Side Pincushion Correction System

This invention relates to a side pincushion correction circuit for use in display systems such as television systems.

It is well recognized that the geometrical relationship of the scanning beam or beams of a cathode ray tube to the inside surface of the viewing screen over which they are scanned causes a raster distortion known as pincushion distortion. This distortion is characterized by a widening of the display in a horizontal direction at the top and bottom of the raster relative to the center of the raster. It is also well recognized that in many situations this distortion can be corrected by modulating the horizontal scanning current at the vertical deflection scanning rate. Frequently the modulation is in parabolic form, causing a maximum decrease in horizontal scanning current at the top and bottom of the raster, corresponding to the beginning and end portions of the vertical trace interval, relative to the horizontal scanning current at the center of the raster.

The vertical rate modulation may be impressed upon the horizontal scanning current in a number of ways. A reactor may be placed in circuit with the horizontal deflection coils and controlled to vary the impedance to horizontal scanning current at the vertical rate. A transformer in circuit with the horizontal deflection coils may also be utilized to achieve the required modulation. A more recent approach is to rectify horizontal rate energy, such as that obtained from a winding of the horizontal output transformer, and utilize the rectified current for supplying the vertical deflection output stage. The vertical stage itself then loads the horizontal winding such that the horizontal scanning current is modulated at the vertical deflection rate. While this arrangement is satisfactory, it would be desirable to provide a raster correction circuit which offers a greater degree of control over the modulation.

In accordance with the invention, a side pincushion correction system is provided including a line rate deflection generator coupled to a line deflection winding and a first winding of an output transformer for inducing energy therein for defining trace and retrace intervals during each line deflection cycle. An inductance and an active current conducting device are serially coupled between a terminal of a second winding of said transformer and a point of reference potential. First waveshaping means are coupled to a winding of said transformer and a control electrode of said device for enabling conduction of said device during said retrace intervals. Second waveshaping means are coupled to a field rate generator and said control electrode for modulating the conduction of said device at said field rate to load said first winding and alter the scanning current in said line deflection winding in a manner to correct for side pincushion distortion.

A more detailed description of the invention is given in the following specification and accompanying drawing of which:

FIGS. 1 and 2 are circuit diagrams of first and second pincushion correction systems embodying the invention.

In FIG. 1, a winding 10a of an input reactance 10 is coupled between a source of voltage B+ and one terminal of a bidirectionally conducting switch 11. Winding 10a is also coupled to a commutating coil 13 and a capacitor 15 to one terminal of a second bidirectionally conducting switch 16. An auxiliary capacitor 14 is coupled between commutating coil 13 and capacitor 15 and ground. Switch 16 is also coupled through horizontal deflection windings 18 and S-shaping capacitor 19 to ground and through a primary winding 20a of a horizontal output transformer 20 and a blocking capacitor 21 to ground. A horizontal oscillator 12 is coupled to the gate electrode of one of the elements of switch 11. A winding 10b of input reactor 10 is coupled through a waveshaping network 17 to provide gating signals for one element of switch 16. The circuit described thus far is a horizontal deflection circuit of the retrace driven SCR type described in U.S. Pat. No. 3,452,244. Since a description of the operation of this circuit is unnecessary for an understanding of the invention, it will suffice to say that the horizontal deflection circuit induces a scanning current through the horizontal deflection coils 18 and induces horizontal rate energy into the primary winding 20a of the horizontal output transformer 20. The horizontal energy comprises a relatively short retrace portion and a trace interval portion. Switch 16 connects deflection windings 18 across current source 19 and winding 20a across source 21 during the scan interval. Switch 11 connects elements 13, 14 and 15 to receive energy from winding 10a during the commutation interval, which includes the retrace interval, to receive energy to transfer this energy to capacitors 19 and 21 to replace losses during said deflection cycle.

A winding 20b of horizontal output transformer 20 supplies horizontal retrace pulses to a conventional high voltage rectifier and multiplier 22 for producing a positive high voltage in the order of 25,000 volts for energizing the ultor terminal of a television picture tube.

The remainder of the circuit in FIG. 1 includes the circuit embodying the invention for providing pincushion distortion correction to the horizontal scanning current.

