Abstract:
An image forming apparatus for forming an image of multilevel image data includes a driving unit for driving an image forming element for image formation, an additional data generating unit for generating a digital signal string based on predetermined additional data, and an input unit for superposing a digital signal string related to the multilevel image data and the digital signal string based on the additional data and inputting the superposed digital signal string to the driving unit.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an image forming apparatus and image forming method and, more particularly, to an image forming apparatus and image forming method capable of contributing to prevention of, e.g., copying of securities.  
       BACKGROUND OF THE INVENTION  
       [0002]     Recently, image forming apparatuses such as printers have been given color capability and used as various expressing means by users. In particular, color page printers are attracting attention because they are silent and capable of high-quality, high-speed printing.  
         [0003]     A multicolor beam printer as one color page printer is characterized by printing a multicolor image by performing first development by scanning a light beam on a photosensitive body in a main scan direction, and then transferring the image onto a printing medium such as a printing paper sheet on a transfer carrier to perform predetermined processing.  
         [0004]     A method of printing a multicolor image by this multicolor beam printer will be described below with reference to  FIGS. 18 and 19 .  
         [0005]      FIG. 18  is a schematic view of a conventional multicolor beam printer.  FIG. 19  is a block diagram of signal processing.  
         [0006]     Referring to  FIG. 18 , a photosensitive drum  201  which rotates in the direction of an arrow at a predetermined constant velocity is charged to a predetermined polarity and a predetermined voltage by a charger  204 .  
         [0007]     Printing sheets P are fed one by one at a predetermined timing from a paper feed cassette  215  by a paper feed roller  214 . When a sensor  202  senses the leading edge of the printing sheet, a laser beam L modulated by an image signal VDO is emitted from a semiconductor laser  205  toward a polygonal mirror  207 .  
         [0008]     This laser beam L is scanned by the polygonal mirror  207  and guided onto the photosensitive drum  201  via a lens  208  and a mirror  209 .  
         [0009]     A signal (to be referred to as TOPSNS hereinafter) from the sensor  202  placed at one end of light scan is output as a vertical sync signal to an image processor  250  ( FIG. 19 ).  
         [0010]     The image signal VDO is sequentially supplied to the semiconductor laser  205  in synchronism with a BD signal (to be described later) which follows the TOPSNS signal. When the laser beam L enters a detector  217 , a beam detection signal (to be referred to as a BD signal hereinafter) serving as a horizontal sync signal is output.  
         [0011]     The polygonal mirror  207  is driven by a scanner motor  206 . This scanner motor  206  is controlled by a motor control circuit  225  so as to rotate at a predetermined constant velocity in accordance with a signal S 2  from a frequency divider  221  which divides the frequency of a signal S 1  from a reference oscillator  220  shown in  FIG. 19 .  
         [0012]     The photosensitive drum  201  is exposed by scan in synchronism with the BD signal, and a developing device  203 Y develops a first electrostatic latent image. After that, a first toner image of yellow is formed on the photosensitive drum  201 .  
         [0013]     Immediately before the leading edge of the printing sheet P fed at a predetermined timing reaches a transfer start position, a predetermined transfer bias voltage having a polarity opposite to that of toner is applied to a transfer drum  216 . Consequently, the first toner image is transferred onto the printing sheet P, and at the same time this printing sheet P is electrostatically attracted to the surface of the transfer drum  216 .  
         [0014]     Subsequently, a second electrostatic latent image is formed on the photosensitive drum  201  by manipulating the laser beam L. A developing device  203 M develops this second electrostatic latent image to form a second toner image of magenta on the photosensitive drum  201 . This second toner image is transferred onto the printing sheet P so as to be aligned with the position of the first toner image previously transferred onto the printing sheet P. Note that the end of the image of each color is defined by the TOPSNS signal.  
         [0015]     Analogously, a third electrostatic latent image is formed and developed by a developing device  203 C, and a cyan toner image formed is aligned with and transferred onto the printing sheet P. A fourth electrostatic latent image is then formed and developed by a developing device  203 K, and a black toner image formed is aligned with and transferred onto the printing sheet P.  
