International projectionist (Jan-Dec 1947)

slide by means of a lens. The light rays transmitted by the slide are condensed and then divided by dichroic (two-part) mirrors which pass one color of light and reflect the others. The use of dichroic mirrors for a light splitter instead of half-silvered mirrors and color filters, reduces light losses and therefore provides a signal with higher signal-to-noise ratio. The divided light beams are further filtered by color-absorption filters, then collected by multiplier type phototubes which convert the varying light intensity of the spot as transmitted by the slide into video signals corresponding to the three primary colors of the slide. The Use of multiplier phototubes provides a high video input to the amplifier. The amplifiers are equalized to correct for the decay characteristic of the phosphor used in the flying-spot kinescope. On the bottom of the rack in the photograph (Fig. 2) is the chassis containing the synchronizing, blanking, and deflection circuits for the flying-spot kinescope. Behind the kinescope is the highvoltage power supply which provides approximately 30 kilovolts for the kinescope and a first anode focusing potential variable between four and seven kilovolts. The flying-spot kinescope has a special short-persistence phosphor whose light intensity drops to less than 1 per cent of the original value in one microsecond. Facing the kinescope screen is the slide holder with an F:2 objective lens. The whole optical assembly is mounted on the same chassis with the three video amplifiers. The beam splitter is at the lower end of this chassis, and a condensing lens system for each channelreduces the beam diameter to that of the photocell aperture after the beam is divided. The photocells are enclosed in shield cans, which also support the color filters. The voltage for the phototube multipliers may be controlled by the variable power supply directly under the beam splitter chassis, and by this means the video levels of all three channels may be varied simultaneously. The supply voltage of the phototube multipliers of the individual channels may be varied individually by the potentiometers visible at the top of the chassis, to provide the desired color balance. Each of the three video amplifiers contains three stages having a flat frequency response to approximately 5.5 megacycles. Included in each of the amplifiers are the equalizing circuits to compensate for the various phosphor persistences. The output level of the amplifiers is approximately 1 volt peak-topeak. The small chassis' above the beam splitter is for the insertion of the synchronizing signals in the green video signals. The quality of the signal from this generator is highly satisfactory not only because of the high resolution, but also because the blacks have the unusual characteristic of being practically free from noise. Noise in the picture therefore has the general appearance of the equivalent effect found in motion pictures from photographic grain and dirt. The registration of the three signals is inherently correct. Motion Picture Film Scanner The motion picture film scanner was built with no attempt at refinement or optimum design, in order to hasten preliminary tests of reception with moving subjects. Its general scheme js the same' as that of the slide scanner, with the film gate replacing the slide holder. The apparatus is shown in Fig. 3. A standard 16-mm home sound film projector waj modified by substituting a synchronous motor drive so that the film speed was changed to 30 frames per second, instead of 24. Each frame was then scanned twice to give 60 fields per second. Tb.e pull-down mechanism, which was unchanged, is so slow that it was necessary to blank approximately 30 per cent of the field time to avoid showing the distorted picture produced during the film pull-down time. The picture therefore actually contained only about 370 lines, although the nominal number of lines was retained at 525. The picture quality was judged to be good, particularly when allowance is made for the fact that part of the picture area was missing, due to the compromise in design of the film projector. The sound was usable, but was not very satisfactory due to the improper film speed. Reproducing Equipment The picture reproducer, as shown in Figs. 4, 5, 6, contains three 3-inch kinescopes arranged side-by-side in an equilateral-triangular group, each having an associated projection lens and deflection yoke. The kinescopes are identical except that phosphors selected for producing red, green, and blue light, respectively, are used. The kinescopes and lenses are mounted in an assembly frame which also holds the yokes in such a manner that each yoke may be adjusted in rotation and height without disturbing the kinescope mounting position. Each kinescope is provided with the video signal corresponding to its par FIGURE 3 (left). Motion picture film scanner. FIGURE 4 (right). Laboratory model simultaneous all-electronic color television receiver.