Abstract:
A circuit and method for generating pulse width modulated signal from an analog video signal. The circuit includes a first circuit portion that includes a first switch, a voltage storage device that stores a portion of the analog video signal as a voltage value according to the first switch activated according to sample time information within a portion of time of the analog video signal, and a second switch that outputs the stored voltage value when activated according to a next portion of time of the analog video signal. The circuit also includes a second circuit portion that converts the outputted voltage value into a pulse width modulated signal.

Description:
[0001]    The present invention is related to the following co-pending U.S. patent applications: ______ entitled “Method of Gray Scale Generation For Displays Using a Binary Weighted Clock;” ______ entitled “Method of Gray Scale Generation For Displays Using a Register and a Binary Weighted Clock;” and ______ entitled “Method of Gray Scale Generation For Displays Using a Sample and Hold Circuit With a Variable Reference Voltage.” 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to displays and more particularly to driving display pixels according to a gray scale value.  
         BACKGROUND OF THE INVENTION  
         [0003]    Most displays must support many levels of brightness, i.e. shades of gray or “gray scale”, for each pixel element. With the exception of the cathode ray tube, the cost of gray scale driver electronics is one of the largest component costs of a display system. This is because of the complexity of generating gray scale as well as the fact that there are far more gray scale drivers needed in a display than any other driver element.  
           [0004]    For example, in an SVGA Field Emission Display, there are 800 columns, each column composed of 3 sub-columns (Red, Green and Blue) and 600 rows or lines. Each row requires a simple ON or OFF driver, essentially a twolevel driver, and there are 600 drivers required per display. Each sub-column, however, requires a gray scale driver that may be required to provide 256 or more different levels of brightness, and there are one gray scale driver required per each sub-pixel or 800×3=2,400 of these drivers required per display. Thus, if the row and column drivers cost exactly the same, there would still be a 4:1 ratio of costs due simply to the number of drivers. However gray scale drivers are actually much more expensive than simple two-level drivers since they contain significantly more circuitry and therefore the additional cost would be much greater than 4:1.  
           [0005]    There are two methods of generating the differing levels of pixel brightness in a gray scale driver. The first method is to vary the output voltage or output current provided by the driver. The higher the voltage or current, the brighter the pixel brightness. However, when the brightness is less than maximum, the excess energy that does not go to lighting the pixel is dissipated across the driver, generating heat. This makes the driver expensive because it must dissipate this heat in order to properly operate and few drivers can be packed in one chip because of this heat problem. It is also very complicated and expensive to build a driver which translates digital picture information into the varying output voltages or currents needed for gray scale. Additionally, when the pixel is driven at a low brightness level with reduced voltage or current, the pixel may not be driven at its full efficiency, causing reduced display efficiency and uneven pixel illumination and sharpness.  
           [0006]    The second method overcomes these heat and efficiency problems by utilizing the fact that the human eye cannot perceive fast changes in brightness and therefore integrates, or averages, the total light received over time and “sees” an average brightness. In this method, known as Pulse-Width Modulation, the pixel is driven at maximum brightness for a certain period of time and then turned off for another period of time. Because the driver circuit is only fully on or fully off, a minimum amount of the energy is lost in the driver and the pixel is always on at full efficiency. By varying the portion of a cycle that the pixel is lit, the perceived brightness can be varied from barely on to fully on.  
           [0007]    However, the circuits to accomplish this second method of gray scale are very complicated. As can be seen in FIG. 1A, a typical gray scale circuit includes a latch or shift register to store the binary gray scale number before it is used, a latch to store the active gray scale number, a counter to generate the time slots, a comparator circuit to determine if the counted number is less than, equal to or greater than the stored gray scale number, and a driver transistor.  
