Minimizing residual charge carryover in an asynchronous raster input scanner

Abstract: A method and apparatus for minimizing residual charges in the shift registers of a CCD type scanning array (35) in an asynchronously operated image input terminal (10). 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.

Description: The invention relates to a raster input scanner, and more particularly, to minimizing residual charge carryover in an asynchronous scanner. Raster input scanners typically employ one or more arrays such as CCD's for scanning. The array converts each scanned image line into a series of charges which, following suitable processing, are output as image signals or pixels to an end user. Scanners of this type often use a transparent platen on which the image bearing document is placed face down. The scanning array for example may be carried on a carriage which traverses back and forth under the platen to provide the necessary relative motion between image and array. Other arrangements such as a fixed scanning array with movable document, etc., may be contemplated instead. An optical system focuses the array onto the image, and one or more lamps are provided to illuminate the image. In the typical scanning process, the image area viewed by each of the array photosensors is converted into a charge potential representative of the image gray level. Scanning takes place during an integration period of preset duration. Following integration, the image charges are transferred to a pair of analog shift registers, the operating sequence being such that during the integration period, the image charges from the previously scanned line are clocked from the shift registers, leaving the shift registers free to receive the image charges from the next integration period. The duration of the integration period, however, must be limited if undesirable saturation of the array photosensors is to be avoided. As a result, the integration/transfer cycle must occur at a rate high enough to preclude over long integration periods with resulting saturation of the array photosensors. To accommodate this situation, where the rate of image signal demand is insufficient to prevent saturation, a periodic clock driven shift signal is imposed. As a result, where there is no immediate demand for image signals by the user, integration nevertheless takes place periodically at a rate sufficient to prevent saturation. Since there is no demand for image signals, the image charges produced are discarded or dumped. Where the scanner operates in a synchronous manner, relative scanning movement between array and image is at a fixed rate. This in turn assures a fixed timing relation between the shift signals and the integration signals issued in response to a demand for image signals, and allows a timing relationship to be established that will assure clearing of image charges from the shift registers before new charges are introduced. However, where the scanner operates asynchronously, the relative scanning movement between array and image is not fixed but can change with demand. As a result, timing of the integration signals can change and thus fall out of synchronism with the periodic shift signals that are designed to protect the photosensors of the array from becoming saturated. When this occurs, the integration period may be too abbreviated to allow the shift registers to be fully and completely cleared of all image charges from the previous line. As a result, new charges combine with leftover charges from the previous line, resulting in a distorted and inaccurate image signal output. In the prior art, US-A-4,541,061 discloses a system where, in a scanning apparatus, the operating clock signals are provided at a rate that matches the varying velocity of the scanning mirror through the use of a memory in which various clock rates are stored based on the known repetitive movement of the mirror, there being addressing means for addressing the memory locations. In US-A-4,587,415, a photodetector with timing control for controlling the information storage and information readout process of the detector is disclosed. In US-A-4,628,368, there is disclosed a system for controlling the scanning rate of a document reader in which the speed, acceleration, and deacceleration of the reader are set according to image information stored in a buffer memory. In US-A-4,367,493, there is disclosed a raster scanner in which lines of pixels output by the scanning array are stored in a buffer pending user demand, and operation of the scanner in response to buffer pixel demands is effected through starting and stopping of a document transport which is provided to convey documents past the scanning array. Provision is made for preventing uneven spacing of scan lines as a result of scanning documents which have been stopped or are being accelerated/decelerated. In contrast, the present invention, in accordance with claim 1, provides a process for minimizing residual charges in a scanning array of the type having at least one row of sensors for scanning at a variable relative speed an image viewed by the array during an integration period of predetermined duration, and at least one shift register for receiving the image charges developed by the sensors following the integration period, in which it is required that the shift register be cleared of residual charges during each integration period to assure an accurate and undistorted image signal output, comprising the steps of: periodically generating shift signals defining an integration period of predetermined duration sufficient to preclude saturation of the array sensors yet sufficient to allow time for the shift register to be cleared of residual charges from the preceding integration cycle; generating integration signals in response to a demand for image signals; at each integration signal, calculating the arrival time of the next integration signal based on the said relative scanning speed; and where the period until the calculated arrival time of the next integration signal is less than the integration period defined by the periodic shift signals, delaying the last one of the shift signals before the next integration signal to coincide with the said next integration signal thereby extending the predetermined integration period provided by the shift signals and assure clearing of residual charges from the shift register. The present invention also provides a raster input scanner in accordance with claim 4. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic view of a raster input scanner of the type adapted for use with the screening circuit of the present invention; Figure 2 is a schematic block view showing the principal scanner operating elements as related to the residual charge minimization system of the present invention; Figure 3 is a schematic view showing details of the scanning array; Figure 4 is a timing chart depicting the timing relationship between the shift signals and the shift register clocking pulses; Figure 5 is a view showing the general timing interrelationship between image line integration and image line discharge; Figure 6 is a schematic view illustrating the effect of mis-alignment between the shift signals and an integration signal in an asynchronously operated scanner with resultant leftover image charges in the array shift registers; Figure 7 is a view illustrating the effect of the residual charge minimization system of the present invention in preventing leftover image charges in the array shift registers of an asynchronously operated scanner; Figure 8 is a view depicting the effect of a change in scan carriage speed between shift signals; and Figure 9 is a flow chart depicting the process by which the arrival time of the next integration signal is calculated. Referring to Figure 1, there is shown an exemplary raster input scanner, designated generally by the numeral 10, of the type with which the residual charge minimization system of the present invention is intended to be used. 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, typically glass, 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 reciprocable movement 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 a clockwise or counter-clockwise direction by a reversible carriage driving step motor 32 serving to move carriage 26 in either a forward or reverse direction as shown by the arrows in drawing Figure 1. A 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. Suitable optical means, exemplified here by lens 37 and mirrors 38, are provided 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. A lamp 40 is provided for illuminating the line-like area on which array 35 is focused. A suitable clock 45 (shown in Figure 2) provides clock signals including clock pulses Φ SH and convert pulses Φ₁, Φ₂ to array 35. The image signals generated by scanning array 35 are converted to digital form by an analog-to-digital(A/D) converter 47 (shown in Figure 3). While a single scanning array 35 is shown and described, plural arrays may instead be envisioned. In the ensuing description, the integration signal (INT) refers to a request for the next line of image signals while the periodic integration that occurs in response to clock pulses Φ SH is referred to as free run integration(FREE RUN INT). Scanner 10 operates at a variable scanning rate in response to the demand for image signals by the user or workstation 48 which may for example comprise a publishing workstation, personal computer, printer, etc. Referring to Figure 2, image signals from array 35, after suitable processing including conversion to digital format, are output to the user through a relatively small First in-First out (FIFO) buffer 50. A buffer capacity sensor 51 continuously monitors the residual capacity of buffer 50, the signal from buffer capacity sensor 51 being input to the scanner controller 53. Controller 53 controls carriage motor 32 through a suitable motor controller 56 to regulate the direction in which scan carriage 26 moves and carriage scanning speed, the latter in order to control the residual storage capacity of buffer 50. As a result, the fuller buffer 50 is, the slower the scanning speed, and vice versa. A suitable carriage position sensor 58 tracks movement of carriage 26, controller 53, on movement of carriage 26 with array 35 through a distance equal to one scan line, outputting an integration signal (INT). As will appear, at each integration signal (INT), there is output a good scan line of image signals to buffer 50. A variable rate scanning system of the type referred to above is shown and disclosed in US Patent No. 4 748 514 (application /s. N. 945,156, filed on December 22, 1986 in the name of Gregory E. Bell). Referring to Figures 3 and 4, shift pulses Φ SH are input periodically at a fixed clock rate to the array 35. The time between successive shift pulses Φ SH determines the length of time (the integration period) during which charges are allowed to accumulate on the photosites 62 of array 35, the charge potentials on the individual photosites being a function of the grey level of the area of the image line viewed by each photosite. At the shift pulse Φ SH ending the integration period, shift gates 60, 61 transfer the image signal charges accumulated by the photosites 62 to an analog shift register pair 63, 64 respectively, charges from the odd numbered photosites (i.e., 1, 3, 5, ...n-1) being transferred to successive stages 65 of shift register 63 while the charges from the even numbered photosites (i.e., 2, 4, 6, ...n) are transferred to successive stages 65 of shift register 64. As depicted in Figure 5, while each image line (i.e., line N + 2) is being scanned and integrated, charges transferred to shift registers 63, 64 from the previous line (i.e. line N + 1) are clocked out in alternating fashion by clock pulses Φ₁, Φ₂ to a multiplexer 67 (MULT) which provides a stream of image signal charges to A/D converter 47. Clearing of shift registers 63, 64 is timed to occur during the integration period so that as one line is being integrated, shift registers 63, 64 are being emptied of the image signal charges from the previous line. As will be understood, to obtain accurate and valid image signals representative of the true image area viewed by each of the photosites 62 of array 35, all charges from the preceding scan line must be cleared from the shift registers 63, 64. Where the scanner operates in a synchronous manner, timing of