Abstract:
A light emitting diode (LED) print head arranged to receive a set of raw print data. The print head includes a plurality of LED array chips each having a plurality of LEDs, wherein each LED of the plurality of LEDs includes a process direction position. The print head further includes at least one matrix drive circuit arranged to control activation of the plurality of LEDs in response to the set of raw print data and at least one real-time print data compensation circuit, wherein the at least one real-time print data compensation circuit receives the set of raw print data and delays transmitting the set of raw print data to the at least one matrix drive circuit based on a relative relationship between the process direction position of each LED.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The following co-pending applications are incorporated herein by reference in their entireties: U.S. patent application Ser. Nos. 12/232,220, filed on Sep. 12, 2008 and 12/858,753, filed on Aug. 18, 2010. The following patent is incorporated herein by reference in its entirety: U.S. Pat. No. 6,172,701, issued on Jan. 9, 2001. 
     
    
     TECHNICAL FIELD 
       [0002]    The presently disclosed embodiments are directed to providing high resolution light emitting diode (LED) print-heads (LPH). 
       BACKGROUND 
       [0003]      FIG. 1  schematically shows prior art image recording apparatus  20  with light-emitting diode (LED) printbar  21 . It should be appreciated that an LED printbar may also be referred to as an LED printhead (LPH). Printbar  21  is an example of an LED full width array imager. An LED full width array imager consists of an arrangement of a large number of closely spaced LEDs in a linear array. By providing relative motion between the LED printbar and a photoreceptor in a process direction, and by selectively energizing the LEDs at the proper times in a scan direction, a desired latent electrostatic image can be produced on the recording member. The production of a desired latent image is usually performed by having each LED expose a corresponding pixel on the recording member in accordance with image-defining video data information applied to the printbar through driver circuitry. Conventionally, digital data signals from a data source, which may be a Raster Input Scanner (RIS), a computer, a word processor or some other source of digitized image data is clocked into a shift register. Some time after the start of a line signal, individual LED drive circuits are then selectively energized to control the on/off timing of currents flowing through the LEDs. The LEDs selectively turn on and off at fixed intervals to form a line exposure pattern on the surface of the photoreceptor. A complete image is formed by successive line exposures. 
         [0004]    The following provides further detail regarding prior art apparatus  20 . Printbar  21  includes: LED&#39;s controlled according to recording signals supplied from an unrepresented external device; a rotary drum  22  provided with a photoreceptor along the periphery thereof; a rod lens array  23  for focusing the light beams of the LEDs in the printing head  21  onto the photoreceptor surface of the drum  22 ; a corona charger  24  for charging the photoreceptor in advance; a developing station  25  for developing an electrostatic latent image with toner; a recording sheet  26 ; a cassette  27  housing a plurality of recording sheets  26 ; a feed roller  28  for feeding the recording sheet  26  from the cassette  27 ; registration rollers  29  for matching the front end of the recording sheet with the leading end of the image formed on the drum  22 ; a transfer charger  30  for transferring the developed image from the drum  22  onto the recording sheet  26 ; a separating roller  31  for separating the recording sheet from the drum  22 ; a belt  32  for transporting the recording sheet; fixing rollers  33 ; discharge rollers  34  for discharging the recording sheet onto a tray  35 ; a blade cleaner  36  for removing the toner remaining on the drum  22 ; a container  37  for the recovered toner; and a lamp  38  for eliminating charge remaining on the drum  22 . 
         [0005]    An LED print-head (LPH), e.g., LPH  50 , is built by assembling a number of LED array chips end-to-end, e.g., LED array chips  52   1  through  52   40 . For example, a 1200 dots per inch (dpi) A3 sized print-head could be constructed by assembling forty (40) individual LED array chips each having three hundred eighty four (384) LEDs, or fifteen thousand three hundred sixty (15,360) total LEDs across the entire LPH, as shown in  FIG. 1A . Typically this type of print-head is driven with a ⅛th matrix drive circuit, e.g., matrix drive circuits  54   1  through  54   48 , (See U.S. Pat. No. 6,172,701) and has an LED arrangement as shown in  FIGS. 1B through 1D . It should be appreciated that the foregoing arrangement includes a single shift register, i.e., shift register  56 , which loads print data into the entire LPH and results in all LEDs within the LPH firing within eight actuations of the strobe. Thus, for example, LED  58   a  of LED array chip  52   1  simultaneously fires as all other LEDs  58   a  on all other LED array chips  52   2  through  52   40  fire. Similarly, all LEDs  58   b,    58   c,    58   d,    58   e,    58   f,    58   g,    58   h  collectively as groups simultaneously fire on all LED array chips  52   1  through  52   40 . 