         [0016]     As described above, a VDO signal of one page is output to the semiconductor laser  205  in each step. Also, whenever the transfer step is performed, a cleaner  210  scrapes off any untransferred toner image.  
         [0017]     After that, when the leading edge of the printing sheet P on which the toner images of four colors are transferred approaches the position of a separation pawl  212 , this separation pawl  212  comes in contact with the surface of the transfer drum  216  to separate the printing sheet P from the transfer drum  216 . The end portion of this separation pawl  212  keeps contacting the transfer drum  216  until the trailing edge of the printing sheet P is separated from the transfer drum  216 . After that, the separation pawl  212  moves away and returns to the original position. A charger  211  removes stored charge on the printing sheet P to facilitate separation of the printing sheet P by the separation pawl  212 , and reduces air discharge during separation.  
         [0018]      FIG. 20  is a timing chart showing the relationship between the TOPSNS signal and the VDO signal described above. Referring to  FIG. 20 , reference symbol A 1  denotes a printing operation of the first color; A 2 , a printing operation of the second color; A 3 , a printing operation of the third color; and A 4 , a printing operation of the fourth color. These sections A 1  to A 4  form a color printing operation of one page.  
         [0019]      FIG. 21  is a block diagram showing the system configuration of a conventional printer.  
         [0020]     Referring to  FIG. 21 , a printer  302  receives a control signal and an image signal  307  from an external apparatus, e.g., a host computer  301 . A printer controller  303  transfers the control signal to a printer control unit  304 . The image signal is supplied to a laser driver  310  of a printer engine via an image processor  305  in the printer controller  303  and drives a semiconductor laser  306 .  
         [0021]      FIG. 22  is a block diagram showing the internal arrangement of the image processor  305  shown in  FIG. 21 . The image processor shown in  FIG. 22  receives an image signal of 8 bits for each of R, G, and B, i.e., a total of 24 bits from the printer controller (not shown). A color processor  351  converts each of Y, M, C, and K signals into the 8-bit VDO signal described above at respective timings ( FIG. 23  is a corresponding timing chart).  
         [0022]     A γ correction unit  325  converts these Y, M, C, and K VDO signals into γ-corrected, 8-bit signals and inputs these signals to a pulse width modulation unit  353  (to be referred to as a PWM unit hereinafter) in the next stage. In this PWM unit  353 , a latch  345  synchronizes the 8-bit image signal with the leading edge of an image clock iVClK. A D/A converter  355  converts the signal into an analog voltage and inputs the voltage to an analog comparator  356 .  
         [0023]     The image clock iVCLK is converted into a triangular wave by a triangular wave generator  358  and input to the analog comparator  356 . This analog comparator  356  compares the two signals and outputs an image signal  309  subjected to PWM. An inverter  357  inverts the output signal to obtain a desired PWM signal.  
         [0024]      FIG. 24  shows a timing chart when the PWM unit  353  generates a PWM signal. As shown in  FIG. 24 , when the input 8-bit image data to the PWM unit  353  is FF[H] (H indicates hexadecimal notation), the widest PWM signal is output. When the image data is 00[H], the narrowest PWM signal is output.  
         [0025]     Unfortunately, the improved printing performance and high-quality printing capability of a conventional image forming apparatus as described above lead to frequent occurrence of forgery of securities such as paper money.  
         [0026]     As the image formation technology improves in the future, the image quality improves accordingly, so this sort of crimes are expected to increase in number.  
       SUMMARY OF THE INVENTION  
       [0027]     It is, therefore, an object of the present invention to provide an image forming apparatus and image forming method capable of adding predetermined information on an image in order to track down perpetrators in the event of such crimes.  
         [0028]     According to the present invention, there is provided an image forming apparatus for forming an image of multilevel image data, comprising driving means for driving an image forming element for image formation, additional data generating means for generating a digital signal string based on predetermined additional data, and input means for superposing a digital signal string related to the multilevel image data and the digital signal string based on the additional data and inputting the superposed digital signal string to the driving means.  
