Abstract:
A convergence method for color television receiver comprises the steps of emitting a test signal from a control circuit to a first convergence channel to generate a current of a predetermined intensity in a convergence coil assigned thereto. The control circuit emitting a zero signal to the other convergence channels to generate zero current in the assigned convergence coils. Measuring and comparing with a reference potential a potential at a reference point of each convergence coil of channels driven by the zero signal. Generating a reference signal responsive to the comparison step b), for each case for the relevant convergence channel and coupling to the control circuit. Repeating steps a) to c) for each convergence channel, the convergence channels being driven in an alternating fashion one after another once in each case by a test signal.

Description:
This application claims the benefit under 35 U.S.C. § 365 of International Application PCT/EP99/05450 filed Jul. 30, 1999, which claims the benefit of German Application No. 198 35 803.2, filed Aug. 7, 1998. 
     The invention relates to a method and an apparatus for convergence correction in a television receiver, in particular in a projection colour television receiver. 
     Convergence correction devices serve to correct parameters in the raster deflection, for example the correction of north/south and east/west distortions, of pin-cushion distortion, of non-linearities in the deflection and other geometric errors in the horizontal or vertical direction. A particular field of application is convergence correction in the case of a projection television receiver, in which the pictures of three monochromatic picture tubes are projected onto a picture area. Here, television receiver is taken to mean any device with electronic, rasterized picture reproduction. The receiver can be fed by a television broadcast signal or also, as a pure monitor, by a RGB signal, an FBAS signal or, separately, with a luminance signal and a colour subcarrier from an arbitrary video signal source. 
     The correction of the deflection parameters is undertaken with the aid of digital correction values which are stored in a convergence circuit. The stored correction values are converted in a digital/analogue converter into an analogue control signal and emitted to the first input of an output amplifier whose output is connected to the first terminal of the respectively assigned convergence coil. The second terminal of the convergence coil is connected, on the one hand, to frame potential via a measuring shunt and, on the other hand, to the second input of the output amplifier via a feedback loop. It is ensured in this way that a current corresponding to the analogue control signal is impressed on the convergence coil. Series resistances, for example due to the resistance of conductor tracks, can have the effect that the measuring shunt is no longer at frame potential, but at a potential differing therefrom. The effect of this is that the current in the convergence coil no longer corresponds exactly to the convergence correction value, but deviates from the desired value. 
     SUMMARY OF THE INVENTION 
     Starting from here, it is the object of the invention to specify a method in which the difficulties described at the beginning and their effects no longer occur in a troublesome fashion. 
     An advantage of the method according to the invention is that the crosstalk of one convergence channel onto the other convergence channels is compensated by a simple electrical measurement. It is possible in this way for the convergence, for example in a projection television receiver, to be balanced substantially more accurately and quickly, and this leads, on the one hand, to an improved picture quality and, on the other hand, to reduced production costs, because the mutual influencing of the convergence channels is eliminated. 
     The object is also achieved by a method based on an optical measurement of the crosstalk of one convergence channel into the other convergence channel. The same advantages as with the prior method are achieved with this method. Moreover, it is possible using the optical measuring method also to take account during compensation of such forms of crosstalk which cannot be embodied in the form of a measurable electric voltage. This holds, for example, for stray magnetic fields which are distributed spatially such that although they influence the electron beam of a picture tube they do not cause any measurable induced voltage. 
     The method according to the invention can be carried out both with optical sensors which are arranged on the picture screen of the television receiver, and by means of an electronic camera. 
     Furthermore, it is an object of the present invention to provide an apparatus for improved convergence correction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawing is a schematic representation of a circuit with the aid of which the method according to the invention is explained. In the drawing: 
     FIG. 1 shows a diagram of a circuit with six convergence channels of a projection television receiver, 
     FIG. 2 shows the circuit of FIG. 1, which is supplemented by a comparitor for each convergence channel, and 
     FIG. 3 shows a diagram of a projection television receiver. 
    
