Abstract:
First, second, and third light sources serve to emit light having three primary colors, respectively. The first light source is activated by a first drive pulse which has a first width and which repetitively occurs at a specified frequency. The second light source is activated by a second drive pulse which has a second width and which repetitively occurs at the specified frequency. The third light source is activated by a third drive pulse which has a third width greater than the first and second widths and which repetitively occurs at the specified frequency. Time positions of front edges of the first, second, and third drive pulses are different. The first drive pulse occupies a time range contained in a time range for which the third drive pulse extends. The second drive pulse occupies a time range contained in the time range for which the third drive pulse extends.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a light source device for a video display. In addition, this invention relates to a method of driving a light source device for a video display. 
         [0003]    2. Description of the Related Art 
         [0004]    Some video displays include a back light formed by a light source device composed of red (R), green (G), and blue (B) light sources. It is known to use LEDs (light emitting diodes) for a back light. Usually, a red LED array, a green LED array, and a blue LED array constitute a back light. 
         [0005]    In conventional back lights, LEDs are driven by pulse signals respectively, and the intensities of light emitted from the LEDs are adjusted by controlling the pulse widths, the pulse numbers, or the pulse voltages (the pulse heights) concerning the pulse signals. 
         [0006]    Japanese patent application publication number 2004-333576 discloses a light source unit for a picture display. The light source unit in Japanese application 2004-333576 includes red, green, and blue LED arrays which are driven by pulse signals. The pulse widths concerning the pulse signals can be changed. According to a first example, the activations of the red, green, and blue LED arrays are on a time sharing basis. According to a second example, the moments of start of every activation of the red and green LED arrays are the same and are preceded by the moment of start of corresponding activation of the blue LED array, and the moments of end of every activation of the green and blue LED arrays are the same and are followed by the moment of end of corresponding activation of the red LED array. In the second example, there are a time range for which all the red, green, and blue LED arrays are activated, a time range for which only the blue LED array is activated, and a time range for which only the red LED array is activated. In the second example, the time range of every other activation of the green LED array is contained in the time rage during which the red LED array is activated and the time range during which the blue LED array is activated. 
         [0007]    In general, red, green, and blue LEDs are different in light emission efficiency. Specifically, the light emission efficiency of the blue LED is lower than those of the red and green LEDs. Accordingly, in the case where the red, green, and blue LEDs are driven by same-frequency same-amplitude pulse signals respectively and the intensities of light emitted therefrom are required to be equal, it is necessary that the pulse width concerning the pulse signal for the blue LED is greater than those concerning the pulse signals for the red and green LEDs. In this case, the blue LED continues to be activated to emit light for every time interval longer than those during which the red and green LEDs remain activated. Such extra-time light emission from the blue LED may cause color breaking when the frequency of the pulse signals is in a certain range. The color breaking means a phenomenon in which non-existent color is observed in an outline of an image indicated by a display, or the tone of an image portion is seen as separate colors. 
         [0008]    Japanese patent application publication number 2001-174782 discloses a color display including a liquid-crystal display panel and a back light. In Japanese application 2001-174782, the back light uses a fluorescent lamp designed to emit light having at least two of red, green, and blue components. The liquid-crystal display panel and the back light are driven by pulse signals respectively. To control a color tone of light outputted through the liquid-crystal display panel, the phase difference between the pulse signals is adjusted. Japanese application 2001-174782 teaches that the back light may use LEDs instead of the fluorescent lamp. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of this invention to provide a light source device for a video display which suppresses color breaking. 
         [0010]    It is another object of this invention to provide a method of driving a light source device for a video display which suppresses color breaking. 
         [0011]    A first aspect of this invention provides a method of driving a light source device for a video display. The light source device includes a first light source for emitting light having a first primary color, a second light source for emitting light having a second primary color different from the first primary color, and a third light source for emitting light having a third primary color different from the first and second primary colors. The method comprises the steps of activating the first light source by a first drive pulse which has a first width and which repetitively occurs at a specified frequency; activating the second light source by a second drive pulse which has a second width and which repetitively occurs at the specified frequency; and activating the third light source by a third drive pulse which has a third width greater than the first and second widths and which repetitively occurs at the specified frequency. Time positions of front edges of the first, second, and third drive pulses are different. The first drive pulse occupies a time range contained in a time range for which the third drive pulse extends, and the second drive pulse occupies a time range contained in the time range for which the third drive pulse extends. 
