Abstract:
The invention provides a circuit for controlling a cathode ray tube. The circuit comprises a source of input colour signals (U in (R), U in (G), U in (B)) for the primary colours of the cathode ray tube. Output amplifiers are coupled to the source of colour input signals and to electron guns of the cathode ray tube. Finally, limiting means are provided to limit the colour input signals to a predetermined threshold value. In this way the output amplifiers are prevented from saturation and undesirable smears do not appear on the screen. In an advantageous embodiment the limiting means are realized by a diode biased in reverse direction.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a circuit for controlling a cathode ray tube (CRT) and a television receiver equipped with such a circuit. A further aspect of the invention is related to control the driving signals for a CRT.  
         BACKGROUND OF THE INVENTION  
         [0002]    For a television receiver the maximum brightness, which can be represented on the viewing screen of the CRT or picture tube, the so-called light at peak white is an important quality characteristic of the image reproduction. In practice, the light at peak white is essentially limited firstly, by the control range or the saturation limit of the RGB output stages controlling the picture tube and, secondly, by the maximum permissible drive of the picture tube.  
           [0003]    In order to achieve the highest possible brightness, it is therefore necessary to set the drive of the picture tube, that is to say the amplification of the RGB signals controlling the picture tube, as high as possible in the direction of white.  
           [0004]    However, when the video output stages are driven in this way they may occasionally reach the saturation limit of the amplifiers in the output stages. Substantial tolerances such as, for example, the cut-off calibration, the transparency of the picture tube glass and fluctuations in the input signal, the dark current regulation compensating the ageing of the picture tube as well as customer settings such as brightness, contrast and colour saturation are responsible for exceeding the saturation limit.  
           [0005]    If an amplifier of a video output stage reaches the saturation limit, so-called smears appear on the viewing screen in the corresponding colour in the form of an extension of a pixel or of an image area in the line direction. These smears occur primarily for the colour red, because the electron gun for the colour red of the picture tube requires the highest drive because the red phosphor is least sensitive.  
           [0006]    In order to avoid such saturation phenomena in the case mentioned where tolerances sum up in an unfavourable way, it would therefore be necessary to observe a certain safety distance towards full output of the output amplifiers. However, this means in turn that the theoretically maximum possible drive to full output is generally not reached. Consequently, it is not possible to reach the maximum brightness of the picture tube.  
           [0007]    U.S. Pat. No. 6,057,887 suggests a circuit to alleviate the problems described above. In the known circuit the output colour signals are limited by Zener diodes to a threshold value that avoids the occurrence of smears on the screen. However, in practice it has been noted that the use of the known circuit in connection with the video amplifier IC TEA 5101 manufactured by ST has led to enhanced smearing effects due to “latching” effects inside the video amplifier IC.  
         SUMMARY OF INVENTION  
         [0008]    Based on these observations it is desirable to provide a circuit, which reliably all the described smears.  
           [0009]    The invention provides a circuit for controlling a cathode ray tube comprising a source of input colour signals (U in (R), U in (G), U in (B)) for the primary colours of the cathode ray tube. Output amplifiers are coupled to the source of colour input signals and to electron guns of the cathode ray tube. Finally, limiting means are provided to limit the colour input signals to a predetermined threshold value.  
           [0010]    In an advantageous embodiment of the inventive circuit the limiting means comprise a diode biased in reverse direction. Preferably a voltage source provides the bias voltage of the diode. The voltage source may be realized by a voltage divider stabilized with a Zener diode.  
           [0011]    In a preferred embodiment of the invention the same voltage source utilized to provide a reference voltage (U ref ) for the output amplifier is also utilized to provide the biasing voltage for the diode.  
           [0012]    It is advantageous to connect the limiting means to a transistor providing a low impedance voltage source in case the input signal exceeds the threshold voltage.  
           [0013]    Further advantages of the invention become apparent when reading the detailed description of the embodiments of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    In the drawing exemplary embodiments of the present invention are shown. Corresponding elements in the drawing are designated with corresponding reference symbols. It shows:  
         [0015]    [0015]FIG. 1 a circuit for controlling a cathode ray tube according to the prior art;  
         [0016]    [0016]FIG. 2 a schematic circuit diagram of a first embodiment of the inventive circuit;  
         [0017]    [0017]FIG. 3 a curve illustrating the effect of the inventive circuit on the input colour signals;  
         [0018]    [0018]FIG. 4 a schematic circuit diagram of a second embodiment of the inventive circuit; and  
         [0019]    [0019]FIG. 5 a schematic circuit diagram of the present invention inside a television receiver. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    [0020]FIG. 1 shows a circuit for controlling a CRT as it is known from U.S. Pat. No. 6,057,883. The known circuit, which is designated as a whole with the reference symbol  1  has three outputs connected to the electron guns of a CRT  2 . Each output of the control circuit  1  is associated with a video amplifier  3 ,  4 ,  5  for each of the primary colours red, green and blue. The amplifiers  3 ,  4 ,  5  receive colour input signals U in (R), U in (G), U in (B) to generate colour output signals U out (R) U out (G) , U out (B)  
         [0021]    The outputs of the amplifiers are each connected to the cathode of a diode  6 ,  7 ,  8 , respectively. The diodes are biased in forward direction by an external voltage U 1 . Thus, the colour output signals U out (R), U out (G), U out (B) are prevented from falling below the external voltage U 1 . In this way the known circuit avoids smears as described in the introduction.  
         [0022]    [0022]FIG. 2 displays an embodiment of the inventive circuit. For the sake of simplicity only the circuit for the primary colour red is shown, because the circuits for the other two primary colours are identical as to the extent it is relevant for the present invention. The circuit designated as a whole with the reference symbol  10  performs the amplification of a colour input signal U in (R) to generate a colour output signal U out (R). This is similar to the function of the amplifier  3  shown in FIG. 1.  
