Patent GB-1294368-A (GB)
Title: Television display apparatus
IPC: H04N3/16
Filed: 1970-05-26
Abstract: 1294368 Cathode-ray tube circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 26 May 1970 [29 May 1969 13 Dec 1969] 25212/70 Heading H4T In an arrangement for correcting scan distortion in a cathode-ray tube television display, particularly that caused by "anisotropic astigmatism" of the deflection coils, a correction current comprising a line frequency sawtooth wave amplitude modulated by a field frequency sawtooth wave and further modified at the beginning and end of each line period by factors proportional to the third power of the line deflection current and the third power of the field deflection current is produced and added to the line deflection current in such manner that it flows in opposite directions in the two halves of the line deflection coil. The field frequency modulation compensates for the distortion over the major position of each line scan and the further modification compensates for errors at the extremities of each line scan throughout the entire field of the display. By causing the correction current to flow in opposite direction in the line deflection coils the correction is achieved without affecting the actual line deflection. Fig. 3 shows the general arrangement in which a modulator 6 supplies a line frequency sawtooth wave amplitude-modulated at field frequency to the (effective) centre of the line scan yoke comprising symmetrical coil halves 2<SP>1</SP>, 2<SP>11</SP> via a saturable inductor 8 which, at high current levels (i.e. levels corresponding to the ends of each line scan) causes the field frequencymodulated line sawtooth current iKt (Fig. 1) to be modified as shown at i<SP>1</SP>K (see also Figs. 2a and 2b). This current then opposes and assists the line scan deflection current iH in coils 2<SP>1</SP> and 2<SP>11</SP>, respectively, and thus, the line scanning is independent of the correction effect. The saturable inductor 8, however, only provides the modification proportional to the third power of the line frequency so that correction by this adjacent to the centre of the display-i.e., the means is effected only at the ends of line scans correction current has the form shown at Fig. 2b. To achieve correction for extremes of field scan, i.e. at the corners of the display, the further correction factor proportional to the third power of the field scan uninform is required and this is produced by causing the modulation envelope to have the form  i K shown in Fig. 2c. Fig. 9 shows the modulator 6 which comprises diodes 47, 48 switched (OFF) by fly-back pulses derived from a winding 45 on the line output transformer, the modulation being effected by supplying to the centre tap on winding 45 (at which a line frequency pulse voltage is produced by the diode switching action) the output of a transistor amplifier 35, 38, 39, 40 supplied with the field sawtooth 32 (from which the spikes are removed by integrator 33, waveform v i ) and having a negative feed-back loop including resistor 41 and a voltage dependent resistor 36 producing the characteristic v o (3rd power), an inductor 44 having a high impedance at line frequency and a low impedance at field frequency being included together with a capacitor 46 which, with the line deflection circuit, constitutes a resonant circuit having a period equal to twice the line fly-back period. Fig. 7 shows a modification in which the saturable reactor 8 of Fig. 3 is replaced by a circuit 8<SP>1</SP> comprising the primary winding 10 of a transformer which is tuned to a frequency a little above line frequency but below twice line frequency (e.g. 19.6 kHz for a line frequency of 15,625 kHz). A mathematical treatment of the equivalent circuit is given with reference to Figs. 5a and 5b (not shown) from which it follows that the action of the circuit is not dependent on the amplitude of the field frequency modulated line sawtooth as is the case with the saturable reactor 8 of Fig. 3. Additionally, a sinusoidal voltage of line frequency amplitude modulated at field frequency 11, is developed across winding 10 and this is combined in secondary winding 15 with line frequency pulses 14 of adjustable polarity and magnitude (derived from a winding in the line transformer) and utilized to effect North-South pincushion distortion correction via transistor amplifier 20, 21 in the field scan coils 25<SP>1</SP>, 2511 supplied from the field deflection generator 28. With this arrangement however, adjustment of the modulator 6 may influence the pincushion correction circuit (and conversely) and to prevent this the circuit is modified by making circuit 8<SP>1</SP> a parallel tuned circuit (instead of a transformer) and employing a separate modulator to provide the required field-frequency-modulated linefrequency waveform (Fig. 10, not shown). The line deflection coils may be connected in parallel instead of in series (Fig. 11, not shown).