Abstract:
A digital television signal receiver includes a decoder, capable of decoding digital television signals having a plurality of different formats, which outputs video information in which image fields having mutually different numbers of scanning lines appear aperiodically when the decoder decodes a digital television signal having a specific one of the different formats; a display device which displays an image based on the video information output from the decoder; and an image controller which sets respective display start positions for the image fields having mutually different numbers of scanning lines to a same display start position in a vertical direction when the display device displays an image based on the video information output from the decoder when the decoder decodes the digital television signal having the specific format.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/139,116 filed on Aug. 24, 1998 now U.S. Pat. No. 6,288,748, the contents of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to a display device for showing video information whose number of scanning lines changes over elapsed time per each field, and relates in particular to a display device which is ideal for preventing vertical jitter from occurring during the display of video information having changes in the number of scanning line that occur at random. This invention is suited for display, for instance, of digital broadcasts, or joint display of both digital broadcasts and analog broadcasts. 
     2. Description of the Related Art 
     Digital technology has been making steady progress in the area of broadcasting in recent years. In order for display equipment such as, for instance, television receivers to keep pace with this progress, the display equipment must be able to display digital broadcast signals as well as analog broadcast signals and VTR playback signals. Circuit systems are therefore being designed to make receivers compatible with digital broadcasts. A decoded digital broadcast signal is therefore written temporarily into a field memory and then utilized in a readout system. Analog signals of the currently used NTSC method are subjected to analog/digital (A/D) conversion and then input to a digital signal processing circuit and written along with the digital broadcast signal into a field memory. 
     The writing and reading of video information into the field memory is performed by utilizing the read block and vertical and horizontal synchronizing signals. However, when the digital broadcast signal is, for instance, an MPEG2 type signal and has been compressed, no synchronizing signal is sent from the transmitter. Accordingly, a synchronizing signal must be generated and a read signal formed by the receiver in order to read out the information from the field memory. However, a difference in frequency sometimes occurs between the write signal for writing into the field memory, and the read signal for readout from the field memory. This difference in frequency results in reversed timing or an opposite phenomenon called “skip” occurring between the memory writing and readout operations. 
     When this memory skip occurs, the current field image on one frame of the screen and the image from one previous field are mixed together, thus requiring some contrivance to prevent memory skip from occurring. The changes particularly in the vertical frequency are particularly large in cases such as the custom playback of VTR signals. One method to prevent memory skip from occurring at such times is by changing the number of scanning lines per field. One example of this is television receivers in Europe capable of receiving broadcast teletext or subscript transmissions. These receivers are set to have alternate scanning lines of 312 lines by 313 lines per field (PAL etc.) or 262 lines by 263 lines (NTSC) with non-interlaced scanning used for display of the teletext broadcast screen. 
     However, a feedback circuit for uniform vertical amplitude is provided in the vertical deflection circuit of the CRT display. Consequently, a vertical jitter at 60 Hz occurs when attempting to display video information in which the alternate scanning lines are 312 lines by 313 lines (or 262 by 263 lines) per field on the CRT display. A technology that has been proposed to reduce this vertical jitter is disclosed in Japanese Examined Utility Model Publication No. 7-44130. This method isolates the circuit for aligning vertical amplitude and the feedback circuit, and utilizes a differentiator circuit consisting of a resistor and capacitor to cut the DC components and apply feedback signals to the vertical amplifier amplitude control terminal. 
     However, in video information formed for a system designed for joint use of conventional analog broadcasts and expanding systems utilizing MPEG 2  for handling digital broadcasts, the number of scanning lines will not always mutually increase and decrease per field. In such cases, the increase or decrease in the number of scanning lines per field will be a factor appearing randomly in the synchronizing signal status of the input signal. Consequently, vertical jitter will still be difficult to suppress, even if the technology disclosed in the above patent is adopted in the above system, since the system permits creation of video information in which changes in the number of scanning lines per field appear at random. 
     SUMMARY OF THE INVENTION 
     This invention, which takes the above problems into account, has the objective of providing a display capable of satisfactory suppression of vertical jitter, even when displaying video information in which changes in the number of scanning lines per field appear at random. 
     In order to achieve the above mentioned objectives, this invention is characterized in being comprised of a receiving means to receive digital broadcast, a decoding means to decode the digital broadcast signal from the receiving means and then output video information containing at least a first and a second field with a mutually different number of scanning lines, a display means to display images utilizing the video information output from the decoding means, and a display control means for setting the same start position for the first and the second fields on the screen of the display means. 
