Abstract:
A pulse-width modulated backlight control for a video display restarts the pulse-width modulated pulse train on occurrence of a video refresh pulse. In order to prevent an undesirable momentary increase in brightness in the event that the last pulse of the pre-refresh pulse train occurs too close to the first pulse of the post-refresh pulse train relative to the normal pulse interval, the width of the first pulse following refresh may be reduced from a first value determined by the desired brightness to a second value that bears the same proportion to the first value that the interval between the beginning of the previous pulse and the occurrence of the refresh pulse bore to the normal pulse interval. In that way, the duty cycle during the shortened pulse interval is the same as during a normal pulse interval, avoiding or minimizing perceptible increase in backlight brightness.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/369,458, filed Feb. 11, 2009 (now allowed), which claims the benefit of commonly-assigned U.S. Provisional Patent Application No. 61/113,141, filed Nov. 10, 2008, both of which are hereby incorporated by reference herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to controlling the backlighting of a video display. In particular, this invention relates to control of backlighting separately from the video display itself without introducing visual artifacts in the display. 
         [0003]    In early video displays based on cathode-ray tube technology, the display generated its own light. However, in many types of solid-state video displays, the elements that display the video data do not generate their own light, and must be coupled with a separate light source. For example, liquid crystal displays operate by selectively lightening and darkening elements in an array, allowing light to shine through from behind the array. In such displays, the light source is generally a backlight, although for some displays, light may be provided from the sides, using reflectors, light pipes, etc., to spread the light out behind the liquid crystal array. 
         [0004]    One type of light source commonly used in such displays includes fluorescent lamps. More recently, however, in order to reduce power consumption, and to save space, thereby allowing thinner displays, solid-state light sources have been introduced. For example, light-emitting diodes can be used, either in an array behind the liquid-crystal image-forming array, or as sidelights, with the light distributed behind the image-forming array using reflectors, light pipes, etc., as described above. 
         [0005]    It is sometimes necessary to vary the brightness of a video display. This may be a function of the image being displayed, or it may be done to conserve power (e.g., in a portable device, the brightness may be reduced when operating on battery power, particularly during idle periods). One way of controlling the brightness is by pulse width modulation, in which a current pulse of duration t 2  is sent during each interval of duration t 1  to power the light source. Maximum brightness may be achieved when t 2  is a certain fraction f of t 1 , where f≦1. By narrowing each pulse—i.e., shortening the pulse width, so that t 2 &lt;ft 1 , the brightness can be reduced. The magnitude of each current pulse may remain constant. 
         [0006]    The video array itself may have a certain refresh rate, which may be determined, for example, by the video standard being displayed, such as, e.g., NTSC, ATSC, VGA, SVGA, XVGA, etc. The video refresh rate may be totally independent of the pulse-width modulation pulse rate 1/t 1 . This complete lack of a fixed-phase relationship between the two signals may result in motion artifacts (e.g., a “waterfall” effect) in the video display when the refresh pulses do not coincide with the pulse-width modulated current pulses, for which there may be a number of solutions. 
         [0007]    One solution is to synchronize the video refresh rate and the pulse-width modulation pulse rate of the backlight control current, so that the pulse-width modulation pulse rate of the backlight control current is an integer multiple of the refresh rate. This solution requires deriving both signals from a common clock source (e.g., a quartz crystal). 
         [0008]    Another solution is to greatly increase the pulse-width modulation pulse rate of the backlight control current, so that when a video refresh pulse occurs, it will be very close to a backlight control current pulse so as to be nearly synchronous to the current pulse. 
         [0009]    For example, a common video refresh rate is 60 Hz, while a common pulse-width modulation pulse rate for the backlight control current is 600 Hz. Either the 600 Hz backlight control current pulses may be synchronized with the 60 Hz refresh pulses, or the pulse-width modulation pulse rate of the backlight control current may be increased to between about 20 kHz and about 30 kHz. However, the video subsystem and the backlight subsystem are typically completely separate, so that synchronizing both rates using a common clock source is not practical or desirable, and very high pulse-width modulated backlight control current pulse rates also are not desirable. 
         [0010]    Another solution might be to restart the pulse-width modulated backlight control current pulse train on each occurrence of a refresh pulse. However, because the pulse rate and the refresh rate may be completely independent, it may occur, by happenstance, depending on how the two cycles overlap, that a current pulse will have occurred just before a refresh pulse. Then, if the pulse train is restarted on occurrence of the refresh pulse, another current pulse will occur. The occurrence of two current pulses close together may cause a noticeable, if momentary, brightness increase or “flicker,” which also is undesirable. 
