Abstract:
An image forming apparatus including plural image forming units arranged for respective color components at predetermined intervals. A transportation unit transports a recording medium to the plural image forming units. A delay memory delays sending of image data for a time required to transport the recording medium between the plural image forming units. A length holding unit stores, for each page, plural pixel counts, the pixel counts representing respective numbers of pixels in rows of pixels spaced in the sub-scan directions, which pixels represent the image data for image formation. A reading control unit controls reading of the image data stored in the delay memory on the basis of the pixel counts of each page stored in the length holding unit.

Description:
This application is a divisional application of U.S. patent application Ser. No. 10/285,459, filed Nov. 1, 2002 now U.S. Pat. No. 7,224,481. 

   This application also claims priority from Japanese Patent Application Nos. 2001-345034, filed Nov. 9, 2001, and 2002-317719, filed Oct. 31, 2002, which are hereby incorporated by reference herein in their entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus which has, e.g., a photosensitive drum for each color and thus, performs color printing. 
   2. Related Background Art 
   Conventionally, a so-called 1D (one-drum) printer, which contains only one photosensitive drum, and a so-called 4D (four-drum) printer (or tandem printer), which contains plural photosensitive drums, have been widely known. 
   The 1D printer performs image formation of four colors of yellow (Y), magenta (M), cyan (C) and black (K) by rotating four times a transfer drum on which a medium, called an intermediate transfer body or a sheet of paper (simply called a sheet hereinafter), is applied. 
   On the other hand, in the 4D printer, since the respective photosensitive drums of the four colors are provided independently, an image including four colors can be formed at one time. Therefore, if a sheet transportation speed is constant, the 4D printer can generally achieve a print speed four times as much as that of the 1D printer. However, as described above, the 4D printer is equipped with the plural photosensitive drums respectively arranged at predetermined intervals. Thus, in the 4D printer, after color image signals of red (R), green (G) and blue (B) read by a scanner, or the like, are subjected to a color process, and then converted into color image signals of Y, M, C and K, it is necessary to delay the image signal for a time required to transport the sheet for the interval (distance) between the adjacent color photosensitive drums. For this reason, a delay memory to be used to delay the image signals is required. Here, it should be noted that the delay memory has a storage capacity according to the distance between the adjacent photosensitive drums. 
   On one hand, if the sheet transportation speed of the 4D printer is constant, it is necessary to shorten a so-called sheet (paper) interval, which is the interval between the trailing edge of a preceding sheet and the leading edge of a succeeding sheet, so as to increase the print speed. 
   In an apparatus, which is called an MFP (Multifunction Peripheral), it is required to output, through a printer, image data obtained from various devices such as a scanner, a PC (personal computer), a FAX (facsimile device), and the like. In such a situation, in order to improve productivity of a system as a whole, it is required to continuously output, through the printer, the image data obtained from the various devices and, respectively, having different image sizes. 
   In this case, data writing and reading in units of a line to and from the delay memory are controlled on the basis of the length of a main-scan valid (or effective) interval signal corresponding to the length of the image data in a main-scan direction and the length of a sub-scan valid (or effective) interval signal corresponding to the length of the image data in a sub-scan direction. 
   However, since the conventional 4D printer is equipped merely with a single set of delay memories, even if a sheet size (image size) is changed while a series of recording operations is being performed, the main-scan valid interval signal and the sub-scan valid interval signal cannot be changed until the delay memories become empty. Therefore, next data cannot be stored in the delay memory until it becomes empty, and it is thus necessary to prolong the sheet interval. That is, the images respectively having the different sizes cannot be simultaneously formed on the respective photosensitive drums of the four colors. 
     FIGS. 17A and 17B  are diagrams showing the relations between the photosensitive drums and the image sizes. In  FIGS. 17A and 17B , numerals  1701  and  1702  respectively denote photosensitive drums of which development colors are different from each other, numeral  1703  denotes a recording sheet (paper) P 1  on which an output image of a first page is recorded, numerals  1704  and  1705  respectively denote recording sheets P 2  on which output images of a second page are recorded, symbol d denotes an interval between the adjacent photosensitive drums, and symbol i denotes an interval between the trailing edge of the recording sheet P 1  of the first page and the leading edge of the recording sheet P 2  of the second page. 
   In  FIG. 17A , the size of the recording sheet P 1  of the first page is the same as the size of the recording sheet P 2  of the second page, whereby the image data can be output at the short sheet interval, even when using the conventional 4D printer. On the other hand, in  FIG. 17B , the size of the recording sheet P 1  of the first page is different from the size of the recording sheet P 2  of the second page. Thus, in the above conventional 4D printer, as described above, the main-scan valid interval signal and the sub-scan valid interval signal cannot be changed until the delay memories become empty, whereby the image data cannot be output in the state that the sheet interval is being shortened. That is, it is necessary in this case to prolong the interval i between the trailing edge of the recording sheet P 1  of the first page and the leading edge of the recording sheet P 2  of the second page more than the interval d between the adjacent photosensitive drums. Incidentally, since the 4D printer contains the four photosensitive drums, the interval between the photosensitive drums at both ends of the printer is three times the interval d between the adjacent photosensitive drums, whereby it is necessary to enlarge or to prolong the sheet interval i more than three times the interval d. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an image forming apparatus, which eliminates such inconveniences as described above, and to provide a control method of the above image forming apparatus. 
   Another object of the present invention is to provide an image forming apparatus, which can shorten a sheet interval between transported successive sheets without any specific setting, by administrating main-scan direction lengths and sub-scan direction lengths of plural pages in the case of controlling delay memories in a 4D printer, and to provide a control method of the above image forming apparatus. 
   Still another object of the present invention is to provide an image forming apparatus, which can shorten a sheet interval between transported successive sheets without any specific setting, by administrating main-scan direction lengths according to a maximum size in the case of controlling the delay memories in the 4D printer, and to provide a control method of the above image forming apparatus. 
   Still another object of the present invention is to provide an image forming apparatus, which can shorten a sheet interval between transported successive sheets by storing a memory end address of each page in the case of image data writing, even in a case where a sub-scan direction length of an image to be output is shorter than an interval between adjacent photosensitive drums, and the sub-scan direction length for each page is different from others. 
