Abstract:
Multiple pulse width and position modulation methods and systems use multiple pulse width and position modulation (PWPM) circuits driven from the same system clock and video data to provide multiple video pulses per clock period. The multiple pulse width and position modulation methods and systems separately provide extremely fine halftone structures from n-bit per pixel video data words. The halftone structures can be provided without the need for extremely high resolution raster output scanners, associated optics and high speed electronics. The multiple pulse width and position modulation system (PWPM) can produce video pulses of variable width and position within a pixel period with extremely high addressability. The pulses output from multiple, independent pulse width and position modulation (PWPM) channels can be combined to form structured multiple video pulse patterns within the video clock or pixel period from a given n bit video data word. The multiple pulse width and position modulation methods and systems can be used with cathode ray tubes (CRT), laser printers, LED BAR printhead systems, an ink jet head with an ink jet printer, a microwave transmission apparatus, data transcription devices or data encryption devices.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates generally to a digital pulse modulator. More particularly, this invention is directed to methods and systems that use multiple pulse width and position modulation (PWPM) circuits driven from the same system clock and video data to provide multiple video pulses per clock period. 
     2. Description of Related Art 
     In a system that uses one or more beams to record information, for example, on a photoreceptor, a digital pulse forming circuit may be used to control the one or more beams. Each beam may vary in intensity and duration according to the pulses used to control that beam. 
     One or more laser beams may be used in a printer or photocopier, for example, for discharging negative image areas on a photoreceptor. The latent electrostatic image formed on the photoreceptor by the one or more beams attracts developing toner in proportion to the latent image charge level in order to develop the image. 
     As another example, a cathode ray tube uses an electron beam to scan a phosphorus screen. The electron beam may be varied in intensity and duration to accurately display information on the phosphorous screen. 
     In both examples, a pulse forming circuit may be used to generate pulses to control the intensity and operation duration of the respective beams. In a high speed image forming system, the reset time of a pulse forming circuit is the time needed for the pulse forming circuit to reset to an initial state before a new pulse can be generated. Thus, the speed of a pulse forming circuit is limited by the amount of time it takes the circuit to form a pulse and reset to its initial state. U.S. Pat. Nos. 4,965,675 to Duke et al., 4,905,023 to Suzuji, 4,375,065 to Ohara, and 4,347,523 to Ohara each describe pulse forming circuits, systems and/or methods. 
     U.S. Pat. No. 5,184,226 to Cianciosi, incorporated herein by reference in its entirety, describes a digital electronics system that generates pulses from a series of data words. The digital electronics system includes lookup tables to translate each data word into a pulse attribute word. Each pulse attribute word includes information to controllably form a corresponding pulse. The digital electronics system also includes one or more multiplexers that split the series of pulse attribute words, generated from the series of data words, into two channels. 
     The digital electronics system of the 226 patent further includes pulse forming circuits for each channel. Each pulse forming circuit receives the pulse attribute word from the corresponding channel. Each pulse forming circuit forms a pulse based on the received pulse attribute word. The digital electronics system of the 226 patent additionally includes control circuits that generate and output the pulses to the beam emitting devices. In particular, in the digital electronics system described in the 226 patent, a first pulse from a first pulse forming circuit is generated while a second pulse from a second pulse forming circuit is being formed. 
     SUMMARY OF THE INVENTION 
     This invention provides methods and systems that provide multiple pulses per clock period. 
     This invention separately provides methods and systems that use multiple pulse width and position modulation (PWPM) circuits driven from the same system clock and data signals. 
     This invention separately provides systems and methods for providing extremely fine halftone structures from n-bit per pixel image data words. 
     One exemplary embodiment of the multiple pulse width and position modulation systems and methods according to this invention includes two or more of the digital electronics systems described in the 226 patent. Each of these two or more digital electronic systems forms one pulse width and position modulation circuit that is able to output a single pulse that is highly accurately positioned within a pixel period and that has a highly accurate pulse width. The pulse output from the two or more digital electronic systems are combined to form a single signal having one or more pulses, where each pulse is highly positionable and each pulse has a highly accurate pulse width. It should be appreciated that any known or later developed pulse width and position modulation method and/or circuit may be used with the multiple pulse width and position modulation systems and methods of this invention. 