         [0006]    Ideally, the process direction profile of the LEDs of the print-head would be flat, i.e., there is no process direction profile in the scan direction; however, this arrangement does not occur as it is not possible to manufacture LED array chips in such a fashion. It should be appreciated that as used herein, the process direction is represented by bi-directional arrow  59   a  while the scan direction is represented by bi-directional arrow  59   b.  As array chips  52   1  through  52   40  are positioned end-to-end, a true straight line is not formed thereby resulting in process direction profile errors. The measured process direction profile of a typical print-head of the type shown in  FIG. 1A  is provided in  FIG. 2A . It has a process direction profile range of 106 μm (+80 μm to −26 μm) across the length of the LPH. Alternately, a chip averaged process direction profile of  FIG. 2A , is shown in  FIG. 2B , and has a range of 88 μm (+69 μm to −19 μm). It should be appreciated that the “chip average” is the average process direction position of all LEDs within a discreet LED array chip. A typical specification range for the measured process direction profile is 100 μm, which is approximately equal to five lines of correction for a 1200 dpi×1200 dpi printing system. 
         [0007]    For a 1200 dpi×1200 dpi printing system, the process direction profile range of a typical LPH represents a misalignment of about five scan lines in a printed image, while for a 1200 dpi×2400 dpi system it represents a misalignment of about eight scan-lines. It should be appreciated that the foregoing misalignments are based on the physical location of the LEDs and are therefore present in a printed image if a method of process direction profile compensation is not implemented. Moreover, the misalignment is further increased based on the fact that between the time that the first LED on a chip is activated and the last LED on that chip is activated, the print media, e.g., toner drum, sheet, image bearing belt, etc., has traveled in the process direction a particular distance based on the media&#39;s travel speed. 
         [0008]    Thus, the process direction profile of an LED print-head impacts the image quality of a printer unless a method of compensation is implemented with the print controller. Although use of the print controller improves image quality, such use is quite complex and results in significant processing overhead which may then impact print speed. In known systems, the print controller variably delays the print data in the scan direction in order to minimize the misalignment, i.e., to compensate for the LPH process direction profile. For a ⅛th matrix driven LPH with print controller process direction profile compensation, the effective process direction profile range can be reduced to approximately one scan-line. Although this is a significant improvement, the improvement is provided at the expense of processing overhead and complexity. 
         [0009]    The present disclosure addresses a system and method for compensating for LED print-head process direction profile variability. 
       SUMMARY 
       [0010]    Broadly, the apparatus discussed infra provides an LED driver chip with configurable print delay that compensates for process direction profile variability of an LED print-head. The present apparatus provides improved image quality without additional overhead or complexity to the printer controller by the addition of various circuits within the data path, which circuits create variable print delays based on the characteristics of the LPH. The present disclosure proposes two embodiments that integrate the process direction profile compensation into the LED driver chips of the print-head, thereby eliminating the need to perform the compensation via the print controller. Each of the embodiments increases the use of the existing compensation memory of an LED print-head to provide additional information to the LED driver chips, thereby enabling the driver chips to perform “line” and “sub-line” process direction profile compensation. In other words, print delay values for the various LED array chips are loaded into memory, typically resident on the LED driver chip, wherein such delay values control where within a chain of shift registers print data is loaded. Thus, the present embodiments determine how many lines of delay are necessary and subsequently load the print data into the various shift registers corresponding to the necessary lines of delay. 
         [0011]    According to aspects illustrated herein, there is provided an apparatus for providing “line” compensation which reduces the effective process direction profile to less than or equal to one scan-line, i.e., the same improvement obtained with print controller based compensation but without the processing overhead. While according to other aspects illustrated herein, there is provided an apparatus for providing “sub-line” compensation which further reduces the effective process direction profile of the print-head to ⅛th of the process direction resolution thereby resulting in an additional eight times (8×) improvement without any additional processing overhead. Such apparatus includes a light emitting diode (LED) print head arranged to receive a set of raw print data. The print head includes a plurality of LED array chips each having a plurality of LEDs, wherein each LED of the plurality of LEDs includes a process direction position. The LED print head further includes at least one matrix drive circuit arranged to control activation of the plurality of LEDs in response to the set of raw print data and at least one real-time print data compensation circuit, wherein the at least one real-time print data compensation circuit receives the set of raw print data and delays transmitting the set of raw print data to the at least one matrix drive circuit based on a relative relationship between the process direction position of each LED. 
         [0012]    According to still other aspects illustrated herein, there is provided a method for calculating the necessary “line” and “sub-line” compensation values used by the present apparatus. Such method is a method for quantifying a process direction profile for a light emitting diode (LED) print head having a plurality of LED array chips, each LED array chip including a plurality of LEDs. The method includes: a) measuring a process direction profile value Y(N, n) for each of the plurality of LEDs of each of the plurality of LED array chips, wherein N is a number associated with each of the plurality of LED array chips and n is a number associated with each of the plurality of LEDs; b) calculating an average process direction profile value Y_AVG(N) for each of the plurality of LED array chips; c) determining a maximum average process direction profile value Y_MAX from Y_AVG(N); d) calculating a line compensation value L(N) for each of the plurality of LED array chips; and, e) storing each line compensation value L(N) in a memory, wherein the LED print head includes the memory. 