A secondary winding 20c of output transformer 20 is coupled to a rectifier 30 and filter network 31 for rectifying the horizontal retrace pulses and providing a source of direct current. Filter network 31 is coupled through series connected transistors 32 and 33 to ground. Transistors 32 and 33 provide a complementary symmetry push-pull output stage for the vertical deflection circuit. A vertical generator and driver stage 29 provides sawtooth drive signals at the vertical deflection rate for causing transistors 32 and 33 to conduct. The common junction of the emitter electrodes of transistors 32 and 33 forms an output terminal which is coupled through a coupling capacitor 34 to a vertical deflection winding 35. The other terminal of vertical deflection winding 35 is coupled through a feedback resistor 36 to ground. An AC feedback signal is taken from the top of resistor 36 and fed back to the vertical generator and driver 29 for linearity correction. The vertical stage described thus far is conventional and any suitable push-pull vertical amplifier may be utilized for the purpose of providing vertical scanning current and for the purpose of providing appropriate vertical signals for use with the remainder of the pincushion correction circuitry.

The output terminal of the vertical deflection stage is coupled through the parallel combination of a variable resistor 37 and a capacitor 38 through an amplitude potentiometer 39 and through an integrating capacitor 40 to ground. As can be seen more clearly in FIG. 2, the waveform obtained at the output terminal of the vertical deflection amplifier is essentially a sawtooth waveform. Variable resistor 37 passes a DC component of this waveform and serves as a width control for the pincushion correction circuitry, as will be explained subsequently. Capacitor 40 integrates the sawtooth waveforms to form essentialy parabolic waveforms at the vertical deflection rate. These parabolic waveforms are coupled through a resistor 41 to the gate electrode of an SCR 42. The main conduction path of SCR 42 is coupled in series with an inductance 45 and this combination is coupled in parallel with a winding 20d of horizontal output transformer 20.

The number of turns of winding 20d is selected to provide a retrace pulse 47 having an amplitude of about 250 volts peak-to-peak, as shown. The positive-going horizontal retrace pulses provide the anode voltage for SCR 42. A series coupled resistor 44 and integrating capacitor 43 are coupled in parallel with SCR 42 and inductance 45. The junction of resistor 44 and capacitor 43 is coupled to the gate electrode of SCR 42. Resistor 44 and capacitor 43 serve to integrate the horizontal retrace pulses to form substantially sawtooth pulses applied to the gate electrode of SCR 42. It is noted that the horizontal rate sawtooth pulses and the vertical rate parabola are combined at the gate electrode. The positive-going vertical rate parabola modulates the horizontal rate sawtooth pulses such that SCR 42 conducts more at the beginning and ends of the parabola, corresponding to the top and bottom portions of the raster, than at the center. When SCR 42 is conducting, it couples inductor 45 in parallel with horizontal winding 20d and loads this winding by diverting current through inductor 45 to ground. The loading of horizontal winding 20d is reflected to the primary winding 20a. This loading decreases the total inductance in the circuit and results in a faster discharge of capacitors 14 and 15 during the horizontal retrace interval. This faster discharge of these capacitors reduces the stored energy in primary winding 20a and thereby reduces the energy available for transfer to deflection winding 18, which in turn reduces the width of the raster.

As this loading occurs during each horizontal deflection cycle modulated in a parabolic manner at a vertical rate as described above, side pincushion correction is thereby achieved. The DC potential at the gate electrode of SCR 42 is determined by the setting of potentiometer 37. This DC potential further biases the gate electrode of SCR 42 to control its conduction during the horizontal retrace interval to alter the amount of current loading and thereby alter the width of the raster. In FIG. 2, another embodiment of the side pincushion distortion correction circuit in accordance with the invention is illustrated. It is noted that the horizontal deflection circuitry to the left of horizontal transformer 20 is omitted, as it may be the same as illustrated in FIG. 1. Those components in FIG. 2 performing the same function as in FIG. 1 are labelled with the same reference numerals as in FIG. 1.

The substantially sawtooth waveform 51 obtained from the output terminal of the vertical deflection output stage is coupled through the parallel combination of resistor 37 and capacitor 38 and through a resistor 39 to an integrating capacitor 40. A potentiometer in parallel with capacitor 40 serves as an amplitude control. The wiper arm of potentiometer 70 is coupled through a resistor 41 to the base electrode of a transistor 62. A vertical rate parabolic waveform 53 is shown at the base electrode of transistor 62. Transistors 62 and 63 serve as a driving stage for the gate electrode of SCR 42. The emitter electrode of transistor 62 is coupled through the parallel combination of a resistor 59 and bypass capacitor 60 to ground. The collector electrode of transistor 62 is coupled to the base of transistor 63 and through a resistor 65 to the emitter electrode of transistor 63. The collector electrode of transistor 63 is coupled through a resistor 64 to provide the drive current for the gate electrode of SCR 42. A tap 46 on winding 20d of horizontal output transformer 20 is coupled to a rectifier diode 67. Horizontal rate retrace pulses 55 having an amplitude of approximately 50 volts peak-to-peak are rectified by diode 67 and coupled through a resistor 68 to a filter capacitor 69, the other terminal of which is coupled to ground. Direct current is obtained from the junction of resistor 68 and capacitor 69. This direct current is coupled to the emitter of transistor 63 to provide operating current thereto and through resistor 65 to the junction of the base of transistor 63 and the collector of transistor 62. The junction of resistor 68 and capacitor 69 is further coupled through a potentiometer 66, which acts as a width control, and through serially coupled resistors 57 and 58 to ground. The junction of resistors 57 and 58 is coupled to the base electrode of transistor 62 to provide direct current bias thereto. Adjustment of potentiometer 66 varies the amount of DC bias applied to the base of transistor 62.

The top terminal of winding 20d, from which horizontal retrace pulses having an amplitude of approximately 250 volts peak-to-peak are obtained, is coupled through a resistor 44 and an integrating capacitor 43 to ground. At the junction of resistor 44 and capacitor 43, horizontal rate sawtooth pulses 54 are obtained. It is noted that the positive peak of the sawtooth waveforms 54 occurs during the horizontal retrace interval. The sawtooth waveforms 54 are coupled through resistor 57 to the base of transistor 62, which amplifies and inverts the waveforms which are then coupled to the base of transistor 63 for further amplification and another inversion, to provide positive-going sawtooth waveforms coupled through resistor 64 to the gate electrode of SCR 42.

It is noted that the vertical rate parabolic waveform 53 is combined with the horizontal rate sawtooth waveforms 54 at the base electrode of transistor 62. Thus, the total waveform coupled through transistors 62 and 63 and applied to the gate electrode of SCR 42 comprises the horizontal rate sawtooth waveforms riding on top of the vertical rate parabolic waveforms. A further component of this combined waveform is the direct current component determined by the setting of width control 66. This combined signal causes SCR 42 to conduct during the latter half of each horizontal deflection cycle. The conduction is greatest at the beginning and end of each vertical deflection cycle because the parabolic waveform 53 is most positive at these portions. The conduction time of SCR 42 is decreased toward the center of the vertical rate parabola 53.

The junction of vertical deflection winding 35 and feedback resistor 36, at which a vertical rate sawtooth waveform 52 is obtained, is coupled through a potentiometer 61 to the emitter electrode of transistor 62. This sawtooth waveform modulates the combined horizontal rate sawtooth and vertical rate parabolic waveforms in a manner to vary the amplitude at the beginning of the vertical parabola relative to the amplitude at the end of the vertical parabolic waveform. This adjustment as determined by resistor 61 serves to adjust the tilt of the raster by adjusting the width of the raster at the top relative to the bottom.

As in FIG. 1, conduction of SCR 42 couples inductance 45 to ground, placing a shunt load on horizontal winding 20d. The current in inductance 45 is illustrated by current waveform 56. The current builds up in a positive manner to reach a peak at the end of the retrace period and when SCR 42 is cut off, the current decreases and reverses itself through inductance 45 returning energy to deflection winding 46 primarily during the first portion of each horizontal scan interval following each retrace interval. The loading of winding 20d by inductance 45 and conducting SCR 42 is reflected to the primary circuit of the horizontal deflection transformer and thereby alters the deflection current in the horizontal deflection winding during the latter half of the horizontal retrace interval at a vertical parabolic rate such that the raster width is reduced at the top and bottom relative to the center of the raster. This corrects for the pincushion distortion.

It is noted that in accordance with a feature of the invention the inductive loading of the horizontal winding 20d by inductance 45 causes the loading to occur during the second half of each retrace interval. This results in minimum effect caused by modulation of the picture tube high voltage derived by rectifying the horizontal retrace pulses because most of the high voltage energy is obtained from the winding 20b during the middle portion of each horizontal retrace interval as the retrace pulse reaches its peak voltage and the capacitance in the multiplier and rectifier assembly 22 charges to the peak voltage.