         [0029]     According to the present invention, there is provided an image forming method of forming an image of multilevel image data by using an image forming element for image formation and driving means for driving the image forming element, comprising the steps of generating a digital signal string based on predetermined additional data, and superposing a digital signal string related to the multilevel image data and the digital signal string based on the additional data and inputting the superposed digital signal string to the driving means.  
         [0030]     According to the present invention, there is provided an image forming apparatus for forming an image of multilevel image data, comprising driving means for receiving a digital signal string related to the multilevel image data and driving an image forming element for image formation, and additional data generating means for generating a digital signal string based on predetermined additional data, wherein the driving means has an input terminal for forcedly controlling light emission of the image forming element, and a digital signal string based on the additional data is input to the input terminal of the additional data generating means.  
         [0031]     According to the present invention, there is provided an image forming apparatus for forming an image of multilevel data, comprising at least two image forming means, each of the image forming means comprising driving means for driving an image forming element for image formation, additional data generating means for generating a digital signal string based on predetermined additional data, and input means for superposing a digital signal string related to the multilevel image data and the digital signal string based on the additional data and inputting the superposed digital signal string to the driving means.  
         [0032]     According to the present invention, there is provided an image forming method of forming an image of multilevel image data by using an image forming element for image formation and driving means for driving the image forming means, the driving means having an input terminal for forcedly controlling light emission of the image forming element, comprising the steps of inputting a digital signal string related to the multilevel image data to the driving means, and generating a digital signal string based on predetermined additional data and inputting the digital signal string to the input terminal.  
         [0033]     According to the present invention, there is provided an image forming method of forming an image of multilevel data by using at least two image forming elements for image formation and at least two driving means for driving the image forming elements, comprising the steps of generating a digital signal string based on predetermined additional data for each of the driving means, and superposing a digital signal string related to the multilevel image data and the digital signal string based on the additional data and inputting the superposed digital signal string to each of the driving means.  
         [0034]     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0036]      FIG. 1  is a sectional view showing the structure of an image forming apparatus A according to one embodiment of the present invention;  
         [0037]      FIG. 2  is a block diagram showing an outline of the arrangement of a printing system using the image forming apparatus A;  
         [0038]      FIG. 3  is an internal block diagram of a printer controller  3 ;  
         [0039]      FIG. 4  is a timing chart showing a VDO signal  6 , a BD signal  423  as a horizontal sync signal, and a PSYNC signal  424  as a vertical sync signal;  
         [0040]      FIG. 5  is an internal block diagram of a signal processor  402  of an engine;  
         [0041]      FIG. 6  is an internal block diagram of a tracking pattern generator  410 ;  
         [0042]      FIG. 7  is a timing chart showing PCLK signal generation in the tracking pattern generator  410 ;  
         [0043]      FIG. 8  is a schematic view showing a unit region representing a number unique to the machine by a tracking pattern;  
         [0044]      FIG. 9  is a view showing examples of MKON[7:0] 443 and MKOFF[7:0]  444  as multilevel signals of tracking pattern dots generated by the tracking pattern generator  410 ;  
         [0045]      FIG. 10  is a view showing an image printed by mixing the tracking pattern shown in  FIG. 9  into the VDO image signal  6 ;  
         [0046]      FIG. 11  is a view showing a case in which the output VDO data  6  from the printer controller is not in synchronism with the tracking pattern;  
         [0047]      FIG. 12  is an internal block diagram of another signal processor  402  of the engine;  
         [0048]      FIG. 13  is an internal block diagram of a tracking pattern generator  502 ;  
         [0049]      FIG. 14  is an internal block diagram of still another signal processor  402  of the engine;  
         [0050]      FIG. 15  is an internal block diagram of tracking pattern generators  503  and  504 ;  
         [0051]      FIG. 16  is an internal block diagram of still another signal processor  402  of the engine;  
         [0052]      FIG. 17  is an internal block diagram of another tracking pattern generator  410 ;  
         [0053]      FIG. 18  is a schematic view of a conventional multicolor beam printer;  
         [0054]      FIG. 19  is a block diagram of signal processing;  
         [0055]      FIG. 20  is a timing chart showing the relationship between a TOPSNS signal and a VDO signal;  
         [0056]      FIG. 21  is a block diagram showing the system configuration of a conventional printer;  
         [0057]      FIG. 22  is a block diagram showing the internal arrangement of an image processor  305 ;  
         [0058]      FIG. 23  is a timing chart showing individual signals; and  
         [0059]      FIG. 24  is a timing chart when a PWM unit  353  generates a PWM signal.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0060]     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.  
