Abstract:
A magnetic field compensation winding arrangement for a cathode ray tube includes a magnetic field compensation winding positioned on the cathode ray tube to compensate for an ambient magnetic field. An operational amplifier is used for generating a magnetic field compensation current in the winding. A pair of digital-to-analog converters are used for generating a pair of signals, respectively, that are coupled to the amplifier to control the magnetic field compensation current.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a non-provisional application which claims the benefit of provisional application Ser. No. 60/373,874, filed Apr. 19, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to a bus controlled arrangement for a video apparatus. In particular, the invention relates to an arrangement for adjusting a current in a winding mounted on a cathode ray tube (CRT) to compensate for the earth&#39;s magnetic field.  
         [0003]     U.S. Pat. No. 5,168,195, entitled, MAGNETIC FIELD COMPENSATION APPARATUS, in the names of Breidigan, et al. describes a compensating coil for a scanning electron beam display, such as a color television tube, that reduces undesirable deflection of the electron beams due to ambient magnetic fields, in particular the geomagnetic field. The coil has a winding disposed to encircle the tube neck, perpendicular to the Z axis. A degaussing coil is positioned on the tube envelope to provide for demagnetizing metal structures within the envelope. In order to provide more complete degaussing, the supply current to the compensating coil is interrupted during the degaussing operation.  
         [0004]     U.S. Pat. No. 5,739,638, in the name of Wilber et al., entitled Bus controlled arrangement using a duty cycle modulated control signal in a CRT, describes an arrangement using a microprocessor that applies a digitally coded signal to a digital-to-analog (D/A) converter. An output voltage of the D/A converter is applied via a power amplifier to the compensating coil.  
         [0005]     In the power amplifier, a differential current sense arrangement is used to control a voltage developed across a current sense resistor that is coupled in series with the compensating coil. Thereby, the current in the compensating coil is made independent of the value of the resistance of the compensating coil. This allows Z coils with significantly differing resistances to achieve identical rotation ranges.  
         [0006]     In carrying out an inventive feature, a pair of differential output signals of a corresponding pair of DAC&#39;s fabricated in an integrated circuit (IC) on a common substrate are coupled to corresponding inverting and non-inverting inputs of the power amplifier. Such arrangement facilitates tracking between the pair of differential output signals.  
         [0007]     During a degaussing interval, the pair of DAC&#39;s are programmed to convert the same digital value, for example a mid-range value. Because of the tracking between the pair of differential output signals, advantageously, accurate zero current is produced in the compensating coil, during the degaussing interval. Outside the degaussing interval, one DAC output signal may remain at the mid-range value while the other one may be adjusted to either a higher or a lower value as required for the aforementioned earth magnetic field compensation.  
       SUMMARY OF THE INVENTION  
       [0008]     A magnetic field compensation apparatus, embodying an inventive feature includes a first digital-to-analog converter responsive to a digitally encoded signal containing magnetic field compensation information for generating a first analog signal containing the magnetic field compensation information from the digitally encoded signal. A magnetic field compensation winding is positioned on a cathode ray tube. An amplifier is responsive to the first analog signal and having an output that is coupled to the magnetic field compensation winding for producing a current in the magnetic field compensation winding. The current produces a magnetic field in a beam path of the cathode ray tube that compensates for an ambient magnetic field. A second digital-to-analog converter generates a second analog signal that is coupled to an input of the amplifier that varies the current in accordance with the second analog signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]      FIGS. 1A and 1B  illustrate a bus controlled Z-axis or tilt compensation arrangement,embodying an aspect of the invention, for a video display. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]      FIGS. 1A and 1B  illustrate a bus controlled Z-axis or tilt compensation arrangement  100 , embodying an aspect of the invention, for a video display. An output signal Z-COIL-REF of a conventional digital-to-analog converter (DAC)  102   a  of  FIG. 1B  is coupled via a resistor R 1  of  FIG. 1A  to a non-inverting input terminal  106  of an operational amplifier U 1 . An output signal Z-COIL of a conventional DAC  102   b  is coupled via a resistor R 9  to an inverting input terminal  104  of operational amplifier U 1 . Each of DAC  102   a  and DAC  102   b  of  FIG. 1B  is responsive to a digitally coded signal  103 , developed on an I 2 C bus BUS, and generates the corresponding output signal Z-COIL-REF or Z-COIL, in accordance with the digital value of digitally coded signal  103 .  
