Television

The transmission of television signals may be accomplished either over wire or wireless channels. In the case of wire trans mission, it has been found most practical to transmit the tele vision signals directly, utilizing a channel which transmits from below 20 cycles per second up to the highest frequency called for by the degree of resolution of the image. In radio transmission, the television signals are placed upon an appropriate high fre quency carrier in the usual manner. In either case, the lowest frequencies—below the repetition frequency of the images—to which are due the general brightness of the picture, are not trans mitted, this element of picture quality being arbitrarily inserted as a direct. current component at the receiving end. This expedient is resorted to because of the serious technical difficulty of ampli fying and transmitting very low frequencies.

In the case of wire transmission, the system must be reasonably free from interference such as produces what in voice transmis sion is called "noise," which in the case of television causes specks or streaks obscuring the image. In the case of wireless transmis sion, there are additional sources of trouble, particularly those associated with fading and multiple reflection of wireless signals in the Kennelly-Heaviside layer. These latter produce multiple images whose prominence varies with the wavelength, the trans mission distance, and the time of day.

Reception of the Image.

The simplest method of recon structing the image from the television signals consists in the use of a disc with a spiral of holes at the receiving end similar to that at the sending end. Combined with this is a light source, which can be viewed through the disc, which will follow faithfully the variations in strength of the incoming signals. The most success ful light source for this purpose is a neon glow lamp furnished with a large flat electrode which when placed behind the disc fills the whole field of view. In the Plate, fig. 2, is shown this arrange ment of disc and neon lamp and in the Plate, fig. 3, a neon lamp of a type commonly used. As the observer watches the rapidly rotating disc, he sees at any instant a bright point of light which is of the intensity corresponding to the incoming signal. This in turn corresponds to the photoelectric current generated at the sending end, that is, to the brightness of the image on the sending disc. Due to persistence of vision, the observer is conscious only of a completely illuminated field differing in brightness in its different elements. If the receiving disc is rotating at exactly the speed of the sending disc and exactly in phase with it, the image seen at the receiving end is similar to that at the sending end.

While nearly all experimenters in television have used receiving apparatus of this general sort, it is recognized that the disc with its spiral of holes is inefficient and unhandy in size where im ages of many elements are desired. Efforts have been made to develop more compact forms of receiving device. An attractive possibility, demonstrated on an experimental scale, is the use of the Braun tube or cathode ray oscillograph. In this a stream of electrons, controlled in intensity by the incoming signals, is caused to sweep over a fluorescent screen by a varying magnetic field.

Apparatus of the sort just described is suitable for viewing only by small groups of observers. One form of apparatus by which the received image may be observed by a large audience is shown in the Plate, fig. I. In place of the small neon lamp with a single electrode, a lamp composed of 5o straight parallel glass tubes is utilized. Each tube is furnished with so electrodes and the incom ing electrical signals are transmitted to each of these electrodes in turn by the distributing device shown in the Plate, fig. 5. Here again, by persistence of vision, the whole area of the grid of neon tubes appears to be continuously illuminated and the image is de veloped by the varying intensity of the glow at each electrode. In another form of apparatus for producing large pictures, the light from a high intensity arc lamp is passed through two polar izing prisms and a Kerr cell. The electric field across the Kerr cell is controlled by the incoming signals, causing the light trans mitted by the cell and polarizing prisms to vary in intensity. A series of small lenses mounted spirally on a rotating disc then co operate with a projection lens system to throw moving images of the arc crater on the rear of a translucent screen.

Synchronization.

In order that the scanning and receiving means which have been described shall faithfully reproduce the scene, it is essential that the speed of operation of the two ends shall be identical to a high degree of accuracy. Deviations from the proper speed produce motion and distortion of the received image. The necessary accuracy of synchronization has been achieved by the use of synchronous motors with a large number of poles, operated by relatively high frequency signals. These high frequency signals (i,000 to 2,000 cycles per second) may be generated by a vacuum tube oscillator, or by light re ceived by an auxiliary photoelectric cell from holes or reflecting spots on the disc. The synchronizing signals may be transmitted either over a separate communication channel, or as has been proved practical, may be produced by piezoelectric crystal oscil lators separately operated at the two ends.