     In accordance with one aspect of the multiple pulse width and position modulation systems and methods of this invention, the halftone structures can be provided without needing extremely high resolution raster output scanners, associated optics and high speed electronics. 
     The multiple pulse width and position modulation systems and methods according to this invention can produce one or more pulses of variable width and position within a pixel period with extremely high addressability. According to the multiple pulse width and position modulation systems and methods of this invention, the pulses output from multiple, independent pulse width and position modulation channels are combined to form structured multiple pulse patterns within the clock or pixel period for a given n-bit data word. 
     These and other features and advantages of this invention are described in or are apparent from the following detailed description of the circuits, systems and methods according to this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of the multiple pulse width and position modulation systems and methods of this invention will be described in detail, with reference to the following figures, wherein: 
     FIG. 1 shows an exemplary embodiment of a raster output scanner-type laser printer; 
     FIG. 2 illustrates an exemplary pulse with variable width, position, and amplitude; 
     FIG. 3 is block circuit diagram of one exemplary embodiment of a multiple pulse width and position modulation system of FIG. 1 according to this invention; 
     FIG. 4 is a schematic diagram showing multiple individual pulses and a combined pulse for a single clock period; 
     FIG. 5 is a block circuit diagram outlining one exemplary embodiment of the pulse width and position modulation circuit of FIG. 3 according to this invention; and 
     FIG. 6 is a block circuit diagram outlining another exemplary embodiment of the pulse width and position modulation circuit of FIG. 3 according to this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows an exemplary embodiment of a raster output scanner-type laser printer  100  that incorporates the multiple pulse width and position modulation system  200  according to this invention. As shown in FIG. 1, the laser printer  100  is connected to an image data source  201  that outputs a series of image data words representative of an image to be printed by the laser printer  100 . The image data source  201  can be any known or later developed device or system capable of generating the image data words. The image data source  201  is connected to the laser printer  100  by a link  202 . The link  202  can be any known or later developed device or system for transmitting the image data words to the laser printer  100 . 
     The laser printer  100 , in addition to the multiple pulse width and position modulation system  200 , also includes a laser modulator subsystem  120  and an image forming subsystem  130 . The laser modulator system  120  includes a laser modulator  122  that generates one or more modulated laser beams  124 , a rotating polygon scanner  126  having a plurality of reflective facets  127  and an optics system  128  that focuses the one or more laser beams  124  onto a photoreceptor  140  of the image forming subsystem  130 . The laser modulator  122  is connected to the multiple pulse width and position modulation system  200  by a signal line  110 . 
     As shown in FIG. 1, one exemplary embodiment of the photoreceptor  140  is an endless belt stretched across a pair of drive and idler belt support rollers  142  and  144 , respectively. Latent electrostatic images representative of the image defined by the image data words received from the image data source  201  are formed on the photoreceptor belt  140  by modulating the laser beam  124 . The belt support rollers  142  and  144  are rotatably mounted in predetermined fixed positions. The support roller  144  is driven by a suitable drive motor to move the photoreceptor  140  in the direction shown by the solid line arrow. While the photoreceptor  140  is illustrated in the form of an endless belt, drum photoreceptors and any other known or later developed photoreceptor can be used with the multiple pulse width and position modulation systems and methods of this invention. 
     A corona charging device  150 , commonly known as a corotron, is operatively disposed adjacent to the photoreceptor  140  at a charging station. The corotron  150 , which is coupled to a suitable negative high voltage source  191 , serves to place a uniform negative charge on the photoreceptor  140  in preparation for imaging. 
     The one or more laser beams  124  of the laser modulator subsystem  120  are incident on the photoreceptor  140  at an exposure point  129 . The intensity and duration of each laser beam  124  is determined based on a corresponding pulse signal output by the multiple pulse width and position modulation system  200 . The one or more laser beams  124  are swept across the photoreceptor  140  transverse to the indicated direction of motion by the rotating polygon mirror  126 . 
