Abstract:
A dynamic focus circuit enables ideal focusing characteristics to be obtained across the entire screen of a wide-angled cathode ray tube, by generating a dynamic focus voltage having a flat-bottomed waveform through the addition of a simple circuit to a conventional analog circuit. If an S-shaping voltage method is used, the voltage of a signal induced in a secondary coil of a step-up transformer for raising the voltage of a parabolic waveform signal having a horizontal deflection period switches the gain for converting the parabolic waveform signal to a dynamic focus voltage, according to whether the induced voltage exceeds a specified reference value. If the induced voltage does not exceed the specified reference value, the parabolic waveform signal is converted to the dynamic focus voltage at a gain of less than one. If a DAF (Dynamic Focus) signal generating IC is used, a gain control voltage for controlling a gain of the parabolic waveform signal generated by the DAF signal generating IC is altered so that the gain is smaller than one at the approximate midpoint of a horizontal deflection period, and increases as it moves out to the edges of the screen.

Description:
This application is based on applications Nos. 10-284900 and 10-300565 filed in Japan, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a dynamic focus circuit generating a dynamic focus voltage, suitable for use in a wide-angled cathode ray tube (referred to as a CRT hereafter). 
     2. Description of the Related Art 
     In a CRT, a direct current (DC) voltage is typically applied as a focus voltage to a focus electrode in an electron gun. This DC voltage is produced by dividing the anode voltage to give a voltage with one quarter to one third times the magnitude. However, the distance the CRT electron beam travels to reach a screen varies between the center and the edges of the screen, so that obtaining satisfactory focus across the whole screen by applying only the DC voltage to the focus electrode is impossible. The amount of voltage applied to the focus electrode needs to be determined according to the distance between the focus electrode and a phosphor surface of the screen, that is according to the screen position at which the electron beam is focused. In a conventional CRT, voltage variations are commonly expressed by parabolic waveforms having horizontal and vertical periods. These waveforms are hereafter referred to as the horizontal and vertical parabolic waveforms. The voltage expressed by these parabolic waveforms is combined with the DC voltage and the resulting voltage is applied to the focus electrode. A circuit for generating modified horizontal and vertical parabolic waveforms so that the appropriate focus voltage can be applied to the focus electrode is known as a dynamic focus circuit. 
     One example of a dynamic focus circuit in the related art is given in Japanese Laid-Open Patent 4-117772 and  Television Gakkai Gijyutsu Hokoku  ( The Television Society&#39;s Technical Report ) Vol 17, No. 71, P 19 to 24 (published Nov. 18, 1993). The dynamic focus circuit disclosed here uses a method in which the voltage of a parabolic waveform signal generated at both ends of an S-shaping capacitor in a horizontal deflection circuit is raised directly using a step-up transformer. This method is hereafter referred to as the ‘S-shaping voltage method’. The following is an explanation of a related art example, a dynamic focus circuit using this S-shaping voltage method, with reference to the drawings. 
     FIG. 1 shows an example of a horizontal deflection circuit, and a related art dynamic focus circuit, which have been integrated to form one circuit. 
     The horizontal deflection circuit shown in the drawing includes a horizontal output transistor  901 , into the base of which a horizontal drive signal is input, a damper diode  902 , a resonance capacitor  903 , a choke coil  904 , a horizontal deflection coil  905 , a S-shaping capacitor  906 , and an alternating current (AC) coupling capacitor  909 . The dynamic focus circuit is structured so that a dynamic focus voltage can be obtained by raising the voltage of a parabolic waveform signal generated at both ends of the S-shaping capacitor  906  using a step-up transformer  908 . 
     The horizontal output transistor  901 , the damper diode  902  and the resonance capacitor  903  are connected in parallel, and the collector side of the horizontal output transistor  901  is connected to a +B power source through the choke coil  904 . The collector side of the horizontal output transistor  901  is also connected to one terminal of the horizontal deflection coil  905 . The other terminal of the horizontal deflection coil  905  is connected to the S-shaping capacitor  906 . 