     The display unit of this invention is comprised of a memory for storing the video information output from the decoding means, a clock generator to generate a read clock for readout of video information stored in the memory, and a D/A (digital/analog) converter to perform digital to analog conversion of the video information read out from the memory and supply the converted information to the display means. 
     The display control means may also be comprised of a vertical ramp waveform generator circuit to generate a vertical ramp waveform for performing vertical deflection, and a clamp circuit to maintain a uniform voltage for the first field and the second field that corresponds to the vertical retrace period of the vertical ramp waveform formed in the vertical ramp waveform generator circuit. 
     This vertical ramp waveform generator circuit operates in synchronization with a vertical synchronizing signal and may contain a switch to charge the capacitor with electrical current from the power supply during the vertical scanning period and to discharge the capacitor during the vertical retrace period. 
     The clamp circuit is synchronized with the vertical synchronizing signal and may include a switch to link the fixed DC voltage to the vertical ramp waveform. 
     The vertical ramp waveform generator circuit is further comprised of a counter to count the read clock generated by the clock generator, and a D/A converter to perform digital to analog conversion of the output signal from the counter and output a vertical ramp waveform. The clamp circuit may include a reset circuit to reset the count from the counter according to the vertical synchronizing signal. 
     The display control means may be comprised of a vertical ramp waveform generator circuit, a comparator to compare a reference voltage corresponding to a reference number of scanning lines per one field versus a voltage corresponding to a number of scanning lines for the first field and the second field read out from the memory, and output a control signal according to the comparison results, and a variable voltage power supply for controlling the direct current component of the vertical ramp waveform for performing vertical deflection according to the control signal output from the comparator. 
     The display control means may be comprised of a vertical ramp waveform generator means to generate a vertical ramp waveform for performing vertical deflection, and an amplifier control means to control feedback of the amplitude of the vertical ramp waveform generated in the vertical ramp waveform generator means. 
     The amplitude control means includes a rectifier to rectify the vertical ramp waveform, and if the rectifier time constant is set at 80-120 ms, then there is no follow-up (slaving) to changes in the field frequency between a plurality of fields. 
     The amplitude control means may also be provided with a rectifier to rectify the vertical ramp waveform, and a time constant adjustment circuit to adjust the time constant of the rectifier according to the status of the display unit. 
     This time constant adjustment circuit may adjust the time constant when power to the display device is turned on in order to make the time constant smaller than in normal operation. In such a case, the time constant may be set at 10 ms or less at the time when the power to the display device is turned on, and set at 400-600 ms during normal operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an essential portion of the display device of this invention. 
     FIG. 2 is a circuit schematic showing a specific example of an image display control unit of the first embodiment of this invention. 
     FIG. 3 is a waveform chart illustrating operation of the circuit shown in FIG.  2 . 
     FIG. 4 is a circuit diagram showing an example of the digital television of the first embodiment of this invention shown in FIG.  2 . 
     FIG. 5 is a circuit diagram showing an example of the image display position control unit of the second embodiment of this invention. 
     FIG. 6 is a waveform diagram illustrating the operation of the circuit shown in FIG.  5 . 
     FIG. 7 is a circuit diagram showing an example of the image display position control unit of the third embodiment of this invention. 
     FIG. 8 is a waveform diagram illustrating the operation of the circuit shown in FIG.  7 . 
     FIG. 9 is a circuit diagram showing an example of the image display position control unit of the fourth embodiment of th is invention. 
     FIG. 10 is a circuit diagram showing an example of the image display position control unit of the fifth embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will now be described with reference to the accompanying drawings. 
     FIG. 1 is a block diagram of an essential portion of the display device of this invention. The display device is mainly comprised of a frame memory  131 , a D/A converter  132 , a synchronizing signal generator  130 , an image display position control unit  152 , and an image display unit  155 . In the frame memory  131 , a digitized image such as a video signal for 1 field is assigned to and written at an address corresponding to a horizontal dot and a vertical line. The synchronizing signal generator  130  is input with a read clock (CK), and generates a horizontal synchronizing signal (H sync) and a vertical synchronizing signal (V sync). The CK, H sync, and V sync are input to the frame memory  131  and the pulses are then counted by an address counter contained in, for instance, the frame memory  131 . The count from the address counter is utilized as address information, digital data of images from the field memory address corresponding to this address information is read out, and the result is converted to an analog video signal (YUV or RGB) by way of the D/A converter  132  and input to the image display unit  155 . Further, the H sync and V sync signal generated in the synchronizing signal generator  130  are also input to the image display position control unit  152  and serve to control the display position of the video of the image display unit  155 . 
     The physical conditions of the display device utilized in the display device configured as described above, and the relation of the fields utilized in this display device versus the number of scanning lines per field, are shown in Table 1. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Item 
                   
