Patent GB-945802-A (GB)
Title: Improvements in or relating to circuit arrangements in colour television receivers
IPC: H04N9/24
Filed: 1960-05-19
Abstract: 945,802. Television. PHILIPS ELECTRICAL INDUSTRIES Ltd. May 19, 1960 [May 22, 1959], No. 17698/60. Heading H4F. In a colour television receiver employing a beam indexing tube 1 (Figs. 1), the indexing pulses derived from resistor 7 are supplied (after amplification at 8, if necessary) to a gate 14 which is opened (i.e. rendered conductive) by rectangular pulses from an oscillator 15 (the frequency of which is equal to, or a submultiple of, the indexing frequency) to gate the indexing pulses to a trigger 16 the pulses produced by which are phased compared with the oscillator output in comparator 17 to produce a signal which controls the frequency of the oscillator via a variable reactance circuit 18. The pulses from 15, thus controlled, are then fed, via a phase shifter 19, to a pulse generator 20 which, during the occurrence of an indexing signal, switches a further gate 21 to interrupt the supply of (colour) video signals from 23 to the control electrode of tube 1 and to supply to the latter via 22 a fixed (reference)voltage which increases the beam intensity to ensure the production of an indexing signal of sufficiently large amplitude. To ensure correct colour reproduction should the frequency of the indexing signals change due to non-linearity in the horizontal deflection waveform, the video signals are mixed in 23 with the output of phase shifter 19. This mixing also effects the necessary conversion of the colour signals when the latter comprise the chrominance subcarrier of the N.T.S.C. system, the sequence of the colour phosphor strips in the screen 5 being arranged in accordance with the phase modulation of the colour signals on this subcarrier so that the signals are decoded by the tube itself. Alternatively, the pulse generator 20, the gate 21 and the mixer circuit 23 may be replaced by four gates having a common output connected to control electrode 3 and to the inputs of which are supplied, respectively, the three demodulated colour signals plus their brightness information and the fixed voltage via 22. These gates are then " opened" successively by the signals on line 25 to pass their input signals to electrode 3. The rectangular pulses derived from oscillator 15 have a width such that gate 14 " opens " at the instants at which the scanning beam leaves a colour strip adjacent an indexing strip and " closes " at the instants at which the beams commences the scanning of the next colour strip, the gate 21 (or its four gate equivalent referred to above) operating to pass the reference voltage when gate 14 is " open " and to pass the video signals when gate 14 is " closed." To ensure correct synchronization the screen includes " run-in indexing strips (Fig. 3, not shown), during the scanning of which delayed fly-back pulses from the line-scanning circuit are supplied to pulse generator 20 via 38 and to gate 14 via 39 so that gate 14 is opened and generator 20 operates gate 21 to supply the fixed reference voltage to electrode 3. In a second embodiment (Fig. 2), the oscillator 15 (Fig. 1), is replaced by a trigger circuit comprising a monostable multivibrator 40 which is tripped by each indexing pulse from gate 14 and returns to its first condition after an interval equal to the time of scanning one indexing strip. The pulse thus produced is delayed in 41 for a period slightly less than the period of the indexing signal and supplied (via an amplifier 49 if necessary) as a gating signal for gate 14 the whole arrangement being " started " by a delayed line fly-back pulse supplied to amplifier 49 via 52. To prevent loss of colour synchronism should some of the indexing pulses fail to be produced the trigger circuit 40 may be replaced by a free-running oscillator synchronized by indexing pulses from 14 so that, if an indexing pulse fails, the oscillator continues running and is brought into synchronism by a subsequent indexing pulse. If desired, a " search voltage " oscillator which varies the frequency of oscillator 40 (or oscillator 15, Fig. 1) at a definite rate may be included in the control loop circuit to enhance the re-establishment of synchronization such oscillator cutting-off automatically when synchronization is achieved. When oscillator 40 is a trigger circuit the failure of indexing signals may be utilized to cause gate 14 to open and gate 21 to pass the reference voltage to electrode 3 to effect the production of indexing signals. The screen 5 (Fig. 5), of the tube comprises red, green and blue phosphor strips 10, 11, 12 respectively separated by guard strips 28 whilst the indexing strips 13 comprise " active " portions 13 3  bordered by " inactive " portions 13 1 , 13 2 . The direction of line scanning may be perpendicular to the strips or, by " spot-wobbling " the beam scanning may be effected in a direction parallel to the strips. The " spot-wobble " frequency should be equal to that of the chrominance sub-carrier (when the signals are of the N.T.S.C. type) and should be controlled by the oscillator 15 (Fig. 1) or 40 (Fig. 2). In this case, to avoid the production of indexing signals having a frequency equal to twice that of the " wobble " frequency (due to the " wobbled " beam impinging on two indexing strips per cycle) the frequency of the oscillators 15 or 40 is made an integral (2) sub-multiple of the frequency of the indexing signals actually produced by the " spot-wobbled " beams. Specification 787,071 is referred to.