     One terminal of the primary coil of the step-up transformer  908  is connected to a node  907 , where the horizontal deflection coil  905  and the upper end of the S-shaping capacitor  906  connect. The AC coupling capacitor  909  is connected to the other terminal of the primary coil of the step-up transformer  908 . 
     In the dynamic focus circuit, a vertical dynamic focus voltage waveform generating circuit  912  (hereafter referred to as the vertical voltage waveform generating circuit  912 ) and a capacitor  914  are connected to one terminal of the secondary coil of the step-up transformer  908 . The other terminal is coupled to a DC focus voltage generating circuit  913  through a resistor  910  and an AC coupling capacitor  911 . The dynamic focus circuit is structured so as to be connected to the focus electrode in the electron gun. 
     FIG. 2 shows waveforms produced in various parts of the above dynamic focus circuit. Horizontal collector pulses  921  are generated at the collector side of the horizontal output transistor  901  by the resonance of the horizontal deflection coil  905 , the choke coil  904  and the resonance capacitor  903 . A secondary integration operation of the horizontal deflection coil  905  and the S-shaping capacitor  906  generates a horizontal parabolic voltage  922  in the S-shaping capacitor  906 . When the horizontal parabolic voltage  922  is applied to the primary coil of the step-up transformer  908 , a dynamic focus voltage  923  for a horizontal deflection period is output from the secondary coil of the step-up transformer  908 . 
     A vertical dynamic focus voltage generated by the vertical voltage waveform generating circuit  912  is added to the horizontal dynamic focus voltage by being input into the other terminal of the secondary coil of the step-up transformer  908 . The dynamic focus voltage obtained is passed through the resistor  910  and the AC coupling capacitor  911  and combined with a DC voltage obtained from the DC focus voltage generating circuit  913 . The resulting voltage is then supplied to the focus electrode in the electron gun, enabling an ideal focus to be obtained across the entire screen. 
     As explained above, the related art dynamic focus circuit uses the S-shaping voltage method to obtain a dynamic focus voltage waveform by raising the voltage of a parabolic waveform signal generated at both ends of an S-shaping capacitor in a horizontal deflection circuit using a step-up transformer. 
     In recent years, however, CRTs with a wider deflection angle and less depth (hereafter referred to as wide-angled CRTs) are increasingly being used to enable space-saving display devices with large screens to be produced. When compared with a conventional device, a wide-angled CRT experiences a sharp increase in distortion between the plane at which the electron beam is focused and the surface of the phosphor layer as the electron beam moves towards the edges of the screen, if a uniform focus voltage is used. Accordingly, when a related art parabolic waveform proportional to the square of the distance from the center of the screen is used as the horizontal focus voltage waveform, obtaining satisfactory focus across the entire screen is problematic. 
     The results of our investigation into the focus characteristics of a wide-angled CRT, in light of the above problems, are shown in FIG.  3 . In the drawing, a dashed line (curve a) represents a quadratic curve of a dynamic focus voltage in the related art. A solid line (curve b) represents the dynamic focus voltage suitable for a wide-angled CRT. Curve a is proportional to the square of the distance from the center of the screen. Curve b is obtained by making a mirror image of a curve proportional to the distance from the center of the screen raised to the power of around 2.5 for the right half of the screen. This produces a curve in which the left and right halves are symmetrical. In a wide-angled CRT, the distortion between the plane at which the electron beam is focused and the surface of the phosphor layer increases sharply as the electron beam moves from the center to the edges of the screen, as shown here. As a result, a dynamic focus voltage having a waveform that is flatter than the related art in the center of the screen, and rises more steeply towards the edges of the screen (this waveform is hereafter referred to as ‘the flat-bottomed waveform’) is required to obtain satisfactory focus characteristics across the entire screen. As shown in FIG. 3, there is a large voltage difference between the two curves at either edge of the screen and in an area from 60 to 140 mm on either side of the center of the screen, so that applying a dynamic focus voltage with a conventional parabolic waveform in a wide-angled CRT leads to a deterioration in focus in these areas of the screen. 
