Patent GB-1459922-A (GB)
Title: Television line-output circuit
IPC: H04N3/18
Filed: 1974-01-09
Abstract: 1459922 EHT supply for TV systems PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 9 Jan 1974 [1 Feb 1973] 00941/74 Heading H2F [Also in Division H4] In a (colour) television system in which the line deflection is amplitude modulated by a (parabolic) function of the field frequency (to compensate for EAST-WEST raster distortion) and/or by some other control function (e.g. to compensate for the effect of supply voltage variations), a first sawtooth wave generator supplies a portion of the line deflection current and a second sawtooth wave generator supplies a further portion which is EAST-WEST correction modulated, both generators, which are fed from the same supply voltage source and are controlled by the same line frequency switch, having the same retrace time and being so arranged that modulation of the line fly-back pulses by the field frequency (or other control function) is prevented. Thus E.H.T. and other supply voltage derived from the fly-back pulses are free from such modulation. Fig. 1, shows one embodiment in which the first sawtooth generator comprises the line deflection coils LY, trace and retrace capacitors C t  and C r  respectively together with diode D whilst the second sawtooth generator comprises inductance L<SP>1</SP>, trace and retrace capacitors C<SP>1</SP> t  and C<SP>1</SP> r  respectively together with diode D<SP>1</SP>, both generators being controlled by transistor switch Tr rendered conducting near the middle of the trace time and supplied from (mains derived) voltage source B via the primary L1 of the line output transformer. To compensate for EAST-WEST raster distortion a modulation circuit M1 comprising a driver circuit Dr<SP>1</SP> supplied with a field-frequency input controls a transistor Tr<SP>1</SP> to produce a parabolically varying load across capacitor C<SP>1</SP> t  which varies the voltage V<SP>1</SP> across this capacitor and likewise the voltage V across capacitor C t  and therefore the deflection current i y  in similar manner. The voltage VA at point A is equal to the sum of the voltages across C r  and C r <SP>1</SP> and the peak value of VA relative to its mean value (the voltage VB) bears the same relationship as the retrace voltage across C r , C,<SP>1</SP> relative to voltages V and V<SP>1</SP>. If VB is constant the peak value VA is also constant so that the amplitude of the voltage across L1 is also constant. Thus voltages derived from windings such as L2 (the EHT voltage) are not field frequency modulated. If VB is not constant the resulting variations in V + V<SP>1</SP> or in the peak voltage of VA may form the input to a stabilization circuit S (Fig. 2) connected to a third sawtooth generator D<SP>11</SP>, C<SP>11</SP> r , C<SP>11</SP> t  and L<SP>11</SP> (similar to the other two sawtooth generators) to produce a compensating variation across inductance L3. In a further arrangement compensation may be effected by controlling the conduction time of transistor Tr (Fig. 3) by means of a comparison stage in driver Dr which compares the peak rectified voltage across L5 with the voltage at the junction of L y  and C t  such voltage being supplied to the comparison stage via a low-pass-filter F which removes line frequency components and passes any field frequency component. When the line deflection coil is in two equal portions L y1 , L y2  (Fig. 4a) anisotropic astigmatic deflection errors may be corrected by means of a modulation source M2 which so varies the voltage across capacitor C 12  that a correction difference current i t  flows in opposite directions in coil portions L y1 , L y2  to produce, in some manner, a quadripolar correction field, the EAST-WEST correction being effected, as in the other embodiments, by means of the modulation source M1. Because a D.C. component of correction current flows through coil portion L y2  but not through portion L y1 , errors may arise and these may be eliminated by connecting the modulation source M2 through a choke L6 (Fig. 4b) which passes field frequency but blocks line frequency. An arrangement for generating a NORTHSOUTH correction is described with reference to Fig. 5 (not shown) and an arrangement in which the "S" correction (provided by capacitor C t ) is more EAST-WEST modulated than the deflection current itself (to improve linearity) is described with reference to Fig. 6 (not shown). When the impedance for line frequency of the inductor between point A and the positive terminal of source B is reduced by the presence of parasitic capacitances the resonant frequencies and consequently the retrace times of the individual sawtooth generators may not be identical and to compensate for this an inductive potential divider including a tap on such impedance may be employed (Fig. 7, not shown).