         [0061]      FIG. 1  is a sectional view showing the structure of an image forming apparatus A according to one embodiment of the present invention.  
         [0062]     In this image printing apparatus A, a gripper  103   f  grips the leading edge of a paper sheet  102  fed from a paper feeder  101 , holding the paper sheet  102  on the circumferential surface of a transfer drum  103 .  
         [0063]     Latent images formed by different colors on an image carrier  100  by an optical unit  107  are developed by developing devices (Dy, Dc, Dm, and Db) of the corresponding colors and transferred a plurality of times onto the paper sheet held on the circumferential surface of the transfer drum  103 , thereby forming a multicolor image.  
         [0064]     After that, the paper sheet  102  is separated from the transfer drum  103 , fixed by a fixing unit  104 , and delivered to a paper delivery tray  106  by a paper delivery unit  105 .  
         [0065]     Each developing device (Dy, Dc, Dm, or Db) has rotating shafts at its two ends and is held by a developing device selection mechanism  108  so as to be rotatable around the shafts.  
         [0066]     Also, each developing device (Dy, Dc, Dm, and Db) is rotated to be selected with its posture maintained constant. After the selected developing device has moved to a development position, a selection mechanism holding frame  109  moves around a supporting point  109   b  by a solenoid  109   a  to position the developing device selection mechanism  108  and the developing device in the direction of the image carrier  100 .  
         [0067]     The operation of the image forming apparatus A constructed as above will be described below.  
         [0068]     First, a charger  111  shown in  FIG. 1  evenly charges the image carrier (photosensitive drum)  100  to a predetermined polarity. A first latent image of magenta is formed on this photosensitive drum  100  by exposure to a laser beam L.  
         [0069]     In this case, a predetermined developing bias voltage is applied only to the magenta developing device Dm to develop the magenta latent image, forming a first toner image of magenta M on the photosensitive drum  100 .  
         [0070]     Meanwhile, a transfer paper sheet P is fed at a predetermined timing. Immediately before the leading edge of this transfer paper sheet P reaches the transfer start position described above, a transfer bias voltage (+1.8 KV) having a polarity (e.g., positive polarity) opposite to that of the toner is applied to the transfer drum  103 . Consequently, the first toner image on the photosensitive drum  100  is transferred onto the transfer paper sheet P. At the same time, the transfer paper sheet P is electrostatically attracted to the surface of the transfer drum  103 . After that, a cleaner  112  removes the residual magenta toner from the photosensitive drum  100  to prepare for the formation and development of a latent image of the next color.  
         [0071]     Subsequently, the laser beam L forms a second latent image of cyan on the photosensitive drum  100 . The cyan developing device Dc develops this second latent image on the photosensitive drum  100  to form a second toner image of cyan C.  
         [0072]     This second toner image of cyan C is transferred onto the transfer paper sheet P so as to be aligned with the position of the first toner image of magenta M previously transferred onto the transfer paper sheet P. In the transfer of this toner image of the second color, a bias voltage of +2.1 KV is applied to the transfer drum  103  immediately before the transfer paper sheet reaches the transfer unit.  
         [0073]     Similarly, third and fourth latent images of yellow and black are sequentially formed on the photosensitive drum  100  and sequentially developed by the developing devices Dy and Db, respectively. Third and fourth toner images of yellow and black thus formed are sequentially transferred so as to be aligned with the toner images previously transferred onto the transfer sheet P. As a consequence, the toner images of the four colors are formed to overlap each other on the transfer paper sheet P.  
         [0074]     In the transfer of the toner images of the third and fourth colors, bias voltages of +2.5 and +3.0 KV, respectively, are applied to the transfer drum  103  immediately before the transfer paper sheet reaches the transfer unit. The transfer bias voltage is raised whenever a toner image of each color is transferred in order to prevent a lowering of the transfer efficiency.  