     One possible approach to resolving this problem is suggested in Japanese Laid Open Patent No. 10-42162. Here, this document describes a dynamic focus circuit shown in FIG. 4, including a ROM  931  for storing function data, a counter  932  initialized by a synchronizing signal S, a RAM  933  for storing waveform data, and a CPU  934  for performing calculations. The CPU  934  uses function data already stored in the ROM  931  to perform computation of waveform data depending on screen positions, and stores the results in the RAM  933 . Next, the waveform data is read from the RAM  933  and output to a D/A converter  935 , which converts it from digital to analog data, and outputs a dynamic focus voltage waveform. In other words, in the above related art dynamic focus circuit, digital processing is used to generate a waveform expressed by a complex function. 
     Digitalization of circuitry in display devices using CRTs has become more common in recent years, but a method for generating a digitalized dynamic focus signal cannot easily be introduced due to cost constraints, and so analog circuits are still generally preferred. Even the dynamic focus circuits used in high-definition computer monitors are mainly analog. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a dynamic focus circuit that can obtain ideal focus characteristics across the whole screen of a wide-angled CRT device by generating a flat-bottomed waveform using an analog method. 
     The above object is achieved by a dynamic focus circuit with the following characteristics. The dynamic focus circuit obtains a dynamic focus signal from a parabolic waveform signal having a horizontal deflection period generated at both ends of an S-shaping capacitor. The dynamic focus signal is supplied to an electron gun in a cathode ray tube to focus an electron beam. The dynamic focus circuit includes a transformer and first and second converting units. The transformer has a primary coil and a secondary coil, and the parabolic waveform signal is applied to the primary coil. The first converting unit converts a signal induced in the secondary coil of the transformer to a dynamic focus signal at a gain of less than one during a first part of a horizontal deflection period. The first part of the horizontal deflection period is when the voltage of the signal induced in the secondary coil does not exceed a specified reference value. The second converting unit converts the signal induced in the secondary coil of the transformer to a dynamic focus signal at a gain greater than the gain of the first converting unit during a remainder of the horizontal deflection period. The remainder of the horizontal deflection period is when the voltage of signal induced in the secondary coil is not less than the specified reference value. 
     Suppose that an S-shaping voltage method is used with this structure. When a parabolic waveform signal is converted to a dynamic focus voltage signal after having its voltage raised using a step-up transformer, the gain when conversion is performed is switched according to whether the value for the parabolic waveform signal has exceeded a specific reference value. This transforms the parabolic waveform signal, creating a flat-bottomed waveform. Accordingly, the flat-bottomed waveform can be obtained by the addition of a simple circuit to the analog circuit in the related art. 
     The above object is also achieved by a dynamic focus circuit with the following characteristics. The dynamic focus circuit obtains a dynamic focus signal from a pulse signal having a horizontal deflection period. The dynamic focus signal is supplied to an electron gun in a cathode ray tube to focus an electron beam. The dynamic focus circuit includes a parabolic waveform generating integrated circuit (IC), and a control voltage generating circuit. The parabolic waveform generating integrated circuit (IC) includes a first circuit part that generates a parabolic waveform signal having a horizontal deflection period from the pulse signal, and a second circuit part that amplifies the parabolic waveform signal. In addition, the parabolic waveform generating IC is provided with a receiving part that receives a control voltage for controlling a gain of the second circuit part. The control voltage generating circuit outputs the control voltage to the receiving part. The control voltage then changes the gain so that the gain is less than one in a central part of the horizontal deflection period, and not less than one in the parts of the horizontal deflection period excluding the central part. 