         [0013]    Other objects, features and advantages of one or more embodiments will be readily appreciable from the following detailed description and from the accompanying drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Various embodiments are disclosed, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols indicate corresponding parts, in which: 
           [0015]      FIG. 1  schematically shows a prior art image recording apparatus with a light-emitting diode (LED) printbar 
           [0016]      FIG. 2A  is a top plan view of an LED print-head; 
           [0017]      FIG. 2B  is a schematic view of a portion of the LED print-head of  FIG. 1A ; 
           [0018]      FIG. 2C  is a schematic view of a portion of an LED array chip; 
           [0019]      FIG. 2D  is a schematic view of a portion of the LED array chip of  FIG. 1C ; 
           [0020]      FIG. 3A  is a graph depicting a typical process direction profile of an LED print-head; 
           [0021]      FIG. 3B  is a graph depicting a typical chip averaged process direction profile of the process direction profile of  FIG. 2A ; 
           [0022]      FIGS. 4A-1  and  4 A- 2  depict an embodiment of a real-time print data compensation circuit arranged to provide line compensation; 
           [0023]      FIG. 4B  is an embodiment of a line compensation register; 
           [0024]      FIG. 5  is a graph depicting a typical chip averaged process direction profile of an LED print-head, a line compensated chip averaged process direction profile of a 1200 dpi LED print-head, and a line compensated chip averaged process direction profile of a 2400 dpi LED print-head; 
           [0025]      FIGS. 6A-1  and  6 A- 2  depict another embodiment of a real-time print data compensation circuit arranged to provide line and sub-line compensation; 
           [0026]      FIG. 6B  is an embodiment of a sub-line compensation register; 
           [0027]      FIG. 6C  is an embodiment of a line compensation register; 
           [0028]      FIG. 7  is a graph depicting a typical chip averaged process direction profile of an LED print-head, a line and sub-line compensated chip averaged process direction profile of a 1200 dpi LED print-head, and a line and sub-line compensated chip averaged process direction profile of a 2400 dpi LED print-head; and, 
           [0029]      FIG. 8  is a block diagram depicting a present method for quantifying a process direction profile for a light emitting diode print head. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the embodiments set forth herein. Furthermore, it is understood that these embodiments are not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the disclosed embodiments, which are limited only by the appended claims. 
         [0031]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which these embodiments belong. The words “printer,” “printer system”, “printing system”, “printer device” and “printing device” as used herein encompasses any apparatus, such as a digital copier, bookmaking machine, facsimile machine, multi-function machine, etc. which performs a print outputting function for any purpose, while “multi-function device” and “MFD” as used herein is intended to mean a device which includes a plurality of different imaging devices, including but not limited to, a printer, a copier, a fax machine and/or a scanner, and may further provide a connection to a local area network, a wide area network, an Ethernet based network or the internet, either via a wired connection or a wireless connection. An MFD can further refer to any hardware that combines several functions in one unit. For example, MFDs may include but are not limited to a standalone printer, one or more personal computers, a standalone scanner, a mobile phone, an MP3 player, audio electronics, video electronics, GPS systems, televisions, recording and/or reproducing media or any other type of consumer or non-consumer analog and/or digital electronics. Additionally, as used herein, “sheet,” “sheet of paper” and “paper” refer to, for example, paper, transparencies, parchment, film, fabric, plastic, photo-finishing papers or other coated or non-coated substrate media in the form of a web upon which information or markings can be visualized and/or reproduced. As used herein, the term ‘average’ shall be construed broadly to include any calculation in which a result datum or decision is obtained based on a plurality of input data, which can include but is not limited to, weighted averages, yes or no decisions based on rolling inputs, etc. Furthermore, as used herein, when referring to logical values, 0 and logical low are used interchangeably while 1 and logical high are also used interchangeably. Still yet further, as used herein, “real-time” is intended to mean data manipulation and/or compensation which occurs with little or no use of a processor, e.g., print controller processor, thereby resulting in efficient data manipulation and/or compensation without added processor overhead, such as delaying raw data transmission without any computational analysis of the same, while “raw data” and “raw print data” are intended to mean data which has not been modified from its original form and sequence, such as a single unmodified printer scan line of data. “Activation” of an LED, as used herein, is intended to mean either providing energy to an LED or not providing energy to an LED depending on whether image data is present at the particular location corresponding to that LED, i.e., in both the process and cross-process directions. 