         [0075]     A primary cause of a lowering of this transfer efficiency is that when the transfer paper sheet separates from the photosensitive drum  100  after transfer, air discharge charges the surface of the sheet to a polarity opposite to that of the transfer bias voltage (because air discharge slightly charges the surface of the transfer drum carrying the transfer paper sheet), and this electric charge builds up each time an image is transferred. If the transfer bias voltage is held constant, the transfer electric field lowers whenever transfer is performed.  
         [0076]     Also, during the transfer of the fourth color described above, when (or immediately before or immediately after) the leading edge of the transfer paper sheet reaches the transfer start position, a DC bias voltage of +3.0 KV having the same polarity and same potential as the transfer bias voltage applied when the fourth toner image is transferred is superposed on an AC voltage of 5.5 KV (an effective value, the frequency is 500 Hz), and the resulting voltage is applied to the charger  111 .  
         [0077]     The charger  111  is thus operated when the leading edge of the transfer paper sheet reaches the transfer start position during the transfer of the fourth color in order to prevent uneven transfer. Especially in transfer of a full-color image, even slight transfer unevenness is conspicuous as a color difference. Therefore, it is necessary to apply a predetermined-bias voltage to the charger  111  to perform discharge as described above.  
         [0078]     After that, as the leading edge of the transfer paper sheet P on which the toner images of the four colors are transferred by superposition moves close to a separation position, a separation pawl  113  approaches, and its end portion comes in contact with the surface of the transfer drum  103  to separate the transfer paper sheet P from the transfer drum  103 . The end portion of this separation pawl  113  keeps contacting the transfer drum surface until the trailing edge of the transfer paper sheet P separates. After that, the separation pawl  103  moves away from the transfer drum  103  and returns to the original position.  
         [0079]     As described above, the charger  111  operates from the time the leading edge of the transfer paper sheet P reaches the transfer start position of the last color to the time the trailing edge of the transfer paper sheet P separates from the transfer drum  103 . In this manner, the charger  111  removes stored charge (having a polarity opposite to that of the toner) on the transfer paper sheet to facilitate the separation of the transfer paper sheet by the separation pawl  113 . Also, the charger  111  reduces air discharge during the separation of the transfer paper sheet.  
         [0080]     Note that when the trailing edge of the transfer paper sheet reaches the transfer end position (the exit of a nip formed by the photosensitive drum  100  and the transfer drum  103 ), the transfer bias voltage (ground potential) to be applied to the transfer drum  103  is turned off. Simultaneously, the bias voltage applied to the charger  111  is turned off.  
         [0081]     The transfer paper sheet P thus separated is conveyed to a fixing device  104  where the toner images on the transfer paper sheet are fixed. After that, the transfer paper sheet P is delivered onto the paper delivery tray  115 .  
         [0082]     The operation of laser beam scanning in the image forming apparatus A will be described below.  
         [0083]     The optical unit  107  as a driving means comprises a semiconductor laser  120  as an image forming device (light-emitting device), a polygonal mirror  121 , a scanner motor  122 , a lens  123 , and a mirror  125 . When the printing sheet P is fed and its leading edge is detected, an image signal VDO of one page is output to the semiconductor laser  120  in synchronism with the detection.  
         [0084]     The light beam L is modulated by the image signal VDO and emitted toward the polygonal mirror  125  which is rotated by the scanner motor  122 . In this way the light beam L is guided to the photosensitive drum  100  by the lens  123  and the mirror  125 . Also, when the light beam L is emitted, a detector (not shown) placed on the scanning axis detects this light beam L and outputs a beam detection signal BD as a horizontal sync signal. Consequently, the light beam L exposes the photosensitive drum  100  by scanning in synchronism with the BD signal to form an electrostatic latent image.  
         [0085]      FIG. 2  is a block diagram showing an outline of the arrangement of a printing system using the image forming apparatus A. As shown in  FIG. 2 , a printer  2  (image forming apparatus A) comprises a printer controller  3  for rasterizing image information in a predetermined descriptive language supplied from a host computer  1 , and a printer engine including a printer control unit  404  and a signal processor  402 .  