         [0012]    A second aspect of this invention is based on the first aspect thereof, and provides a method wherein time positions of centers of the first, second, and third drive pulses are equal. 
         [0013]    A third aspect of this invention is based on the first aspect thereof, and provides a method wherein time positions of rear edges of the first, second, and third drive pulses are different. 
         [0014]    A fourth aspect of this invention provides a light source device for a video display which comprises a first light source for emitting light having a first primary color; a second light source for emitting light having a second primary color different from the first primary color; a third light source for emitting light having a third primary color different from the first and second primary colors; means for activating the first light source by a first drive pulse which has a first width and which repetitively occurs at a specified frequency; means for activating the second light source by a second drive pulse which has a second width and which repetitively occurs at the specified frequency; and means for activating the third light source by a third drive pulse which has a third width greater than the first and second widths and which repetitively occurs at the specified frequency. Time positions of front edges of the first, second, and third drive pulses are different. The first drive pulse occupies a time range contained in a time range for which the third drive pulse extends, and the second drive pulse occupies a time range contained in the time range for which the third drive pulse extends. 
         [0015]    A fifth aspect of this invention is based on the fourth aspect thereof, and provides a light source device wherein time positions of centers of the first, second, and third drive pulses are equal. 
         [0016]    A sixth aspect of this invention is based on the fourth aspect thereof, and provides a light source device wherein time positions of rear edges of the first, second, and third drive pulses are different. 
         [0017]    A seventh aspect of this invention is based on the fourth aspect thereof, and provides a light source device wherein each of the first, second, and third light sources comprises an array of LEDs. 
         [0018]    An eighth aspect of this invention provides a method of driving a back light device for a liquid-crystal display. The back light device includes a first light source for emitting light having a first color, and a second light source for emitting light having a second color different from the first color. The method comprises the steps of activating the first light source by a first drive pulse which has a first width and which repetitively occurs at a specified frequency; and activating the second light source by a second drive pulse which has a second width greater than the first width and which repetitively occurs at the specified frequency; wherein time positions of front edges of the first and second drive pulses are different, and time positions of rear edges of the first and second drive pulses are different, and wherein the first drive pulse occupies a time range contained in a time range for which the second drive pulse extends. 
         [0019]    A ninth aspect of this invention is based on the eighth aspect thereof, and provides a method wherein each of the first and second light sources comprises an array of LEDs. 
         [0020]    A tenth aspect of this invention is based on the eighth aspect thereof, and provides a method wherein time positions of centers of the first and second drive pulses are equal. 
         [0021]    An eleventh aspect of this invention provides a back light device for a liquid-crystal display. The back light device comprises a first light source for emitting light having a first color; a second light source for emitting light having a second color different from the first color; means for activating the first light source by a first drive pulse which has a first width and which repetitively occurs at a specified frequency; and means for activating the second light source by a second drive pulse which has a second width greater than the first width and which repetitively occurs at the specified frequency; wherein time positions of front edges of the first and second drive pulses are different, and time positions of rear edges of the first and second drive pulses are different, and wherein the first drive pulse occupies a time range contained in a time range for which the second drive pulse extends. 
         [0022]    A twelfth aspect of this invention is based on the eleventh aspect thereof, and provides a back light device wherein each of the first and second light sources comprises an array of LEDs. 
         [0023]    A thirteenth aspect of this invention is based on the eleventh aspect thereof, and provides a back light device wherein time positions of centers of the first and second drive pulses are equal. 