           [0008]    In the operation of the circuit shown in FIG. 1A, the binary gray scale number is first stored in the latch or shift register for later transfer to the active latch. After the data is transferred to the active latch, the counter is reset to zero and then begins counting up to a maximum number, which defines one complete brightness cycle, defined as T in FIG. 1B. Each time the counter counts up, its output is compared by the comparator circuit with the gray scale number stored in the active latch. If the stored number in the active latch is lower than the count number from the counter, the comparator circuit will set the driver transistor to ON. When the gray scale number becomes equal to or greater than the count from the counter, the comparator circuit turns the driver transistor to OFF. The period of time when the driver is ON is shown as X in FIG. 1B. The overall brightness of the pixel in the typical gray scale circuit described in FIG. 1A is defined by the ratio of X to T shown in FIG. 1B, where X is defined as the time the driver is ON and T is defined as the total time period for one complete brightness cycle. This solution requires a large amount of circuitry to drive a pixel according to gray scale.  
           [0009]    Therefore, there exists a need to reduce the amount of gray scale circuitry to drive a pixel for various types of flat panel displays.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides a circuit for generating pulse width modulated signal from an analog video signal. The circuit includes a first circuit portion that includes a first switch, a voltage storage device for storing a portion of the analog video signal as a voltage value according to the first switch activated according to sample time information within a portion of time of the analog video signal, and a second switch for outputting the stored voltage value when activated according to a next portion of time of the analog video signal. The circuit also includes a second circuit portion for converting the outputted voltage value into a pulse width modulated signal.  
           [0011]    In accordance with further aspects of the invention, the second circuit portion is a Schmidt trigger with a preset bias voltage value.  
           [0012]    In accordance with other aspects of the invention, the second circuit portion is a comparator.  
           [0013]    In accordance with still further aspects of the invention, the stored portion of the analog video signal represents display element information.  
           [0014]    In an alternate embodiment, the present invention provides a circuit that includes for generating pulse width modulated signal from an analog video signal that includes a first and second circuit portion. The first circuit portion includes a first and second subcircuit portion. The first subcircuit portion includes a first switch, a voltage storage device that stores a portion of the analog video signal as a voltage value within a period of time according to the first switch activated according to sample time information within the period of time, and a second switch that outputs the stored voltage value when activated during a subsequent period of time. The second subcircuit portion includes a first switch, a voltage storage device that stores a next portion of the analog video signal as a voltage value within the subsequent period of time according to the first switch activated according to sample time information within the subsequent period of time, and a second switch that outputs a previously stored voltage value when activated during the period of time. The second circuit portion converts the outputted voltage values from the first and second circuit portions into pulse width modulated signals. This alternate embodiment allows the gray scale information for a display element to be outputted while the gray scale information for a display element is a next row is stored.  
           [0015]    As will be readily appreciated from the foregoing summary, the invention provides an improved circuit for generating a pulse width modulated signal for sending gray scale information to a display. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0017]    [0017]FIGS. 1A and B are diagrams illustrating the prior art;  
         [0018]    FIGS.  2 A-C illustrates a first embodiment of the present invention;  
         [0019]    [0019]FIG. 3 is a circuit diagram of a video display system formed in accordance with the present invention;  
         [0020]    [0020]FIG. 4 is a block circuit diagram of the present invention;  
         [0021]    [0021]FIGS. 5 and 6 are detailed circuit diagrams of the block circuit diagram shown in FIG. 4;  
         [0022]    [0022]FIGS. 7 and 8 are timing diagrams of an example of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    The present invention is an analog to pulse width modulated signal generator (APWM). One use of the APWM is to drive the gray scale exhibited by phosphor pixels in a display. In an first embodiment, the present invention provides a separate set of circuitry for each pixel element or sub-pixel, thereby providing separate driving circuitry for each pixel element or sub-pixel (i.e. an active matrix display). As shown in FIG. 2A, each pixel element or sub-pixel includes its own circuitry  10  that includes a pixel data storage circuit  12  and a pulse width modulation (PWM) generator circuit  14 . The pixel data storage circuit  12  samples and stores a data portion of the analog video signal. The PWM generator circuit  14 , at a preset time, discharges the stored data portion to a driver  16  that then drives a pixel or subpixel  18 .  
         [0024]    An example circuit suitable for implementing the first embodiment is illustrated in FIG. 2B. The pixel data storage circuit  12  includes a transistor Q 1   20  with its source  22  coupled to a video bus  24  and its drain  26  connected to a first end  30  of a resistor R 1   32 . A gate  44  of the transistor Q 1   20  is connected to a sampling signal S. A second end  34  of the resistor R 1   32  is coupled to a first side  38  of a capacitor  40 . A second side  42  of the capacitor  40  is tied to ground.  