         [0086]     The host computer  1  also supplies bit data of, e.g., RGB read by an image reader or the like.  
         [0087]     An image processor  401  in the printer controller  3  converts an RGB image into a YMCK image and performs pulse width modulation and dither processing for data by using the multilevel image, thereby generating a VDO signal  6  as a 1-bit image data string.  
         [0088]      FIG. 3  is an internal block diagram of the printer controller  3 . As shown in  FIG. 3 , this printer controller  3  comprises an image rasterizer  406 , a page memory  407 , and the image processor  401 . The image rasterizer  406  converts information of a printer language supplied from the host computer  1  into bit map data. The page memory  407  stores the data of one page. The image processor  401  converts RGB information supplied from the page memory into YMCK information and generates the VDO signal  6  converted to have a pulse width corresponding to the multilevel density. This VDO signal  6  is one hard signal. The image processor  401  can be controlled by a clock signal corresponding to one dot of 600 Dpi as printing dots.  
         [0089]      FIG. 4  is a timing chart showing the VDO signal  6  supplied from the printer controller  3 , a BD signal  423  as a horizontal sync signal supplied from the engine to the printer controller, and a PSYNC signal  424  as a vertical sync signal. As shown in  FIG. 4 , magenta data, cyan data, yellow data, and black data are output in this order in synchronism with the PSYN signal  424 .  
         [0090]      FIGS. 5, 12 , and  14  are internal block diagrams showing examples of the signal processor  402  of the engine. The example shown in  FIG. 5  will be described first.  
         [0091]     The VDO signal  6  supplied from the printer controller  3  is transferred to a laser driving circuit  500  via an OR gate  414  and an AND gate  415 .  
         [0092]     An image mask signal generator  411  is a block for generating a MASK signal  419  as a control signal for forcedly turning off a laser outside a printing region.  
         [0093]     This MASK signal is “1” outside a printing region and “0” in a printing region. The MASK signal is generated on the basis of the BD signal and PSYNC signal by receiving desired information from a CPU  412 .  
         [0094]     A tracking pattern generator  410  as an additional data generating means is a block for generating a signal by which dots representing a number unique to the machine are printed on printed matter by yellow toner difficult to see. A code is expressed by the arrangement of this tracking pattern on printed matter.  
         [0095]     The tracking pattern generator  410  receives an output clock signal CCLK from a quartz oscillator  413  installed in the engine, the BD signal  423 , and the PSYNC signal  424 , and generates a signal MKON for forcedly turning on the laser and a signal MKOFF for forcedly turning off the laser. These signals MKON and MKOFF can be asynchronous to the VDO signal  6  from the printer controller  3 .  
         [0096]     Note that the tracking pattern generator  410  is given arrangement information  421  of the tracking pattern by the CPU  412 . The CPU  412  reads out a number unique to the machine from a memory  420  and encodes the number to generate the arrangement information  421  of the tracking pattern.  
         [0097]     Note also that this tracking pattern is added to the VDO signal  6  when a yellow plane is printed; the tracking pattern is desirably not added in a plane of another color.  
         [0098]      FIG. 6  is an internal block diagram of the tracking pattern generator  410  shown in  FIG. 5 .  
         [0099]     The frequency of the clock CCLK of the quartz oscillator  413  is the same as or close to the image transfer rate of the printer controller  3 .  
         [0100]     The frequency of this CCLK signal is multiplied by 8 by a frequency multiplier  434 . A clock signal  445  having this eightfold frequency is output to shift registers  432  and  433  and a frequency divider  435 . In synchronism with the leading edge of the BD signal  423 , the frequency divider  435  generates a clock PCLK, which synchronizes with the BD signal, at the same frequency as the quartz oscillator  413 .  FIG. 7  is a timing chart showing these signals.  
         [0101]     A counter  426  is a 4-bit counter for counting the image clocks PCLK in a main scan direction. This counter  426  is reset by the BD signal  423  to start counting from 0h to Bh repeatedly.  
         [0102]     A counter  427  is a 5-bit counter for counting the BD signal  423  in a sub-scan direction. This counter  427  is reset by the PSYNC signal  424  to start counting from 0h to 1Fh repeatedly.  