         [0024]    This invention has advantages indicated below. Since the time positions of the front edges of the first, second, and third drive pulses are different, the sum of the electric powers consumed by the first, second, and third light sources gradually increases to the maximum value. Therefore, the load applied to a power supply for the first, second, and third light sources gradually increases to the maximum level. Basically, such a gradually-increasing applied load is acceptable to the power supply. In the case where the time positions of the rear edges of the first, second, and third drive pulses are different, it is possible to suppress observable color breaking in an image indicated by the video display. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a block diagram of a video display including a back light device according to a first embodiment of this invention. 
           [0026]      FIG. 2  is a time-domain diagram showing a first example of the waveforms of a vertical sync signal, and PWM signals for driving red, green, and blue LED arrays in  FIG. 1 . 
           [0027]      FIG. 3  is a time-domain diagram showing a second example of the waveforms of the vertical sync signal and the PWM signals. 
           [0028]      FIG. 4  is a time-domain diagram showing variations in the electric powers consumed by the red, green, and blue LED arrays, the sum of the consumed electric powers, the luminance provided by the light applied from the back light device to a display panel in  FIG. 1 , and the color of the light applied from the back light device to the display panel which occur in the case where the waveforms of the vertical sync signal and the PWM signals, and the phase relation thereamong are in the conditions of  FIG. 3 . 
           [0029]      FIG. 5  is a time-domain diagram showing a third example of the waveforms of the vertical sync signal and the PWM signals. 
           [0030]      FIG. 6  is a time-domain diagram showing variations in the electric powers consumed by the red, green, and blue LED arrays, the sum of the consumed electric powers, the luminance provided by the light applied from the back light device to the display panel in  FIG. 1 , and the color of the light applied from the back light device to the display panel which occur in the case where the waveforms of the vertical sync signal and the PWM signals, and the phase relation thereamong are in the conditions of  FIG. 5 . 
           [0031]      FIG. 7  is a time-domain diagram showing the third example of the waveforms of the PWM signals. 
           [0032]      FIG. 8  is a time-domain diagram showing a fourth example of the waveforms of the vertical sync signal and the PWM signals. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
       [0033]    With reference to  FIG. 1 , a video display includes a display panel  11 , and a back light device  12  for illuminating the display panel  11 . The display panel  11  is, for example, a color liquid-crystal display panel having a color filter. Basically, the back light device  12  is designed to apply white light to the display panel  11 . A video signal having red (R), green (G), and blue (B) signals and vertical and horizontal sync signals is fed to the display panel  11  and the back light device  12 . 
         [0034]    The back light device  12  includes a control circuit  13  and a light source unit  14 . The control circuit  13  has a timing circuit  15  and PWM (pulse-width modulation) signal generators  16 R,  16 G, and  16 B. The light source unit  14  has an array  14 R of red LEDs (light emitting diodes), an array  14 G of green LEDs, and an array  14 B of blue LEDs. Preferably, the red, green, and blue LED arrays  14 R,  14 G, and  14 B are equal in total LED number. Alternatively, the red, green, and blue LED arrays  14 R,  14 G, and  14 B may be different in total LED number. 
         [0035]    The timing circuit  15  in the control circuit  13  receives the video signal. The timing circuit  15  includes a sync detector or a sync separator for detecting the vertical sync signal in the video signal, and signal generators for producing timing signal  15 R,  15 G, and  15 B in response to the detected vertical sync signal. Preferably, adjustable signal delay sections are provided in the connections of the sync detector (or the sync separator) with the signal generators respectively. The produced timing signals  15 R,  15 G, and  15 B are synchronized with the vertical sync signal. In other words, the timing signals  15 R,  15 G, and  15 B are synchronized with frames represented by the video signal. The timing circuit  15  outputs the timing signals  15 R,  15 G, and  15 B to the PWM signal generators  16 R,  16 G, and  16 B respectively. 