   Other objects of the present invention will be apparent from the following explanation based on the attached drawings and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the structure of an image forming system according to the embodiments; 
       FIG. 2  is a block diagram showing the structure of an MFP  104 ; 
       FIG. 3  is a cross-sectional diagram showing the structure of a scanner unit  201 ; 
       FIG. 4  is a block diagram showing the structure of a scanner IP (Information Processing) unit  202 ; 
       FIG. 5  is a block diagram showing the structures of an NIC (Network Interface Card) unit  204  and a PDL (Page Description Language) unit  205 ; 
       FIG. 6  is a block diagram showing the structure of a core unit  206 ; 
       FIG. 7  is a block diagram showing the structure of a printer IP unit  207 ; 
       FIGS. 8A and 8B  are, respectively, a block diagram showing the structure of a PWM (Pulse Width Modulation) unit  208  and a timing chart for explaining the operation of the PWM unit  208 ; 
       FIG. 9  is a cross-sectional diagram showing the structure of a four-drum type color printer unit  209 ; 
       FIG. 10  is a block diagram showing the structure of a display unit  211 ; 
       FIG. 11  is a cross-sectional diagram showing the structure of a finisher unit  210 ; 
       FIG. 12  is a block diagram showing the internal structure of a drum delay unit  704 ; 
       FIG. 13  is a timing chart showing changes of control signals of the drum delay unit  704  in its main-scan direction; 
       FIG. 14  is a timing chart showing changes of the control signals in units of a clock in the drum delay unit  704 ; 
       FIG. 15  is a timing chart showing changes of the control signals of the drum delay unit  704  in its sub-scan direction; 
       FIG. 16  is a timing chart showing changes of the control signals of the drum delay unit  704  in its sub-scan direction; 
       FIGS. 17A and 17B  are diagrams showing the relations between photosensitive drums and image sizes; 
       FIG. 18  is a timing chart showing changes of control signals of a drum delay unit  704  in its main-scan direction, according to the second embodiment; 
       FIG. 19  is a block diagram showing the internal structure of a drum delay unit  704  according to the third embodiment; and 
       FIG. 20  is a diagram showing an address map of an image delay memory according to the third embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, the embodiments of the present invention concerning an image forming apparatus and an image forming method will be explained with reference to the accompanying drawings. Here, it should be noted that the image forming apparatus according to the embodiments of the present invention is applied to a Multifunction Peripheral (an MFP). 
   First Embodiment 
     FIG. 1  is a block diagram showing an image forming system according to the first embodiment of the present invention. In  FIG. 1 , a computer  102  is a server, which is connected to a network  101 , and computers  103   a  and  103   b  are clients, which are also connected to the network  101 . Although it is not shown in  FIG. 1 , it should be noted that a lot of other clients are connected to the network  101 , and all of these clients are representatively called a client  103  hereinafter. 
   Moreover, an MFP  104 , which is a color-four-drum MFP capable of performing full-color image reading, full-color printing, facsimile transmission, facsimile reception, and the like, is connected to the network  101 . Moreover, although it is not shown in  FIG. 1 , various other apparatuses, such as another MFP, a scanner, a printer, a facsimile machine, and the like, are connected to the network  101 . 
   Here, on the computer  103 , application software for executing so-called DTP (Desktop Publishing) operates, whereby various texts and figures are formed and edited. Then, the computer  103  converts the formed texts and figures into PDL data, and transmits the obtained PDL data to the MFP  104  through the network  101 , whereby the transmitted data is printed out. 
   The MFP  104  includes a communication means capable of exchanging information to the computers  102  and  103  through the network  101 , whereby the information and the state of the MFP  104  are sequentially notified to the computers  102  and  103 . Moreover, each of the computers  102  and  103  includes utility software, which operates in response to the information notified by the MFP  104 , and thus, the MFP  104  is administrated by the computers  102  and  103 . 
   &lt;Structure of MFP  104 &gt; 
     FIG. 2  is a block diagram showing the structure of the MFP  104 , which includes a four-drum-type full-color printer. More specifically, the MFP  104  includes a scanner unit  201 , which performs image reading and generates image data, a scanner IP unit  202 , which performs an image process to the image data obtained by the scanner unit  201 , a FAX unit  203 , such as a facsimile machine, or the like, which transmits and receives image data by using a telephone line, an NIC (Network Interface Card) unit  204 , which exchanges image data and device information by using the network, and a PDL unit  205 , which expands the PDL data transmitted from the computer  103  into an image signal. A core unit  206  temporarily stores the image signal and determines a path in accordance with how to use the MFP  104 . 
   The image data output from the core unit  206  is subjected to an image process by a printer IP unit  207  and then transferred to a printer unit  209  through a PWM unit  208 , whereby the image data is actually printed out on a sheet. Then, the sheet discharged from the printer unit  209  is transported to a finisher unit  210  for a sheet sorting process and a sheet finishing process. 
   On one hand, a display unit  211  is used to a so-called preview process. In the preview process, it is possible to confirm the contents of the image without printing it out, and it is also possible to confirm the state of the image before printing it out. 
   &lt;Structure of Scanner Unit  201 &gt; 
     FIG. 3  is a cross-sectional diagram showing the structure of the scanner unit  201 . In  FIG. 3 , numeral  301  denotes an original mounting board glass on which an original  302  to be read is put. The original  302 , put on the original mounting board glass  301 , is illuminated by an illumination lamp  303 , and its reflection light is guided to a CCD sensor  308  through mirrors  304 ,  305  and  306  and a lens  307 , whereby the image based on the guided reflection light is formed on the CCD sensor  308 . A first mirror unit  310 , including the mirror  304  and the illumination lamp  303 , moves at a speed v, and a second mirror unit  311 , including the mirrors  305  and  306 , moves at a speed v/2, whereby the surface of the original  302  is scanned as a whole. Here, the first mirror unit  310  and the second mirror unit  311  are driven by a motor  309 . 
   &lt;Structure of Scanner IP Unit  202 &gt; 
     FIG. 4  is a block diagram showing the structure of the scanner IP unit  202 . An optical signal obtained by the scanner unit  201  is converted into an electrical signal by the CCD sensor  308  and input to the scanner IP unit  202 . Here, the CCD sensor  308  is an R, G and B three-line color sensor. In the scanner IP unit  202 , each of the input R, G and B image signals is converted into an eight-bit digital image signal by an A/D conversion unit  401 . The obtained digital image signals are respectively called RO, GO and BO image signals. 
   Then, a known shading correction process using a signal read from a reference white plate is performed for each color by a shading correction circuit  402 . Moreover, since the color line sensors constituting the CCD sensor  308  are arranged mutually apart from each other by a predetermined distance, spatial aberration (or misregistration) in the sub-scan direction is corrected by a line delay adjustment circuit (line interpolation unit)  403 . 
   An input masking unit  404  is to convert a reading color space determined by spectral characteristics of the R, G and B filters of the CCD sensor  308  into an NTSC (National Television System Committee) standard color space. That is, in the input masking unit  404 , a 3×3 matrix operation using a constant inherent in the apparatus is performed in consideration of various characteristics, such as a sensitivity characteristic of the CCD sensor  308 , a spectral characteristic of the illumination lamp, and the like, whereby the input RO, GO and BO image signals are respectively converted into the standard R, G and B signals. 
   A brightness/density conversion unit (LOG conversion unit)  405  includes an LUT (Look-Up Table) RAM. In the LOG conversion unit  405 , the R, G and B brightness signals are converted into C, M and Y density signals (called C1, M1 and Y1 signals), and then, the C1, M1 and Y1 image signals are processed and transferred to the core unit  206  through an output masking unit  406 , a gamma conversion unit  407  and a spatial filter  408 . 