     A development subsystem  170  is disposed in operative contact with the photoreceptor  140  downstream of the contact point  129  of the one or more laser beams  124 . The development subsystem  170  preferably comprises a non-scavenging development system using a mono-component developer. The mono-component developer is preferably a relatively small colorant material, referred to as a toner. Due to electrostatic forces, the toner is drawn to the latent electrostatic image formed on the photoreceptor  140  in proportion to the charge level of the latent image to develop the image. In this exemplary embodiment, a discharge development system is used as the development subsystem  170 . 
     Thus, following negative charging of the photoreceptor  140  by the corotron  150 , image areas are discharged by the laser beam  124  in accordance with the pulse signals from the multiple pulse width and position modulation system  200 . The developing toner is negatively charged and is therefore attracted to the discharged image areas while being repelled from the non-discharged areas. The development subsystem  170  includes a suitable developer housing (not shown) within which a supply of developer is provided together with any known or later developed device for supplying the developer to the photoreceptor  140 . However, it should be appreciated that any other known or later developed development system could equivalently be used in place of this discharge development system. 
     In the discharge development subsystem  170 , when the intensity of any particular laser beam  124  is at a maximum value, maximum development occurs and a fully black pixel is obtained. When that laser beam  124  is turned off, no development occurs and a white pixel is obtained. In the exemplary embodiment of the laser printer  100 , multiple intermediate gray pixel levels are provided. These levels are obtained by providing intermediate intensity levels, so that corresponding intermediate amounts of development take place to provide predetermined light gray and dark gray pixels. The image data words received from the image data source  201  contain the information for controlling the beam intensity and duration. 
     Following development of the latent electrostatic image on the photoreceptor  140  by the developing subsystem  170 , the developed image is transferred to a suitable copy or print substrate material  190 , such as paper, at a transfer station. To facilitate transfer, a transfer corotron  182 , which is coupled to a high voltage power source  189 , is provided to attract the developed image from the photoreceptor  140  onto the copy substrate material  190 . Following transfer, the developed image is fixed by fusing the toner onto the substrate  190 . Any residual charges and/or developing material left on the photoreceptor  140  are removed at cleaning station by an erase lamp and cleaning brush  160 . 
     While the laser printer  100  illustrated in FIG. 1 is a raster output scanner, any known or later developed image forming device, or any other known or later developed type of device that uses a modulated pulse to energize a beam or output element can be used with the multiple pulse width and position modulation system  200  according to this invention. Thus, the image forming device could be a digital photocopier, a liquid image development-type printer or photocopier, a full-width array type printer or photocopier, a pulse-driven ink jet printer of any type, or any other known or later developed pulse-driven image output device. It should also be appreciated that, in general, any known or later developed pulse-width modulated light emitting device can be used in place of the laser modulator  122 . In particular, the light emitting device can include one or more of a laser, a semiconductor laser, a light emitting diode, an organic light emitting diode, or a laser diode. 
     Similarly, instead of an image output device, the pulse width and position modulation system  200  can be incorporated into any other known or later modulated-pulse-driven device, such as a cathode ray tube (CRT), light-emitting diode-type display, liquid crystal display, or the like. 
     Moreover, other pulse driven devices that can incorporate the pulse width and position modulation system  200  include acousto-optic modulators, which are commonly used to modulate helium-neon or other gas lasers. In particular, these devices use modulated radio frequency signals to modulate the laser beam output by the helium-neon or other gas lasers. In this system, the pulses output by the pulse width and position modulation system  200  would be used to modulate the radio frequency signals. 
     Similarly, other pulse driven devices that can incorporate the pulse width and position modulation system  200  include devices that modulate radio frequency signals for direct transmission or propagation as electromagnetic waves. For example, systems that generate coded transmissions of digital data can use the pulse width and position modulation system  200  to control pulses similar to the pulses that are used for radar etc. 
     Furthermore, any device where a precise pulse train is used to create a specialized analog pulse waveform can use the pulse width and position modulation system  200 . For example, the pulse width and position modulation system  200  can be used to generate a pulse train input to a charge pump device, an integrator, or an electrical, optical or other known or later developed filter, to create a specialized analog waveform. One example waveform is a gaussian shaped pulse. The pulse width and position modulation system  200  can be used to control the pulse width and position relative to the time constants of a device like the charge pump, the integrator, or the filter to determine the shape of the output analog signal. 