         [0036]    The PWM signal generators  16 R,  16 G, and  16 B are assigned to the red LED array  14 R, the green LED array  14 G, and the blue LED array  14 B in the light source unit  14 , respectively. The PWM signal generator  16 R produces a PWM signal SR in response to the timing signal  15 R. The produced PWM signal SR has a specified phase relation with the timing signal  15 R or the vertical sync signal. Preferably, the PWM signal SR has a duty cycle less than 100%. The PWM signal generator  16 G produces a PWM signal SG in response to the timing signal  15 G. The produced PWM signal SG has a specified phase relation with the timing signal SG or the vertical sync signal. Preferably, the PWM signal SG has a duty cycle less than 100%. The PWM signal generator  16 B produces a PWM signal SB in response to the timing signal  15 B. The produced PWM signal SB has a specified phase relation with the timing signal  15 B or the vertical sync signal. Preferably, the PWM signal SB has a duty cycle less than 100%. The PWM signal generators  16 R,  16 G, and  16 B feed the PWM signals SR, SG, and SB to the red LED array  14 R, the green LED array  14 G, and the blue LED array  14 B, respectively. 
         [0037]    The red LED array  14 R emits red light while being driven by the PWM signal SR. The green LED array  14 G emits green light while being driven by the PWM signal SG. The blue LED array  14 B emits blue light while being driven by the PWM signal SB. Basically, the emitted red light, the emitted green light, and the emitted blue light mix with each other, constituting white light applied to the display panel  11 . 
         [0038]    The PWM signal SR alternates between a high level state and a low level state. The red LED array  14 R is activated and deactivated when the PWM signal SR is in its high level state and its low level state, respectively. The red LED array  14 R emits the red light only when being activated. Thus, every positive-going pulse in the PWM signal SR serves as a drive pulse for the red LED array  14 R. The PWM signal SG alternates between a high level state and a low level state. The green LED array  14 G is activated and deactivated when the PWM signal SG is in its high level state and its low level state, respectively. The green LED array  14 G emits the green light only when being activated. Thus, every positive-going pulse in the PWM signal SG serves as a drive pulse for the green LED array  14 G. The PWM signal SB alternates between a high level state and a low level state. The blue LED array  14 B is activated and deactivated when the PWM signal SB is in its high level state and its low level state, respectively. The blue LED array  14 B emits the blue light only when being activated. Thus, every positive-going pulse in the PWM signal SB serves as a drive pulse for the blue LED array  14 B. 
         [0039]    The time position of every drive pulse in the PWM signal SR relative to the vertical sync signal, and the width thereof are determined by the timing signal  15 R. Accordingly, the timing and duration of every activation of the red LED array  14 R are determined by the timing signal  15 R. The time position of every drive pulse in the PWM signal SG relative to the vertical sync signal, and the width thereof are determined by the timing signal  15 G. Accordingly, the timing and duration of every activation of the green LED array  14 G are determined by the timing signal  15 G. The time position of every drive pulse in the PWM signal SB relative to the vertical sync signal, and the width thereof are determined by the timing signal  15 B. Accordingly, the timing and duration of every activation of the blue LED array  14 B are determined by the timing signal  15 B. 
         [0040]      FIG. 2  shows a first example of the waveforms of the vertical sync signal and the PWM signals SR, SG, and SB, and the phase relation thereamong. In  FIG. 2 , each of the PWM signals SR, SG, and SB has “n” positive-going pulse or pulses (drive pulse or pulses) during every 1-frame time interval defined by the vertical sync signal, where “n” denotes an integer equal to or greater than “1”. The PWM signals SR,SG, and SB are the same in waveform, pulse frequency, and PWM period. The PWM signals SR, SG, and SB have equal phases “φ” relative to the vertical sync signal. Every positive-going pulse in the PWM signal SR and those in the PWM signals SG and SB are equal in timing and width. 
         [0041]      FIG. 3  shows a second example of the waveforms of the vertical sync signal and the PWM signals SR, SG, and SB, and the phase relation thereamong for use with, for example, non-field sequential drive or impulse drive of the display panel  11 . The impulse drive is of not only a true type but also a pseudo type based on back light control.  FIG. 4  shows time-domain variations in the electric powers consumed by the red, green, and blue LED arrays  14 R,  14 G, and  14 B, the sum of the consumed electric powers, the luminance provided by the light applied from the back light device  12  to the display panel  11 , and the color of the light applied from the back light device  12  to the display panel  11  which occur in the case where the waveforms of the vertical sync signal and the PWM signals SR, SG, and SB, and the phase relation thereamong are in the conditions of  FIG. 3 . 
