IE43170B1 - Improvements in pincushion distortion correction circuits - Google Patents

Description

This invention relates to a pincushion distortion correction circuit.

In a magnetically deflected cathode ray tube, as used in a television receiver for instance, the picture shape may be distorted due to the face-plate being nearly flat. This is commonly known as pincushion distortion, and is generally considered to have two main components, namely East-West and Worth-South distortion. The North-South distortion is seen as relatively more picture height (i.e. vertical scan amplitude) towards the left and right edges of the display. This specification describes an electronic circuit which corrects for North-South distortion. Other circuits are used at present which use either a saturable reactor (transductor) or a waveform generator circuit followed by a power amplifier. The circuit to be described is simpler, cheaper and performs at least·as well as either of these known circuits.

To achieve North-South pincushion correction, a sinusoidal current at line frequency (typically 15,625 Hz) which varies in amplitude with the vertical· deflection current has to flow in the deflection coils. This correction current is in some cases referred to as the bowtie because of the shape as seen on an oscilloscope.

In Eig. 1(a) of the accompanying drawings a switch Sw is opened and closed repeatedly at the natural resonant frequency of the resonant circuit. This can take the form as shown in Eig. 1(b) where an inductor L and a capacitor c -243170 form a series resonant circuit. While the switch is closed, L and C can resonate, their circulating current flowing through the switch. When the switch is opened, a current I flows into the resonant circuit from a source S. This has the action of maintaining the oscillating current in g the resonant circuit. Further, the oscillating current is directly proportional to the current X. Consequently the sinusoidal voltage across L or C is directly proportional to the current I. If the direction of the current I flow is reversed, then the oscillations undergo a 180° phase change. 1q If the source S is a source of television field deflection current and the switch Sw is taken through repeated cycles of opening and closing at a rate equal to a television line frequency, the current I appears as a .lowlyvarying current to the switch Sw and the resonant circuit. The shape of an unconreci ed television field deflection current is a sawtooth wave which passes through zero at the midpoint of the slope of each sawtooth.

According to the present invention there is provided a field scan deflection circuit for correcting pin cushion distortion in a cathode ray tube, the circuit having one or more field deflection coils, and means for driving deflection current through the deflection coils, the said means 2o including a resonant circuit adapted to add North-South distortion correction current to the deflection current for correcting the pin cushion distortion, the resonant circuit including switching means arranged to short circuit the resonant circuit when the switching means is in a - 3 [) closed, condition, the switching means in operation being in the closed condition during each line scan interval and in an open condition, during each line flyback interval The switching means may be adjustable l.o, determine the on-off ratio thereof in operation.

Examples of embodiment of the invention will now be described with reference to the accompanying drawings in which:Figs 2a2b and 2c are circuit diagrams of simple embodiments of the invention, Fig. J is a circuit diagram of another embodiment of the invention, Figs. 4a, 4b, 4c and 4d are circuit diagrams of switching means for embodiments of the invention, Figs. 5a, 5b, 5£, 5d, 5θ and 5f are circuit diagrams of adjustable resonant circuits for embodiments of the invention invention, Fig. 6 is a circuit diagram of an adjustable switching means for an embodiment of the invention, Fig. 7 is a circuit diagram of another adjustable switching means for an embodiment of the invention, and Fig. 8 is a waveform diagram illustrating waveforms concerned in the operation of another embodiment.

Figs· 2(a), (b) and (c) of the accompanying drawings are circuit diagrams of corrected field deflection circuits embodying the present invention. Each of these circuits includes a field output amplifier A, field deflection coils _44317 Ly, a transformer TI having a primary winding Mp and a secondary winding Ns, a capacitor C, and a switch. In Fig. 2(a). the field deflection current flows into the field deflection coils Ly via the secondary winding Ns, and out via the switch. Sv; when it is closed, or the series resonant circuit formed by the capacitor C and the primary winding Np, when the switch is open. Each circuit has four main functional·, areas, they are:1. The power amplifier A needed for field deflection. 2. The field deflection coils Ly. 3, A line frequency sinewave generating circuit composed of the capacitor 0, the primary winding Mp, and the switch Sw. 4. A winding Ns to couple the line frequency sinewaves to> the field deflection coils Ly.

The sinewave generating circuit Np - 0 - Sw is connected in series with the field deflection coils so that it can generate a bowtie waveform. (N.B. Np and 0 can be interchanged and the circuit still functions correctly).