         [0011]     An output terminal U 1   a  of amplifier U 1  of  FIG. 1A  is coupled to a base electrode of an emitter follower PNP transistor Q 1  that is capable of sinking current at its emitter. Output terminal U 1   a  is also coupled to a base electrode of an emitter follower NPN transistor Q 2  that is capable of sourcing current at its emitter. The collector electrode of transistor Q 1  is coupled to a common potential or ground via a current limiting resistor R 4 . The collector electrode of transistor Q 2  is coupled to a supply voltage V 1  of amplifier U 1  via a current limiting resistor R 5 .  
         [0012]     A junction terminal  101 , between the emitters of transistors Q 1  and Q 2 , is coupled via a feedback resistor R 7  to inverting input terminal  104  of amplifier U 1 . A terminal  105  of a current sense resistor R 6  is coupled via a feedback resistor R 2  to non-inverting input terminal  106  of amplifier U 1 . Feedback resistors R 7  and R 2  cause a voltage V 3  at inverting input terminal  104  to be equal to a voltage V 4  at non-inverting input terminal  106 . A difference between a voltage V 101  developed at terminal  101  and a voltage V 105  developed at terminal  105  is controlled in a feedback manner by a difference between signals Z-COIL-REF and Z-COIL.  
         [0013]     Junction terminal  101 , formed between the emitters of transistors Q 1  and Q 2 , is coupled via current sensing resistor R 6  to a compensating or Z coil W 1 . Coil W 1  acts as a transducer for producing a field in a vicinity of a beam in a cathode ray tube (CRT)  22 . The operation of coil W 1  for compensating the earth&#39;s magnetic field is well known, as discussed in, for example, U.S. Pat. No. 5,015,915 in the names of Hartmann et al. A second end terminal of coil W 1  is coupled to a supply voltage V 2  that is approximately one half of supply voltage V 1 .  
         [0014]     When a voltage V 101  at terminal  101  is more positive than voltage V 2 , a current iW 1  in coil W 1  is positive. Conversely, when voltage V 101  is less positive than voltage V 2 , current iW 1  in coil W 1  is negative. Therefore, the two polarities of current iW 1  are obtained using supply voltages V 1  and V 2  that are both positive voltages.  
         [0015]     A differential current sense arrangement formed by resistors R 7  and R 2  is used to control the voltage difference between voltages V 101  and V 105  developed across current sense resistor R 6  that is coupled in series with compensating coil W 1 . Thereby, current iW 1  in compensating coil W 1  is made independent of the value of an inherent resistance of compensating coil W 1 . Advantageously, this allows Z coils with significantly differing resistances to achieve identical rotation ranges.  
         [0016]     The pair of differential output signals Z-COIL-REF and Z-COIL of DAC  102   a  and DAC  102   b  of  FIG. 1B , fabricated in an integrated circuit (IC)  102  of the type TDA8444 on a common substrate, are coupled to non-inverting and inverting inputs  106  and  104 , respectively, of amplifier U 1  of  FIG. 1A . A supply voltage V 5  is also coupled in common to DAC  102   a  and to DAC  102   b.  Such arrangement facilitates tracking between the pair of differential output signals Z-COIL-REF and Z-COIL. For example, signal Z-COIL may contain magnetic field compensation information; whereas, signal Z-COIL-REF may not contain any magnetic field compensation information. Instead, signal Z-COIL-REF may track variation in signal Z-COIL introduced by temperature variations, variations of supply voltage V 5  or variations related to component aging of IC  102 . Thereby, compensation by common mode rejection is, advantageously, provided.  
         [0017]     During a degaussing interval, not shown, the pair of DAC  102   a  and DAC  102   b  of  FIG. 1B  are programmed to convert the same digital value, for example a mid-range value of signal  103 . Because of the tracking between the pair of differential output signals Z-COIL-REF and Z-COIL, advantageously, accurate zero current iW 1  of  FIG. 1A  is produced in compensating coil W 1 , during the degaussing interval. Outside the degaussing interval, one of output signals Z-COIL-REF and Z-COIL may remain at the mid-range value while the other one may be adjusted to either a higher or a lower value as required for the aforementioned earth magnetic field compensation.