Patent GB-799853-A (GB)
Title: Colour television receiver
IPC: H04N9/24
Filed: 1956-07-18
Abstract: 799,853. Colour television. SYLVANIA ELECTRIC PRODUCTS, Inc. July 18, 1956 [July 19, 1955], No. 22298/56. Class 40 (3). Relates to a colour television receiver, Fig. 1, employing a cathode-ray tube 24 having a screen 50 formed of a repeating sequence of red, green and blue horizontal phosphor strips and incorporating indexing means for indicating the instantaneous beam position, such as secondary electron-emissive strips positioned over the green strips. The tube is scanned with the raster lines generally parallel to and located on the green phosphor strips and the beam is subjected to a transverse deflection (preferably of sawtooth form) at a high frequency under the control of a spot wobble generator 40 so as to cause it, during each line scan, to traverse also the adjacent red and blue phosphor strips. In accordance with the invention, the resulting indexing pulses, produced across a resistor 54 each time the beam impinges a region of green phosphor, are employed not to correct any misalignment of the raster lines, but to control appropriately the times at which colour information signals are applied to modulate the intensity of the beam. As illustrated in Fig. 1, the indexing signals are applied to an amplifier and phase-shifter 58, where they are converted from pulse to sinusoidal form by limitation of band-width, and the applied with appropriate phase to control the opening of gate stages 18, 20, 22 receiving blue, red and green component colour signals. The signals transmitted through the gates are proportionately combined in accordance with the transmitted signal specifications and applied to the cathode-ray tube grid 30. The operation of the invention is illustrated in Fig. 3, where curves C and D show the effect when the raster is perfect, curves A and B the effect when a raster line as it progresses encroaches on phosphor strips below the correct ones, and curves E and F the effect when a raster line encroaches on phosphor strips above the correct ones. The indexing signals vary in phase in accordance with any departure of the line from correct alignment and control the timing of the applied signals correspondingly so that each modulates the tube beam correctly as the correspondingly coloured phosphor is impinged. A second embodiment, Fig. 2 (not shown), operates the signal of the type comprising a luminance component together with a phase and amplitude modulated colour subcarrier. The spot wobble deflection is applied at twice the subcarrier frequency, and the indexing signal, by means of a heterodyning process, is employed to transfer the subcarrier modulation to a carrier nominally of twice subcarrier frequency but varying in phase in accordance with the indexing information. The subcarrier considered is one of asymmetrical form and a modifier circuit is included to first remould the subcarrier to an equi-angle form as described in Proc. I.R.E., Vol. 42 at pages 299-308. The luminance signal (Y) is also converted to a brightness signal (M) with the aid of a so-called M-Y detector operating on the subcarrier. The cathode-ray tube incorporates two beams which are scanned synchronously, one being modulated conventionally by the image signals and the other by a pilot signal at the eleventh harmonic of the subcarrier frequency. The indexing signal is then selected as the upper sideband at the thirteenth harmonic frequency due to the modulation of the pilot signal by the spot wobble. It is stated that the pilot beam may be spaced slightly ahead of the signal beam in order to compensate for delays in the indexing signal circuit, a quarter of an inch being suggested for a 21-inch tube.