Document scanners

Abstract: An asynchronous document scanner (10) with a one-dimensional scanning array (35), a movable scan carriage (26) supporting the array for scanning movement, a variable interpolator (70) for interpolating partial lines of image signals from the array with preceding lines of image signals to provide composite image signal lines, and a control (44) responsive to the timing of the start integration signal to the array to vary the interpolation coefficient of the interpolator.

Description: The invention relates to document scanners, and more particularly, to combined asynchronous-synchronous document scanners. When scanning documents, the document being scanned is typically moved in one direction, called the slow-scan direction, by a document transport mechanism past a one-dimensional scanning device which scans the image. A one dimensional imaging device of this type can be operated only "synchronously". Typically, the scanning device used is a charge-coupled device (CCD) oriented orthogonally to the direction of motion of the document. Present-day document scanners fall into one of two types, asynchronous or synchronous. In a synchronous document scanner, the image signals generated by the scanning device are removed at the rate at which the scanning device generates the image signals. That is, the downstream image signal processing circuitry must pass the image data along to the host at the same rate as the image data are generated, and the host must keep up with the rate. If the downstream image processing circuitry cannot receive image data at the rate at which they are generated, some image data are lost, since the one-dimensional scanning device cannot speed up or slow down. If a document scanner supplies image data to the host at a rate determined by the host, this type of scanner is said to be asynchronous. Typically, there are two types of asynchronous operation. In one, a very large first-in first-out (FIFO) buffer is inserted in the data path to allow the one-dimensional scanning device to be run synchronously. However, this method can entail high cost since the size of the FIFO buffer required to store the image data, particularly when the host slows down or stops, is large. In the other type of asynchronous operation, integration is started 'on demand' when data are required by the host. The single line of image data generated can then be stored in a small FIFO buffer to provide byte asynchronous operation with the host. However, the fastest possible rate in this type of operation is only half that of a synchronous scanner, since the one-dimensional scanning device must first integrate before the image signals can be shifted out of the scanning device to the downstream signal processing circuitry and the host. Since the shift-out time is equal to the integration time, it necessarily takes at least twice as long to obtain a line of image data in an asynchronous document scanner. It would be desirable if the advantages of both scanner types, that is, the high speed of the synchronous scanner and the minimal memory of the asynchronous scanner, could be combined into one scanner. Referring to the prior art, US-A-4,591,727 discloses a solid-state scanner with variable speed transport. The integration period of the detector array is varied by varying the clock rate at which the array is driven to accommodate of the speed of the document transport while correcting the signals produced by multiplying the signal output by a correction factor which is chosen based on the frequency of the clock signals. US-A-4,511,989 discloses a multi-dimensional interpolation system for a color scanner, while US-A-4,712,141 discloses an interpolation method in which pairs of pixels are generated using an arithmetic circuit to interpolate, and US-A-4,680,720 discloses a system for effecting dot interpolation from pattern data stored in memory. Further, US-A-4,644,409 discloses a system employing dual scanning devices for input and output, with selectable scanning resolutions for determining the ratio of output resolution to input resolution, while US-A-4,651,223 discloses a recorder incorporating magnification changing means allowing the number of pixels output to the print unit for printing to be different from the number of pixels input. EP-A-295,875 discloses a method and apparatus for minimizing residual charges in the shift registers of a CCD type scanning array in an asynchronously operated image input terminal. The unknown arrival time of the next integration signal is predicted and based on that prediction, the last periodic dumping pulse before the integration signal is delayed a required period of time to assure clearing of residual charges from the shift registers. In contrast to the prior art, the present invention provides a document scanner operative either asynchronously or in a combined asynchronous and synchronous manner to scan image-bearing documents and provide lines of image data to a work station, comprising: a one-dimensional array for scanning image-bearing documents line-by-line to provide image signals representative of the image content of each image line scanned, the array including a plurality of image detectors for generating image signals in response to integration signals, and shift register means for receiving the image signals from the detectors, the shift register means outputting the image signals in serial form to the workstation, the integration signals being input at random times when the scanner is operating asynchronously, resulting in at least a part of the line of image signals produced by the array being received by the shift register means prior to output of the last image signal from the preceding line of image signals from the shift register means; and control means; characterised by further including variable interpolator means for interpolating the line of image signals with the preceding line of image signals to provide a composite line of image signals for the workstation the interpolator means including means for subtracting from an image signal associated with a pixel in a scan line the value of an image signal associated with a pixel in a previous scan line, and in that the control means is adapted for establishing the interpolation coefficient for the variable interpolator means in response to timing of the integration signal, the interpolation coefficient being used to determine the value to be subtracted from an image signal in the interpolation means. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic view of the asynchronous or combination asynchronous-synchronous document scanner of the present invention; Figure 2 is a schematic block view showing the principal operating components of the scanner shown in Figure 1; Figure 3 is a schematic view showing details of the variable interpolator shown in Figure 2; Figure 4 is a graphical representation depicting the scanner operating in the full synchronous mode; Figure 5 is a graphical representation depicting the scanner operating in the asynchronous mode; Figure 6 is a graphical representation depicting the scanner operating in the fastest asynchronous mode; Figure 7 is a graphical representation depicting the partial synchronous mode at various exemplary speeds ranging from asynchronous mode to full synchronous mode, and Figure 8 is a graph depicting the relation between scan carriage speed and scanner operating modes. Referring to Figures 1-3, there is shown an exemplary document scanner 10, of the type adapted for asynchronous or combined asynchronous-synchronous operation in accordance with the present invention. Scanner 10 includes a housing 12 with base 13, sides 15, and ends 16. The top 18 of housing 12 incorporates a generally rectangular transparent platen 20. Platen 20 is formed from a relatively larger piece of transparent material, typically glass, having an opaque border delineating a generally rectangular viewing window sized to accommodate the largest document original 22 to be scanned. As will be understood, a document 22 to be scanned is located either manually or by a suitable automatic document handler or feeder (not shown) on platen 20 for scanning. A scan carriage 26 is movably supported within housing 12 on base 13 for reciprocal movement in the slow-scan direction underneath platen 20 on one or more longitudinally-extending rails 29. Carriage driving means in the form of a drive screw 30 threadedly engaged with carriage 26 is provided, rotation of screw 30 in either sense by a reversible carriage driving step motor 32 serving to move carriage 26 in either a scan or pre-scan direction, as shown by the dotted line and solid line arrows in Figure 1 respectively. A one-dimensional linear scanning array 35, such as a CCD, is suitably mounted on scan carriage 26 in predetermined operative relation with platen 20 so as to scan the document resting thereon. Array 35, which is disposed so that the scan axis (designated the fast-scan direction) is substantially perpendicular to the direction of movement of carriage 26 on rails 29, has a linear row of photosensitive detectors 33 flanked by parallel-in, serial-out shift registers 34. A suitable optical system is provided for transmitting light rays from the document line being scanned to array 35, the optical system being exemplified by lens 37, mirrors 38, and 1amp 40. Lens 37 and mirrors 38 cooperate to focus array 35 on a line-like area extending across the width of platen 20 and perpendicular to the direction of movement of carriage 26, while light from lamp 40 illuminates the line-like area on which array 35 is focused. A suitable clock 41 provides clock signals for operating the system including driving array 35 through timed scan cycles. Clock signals from pixel clock 41 additionally drive the pixel counter 72 of a variable interpolator 70 which counts the number of pixels output in each image line. The output of pixel counter 72 is fed to latch 74. Each scan cycle consists of an integration period during which the photosensitive detectors 33 of array 35 obtain signals reflecting the image density of the line viewed by the array; a transfer cycle during which the image signals are transferred in parallel from detector 33 to shift registers 34, and a shift cycle during which the image signals in shift registers 34 are shifted serially out of the shift registers to a variable interpolator 70. Each time scan carriage 26 moves to the next image line in response to a demand for image data by workstation 48, controller 44 outputs an integration signal (INT) to clock 41 to start a scan cycle. Where the current scan cycle is out of synchronism with the integration signal (INT), the integration signal effectively resets the timing of the scan cycle. The integration signal (INT) is also input to position counter 76 of interpolator 70 to actuate latch 74 and latch the count on pixel counter 72 at the instant the integration signal is received. In the embodiment shown and described, scanner 10 operates in a combined asynchronous/synchronous manner where the scanning speed of scan carriage 26 varies from zero at startup to a maximum as scan carriage 26 accelerates at start-of-scan from rest up to the maximum scanning speed of which scanner 10 is capable. In the acceleration period, as well as at maximum scanning speed, scanner 10 scans the document original on platen 20 and provides image data to workstation 48 on demand each time array 35 is moved one line. As will be understood, scanner 10 may operate both asynchronously at a variable scanning rate during acceleration, and synchronously at a constant scanning rate in response to the demand for image data by workstation 48. The determination of whether scanner 10 is in the asynchronous or synchronous mode is dependent on the scan speed of carriage 26. Referring now to Figure 4, when scanner 10 is operating in the synchronous mode at the scanner maximum scanning speed, the rate at which the image data generated must be removed is equal to the rate at which array 35 generates the image data. In this case, the scan cycles, once established, are and remain synchronous. When scanner 10 operates in the asynchronous mode, as shown in Figures 5 and 6, the rate at which image data are generated normally varies with the speed at which scan carriage 26 is currently moving. As will be understood, the speed of scan carriage 26 varies from zero at the instant scanning starts, up to the maximum scanning speed that asynchronous operation allows. As a result, the rate at which the image data are output will vary, and the scan cycles may not be synchronous with one another. It will be