    
     DETAILED DESCRIPTION 
     A circuit denoted as a whole by  1  and which comprises six convergence channels  2   a - 2   f  is represented diagrammatically in FIG.  1 . Each of the three monochromatic picture tubes of a projection receiver for the primary colours of red, green and blue are respectively assigned two convergence channels for setting the horizontal and vertical convergence. The convergence channels  2   a - 2   f  are of corresponding design, and so for the sake of brevity only the convergence channel  2   a  is described below as a representative for all convergence channels. 
     The convergence channel  2   a  comprises an output amplifier  3   a  whose output  4   a  is connected to a convergence coil  6   a . With its second terminal, the convergence coil  6   a  is connected via a measuring shunt  7   a  to a frame terminal  8   a  of the circuit  1 . The frame terminal  8   a  is connected to frame potential via a conductor track, the resistance of the conductor track being illustrated by the series resistor Ra illustrated with dashes. Of course, the magnitude of the resistance Ra is also influenced by other properties of the circuit  1 , for example by contact resistances. 
     Connected to a first input  11   a  of the amplifier  3   a  is an input signal U-RH which generates in the convergence coil  6   a  a convergence current which brings about the horizontal convergence correction of the red picture tube. A second input  12   a  of the amplifier  3   a  is connected via a feedback loop  13   a  to a reference point Ba between the convergence coil  6   a  and the measuring shunt  7   a . The amplifier  3   a  raises the convergence current until there is no longer any potential difference present between its inputs  11   a  and  12   a , that is to say the feedback signal corresponds to the input signal U-RH. 
     In the present exemplary embodiment, the convergence values are stored as digital values which are converted into analogue output signals by means of A/D converters. However, it is also possible to implement the invention in a completely analogue circuit. 
     If, because of the input signal U-RH, a current of 1 A, for example flows in the convergence channel  2   a , because of the voltage drop which caused by the convergence current in the series resistor Ra, a potential which is different from 0 V occurs at the frame terminal  8   a . This voltage drop Ua leads to crosstalking into the other convergence channels, which becomes clear most simply from the following consideration: if the input signals U-RH to U-BV of all convergence channels  2   a - 2   f  are equal to 0 V, the respective convergence current is equal to 0 A in all convergence coils  6   a - 6   f . While the input signals U-RV to U-BV of the convergence channels  2   b - 2   f  remain equal to 0 V, one input signal U-RH≠0 V is to generate a current of, eg 1 A, in the convergence channel  2 . The voltage drop Ua is passed via the series resistor Rb and the measuring shunt  7   b  to the input  12   b  of the amplifier  3   b  of the convergence channel  2   b . The potential difference at the input  11   b  and  12   b  of the amplifier  3   b  causes a current to be impressed on the convergence coil  6   b  although it holds for the input signal U-RV at the input  11   b  that: U-RV=0 V. This consideration can be carried over correspondingly to the other convergence channels  2   c - 2   f.    
     It is therefore easy to see that the currents in the convergence channels  2   b - 2   f  depend on the intensity of the current in the convergence channel  2   a . The electric crosstalk, described with the aid of the convergence channel  2   a , between the individual convergence channels on the basis of an unavoidable series resistance which is connected downstream of the measuring shunt  7   a , also holds in an entirely corresponding fashion for all other convergence channels  2   b - 2   f , the further series resistances Rb-Rf then playing a role, as the case may be. 
     The situation, for example for the convergence channel  2   b , can be formulated mathematically as follows. Without series resistances, it would hold that: 
     
       
           I   (L-RV)   =U   (U-RV)   /R   (R-RV)   (1) 
       
     
     The series resistances change this equation in such a way that it holds that: 
     
       
           I   (LRV) =( U   (U-RV)   −Ua−Ub )/ R   (R-RV)   (2) 
       