     Another related art method for obtaining the dynamic focus waveform uses an integrated circuit (IC) used exclusively for dynamic focusing (hereafter referred to as a ‘DAF (Dynamic Focus) signal generating IC’). In the above structure, a parabolic waveform signal generated by the DAF signal generating IC is amplified and output. When this happens, the flat-bottomed waveform is obtained by successively increasing the gain while moving from the center to the edges of the horizontal deflection period. Accordingly, the flat-bottomed waveform can be obtained by the addition of a simple circuit to the analog circuit in the related art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. 
     In the drawings: 
     FIG. 1 shows an example of a unified circuit composed of a horizontal deflection circuit, and a dynamic focus circuit in the related art using a S-shaping voltage method; 
     FIG. 2 shows waveforms produced at various parts of the circuit shown in FIG. 1; 
     FIG. 3 shows the results of an experiment investigating the focus characteristics of a wide-angled CRT; 
     FIG. 4 shows a structure for a related art dynamic focus circuit using digital processing; 
     FIG. 5 shows an example of an integrated circuit composed of the dynamic focus circuit of the first embodiment, and a horizontal deflection circuit; 
     FIG. 6 illustrates the operation of a nonlinear circuit  127  when the value of the parabolic waveform signal is no less than the average value for the same; 
     FIG. 7 illustrates the operation of the nonlinear circuit  127  when the value of the parabolic waveform signal is lower than the average value for the same; 
     FIG. 8 shows the values of the parabolic waveform signal input in the nonlinear circuit  127  and the waveform for the output dynamic focus voltage in the embodiments of the present invention; 
     FIG. 9 is a block diagram showing a structure of the dynamic focus circuit in the second embodiment of the invention; 
     FIG. 10 shows another example structure for the dynamic focus circuit in the second embodiment; 
     FIG. 11 shows another example structure for the dynamic focus circuit in the second embodiment; 
     FIG. 12 shows an embodiment in which the dynamic focus circuit of the second embodiment, having a structure as shown in FIG. 10, is structured using a DAF signal generating circuit manufactured by Mitsubishi Electric Corp.; 
     FIG. 13 shows waveforms produced at various parts of the dynamic focus circuit in the second embodiment, having a structure as in FIG. 10; and 
     FIG. 14 shows the relationship between the gain control voltage input into pin  6  and the amplitude of the parabolic waveform signal output from pin  7  in the DAF signal generating IC  210  used as an embodiment of the dynamic focus circuit in the second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is a description of the embodiments of the present invention, with reference to the drawings. 
     First Embodiment 
     FIG. 5 shows an example of an integrated circuit composed of a dynamic focus circuit in the first embodiment of the invention, and a horizontal deflection circuit. Note that the part of the circuit formed by the horizontal deflection circuit, that is the part to the left of the transformer  108  in the drawing, is identical to the circuit in the related ID art. As a result, detailed explanation of this part of the circuit is omitted. 
     In the dynamic focus circuit of the present embodiment, a resistor  121  and a diode  122  are connected in parallel through a resistor  120  to one terminal of the secondary coil of a step-up transformer  108 . The other terminal of the secondary coil is connected to ground through a capacitor  114 , as well as being connected to a vertical dynamic focus voltage waveform generating circuit  112 . A diode  123  is connected to a node through a resistor  124 . The capacitor  114  and the secondary coil of the step-up transformer  108  are connected at this node. The other terminal of the diode  123  is connected to a node  126 , at which the resistor  121  and the diode  122  are connected. A dynamic focus voltage obtained at the node  126  is input into a resistor  110 , connected to a DC voltage generating circuit  113  through an AC coupling capacitor  111 , and connected to a focus electrode in an electron gun. 
     The dynamic focus circuit in the present embodiment differs from the related art in that a nonlinear circuit  127 , composed of the resistor  121 , the diodes  122  and  123 , and the resistor  124  has been added. 