         [0032]    Moreover, although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of these embodiments, some embodiments of methods, devices, and materials are now described. 
         [0033]    To address the issues described above, in an embodiment, the present apparatus includes a new LED matrix driver architecture that automatically compensates the process direction profile of an LPH to within one scan line while causing virtually no change to print controller processing overhead. Additionally, in an embodiment, the present apparatus includes a new LED matrix driver modified to further reduce the process direction profile to ⅛th of the resolution of a scan line, i.e., an eight times (8×) improvement in resolution compared with known device resolutions, while also causing virtually no change to print controller processing overhead. 
         [0034]    To implement the first embodiment, “serial-in-parallel-out” (SIPO) shift register  56  of LPH  50  ( FIG. 1B ) is replaced by combinations  60   a  through  60   g  of SIPOs and “parallel-in-parallel-out” PIPO shift registers, e.g., one SIPO  62  and seven PIPO shift registers  64 , where there is one combination for each scan line of desired process direction profile compensation. Collectively, these combinations of SIPO shift registers and PIPO shift registers are referred to as “line” compensation shift register assembly  66 . It should be appreciated that in the present embodiments, a shift register assembly  66  is associated with each discreet LED chip  52   1  through  52   40 . It should be further appreciated that more or less than seven PIPO shift registers may be included depending on how many LEDs form an LED chip, e.g., if each LED chip is formed from ten LEDs then nine PIPO shift registers will be needed. Thus, the seven PIPO shift registers  64  of a combination, e.g., combination  60   a,  are required in order to retain the print data due to the sequential actuation of LEDs  58   a  through  58   h  via a matrix driver, e.g., matrix driver  54   k . Since the typical specification of process direction profile range requires five scan-lines of compensation for 1200 dpi×1200 dpi and eight scan-lines of compensation for 1200 dpi×2400 dpi a practical quantity of shift register combinations is eight, as is shown in  FIG. 3A . It should be further appreciated that more or less than eight shift register combinations may be used based on the number of lines of compensation which are needed, e.g., ten shift register combinations provide ten lines of compensation, and such variations are within the spirit and scope of the claims. 
         [0035]    The print data is applied in parallel to the all SIPOs  62  of all combinations  60   a  through  60   g  but is only loaded into one of the SIPOs  62  based on the value of “line” compensation register  68 . In this example, “line” compensation register  68  provides a 3-bit binary value including a most significant bit (MSB) and a least significant bit (LSB) to “line” compensation decoder  70 , which in turn decodes the binary value and outputs logical values for L 0 , L 2 , L 1 , L 3 , L 4 , L 5 , L 6  and L 7 . In other words, as shift register assembly  66  receives an input from compensation register  68  in accordance with Table 1 below, the respective SIPO shift register associated with desired amount of “line” compensation is enabled, thereby permitting the receipt of print data therein. For example, if two scan-lines of compensation are needed, L 2  is set to 1 (or logical high) while L 0 , L 1 , L 3 , L 4 , L 5 , L 6  and L 7  are set to 0 (or logical low), thereby enabling serial data input (DI_S) of the SIPO shift register associated with L 2 , i.e., SIPO shift register  62   a.  In other words, as print data is applied in parallel to all SIPO shift registers via line DI, the print data is only received by the enable SIPO shift register. Simultaneously, all SIPO shift registers and PIPO shift registers located above the enabled SIPO shift register have their respective parallel data inputs (DI_P) and parallel data outputs (DO_P) enabled, thereby permitting the parallel transfer of print data through shift register assembly  66  towards the parallel data output (DO_P) of the upper most SIPO shift register, i.e., SIPO shift register  62   b.  In the embodiment shown in  FIG. 3A , the simultaneous enablement of the parallel data inputs occurs due to the arrangement of OR gates  72   a  through  72   m  and the output from “line” compensation register  68 . For example, when two scan-lines of compensation are needed, OR gates  72   i  through  72   m  each output a 1 as each OR gate receives an input from L 2  or from a prior OR gate, e.g., OR gates  72   j  and  72   L.  Hence, all shift registers above and including SIPO shift register  62   a  are parallel enabled by the outputs of OR gates  72   i  through  72   m.  Load line (L) is connected to load data input (LD) of each SIPO shift register and PIPO shift register in shift register assembly  66 . As the load line receives a rising signal, or is pulsed high, data is output from the enabled shift registers, while as the load line receives a falling signal, or is pulsed low, data is loaded into the enabled shift registers. It should be appreciated that the terms “above” and “upper most” are relative terms used with respect to  FIG. 3A  and that the actual orientation of shift register assembly  66  may be different, e.g., horizontally arranged. Clock line CL receives a clock signal which is thereby provided to each of the SIPO shift registers, and causes print data in the serially enabled SIPO shift register to transfer out of that shift register via the serial data output (DO_S) to the serial data output line (DO S ). The serial data output line is arranged to pass print data from the shift register assembly of one LED chip to the shift register assembly of the next LED chip in sequence on the LPH. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Line Comp. Register MSB→LSB 
               