         [0103]     An output signal  426  from these counters is information representing the coordinates of a printed dot. If coincidence circuits  428  and  429  in the subsequent stage determine that the information is a desired coordinate position, coincidence signals  447  and  448  are “1”. Selectors  430  and  431  select A inputs if the coincidence signals  447  and  448  are “1”, select B inputs if the coincidence signals  447  and  448  are “0”, and output the A or B inputs from Y.  
         [0104]     As shown in  FIG. 10 , the basic pixels of the tracking pattern are such that forced OFF dots are arranged on the two sides of a forced ON dot. The selector  430  in  FIG. 6  outputs multilevel information  443  which indicates a forced ON dot, i.e., outputs FCh at a timing at which a forced ON dot is printed and outputs 00h in other cases. The selector  431  outputs multilevel information  444  which indicates a forced OFF dot, i.e., outputs F8h at a timing at which a forced OFF dot is printed and outputs 00h in other cases. The output 8-bit signals from these selectors are converted into serial data output by parallel-serial converters  432  and  433 .  
         [0105]     Coordinate data ( 437  and  438 ) for printing tracking patterns to be set in the coincidence circuits are previously set by a CPU (not shown).  
         [0106]     Note that the circuit for mixing the tracking pattern in the VDO signal can also be an EX-OR gate, rather than an AND gate or an OR gate. When this is the case, tracking dots are not constituted by forced ON and OFF dots; they form an inverted print of an original image.  
         [0107]     Another example of the signal processor  402  shown in  FIG. 12  will be described below.  
         [0108]     The difference from  FIG. 5  is that the tracking pattern is neither ANDed nor ORed in the input stage of a laser driving circuit  501  but superposed on an image signal by using terminals (a forced ON port ON and a forced OFF port OFF) which the laser driving circuit  501  has to forcedly control a laser.  
         [0109]     In the example shown in  FIG. 12 , the image signals  6  supplied from the printer controller  3  are operation signals/VDO and VDO.  
         [0110]      FIG. 13  is an internal block diagram of a tracking pattern generator  502  shown in  FIG. 12 . Referring to  FIG. 13 , a tracking pattern is generated by a clock from a quartz oscillator  413 , so the tracking pattern has jitter of one clock. Also, since dots forming the tracking pattern are controlled in units of dots, no P-S converter is necessary. As described above, the circuit shown in  FIG. 12  is simple and hence can be realized at low cost.  
         [0111]     Still another example of the signal processor  402  shown in  FIG. 14  will be described below.  
         [0112]     This example is an embodiment of a laser beam printer which performs laser scan in a main scan direction by using two or more lasers (in this example, two).  
         [0113]     As shown in  FIG. 14 , exclusive OR (EX-OR) gates  507  and  508  for superposing tracking patterns are placed before laser driving circuits  505  and  506  for driving the lasers.  
         [0114]      FIG. 15  is an internal block diagram of tracking pattern generators  503  and  504 . In this example, tracking patterns are expressed by inverting image data VDO  513  and  514 . So, output signals are only MKOT  511  and  512 .  
         [0115]      FIG. 8  is a view schematically showing a unit region representing a number unique to the machine by using tracking patterns. As shown in  FIG. 8 , a predetermined code is expressed by nine patterns in a region indicated by the broken lines. Of these nine patterns, two patterns are reference patterns. The positions of the seven remaining patterns represent codes “0” to “3” (two bits), so these seven patterns express a total of 14 bits; in decimal notation, 0 to 16383.  FIG. 8  expresses 11384. This pattern is repeated in the main scan and sub-scan directions.  
         [0116]      FIG. 9  shows examples of the MKON[7:0]  443  and MKOFF[7:0]  444  as multilevel signals of tracking pattern dots generated by the tracking pattern generator  410 .  
         [0117]     In  FIG. 9 , if MKON[7:0] is FCh, 11111100B is converted into serial data and output as MKON to the OR gate  414  (not shown). That is, a 6/8 dot of one dot is forcedly printed. If MKOFF[7:0] is F8h, 11111000B is converted into serial data and output as MKOFF to the AND gate  415  (not shown).  