     The following is an explanation of the operation of the dynamic focus circuit in the present embodiment. A voltage expressed by a parabolic waveform signal having a horizontal period (hereafter referred to as the horizontal parabolic waveform signal) is applied to the primary coil of the step-up transformer  108 . The voltage of the horizontal parabolic waveform signal is raised before it is output from the secondary coil of the step-up transformer  108 . These processes use the related art technology described using FIG. 2, so further explanation is omitted. Here the horizontal period is equivalent to one horizontal scan of the screen by the electron beam. The following explanation concentrates on the operation of the nonlinear circuit  127  connected to the secondary coil of the step-up transformer  108 , which is constructed from the resistors  121  and  124  and the diodes  122  and  123 . 
     As illustrated in FIG. 6A, when a voltage expressed by a parabolic waveform signal input into the nonlinear circuit  127  is no less than an average value (the part of the curve shown by the solid lines in the drawing) the diode  122  is ON and diode  123  OFF. The average value obtained is an average value c obtained from an integral average value theorem f(c)=1/(b−a)∫f(x)dx, Accordingly, the nonlinear circuit  127  operates as the circuit shown in  6 B and the parabolic waveform signal is output without alteration. 
     When, on the other hand, the voltage expressed by the parabolic waveform signal is less than the average value, as shown by the part of the curve drawn with a solid line in FIG. 7A, the diode  122  is OFF and the diode  123  ON. The nonlinear circuit  127  operates as the circuit shown in FIG. 7B, and a parabolic waveform signal whose level is set by the gain determined by the voltage division ratio of the resistors  120 ,  121  and  124 . 
     By using the average value of the voltage expressed by the horizontal parabolic waveform signal as a reference, and switching the gain of the nonlinear circuit  127  when the value is at least as great as the average value and when the value is less than the average value, a flat-bottomed curve with a flatter central part than the dynamic focus voltage expressed by the quadratic curve in the related art can be generated. Ideal focus characteristics can thus be obtained across the entire screen. 
     Here, actual examples of resistance values and the like occurring in the dynamic focus circuit shown in FIG. 5 are given as one way of realizing the first embodiment. In this example, the resistor  120  has a resistance of 1.2kΩ (¼W), the resistor  121  of 33kΩ (2W), resistor  124  of 49kΩ (½W) and resistor  110  of 1kΩ (½W). An RU1C diode (1000V/0.2A) manufactured by Sanken Electric Co., Ltd. is used for the diodes  122  and  123 . The capacitor  114  has a capacitance of 220 pF (1kV). 
     In the above example, since the resistances of the resistors  120 ,  121  and  124  are 1.2kΩ, 33kΩ and 49kΩ respectively, the gain of the nonlinear circuit  127  when the dynamic focus voltage is less than the average value is 49/(1.2+33+49), that is 0.59. 
     The following is an explanation of the gain adjustment method. Taking the center point of the screen as the origin, points are determined along the horizontal axis so as to divide the right half (or the left half) of the screen into four parts. If a 19-inch CRT is used, these points will be 45 mm, 90 mm, 135 mm and 180 mm from the center of the screen. The focus voltage is then adjusted so that the R, G and B focuses are ideal at each of these four points, in other words so that the spot formed by the electron beam is as small as possible. The relation between the focus voltage and screen position at these points is plotted on a graph, and an approximation curve is found. The approximation curve obtained in this way establishes the values of the dynamic focus voltage signal to be output by the nonlinear circuit  127 . The input voltage expressed by the parabolic waveform signal generated by the secondary coil of the step-up transformer  108  may be suitably adjusted via the resistance values of the resistors  121  and  124  and the like to produce the dynamic focus voltage expressed by the approximation curve. 
     FIG. 8 shows a waveform for the values of the parabolic waveform signal input into the nonlinear circuit  127  (the curve represented by a dashed line), and a waveform for the dynamic focus voltage output to the focus electrode in the electron gun (the curve represented by a solid line) in the above embodiment. It can be seen that a dynamic focus voltage with a flat-bottomed waveform is output in the horizontal deflection range, rather than the quadratic curve of the related art. The dynamic focus voltage obtained by the dynamic focus circuit in the present example has a curve proportional to the distance from the center of the screen raised for to a power of around 2.5 for the right half of the screen. A mirror image of this curve forms the curve for the left half of the screen. 