             
          
           
               
                   
                 L(N) 
                 L 0   
                 L 1   
                 L 2   
                 L 3   
                 L 4   
                 L 5   
                 L 6   
                 L 7   
               
               
                   
                   
               
             
          
           
               
                 000 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 001 
                 1 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 010 
                 2 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 011 
                 3 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
               
               
                 100 
                 4 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
               
               
                 101 
                 5 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
               
               
                 110 
                 6 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
               
               
                 111 
                 7 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                   
               
             
          
         
       
     
         [0036]    The print data is moved by whole lines though the shift registers of shift register assembly  66 . After the print data is serially loaded into the desired SIPO shift register, i.e., shift register  62   a  in the example above, the data is shifted in parallel to the following PIPO shift registers in shift register assembly  66 . The print data is moved in parallel up assembly  66  one shift register at a time every serial load, until the print data reaches the last register, where it is applied to LED driver circuit  54   1  (See U.S. patent application Ser. No. 12/232,220, filed on Sep. 12, 2008). Applying the foregoing embodiment to the process direction profile of  FIG. 2B , the process direction profile is reduced by approximately 76% to 21 μm for 1200 dpi×1200 dpi and by approximately 88% to 10.4 μm for 1200 dpi×2400 dpi. The “line” compensated process direction profiles versus the chip averaged process direction profile are shown in  FIG. 4 . 
         [0037]    The second embodiment of the present apparatus includes a new LED matrix driver modified to further reduce the process direction profile to ⅛th of the resolution of a scan line, i.e., an eight times (8×) improvement in resolution compared with known device resolutions, while causing virtually no change to print controller processing overhead. In this embodiment, combinations  80   a  through  80   h  each include eight SIPO shift registers  82 , as opposed to one SIPO shift register and seven PIPO shift registers, while at the same time adding additional “line” compensation register  84  and “sub-line” compensation register  86 . In this example, “line” compensation register  84  provides a 3-bit binary value including a most significant bit (MSB) and a least significant bit (LSB) to “line” compensation decoder  88 , which in turn decodes the binary value and outputs logical values for L 0 , L 2 , L 1 , L 3 , L 4 , L 5 , L 6  and L 7 . Similarly, “sub-line” compensation register  86  provides a 3-bit binary value including a most significant bit (MSB) and a least significant bit (LSB) to “sub-line” compensation decoder  90 , which in turn decodes the binary value and outputs logical values for SL 0 , SL 2 , SL 1 , SL 3 , SL 4 , SL 5 , SL 6  and SL 7 . Again, it should be appreciated that more or less than eight shift register combinations may be used based on the number of lines of compensation which are needed, e.g., ten shift register combinations provides ten lines of compensation, and that more or less than eight SIPOs may be included in each combination depending on how many LEDs form an LED chip, e.g., if each LED chip is formed from ten LEDs then ten SIPOs within each combination will be needed, and such variations are within the spirit and scope of the claims. 
         [0038]    The print data is applied in parallel to all SIPOs  82  of all combinations  80   a  through  80   h  but is only loaded into one of the SIPOs  82  based on the value of “line” compensation register  84  and “sub-line” compensation register  86 . In other words, as shift register assembly  92  receives an input from “line” compensation register  84  and “sub-line” compensation register  86  in accordance with Tables 2 and  3   below, the respective SIPO shift register associated with desired amount of compensation is enabled, thereby permitting the receipt of print data therein. For example, if  0 and 4/8ths scan-lines of compensation are needed, L 0  is set to 1, L 1 , L 2 , L 3 , L 4 , L 5 , L 6  and L 7  are set to 0, SL 4  is set to 1, SL 0 , SL 1 , SL 2 , SL 3 , SL 5 , SL 6  and SL 7  are set to 0 thereby enabling serial data input (DI_S) of the SIPO shift register associated with L 0  and SL 4 , i.e., SIPO shift register  82   a.  As print data is applied in parallel to all SIPO shift registers via line DI, the print data is only received by the enable SIPO shift register. Simultaneously, all SIPO shift registers including and located above the enabled SIPO shift register have their respective parallel data inputs (DI_P) enabled, thereby permitting the parallel transfer of print data through shift register assembly  92  towards the parallel data output (DO_P) of the upper most SIPO shift register, i.e., SIPO shift register  82   e.  In the embodiment shown in  FIG. 5A , the simultaneous enablement of the parallel data inputs occurs due to the arrangement of OR gates  94   a  through  94   i  and AND gates  96   a  through  96   h.  For example, when 0 and 4/8ths scan-lines of compensation are needed, OR gates  94   e  through  94   i  each output a 1 as each OR gate receives an input of 1 from at least one of OR gates  94   e  through  94   h  and/or AND gates  96   d.    
         [0039]    Hence, all shift registers including and above SIPO shift register  82   a  are parallel enabled by the outputs of OR gates  94   e  through  94   i.  Load line (L) is connected to load data input (LD) of each SIPO shift register in shift register assembly  92 . As the load line receives a rising signal, or is pulsed high, data is output from the enabled shift registers, while as the load line receives a falling signal, or is pulsed low, data is loaded into the enabled shift registers. It should be appreciated that the terms “above” and “upper most” are relative terms used with respect to  FIG. 5A  and that the actual orientation of shift register assembly  92  may be different, e.g., horizontally arranged. Clock line CL receives a clock signal which is thereby provided to each of the SIPO shift registers, and causes print data in the serially enabled SIPO shift register to transfer out of that shift register via the serial data output (DO_S) to the serial data output line (DO S ). The serial data output line is arranged to pass print data from the shift register assembly of one LED chip to the shift register assembly of the next LED chip in sequence. 
         [0040]    As described above, the print data is applied in parallel but the additional “line” compensation register  84  and “sub-line” compensation register  86  selects the appropriate shift register to enable and thereby permit loading. Based on the values provided by “line” compensation register  84  and “sub-line” compensation register  86 , as shown in Tables 2 and 3 below, in combination with the logic gate arrangement shown in  FIG. 5A , the print data is received by the appropriate SIPO shift register. For example, if 0 and 4/8th lines of compensation is necessary, the print data is loaded into SIPO shift register  82   a.    
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Line Comp. Register MSB→LSB 
               
             
          
           
               
                   
                 L(N) 
                 L 0   
                 L 1   
                 L 2   
                 L 3   
                 L 4   
                 L 5   
                 L 6   
                 L 7   
               
               
                   
                   
               
             
          
           
               
                 000 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 001 
                 1 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 010 
                 2 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 011 
                 3 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
               
               
                 100 
                 4 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
               
               
                 101 
                 5 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
               
               
                 110 
                 6 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
               
               
                 111 
                 7 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Sub-Line Comp. Register MSB→LSB 
               
             
          
           
               
                   
                 SL(N) 
                 SL 0   
                 SL 1   
                 SL 2   
                 SL 3   
                 SL 4   
                 SL 5   
                 SL 6   
                 SL 7   
               
               
                   
                   
               
             
          
           
               
                 000 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 001 
                 1 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 010 
                 2 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 011 
                 3 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
               
               
                 100 
                 4 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
               
               
                 101 
                 5 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
               
               
                 110 
                 6 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
               
               
                 111 
                 7 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                   
               
             
          
         
       