   &lt;Structure of NIC Unit  204 &gt; 
     FIG. 5  is a block diagram showing the structures of the NIC unit  204  and the PDL unit  205 . The NIC unit  204  has an interface function to the network  101  and thus, functions to obtain external information and to transmit information outward by using, e.g., an Ethernet™ cable, such as a 10Base-T, a 100Base-TX, or the like. 
   In the case of obtaining the information externally, the data representing information is first subjected to voltage transformation by a transformer unit  501  and then transferred to a LAN controller unit  502 , which includes a first buffer memory (not shown) to store the transferred data. Then, it is judged whether or not the information represented by the transferred data is necessary information. If it is judged that the information in question is the necessary information, the data is transferred to a second buffer memory (not shown), and then transferred to the PDL unit  205 . 
   On the other hand, in the case of transmitting the information outward, necessary information is added to the data transferred from the PDL unit  205  by the LAN controller unit  502 , and then, the obtained data is output to the network  101  through the transformer unit  501 . 
   &lt;Structure of PDL Unit  205 &gt; 
   The image data generated by the application software operating on the computer  103  represents texts, figures, photographs, and the like, and is thus composed of the combination of image description elements, such as character codes, graphic codes, raster image data, and the like. This is the so-called PDL, which is represented by, e.g., PostScript™ by Adobe. 
   The PDL unit  205  converts the PDL data into raster image data. That is, the PDL data transferred from the NIC unit  204  is once stored in a large-capacity memory  504 , such as an HDD (Hard Disk Drive), and the stored data is administrated and held for each job. 
   A CPU  503  performs a rasterizing image process, which is called RIP (Raster Image Processing) according to need, and expands the PDL data into the raster image data. The expanded raster image data is stored in a high-speed accessible memory  505 , such as a DRAM, or the like, for each of the C, M, Y and K color components. Here, it should be noted that, in each job, the raster image data is stored for each page. After that, the raster image data is transferred to the core unit  206  by the CPU  503 , in accordance with the state of the printer unit  209 . 
   &lt;Structure of Core Unit  206 &gt; 
     FIG. 6  is a block diagram showing the structure of the core unit  206 . In the core unit  206 , a bus selector unit  601  saves as, in a manner of speaking, data traffic control in the MFP  104 . That is, the bus selector unit  601  switches over the buses in accordance with various functions in the MFP  104 , such as a copying function, network scanning, network printing, facsimile transmission, facsimile reception, display, and the like. 
   The followings are the bus switching patterns for executing the respective functions. 
   copying function: scanner unit  201 →core unit  206 →printer unit  209 ; 
   network scanning: scanner unit  201 →core unit  206 →NIC unit  204 ; 
   network printing: NIC unit  204 →core unit  206 →printer unit  209 ; 
   facsimile transmission function: scanner unit  201 →core unit  206 →FAX unit  203 ; 
   facsimile reception function: FAX unit  203 →core unit  206 →printer unit  209 ; and 
   display function: scanner unit  201 , FAX unit  203  or NIC unit  204 →core unit  206 →display unit  211 . 
   Next, the image data output from the bus selector unit  601  is transferred to the printer unit  209  (PWM unit  208 ) or the display unit  211  through a compression unit  602 , a memory unit  603  including a large-capacity memory such as an HDD, or the like, and a decompression unit  604 . 
   A compression system to be used in the compression unit  602  may be a general system, such as a JPEG (Joint Photographic Experts Group) system, a JBIG (Joint Bi-level Image experts Group) system, a ZIP system, or the like. The compressed image data is administrated for each job, and stored together with additional data representing a file name, an author, generation date and time, a file size, and the like. 
   Moreover, a job number and a job password are provided and stored together with the image data, whereby it is possible to support a personal box function. This is the function by which the data can be temporarily stored or only a certain user can print out the stored data (i.e., data reading from the HDD). If it is instructed to print out the job stored in the memory unit  603 , authentication is performed on the basis of the password, the image data is read from the memory  603  if the password is correct, the read image data is decompressed or expanded by the decompression unit  604  to obtain the raster image data, and then the obtained raster image data is transferred to the printer IP unit  207 . 
   Structure of Printer IP Unit  207 &gt; 
     FIG. 7  is a block diagram showing the structure of the printer IP unit  207 . In  FIG. 7 , numeral  701  denotes an output masking/UCR (under color removal) circuit, which corrects the C1, M1 and Y1 signals and thus, obtains the C, M, Y and K signals respectively representing toner colors of the image forming apparatus. Then, the output masking/UCR circuit  701  outputs the C, M, Y and K signals. 
   Numeral  702  denotes a gamma correction (conversion) unit, which converts the input C, M, Y and K signals into C, M, Y and K data for image output by using an LUT RAM provided in consideration of the various color characteristics of toners. Numeral  703  denotes a spatial filter by which an edge emphasizing process or a smoothing process is performed to the C, M, Y and K data. 
   Numeral  704  denotes a drum delay unit, which includes a memory necessary in a case of delaying the image data for each color according to the distance between the photosensitive drums. The image signal, which has been delayed for each color by the drum delay unit  704 , is then transferred to the PWM unit  208 . 
   &lt;Structure of PWM Unit  208 &gt; 
     FIGS. 8A and 8B  are, respectively, a block diagram showing the structure of the PWM unit  208  and a timing chart for explaining the operation of the PWM unit  208 . The multivalued image data representing each of the separated Y, M, C and K output from the printer IP unit  207  is subjected to the image formation through the PWM unit  208 . 
   In  FIG. 8A , numeral  801  denotes a triangular wave generation unit, and numeral  802  denotes a D/A converter (D/A conversion unit), which converts the input digital image signal into an analog signal. An output signal a, generated from the triangular wave generation unit  801 , and an output signal b, generated from the D/A conversion unit  802 , are compared with each other by a comparator  803 , and an output signal c, representing the compared result, is transferred to a laser driving unit  804 . Here, for each of C, M, Y and K, a laser beam output from a laser oscillator  805  driven by the laser driving unit  804  is modulated on the basis of the output signal c of the comparator  803 . 
   The laser beams of C, M, Y and K output from the laser oscillator  805  are scanned by a polygonal scanner  913  and thus, irradiated to photosensitive drums  917 ,  921 ,  925  and  929  respectively. 
   &lt;Structure of Printer Unit  209  of MFP  104  (Four-Drum Type Printer)&gt; 
     FIG. 9  is a cross-sectional diagram showing the structure of a four-drum type color printer unit  209 . In  FIG. 9 , numeral  913  denotes a polygonal mirror (polygonal scanner), which receives the four laser beams respectively generated from the four semiconductor laser oscillators  805  for C, M, Y and K. One of these four laser beams is scanned on the photosensitive drum  917  through mirrors  914 ,  915  and  916 , another laser beam is scanned on the photosensitive drum  921  through mirrors  918 ,  919  and  920 , another laser beam is scanned on the photosensitive drum  925  through mirrors  922 ,  923  and  924 , and the remaining one of these four laser beams is scanned on the photosensitive drum  929  through mirrors  926 ,  927  and  928 . 