     Finally, the pulse width and position modulation system  200  can be used in automatic test equipment to create precise pulses for testing and characterizing electronic devices and systems. 
     FIG. 2 shows how a single pulse of a pulse signal output by the multiple pulse width and position modulation system  200  is generated based on an image data word. As shown in FIG. 2, the width and position of a pulse  520  within a pixel period  500  may be varied with separate, independently-variable, delays for the leading edge  522  and the trailing edge  524  of the pulse  500 . A leading edge delay  530  is defined from the beginning  510  of the pixel period  500  to the leading edge  522  of the pulse  520 . A trailing edge delay  540  is defined from the leading edge  522  of the pulse  520  to the trailing edge  524  of the pulse  520 . Alternately, the trailing edge delay can be defined from the beginning  510  of the pixel period  500 . 
     FIG. 3 shows one exemplary embodiment of the multiple channeling of pulse width and position modulation system  200  according to this invention. As shown in FIG. 3, the pulse width and position modulation circuit includes a number i of individual pulse width and position modulation circuits  210 - 240 . Each of the i pulse width and position modulation circuits  210 - 240  includes a pulse width and position modulation lookup table  212 - 242  and a pulse width and pulse modulation processing circuit  214 - 244 . The i pulse width and position modulation circuits  210 - 240  are each connected to the image data source  201  to input over a data bus  205  the same image data word, as each image data word is received from the image data source  201  over the signal line  202 . 
     Each image data word is an n-bit image data word. The n-bit image data words enter the i pulse width and position modulation (PWPM) lookup tables  212 - 242 . Each data word represents an address within each of the lookup tables  212 - 242 . Each of the i lookup tables  212 - 242  outputs a distinct data attribute from the address defined by the image data word that is input to all of the i lookup tables  212 - 242  at the same time. Each distinct data attribute word output from the i lookup tables  212 - 242  is input by the corresponding pulse width and position modulation processing circuit  214 - 244 . Each of the i pulse width and position modulation processing circuits  214 - 244  converts the distinct data attribute word received from the corresponding pulse width and position modulation lookup table  212 - 242  into a distinct pulse signal having a pulse. That pulse has a defined pulse width and a defined position within the pixel period corresponding to the input image data word. The i distinct pulse signals generated by the i pulse width and position modulation processing circuits  214 - 244  are output over i signal lines  216 - 246  to a signal combining circuit  250 . 
     As shown in FIG. 3, one exemplary embodiment of the signal combining circuit  250  is an i-input OR gate. However, any known or later developed device or circuit that is capable of combining the i distinct pulse signals output from the i pulse width and position modulation processing circuits  214 - 244  can also be used as the signal combining circuit  250 . The signal combining circuit  250  combines the i distinct pulse signals into a single pulse signal having up to i distinct pulses. The single pulse signal is then output from the signal combining circuit  250  onto the signal line  110 . 
     FIG. 4 shows a number of individual pulse width and position modulated signals  620 - 650  and the resulting composite multiple-pulse pulse width and position modulated signal  660  according to the multiple pulse width and position modulation systems and methods of this invention. As shown in FIG. 4, within each clock period  612  occurring in the clock signal  610 , each of the i individual pulse width and position modulated signals  620 - 650  will contain at most one pulse  622 - 652 , although none of the i signals  620 - 650  needs to have a pulse. That is, any particular signal  620 - 650  may have 0 pulses. 
     As shown in FIG. 4, a particular clock period  612  begins at time t 1 , and extends until time t 10 . Within this particular clock period  612 , the pulse  622  occurring in the signal  620  extends from time t 2  to time t 3 . Similarly, the pulse  632  in the signal  630  begins at time t 4  and continues until time t 5 . The pulse  642  in the signal  640  occurs between times t 6  and t 7 . Finally, shown in FIG. 4, the i th  pulse  652  in the i th  signal  650  occurs between times t 8  and t 9 . Accordingly, when the pulses occurring in the clock period  612  in the i signals  620 - 650  are combined to form the composite signal  660 , the composite signal  660  includes four pulses  662 ,  664 ,  666  and  668 , corresponding to the four pulses  622 ,  632 ,  642  and  652 , respectively. In particular, the pulse  662  occurs between times t 2  and t 3 , while the pulse  664  occurs between times t 4  and t 5 , the pulse  666  occurs between times t 6  and t 7 , and the pulse  668  occurs between times t 8  and t 9 . 