     
         [0041]    In this embodiment, the correction data is in the form of two integers, where the first integer represents the full line correction value and the second integer represents the fractional line correction value. For example, a correction of 0 and 4/8th would result in a value of 000 being stored in “line” compensation register  84  and a value of 100 being stored in the “sub-line” compensation register  86 . The print data then moves in parallel up the shift register chain to the LED driver circuit just as described above. In other words, every time print data is written to the appropriate SIPO shift register, i.e., the shift register corresponding to the necessary correction value, the previous print data shifts up one eighth of a line, i.e., one SIPO shift register, until all print data passes from SIPO shift register  82   e  to LED driver chip  54   1 . This embodiment further reduces the compensation range from one scan-line to ⅛th of a scan-line, i.e., approximately 2.6 μm for 1200 dpi×1200 dpi and approximately 1.3 μm for 1200 dpi×2400 dpi. The “sub-line” compensated process direction profiles versus the chip averaged process direction profile are shown in  FIG. 6 . 
         [0042]    It should be appreciated that the embodiment of the logic gates shown in  FIG. 5A  are but one arrangement of logic gates capable of enabling the necessary shift registers. The foregoing example of 0 and 4/8ths line compensation is now further explained in view of the arrangement shown in  FIG. 5A . As described above, when 0 and 4/8ths line compensation are needed, L 0  and SL 4  are set to 1 with all other L x s and SL x s are set to 0. As all inputs to OR gate  94   a  are set to 0, OR gate  94   a  outputs a 0. Similarly, as AND gates  96   a,    96   b,    96   c,    96   e,    96   f,    96   g  and  96   h  have inputs of 1 and 0, each of AND gates  96   a,    96   b,    96   c,    96   e,    96   f,    96   g  and  96   h  outputs a 0. AND gate  96   d,  which receives two inputs of 1, i.e., L 0  and SL 4 , outputs a 1, and thus, SIPO shift register  82   a  receives a 1 at EN_S. Additionally, the output from AND gate  96   d  is provided to OR gate  94   e  thereby causing OR gate  94   e  to output a 1, and thus, SIPO shift register  82   a  receives a 1 at EN_P. Each of OR gates  94   f through  94   i  receives a 1 from an AND gate, in this example AND gate  96   d,  and/or receives a 1 from the previous OR gate in sequence, in this example, OR gate  94   f  receives a 1 from OR gate  94   e,  OR gate  94   g  receives a 1 from OR gate  94   f,  OR gate  94   h  receives a 1 from OR gate  94   g  and OR gate  94   i  receives a 1 from OR gate  94   h,  and thus, SIPO shift registers  82   a  through  82   e  receive a 1 at each respective EN_P. It should be appreciated that each combination  80   a  through  80   h  in assembly  92  includes a similar logic gate arrangement. 
         [0043]    Both embodiments require the application of an algorithm to determine the “line” and “sub-line” compensation values of each chip “N”, i.e., L(N) and SL(N). Such an algorithm is shown in  FIG. 7 , and is typically performed at the time of manufacturing the LPH. Algorithm  100  is based on the calculation of the average process direction position for each LED chip, i.e., Y_AVG(N). First, the process direction profile of the entire LPH is measured, i.e., Y(N,n), where “N” is the LED chip number with the LPH, e.g., LED chip numbers  1  through  40 , and “n” is the LED number within each respective LED chip, e.g., LED number  1  through  384  (See Step  102 ). Next, Y_AVG(N) is calculated for each respective LED chip, where such average value is the average process direction position for all LEDs with a respective LED chip, e.g., LEDs  1  through  384  of LED chip number  1  (See Step  104 ). Then, the most positive or maximum value of Y_AVG(N) within the LPH is determined for later use as the reference, i.e., zero, position (See Step  106 ). It should be appreciated that the most positive position is chosen as the reference position because it is more convenient to add a delay to all print data sent to each respective LED chip as opposed to increasing the speed by which data is sent the LED chips. In other words, each respective LED chip is delayed to coincide with the most positive LED chip. Next, the algorithm determines the amount of delay or offset that needs to be applied to the print data for each LED chip relative to the reference position in order to align the print data for an entire scan-line. In other words, the difference between the Y_AVG(N) value of a particular LED chip is subtracted from the Y_MAX value, and the resulting value is then divided by process direction resolution in microns (See Step  108 ). For example, if the process direction resolution is 1200 dpi then that is equivalent to 21.1667 microns per dot (mpd), or microns for short, as there are 25,400 microns per inch. Thus, the difference between Y_AVG(N) and Y_MAX, a value having units of microns in this example, is divided by 21.1667 mpd. Similarly, if the process direction resolution is 2400 dpi then that is equivalent to 10.5831 microns per dot (mpd), as there are 25,400 microns per inch. Thus, the difference between Y_AVG(N) and Y_MAX, a value having units of microns in this example, is divided by 10.5831 mpd. 
         [0044]    Next, based on whether “line” compensation is used or “sub-line” compensation is desired, algorithm  100  results in the calculation of the amount of delay in scan-lines for “line” compensation, i.e., L(N), or alternatively, the amount of delay in scan-lines for “sub-line” compensation, i.e., L(N) and SL(N) (See Step  110 ). For “line” compensation, L(N) is rounded to the nearest integer (See Step  112 ), while for “sub-line” compensation, L(N) is truncated to remove any fractional portion of a scan-line (See Step  114 ). Subsequently, SL(N) is calculated by taking the difference between Y_MAX and Y_AVG(N), multiplying L(N) by the process direction resolution in microns, dividing that value by ⅛ of the process direction resolution and lastly taking the difference between the first difference and the second calculated value (See Steps  116  and  118 ). Algorithm  100  was applied to the chip averaged process direction profile shown in  FIG. 2B , and the results are shown in Table 4 below. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Compensation 
                 Line 
                 Sub-Line 
                 Line 
                 Sub-Line 
               
