Patent GB-379552-A (GB)
Title: Improvements in and relating to television, electrical transmission of pictures and the like
IPC: H04N5/073
Filed: 1931-10-23
Abstract: 379,552. Television &c. WALTON, G. W., 2, Dean Street, Soho, London. Oct. 23, 1931, No. 29504. [Classes 40 (iii) and 40 (v).] In a television or like system one or more alternating control currents are transmitted simultaneously and continuously with the picture currents and at the receiver are utilized for synchronization, phase adjustment, &c., the control currents being cancelled out from the picture as viewed by the addition of counterbalancing effects at the frequency &c. of the control currents, such effects being produced, e.g. (1) by the use of optical filters, apertures, or stops, which are the negatives of those employed at the transmitter, (2) by the superimposition of balancing currents of the same frequency but opposite phase to those transmitted, or (3) where there is no electrical balancing, by using control frequencies that have a definite fractional relation to the image frequency so that, by virtue of the phase changes of the control frequencies in successive pictures, their additive effect over a certain number of pictures, succeeding each other with suificient rapidity, is zero. In some cases it may be desirable to effect a preliminary separation of the control frequencies from each other and from the picture-detail frequencies by parallel tuned circuits disposed between the input and output terminals. Synchronizing &c. by use of optical filters. In employing a stationary optical filter to produce the control frequencies, the filter is preferably located in the focal plane of the image at the transmitter, one example of filter, having a sinusoidal shade variation with a maximum at its centre A being shown in Fig. 14. If the picture is scanned, say, in four successive strips, a control frequency equal to four times the image frequency is produced. At the receiver a filter B, Fig. 15, which is the negative of that shown in Fig. 14, is employed to cancel the control frequency from the picture as viewed. By shading the filters additionally from top 44 to bottom 47, Fig. 28, a second control frequency, Fig. 30 (not shown), equal to the image frequency may be produced and utilized for framing purposes. The use of negative filters at the receiver may, however, be obviated if the scanning at the transmitter is effected in successive strips, in which one strip compensates the succeeding strip, and the image is viewed, through apertures that reconstitute two, preferably, or more strips simultaneously. Movable optical filters may be employed in place of the stationary ones described, and be produced by combining a Nipkow disc C, Fig. 35, with a filter in which the shade intensity changes with the angle, a similar filter with a phase displacement of 180‹ being used at the receiver. The filters may be either continuous annuli or composed of separate shades covering the apertures only. Alternatively, the size of the apertures may be arranged to vary in a continuously changing manner. Filters may also be employed to modulate the picture currents with a changing frequency of constant amplitude, the period of a complete cycle of such changes determining the control frequency. Synchronizing &c. by use of optical apertures and stops. Between the Nipkow disc 7, Fig. 20, and the photo-cells 10 at the transmitter are disposed lenses 9 and an apertured plate 12. When a scanning aperture of the disc is at positions 5 or 11, approximately one-half of the light is cut off from cell 10 by the edges of the aperture ; midway between these positions the cell receives a maximum of light. Consequently a control frequency is imposed on the picture currents equal to the product of the image frequency and the number of apertures in the disc 7. A similar effect may be obtained in oscillatory scanning by a mirror 14 by using either an aperture, Fig. 21 (not shown), or a stop 18, Fig. 22. If the aperture as shown at 20, Fig. 23, is smaller than the area 21, 22, 23 framed by the pencil of light, a second control frequency equal to the image frequency results from the top to bottom movement of the pencil. At the receiver the control frequencies produced by the aperture 20 would be cancelled by a stop shaped as shown at 24, Fig. 24. The use of stops of the latter and other shapes at the transmitter is also described. Circuits, thermionic-valve and filter. When the scanning at the receiver is effected by cathode rays under the control of additional deflecting frequencies, it is necessary, since the rays have no inertia, to separate the picture impulses from the deflecting frequencies, one arrangement for this purpose being illustrated in Fig. 26. From the input terminals 32 the high picture-detail frequencies are applied to terminals 34 through a capacity 33, while the higher and lower deflecting frequencies are passed to a transformer coupled circuit where they are separated by condenser 39 and inductance 40 and applied separately to regulate the voltages of deflecting plates 38, 38<1> of the cathode-ray tube. In another arrangement, Fig. 32, the input from terminals 58 is applied to a filter circuit comprising valves 60, 62, 65 with grids arranged in parallel and with interlocked negative reaction. Suitable condensers 64, 67 and inductances 63, 66 are disposed in the grid circuits so that valve 60 receives mainly the picture-detail frequencies, valve 62 the high control frequencies and valve 65 the low control frequencies. At each grid any of the unwanted frequencies are balanced out by potentiometer tappings with phase reversal taken from the output circuits of the other two valves so that, in the result, picture-detail changes, high frequency control and low frequency control appear separated at terminals 75, 76, 77 respectively. Light-valves, electro-optical, &c.-The cancellation of the synchronizing &c. frequencies at the receiver may also be effected by modulating the received signals by applying to Kerr's cells, neon lamps, or to apparatus utilizing the Faraday rotation of polarized light counterbalancing frequencies, either to the same electrode as the picture currents are applied or to additional electrodes, several arrangements, Figs. 36-43 (not shown) being described. Uneven illumination, compensating. In oscillatory scanning the reduced illumination at the centre of the picture may be counteracted by the use of optical stops or by filters produced on photographic surfaces by operating the scanning apparatus for a suitable period of time, or by the direct application of alternating currents of the required frequency, without hindering the use of the produced control frequency for synchronizing, phasing, &c. When the pictures &c. are transmitted as modulations of a carrier frequency, rectification is unnecessary, the modulations being applied directly to vary the intensity of a light source and either a compensating or a phasecancellation method as described being used to balance out the carrier frequency. In the case of modulation produced by filters or stops, a resultant control frequency may serve as the carrier and be received and applied without rectification. In picture telegraphy, since the phase-cancellation method would necessitate the production and superimposition of two or more pictures, the other methods are preferable, e.g. by mounting suitable filters on the picture drum. Specification 218,766, [Class 40 (iii), Telegraphs, Electric), is referred to