       SUMMARY OF THE INVENTION 
       [0011]    In accordance with the present invention, in a pulse-width modulated backlight control for a video display, the pulse-width modulated backlight control current pulse train is resynched to the video refresh rate on occurrence of a video refresh pulse. In order to prevent an undesirable momentary increase in brightness, or flicker, in the event that the pulse train is restarted soon after a pulse has occurred relative to the normal pulse interval, this resynchronization does not result in immediate restart of the pulse-width modulated backlight control current pulse train at full pulse width. Instead, the width, and possibly the timing, of the first backlight control current pulse after resynchronization are adjusted so that the duty cycle remains constant, to avoid any perceptible flicker. 
         [0012]    In one embodiment, the first backlight control current pulse after resynchronization occurs essentially on occurrence of the video refresh pulse, so that it is closer to the previous backlight control current pulse than a normal pulse-width modulation pulse interval. In this embodiment, to mitigate the flicker effect of having the first backlight control current pulse after resynchronization closer than normal to the previous pulse, the width of the first backlight control current pulse after resynchronization is reduced from a first value determined by the desired brightness to a second value that bears the same proportion to the first value that the interval between the beginning of the previous backlight control current pulse and the beginning of the first backlight control current pulse bears to the normal pulse interval. Thus, the average duty cycle, over the combination of the shortened pulse interval before the video refresh pulse and the first pulse interval after the video refresh pulse, is constant, minimizing or eliminating perceptible flicker. 
         [0013]    In a second embodiment, the first backlight control current pulse after resynchronization still occurs one pulse-width modulation pulse interval after the rising edge of the previous backlight control current pulse, while the second backlight control current pulse after resynchronization occurs one pulse-width modulation pulse interval after the rising edge of the video refresh pulse. To mitigate the flicker effect from having those first and second pulses closer together than one pulse-width modulation pulse interval, the width of the first backlight control current pulse may be reduced from a first value determined by the desired brightness to a second value that bears the same proportion to the first value that the interval between the beginning of the first backlight control current pulse and the beginning of the second backlight control current pulse bears to the normal pulse interval. In that way, the duty cycle during the shortened pulse interval is the same as that during a normal pulse interval, and there is no perceptible increase in backlight brightness. 
         [0014]    By definition, the first backlight control current pulse after resynchronization occurs during a new video frame. In some cases, characteristics of the next video frame may require a brightness change. Therefore, the unadjusted pulse width after resynchronization may be different from the unadjusted pulse width before resynchronization. In either of the two embodiments described above, the pulse width update for any brightness change that may be required can occur either in the first backlight control current pulse after resynchronization or the second backlight control current pulse after resynchronization. In the former case, the adjustment of the pulse width may be applied to the updated pulse width. 
         [0015]    Thus, in accordance with the present invention there is provided a method for controlling a backlight associated with a video display, the video display having a refresh rate. The method includes generating a train of pulse-width modulated current pulses, each of the pulse-width modulated current pulses having a pulse width determined by a leading edge and a trailing edge (i.e., a rising edge and falling edge for a positive-going pulse, or a falling edge and a rising edge for a negative-going pulse) and determined by desired backlighting parameters of a present frame, and leading edges of successive ones of the pulses being separated by a uniform pulse interval. On occurrence of a refresh of the video display after an incomplete pulse interval following one of the pulse-width modulated current pulses to start a next frame, the pulse train is restarted and the pulse width of a subsequent pulse is shortened so that the pulse width of the subsequent pulse is reduced from a first value determined by the desired backlighting parameters to a second value that bears a same proportion to the first value that the duration between the beginning of the incomplete pulse interval and a leading edge of the refresh pulse bears to the uniform pulse interval. A pulse following the subsequent pulse occurs one uniform pulse interval following the leading edge of the refresh pulse. 
         [0016]    A video display operated in accordance with the method also is provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Further features of the invention, its nature and various advantages will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
           [0018]      FIG. 1  is a simplified schematic representation of a video display with which the present invention may be used; 
           [0019]      FIG. 2  is a graphical representation showing a backlighting control pulse train that is synchronized with a video refresh signal; 
           [0020]      FIG. 3  is a graphical representation showing a backlighting control pulse train that is not synchronized with a video refresh signal; 
           [0021]      FIG. 4  is a graphical representation of a first embodiment of a method according to the present invention; 
           [0022]      FIG. 5  is a graphical representation of a second embodiment of a method according to the present invention; and 
           [0023]      FIG. 6  is a diagram of circuitry according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]      FIG. 1  shows, schematically, a video display  100  of the type with which the present invention may be used. Video display  100  includes a video array  101  which may be, e.g., a liquid crystal array as described above, and a backlighting source  102 . Although backlighting source  102  is shown as having an area coextensive with that of video array  101 , the actual light source of backlighting source  102  may occupy a small area, and its output may be spread over video array  101  using appropriate reflectors, light pipes, etc. (not shown). 