         [0042]    With reference to  FIGS. 3 and 4 , each of the PWM signals SR, SG, and SB has one or more positive-going pulses (drive pulses) during every 1-frame time interval defined by the vertical sync signal. The PWM signals SR, SG, and SB are the same in pulse frequency and PWM period. The PWM signals SR, SG, and SB have equal phases “φ” relative to the vertical sync signal. Every positive-going pulse in the PWM signal SR and those in the PWM signals SG and SB are equal in rising-edge timing. Every positive-going pulse in the PWM signal SR and those in the PWM signals SG and SB are different in falling-edge timing. Thus, every positive-going pulse in the PWM signal SR and those in the PWM signals SG and SB are different in width. Specifically, the time position of the falling edge of every positive-going pulse in the PWM signal SG is later than that of the falling edge of a corresponding positive-going pulse in the PWM signal SR by a time interval T 1 . The time position of the falling edge of every positive-going pulse in the PWM signal SB is later than that of the falling edge of a corresponding positive-going pulse in the PWM signal SG by a time interval T 2 . Therefore, every positive-going pulse in the PWM signal SG is wider than a corresponding positive-going pulse in the PWM signal SR by the time interval T 1 . Every positive-going pulse in the PWM signal SB is wider than a corresponding positive-going pulse in the PWM signal SG by the time interval T 2 . The waveforms of the vertical sync signal and the PWM signals SR, SG, and SB, and the phase relation thereamong in  FIG. 3  are to compensate for differences in light emission efficiency among red, green, and blue LEDs. 
         [0043]    Under the signal conditions of  FIG. 3 , the timings of start of the corresponding light emissions from the red, green, and blue LED arrays  14 R,  14 G, and  14 B are the same. On the other hand, the timings of end of the corresponding light emissions from the red, green, and blue LED arrays  14 R,  14 G, and  14 B are different. Specifically, the light emission from the green LED array  14 G terminates the time interval T 1  after the end of the corresponding light emission from the red LED array  14 R. The light emission from the green LED array  14 G continues until the time interval T 1  has lapsed since the moment of end of the corresponding light emission from the red LED array  14 R. The light emission from the blue LED array  14 B terminates the time interval T 2  after the end of the corresponding light emission from the green LED array  14 G. The light emission from the blue LED array  14 B continues until the time interval T 2  has lapsed since the moment of end of the corresponding light emission from the green LED array  14 G. Thus, for the time intervals T 1  and T 2 , the light emission from the blue LED array  14 B lasts. During the time interval T 1 , the red light is absent so that the color of the light applied from the back light device  12  to the display panel I 1  is cyan as shown in  FIG. 4 . During the time interval T 2 , the red light and the green light are absent so that the color of the light applied from the back light device  12  to the display panel  11  is blue as shown in  FIG. 4 . In general, as the time intervals T 1  and T 2  are longer, color breaking in an image indicated by the display panel  11  is more observable. 
         [0044]    With reference to  FIGS. 3 and 4 , the timings of start of the corresponding light emissions from the red, green, and blue LED arrays  14 R,  14 G, and  14 B are the same so that the sum of the electric powers consumed by the red, green, and blue LED arrays  14 R,  14 G, and  14 B instantly takes the maximum value at that timing. Therefore, the maximum load is instantly applied to a power supply for the red, green, and blue LED arrays  14 R,  14 G, and  14 B. Such an instantly-applied maximum load may damage the power supply or shorten the life thereof. During the time intervals T 1  and T 2 , the luminance provided by the light applied from the back light device  12  to the display panel  11  has appreciable values and hence after-light exists so that an after-image may be indicated by the display panel  11 . The after-light or the indicated after-image may cancel the advantage provided by the impulse drive of the display panel  11 . 