         [0023]    The circuit  10  receives at input terminal  11  the colour input signal U in (R), which is coupled via a voltage divider composed of resistors  12  and  13  to the inverting input of amplifier  14 . The amplifier generates the colour output signal U out (R) supplied to the output terminal  16  and coupled back by resistor  17  to the inverting input of the amplifier  14 . The non-inverting input of the amplifier  14  is supplied with a reference voltage U ref . The reference voltage U ref  is supplied and stabilized by a series connection of a first voltage source  18 , resistor  19  and Zener diode  21 . In a specific embodiment the first voltage source  18  provides essentially 12 V and the reference voltage is essentially equal to 3 V.  
         [0024]    The input terminal  11  is further connected to the anode of a diode  22 . The diode  22  is biased in reverse direction by a voltage U 1  provided by a second voltage source  23 . In a specific embodiment the voltage U 1  is about 4.9 V and the turn-on voltage U d  of the diode  22  is 0.6 V.  
         [0025]    In consequence, the diode  22  becomes conducting when the colour input signal U in (R) exceeds 5.5 V and hence effectively limits the maximum level of the input signal. The dimensions of the circuit and in particular the amount of feedback are selected such that the amplifier  14  does not reach saturation. Smear effects on the screen are consequently avoided.  
         [0026]    In FIG. 3 the effect of the inventive circuit on the input signal U in (R) is visualized. The maximal amplitude of the input signal U in (R) is limited to a threshold voltage of U 1 +U d , which is the bias voltage of the second voltage source  23  and the turn-on voltage of the diode  22 . The threshold voltage is represented by a dashed line in FIG. 2. The curve the input signal would follow without limitation by diode  22  is indicated with a dotted line. It is understood that the input signal U in (R) is representative for all other input signals U in  (G), U in (B).  
         [0027]    [0027]FIG. 4 shows a second embodiment of the inventive circuit, which is identified as a whole with the reference symbol  40 . Corresponding components and elements are identified with the same or corresponding reference symbols.  
         [0028]    Contrary to the circuit  10  shown in FIG. 2 in circuit  40  the voltage source  18  is also used to supply the bias voltage U 1  for the diode  22 . To enable this, the cathode of diode  22  is tapped to a series connection of resistors  19   a,    19   b  essentially replacing the resistor  19  of circuit  10 . The resistors  19   a  and  19   b  are selected such that diode  22  is biased in reverse direction by a voltage U 1 , similar to the configuration in FIG. 2.  
         [0029]    The limiting voltage in this embodiment is derived from the same source  18  that supplies the reference voltage. In case the voltage source  18  provides a higher than nominal voltage both the reference voltage U ref  and the bias voltage U 1  are increasing. A higher reference voltage U ref  reduces the output signal U out (R) for a given input signal U in (R). However, if the input signal U in (R) exceeds the threshold voltage U 1 +U d , the increased U 1  (i.e. U 1  is higher than its nominal value) allows a higher maximum input signal level. In this case the two effects are counteracting and the tolerance of the output signal level is reduced compared to the circuit  10 .  
         [0030]    [0030]FIG. 5 displays a further schematic diagram of the inventive circuit emphasizing some other aspects of the invention not shown in FIG. 2 and FIG. 4, but suppressing some details already described there. The video signals for the three primary colours are received from a video processing board (not shown in the drawing) at connector  51 . The signals of the primary colours are processed individually in delay lines  52   a,    52   b,    52   c  and preamplifiers  53   a,    53   b,    53   c.  The output signals of the preamplifiers provide the video input signals U in (R), U in (G), U in (B) for the video end stages  14   a,    14   b,    14   c  at their respective inverting inputs.  
         [0031]    In one embodiment of the invention the video end stage is the integrated circuit TDA 6111 Q manufactured by Philips. The non-inverting input of the video end stages  14   a,    14   b,    14   c  is supplied with a reference voltage U ref .  
         [0032]    The inverting input of each video end stage  14   a,    14   b,    14   c  is connected with the anode of a diode  22   a,    22   b,    22   c,  respectively. The cathodes of the diodes  22   a,    22   b,    22   c  are connected in parallel with the emitter of a pnp transistor  54 . The base of the transistor  54  is biased with a positive voltage U 1  taken from a tap of the series connection of resistors  19   a,    19   b,  similar to the configuration displayed in FIG. 4. Hence, the transistor  54  is turned on when the voltage at its emitter exceeds U 1 +U be  (U be =base-emitter voltage of transistor) and operates then as a emitter follower decoupling the diodes  22   a,    22   b  and  22   c  from the voltage divider  19   a,    19   b.  It is noted that in this particular embodiment of the inventive circuit the video input voltages U in (R), U in (G), U in (B) are limited to a maximum value equal to U 1 +U be +U d , where U d  is the diode voltage of the diodes  22   a,    22   b,    22   c.    
         [0033]    The skilled person realizes that the presence of transistor  54  is not absolutely necessary. It would also be possible to connect the cathodes of the diodes  22   a,    22   b  and  22   c  directly to the tap of the voltage divider  19   a,    19   b.  In order to keep the same threshold voltage it would only be necessary to adapt the values of resistors  19   a,    19   b  to compensate for the absence of the base-emitter voltage U be . However, this simplification of the circuit would sacrifice the decoupling effect described above.  
         [0034]    In another embodiment of the invention not shown in the drawing the diodes  22 ,  22   a,    22   b,    22   c  may be replaced by appropriate transistors. In this case the biasing voltage U 1  is used to control the base of the respective transistors to achieve the clipping of the video input signals. Thus, the threshold voltages is U 1 +U BE .