     FIG. 5 shows one exemplary embodiment of a pulse width and position modulator  300  used to form each of the i pulse width and position modulation circuits  210 - 240 . As shown in FIG. 5, each pulse width and position modulator  300  inputs n-bit data words from the image data source  201 . Each input data word is provided over a data bus  305  to a plurality of lookup tables  310 ,  320 ,  330 , and  340  that together form one of the lookup tables  212 ,  222 ,  232  or  242 . 
     The data bus  305  is connected to the lookup tables  310 - 340  through a lookup table data/image data multiplexer  304 . Each data word represents an address within the four lookup tables  310 ,  320 ,  330 , and  340 . In this exemplary embodiment, two pairs of 256x4 ECL RAM memories are used to form the lookup tables  310 - 340 . Each pair of lookup tables  310 , and  320 , or  330  and  340 , is used to generate a pulse attribute word for each pulse attribute sought to be controlled. Pulse attributes may include leading edge delay, trailing edge delay, amplitude of the pulse to be formed, and other special features. Alternatively, a single 256x8 ECL RAM memory may be used in place of each pair of 256x8 ECL RAM memories. The exemplary embodiment of the pulse width and position modulator  300  shown in FIG. 5 includes two pairs of 256x4 RAM memories forming the lookup tables  310  and  320 , and  330  and  340 , respectively, corresponding to the two pulse attributes of leading edge delay and trailing edge delay to be generated for each phase. The pulse width and position modulator  300  will accommodate as many pairs of lookup tables as there are desired pulse attributes. For example, a third pair of lookup tables may be used to control the amplitude of a pulse to be formed. 
     Once an address in each lookup table  310 - 340  is accessed by the data word, each lookup table  310 - 340  generates a nibble (4 bits) of information. Thus, each pair of lookup tables  310  and  320 , and  330  and  340 , respectively, generates an 8-bit pulse attribute word corresponding to the pulse attribute sought to be controlled. 
     Characteristic data indicative of the pulse attributes sought to be controlled in a pulse width and position modulator  300  may be downloaded from the image data source  201  or another data source and stored into the lookup tables  310 ,  320 ,  330 , and  340  through the lookup table download interface  301 . By using a lookup table download interface  301  for the lookup tables  310 ,  320 ,  330 , and  340 , the pulse attributes can be changed by loading a new set of pulse attribute data into the lookup tables  310 ,  320 ,  330 , and  340  before printing. 
     To load data into the lookup tables  310 ,  320 ,  330 , and  340 , the lookup table download interface  301  first instructs the lookup table data/image data multiplexer  304  to connect the lookup table address bus  302 , instead of the data bus  305 , to the lookup tables  310 - 340 . In this way, the lookup table download interface  301  may designate the addresses that correspond to the memory locations in the lookup tables  310 , 320 ,  330 , and  340  that are to be changed. 
     Once the lookup table download interface  301  accesses an address of one of the lookup tables  310 - 340 , a pulse attribute data nibble may be loaded into that lookup table  310 ,  320 ,  330  or  340  through the lookup table data bus  303  from the lookup table download interface  301 . This allows for different mapping functions to be provided in the pulse width and position modulator  300  for different printing characteristics, such as, for example, font smoothing, graphics, etc.. This further facilitates maintenance of print quality as the components of the system age. Thus, for example, the pulse characteristics can be changed as the photoreceptor  140  ages. After the lookup tables  310 ,  320 ,  330 , and  340  are loaded, the lookup table download interface  301  instructs the lookup table data/image data multiplexer  304  to receive data from the data bus  305 . 