         [0025]    The PWM signal generator circuit  14  includes a transistor Q 2   46  that has its source  48  connected to the first side  38  of the capacitor  40 . A gate  52  of the transistor Q 2   46  is connected to a discharge signal D. A drain  54  of the transistor Q 2   46  is coupled to a first end  56  of a resistor R 2   58 . A second end  60  of the resistor R 2   58  is tied to ground. The first end  56  of the resistor R 2   58  is also connected to a first input  62  of a Schmidt trigger (or a comparator)  64 . The second input  66  of the Schmidt trigger is tied to ground or an appropriate reference voltage.  
         [0026]    In an alternate embodiment, the resistors R 1   32  and R 2   58  are replace with constant current sources.  
         [0027]    An example timing diagram of the sampling signal S and the discharge signal D are shown in FIG. 2C. Time width  70  corresponds to the horizontal sync pulses of the analog video signal. Because the circuitry of this first embodiment produces a digital PWM signal for a single pixel, the transistor Q 1   20  allows the capacitor  40  to charge to a voltage value that is an approximate average of a sample period, as determined by the sampling signal S, of the analog video signal that corresponds to the pixel. The transistor Q 2   46  allows the capacitor  40  to discharge the stored, sampled voltage value, as determined by the discharge signal D, to the driver  16 .  
         [0028]    In a second embodiment, as shown in FIG. 3, a display  110  has a plurality of pixels  112   a - d.  The display may be monochrome or color. When the display is color each pixel  112   a - d  comprises three sub pixels: red (R)  114   a - d,  green (G)  116   a - d  and blue (B)  118   a - d.  To simplify the discussion, the following discussion will mostly refer to the pixels  112   a - d  as if they are monochrome, with the understanding that invention can also be applied in the manner described to sub-pixels  114   a - d,    116   a - d,    118   a - d  in a color display.  
         [0029]    As is well known in the art, each pixel  112   a - d  may be electrically coupled to display drivers through scan line or active matrix addressing. The scan line configuration is illustrated in FIG. 3 and used in the following description. The present invention may also be coupled to the pixels  112   a - d  in active matrix fashion, as will be apparent to one skilled in the art. In a scan line configuration, each pixel  112   a - d  is addressed by the correspondence of a line  120   a - b  and a column  122   a - f.  A pixel  114   a  is activated when a line  120   a  (acting as a cathode) and a column  122   a  (acting as an anode) provide an electrical path for current to excite a phosphor pixel to throw off photons. An example display  110  has 480 lines that are sequentially activated so that each line is accessed once in a period of approximately {fraction (1/30)} th  of a second. This “paints” the screen in a short enough period that the human eye does not perceive the scan of the individual lines. The activation of each line  120   a - b  is controlled by a line sequencer  124  that addresses each line according to timing provided by a line clock  126 .  
         [0030]    As each line  120   a - b  is activated, the corresponding column  122   a - b  is activated with a pulse width modulated signal that supplies power to the pixel  112   a - d.  A pulse width modulated signal is a signal that provides power through one or more pulses that occur during a signal period, which in this use corresponds approximately to the time that the column is activated to control the pixel. The power supplied by the pulse width modulated signal is described as a proportion of total available power, or duty cycle. The pulse width modulated signal is provided by an analog to pulse width modulated signal generator (APWM)  128   a - f.  An APWM  128   a - f  is coupled to each column  122   a - f.  In an active matrix configuration, an APWM  128   a - f  is coupled to each pixel  112   a  or sub-pixel  114   a - d,    116   a - d  or  118   a - d.  Each APWM  128   a - f  is coupled to a column sequencer  128  that controls the activation of the APWM  128   a - f  to correspond with the column timing. The column timing is provided by a column clock  132 , that is coupled to the column sequencer  130 . Generally, the column clock  132  is derived from the line clock  126 . For instance, an example display will have 640 columns for each line, or 640 column timing pulses occurring during each of the 480 line pulses generated by the line clock  126 .  