     
     In the case of the circuit represented in FIG. 2, each convergence channel  2   a - 2   f  is extended by an additional comparator  16   a - 16   f  whose first input is connected to the feedback loop  13   a - 13   f . The second input of the comparator  16   a  is connected to frame potential. The outputs of the comparators  16   a - 16   f  are connected to a convergence circuit  17  which has a memory M and computing means L. The output signals of the comparators  16   a - 16   f , which compare the voltages at the reference points Ba . . . Bf with frame potential, are denoted by U-RH(comp) to U-BV(comp). 
     In order to be able to compensate the crosstalking of the individual convergence channels, the method described below is carried out at the start of convergence setting with a television receiver and after, as the case may be, existing offset voltages have been balanced. A method for balancing offset voltages is known, for example, from the German Patent Application with the file reference 42 14 317.9 from the same Applicant. 
     The first step is for the convergence circuit  17  to emit to the convergence channel  2   a  an output signal U-RH of, for example, 2 V, which causes a current of 1 A in the convergence coil  6   a . The output signals U-RV, U-GH, U-GV, U-BH and U-BV emitted to the convergence channels  2   b - 2   f  are equal to 0 V in this case. The reference signal of the comparator  16   b  is now interrogated. If the comparator  16   b  emits a “1”, the potential at the non-inverting input of the comparator is greater than 0 V. In this case, the convergence value, which determines the output signal U-RV in the convergence circuit  17  is decremented by one step. Thereafter, the reference signal of the comparator  16   b  is interrogated once again. This cycle is repeated until the reference signal of the comparator  16   b  changes from “1” to “0”. The associated digital convergence value is then incremented again. As soon as the convergence circuit  17  establishes that the comparator  16   b  is exactly at its operating point, a progression is made to the next convergence channel and the output signal of the next comparator  16   c  is evaluated in the same way. In an entirely corresponding fashion, the digital convergence value is incremented by one step when the comparator emits a “0” at the start of the cycle. This happens until the reference signal changes from “0” to “1” and subsequently oscillates between “0” and “1”. Such evaluation methods have already been described in the German Patent Applications with file references 42 14 317.9 and 198 01 968.8 of the same Applicant. 
     This cycle is repeated for all five convergence channels for which an output signal of 0 V is emitted at the start of the test mode. As soon as this first pass is concluded, the next convergence channel is driven by an output signal of, for example, 2 V, for example the convergence channel  2   b . Once again, all other convergence channels  2   a ,  2   c - 2   f  are driven by an output signal of 0 V and subsequently balanced such that the associated comparitors  16   a ,  16   c - 16   f  are exactly at their operating point. This cycle is also repeated until all convergence channels have respectively alternately had a current applied to them once while the other convergence channels were balanced with respect to this one convergence channel. 
     This method supplies parameters which describe the crosstalk of each convergence channel onto the respective other convergence channels. By taking into account the parameters thus obtained in the case of the output signals for the individual convergence channels, it is possible to compensate crosstalking from one convergence channel to the others. The advantage of this is that from now on the setting of a specific convergence channel can be varied without this influencing the other convergence channels. 
     With reference to equation (2), the result of the compensation can be formulated such that: 
     
       
           U   (U-RV)*   =U   (U-RV)   +Ua+Ub   (3) 
       
