Television

Transformation of Light into Electrical Signals.—In order to produce electrical currents for transmission purposes from the light passing through the scanning disc, use is made of some light sensitive device. This light sensitive device must be extremely rapid in its action and should, if possible, give currents of a magnitude acceptable for transmission over ordinary trans mission channels such as telephone lines. For this purpose earlier experimenters used selenium, which, however, is relatively sluggish in its response. More recently, in the most successful experiments on television, a photoelectric cell has been used. This consists of a glass tube on whose inner wall is a layer of alkali metal such as potassium. The incidence of light on the sensitive surface causes the emission of electrons, thus producing an electric current which, in the case of properly designed photoelectric cells is directly pro portional to the intensity of the light, and follows variations of light intensity at very high speeds.

Photoelectric currents as excited by the light available from ordinarily illuminated objects suffer from the disadvantage that they are excessively minute. It is necessary to amplify these cur rents by means of vacuum tube amplifiers before they are avail able for transmission purposes. It is, in fact, largely owing to the recent development of vacuum tube amplifiers, that it has been possible to bring television to a successful stage, since the general principles by which television might be accomplished have been recognized for a long time. Even, however, when image-forming lenses of the largest light gathering power are used it is necessary for the original object to be very brilliantly illuminated if the method of image formation and scanning shown in Fig. i is to be employed. It has been possible, by constructing the television apparatus of large dimensions, whereby more light can be gath ered, and by the development of very sensitive photoelectric cells, to demonstrate this "direct" method of scanning with objects in the relatively intense illumination of outdoor daylight. An alter native method of scanning, which utilizes the available light much more efficiently although limited to indoor scenes, is the method of "beam" scanning. In this (Fig. 2) the relative positions of the photoelectric cell and the light sources as shown in Fig. i are reversed, and a narrow beam of light is projected through the hole in the scanning disc and traverses the object rapidly from side to side. The light reflected from the object is then picked up by the photoelectric cells, which may be made of very large area so as to collect the maximum amount of light.

An exemplification of the direct method of scanning is shown in Plate, fig. 4 where a scanning disc of 3 ft. diameter containing

a spiral of 5o scanning holes is utilized to produce television sig nals from a full length human figure. Plate, fig. 6 shows in skel etonized form a practical exemplification of apparatus for the beam method of scanning. Here the light from an arc lamp is condensed upon the back of a scanning disc which carries a spiral of 5o holes, and a narrow beam of light is projected by means of a lens to the face of the sitter. Three large photoelectric cells collect the reflected light, and the electric currents thus produced are amplified by means of vacuum tube amplifiers close to the cells. Subsequent amplification to the order of several thousand million fold raises these currents to sufficient strength for transmission.

Transmission.

The problem of transmitting television signals presents considerable difficulty for the reason that an exceedingly wide band of frequencies must be transmitted, and all parts of this band must be of uniform efficiency and in proper phase rela tion with each other. The nature of the problem may be compre hended by considering a typical case. If the image to be trans mitted is scanned by a disc apparatus having 5o holes, the analysis of the entire image corresponds to dividing it into 5o times 5o, or 2,500 elements. In order to create the illusion of continuity the image must be completely scanned about 20 times a second. This means that signals corresponding to 5o,000 elements per second must be transmitted. In terms of alternating current frequencies, this means approximately 25,000 cycles per second. Now speech transmission by the telephone is successfully accomplished by a frequency band of approximately 2,000 cycles, and the best radio broadcasting stations use "sidebands" about 5,000 cycles in width. It is, therefore, obvious that the transmission of a relatively crude image, corresponding in quality to considerably less than a square inch of newspaper halftone, requires transmission facil ities of extraordinary character. Furthermore, the requirements increase rapidly with the complexity of the scene to be trans mitted. While a single face may be recognizably rendered with the 2,500 elements just considered, the amount of detail shown in the very coarsest newspaper halftones of news events demand Ioo,000 or more elements. While with sound the same apparatus and transmission channel will handle a chorus as well as a solo, in television the width of transmission channel increases with the number of performers. The transmission of an extended scene in any detail requires transmission facilities equivalent to a very large number of radio broadcasting channels.