the integration signal (INT) pulses would normally be fixed, and accordingly, clearing of charges from the shift register stages 65 can be assured by properly synchronizing the clock pulses Φ SH Φ₁, Φ₂ with the integration signals (INT). Photosites 62 of array 35 can become saturated if not cleared of image signal charges within a predetermined time interval or window. A typical saturation interval for an array such as array 35 is 6 ms. In order to prevent saturation of the photosites 62, shift pulses Φ SH (which define the FREE RUN INT periods) are normally generated at a rate greater than that of the saturation interval. A typical rate for the shift pulses Φ SH is 2 ms. The digital image signals output from A/D converter 47 are further processed as by shading circuit 70 (shown in Figure 2). Shading circuit 70 responds to the integration signal (INT) to transfer the line of image signals received from array 35 through any subsequent processing circuits (not shown) to buffer 50 and work station 48. In the absence of an integration signal(INT), the image signals received from array 35, which is continuously scanning, are discarded or dumped. For example, where the residual capacity of buffer 50 is reduced below a preset minimum, the scan carriage ceases to move and hence integration signals (INT) are not generated. Accordingly, the image signals received from the array are discarded. Referring now to Figure 6, since scanner 10 operates asynchronously, the speed at which carriage 26 moves (and hence the rate at which integration signals are generated) can change in response to user demand as reflected by the residual capacity of buffer 50. Thus, irregular intervals between integration periods are the norm during acceleration, deacceleration, and even steady state scanning speed of scan carriage 26. As a result, the integration period will almost never be an integral multiple of the time between good scan lines in an asynchronous scanner. In cases where the integration signal (INT) does not coincide with the integration periods (FREE RUN INT) triggered by clock pulses Φ SH , the integration signal (INT) occurs at some point before the current integration period between successive clock pulses Φ SH has expired. In that event, a partial integration (PART INT) takes place which is too short to complete the clearing of all stages of the shift registers 63, 64 of image signal charges from the previous line. Where this occurs, there results inaccurate and distorted image signals caused by fresh image signal charges being introduced into shift register stages where image signal charges from the previous line are still present. Referring to Figures 7-9, in order to assure that shift registers 63, 64 are cleared of all image signal charges from the previous scan before transfer of the image signal charges from the next line, the arrival time of each integration signal (INT) is predicted. Based on the projected arrival time of the next integration signal (INT), the clock pulse Φ SH is held (HOLD DELAY) for an interval sufficient to delay transfer of the image signal charges to shift registers 63, 64 to coincide with the integration signal (INT). Thus, where the next integration signal (INT) is predicted to occur in less than that the free run integration period (i.e., 2 ms.) from the current Φ SH pulse, the hold delay assures that the prior charges are transferred out of the shift registers 63, 64 before the next integration signal (INT) is received and the new integration interval started. At the interrupt triggered by each integration signal (INT), the predicted arrival time of the next integrate signal (INT) is calculated in accordance with the following relationship: ( A ) EST.TIME = SCAN LINE LENGTH CURRENT VELOCITY where EST. TIME is the predicted time for carriage 26 to move to the next line; SCAN LINE LENGTH is the distance between successive scan lines which is normally fixed (for example, 1,200/in.); and VELOCITY is the velocity or speed at which carriage 26 moves. Using the EST. TIME, a delay count (HOLD DELAY), which is an integer number equal to the number of integrations by clock pulses Φ SH that will occur within the period EST. TIME, is obtained from the following relationship: ( B ) HOLD DELAY = EST.TIME SCAN TIME where SCAN TIME is time required for array 35 to scan a line and is determined from the following relationship: ( C ) SCAN TIME = N x 2 f where N is the number of photosites 38 in the array; and f is the array operating frequency. Additionally, a snapshot at the time of the current of integration signal (INT) is saved. Distance and total time (TTL) counts, which are used for tracking the distance moved and the total time from the start of the integration signal (INT) respectively, are set to zero and a FREE RUN INT count, which counts the number of integrations resulting from clock pulses Φ SH , is set to zero. The previous hold line (HOLD) is cleared. As scanning proceeds, the FREE RUN INT count is incremented by 1 on each occurrence of a clock pulse Φ SH while the HOLD DELAY count is decremented by 1. Where the HOLD DELAY counted equals 0, the hold line (HOLD) is set to delay (i.e. hold) the shift pulse Φ SH until the integration signal (INT) is received. In the asynchronously operated scanner 10, the speed of scan carriage 26 is updated (UPDATE) periodically (i.e., every 8.333 msecs.) to reflect any charge in the velocity of scan carriage 26. Thus, where the residual capacity of buffer 50 increases, the speed of scanning carriage 26 is increased in an attempt to return the residual buffer capacity back to the desired level, and vice versa. Accordingly, when predicting the next integration signal (INT), it is necessary to see if there has been a change in the velocity of carriage 26 as a result of the update signal (UPDATE). If no change in velocity has occurred, the current velocity (OLD SPEED) of carriage 26 is used in the calculations. Where a change in velocity of carriage 26, which can be either an increase or decrease in carriage speed, has occurred, the velocity change (NEW SPEED) is factored into the calculations. For this, the updated speed (NEW SPEED) is compared with the previous speed (OLD SPEED). If the updated speed (NEW SPEED) is equal to