         [0045]      FIG. 5  shows a third example of the waveforms of the vertical sync signal and the PWM signals SR, SG, and SB, and the phase relation thereamong for use with, for example, non-field sequential drive or impulse drive of the display panel  11 .  FIG. 6  shows time-domain variations in the electric powers consumed by the red, green, and blue LED arrays  14 R,  14 G, and  14 B, the sum of the consumed electric powers, the luminance provided by the light applied from the back light device  12  to the display panel  11 , and the color of the light applied from the back light device  12  to the display panel  11  which occur in the case where the waveforms of the vertical sync signal and the PWM signals SR, SG, and SB, and the phase relation thereamong are in the conditions of  FIG. 5 . 
         [0046]    With reference to  FIGS. 5 and 6 , each of the PWM signals SR, SG, and SB has one or more positive-going pulses (drive pulses) during every 1-frame time interval defined by the vertical sync signal. The PWM signals SR, SG, and SB are the same in pulse frequency and PWM period. The PWM signals SR, SG, and SB have different phases φ 1 , φ 2 , and φ 3  relative to the vertical sync signal. Every positive-going pulse in the PWM signal SR and those in the PWM signals SG and SB are different in rising-edge timing and falling-edge timing. The time positions of the centers of corresponding positive-going pulses in the PWM signals SR, SG, and SB are the same. Thus, every positive-going pulse in the PWM signal SR and those in the PWM signals SG and SB are different in width. Specifically, the time position of the rising edge of every positive-going pulse in the PWM signal SG is later than that of the rising edge of a corresponding positive-going pulse in the PWM signal SB by a time interval T 3 . The time position of the rising edge of every positive-going pulse in the PWM signal SR is later than that of the rising edge of a corresponding positive-going pulse in the PWM signal SG by a time interval T 4 . The time position of the falling edge of every positive-going pulse in the PWM signal SG is later than that of the falling edge of a corresponding positive-going pulse in the PWM signal SR by a time interval T 5 . The time position of the falling edge of every positive-going pulse in the PWM signal SB is later than that of the falling edge of a corresponding positive-going pulse in the PWM signal SG by a time interval T 6 . Therefore, every positive-going pulse in the PWM signal SG is wider than a corresponding positive-going pulse in the PWM signal SR by the sum of the time intervals T 4  and T 5 . Every positive-going pulse in the PWM signal SB is wider than a corresponding positive-going pulse in the PWM signal SG by the sum of the time intervals T 3  and T 6 . Every positive-going pulse in the PWM signal SR occupies a time range contained in a time range for which a corresponding positive-going pulse in the PWM signal SB extends. Similarly, every positive-going pulse in the PWM signal SG occupies a time range contained in a time range for which a corresponding positive-going pulse in the PWM signal SB extends. Thus, it is possible to maximize the length of every term during which all the red, green, and blue LED arrays  14 R,  14 G, and  14 B are deactivated. This term-length maximization promotes the advantage provided by the impulse drive of the display panel  11 . The waveforms of the vertical sync signal and the PWM signals SR, SG, and SB, and the phase relation thereamong in  FIG. 5  are to compensate for differences in light emission efficiency among red, green, and blue LEDs. 
         [0047]    Under the signal conditions of  FIG. 5 , the timings of start of the corresponding light emissions from the red, green, and blue LED arrays  14 R,  14 G, and  14 B are different. Furthermore, the timings of end of the corresponding light emissions from the red, green, and blue LED arrays  14 R,  14 G, and  14 B are different. Specifically, the light emission from the blue LED array  14 B starts the time interval T 3  before the start of the corresponding light emission from the green LED array  14 G. The light emission from the green LED array  14 G starts the time interval T 4  before the start of the corresponding light emission from the red LED array  14 R. For the time interval T 3 , the light emission from the blue LED array  14 B lasts. For the time interval T 4 , the light emissions from the green and blue LED arrays  14 G and  14 B last. Therefore, during the time interval T 3 , the red light and the green light are absent so that the color of the light applied from the back light device  12  to the display panel  11  is blue as shown in  FIG. 6 . During the time interval T 4 , the red light is absent so that the color of the light applied from the back light device  12  to the display panel  11  is cyan as shown in  FIG. 6 . The light emission from the green LED array  14 G terminates the time interval T 5  after the end of the corresponding light emission from the red LED array  14 R. The light emission from the green LED array  14 G continues until the time interval T 5  has lapsed since the moment of end of the corresponding light emission from the red LED array  14 R. The light emission from the blue LED array  14 B terminates the time interval T 6  after the end of the corresponding light emission from the green LED array  14 G. The light emission from the blue LED array  14 B continues until the time interval T 6  has lapsed since the moment of end of the corresponding light emission from the green LED array  14 G. Thus, for the time intervals T 5  and T 6 , the light emission from the blue LED array  14 B lasts. During the time interval T 5 , the red light is absent so that the color of the light applied from the back light device  12  to the display panel  11  is cyan as shown in  FIG. 6 . During the time interval T 6 , the red light and the green light are absent so that the color of the light applied from the back light device  12  to the display panel  11  is blue as shown in  FIG. 6 . It is thought that the time interval T 1  in  FIG. 3  is halved into the time intervals T 4  and T 5  in  FIG. 5 , and that the time interval T 2  in  FIG. 3  is halved into the time intervals T 3  and T 6  in  FIG. 5 . 
