Patent GB-1051522-A (GB)
IPC: H04N9/26
Abstract: 1,051,522. Colour television. YAOU ELECTRIC CO. Ltd. Feb. 11, 1965 [May 5, 1964], No. 5951/65. Heading H4F. To reproduce N.T.S.C. type colour signals using a single glm cathode-ray tube of the type having a line phosphor screen and postdeflection switching grids for directing the beam on to the respective phosphors, a voltage having three steps in its waveform and a frequency of one-third of the horizontal scanning frequency of the raster on the screen of the CRT is applied to the switching grids and a reference signal derived from the colour burst signal and having the colour subcarrier frequency and a predetermined phase with respect to the burst signal is phase-modulated with said three-stepped waveform voltage, the signal thus phase-modulated being adapted to demodulate the chrominance signal component thereby developing a line-sequential colour difference signal. The luminance signal component of the N.T.S.C. signal is applied to the cathode and the colour-difference signal to the first grid of the CRT or vice versa. The linesequential colour difference signal comprises three sequential primary colour-difference signals and thus the electron gun of the CRT emits red, green and blue component beam sequentially each for one horizontal scanning period. and the beams are caused to impinge upon the respective phosphor strips on the image screen which are at right angles to the direction of line scan by applying said three-stepped waveform voltage to the switching grids (two sets of grids), each step of said three-stepped waveform corresponding to one horizontal scanning period. The three-stepped waveform voltage generator may comprise either a steppedwaveform shaper circuit 65 (Fig. 2, not shown), adapted to superimpose the outputs from a ring counter 64 operating with the pulses from the horizontal raster deflection output circuit 58, or integration circuits 75 and 76 (Fig. 3, not shown), developing synchronous sawtooth waves by integrating the pulses from the horizontal raster deflection output circuit 58 and from a blocking oscillator 74 adapted to produce a pulse having a cycle corresponding to three horizontal scanning cycles and a matrix circuit 77 to combine the two sawtooth waves to develop the three-stepped waveform. The phase modulator 67 (Fig. 2, not shown), in which the reference subcarrier frequency signal is phase modulated with the three-stepped waveform voltage comprises a continuous wave transformer 67 2  having an intermediate tap 154 at the secondary winding connected to the demodulator circuit 69 1  of the demodulation and colour-difference signal amplifier 69, and having coupled between the ends of the secondary winding a variable reactance circuit 67 3  comprising a condenser 160, a coil 161 and a variable-capacity diode 164. With the threestepped waveform voltage applied to the diode 164, the composite reactance of the variable reactance circuit changes likewise in three steps and thus the phase modulator applies three sequential signals of constant frequency and magnitude but different phases to the demodulator 69. By utilizing the linear portion of the graph (Fig. 15, not shown) of the phase angles of the output voltage against the diode biasing voltage, and a stepped waveform having three equal steps (Fig. 7a, not shown), the detection axes are 120 degrees apart (Fig. 8, not shown). With N.T.S.C. signals the detection axes have different phase angles (Fig. 9, not shown) and thus in order that a stepped waveform having three equally high steps may be used (since it is applied to the switching grids) the non-linear portion of the modulation characteristic (Fig. 15, not shown) is used for phase modulation this being equivalent to using a stepped waveform of unequally high steps in the modulator.