               
                 Resolution 
                 1200 
                 1200 
                 2400 
                 2400 
               
             
          
           
               
                 Chip # 
                 Y_avg 
                 L(N) 
                 Y(N) 
                 L(N) 
                 SL(N) 
                 Y(N) 
                 L(N) 
                 Y(N) 
                 L(N) 
                 SL(N) 
                 Y(N) 
               
               
                   
               
             
          
           
               
                 1 
                 5.15 
                 3 
                 0.08 
                 3 
                 0 
                 0.08 
                 6 
                 0.08 
                 6 
                 0 
                 0.08 
               
               
                 2 
                 1.65 
                 3 
                 3.58 
                 3 
                 1 
                 0.94 
                 6 
                 3.58 
                 6 
                 3 
                 −0.39 
               
               
                 3 
                 1.24 
                 3 
                 3.99 
                 3 
                 2 
                 −1.30 
                 6 
                 3.99 
                 6 
                 3 
                 0.02 
               
               
                 4 
                 −2.26 
                 3 
                 7.49 
                 3 
                 3 
                 −0.44 
                 7 
                 −3.09 
                 6 
                 6 
                 −0.44 
               
               
                 5 
                 −2.57 
                 3 
                 7.80 
                 3 
                 3 
                 −0.14 
                 7 
                 −2.78 
                 6 
                 6 
                 −0.14 
               
               
                 6 
                 −7.62 
                 4 
                 −8.32 
                 3 
                 5 
                 −0.38 
                 7 
                 2.26 
                 7 
                 2 
                 −0.38 
               
               
                 7 
                 −9.50 
                 4 
                 −6.44 
                 3 
                 6 
                 −1.15 
                 7 
                 4.14 
                 7 
                 3 
                 0.17 
               
               
                 8 
                 −12.76 
                 4 
                 −3.17 
                 3 
                 7 
                 −0.53 
                 8 
                 −3.17 
                 7 
                 6 
                 −0.53 
               
               
                 9 
                 −16.62 
                 4 
                 0.68 
                 4 
                 0 
                 0.68 
                 8 
                 0.68 
                 8 
                 1 
                 −0.64 
               
               
                 10 
                 −18.98 
                 4 
                 3.04 
                 4 
                 1 
                 0.40 
                 8 
                 3.04 
                 8 
                 2 
                 0.40 
               
               
                 11 
                 −15.83 
                 4 
                 −0.11 
                 3 
                 8 
                 −0.11 
                 8 
                 −0.11 
                 7 
                 8 
                 −0.11 
               
               
                 12 
                 −17.44 
                 4 
                 1.50 
                 4 
                 1 
                 −1.14 
                 8 
                 1.50 
                 8 
                 1 
                 0.18 
               
               
                 13 
                 −18.63 
                 4 
                 2.69 
                 4 
                 1 
                 0.05 
                 8 
                 2.69 
                 8 
                 2 
                 0.05 
               
               
                 14 
                 −13.91 
                 4 
                 −2.03 
                 3 
                 7 
                 0.62 
                 8 
                 −2.03 
                 7 
                 6 
                 0.62 
               
               
                 15 
                 −11.00 
                 4 
                 −4.93 
                 3 
                 6 
                 0.36 
                 8 
                 −4.93 
                 7 
                 4 
                 0.36 
               
               
                 16 
                 −10.77 
                 4 
                 −5.17 
                 3 
                 6 
                 0.12 
                 8 
                 −5.17 
                 7 
                 4 
                 0.12 
               
               
                 17 
                 −8.36 
                 4 
                 −7.58 
                 3 
                 5 
                 0.36 
                 7 
                 3.01 
                 7 
                 2 
                 0.36 
               
               
                 18 
                 −6.22 
                 4 
                 −9.72 
                 3 
                 4 
                 0.87 
                 7 
                 0.87 
                 7 
                 1 
                 −0.46 
               
               
                 19 
                 −5.35 
                 3 
                 10.58 
                 3 
                 4 
                 0.00 
                 7 
                 0.00 
                 6 
                 8 
                 0.00 
               
               
                 20 
                 −4.82 
                 3 
                 10.05 
                 3 
                 4 
                 −0.53 
                 7 
                 −0.53 
                 6 
                 8 
                 −0.53 
               
               
                 21 
                 −6.75 
                 4 
                 −9.18 
                 3 
                 5 
                 −1.25 
                 7 
                 1.40 
                 7 
                 1 
                 0.08 
               
               
                 22 
                 −4.62 
                 3 
                 9.85 
                 3 
                 4 
                 −0.73 
                 7 
                 −0.73 
                 6 
                 7 
                 0.59 
               
               
                 23 
                 −3.95 
                 3 
                 9.18 
                 3 
                 3 
                 1.24 
                 7 
                 −1.40 
                 6 
                 7 
                 −0.08 
               
               
                 24 
                 −5.05 
                 3 
                 10.28 
                 3 
                 4 
                 −0.31 
                 7 
                 −0.31 
                 6 
                 8 
                 −0.31 
               
