DK145840B - CONCAVE PREVENTION CORRECTION CIRCUIT - Google Patents

Description

and 145840

The present invention relates to a concave distortion correction circuit.

In a magnetically deflected cathode ray tube, such as used in a television receiver, the picture shape may be distorted due to the fact that the picture surface is almost flat. This is commonly known as concave distortion or cushion distortion and is commonly considered to have two main components, namely east-west and north-south distortion. The north-south distortion is seen as relatively more image height (ie, low vertical scan amplitude) toward the left and right edges of the screen. In the present description, an electronic circuit is provided which corrects for north-south distortion. Currently, other circuits are used which make use of either a saturable reactor (transducer) or a waveform generator circuit followed by a power amplifier.

British Patent Specification No. 1,638,3o7 discloses the use of a line frequency switch comprising two separate switching means in connection with two resonant circuits.

Each of these switching means uses voltage pulses 20 equal to the difference between a sawtooth voltage at field scan speed (one facsimile of field scan current) and a positive and negative voltage supply. The energy to estimate the resonant circuits is thus obtained from the direct current supplies. The field scan voltage therefore modulates only the amplitude 25 of the applied pulses.

The two resonant circuits are used to create positive and negative parabolas, and a third resonant circuit then combines these two waveforms to produce the necessary sinusoidal diagonal raster correction-3 currents.

French Utility Model No. 2,o41,728 discloses a circuit in which energy from the line return generator is used to estimate the resonant circuit and the transistors are operated with a modulating field waveform. The transistors 35 operate in a linear state exactly as in a conventional balanced modulator.

It is the object of the invention to provide a simpler and cheaper circuit which operates at least as well as these known circuits, and this is achieved by what is stated in claim 1.

According to the present invention, there is provided a field scan deflection circuit for a field and line deflection cathode ray tube having one or more field deflection coils, means for driving deflection current through the deflection coils and resonant circuit means for adding a north deflection. the deflection current, and the characteristic of the circuit according to the invention is that the resonant circuit means comprise a series resonant circuit connected in series with the field deflection coil or coils and switching means arranged to substantially short-circuit the resonant circuit means when the switching means and is in the closed state during each line scan interval and in the open state during each line return interval.

The invention will now be explained in more detail with reference to the drawing, in which 20o fig. 1a and 1b show circuit diagrams which schematically show the above-mentioned embodiments, fig. 2a, 2b and 2c are circuit diagrams of simple embodiments of the circuit 25 according to the invention, fig. Fig. 3 is a circuit diagram of a further embodiment according to the invention; 4a, b, c, d and e are circuit diagrams of circuit breakers for embodiments according to the invention, fig. Figs. 5a, b, c, d, e and f are circuit diagrams of adjustable resonant circuit means for embodiments according to the invention, fig. Fig. 6 is a circuit diagram of an adjustable switching means for an embodiment according to the invention; 7 is a circuit diagram of a further adjustable switching means for an embodiment according to the invention, and FIG. 8 is a waveform diagram showing waveforms 5 which occur in the operation of a further embodiment.

To achieve the north-south concave correction, a sinusoidal current at line frequency (typically 15,625 Hz), which varies in amplitude with the vertical deflection current, lo must run in the deflection coils. This current is a sinusoidal two-way correction current, which is generated by the combination of two positive and negative dishes, hereinafter referred to as "butterfly".

In FIG. 1a in the drawing, a switch Sw is opened and closed repeatedly at the natural resonant frequency of a resonant circuit. This may have a shape as shown in FIG. Ib, where an induction coil L and a capacitor C form a series resonant circuit. While the switch is closed, L and C may oscillate as their circulating current 20 runs through the switch. When the switch is open, a current I flows into the resonant circuit from a current source S.

This has the effect of maintaining the oscillating current in the resonant circuit. Furthermore, the oscillating current is directly proportional to the current I. Consequently, the sinusoidal voltage across L and C is directly proportional to the current I. If the direction of the current I is reversed, the oscillations undergo a phase change of 18 °.

If the current source S is a source of television field deflection current and the switch Sw is subjected to repeated opening and closing periods at a rate equal to a television line frequency, the current I appears as a slowly varying current for the switch Sw and the resonant circuit.