                 Field No. 
               
             
          
           
               
                 No. 
                 Conditions 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                   
               
             
          
           
               
                 1 
                 1080 i, 30 fps 
                 562.5 
                 562.5 
                 562.5 
                 562.5 
                 562.5 
                 — 
               
               
                 2 
                 1080 i, 29.97 fps 
                 562.5 
                 563.5 
                 562.5 
                 562.5 
                 562.5 
                 — 
               
               
                 3 
                 540 P, 60 fps 
                 562 
                 563 
                 562 
                 563 
                 562 
                 — 
               
               
                 4 
                 540 P, 59.94 fps 
                 563 
                 563 
                 562 
                 563 
                 562 
                 — 
               
               
                   
               
             
          
         
       
     
     As shown in Item No. 1, when using a display with a total of 1125 scanning lines and an interlaced display with 1080 effective scanning lines at a vertical synchronizing frequency of 60 Hz in which case the frame frequency will be 30 fps, the scanning lines per field will be a repetitive 562.5 lines. Here, fps is an abbreviation signifying units of frames per second, i is skip scanning (interlaced scanning), and p is sequential scanning (progressive scanning). In a digital type high vision system, the vertical synchronizing frequency may be 59.94 Hz in which case the frame frequency will be 29.97 fps. As shown in Item No. 2, in this case a field having 562.5 scanning lines is present along with a field having 563.5 scanning lines for handling a frame frequency that averages 29.97 fps. Further, when using a non-interlaced display of 540 effective scanning lines, a total of 1125 scanning lines, and a vertical synchronizing frequency of 60 Hz in which case the frame frequency will be 60 fps, the scanning lines per field will be a repetitive 562 by 563 lines per field as shown in Item No. 3. Additionally, when the vertical synchronizing frequency is 59.94 Hz in which case the frame frequency will be 59.94 fps under the same conditions for effective scanning lines and the scanning system, then, as shown in Item No. 4, the number of fields having 562 scanning lines is fewer than shown in Item No. 3 (the ratio of the number of fields having 563 scanning lines to the number of fields having 562 scanning lines is smaller than in Item No. 3), and the frame frequency can be set to an average of 59.94 fps. In this kind of system, the read clock can be just a single frequency and the cost will be economical. 
     In this system, the number of scanning lines per one field changes over elapsed time so that accurate control of the video image position with the image display position control unit  152  is necessary. A specific example of this image display position control unit  152 , in other words, the first embodiment of this invention, is shown in FIG.  2 . 
     FIG. 2 is a circuit diagram showing the image display position control unit  152 , and in particular shows the vertical deflection circuit operated by V sync. This circuit is comprised of a vertical ramp waveform generator  1  to generate a vertical ramp waveform to drive the vertical deflection coil  4 , a clamp circuit  2  to clamp the voltage for the vertical retrace period of the vertical ramp waveform, an amplifier  3  for amplifying the vertical ramp waveform, a vertical deflection coil  4  for performing vertical deflection of the electron beam, resistors  5  and  6 , and a cathode ray tube  7 . Also, the vertical ramp waveform generator  1  is comprised of a power supply terminal  11 , a current source  12 , a first switch element (SW 1 )  13 , and a capacitor  14 . The clamp circuit  2  is comprised of a resistor  21 , a second switch element (SW 2 )  22 , and a power supply  23 . The operation of this circuit is described using FIG.  3 . 
     The operation of the vertical ramp waveform generator  1  is first explained. The waveform  31  of FIG. 3 is for V sync and is input to the first switch element  13 . The terminal voltage at the capacitor  14  is zero (0) when the V sync is high (period  36  ) and the first switch element  13  is on during the vertical retrace period. On the other hand, the first switch element  13  is off when the V sync is low (period  33  ) and the capacitor  14  is charged by the current source  12 . Consequently, the voltage at the terminal of the capacitor rises at a fixed slope, and a ramp waveform having a fixed slope is generated as shown by  32  in FIG.  3 . The V sync then returns to a high level (period  37  ) and the first switch element  13  closes, the charge stored in the capacitor  14  is discharged, and the voltage at the capacitor terminal becomes zero. The process continually repeats in order to generate the ramp waveform. 