         [0118]     That is, a 5/8 dot of one dot is forcedly printed. If the signal is 00h, the VDO signal  6  is directly output to a laser driver. Also, this tracking pattern is printed every four lines.  
         [0119]      FIG. 10  is a view showing an image printed by mixing the tracking pattern shown in  FIG. 9  into the image signal VDO  6 . In  FIG. 10 , the VDO data  6  output from the printer controller is in phase with the tracking pattern. That is, the frequency of the control clock of the image processor  401  in the printer controller  3  perfectly matches the frequency of the control clock CCLK of the tracking pattern generator  410 . Assume that the VDO signal is printing an even intermediate density.  
         [0120]      FIG. 11  is a view when the output VDO data  6  from the printer controller is not in synchronism with the tracking pattern. That is, the frequency of the control clock of the image processor  401  in the printer controller does not perfectly match the frequency of the control clock CCLK of the tracking pattern generator  410 .  
         [0121]     This can happen because, as described earlier, both of the printer controller  3  and the tracking pattern generator  410  of the engine have a circuit for generating a clock signal synchronized with the horizontal sync signal BD. Referring to  FIG. 11 , the frequency of the control clock CCLK of the tracking pattern generator  410  is 1/1.5 the frequency of the control clock of the image processor  401 .  
         [0122]     As a modification of this embodiment, the frequency of the quartz oscillator of the engine can be made different from the image transfer rate of the controller. If a quartz oscillator having a frequency several times as high as the image transfer rate is used, the frequency multiplier  434  is unnecessary. Also, a frequency lower than the image transfer rate can be multiplied by the frequency multiplier  434  to obtain a clock having a desired frequency.  
         [0123]      FIG. 16  is an internal block diagram of the signal processor  402  in this case. The difference from the above case is that instead of a quartz oscillator being included in the engine, an image transfer clock signal VCLK  449  of the printer controller is output from the engine and used in the tracking pattern generator.  
         [0124]     Analogously,  FIG. 17  is an internal block diagram of the tracking pattern generator  410  in a case like this. This circuit obviates the need for a frequency divider for dividing the frequency of a block in synchronism with the BD signal, which is necessary in the above example.  
         [0125]     A tracking pattern mixed in this example is in synchronism with the VDO signal  6  as an image signal. So, the printed state is as shown in  FIG. 10 .  
         [0126]     Although a preferred embodiment of the present invention has been described above, the present invention naturally includes arbitrary combinations of some of the abovementioned arrangements.  
         [0127]     Also, both the forced ON dot and the forced OFF dot are smaller than one dot in the above embodiment. However, these dots can be larger than one dot, e.g., can be 11/8 dots or 5/4 dots.  
         [0128]     Furthermore, PCLK output from the frequency divider  435  can be entirely different from the image transfer rate. If this is the case, the size and interval of tracking pattern dots are not integral multiples of an image dot. This is established because not absolute dimensions but a printing interval ratio is used to extract codes from the positions of tracking pattern dots. The use of a clock signal of a quartz oscillator by another circuit makes any additional quartz oscillator unnecessary. This can realize an inexpensive arrangement.  
         [0129]     In the above embodiment, after the frequency of the output clock from the quartz oscillator  413  is raised by the frequency multiplier  434 , the clock is synchronized with the horizontal sync signal to set the phase jitter of a tracking pattern to be several times as small as one dot. However, the phase jitter can also be set to be equal to one dot without using the frequency multiplier  434 . Since the frequency multiplier and the like are unnecessary, an inexpensive arrangement can be realized.  
         [0130]     Furthermore, although a P-S converter is used to divide one tracking dot into eight portions, PWM (Pulse Width Modulation) is also usable.  
         [0131]     In  FIG. 6 , the counters  426  and  427 , the coincidence circuits  428  and  429 , the selectors  430  and  431 , the P-S circuits  432  and  433 , the frequency multiplier  434 , the frequency divider  435 , the OR gate  415  (not shown), and the AND gate  416  (not shown) can be contained in an ASIC. Additionally, although the OR gate and AND gate are used to mix tracking dots in the VDO signal  6 , a selector circuit can also be used.  
         [0132]     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.