         [0031]    Each APWM  128   a - f  is coupled to a data bus  134   a - c  that supplies an analog video signal, such as NTSC or PAL, to the APWN  128   a - f.  The analog signal has a voltage that varies over time within known parameters. By sampling the voltage at a given time in the analog signal, a gray scale value for a particular pixel  112   a - d  can be determined. In an embodiment of the invention, a composite video signal is divided into an analog gray scale signal for each of the primary colors RGB and placed onto a video in signal bus R  134   a,  G  134   b  and B  134   c.  Each APWM&#39;s is coupled to the data bus that corresponds with the color of their sub-pixel, i.e., APWM  128   a  &amp;  d  to R data bus  134   a,  APWM  128   b  &amp;  e  to G data bus  134   b,  and APWM  128   d  &amp;  f  to B data bus  134   c.  Only a single data bus is necessary for a monochrome display.  
         [0032]    In FIG. 4, the present invention is illustrated in block format. A video source block  210  supplies an analog video signal. A column sequencer  212  determines the appropriate time during a video line to activate an AWPM  128   a  to sample the analog video signal. The AWPM  128   a  comprises a pixel data storage “A” circuit  214   a  and a pixel data storage “B” circuit  214   b  that are alternately coupled to the analog video signal by a line A/B sequencer circuit  216 . The A/B sequencer circuit  216  also alternately activates a multiplexer (mux) “B” circuit  218   a  and a mux “A” circuit  218   b.  The A/B sequencer determines the time that a current line is active and changes states at a next line. During a current line, the A/B sequencer enters an “A” state during which the pixel data storage A circuit  214   a  and the mux B  218   b  circuit are active. When a next line becomes the current line, the A/B sequencer circuit  216  enters a “B” state during which the pixel data storage B circuit  214   b  and the mux A circuit  214   b  are active. A next line alternates the A/B sequencer circuit  216  back to the “A” state, and so on.  
         [0033]    The mux B circuit  218   a  at the appropriate time connects to PWM Generator  223  to generate a pixel data value or voltage value stored by the pixel data storage B circuit  214   b  to PWM generator  223  comparison to a voltage reference signal V ref    219  that is supplied to the PWM Generator  223  circuit which at the appropriate time outputs the PWM signal to a driver circuit  220 . Similarly, the mux A circuit  218   b  connects a pixel data value stored by the pixel data storage A circuit  214   a  to PWM generator  223  for comparison to the voltage reference signal V ref    219  that is supplied to the PWM Generator circuit  223  which outputs the PWM signal to the driver circuit  220 . When the A/B sequencer circuit  216  is in the A state, the pixel data storage A circuit  214   a  samples and holds the pixel data (voltage) value from the input video signal  210  and the PWM generator circuit  223  generates a PWM signal based on the pixel data value stored in the pixel data storage B circuit  214   b —stored during a previous “B” state, and now connected to the PWM generator  223  via mux B circuit  218   a.  At the next line, the A/B sequencer circuit  216  transitions to the B state where the pixel data storage B circuit  214   b  samples and holds the pixel data value from video signal  210  and the PWM generator circuit  223  generates a PWM signal based on the pixel data value stored in the pixel data storage A circuit  214   a —stored during a previous “A” state, and now connected to the PWM generation circuit  223  via mux A circuit  218   b.  A pixel  222  (or other load) is driven by the driver circuit  220  when the column sequencer  212  activates the APWM  128   a  with either the mux B circuit  218   a  or the mux A circuit  218   b,  which alternately provide the PWM generator circuit  223  with a stored pixel data values or voltages for generation of PWM signals which form the inputs to the driver circuit  220 .  
         [0034]    In an alternate embodiment for an active matrix display, a pixel element or sub-pixel includes its own circuitry, i.e. one pixel data storage circuit and one PWM generator circuit. The only other component needed for this alternate embodiment is a column sequencer coupled to the pixel data storage circuit.  