     The slope for the period  33  of the ramp waveform  32  and the slope for the period  34  are identical, but a difference  35  occurs in the peak values when the period  34  is longer than the period  33  (in other words, when the number of scanning lines between adjacent fields is different). The difference in peak values causes a voltage fluctuation in the vertical retrace period of the vertical ramp waveform. When the voltage fluctuates during this period, the display position on the screen varies up and down (vertically) and the so-called vertical jitter occurs. In order to prevent this vertical jitter, this invention provides a clamp circuit to fix the voltage in the vertical retrace period of the vertical ramp waveform. The operation of this clamp circuit is discussed next. 
     The V sync shown in the waveform in FIG. 3 also receives an input from the second switch element  22 . The second switch element  22  closes when the V sync is high, in the vertical retrace period (periods  36 ,  37 ) and the direct current voltage output from the power supply  23  is coupled with the vertical ramp waveform  32  output from the vertical ramp waveform generator circuit  1  by way of the resistor  21 . Also, the second switch element  22  opens in the period when the V sync is low (periods  33 ,  34 ) and the coupling between the vertical ramp waveform and the power supply  23  is eliminated. Therefore, even if a fluctuation in the peak value of the vertical ramp waveform occurs, the voltage in the vertical retrace period can be clamped to a fixed value by means of the direct current voltage output from the power supply  23  so that voltage fluctuations within the period can be prevented. 
     This clamped vertical ramp waveform is input to the amplifier  3  and current conversion is performed in the vertical deflection coil  4  by way of the vertical deflection coil  4  and the resistors  5 ,  6 . Accordingly, the vertical start position of the cathode ray tube  7  can be constantly maintained at a fixed position. As a result, even when the number of scanning lines per field has changed, the scanning start position of the electron beam will be maintained at a constant fixed position so that a satisfactory screen image without vertical jitter can be obtained. 
     FIG. 4 is a circuit diagram showing an example of the image display position control unit adopted for digital television in the embodiment of this invention shown in FIG.  1  and FIG.  2 . In FIG. 4,  101  denotes the input terminal for a received digital broadcast signal,  102  denotes the input terminal for video and audio signals of the NTSC method,  103  is the input terminal for the AC power supply for inputting 100 volts AC, for instance, in Japan,  110  denotes the decoder mainly for decoding the video signals and audio signals compressed with the MPEG 2  method,  111  denotes the digital broadcast tuner,  112  is the demodulator,  113  is the error correction circuit,  114  denotes the demultiplexer,  115  denotes the audio data buffer,  116  denotes the audio decoder,  117  denotes the video data buffer,  118  is the video decoder,  119  denotes the system data buffer,  120  denotes the system decoder,  121  is the time control counter,  122  denotes the system clock generator for generating a main clock pulse for the decoder  110 ,  123  denotes the oscillator,  124  denotes the first control circuit such as a microprocessor for controlling each section mainly in the decoder  110 ,  125  denotes the custom readout memory (hereafter referred to as a ROM) for storing the basic control information of each section of the decoder  110 , numeral  126  denotes the nonvolatile read/write memory (hereafter referred to as a RAM),  127  denotes a first switch,  128  denotes the audio signal processor,  129  denotes a second switch,  130  denotes a synchronizing signal generator for generating a horizontal synchronizing signal as well as a vertical synchronizing signal, etc., the numeral  131  denotes a display memory for interpolation processing of the digital video signal that was input and for performing luminance level control of the frame signal when a frame signal has been input, the numeral  132  denotes a D/A converter,  141  denotes an A/D converter and  142  denotes a signal processing circuit for processing signals such as by Y/C isolation and A/D conversion of analog video and audio signals of the NTSC method input from the terminal  102 , numeral  150  denotes a display device for displaying video signals formed by specified processing,  151  denotes a video signal processor,  152  denotes the deflection circuit,  153  denotes the digital convergence circuit,  154  denotes the brightness control circuit,  155  denotes the CRT for the 16:9 aspect ratio,  161  denotes the second control circuit such as a microprocessor for performing control of the overall system of the video signal display unit of this embodiment,  171  denotes the power supply circuit for generating DC power from the 100 volts AC supplied from the input terminal  103 ,  172  denotes the regulator,  173  denotes the detector circuit for monitoring the input/output of the power supply  171  and the regulator  172  and detecting power supply abnormalities, numeral  174  denotes the power supply switch for cutting off AC input power from the input terminal  103 , and numeral  181  denotes the speaker for outputting the audio. The frame memory  131 , the D/A converter  132 , the synchronizing signal generator  130 , and the image display unit  155  shown in FIG. 1 respectively correspond to the display memory  131 , the D/A converter  132 , the synchronizing signal generator  130 , and the CRT  155  in FIG. 4, and the read clock signal CK input to the synchronizing signal generator  130  in FIG. 1 is formed based on the system clock from the system clock generator circuit  122 . Further, the image display position control unit  152  explained using FIG.  1  and FIG. 2 is contained inside the deflection circuit  152  in FIG.  4 . 