The shape of an uncorrected television field deflection current is a sawtooth wave which passes through zero at the midpoint 35 on the flank of each sawtooth.

In the drawing, fig. 2a, b and c are circuit diagrams for corrective field deflection circuits according to the present invention. Each of these circuits contains a field output amplifier A, field deflection coils Ly, a transformer T1 with a primary winding Np and a secondary winding Ns, a capacitor C and a switch Sw. In FIG.

2a, the field deflection current flows into the field deflection coils Ly over the secondary winding Ns and out through the switch Sw when 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, namely lo 1. The power amplifier A, which is needed for field deflection.

2. The field deflection coils Ly.

A line frequency sine wave generating circuit formed by the capacitor C, the primary winding Np and the switch Sw.

15 4. A winding Ns to connect the line frequency sine waves to the field deflection coils Ly.

The sine wave generating circuit Np - C - Sw is connected in series with the field deflection coils so that it can produce a butterfly curve shape, noting that the circuit is still functioning properly if Np and C are exchanged.

The secondary winding Ns must be connected in series with the field deflection coils Ly so that the butterfly can be applied to the amplifier output voltage which is applied to the field-25 deflection coils Ly. However, it is irrelevant in what order they are coupled as long as they are in series (see, for example, the circuits in Figs. 2b and 2c).

In the circuit of FIG. 2c, the field deflection coil Ly is in two halves, the butterfly being applied at the center of 3o Ly by means of the secondary winding Ns.

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

Other combinations are possible as the circuits will operate as long as the sine wave generating circuit is in series with the field deflection coils and the secondary winding is in series with the field deflection coils. FIG. 3 of the drawing is a circuit diagram of a practical embodiment 145840 5 according to the invention.

The switch Sw must be able to work in two directions, ie. it must be able to conduct electric current in both directions, and it must be able to connect 5 and break faster than the line frequency (15,625 Hz in England), as the switch Sw must be in the closed state during each line scan interval and in the open state during each line return interval.

There is the possibility of different circuits that can serve as the switch Sw and that use diodes (either semiconductor or tube diodes). Two such are shown in fig.

4a and 4b in the drawing.

In both circuits there is a winding Nt on a transformer (eg the line output transformer). 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, there is a current path between two terminals A and B, and the switch can be said to be connected. During the line return interval-20o interval, a large pulse voltage with opposite polarity is induced in Nt. The diodes are therefore biased in the blocking direction, and there is now no path for current passage. The switch can now be said to be open.

A transistor which is capable of switching current 25 in one and the other direction can be used as the switch Sw or also two transistors, one NPN and the other PNP. An example of a transistor diode switch is shown in FIG. 4d in the drawing, and a tube diode switch is shown in fig. 4th.

One or more SCR (controllable silicon rectifiers) could be considered for use, possibly together with other previously mentioned means.

The amplitude of the correction, i.e. butterfly curve shape amplitude, can be controlled by setting the value of the capacitor C. This can be in the form of a variable capacitor, as in fig. 5a, or alternatively as one or more additional capacitors added in parallel with the fixed tuning capacitor C, as in FIG. 5b.

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The principle of its ability to control the amplitude is as follows:

If the capacitor value e.g. increased, the inductance value must be reduced to resonate again. Since the inductance is now smaller, its inductive reactance is smaller, and thus the sinusoidal voltage which occurs across it will be smaller. With the same reasoning, a smaller capacitor will cause more correction.

By changing the winding ratio between the primary winding layer Np and the secondary winding Ns, the output voltage will be greater or less at normal transformer operation. FIG.

5c shows that Ns is variable, and fig. 5d shows that Np is variable.

In the circuit of FIG. 5e in the drawing, the total tuning inductance in the resonant circuit consists of L + L, in which L is a small extra variable induction coil.

If it e.g. made smaller, L must be made larger to maintain resonance. But more of the available output voltage now appears across L and thus at the output.

2o If L is made larger, the output voltage becomes smaller.

If a resistor is connected across the switch Sw as shown in the circuit of fig. 5f, when the switch Sw is open, some of the current which would otherwise flow into the resonant circuit will now flow through the resistor, and the re-suiting output current is smaller. Some of the current has therefore been shunted by the resistor. The shunt does not have to be a resistor, but can e.g. be a transistor, a tube, a voltage-dependent resistor or a thermistor.

The switch Sw can be modified as shown in fig. 6, 3o showing the circuit for a top-bottom balance adjustment.

The control adds equal amounts of positive and negative shift curve shape (usually line return pulses) when a slider X is set to the center of a resistor Rx. These cancel each other out and give no 35 net effect. When the slider X is moved to one side, either the positive or negative impulse will prevail and be introduced into the resonant circuit. This has the effect of moving the butterfly crossing point to one side and thus changing the relative correction peak bottom of the screen. FIG. 6 otherwise shows the same as fig.

4a.

Regulation of the pulse input does not have to take place by means of the slider, and in fact it does not have to be performed mechanically. Any means or circuit which introduces an amount of positive or negative shift waveform into the resonant circuit will have the same effect as the circuit of FIG. 6.

lo The circuits of FIG. 5 and 6 can be combined into one circuit to provide combined amplitude and balance control. An example of such a combined circuit is shown in FIG. 7.

Briefly stated, the present invention provides a north-south (i.e., top and bottom) distortion circuit on magnetically deflected cathode ray tubes. This circuit is simple, inexpensive and has excellent function.

The embodiment shown in fig. 3 in the drawing, 2o must now be described.

An output terminal 11 of the field output amplifier A is connected over a first half of the field deflection coil Ly, the secondary winding Ns in the transformer T1 and the second half of the field deflection coil Ly to a circuit 25 point 12. The setting of the slider on the voltage divider 14 determines the vertical position of the television screen on the screen (not shown). The circuit point 12 is also connected to a circuit point 12 'above the series resonant circuit 30 consisting of the capacitor C and the primary winding Np of the transformer T1. The circuit points 12 and 12 'are connected to each other by means of the switch Sw, which is a diode switch as described above with reference to fig. 4a and contains 35 a secondary winding Nt on a line output transformer (not shown). The circuit point 12 'is connected to ground via an electrolytic DC blocking capacitor 15 in series with a resistor 16 which serves as a means for 8 145840

sensing current. A circuit point 17, where the capacitor 15 is connected to one end of the resistor 16, is connected to an input terminal 18 of the amplifier A to provide voltage feedback. The purpose of the 5 voltage feedback is to ensure that the amplifier A

provides the desired output current waveform. A second input terminal 19 on the amplifier A receives during operation a voltage cut-off waveform, of which the amplifier A on its output terminal 11 produces a field deflection current curve lo shape with an amplitude from tip to tip of 3 amps. A capacitor 20 connects the terminal 11 to the circuit point 12 'to divert line frequency current so that the output current from the amplifier A is not disturbed by unwanted line frequency current. Line frequency current circulating in the switch Sw is limited by two equal resistors Rs. Actual values of components used are as follows:

Capacitor C .................. o, 5pF

Primary winding Np ............... 2oojUH

Each resistor Rs ............... o, l / l

2o Capacitor 2o ................. ljuF

Capacitor 15 ................. 22θθμΈ

Resistance 16 ....... o, 5fl

Winding ratio on transformer T1 ............... 1: 1 25 All the embodiments described above have had a series resonant circuit which is in resonance at the line frequency. These embodiments are suitable for correcting north-south concave distortion in many television cathode ray tubes and in particular those cathode ray tubes where the required 30 correction current is referred to as the butterfly current.

However, other cathode ray tubes are known, e.g.

Mullard 2oAX cathode ray tube where a different type of correction current is required to correct for north-south distortion. In such cases, other embodiments of the invention may be used where the series resonant circuit is in resonance at the second harmonic frequency of the line frequency.

For example, the embodiment of FIG. 2b can be adjusted 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 5 embodiments with circuit diagrams corresponding to fig. 2a, 2c and 3 can of course be performed with series resonant circuits having resonance at twice the line frequency.

FIG. 8 shows the conditions in operation in an embodiment with a second harmonic series resonant circuit, the 10 points A and B of two line return voltages being separated by a line period T and the simultaneous sinusoidal current I circulating in the series resonant circuit resonating at twice the line frequency. .