     As explained above, if the dynamic focus circuit of the present embodiment is used in a wide-angled CRT, a dynamic focus voltage waveform with a flat-bottomed shape can be generated by the addition of a simple and low-cost circuit to the related art analog circuit. This enables ideal focus characteristics to be realized across the entire screen. 
     In the nonlinear circuit  127  of the present embodiment, diodes were used to switch the gain between that when a voltage value at least as great as the average value and that when a voltage value is less than the average value, but other components such as transistors and thyristors may be used, provided that they have a switching capability. At present, such substitute components are slightly more expensive than diodes and transistors, but this may change in the future. Other components which possess impedance towards an AC power source may be used instead of the resistors. The use of components such as coils may be considered, although this may require research into the conditions necessary to avoid the generation of undesirable resonance. 
     The dynamic focus circuit in the present embodiment may be adapted to vertical deflection as well as horizontal deflection. 
     Second Embodiment 
     The following is an explanation of the second embodiment of the present invention. This embodiment describes a method for obtaining a flat-bottomed waveform using a DAF (Dynamic Focus) signal generating IC method. 
     FIG. 9 is a block diagram showing a structure for a dynamic focus circuit in the present embodiment. The dynamic focus circuit of the present embodiment uses a DAF signal generating IC  210 , and includes a parabolic waveform signal generating circuit  201 , a gain control unit  202  for controlling a gain for the parabolic waveform signal, a gain control voltage generating circuit  203  for generating a gain control voltage used to control the gain control unit  202 , and an output amplifying circuit  204  for amplifying the output voltage from the gain control unit  202  to generate a dynamic focus voltage. The DAF signal generating IC  210  in the present embodiment is composed from the parabolic waveform signal generating circuit  201  and the gain control unit  202 , as shown in the drawing. The gain control unit  202  includes a gain control circuit  207  and an amplifier  208 . 
     The following is an explanation of the operation of the dynamic focus circuit in the present embodiment. The parabolic waveform signal generating circuit  201  receives a horizontal pulse signal input and generates a horizontal parabolic waveform signal, which is then input into the gain control circuit  207  in the gain control unit  202 . The gain control circuit  207  successively changes the gain of the parabolic waveform signal according to the voltage values expressed by the parabolic waveform signal, using the gain control voltage generated by the gain control voltage generating circuit  203 . This changes the parabolic wave to a shape having a non-parabolic wave. In the present embodiment, the gain control voltage generated by the gain control voltage generating circuit  203  is not uniform, but changes in response to the voltage values expressed by a parabolic waveform signal, that is in response to screen positions along a horizontal axis. The gain control unit  202  changes the parabolic wave to a specified waveform by multiplying the gain control voltage and the parabolic waveform signal together. This means that if the gain is increased by the gain control circuit  207  as the gain control voltage rises, a situation where the gain control voltage is set at a low level in the central part of the screen and raised continuously moving towards either edge of the screen may be envisaged. However, if the gain control voltage is already formed so that it rises as it moves towards the edges of the screen, there is no need to alter it continuously. Depending on the shape of the CRT, this pulse signal can be set, for example, at LOW in the center of the screen and HIGH at the edges of the screen. 
     A circuit for outputting a gain control voltage as described above, such as a function generating circuit or similar, may be used as the gain control voltage circuit  203 . The gain control voltage is output synchronized with the horizontal pulse signal. The DAF signal generating IC  210  in the present embodiment is provided with a pin for receiving the input gain control voltage. The dynamic focus circuit of the present embodiment is realized by guiding the output from the gain control voltage generating circuit  203  to the input pin for the gain control voltage. 