     FIG. 5 is a circuit diagram showing an example of the image display position control unit of the second embodiment of this invention. The circuit of the image display position control unit is comprised of a vertical ramp waveform generator  41 , an amplifier  3 , a vertical deflection coil  4 , resistors  5  and  6 , and a cathode ray tube  7 . The vertical ramp waveform generator  41  is comprised of a reset element  42 , a counter  43 , and a D/A converter  44 . Circuit operation is explained while referring to FIG.  6 . When the clock signal (for instance, the read clock CK) is input to the counter  43 , the pulses of the clock signal are counted in the scanning period  49  and a waveform  46  is output. The waveform  46  is input to the D/A converter  44  and the analog conversion results are output as a waveform such as shown by  52  in FIG.  6 . At this time, a difference in the clock count for each period occurs when the period  51  is greater than the period  49  (in other words, when the number of scanning lines for adjacent fields is different). Consequently, a difference  47  occurs between the peak value of the waveform for the field in the period  49  and the peak waveform for the field in the period  51 . However, in this embodiment, the reset element  42  triggers reset of the counter  43  in the periods  48  and  50  when the V sync signal  45  is high so that the voltage for the applicable period is constantly kept at a value corresponding to the count reset value, and the voltage of the vertical retrace period is constantly maintained at this value. The ramp waveform having this direct current information is current converted in the vertical deflection coil  4  by way of the deflection coil  4  and the resistors  5 ,  6 . Consequently, the electron beam of the cathode ray tube  7  can be constantly maintained at the vertical start position. As a result, even if a change occurs in the number of horizontal scanning lines per field, a fixed scanning start position for the electron beam is constantly maintained so that a satisfactory screen without jitter can be obtained. The circuit described here is of course applicable to the system in FIG.  4 . 
     FIG. 7 is a circuit diagram showing an example of the image display position control circuit of the third embodiment of this invention. The circuit of the image display position control circuit is comprised of a ramp generator  61 , an amplifier  3 , a vertical deflection coil  4 , resistors  5  and  6 , a coupling capacitor  70 , a cathode ray tube  7 , a variable voltage power supply  63 , and a comparator  62 . The circuit operation is explained while referring to FIG.  8 . The ramp generator  61  input with the V sync signal is a ramp generator of the capacitor-discharge type of the conventional art. The portion of the image display position control unit comprised of an amplifier  3 , a vertical deflection coil  4 , resistors  5  and  6 , a coupling capacitor  70 , and a cathode ray tube  7  is a typical AC coupling vertical deflection circuit. When a V sync signal in which the period  69  is greater than the period  68  is input to the ramp generator  61 , the current flowing in the vertical deflection coil  4  forms the ramp waveform  65 . On the other hand, the comparator  62  is input at one terminal with a voltage equivalent, for instance, to 562 horizontal scanning lines per field as a reference voltage Href. At the other terminal, an input voltage Hcount is equivalent to the number of horizontal scanning lines per field (for instance, 563 lines) read out from the field memory. The voltages at both inputs are then compared and a control signal is output as shown by the waveform  67  in FIG.  8 . This control signal is high for fields with a large number of scanning lines, in other words, when Href is less than Hcount. For all other fields, the signal is low, for instance, when Href is greater than Hcount, or when Href equals Hcount. By inputting this control signal to the variable voltage power supply  63 , the amount of current bypass of the vertical deflection coil  4  can be regulated. Accordingly, the electrical current value of the vertical deflection coil  4  can be compensated (offset) to match the dotted line  66  for the period  69 . Consequently, even if the number of horizontal scanning lines per field is changed, a satisfactory screen image without jitter can be achieved. This circuit arrangement can also of course be applied to FIG.  4 . 