         [0025]    Driver circuitry  103  may include a video driver  104  that drives video array  101 , and a separate backlight control  105  for backlighting source  102 . 
         [0026]      FIG. 2  shows a train  201  of pulse-width modulated current pulses  202  for controlling backlighting unit  102 . The pulse interval t 1  is the duration between the rising edges  212  of adjacent pulses  202 . The brightness of backlighting source  102  may be determined by the duty cycle of pulse train  201 —i.e., the proportion that the width or duration t 2  of each pulse  202  bears to the pulse interval t 1 , which may be expressed as a percentage, or as a fraction f. Although the accompanying drawings show positive-going pulses and the discussion that follows refers to the rising and falling edges of those pulses, the invention also applies where the pulses are negative-going. Accordingly, any discussion of rising and falling edges should be considered a discussion of falling and rising edges in the case of negative-going pulses. More generally, one may refer to the leading and trailing edges of the pulses. 
         [0027]    Also shown in  FIG. 2  is a voltage waveform v sync , made up of a train  203  of refresh pulses  204  occurring at a refresh interval t 3  which generally is much longer than pulse interval t 1 . Generally, one may expect refresh interval t 3  to be at least about ten times as long as pulse interval t 1 . 
         [0028]    In the example of  FIG. 2 , pulse-width modulated pulse train  201  is synchronized with v sync  or refresh pulse train  203 , with each refresh pulse  204  coinciding with a pulse-width modulated current pulse  202 . In this case, no visual artifacts are produced. 
         [0029]    In the situation shown in  FIG. 3 , on the other hand, the refresh pulses do not always coincide with one of the pulse-width modulated current pulses of pulse train  301 . In particular, while refresh pulse  302  coincides with current pulse  303 , refresh pulse  304  occurs shortly after current pulse  305  and refresh pulse  307  occurs shortly after current pulse  308 . The resulting restart of the pulse-width modulated current pulse train causes current pulse  306  to occur much closer than normal to current pulse  305  and current pulse  309  to occur much closer than normal to current pulse  308 , giving rise to the increased brightness, and therefore flicker, referred to above. It will be appreciated, of course, that refresh pulse  304  could occur at any time relative to the current pulses of pulse train  301 . Nevertheless, any degree of closeness between pulses  305  and  306  could give rise to at least some visual artifact. 
         [0030]    In an alternative situation (not shown), the pulse-width modulated current pulse train is not synchronized with the refresh pulse train, and is not restarted on occurrence of a refresh pulse. That situation would give rise to the aforementioned “waterfall” effect. 
         [0031]    In accordance with one embodiment of the invention, as shown in  FIG. 4 , first backlight control current pulse  401  after resynchronization occurs at interval t 1  following the previous backlight control current pulse  402  before resynchronization, and the second backlight control current pulse  403  after resynchronization occurs at interval t 1  following refresh pulse  304 . In this embodiment, even though pulse  402  occurs after refresh pulse  304 , it may be considered the last pulse of the pre-refresh pulse train. This results in pulse  401  and pulse  403  being separated by an interval equal to the interval between the rising edge of pulse  402  and the rising edge of refresh pulse  304 , rather than interval t 1 . The amount by which this closer than interval t 1  depends on how soon pulse  304  occurred after pulse  402 . 
         [0032]    In any event, according to this embodiment, the width of pulse  401  is shortened so that the proportion that the adjusted width  411  of pulse  401  bears to the unadjusted width  421  of pulse  401  is the same as the proportion that the interval between the rising edge  431  of pulse  401  and the rising edge  433  of pulse  403  bears to the uniform or standard pulse interval t 1 , which is the same as the proportion that the interval between the rising edge  432  of pulse  402  and the rising edge  334  of pulse  304  bears to the standard pulse interval t 1 . Accordingly, the duty cycle over the shortened interval t 4  between pulse  401  and pulse  403  is the same as during a standard pulse interval t 1 . 
         [0033]    According to a second embodiment shown in  FIG. 5 , first backlight control current pulse  501  after resynchronization occurs substantially at the same time as refresh pulse  304 , and all subsequent pulses are spaced apart by the standard pulse interval t 1 . In this embodiment, pulse  502  may be considered the first pulse of the post-refresh pulse train. This results in pulse  501  and previous pulse  502  being separated by an interval equal to the interval between the rising edge of pulse  502  and the rising edge of refresh pulse  304 , rather than interval t 1 . The amount by which this closer than interval t 1  depends on how soon pulse  304  occurred after pulse  402 . 