     The output voltage from the gain control circuit  207  is input into the output amplifying unit  204  via the amplifier  208 . The voltage is amplified to a specified voltage in the output amplifying unit  204 , forming the dynamic focus voltage. The dynamic focus voltage is supplied to a focus electrode  205  in the electron gun of a CRT  206 . 
     FIG. 10 shows an example of another structure for the dynamic focus circuit in the present embodiment. In this example, the gain control voltage generating circuit is formed from an inverting amplifier circuit  209 , and the output voltage from an amplifier  208  is fed back into the gain control circuit  207  via the inverting amplifier circuit  209 . An example of an actual circuit with this structure will be explained later in this specification. In the example structure shown in the drawing, the DAF signal generating IC  210  inverts and outputs the parabolic waveform generated by the parabolic waveform generating circuit  201 . As a result, the inverting amplifier circuit  209  is used to obtain positive feedback. However, if the DAF signal generating IC  210  does not invert the output, there is no need to use an inverting amplifier circuit and a regular amplifying circuit may be used. This is because the gain control voltage rises continuously as it moves from the center to the edges of the screen in either case. Note that the integral power of the flat-bottomed waveform may be regulated by adjusting the gain of the inverting amplifier circuit  209 . 
     An alternative structure in which a dynamic focus voltage obtained from the output amplifying unit  204  is input into the inverting amplifier circuit  209  and fed back to the gain control circuit  207 , as shown in FIG. 11, may also be used. 
     The following is an explanation of an actual method for realizing the dynamic focus circuit of the present embodiment, using a commercial DAF signal generating IC. FIG. 12 shows an example structure for the dynamic focus circuit of the present embodiment, structured as shown in FIG. 10, using a DAF signal generating IC manufactured by Mitsubishi Electric Corp. (product number M52723SP). 
     As stated above, the DAF signal generating IC  210  in the present embodiment includes the parabolic waveform signal generating circuit  201  and the gain control unit  202  shown in FIGS. 9 to  11 . The part of the circuit in FIG. 12 that is enclosed by dashed lines corresponds to the inverting amplifier circuit  209  shown in FIG.  10 . Pin  7  of the DAF signal generating IC  210  is a terminal for outputting the parabolic waveform signal, and is connected to the output amplifying unit  204  (not shown in FIG. 12) through a resistor  223  and a coupling capacitor  224 , as well as being connected to the inverting amplifier circuit  209  via a pathway that branches off from a node  212  to which the resistor  223  is also connected. Pin  14  of the DAF signal generating IC  210  is connected to ground through resistors  221  and  222 . 
     The gain control generating circuit, here the inverting amplifier circuit  209 , is composed of a transistor  218  and a plurality of resistors. A coupling capacitor  211  is provided as an input unit. The other terminal of the coupling capacitor  211  is connected to a node, at which resistors  214  and  215  connect, through a resistor  213 . The other terminal of the resistor  214  is connected to a 12V power source and the other terminal of the resistor  215  to ground. The base terminal of the transistor  218  is connected to the node to which the resistors  213 ,  214  and  215  are connected, the emitter terminal is connected to ground through a resistor  217  and the collector terminal is connected to the 12V power source through a resistor  216 . A coupling capacitor  220  is connected to the collector terminal, forming an output unit, which is connected to the node at which the resistors  221  and  222  are connected and to pin  6  of the DAF signal generating IC  210 . 
     The following is a detailed explanation of the operation of the dynamic focus circuit in the present embodiment. 
     A horizontal parabolic waveform signal is generated by the parabolic waveform generating circuit  201  (not shown in the drawing) in the DAF signal generating IC  210  using a horizontal pulse signal input into pin  17  of the DAF signal generating IC  210 . The parabolic waveform signal is inversed and output from pin  7  of the DAF signal generating IC  210 . This is the voltage at point A in FIG.  10 . Here, the parabolic waveform signal has a peak voltage of 8.25V (quadratic wave), as shown in FIG.  13 A. This parabolic waveform signal is input into the inverting amplifier circuit  209 , where it is input into the transistor  218  through the coupling capacitor  211  and the resistor  213 . The base terminal of the transistor  218  is biased by the resistors  214  and  215 , and a horizontal parabolic waveform signal is amplified according to the ratio of the resistance values of the resistors  216  and  217 , inversed and output by the collector terminal of the transistor  218 . 