the previous speed (OLD SPEED), the time at speed is equal to the length of the clock interval between update speed pulses (i.e., 8.333 msec.) Next a check is made of the current state of the INT HOLD delay count. If the count is 1, then return is made. If not, the update speed time is saved for use for predicting the next integration signal (INT). Referring particularly to Figure 8, if the updated speed (NEW SPEED) is not equal to the previous speed (OLD SPEED), the distance moved by carriage 26 at the old speed (DIST MOVED) is determined using the following relationship: (D) Dist @ Speed = (OLD SPEED)(time @ speed) Total Time (TTL) at the previous speed (OLD SPEED) is next determined from the following relationship: ( E ) TOTAL TIME ( TTL ) + = Dist @ Speed OLD SPEED The time at the updated speed (NEW SPEED) is found by subtracting the fixed distance between scan lines (i.e., 1200) from the distance moved at the old speed (DIST. MOVED) and dividing by the new speed as per the following: ( F ) Time @ New Speed ( T′ ) = 1200 - Distance @ OLD SPEED NEW SPEED The sum of TTL and T′ gives the EST TIME. Using equation B, the HOLD DELAY is calculated, i.e.: HOLD DELAY = EST.TIME SCAN TIME = TTL + T′ SCAN TIME While an exemplary scanner is shown and described, other suitable scanner designs and types for effecting relative scanning movement between the image bearing document and the scanning array may be contemplated such as for example a scanner of the type where the image document is moved past a scan slit by a document transport for scanning by a stationary array. Further, the invention is not limited to the particular array type described but instead contemplates any array type in which it is desirable to minimize residual charges in the array shift register or shift registers due to asynchronous operation.

Claims: A process for minimizing residual charges in a scanning array of the type having at least one row of sensors for scanning at a variable relative speed an image viewed by the array during an integration period of predetermined duration, and at least one shift register for receiving the image charges developed by the sensors following the integration period, in which it is required that the shift register be cleared of residual charges during each integration period to assure an accurate and undistorted image signal output, comprising the steps of: a) periodically generating shift signals (Φ SH ) defining an integration period of predetermined duration sufficient to preclude saturation of said array sensors yet sufficient to allow time for said shift register to be cleared of residual charges from the preceding integration cycle; b) generating integration signals (INT) in response to a demand for image signals for respectively initiating said integration periods, said demand starting the scanning of said image and said integration signal generation at a rate which corresponds to said variable relative speed; c) at each integration signal, calculating the arrival time of the next integration signal based on the said relative scanning speed; and d) where the period until the calculated arrival time of said next integration signal is less than the integration period defined by said periodic shift signals, delaying the last one of said shift signals before said next integration signal to coincide with the said next integration signal thereby extending the predetermined integration period provided by said shift signals and assure clearing of residual charges from said shift register. A process according to claim 1, including the steps of: moving one of said array and image relative to the other during scanning; and calculating the arrival time of said next integration signal from the rate at which said array or image is moved. A process according to claim 2, including the steps of: periodically updating the rate at which said array or image is moved to accommodate changes in image signal demand; and where the rate at which said array or image is moved changes, recalculating the arrival time of said next integration signal. A raster input scanner, comprising: a) an array (35) having at least one row of photosensors for scanning an image line by line at a variable relative speed, and at least one shift register (63,64) to which image charges from said photosensors are transferred, said photosensors constantly integrating the instantaneous image viewed by said photosensors; b) clock means (45) for driving said array, said clock means providing periodically timed shift pulses defining an integration period during which said sensors integrate the image viewed and provide charges representative thereof to said shift register, said clock means also providing periodic shift register drive pulses for clocking charges from the previous integration period from said shift register to clear said shift register to receive charges from the next integration period; c) means (58) for generating integration pulses in response to a demand for image signals for respectively initiating said integration periods, said demand starting the scanning of said image and said integration signal generation at a rate which corresponds to said variable relative speed; d) means operable at each integration pulse to calculate the arrival time of said next integration pulse, based on the said relative scanning speed; and e) means operable, when the period until the calculated arrival time of said next integration pulse is less than said integration period to delay the last shift pulse before said next integration pulse whereby to synchronize said last shift pulse with the calculated arrival of said next integration pulse. A scanner according to claim 4, including a) a movable carriage (26) for supporting said array for scanning movement; b) means (30) for driving said carriage; c) control means (53) for controlling said driving means and varying said carriage speed in response to a demand for image signals; said calculating means being effective on a change in carriage speed to recalculate the arrival time of said next integration pulse

Inventors: MICHAELS THOMAS B, MICHAELS, THOMAS B.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1988
Main classification: H04N1
IPC codes: H01L27, H04N5, H04N1