     
     U (U-RV) * being the compensated input signal of the convergence channel  2   b.    
     The described crosstalk based on a displacement of the reference potential of the convergence coils is denoted below for short as electric crosstalk. In addition to this electric crosstalk, there is, moreover, crosstalk which is transmitted by magnetically induced interference voltages from one convergence channel to another. This second form of crosstalk is denoted as magnetic crosstalk for short and occurs principally between the horizontal and the vertical convergence channels of the same picture tube, but is also present in principle between the convergence channels of different picture tubes. Furthermore, it is to be noted that the said interference voltages can be induced not only in the convergence coils, but also in parts of the convergence circuit. As regards the compensation of these interference voltages, the cause of the occurrence does not, however, play any role. Magnetic crosstalk can be compensated in the same way as electric crosstalk. However, because of the transient character of the induced interference voltages the evaluation of the comparators  16   a - 16   f  must be performed with adequate speed, but this does not present any difficulty in principle. 
     The described compensation of the crosstalk by means of electric measurements is carried out first and foremost at the manufacturers. Furthermore, it is also possible for the service engineer to carry out such a compensation routine after repairing the television receiver, in order to achieve the optimum convergence setting of the receiver. 
     In the case of receivers which are provided with optical measuring means, it is also possible to carry out the compensation of crosstalk on the basis of an optical measurement. For other receivers, this is possible at the manufacturers, at least. An optical measurement is certainly mostly relatively complicated, but has the advantage that all forms of crosstalk between the convergence channels are detected even when they cannot be detected in the form of a voltage which is measurable in the convergence channels. A further difference by comparison with the electric measurement is that the effect of the crosstalk on the deflection of the electron beam is detected directly without the need to assume a known relationship between a measured potential displacement and/or interference voltage and the deflection of an electron beam. 
     Optical measurement is described below with the aid of FIG. 3. A video generator  19  supplies a three-component RGB video signal, the red component being fed to the picture tube  18 . The video generator can also be integrated in the convergence circuit  17 . For the sake of clarity, the other four convergence channels and two other monochromatic picture tubes, which are designed in an entirely corresponding fashion, are omitted. 
     The pictures of the red picture tube  18  and of the green and blue picture tubes (not represented) are projected onto a screen  21 . Arranged on the screen  21  are four optical sensors  22   a - 22   d  which in each case are connected to an evaluation circuit  23  which is connected, for its part, to a logic circuit L of the convergence circuit  17 . The logic circuit L can also cooperate with an internal or external microprocessor. Suitable optical sensors are disclosed, for example, in the German Patent Application with the file reference 197 00 204.8 of the same Applicant. 
     The optical measuring method functions as follows: 
     Only one colour, for example red, is emitted by the video generator  19 . Furthermore, for example, a vertical cursor in the form of a red horizontal line is represented on the screen  21  and successively balanced such that, for example, the sensor  22   a  and the assigned evaluation circuit  23  are exactly at their response threshold. This is achieved by virtue of the fact that the convergence correction values stored in the memory M of the convergence circuit  17  are varied until the comparators assigned to the sensors  22   a ,  22   c  respond. The position thus determined corresponds to a desired position of the cursor, and the associated convergence value is stored in the memory M of the convergence circuit  17 . This method is now repeated with a vertical red line to balance the horizontal convergence of the red picture tube  18 , the sensors  22   b ,  22   c  serving to detect position. In this way, the convergence channels are sequentially balanced for horizontal and vertical convergence of all three picture tubes, and the associated convergence values, which are denoted as primary convergence values, are stored. 
     Starting from the convergence current which corresponds to the desired position determined, the convergence current is varied by a specific amount in a first convergence channel. Whereas the change in the convergence current in the first convergence channel is maintained, all other convergence channels are balanced once again such that the optical sensors and the assigned comparator stages  23  are at their operating points. The new convergence value found in this way is stored as a secondary convergence value. After this method has been carried out iteratively for all convergence channels, six measured values are to hand for each convergence channel: a primary convergence value and five further, secondary convergence values. The differences between the primary and the secondary convergence values specify a measure of the crosstalk of the other five convergence channels into this one convergence channel. Using these values, the output signals emitted by the convergence circuit  17  to the convergence channels are varied such that the crosstalk between the individual convergence channels is completely compensated in the optimum case. 
     Instead of the optical sensors, it is also possible to use an electronic camera to evaluate the position of a cursor on the picture screen  21 . Such a method is described, for example, in the German Patent Application with the file reference 198 01 966.1 of the same Applicant.