         [0048]    As shown in  FIG. 7 , there are full activation time ranges TA and full deactivation time ranges TB. The full activation time range TA means a term during which all the red, green, and blue LED arrays  14 R,  14 G, and  14 B are activated so that all the red light, the green light, and the blue light are present. The full deactivation time range TB means a term during which all the red, green, and blue LED arrays  14 R,  14 G, and  14 B are deactivated so that all the red light, the green light, and the blue light are absent. There is a full activation time range TA or a full deactivation time range TB between the neighboring time intervals T 4  and T 5 . Similarly, there is a full activation time range TA or a full deactivation time range TB between the neighboring time intervals T 3  and T 6 . Therefore, the time intervals T 4  and T 5  are recognized as separate ones. Similarly, the time intervals T 3  and T 6  are recognized as separate ones. Accordingly, during the time intervals T 3 , T 4 , T 5 , and T 6 , color breaking in an image indicated by the display panel  11  is less observable. 
         [0049]    With reference to  FIGS. 5-7 , the timings of start of the corresponding light emissions from the red, green, and blue LED arrays  14 R,  14 G, and  14 B are different so that the sum of the electric powers consumed by the red, green, and blue LED arrays  14 R,  14 G, and  14 B gradually increases to the maximum value. Therefore, the load applied to the power supply for the red, green, and blue LED arrays  14 R,  14 G, and  14 B gradually increases to the maximum level. Basically, such a gradually-increasing applied load is acceptable to the power supply. As previously mentioned, the time interval T 1  in  FIG. 3  is halved into the time intervals T 4  and T 5  in  FIG. 5 , and the time interval T 2  in  FIG. 3  is halved into the time intervals T 3  and T 6  in  FIG. 5 . There is a full activation time range TA or a full deactivation time range TB between the neighboring time intervals T 4  and T 5 . Similarly, there is a full activation time range TA or a full deactivation time range TB between the neighboring time intervals T 3  and T 6 . Accordingly, after-light exists only for shorter time intervals (the time intervals T 5  and T 6 ). Therefore, it is possible to enhance the quality of moving pictures indicated by the display panel  11  even in the case of the impulse drive of the display panel  11 . 
         [0050]      FIG. 8  shows a fourth example of the waveforms of the vertical sync signal and the PWM signals SR, SG, and SB, and the phase relation thereamong for use with, for example, non-field sequential drive or impulse drive of the display panel  11 . 