   Numeral  930  denotes a development unit, which supplies Y toner, and thus, forms a yellow toner image based on an electrostatic latent image formed by the laser beam on the photosensitive drum  917 . Numeral  931  denotes a development unit, which supplies M toner, and thus, forms a magenta toner image based on an electrostatic latent image formed by the laser beam on the photosensitive drum  921 . Numeral  932  denotes a development unit, which supplies C toner, and thus, forms a cyan toner image based on an electrostatic latent image formed by the laser beam on the photosensitive drum  925 . Numeral  933  denotes a development unit, which supplies K toner, and thus, forms a black toner image based on an electrostatic latent image formed by the laser beam on the photosensitive drum  929 . When these toner images of four colors Y, M, C and K are transferred onto a sheet, a full-color output image can be obtained. 
   A sheet supplied from any one of sheet cassettes  934  and  935  and a manual feed tray  936  is adsorbed onto a transfer belt  938  through a registration roller  937 , whereby the sheet is transported by the transfer belt  938 . On the other hand, the respective toner images have been previously developed on the photosensitive drums  917 ,  921 ,  925  and  929  in synchronism with sheet transportation timing, whereby the toner images are transferred to the sheet while the sheet is being transported. 
   The sheet on which the toner images of the respective colors have been transferred is separated from the transfer belt  938  and then further transported by a transfer belt  939 , and the toners on the sheet are fixed by a fixing unit  940 . The sheet, which passed the fixing unit  940 , is once guided downward by a flapper  950 , and then, the sheet is switched back after the trailing edge of the sheet has passed the flapper  950 . After that, the switched-back sheet is discharged outward. As a result, the sheet is discharged face down, so that the pages can be set in due order when the images are printed out from the first page thereof. 
   It should be noted that the four photosensitive drums  917 ,  921 ,  925  and  929  are arranged at an identical interval (distance) d. If the sheet is transported by the transportation belt  939  at a certain speed v, the above operations are synchronously performed as a whole, and also, the four semiconductor laser oscillators  805  for C, M, Y and K are synchronously driven. 
   &lt;Structure of Display Unit  211 &gt; 
     FIG. 10  is a block diagram showing the structure of the display unit  211 . In the display unit  211 , the image signal output from the core unit  206  is equivalent to the C, M, Y and K data, whereby it is necessary to again convert the input C, M, Y and K data into the R, G and B data by an inverse LOG conversion unit  1001 . Then, in a gamma conversion unit  1002 , output conversion is performed to the R, G and B data by using an LUT to cope with color characteristics of a display (monitor)  1004 , such as a CRT, or the like. More specifically, the converted image data is once stored in a memory unit  1003  and then displayed on the display  1004 . 
   Here, it should be noted that the object of use of the display  211  is to execute a preview function, to execute a proof function, or to confirm beforehand whether or not it is necessary to perform actual printing. That is, a user can confirm by using the preview function the image to be output prior to actual printing, and also, can judge, by using the proof function, whether or not the image to be output is equivalent to the intended image. Thus, it is possible to prevent wasteful use of recording sheets. 
   &lt;Structure of Finisher Unit  210 &gt; 
     FIG. 11  is a cross-sectional diagram showing the structure of the finisher unit  210 . The sheets discharged from the fixing unit  940  of the printer unit  209  are entered into the finisher unit  210 . Incidentally, since a sample tray  1101  and a stack tray  1102  are provided on the finisher unit  210 , the tray to be used is selected and changed according to a kind of job and the number of sheets to be discharged, whereby the sheets are actually discharged to the selected tray. 
   In a case in which a sorting operation is performed, it is possible to use one of two sorting systems, that is, one is a bin sorting system to distribute the output sheets to plural bins by using a sorter having the plural bins, and the other is a shift sorting system to distribute the output sheets for each job by shifting the bin (or the tray) toward near and far sides. 
   If the above core unit  206  has a large-capacity memory, the function of electronic sorting can be supported by using a so-called collating function to make the discharged page order different from the buffered page order, with use of the large-capacity memory. On one hand, a group function is a function to perform the sorting for each page. 
   Moreover, in a case in which the sheets are discharged to the stack tray  1102 , it is possible to stock, for each job, the sheets before they are discharged, and then, to bind the stocked sheets by a stapler  1105  immediately before they are discharged. 
   Besides, a Z folder  1104  and a puncher  1106  are provided in front of the sample tray  1101  and the stack tray  1102 . Thus, the Z folder  1104  folds the sheet like a character “Z”, and the puncher  1106  punches two (or three) holes on the sheets for filing them. It should be noted that the sheet folding process and/or the punching process are performed in accordance with the kind of job. 
   Besides, a saddle stitcher  1107 , which performs a saddle stitching process, is provided in front of a booklet tray  1108 . That is, the saddle stitcher  1107  first binds the sheets at the central two points thereof, center-folds the sheets by applying the center portion of the sheets to a roller, and thus, forms a booklet, such as a weekly magazine, a pamphlet, and the like. Then, the sheets (booklet) subjected to the saddle stitching process by the saddle stitcher  1107  are discharged to the booklet tray  1108 . 
   An inserter  1103  is provided to feed the sheet set on a tray  1110  directly to any one of the sample tray  1101 , the stack tray  1102  and the booklet tray  1108  without printer intervention. Thus, the sheet set on the inserter  1103  can be inserted between the successive sheets transported into the finisher  210 . 
   It is assumed that the sheets are set face up on the tray  1110  by a user. Then, the set sheets are sequentially fed from the uppermost one by a pickup roller  1111 , and the sheets fed from the inserter  1103  are directly discharged face down to the sample tray  1101  or the stack tray  1102 , whereby these sheets are discharged face down. When the sheets are transported to the saddle stitcher  1107 , the sheets are once transported to the side of the puncher  1106  and then switched back toward the saddle stitcher  1107 , whereby the faces of the sheets respectively transported from the inserter  1103  and the printer are consolidated. 
   &lt;Structure of Drum Delay Unit  704 &gt; 
     FIG. 12  is a block diagram showing the internal structure of the drum delay unit  704 . In  FIG. 12 , numeral  1201  denotes a sync control unit, which includes a main-scan valid interval holding unit  1201   a  and a sub-scan valid interval holding unit  1201   b . The main-scan valid interval holding unit  1201   a  can hold a main-scan valid interval signal henb_in of the length corresponding to the three pages, and similarly, the sub-scan valid interval holding unit  1201   b  can hold a sub-scan valid interval signal venb_in of the length corresponding to the three pages. Moreover, numeral  1202  denotes a data conversion unit, numeral  1203  denotes an address generation unit, numeral  1204  denotes a RAM control unit, and numeral  1205  denotes an SDRAM (synchronous dynamic RAM) acting as a delay memory. 