     The parabolic waveform signal output from the collector terminal of the transistor  218  is output through the coupling capacitor  220 , so that the output of the inverting amplifier circuit  209  forms the AC component of the parabolic waveform signal. This AC wave, is coupled to a DC voltage of, for example, 3V, obtained by dividing a 7V DC voltage output from pin  14  of the DAF signal generating IC  210  using the resistors  221  and  222 . The resulting voltage is fed back to pin  6  of the DAF signal generating IC  210  as the gain control voltage. This voltage occurs at point B in FIG. 10, and the parabolic waveform signal has a peak voltage of approximately 12V, as shown in FIG.  13 B. In the method of the present embodiment, the waveform of the parabolic waveform signal is actually changed in the gain control circuit  207 , as explained below. This means that neither the output signal of the inverting amplifier circuit  209  nor the gain control voltage is a parabolic wave (quadratic wave). 
     Pin  6  of the DAF signal generating IC  210  is the input pin for the gain control voltage, that is the voltage input pin controlling the amplitude of the horizontal parabolic wave. FIG. 14 shows the relation between the gain control voltage input into pin  6  of the DAF signal generating IC  210  used in the present embodiment, and the amplitude of the parabolic waveform signal output from pin  7  of the DAF signal generating IC  210 . In the DAF signal generating IC  210  of the present embodiment, a DC voltage in the range of 1.0V to 4.0V may be input, as shown in the drawing. Amplitude is controlled so that it increases as the gain control voltage rises, provided that the limit level of 8.76Vp-p is not exceeded. 
     In the method of the present embodiment, the gain control voltage input into pin  6  of the DAF signal generating IC  210  is modulated by its AC component, so that the gain applied to the parabolic waveform signal passing through the gain control circuit  207  is continuously changed. This means that gain is controlled so that it changes as the electron beam is scanned horizontally across the screen from the center to the edges, increasing amplitude. Accordingly, the voltage output from pin  7  of the DAF signal generating IC  210  is controlled so that amplitude gain in the center of the screen is small and amplitude gain at the edges of the screen is large. The voltage is thus shaped so that it is constant in the center of the screen and rises steeply at the edges of the screen. This voltage occurs at point C in FIG. 10, and forms a flat-bottomed waveform with a peak voltage of approximately 8.25V, as shown in FIG.  13 C. The voltage is amplified by the output amplifying unit  204 , becoming the voltage occurring at point D in FIG. 10, in other words a dynamic focus voltage with a peak voltage of approximately 440V, as shown in FIG.  13 D. 
     By performing the above operations, a dynamic focus voltage having a flat-bottomed waveform identical to the one in the first embodiment illustrated in FIG. 8 is output by the dynamic focus circuit in the present embodiment. This means that the dynamic focus voltage in this embodiment is a curve proportional to the distance from the center of the screen raised to a power of around 2.5 for the right half of the screen, with the left half of the screen being a mirror image of the curve for the right half of the screen. 
     As explained previously, in the dynamic focus circuit of the present embodiment, the dynamic focus voltage required to obtain an ideal focus across the entire screen of a wide-angled CRT device can be produced by the addition of a simple and low-cost circuit to the analog circuit in the related art. This means that a simple circuit composed of transistors and resistors need only be added to a related art dynamic focus circuit that uses an existing general-purpose DAF signal generating IC to enable a circuit capable of generating only related art parabolic waves to generate waveforms expressed by a complex function. 
     In the present embodiment, the DAF signal generating IC used was one manufactured by Mitsubishi Electric Corp., but any IC with similar capabilities may be used. 
     Although the present invention has been fully described by way of examples with reference to accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.