Patent GB-787071-A (GB)
Title: Improvements in and relating to cathode ray tube apparatus
IPC: H01J29/32
Filed: 1953-12-04
Abstract: 787,071. Colour television. PHILCO CORPORATION. Dec. 4, 1953 [Dec. 5, 1952 ; June 29, 1953], No. 33796/53. Drawings to Specification. Class 40 (3). [Also in Group XL (a)] Relates to colour television cathode-ray tube system utilizing a tube with a screen made up of a repeating sequence of red, green and blue vertical phosphor strips and in which indexing means are provided between each group of three strips to provide a signal indicating the instantaneous position of the cathode-ray tube beam on the screen. In accordance with the invention, the video colour wave applied to the tube is interrupted during selected periods and during such periods the intensity of the beam is maintained at a predetermined level. The indexing signal output is interrupted except during these periods and thus provides a signal free from spurious video signals and determined solely by the characteristics of the indexing means and the constant beam intensity. The invention is described as applied to systems utilizing a cathode-ray tube as described in Specification 781,911, [Group XL (a)], in which the indexing means comprise regions of secondary electron emissivity different from the remainder of the screen, although reference is also made to the use of a tube incorporating a photo-cell and in which the indexing regions produce an output of light. In a first embodiment, Fig. I (not shown), the video signal is interrupted by gating at a frequency of 31 Mc/s. whereby the indexing output which is synchronously gated comprises 31 Mc/s. pulses recurring in groups at the frequency of scanning the indexing regions, approximately 7 Mc/s. in the example disclosed. The signal is passed through a filter having a pass-band of 6-8 Mc/s. which integrates the pulses to produce a signal at the indexing frequency. In a second embodiment, Fig. 3 (not shown), the video wave is interrupted by means of a timing circuit at the moment of switching on the receiver for the period of one or a few complete frames. The indexing output is similarly gated only during this period and is applied to a cathode-ray storage tube (see Group XL (a)) scanned synchronously with the image reproducing tube where it is recorded as a change pattern. The tube has a long memory and the signal is repeatedly regenerated for use during the remainder of the programme received. If desired, the change pattern may be replaced at intervals throughout the programme by means of a counting circuit responsive to the line or frame synchronizing pulses and arranged to control the gating operation. In a third embodiment, Fig. 5 (not shown), the absence of image information during the line flyback is used. The tube bias is switched to a fixed level during flyback and the indexing output gated to a short memory storage tube from which the indexing signal is regenerated during the immediately following line scan. Since the storage tube is here only required to store the signal derived during one line scan it is constructed with a target having only a limited vertical dimension and is scanned only by the linescanning signals. In a fourth embodiment, Fig. 6 (not shown), the video signal is interrupted and the indexing output gated for the duration of one line per frame with a different line in each frame. The complete picture is covered in about 8 secs. The indexing signal is again recorded on a long memory tube and regenerated continuously. In the second and third embodiments, the scanning signal is employed to control the modulation of colour signal trains received simultaneously and thereby produces a video wave for application to the cathode-ray tube in which the colour components appear in step with the scanning of the phosphors. In the first and fourth embodiments the received signal includes a 3.89 Mc/s. sub-carrier modulated in phase and amplitude in accordance with the hue and saturation of the image. The indexing signal at 7 Mc/s. is here mixed with a synchronized 3.89 Mc/s. oscillation to obtain a 10.89 Mc/s. sum signal and this signal is then mixed with the sub-carrier to obtain a difference signal of 7 Mc/s. bearing the colour phase and amplitude modulation. This signal, together with a received brightness signal, is then applied as the video signal to the cathode-ray tube. Reference is also made to the use of the indexing signal to control the scanning to maintain it in phase with the colour signals.