understood that where the next scan cycle is commenced before the preceding scan cycle is completed, the image signals from the preceding line are not completely shifted out of the array shift registers. As a result, image signals from the new line are intermixed with leftover signals from the preceding line, leading to image signal distortion in the overlapping areas. As can be seen in Figures 7 and 8, where scan carriage 26 has reached the maximum scan speed possible in the asynchronous mode, i.e., a speed slightly less than one-half the speed required for full synchronous operation, there remains an interval between maximum asynchronous and full synchronous modes where the scan carriage is moving at a speed too fast for asynchronous scanning and too slow for full synchronous scanning. In this interval, the scanner 10 operates in a partial synchronous mode as the scan carriage accelerates up to the maximum scanning speed required for synchronous operation. During the partial synchronous mode, a varying degree of distortion of the image data can result since there is insufficient time to clear the array shift registers fully before the next integration signal (INT) arrives. As a result, image signals from the partial integration period that occur are combined with the remaining uncleared image signals from the preceding integration period. It would be advantageous if both asynchronous and synchronous operation could be combined without distortion. However, this is not possible because of the inability of scan carriage 26 to change speed instantaneously. To overcome possible image signal distortion, variable interpolator 70 is provided. Interpolator 70 employs a variable interpolation coefficient that is established at the instant interpolation begins, the particular interpolation coefficient used being a function of the point in the current scan cycle where the current scan cycle is interrupted and the new scan cycle is started in response to an integration signal (INT). Referring particularly to Figure 3, the count output M on latch 74 of interpolator 70 is fed to a suitable divide-by-N circuit 78, where the count is divided by a count N representing the total number of pixels in the scan line to provide the specific interpolation coefficient M/N for the line of image signals being output. The new interpolation coefficient M/N is fed to a subtracting circuit 80 where the coefficient M/N is subtracted from unity to provide an inverse interpolation coefficient (1-M/N). The image signals (P′ N ) from the preceding line are held in scan line delay buffer 88. The signals from the preceding line output from buffer 88 are multiplied by interpolation coefficient M/N in multiplier 82 and the result fed to one inlet of differencing circuit 84. The corresponding pixel (P N ) in the line of image signals currently output by array 33 is multiplied by the inverse interpolation coefficient (1-M/N) by multiplier 86, and the result fed to a second inlet of differencing circuit 84. Circuit 84 subtracts the two interpolated values from one another to provide the image signal representation for the pixel being processed. Referring to Figure 2, the image signals from array 35 are input to digitizer 42 where they are converted to digital format. From digitizer 42, the image signals are output to interpolator 70, and from interpolator 70 to line buffer 45 where the processed image signals are held pending output to workstation 42. A suitable carriage position sensor 51 is provided to track movement of carriage 26. On movement of carriage 26 with array 35 through a distance equal to one scan line, sensor 51 outputs a signal to controller 44, which responds by sending the integration signal (INT) to pixel clock 41, in effect requesting output of the next valid line of image signals. A bi-directional scan counter 52, which is preset to a count equal to the total number of scan lines required to scan the operating length of platen 20, is provided. Counter 52 is indexed by one each time carriage 26 moves one scan line as sensed by position sensor 51. Control signals output by counter 52 are fed to controller 44 to regulate the start-of-scan, end-of-scan, and the direction of scanning movement of scan carriage 26. While scanner 10 has been shown and described as operating in a combined asynchronous/synchronous manner, it will be understood that scanner 10 may be operated solely in an asynchronous manner.

Claims: A document scanner (10) operative either asynchronously or in a combined asynchronous and synchronous manner to scan image-bearing documents and provide lines of image data to a work station, comprising: a one-dimensional array (35) for scanning image-bearing documents line-by-line to provide image signals representative of the image content of each image line scanned, the array including a plurality of image detectors (33) for generating image signals in response to integration signals, and shift register means (34) for receiving the image signals from the detectors, the shift register means outputting the image signals in serial form to the workstation, the integration signals being input at random times when the scanner is operating asynchronously, resulting in at least a part of the line of image signals produced by the array being received by the shift register means prior to output of the last image signal from the preceding line of image signals from the shift register means; and control means (44); characterised by further including variable interpolator means (70) for interpolating the line of image signals with the preceding line of image signals to provide a composite line of image signals for the workstation the interpolator means including means (84) for subtracting from an image signal associated with a pixel in a scan line the value of an image signal associated with a pixel in a previous scan line, and in that the control means (44) is adapted for establishing the interpolation coefficient for the variable interpolator means in response to timing of the integration signal, the interpolation coefficient being used to determine the value to be subtracted from an image signal in the interpolation means.

Inventors: BEIKIRCH THOMAS R, BEIKIRCH, THOMAS R.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1990
Main classification: H04N1
IPC codes: H04N1