   Here, it should be noted that the following image sync signals are input to the sync control unit  1201 . 
   vsync_in: a sync start signal in the sub-scan direction; 
   venb_in: the valid interval signal in the sub-scan direction; 
   hsync_in: a sync start signal in the main-scan direction; 
   henb_in: the valid interval signal in the main-scan direction; 
   pvsync_in: a sub-scan activation signal of the printer; and 
   phsync_in: a main-scan activation signal of the printer. 
   Each of these signals becomes active at its low level. Here, the valid interval signal venb_in in the sub-scan direction and the valid interval signal henb_in in the main-scan direction change for each page in accordance with an input image size. Moreover, the sub-scan activation signal pvsync_in of the printer and the main-scan activation signal phsync_in of the printer are the activation signals for requesting an image signal input from the printer, and the input timing of each of these signals is different for each color. Incidentally, the following controls signals are output from the sync control unit  1201 . 
   count 16 : a 16 counter signal composed of four-bit counters and counting 16 cycles; 
   venb_wr: a sub-scan valid interval signal for data writing; 
   henb_wr: a main-scan valid interval signal for data writing; 
   venb_rd: a sub-scan valid interval signal for data reading; 
   henb_rd: a main-scan valid interval signal for data reading; 
   henb_vo: a main-scan valid interval signal for data output; and 
   henb_ref: a main-scan valid interval signal for refresh control of the SDRAM. 
   Each of these signals, except for the counter signal count 16 , becomes active at its high level. Moreover, four-bit image data data_in is input from the spatial filter  703  to the data conversion unit  1202 , the input image data is temporarily stored as 16-bit data for every four pixels, the temporarily stored 16-bit image data is then transferred to the SDRAM  1205  at a predetermined timing in response to a sync signal transferred from the sync control unit  1201 , and the transferred image data is written in the SDRAM  1205 . On the other hand, 16-bit image data ram_data read from the SDRAM  1205  is converted into four-bit image data of four pixels, and the obtained data is output as data data_out. Moreover, signals vsync_out, venb_out, hsync_out and henb_out are output from the sync control unit  1201 . 
   The address generation unit  1203  generates a writing address and a reading address of the SDRAM  1205  in response to the control signal transferred from the sync control unit  1201 . When the sub-scan valid interval signal venb_wr for the data writing and the main-scan valid interval signal henb_wr for the data writing both transferred from the sync control unit  1201  are active, the writing address is updated for every sixteen cycles. Similarly, when the sub-scan valid interval signal venb_rd for the data reading and the main-scan valid interval signal henb_rd for the data reading are active, the reading address is updated for every sixteen cycles. 
   The respective addresses are converted into the address format suitable for the SDRAM  1205 , and then, output as an SDRAM address ram_ad and an SDRAM bank address ram_ba. Moreover, the address generation unit  1203  generates a signal mad_cs. 
   The RAM control unit  1204  generates a chip select signal ram_cs, an RAS (Row Address Strobe) signal ram_ras, a CAS (Conditional Access System) signal ram_cas, and a writing signal ram_we, which are necessary for command control of data reading and writing of the SDRAM  1205 . Moreover, the RAM control unit  1204  generates a command control signal, which is necessary for a refresh operation of the SDRAM  1205 . 
   &lt;Control Timing of Drum Delay Unit  704 &gt; 
     FIG. 13  is a timing chart showing changes of the control signals of the drum delay unit  704  in its main-scan direction. As described above, the drum delay unit  704  includes the main-scan valid interval holding unit  1201   a  for holding and storing the length of the main-scan valid interval signal henb_in for each page, whereby the drum delay unit  704  can resultingly store the image data of three pages. 
   First, the sub-scan valid interval signal venb_in of the first page is input (page 0 _wr), and then, the image signal data_in is input simultaneously with the main-scan valid interval signal henb_in. The data are input for a main-scan valid interval (data_valid_p 0 ) of the first page. In this interval, the data of sixteen pixel clocks are temporarily stored as one block (data w 0 ), and then, the image data are collected into the block for every sixteen clocks (data w 1 , data w 2 ). Here, it should be noted that, to simplify the explanation, the number of blocks for one line in the main-scan direction is set to three, however, more blocks actually exist. 
   The temporarily stored image data w 0 , w 1  and w 2  are output as the data ram_data to the SDRAM  1205  in the interval of the main-scan valid interval signal henb_wr for the data writing, and the image data of each block is stored in the SDRAM  1205 . Here, as described above, the length of the main-scan valid interval (data_valid_p 0 ) is stored in the main-scan valid interval holding unit  1201   a . Then, data w 3 , w 4  and w 5  of the next line are similarly input and written in the SDRAM  1205 . 
   Next, if the sub-scan activation signal pvsync_in and the main-scan activation signal phsync_in of the first page are input from the printer (page 0 _rd), the sync control unit  1201  generates the main-scan valid interval signal henb_rd for the data reading. The main-scan valid interval signal henb_rd for the data reading is the signal having the same interval length as the main-scan valid interval (data_valid_p 0 ) stored in the main-scan valid interval holding unit  1201   a.    
   In the interval where no writing operation is performed in the main-scan valid interval signal henb_rd for the data reading, the data reading operation from the SDRAM  1205  is performed, and 16-bit image data r 0  corresponding to the image data w 0  is read. Then, the read image data r 0  is converted into the four-bit image data by the data conversion unit  1202 , and the image data thus obtained is output as the data data_out. Then, data r 1  and r 2  are similarly output for every sixteen clocks. 
   Next, a case in which the sub-scan valid interval signal venb_in of the second page is input will be described. Here, a case wherein the length (data_valid_p 1 ) of the main-scan valid interval signal of the second page is different from the length (data_valid_p 0 ) of the main-scan valid interval signal of the first page will be described. The image data are input for the main-scan valid interval (data_valid_p 1 ) of the second page, the data of sixteen clocks are temporarily stored as one block (data wa), and then the image data are collected into the block for every sixteen clocks (data wa, data wb). The temporarily stored image data are output to the SDRAM  1205  as the data ram_data in the interval of the main-scan valid interval signal henb_wr for the data writing, whereby the image data of the plural blocks are stored in the SDRAM  1205 . Here, as described above, the length (data_valid_p 1 ) of the main-scan valid interval signal is stored in the main-scan valid interval holding unit  1201   a.    
   If the main-scan activation signal phsync_in is input from the printer, 16-bit image data r 3  corresponding to the image data w 3  of the first page is read from the SDRAM  1205 , the read image data is converted into four-bit image data, and then the obtained image data is output as the data data_out. Then, image data r 4  and r 5  are similarly read out. Here, it should be noted that the main-scan valid interval signal henb_rd for the data reading is output by the length (data_valid_p 0 ) of the stored main-scan valid interval signal of the first page. 