     FIG. 9 is a circuit diagram showing the image display position control unit  152  of the fourth embodiment of this invention. The circuit of the image display position control unit is comprised of a ramp generator  81 , amplifiers  3 ,  82 ,  85 ,  88 , a vertical deflection coil  4 , resistors  5 ,  6 ,  83 ,  84 , a capacitor  87 , a cathode ray tube  7 , a variable voltage power supply  89 , and a diode  86 . Next, the circuit operation will be explained while referring to FIG. 9. A clamped vertical ramp waveform is output at the output of the ramp generator  81  which is input with the V sync signal. When this signal output is input to a buffer circuit comprised of an amplifier  82  and resistors  83  and  84 , a vertical ramp waveform is output at both ends of the resistor  84 . A direct current voltage can be obtained according to the amplitude of the vertical ramp waveform by utilizing a rectifier circuit comprised of the amplifier  85 , the diode  86 , and the capacitor  87  on the voltage at both ends of the resistor  84 . This direct current voltage value is input to an amplitude adjustment terminal of the ramp generator  81  by way of the amplifier  88  so that the amplitude of the vertical ramp waveform is always maintained at a fixed value. The variable voltage power supply  89  is connected to one side of the amplifier  88  to permit amplitude adjustment. In this circuit, a large value is set for the capacitor  87  and if a long time constant of, for instance, 80-120 ms or more preferably looms is set, then there is no follow-up of changes in field frequency for an interval of five to six fields. Consequently, the start position of the electron beam of the cathode ray tube  7  can be constantly maintained at a fixed location. Further, the raster size can also be maintained at a fixed value even if changes occur in the field frequency reference. Accordingly, there will be no change in screen size even in cases such as custom VTR playback. Also, even if changes occur over a short time in the number of horizontal scanning lines per field, the scan start position of the electron beam is constantly held at a fixed position so that a satisfactory image without jitter is obtained. 
     FIG. 10 is a circuit diagram showing an example of the image display position control unit of the fifth embodiment of this invention. This circuit is basically the circuit of FIG. 9 to which a time constant stabilizer circuit  97  has been added. In contrast to the circuit operation of FIG. 9, when the value of the capacitor  87  has been increased in order to obtain a sufficient time constant, the time until the vertical ramp waveform amplitude is sufficient becomes long, and the load on the cathode ray tube  7  is therefore large versus the on and off operation of the power supply. The time constant stabilizer circuit  97  is utilized in order to improve this situation. The circuit operation is next described. The time constant stabilizer circuit  97  is comprised of a transistor  96 , resistors  94  and  95 , a diode  92 , a capacitor  93 , and a power supply terminal  91 . The power supply terminal  91  has zero volts and the charge on the capacitor  93  is zero with the power supply off. When the power supply is on, or in other words when it is turned on, power is supplied to the power terminal  91  and the capacitor  93  is charged by way of the resistor  94 . At this same time, the transistor  96  turns on and the capacitor  87  is rapidly charged by way of the resistor  95  which speeds up the time needed for the vertical ramp waveform to reach the correct amplitude. In other words, when the power supply is turned on, the time needed to charge the capacitor  87  is less than in normal operation (for instance, 10 ms or less) and the follow-up time is shortened. 
     When the capacitor  93  has completely charged, the emitter and base of the transistor  96  are at the same electrical potential, and the transistor reaches cutoff status with no effect on the operation of the capacitor  87 . Compared to the circuit status of the fourth embodiment shown in FIG. 9, the capacitor  87  of the circuit of FIG. 10 is set to an even larger value, the time constant is long and if, for instance, the time constant is set to 400-600 ms or more preferably to approximately 500 ms, then there is no follow-up of changes in field frequency over 50 to 60 fields so that just as in the first embodiment, the vertical start position of the electron beam of the cathode ray tube  7  can be constantly maintained at a fixed position. Further, the raster size can also be maintained at a fixed value even if changes occur in the field frequency reference. Accordingly, there will be no change in screen size even in cases such as custom VTR playback. Also, even if changes occur over a short time in the number of horizontal scanning lines per field, the scan start position of the electron beam is constantly held at a fixed position so that a satisfactory image without jitter is obtained. 
     As related in the above explanation, in this invention, even during display of video information in which irregular changes in the number of field scanning lines occur, the vertical jitter can be satisfactorily suppressed. 
     Although the above described embodiments represent preferred forms of the invention, it should be understood that modifications and changes will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of this invention is therefore to be determined solely by the appended claims.