         [0051]    With reference to  FIG. 8 , the PWM signal SB is a reference for designing and setting the PWM signals SR and SG. Each of the PWM signals SR, SG, and SB has one or more positive-going pulses (drive pulses) during every 1-frame time interval defined by the vertical sync signal. The PWM signals SR, SG, and SB are the same in pulse frequency and PWM period. The PWM signals SR, SG, and SB have different phases φ 4 , φ 5 , and φ 6  relative to the vertical sync signal. Every positive-going pulse in the PWM signal SR and those in the PWM signals SG and SB are different in rising-edge timing, falling-edge timing, and width. Specifically, the time position of the rising edge of every positive-going pulse in the PWM signal SR is later than that of the rising edge of a corresponding positive-going pulse in the PWM signal SB. The time position of the rising edge of every positive-going pulse in the PWM signal SG is later than that of the rising edge of a corresponding positive-going pulse in the PWM signal SR. The time position of the falling edge of every positive-going pulse in the PWM signal SG is later than that of the falling edge of a corresponding positive-going pulse in the PWM signal SR. The time position of the falling edge of every positive-going pulse in the PWM signal SB is later than that of the falling edge of a corresponding positive-going pulse in the PWM signal SG. Every positive-going pulse in the PWM signal SG is wider than a corresponding positive-going pulse in the PWM signal SR. Every positive-going pulse in the PWM signal SB is wider than a corresponding positive-going pulse in the PWM signal SG. Every positive-going pulse in the PWM signal SR occupies a time range contained in a time range for which a corresponding positive-going pulse in the PWM signal SB extends. Similarly, every positive-going pulse in the PWM signal SG occupies a time range contained in a time range for which a corresponding positive-going pulse in the PWM signal SB extends. Thus, it is possible to maximize the length of every term during which all the red, green, and blue LED arrays  14 R,  14 G, and  14 B are deactivated. This term-length maximization promotes the advantage provided by the impulse drive of the display panel  11 . The waveforms of the vertical sync signal and the PWM signals SR, SG, and SB, and the phase relation thereamong in  FIG. 8  are to compensate for differences in light emission efficiency among red, green, and blue LEDs. 
         [0052]    When the vertical sync signal and the PWM signals SR, SG, and SB are in the conditions of  FIG. 8 , the timings of start of the corresponding light emissions from the red, green, and blue LED arrays  14 R,  14 G, and  14 B are different. Furthermore, the timings of end of the corresponding light emissions from the red, green, and blue LED arrays  14 R,  14 G, and  14 B are different. Specifically, the light emission from the blue LED array  14 B starts before the start of the corresponding light emission from the red LED array  14 R. The light emission from the red LED array  14 R starts before the start of the corresponding light emission from the green LED array  14 G. The light emission from the green LED array  14 G terminates after the end of the corresponding light emission from the red LED array  14 R. The light emission from the blue LED array  14 B terminates after the end of the corresponding light emission from the green LED array  14 G. It is thought that the time interval T 1  in  FIG. 3  is divided into separate portions, and that the time interval T 2  in  FIG. 3  is divided into separate portions. Accordingly, color breaking in an image indicated by the display panel  11  is less observable. Since the timings of start of the corresponding light emissions from the red, green, and blue LED arrays  14 R,  14 G, and  14 B are different, the sum of the electric powers consumed by the red, green, and blue LED arrays  14 R,  14 G, and  14 B gradually increases to the maximum value. Therefore, the load applied to the power supply for the red, green, and blue LED arrays  14 R,  14 G, and  14 B gradually increases to the maximum level. Basically, such a gradually-increasing applied load is acceptable to the power supply. 
       Second Embodiment 
       [0053]    A second embodiment of this invention is similar to the first embodiment thereof except that one or two of the red, green, and blue LED arrays  14 R,  14 G, and  14 B are omitted. 
       Third Embodiment 
       [0054]    A third embodiment of this invention is similar to the first embodiment thereof except that an LED array or arrays for emitting light having a color or colors different from red, green, and blue are added. 
       Fourth Embodiment 
       [0055]    A fourth embodiment of this invention is similar to the first embodiment thereof except that the PWM signal SR is wider in drive pulse width than the PWM signals SG and SB. 
       Fifth Embodiment 
       [0056]    A fifth embodiment of this invention is similar to the first embodiment thereof except that the PWM signal SG is wider in drive pulse width than the PWM signals SR and SB. 
       Sixth Embodiment 
       [0057]    A sixth embodiment of this invention is similar to the first embodiment thereof except that the red, green, and blue LED arrays  14 R,  14 G, and  14 B are replaced by red, green, and blue light sources exclusive of LEDs.