   Moreover, if the sub-scan activation signal pvsync_in and the main-scan activation signal phsync_in of the second page are input from the printer (page 1 _rd), 16-bit image data ra corresponding to the image data wa of the second page is read from the SDRAM  1205 , the read image data is converted into four-bit image data, and then the obtained image data is output as the data data_out. Then, the image data wb is similarly read out. Here, it should be noted that the main-scan valid interval signal henb_rd for the data reading is output by the length (data_valid_p 1 ) of the stored main-scan valid interval signal of the second page. 
   Thus, according to the present embodiment, the lengths (data_valid_p 0 , data_valid_p 1 , data_valid_p 2 ) of the main-scan valid interval signals of three pages in the data writing are stored beforehand, and the main-scan valid interval signal henb_rd for the data reading corresponding to each page is generated in the data reading. Therefore, even if the data writing of a certain page into the SDRAM  1205  and the data reading of another page from the SDRAM  1205  occur in the same interval, the recording operation can be continued without changing memory access setting on the way and without prolonging the sheet interval. 
   &lt;Clock Control of Drum Delay Unit  704 &gt; 
     FIG. 14  is a timing chart showing changes of the control signals in units of pixel clocks in the drum delay unit  704 . Incidentally,  FIG. 14  shows control timing in units of a clock with respect to the timing chart shown in  FIG. 13 . In the SDRAM  1205 , it is assumed that a necessary initial setting and mode setting previously defined are performed by a not-shown circuit, here, CAS latency has been set to “3” and a burst mode has been set to “4”. 
   First, the counter signal count 16  output from the sync control unit  1201  repeatedly counts (represents) the values from “0” to “15”. When the main-scan valid interval signal henb_rd for the data reading is active, that is, while the counter signal count 16  represents the values from “0” to “7”, the reading operation is performed. On the other hand, when the main-scan valid interval signal henb_wr for the data writing is active, that is, while the counter signal count 16  represents the values from “8” to “15”, the writing operation is performed. Moreover, when the main-scan valid interval signal henb_ref for refresh control of the SDRAM (also called refresh valid interval signal henb_ref) is active, that is, while the counter signal count 16  represents the values from “8” to “15”, the refresh operation of the SDRAM  1205  is performed. However, it is set that, if the main-scan valid interval signal henb_wr for the data writing is active, the refresh valid interval signal henb_ref does not become active. 
   When the counter signal count 16  represents the value “0”, in order to transfer an active command ACT defined in the specification of the SDRAM, the control signals ram_cs, ram_ras, ram_cas and ram_we are respectively output from the RAM control unit  1204 , and also, a signal corresponding to the row address included in the RAM address signal ram_ad for the data reading and the bank address signal ram_ba are transferred from the address generation unit  1203 . 
   Next, when the counter signal count 16  represents the value “1”, in order to transfer a read command RDA defined in the specification of the SDRAM, the control signals ram_cs, ram_ras, ram_cas and ram_we are output from the RAM control unit  1204 , and also, a signal corresponding to the column address included in the RAM address signal ram_ad for the data reading and the bank address signal ram_ba are transferred from the address generation unit  1203 . Moreover, when the counter signal count 16  represents the value “4”, the 16-bit RAM image data ram_data is output from the SDRAM  1205  as read data Di 0 . After that, the 16-bit image data are continuously output as read data Di 1 , read data Di 2  and read data Di 3 . 
   When the counter signal count 16  represents the value “8”, in order to transfer the active command ACT defined in the specification of the SDRAM, the control signals ram_cs, ram_ras, ram_cas and ram_we are respectively output from the RAM control unit  1204 , and also, a signal corresponding to the row address included in the RAM address signal ram_ad for the data writing and the bank address signal ram_ba are transferred from the address generation unit  1203 . 
   When the counter signal count 16  represents the value “9”, in order to transfer a write command WRA defined in the specification of the SDRAM, the control signals ram_cs, ram_ras, ram_cas and ram_we are respectively output from the RAM control unit  1204 , and also, the signal corresponding to the column address included in the RAM address signal ram_ad for the data writing and the bank address signal ram_ba are transferred from the address generation unit  1203 . At the same time, 16-bit write data Do 0  is output to the SDRAM  1205 . After that, the 16-bit image data are continuously output to the SDRAM  1205  as write data Do 1 , write data Do 2  and write data Do 3 . If the value of the counter signal count 16  is returned to “0”, the same read operation is performed while the counter signal count 16  represents the values from “0” to “7”. 
   When the counter signal count 16  represents the value “8”, the refresh valid interval signal henb_ref is active. Thus, in order to transfer a refresh command REF defined in the specification of the SDRAM, the control signals ram_cs, ram_ras, ram_cas and ram_we are respectively output from the RAM control unit  1204 . After that, the refresh command REF is similarly transferred when the counter signal count 16  represents the value “12”. 
   &lt;Sub-Scan Control of Drum Delay Unit  704 &gt; 
     FIGS. 15 and 16  are timing charts showing changes of the control signals of the drum delay unit  704  in its sub-scan direction. In  FIGS. 15 and 16 , symbol LINE (“0”, “1”, “2”, . . . ) indicates the number of lines in the sub-scan direction, and symbols page_wr and page_rd respectively denote counter data for administrating the pages in the data writing and the data reading. Here, the value of each of the data page_wr and page_rd is counted up like “0”, “1”, “2”, “0”, . . . , for each page. However, in the example shown in  FIG. 15 , the value of the counter data is maintained as “0” because the number of pages is “1”. 
   Moreover, symbols venb_rd_cntr 0 , venb_rd_cntr 1  and venb_rd_cntr 2  respectively denote up-and-down counter data for administrating the reading and writing operations in the sub-scan direction corresponding to each counted value (page) of the counter data page_wr and page_rd. That is, each of the up-and-down counter data venb_rd_cntr 0 , venb_rd_cntr 1  and venb_rd_cntr 2  is counted up for each line in the writing operation and counted down for each line in the reading operation. 
   For example, the up-and-down counter data venb_rd_cntr 2  is counted up for each line when the data page_wr is “2” and the sub-scan valid interval signal venb_in is “0”, and is counted down for each line when the data page_rd is “2” and the up-and-down counter data venb_rd_cntr 2  is not “0”. 
   Moreover, symbols ad_wr and ad_rd respectively denote counter data representing the writing address and the reading address, which change for each line. That is, the value of each of the counter data ad_wr and ad_rd returns to “0” after it reaches the maximum value “9”. 
     FIG. 15  shows a case where the length of the output image in its sub-scan direction is longer than the drum interval, that is, the drum interval is equivalent to five lines and the length of the output image in the sub-scan direction is equivalent to sixteen lines. It should be noted that, although the number of lines of each of the length and the interval is set to be small for simplification of the explanation, the actual number of lines is larger. 
   At the line  0 , if the sub-scan valid interval signal venb_in and the image data data_in are input, the up-and-down counter data venb_rd_cntr 0  is counted up for each line when the sub-scan valid interval signal venb_in is “0” (i.e., the image data is input). Then, the up-and-down counter data venb_rd_cntr 0  is counted up to the value “5” at the line  7 . 
