November 1936 the London BBC television station was inaugurated at Alexandra Palace. It operated with both electronic and mechanical scanners on an alternate basis. By 19 December 1936 it was apparent that the emitron cameras were greatly superior to those that used Nipkow disks. As a consequence, after 2 January 2 1937, only the 405-line, all-electronic system of EMI was operational.