     The four lookup tables  310 ,  320 ,  330 , and  340  perform a logic mapping function, which translates the incoming data word into two pulse attribute words to control the formation of a pulse. The two lookup tables  310  and  320  generate separate nibbles of pulse attribute information which combine to form a pulse attribute word for the leading edge delay of a pulse to be formed. The two lookup tables  330  and  340  generate separate nibbles of pulse attribute information which combine to form a pulse attribute word for the trailing edge delay of a pulse to be formed. 
     Each pulse attribute word from the pairs of lookup tables  310 - 320  and  330 - 340  are input to one of the two multiplexer and latch blocks  350  and  360 , respectively. Each multiplexer and latch block  350  and  360  contains two latches, one for each of the phase  1  buses  352 , and  362 , and the phase  2  buses  354  and  364 . The two pulse attribute words generated in the pairs of lookup tables  310 - 320 , and  330 - 340  from a data word are latched onto the phase  1  buses  352  and  362  by the respective multiplexers  350  and  360  at a leading edge of a pulse on the phase  1  clock signal output on a signal line  372  from the training and control circuit  370 . 
     The two data words latched on the respective phase  1  buses  352  and  362  are further processed on separate channels of a delay logic circuit  380  and a pulse forming logic circuit  390 . The delay logic circuit  380  forms separate leading and trailing edge delayed pulses. The pulse forming logic circuit  390  forms a single pulse from the leading and trailing edge delay pulses. The multiplexer and latch blocks  350  and  360 , the timing and control circuit  370 , the delay logic circuit  380  and the pulse forming circuit  390  together form one of the pulse width and position modulation processing circuits  214 ,  224 ,  234  or  244 . 
     FIG. 6 shows a second exemplary embodiment of a pulse width and position modulator  400  usable as one of the pulse width and position modulation circuits  210 - 240 . As shown in FIG. 6, the pulse width and position modulator  400  is similar to the pulse width and position modulator  300 , except that the data words are immediately channeled onto phased buses  452 ,  454 ,  462  and  464  before the data words are sent to the lookup tables  410 ,  420 ,  430 , and  440 . 
     The phasing of the data words in the pulse width and position modulator  400  operates in the same manner as in the pulse width and position modulator  300 . In the pulse width and position modulator  400 , four 256x8 ECL RAM lookup tables are used to form the lookup tables  410 - 440 . Alternatively, 4 pairs of 256x4 ECL RAM lookup tables may be used. A first data word is phased onto the phase  1  buses  452  and  462  and sent to the phase  1  lookup tables  410  and  430  under control of the phase  1  clock signal output on a signal line  472  from the timing and control circuit  470 . 
     Then, a next data word is phased by the address multiplexers and latches  450  and  460  onto the phase  2  buses  454  and  464  and sent to the phase  2  lookup tables  420  and  440  under control of the phase  2  clock signal output on the signal line  474 . A data word addresses two pulse attribute words, one corresponding to each pulse attribute, from the phase  1  lookup tables  410  and  430 . These two pulse attribute words are sent into the delay logic circuit  480 , still under control of the phase  1  clock signal. Likewise, the second data word addresses two pulse attribute words from the phase  2  lookup tables  420  and  440 . Those two pulse attribute words are sent into the delay logic circuit  480 . Again, the pulse width and position modulator  400  may be expanded to accommodate a greater number of pulse attribute words and a greater number of phased routes into the lookup tables  410 ,  420 ,  430  and  440  and into the delay and pulse forming logic circuits  480  and  490 . 
     Because the data words are channeled before the lookup tables  410 ,  420 , 430  and  440 , slower lookup tables may be used. As in the case of separate phased channels going into the delay and pulse forming logic circuits  480  and  490 , performance equivalent to that of faster, more expensive circuitry may be achieved with slower, less expensive lookup tables. The cost trade off may be balanced by the cost of fewer faster circuit components versus the cost of several slower circuit components. In the pulse width and position modulator  400 , for example, twice as many RAM lookup tables are required to replace the lookup tables of the pulse width and position modulator  300 . Similarly, in the delay and pulse forming circuits  480  and  490 , and also in the delay and pulse forming logic blocks  380  and  390  of FIG. 5, twice as many delay and pulse forming circuits are required for two-phased processing, and three times as many delay and pulse forming circuits are required for three-phased processing, than are required for single-phased processing. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.