   Next, if the sub-scan activation signal pvsync_in of the printer is input, the sub-scan valid interval signal is set to “1”. Moreover, the up-and-down counter data venb_rd_cntr 0  is once counted up to the value “6” at the line  8 , but is then counted down to the value “5” at the same line, because the reading operation is performed. 
   After that, since the writing operation and the reading operation are simultaneously performed until the line  18 , the up-and-down counter data venb_rd_cntr 0  is counted up and down only to the values “6” and “5”. Furthermore, since the writing operation ends at the line  19  (because no image data is input), the up-and-down counter data venb_rd_cntr 0  is counted down to the value “4” and then, counted down until the value becomes “0” at the line  23 . At the line  24 , subsequent to the line where the value of the up-and-down counter data venb_rd_cntr 0  became “0”, the sub-scan valid interval signal venb_rd for the data reading is set to “0”, and the reading operation ends. 
     FIG. 16  shows a case in which the length of the output image in its sub-scan direction is shorter than the drum interval and the length of the output image in its sub-scan direction is different for each page. That is, the drum interval is equivalent to seven lines, and the length of the output image in the sub-scan direction is equivalent to three lines at the first page, six lines at the second page, four lines at the third page, two lines at the fourth page, and two lines at the fifth page. It should be noted that, although the number of lines of each of the length and the interval is set to be small for simplification of the explanation, the actual number of lines is larger. 
   From the line  1  to the line  3 , the image data of the first page is input, the value of the counter data page_wr is “0”, and the up-and-down counter data venb_rd_cntr 0  is counted up to the value “3”. Next, from the line  6  to the line  11 , the image data of the second page is input, the counter data page_wr is counted up to the value “1”, and also the up-and-down counter data venb_rd_cntr 1  is counted up to the value “6”. 
   Then, the data reading of the first page starts from the line  8 , the data page_rd is set to “0”, and then the up-and-down counter data venb_rd_cntr 0  is counted down. The obtained value becomes “0” at the line  10 , and the sub-scan valid interval signal venb_rd for the data reading is set to “0” at the line  11 , whereby the reading operation of the first page is stopped. Moreover, the data reading of the second page starts from the line  13 , the data page_rd is counted up to the value “1”, and then the up-and-down counter data venb_rd_cntr 1  is counted down. Thus, the obtained value becomes “0” at the line  18 , and the sub-scan valid interval signal venb_rd for the data reading is set to “0” at the line  19 , whereby the reading operation of the second page is stopped. 
   From the line  14  to the line  17 , the image data of the third page is input, the counter data page_wr is counted up to the value “2”, and the up-and-down counter data venb_rd_cntr 2  is counted up to the value “4”. Next, from the line  20  to the line  21 , the image data of the fourth page is input, the counter data page_wr is returned to the value “0”, and also the up-and-down counter data venb_rd_cntr 0  is counted up to the value “2”. After that, similarly, the writing operation and the reading operation due to the data input are repeated. 
   As above, the sub-scan valid interval signal venb_in in the data reading is administrated on the basis of the three up-and-down counter data (venb_rd_cntr 0 , venb_rd_cntr 1  and venb_rd_cntr 2 ). Thus, even in the case wherein the length of the output image in its sub-scan direction is shorter than the drum interval and the length of the output image in its sub-scan direction is different for each page, it is possible to continue the recording operation without changing the memory control setting for each page. Thus, it is unnecessary to wait until the memory becomes vacant and also, to prolong the sheet interval. 
   Second Embodiment 
     FIG. 18  is a timing chart showing changes of control signals of a drum delay unit  704  in its main-scan direction, according to the second embodiment. It should be noted that output of 16-bit image data ram_data shown in  FIG. 18  is different from that shown in  FIG. 13 . 
   Moreover, it should be noted that inputs of a sub-scan valid interval signal venb_in and four-bit image data data_in are the same as those shown in  FIG. 13 . The data are input for a main-scan valid interval (data_valid_p 0 ) of the first page. In this interval, the data of sixteen pixel clocks are temporarily stored as one block (data w 0 ), and then, the image data are collected into the block for every sixteen clocks (data w 1 , data w 2 ). 
   The temporarily stored image data w 0 , w 1  and w 2  are output as the data ram_data to an SDRAM  1205  in the interval of a main-scan valid interval signal henb_wr for the data writing, and the plural image data are stored in the SDRAM  1205 . Here, as described above, the length of the main-scan valid interval (data_valid_P 0 ) is stored in a main-scan valid interval holding unit  1201   a . It should be noted that the main-scan valid interval signal henb_wr for the data writing is fixed to the maximum output image size. That is, if an A3 size is the maximum image size, the length of the main-scan valid interval signal henb_wr for the data writing is “7024”. When the main-scan valid interval (data_valid_p 0 ) is smaller than the main-scan valid interval signal henb_wr for the data writing, dummy image data fixed to “0” as data w 3  of the image data ram_data is stored in the SDRAM  1205 . Then, data w 4 , w 5 , w 6  and w 7  of next lines are similarly written in the SDRAM  1205 . 
   With respect to the data of the second page, as with the data of the first page, the main-scan valid interval signal henb_wr for the data writing is fixed to the maximum output image size, and dummy data wc and wd of the value “0” are written in the SDRAM. 
   Then, if a main-scan activation signal phsync_in is input from the printer, 16-bit image data r 4  corresponding to the image data w 4  of the first page is read from the SDRAM  1205  by controlling an address value, the read image data is converted into four-bit image data, and then the converted data is output as data data_out. After that, the processes, which are the same as those shown in  FIG. 13 , are performed. 
   In the second embodiment, the length of the writing of the main-scan valid interval signal is always set to the maximum value of the image size, and only the length of the reading of the main-scan valid interval signal is administrated. However, it is thought that the lengths of the writing and the reading of the main-scan valid interval signal are always set with respect to the maximum values of the image size of the main-scan valid interval signal. In this case, since the reading control and the writing control for the main-scan interval are always the same, the control is simplified as a whole. Moreover, if it is set to write “0” as the dummy data to the data outside the valid area in the image data to be written in the memory according to need, no unnecessary data is output to the printer, and the like. Thus, according to the present embodiment, although the accessing for reading or writing the data to the memory increases by the amount corresponding to the data outside the valid interval, it is unnecessary to administrate the length of the data in the main-scan direction, whereby the control can be simplified. 
   Third Embodiment 
   In the first and second embodiments, the data writing and the data reading to and from the memory are controlled by storing and administrating the length of the data in the main-scan direction or in the sub-scan direction. On one hand, memory writing and reading also can be controlled by beforehand setting an end address representing the end of the memory writing for each page. For example, in a case in which the data writing and the data reading of the main-scan interval to and from a memory is always set to the maximum values of the image size, the following control is performed. 