The secondary winding Ns must be connected in series with the field deflection coils Ly so that the bowtie can be added to the amplifier output voltage fed to the field deflection coils Ly. It does not matter, however, in what order they are connected up, as long as they are in series (see, for example, the circuits of Figs. 2(b) and 2(c).) In the circuit of Fig. 2(c), the field deflection coiL Ly is in two halves, the bowtie being fed in at the middle -5170 of Ly by the secondary winding Ns.

In the embodiments of Figs. 2a, 2b and 2c, the series resonant circuit formed by the primary winding Np and the capacitor C is series resonant at the line frequency.

Other combinations are possible, since as long as the sinewave generating circuit i.s in series with the field deflection coils, and the secondary winding is in series with the field deflection coils, the circuits will work.

Fig. 3 of tho accompanying drawings is a circuit diagram of a practical embodiment of the invention.

The s-witch Sw must be bidirectional·, that is, it must be able to conduct electrical current of either direction, and must be able to turn on and off faster than the line frequency (15&2> -Hs in the U.K.), since the switch Sw must be in the closed state during each line scan interval and in the open state during each line flyback interval.

Various circuits, to serve· as the switch Sw are possible using diodes (.either, semiconductor or thermionic). Two such are shown in Figs. 4(a) and (b) of the accompanying drawings In both circuits there is a winding Nt on a transformer (the line output transformer for example). During the line scan interval., the voltage induced in Nt causes a current to flow in the diodes, which is limited by one or more resistors. Under these conditions a current path exists between two terminals A and B, and the switch can be said to be closed. During the line flyback interval, a large pulse voltage of opposite polarity is induced in Nt. The diodes -64 3Γ70 are therefore reverse biassed, and so no path now exists for current flow.

The switch can now be said to be open.

A transistor capable of switching current in either direction could be used as the switch Sw, or two transistors, one NPN tho other PNP. An example of a transistordiode switch is shown in Fig, 4(c) of the drawings, and a thermionic valve-diode switch is shown in Fig. 4(d).

One or more SOS's could conceivably be used, possibly together with other devices previously mentioned, and one or more gas filled valves could conceivably be used, possibly together with other devices previously mentioned.

The amplitude of correction (i.e. the bowtie waveform amplitude) can be controlled by adjusting the value, of the capacitor C. This could take the form of a variable capa-. citor as in Fig. 5(a) or alternatively an extra capacitortor capacitors) which is (are) added in parallel to the fixed tuning capacitor C as in Fig. 5(b).

The principle of its ability to control amplitude is as follows:If the capacitor value is increased, for example, then the inductance value has to be reduced to re-attain resonance. Since the inductance is now less, its inductive reactance is less, and so the sinusoidal voltage appearing across it will -770 be less. Bj the same reasoning a smaller capacitor will cause more correction.

By altering the turns ratio between the primary winding N'y and the secondary winding Ms, the output voltage will be greater op less by normal transformer, action. Pig. 5(c) shows Ks variable, and Pig. 5(<1) shows Ίίρ variable.

In the circuit of Pig. 5(c) of the drawings, the total tuning inductance in the resonant circuit is made up of Ί Ί L + I ', where 1 is a small extra variable inductor. If it is made say smaller, then L has to be adjusted greater ta maintain resonance. But now more of the available output, appears across L and so at the output. If L is made larger, the output becomes less.

If a resistor is connected across the switch Sw as shown in the circuit of Pig. 5(f)> then when the switch Sw is open, some of the current v/hich would otherwise flow into the resonant circuit now flows through the resistor, and the resulting output is less. Some of the current has therefore been shunted away by the resistor. The shunt need not be a resistor, it could be a transistor, valve, voltage dependent resistor, or thermistor, for example.

The switch Sw can be modified as shown in Pig. 6 which shows the circuit of a top-bottom balance control. The control adds equal amounts of positive and negative switching waveform (lino flyback pulses generally) when a slider X is set in the centre of a resistor fix. These cancel out and -843170 give no net effect. With the slider X more to one side, then either the positive or negative pulse predominates and is injected into the resonant circuit. This has the effect of moving the bowtie cross-over point to one side and so altering the relative correction top to bottom of the display. Eig. 6 is otherwise the same as Eig. 4(a).

Any device or circuit which injects an amount of switching waveform of one phase or the other into the resonant circuit will have the same effect as the circuit .10 of Eig. 6.

Circuits of Figs. 5 and 6 can be combined into one circuit to give combined amplitude and balance control.

Fig. 7 shows a switching means which incorporates both amplitude and i,.dance control.

Broadly, the present invention provides a circuit for correcting North-South (i.e* top and bottom) distortion on magnetically deflected cathode ray tubes. This circuit is simple, cheap, and has excellent performance.

The embodiment shown in Eig. 3 of the accompanying 2o drawings will now be described.

An output terminal 11 of the field output amplifier A is connected through a first half of the field deflection coil Ly, the secondary winding Ns of the transformer TI and the second half of the field deflection coil Ly to a circuit point 12. The circuit point 12 is connected through a resistor 15 to the slider of a potential divider 14. The setting of the slider of the divider 14 determines the vertical position of the television raster on the screen -970 (not shown). Tho circuit point 12 is also connected through the series resonant circuit consisting of the capacitor 0, and the primary winding Np of the transformer' T1 to a circuit point 12'. The circuit points 12 ’and 12' are bridged by the switch Cw which is a diode switch as described hereinbeforewith reference to Fig. 4(a), and includes a secondary winding Nt of a line output transformer (not shown). The circuit point 12' is connected to ground through an electrolytic D.O» blocking .capacitor 15 in sqries v/ith a resistor 16, which serves as a means for sensing current. A circuit point. 17 where the capacitor. 15 is connected to one end of the resistor 16 is connected tp an input terminal 18 of the amplifier A to provide voltage feedback. The purpose Of the voltage feedback is to ensure that the amplifier A provides the desired output current waveform. Another input terminal 19 of the amplifier A receives, in operation, a voltage ramp waveform from which the amplifier A produces at its output terminal 11 a field deflection current waveform of 5 amps peak-to-peak amplitude. A capacitor 20 connects the terminal 11 to the circuit point 12' to by-pass line frequency current so that the output of the amplifier A is not contaminated by unwanted line frequency current. Line frequency current ' circulating within the switch Sw is limited by two equal resistors· ks. Actual values of components used are as follows:Capacitor 0 .................. 0.5/F primary winding Np ........... 20^aH -1043170 each resistor lis ............. 0.1/1 capacitor 20................ 1/$’ capacitor 15 ................. 220i)*F resistor 16...... 0.5 Jl turns ratio of transformer T1 1:1 All the embodiments described heretofore have had a series resonant circuit which is resonant at the line frequency. These embodiments are suitable for correcting North-South pincushion distortion in many television cathode ray tubes, and in particular, those cathode ray tubes where the required correction current is referred to as the bowtie current.

However, other cauhode ray tubes are known, for example the Mullard 20ΛΧ Cathode fiay Tube, in which a different form of correction current is required to correct for NorthcSouth distortion. In such cases, other embodiments of the invention may be used in which the series resonant circuit is resonant at the second harmonic frequency of the line frequency.

For example, the embodiment of Fig. 2b may be adapted by providing suitable values of the capacitor C and the inductance of the primary winding Np in series therewith to provide series resonance at twice the television line frequency. Other embodiments, including ones having circuit diagrams corresponding to Figs. 2a, 2c. and 3, can of course be constructed with series resonance circuits resonant at twice line frequency. -11· Fig. 8 shows the relationship in operation in an embodiment having a second harmonic series resonance circuit, the peaks A and B of two line flyback voltages being separated by one line period Ίί, and the contemporaneous sinusoidal current I circulating in the series resonant circuit which is resonant at twice line frequency

Claims (3)

CLAIMS:

1. A field scan deflection circuit for correcting pin cushion distortion i.n a cathode ray tube, the circuit having one or more field deflection coils, and means for driving deflection current through the deflection coils, the said means including a resonant circuit adapted to add North-South dis5 tortion correction current to the deflection current, for correcting the pin cushion distortion,the resonant circuit including switching means arranged to short-circuit the resonant circuit when the switching means is in a closed condition, the switching means in operation being in the closed condition during each line scan interval and in an open condition during each 10 line flyback interval. A deflection circuit according to claim 1, wherein the resonant circuit •.omprises a series resonant circuit. 'i. A deflection circuit according to claim 2, wherein the series resonant circuit is resonant at line frequency. 15 4. A deflection circuit according to claim 2, wherein the series resonant circuit is resonant at twice line frequency. - 13 ro

2. 5. Λ deflection circuit according to any preceding claim, wherein the resonant circuit is tunable. o. A deflection circuit according to any preceding claim, wherein the switching means comprises oppositely poled diode means coupled to a source of biassing pulses, z. A deflection circuit according to any preceding claim, wherein the switching means is adjustable to determine the on-off ratio thereof in operation.

3. 8. A field scan deflection circuit substantially as described hereinbefore with reference to any one of Eigs. 2a to 7»