               
                 25 
                 −3.75 
                 3 
                 8.98 
                 3 
                 3 
                 1.04 
                 7 
                 −1.60 
                 6 
                 7 
                 −0.28 
               
               
                 26 
                 −2.45 
                 3 
                 7.68 
                 3 
                 3 
                 −0.26 
                 7 
                 −2.91 
                 6 
                 6 
                 −0.26 
               
               
                 27 
                 0.09 
                 3 
                 5.14 
                 3 
                 2 
                 −0.15 
                 6 
                 5.14 
                 6 
                 4 
                 −0.15 
               
               
                 28 
                 2.76 
                 3 
                 2.47 
                 3 
                 1 
                 −0.17 
                 6 
                 2.47 
                 6 
                 2 
                 −0.17 
               
               
                 29 
                 3.92 
                 3 
                 1.31 
                 3 
                 0 
                 1.31 
                 6 
                 1.31 
                 6 
                 1 
                 −0.01 
               
               
                 30 
                 5.86 
                 3 
                 −0.63 
                 2 
                 8 
                 −0.63 
                 6 
                 −0.63 
                 5 
                 8 
                 −0.63 
               
               
                 31 
                 9.78 
                 3 
                 −4.55 
                 2 
                 6 
                 0.75 
                 6 
                 −4.55 
                 5 
                 5 
                 −0.58 
               
               
                 32 
                 14.47 
                 3 
                 −9.24 
                 2 
                 5 
                 −1.30 
                 5 
                 1.34 
                 5 
                 1 
                 0.02 
               
               
                 33 
                 20.76 
                 2 
                 5.63 
                 2 
                 2 
                 0.34 
                 5 
                 −4.95 
                 4 
                 4 
                 0.34 
               
               
                 34 
                 27.12 
                 2 
                 −0.72 
                 1 
                 8 
                 −0.72 
                 4 
                 −0.72 
                 3 
                 7 
                 0.60 
               
               
                 35 
                 31.98 
                 2 
                 −5.59 
                 1 
                 6 
                 −0.29 
                 3 
                 5.00 
                 3 
                 4 
                 −0.29 
               
               
                 36 
                 36.80 
                 2 
                 −10.40 
                 1 
                 4 
                 0.18 
                 3 
                 0.18 
                 3 
                 0 
                 0.18 
               
               
                 37 
                 46.38 
                 1 
                 1.18 
                 1 
                 0 
                 1.18 
                 2 
                 1.18 
                 2 
                 1 
                 −0.14 
               
               
                 38 
                 53.10 
                 1 
                 −5.54 
                 0 
                 6 
                 −0.25 
                 1 
                 5.05 
                 1 
                 4 
                 −0.25 
               
               
                 39 
                 62.37 
                 0 
                 6.36 
                 0 
                 2 
                 1.07 
                 1 
                 −4.22 
                 0 
                 5 
                 −0.25 
               
               
                 40 
                 68.73 
                 0 
                 0.00 
                 0 
                 0 
                 0.00 
                 0 
                 0.00 
                 0 
                 0 
                 0.00 
               
               
                   
               
             
          
         
       
     
         [0045]    The determination of L(N) and SL(N), e.g., by performing algorithm  100 , is done during final testing of an LED print-head using know testing techniques and common data analysis devices such as a personal computer. For example, discreet LEDs or combinations of LEDs may be illuminated and their respective positions measured using a stereo microscope, and subsequently the position information may be entered or received into a personal computer and analyzed in accordance with the steps described above. The personal computer can include but is not limited to an arithmetic logic unit, a central processing unit, memory, input devices, output devices, etc. The delay data, i.e., line delay data and/or sub-line delay data, is stored in the print-head&#39;s nonvolatile memory  200  and then readout and applied to the print-head similar to techniques involving power compensation. If four bits are used for both sets of delay data, only  40  additional bytes of memory are required to be loaded into the print-head prior to a print job. This represents less than 0.4% of the total compensation memory of a typical print-head. 
         [0046]    An LED driver chip with configurable print delay that compensates for the process direction profile of an LED print-head has been set forth herein. The LED driver chip provides electronic correction for process direction position errors of LED chips within LED print heads. This correction is implemented by the present LED driver chips, which are integrated within the print head. The present LED driver chip can either correct to the nearest scan line, i.e., nearest process direction pixel position, or correct to within ⅛ of a scan line, assuming there is 1:8 multiplexed drive of the LEDs. Such correction is applied per LED array chip, or in other words, each discreet LED within an LED array chip is corrected by the same amount. The two embodiments set forth above expand the use of the existing compensation memory of a print-head to provide additional information to the LED driver chips, enabling them to perform “line” and “sub-line” process direction profile compensation, respectively. “Line” compensation reduces the effective process direction profile to less than or equal to one scan-line, the same as with print controller based compensation but without the processor overhead. 
         [0047]    “Sub-line” compensation further reduces the effective process direction profile of the print-head to ⅛th of the process direction resolution, which is an additional eight times (8×) improvement, also without adding print controller processor overhead. Furthermore, a method to calculate the necessary “line” and “sub-line” compensation values used by the foregoing embodiments is also set forth herein. The foregoing process direction profile correction of LED print heads improves image quality. The value of sub-line correction decreases as the process direction resolution increases, but is still of value. Correcting process direction profile errors within LED print heads removes a processor intensive task from the print controller, and is the only feasible option for implementing sub-line correction. 
         [0048]    It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.