     FIG. 19  is a block diagram showing the internal structure of a drum delay unit  704  according to the third embodiment. In  FIG. 19 , numeral  1901  denotes a sync control unit, which does not include a main-scan valid interval holding unit, and a sub-scan valid interval holding unit, as in the first embodiment. Numeral  1902  denotes a data conversion unit, and numeral  1903  denotes an address generation unit, which includes an address end value holding unit  1903   a  for storing a last memory address of each page in the data writing. Here, it should be noted that the address end value holding unit  1903   a  can store the memory addresses of three pages, at a maximum. Numeral  1904  denotes a RAM control unit, and numeral  1905  denotes an SDRAM acting as a delay memory. 
   As compared with  FIG. 12 , a page end signal page_end is newly added in the structure shown in  FIG. 19 . On the other hand, other sync signals are the same as those in  FIG. 12 . Incidentally, it should be noted that a sub-scan valid interval signal venb_rd for data reading becomes inactive when the page end signal page_end is output. 
   Four-bit image data data_in is input from a spatial filter  703  to the data conversion unit  1902 , the input image data is temporarily stored as 16-bit data for every four pixels, the temporarily stored 16-bit image data is then transferred to the SDRAM  1905  at predetermined timing in response to a sync signal transferred from the sync control unit  1901 , and the transferred image data is written in the SDRAM  1905 . On the other hand, 16-bit image data read from the SDRAM  1905  is converted into four-bit image data of four pixels, and the obtained data is then output as data data_out. 
   The address generation unit  1903  generates a writing address and a reading address of the SDRAM  1905  in response to a control signal transferred from the sync control unit  1901 . When a sub-scan valid interval signal venb_wr for the data writing and a main-scan valid interval signal henb_wr for the data writing, both transferred from the sync control unit  1901 , are active, the writing address is updated for every sixteen cycles. The start value of the writing address of the first page is “0”, and then, the address when the sub-scan valid interval signal venb_wr for the data writing and the main-scan valid interval signal henb_wr for the data writing become inactive is held as the end address. Similarly, the end addresses of the second page and the third page are held. When the sub-scan valid interval signal venb_rd for the data reading and a main-scan valid interval signal henb_rd for the data reading are active, the reading address is updated for every sixteen cycles. Also, the start value of the reading address is “0”, the page_end signal page_end is output to the sync control unit  1901  when the value of the reading address becomes the address value held at a time of the data writing. 
   The respective addresses are converted into the address format suitable for the SDRAM  1905 , and then, output as an SDRAM address ram_ad and an SDRAM bank address ram_ba. Moreover, the address generation unit  1903  generates a signal mad_cs. 
   The RAM control unit  1904  generates a chip select signal ram_cs, an RAS signal ram_ras, a CAS signal ram_cas, and a writing signal ram_we, which are necessary for command control of data reading and writing of the SDRAM  1905 . Moreover, the RAM control unit  1904  generates a command control signal, which is necessary for a refresh operation of the SDRAM  1905 , in response to a main-scan valid interval signal henb_ref for refresh control of the SDRAM  1905 . 
     FIG. 20  is a diagram simply showing an address map of the image delay memory according to the third embodiment. To simplify the explanation, it is shown in  FIG. 20  that one address corresponds to one pixel. A length X of the image data in the main-scan direction is fixed to the maximum value of the image size capable of being input, whereby the length X is always constant, irrespective of the input data size. On the other hand, a length of the image data in the sub-scan direction is given as Ya for the first page and as Yb for the second page, and the sum of the lengths Ya and Yb is less than a drum interval d. 
   First, if the image data of the first page is input, the writing address of the memory is counted up from the value “0”, and then, the writing address is further counted up by one, every time the image data is input. At a time when all the data of the first page are written in the SDRAM  1905 , the writing address of the memory has the value (X×Ya−1), and this address value is stored in the address end value holding unit  1903   a  as the end address value of the first page. 
   Next, if the image data of the second page is input, the writing address of the memory is counted up from the value (X×Ya), and then, the writing address is further counted up in the same manner. At a time when all the data of the second page are written in the SDRAM  1905 , the writing address of the memory has the value (X×Ya+X×Yb−1), and this address value is stored as the end address value of the second page. 
   Then, similarly, if a sub-scan activation signal pvsync_in of the printer is input while the writing address of the memory is being counted up, the data of the first page is counted up one by one from the reading address “0” of the memory as the data is being read from the SDRAM  1905 . If the reading address of the memory is counted up until it reaches the end address (X×Ya−1) of the first page, the data reading of the first page ends. Then, if the next sub-scan activation signal pvsync_in of the printer is input, the memory reading operation is performed from the start value (X×Ya) of the reading address of the second page obtained by counting up the end address of the first page by one. Then, similarly, if the reading address of the memory is counted up until it reaches the value (X×Ya+X×Yb−1) of the end address of the second page, the data reading of the second page ends. Incidentally, when the reading address of the memory is counted up to the maximum value of the memory, the memory address is then returned to “0”, and the above operations are repeated hereafter. 
   As has been described above, the memory end address of each page is stored when the data writing is performed. Thus, even in the case when the length of the output image in its sub-scan direction is shorter than the drum interval, and the length of the output image in its sub-scan direction is different for each page, the recording operation can be continued without changing the memory control setting for each page. Therefore, it is unnecessary to wait until the memory becomes vacant and also, to prolong the sheet interval. 
   In each of the above embodiments, as the situation that the length of the image in the main-scan direction and the length of the image in the sub-scan direction change during the continuous recording operations, the following cases arise. 
   That is, there is the case in which, when the plural originals are subjected to the copying by using the original feeder, the size is detected for each original when the original is actually fed, and the recording sheet is selected based on the detected size. 
   Further, there is the case wherein, in the state that the printer is connected to the network or the computer, the data of the different sheet sizes exist in one print job, or the sheet sizes are different between successive two print jobs. 
   Furthermore, there is the case in which interruption printing of the different sheet sizes is performed to the running copying process or the running printing process. 
   In addition to these examples, the same situation occurs in a case in which a FAX-received image is printed. 
   Therefore, the function to detect the size of each of the fed originals and to automatically select the sheet on the basis of the detected size and the interruption printing function are provided in the MFP  104  of  FIG. 1 . 
   While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
   For example, in the above embodiments, the lengths of the main-scan valid interval signals of the three pages and the lengths of the sub-scan valid interval signals of the three pages are respectively administrated. However, it is possible to increase the number of pages to more than the three pages. In this case, even if the drum interval is three times the length of the output image in the sub-scan direction, the printer can be operated without any trouble. That is, it is possible to cope with a large-scale apparatus, in which the drum interval is wider, or with a case in which the sheet to be output is made smaller. 
   Moreover, in the above embodiments, the control to be performed to the delay memory on the basis of the length of the main-scan valid interval and the length of the sub-scan valid interval both administrated for each page in the drum delay unit is performed completely, according to the hardware control. However, at least a part of the above control may be performed according to software control based on a CPU.