         [0034]    In any event, according to this embodiment, the width of pulse  501  is shortened so that the proportion that the adjusted width  511  of pulse  501  bears to the unadjusted width  521  of pulse  501  is the same as the proportion that the interval between the rising edge  532  of pulse  502  and the rising edge  531  of pulse  501  bears to the standard pulse interval t 1 , which is the same as the proportion that the interval between the rising edge  532  of pulse  502  and the rising edge  334  of pulse  304  bears to the standard pulse interval t 1 . Accordingly, the average duty cycle over the shortened interval t 5  between pulse  502  and pulse  501  and the following pulse interval is the same as during a standard pulse interval t 1 . 
         [0035]    If in the embodiment of  FIG. 5 , refresh pulse  304  occurs before pulse  502  is complete, pulse  502  is allowed to complete, and then is immediately followed by pulse  501 . This again maintains the average duty cycle over the two intervals containing pulse  502  and pulse  501  the same as during a standard pulse interval t 1 . 
         [0036]    In either embodiment, the second pulse  403 ,  503  following refresh pulse  304  occurs one standard pulse interval t 1  following refresh pulse  304 . 
         [0037]    It may be that the requirements of the video frame to be displayed following refresh pulse  304  require a brightness change. This can be handled in two ways, regardless of whether the embodiment of  FIG. 4  or the embodiment of  FIG. 5  is being used. According to one variant, the brightness change is introduced immediately on occurrence of refresh pulse  304 . According to this variant, the adjustment of pulse width  411  or  511  must also account for the brightness change. Although this is possible, it may be mathematically complex. Therefore, according to a second variant, the brightness change is not applied until pulse  403 ,  503 . This one-pulse delay in applying the brightness change should not cause any perceptible visual artifact. 
         [0038]      FIG. 6  shows an embodiment of a counter arrangement  600  for implementing the invention. c 1  counter  601  controls the standard pulse interval t 1  between the pulse-width modulated backlight control current pulses in pulse train  301 , while c 2  counter  602  controls the pulse width of the individual pulse-width modulated backlight control current pulses. 
         [0039]    c 0  counter  610  supplies CLK 1  clock  620  which clocks counters  601 ,  602 . As such, CLK 1  is significantly faster (e.g., between about 100 times and several tens of thousands of times faster) than the pulse rate of pulse train  301 . For example, the pulse rate of pulse train  301  may be between about 300 Hz and about 30 kHz, while CLK 1  may be between about 4096 times faster and about 16,384 times faster. Counter  610  is itself clocked by an even higher-speed system clock CLK 0  (e.g., a 200 MHz clock) to generate CLK 1 . Counter  610  may be loaded with a maximum count value r 0  that determines CLK 1 =CLK 0 /r 0 . Alternatively, an independent clock circuit (not shown) may generate CLK 1  directly. 
         [0040]    Similarly, counter  601  may be loaded with a maximum count value r 1  that determines t 1 =1/(CLK 1 /r 1 ), while counter  602  may be loaded with a maximum count value r 2  that determines t 2 =1/(CLK 1 /r 2 )=t 1 (r 2 /r 1 ). r2 may be changed by driver circuitry  103  according to the brightness requirements of the present image. 
         [0041]    These relationships hold as long as no v sync  refresh pulse  304  occurs. However, once a refresh pulse  304  occurs, the instantaneous count value c 1  in counter  601  is stored at  611  as m 1 , and used at  603  to derive a new temporary maximum count m 2  for counter  602 , such that m 2  bears the same proportion to r 2  that m 1  bears to r 1 , or m 2 =r 2 (m 1 /r 1 ). 
         [0042]    If operating according to the method shown in  FIG. 4 , counter  601  is not reset by pulse  304 . Instead, it continues to run until c 1 =r 1 , at which point it triggers counter  602 . Because there is a value in m 2 , m 2  is used instead of r 2  (otherwise r 2  would be used) causing a pulse to be output for a duration indicated by m 2 . m 2  is cleared whenever it is used, and therefore the next pulse will again have the duration indicated by r 2 . Counter  601  then starts again. Because there is a value in m 1 , m 1  is used instead of r 1 , and counter  602  is triggered after a shortened interval t 4  indicated by m 1 . m 1  is then cleared, having been used, so that the next pulse interval will be determined by r 1 . 
         [0043]    If operating according to the method shown in  FIG. 5 , counter  601 , including m 1 , is reset by pulse  304  after m 2  has been stored. This triggers counter  602 . Because there is a value in m 2 , m 2  is used instead of r 2  causing a pulse to be output for a duration indicated by m 2 . m 2  is cleared whenever it is used, and therefore the next pulse will again have the duration indicated by r 2 . Because m 1  has been reset, that next pulse will occur after the standard interval determined by r 1 . 
         [0044]    It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. One skilled in the art will appreciate that the present invention is not limited by the disclosed embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims that follow.