         [0035]    An example circuit suitable for implementing the present invention is illustrated in FIG. 5. The pixel data storage A circuit  214   a  includes a transistor Q 1   310  with its source  312  coupled to a video bus  210  and its drain  314  connected to a first end  316  of a resistor R 1   318 . A second end  320  of the resistor R 1   318  is coupled to a first side  322  of a capacitor  324 . A second side  326  of the capacitor  324  is tied to ground. A gate  328  of the transistor Q 1   310  is connected to a drain  330  of a transistor Q 2   332 . A source  334  of the transistor  332  is coupled to a non-inverting output Q of a Flip Flop  338  (Sequencer  216 ). A gate  340  of the transistor Q 2   332  is connected to the column sequencer  212 . The column sequencer  212  is connected to a column clock  132  (FIG. 3) and the Flip Flop  338  is connected to the line clock  126  (FIG. 3).  
         [0036]    The PWM signal generator A circuit  218   b  includes a transistor Q 3   342  that has its source  341  connected to the first side  322  of the capacitor  324 . A gate  344  of the transistor Q 3   342  is connected to an inverting output  346  of the Flip Flop  338 . A drain  348  of the transistor Q 3   342  is coupled to a first end  350  of a resistor R 2   352 . A second end  354  of the resistor R 2  is tied to ground. The first end  350  of the resistor R 2   352  is also connected to a first input  354  of a Schmidt trigger S 1  (or a comparator)  356 . The second input  358  of the Schmidt trigger S 1  is tied to ground or an appropriate reference voltage. In the case where a comparator  356  is used, the second input  358  to comparator  356  would be a connected to a Vref Generator  219  (as shown in FIG. 4), which would supply a reference voltage.  
         [0037]    The pixel data storage B circuit  214   b  includes a transistor Q 4   410  with its source  412  coupled to the video bus  210  and its drain  414  connected to a first end  416  of a resistor R 3   418 . A second end  420  of the resistor R 3   418  is coupled to a first side  422  of a capacitor  424 . A second side  426  of the capacitor  424  is tied to ground. A gate  428  of the transistor Q 4   410  is connected to a drain  430  of a transistor Q 5   432 . A source  434  of the transistor Q 5   432  is coupled to the inverting output/Q  346  of the Flip Flop  338 . A gate  440  of the transistor Q 5   432  is connected to the column sequencer  212 .  
         [0038]    The PWM signal generator B circuit  218   a  includes a transistor Q 6   442  that has its source  441  connected to the first side  422  of the capacitor  424 . A gate  444  of the transistor Q 6   442  is connected to a non-inverting output Q  346  of the Flip Flop  338 . A drain  448  of the transistor Q 6   442  is coupled to a first end  450  of a resistor R 2   452 . A second end  454  of the resistor R 2  is tied to ground. The first end  450  of the resistor R 2   452  is also connected to the first input  354  of a Schmidt trigger S 1  (or a comparator)  356 .  
         [0039]    Another embodiment of a suitable circuit for practicing the invention is shown in FIG. 6. In this embodiment, the resistor R 2   452  is replaced with constant current sources  516 . FIG. 7 illustrates a timing diagram of the sequencers&#39; clocks and the transistors&#39; enabling signals for the A and B states of the circuit shown in FIGS. 4 and 5. FIGS. 7 and 8 illustrate timing diagram of an example sampling and PWM signal generation. In a display line time  700 , Q 1   310  is enabled for a sample period  702  of the analog video signal. Since Q 3   342  is disabled, C 1   310  stores the voltage value of the analog video signal within the sample period  702 . Still within the display line time  700 , Q 4   410  is disabled and Q 6  is enabled, thereby allowing any voltage stored at C 2   424  to discharge through resistor  352  and resistor  452  which are connected to input  354  of the Schmidt trigger S 1   356 . The Schmidt trigger S 1   346  produces a PWM signal by comparing the discharging voltage value from C 2   424  to a bias voltage value  710 . When the discharging voltage value is at or above the bias voltage value  710 , the PWM signal generated is HIGH, otherwise the PWM signal generated is LOW.  
         [0040]    Given resistor R 2   352  and resistor R 4   452  are connected on one end to input  354  and on the other end to ground, it can be appreciated that the circuit can be implemented with only one resistor.  
         [0041]    In the case where a comparator is used the discharging voltage value is compared to a received bias voltage value.  
         [0042]    While the preferred embodiment of the invention has been illustrated and described, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.  
         [0043]    The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: