Sensing module for accelerating signal readout from image sensors

The present invention has been made in consideration of accommodating a higher sensor clock signal to increase the pixel readout rate from a regular image sensor and has particular applications to generating high-resolution and high-speed images from scanning objects. The sensing module in the present invention uses a number of readout passages in parallel to produce several segmented outputs from the image sensor and subsequently combine the outputs to produce an interleaved scanning signal under a sequence of control signals derived from a sensor clock signal.

FIELD OF INVENTION 
The present invention relates to monochrome and color scanning systems and 
more particularly relates to a scanning mechanism for producing multiple 
outputs in parallel from corresponding multiple sensing segments and 
combining the multiple outputs thereafter to increase signal readout rate 
from the scanning mechanism. 
DESCRIPTION OF THE RELATED ART 
There are many applications that need optical scanners to convert 
paper-based objects, such as texts and graphics, to an electronic format 
that can be subsequently analyzed, distributed and archived. One of the 
most popular optical scanners is flatbed scanners that convert objects, 
including pictures and papers, to images that can be used, for example, 
for building Web pages and optical character recognition. Another emerging 
optical scanner is what is called sheet-fed scanners that are small and 
unobtrusive enough to sit between a keyboard and a computer monitor or 
integrated into a keyboard to provide a handy scanning means. Most optical 
scanners are referred to as image scanners as the output thereof is 
generally in digital image format. 
An image scanner generally includes a sensing module that converts scanning 
objects optically into electronic images. The sensing module comprises an 
illumination system, an optical system, an image sensor and an output 
circuit. The illumination system is used to illuminate an object that is 
being scanned. The optical system is used to direct and focus the light 
reflected from the scanning object onto the image sensor. The image sensor 
comprises a plurality of photodiodes or photocapacitors, referred to as 
photodetectors hereafter, that are sensitive to light and produce 
proportional pixel signals accordingly. Therefore corresponding pixel 
signals are produced in the image sensor when the reflected light is 
focused thereon and the output circuit is used to convert the pixel 
signals to an appropriate format to be processed or stored in subsequent 
systems. 
The image sensor is generally in the form of Complementary Metal-Oxide 
Semiconductor (CMOS) or charged couple device (CCD) and fabricated in 
either a one-dimensional array or two-dimensional array. The operation of 
the image sensor often comprises two processes, the first being the light 
integration process and the second being the readout process. In the light 
integration process, each photodetector captures the incident photons of 
the reflected light and records the total amount of these photons as a 
charge or pixel signal. After the light integration process the 
photodetector is masked so that no further photons are captured and 
meanwhile the photodetectors start the readout process during which the 
pixel signal stored in each photodetector is individually readout, via a 
readout passage, to a data bus or video bus. The readout passage is an 
intermediate process that transports the pixel signals to the data bus. To 
be more specific, in the case of CMOS, the readout passage is a switch 
array comprising a plurality of readout switches, each responsible for 
coupling one of the photodetectors to the data bus. The pixel signals in 
the photodetectors are readout, by turning on sequentially the readout 
switches, to the data bus. If there are N photodetectors in the image 
array, hence N readout switches in a readout passage, and it takes one 
clock cycle of a sensor clock signal to turn on one switch and read out 
one pixel signal onto the data bus, it will consequently take N clock 
cycles of the sensor clock signal to read out all of the N pixel signals. 
In the case of CCD, the readout passage is a shift register. The shift 
register comprises the same number of memory cells as the number of 
photodetectors in the image sensor, each memory cell holding one pixel 
signal from a respective photodetector. The pixel signals are dumped into 
the shift register coupled thereto in parallel. Then the pixel signals in 
the shift register are serially shifted out, one pixel at a time from one 
memory cell to another, from the register into the data bus. In other 
words, if there are N photodetectors in the image array and it takes one 
clock cycle to read out one pixel signal, it will then take N clock cycles 
of the sensor clock signal to read out all of the N pixels. In reality, N 
is generally a large number and the readout time is proportional to N. To 
increase the pixel signal readout, an often used approach is to increase 
the clock cycle of the sensor clock signal. 
Many flatbed and sheet-fed scanners use one-dimensional image sensor. This 
requires either the image sensor or the scanning object to move against 
each other so as to get the scanning object completely scanned. If the 
scanning object is a piece of paper having the standard size of 8.5 inch 
by 11 inch and the resultant image resolution is 300 dot-per-inch (dpi), N 
will be required to be at least 2550 or larger if the paper margins are 
considered. When the scanners are capable of reproducing color, the same 
scanning object has to be scanned multiple times, the readout time can be 
much prolonged. For example, a contact image sensor module SV351A4C from 
Scan Vision Inc. takes 1.5 msec to scan a line of 9 inch wide at 300 dpi 
for a gray image but 7.5 mesc to scan the same for a color image. If a 
scanning object has a long size, the time accumulated for a whole image 
thereof can be significant. Although the readout time may be reduced by 
increasing the clock cycle of the sensor clock signal, the readout speed 
is eventually limited by the internal mechanism of the readout passage. It 
is well understood in semiconductors that a large number of readout 
switches in parallel inherently form a capacitor with large capacitance, 
which significantly retards the charging speed in a following signal 
amplifier when the pixel signals are readout to be strengthened by the 
signal amplifier. Similarly a large number of memory cells in a shift 
register may cause the pixel signals to degrade in shifting from one 
memory cell to another. There is thus a great need for a sensing module 
that can accommodate a sensor clock signal having higher clock cycle to 
increase the readout rate without demanding for costly high speed image 
sensors. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above described 
problems and has particular applications to high-resolution scanners. The 
disclosed invention yields significant improvement in the pixel readout 
time when a high clock cycle signal is applied to produce a 
high-resolution image of scanning objects. Current marketed scanners begin 
to experience noticeable delay when the image resolution reaches a certain 
value in spite of high clock cycle signal applied due to the internal 
mechanism of the readout passage in the sensing module. The sensing module 
in the present invention uses a number of readout passages in parallel to 
concurrently produce several segmented outputs from the image sensor and 
subsequently combines the outputs to produce an interleaved scanning 
signal under a sequence of control signals derived from the sensor clock 
signal. The composition of the several parallel segmented outputs using 
the timing in the sensor clock signal is a radical shift from the 
traditional readouts in image sensing module and imposes no additional 
demand for an even higher sensor clock signal while the signal readout 
rate thereof is significant increased. 
The disclosed sensing module comprises an image sensor generating pixel 
signals, a number of readout passages to generate segmented scanning 
signals from the pixel signals, a multiplexer to combine the segmented 
scanning signals and a timing control circuit that generates a number of 
control signals derived from a sensor clock signal. The image sensor 
comprises a plurality of photodetectors and is preferably equally divided 
into virtual groups; pixel signals in each of the virtual groups are 
readout by one of the readout passages concurrently. According to one 
aspect of the present invention, the readout passages are switch arrays 
coupled to local video buses, each of the switch arrays having a plurality 
of readout switches and each of the readout switches couples respectively 
one photodetector to the respective local video bus. Controlled by the 
sequence of the control signals, the readout switches in each of the 
switch arrays are successively turned on, namely permitted for passing 
therethrough, to readout respective pixel signals in the photodetectors in 
one virtual group at the clock cycle rate of the control signals to the 
respective local video bus to produce one segmented scanning signal. All 
the segmented scanning signals are then multiplexed by the multiplexer to 
subsequently generate an interleaved scanning signal. 
According to another aspect of the present invention, the readout passages 
are shift registers. Each of the shift registers comprises a plurality of 
memory cells and the number of the memory cells in each of the shift 
registers is the same as the number of the photodetectors in each virtual 
group. The pixel signals in the photodetectors in each of the virtual 
groups are dumped respectively to the memory cells and then start to shift 
out, from one cell to another in each of the shift registers to 
concurrently produce a segmented scanning signal and thus there are the 
same number of the scanning signals as the number of the shift registers. 
The multiplexer receives each of the scanning signals from the shift 
registers and multiplexes, under the control of the timing control 
circuit, the scanning signals sequentially and produces a complete 
interleaved scanning signal. 
In addition, an order circuitry reorders the pixels in the interleaved 
scanning signal to a normal scanning signal. 
According to one embodiment of the present invention, the sensing module 
for accelerating signal readout rate comprises: 
an image sensor; 
a timing circuit, in response to a clock signal having a clock cycle T, 
producing a plurality of control signals; 
a number of readout passages, coupled to the image sensor and producing the 
same number of segmented scanning signals, each of the readout passages 
respectively and independently controlled by one of the control signals 
and producing one of the segmented scanning signals; 
a multiplexer having the number of inputs, each of the inputs respectively, 
receiving one of the segmented scanning signals and the multiplexer, in 
response to the clock signal, successively sampling the segmented scanning 
signals to produce an interleaved scanning signal. 
Accordingly, an important object of the present invention is to provide a 
generic solution for increasing the capability of a regular image sensor 
to accommodate a higher clock signal so as to increase the signal readout 
rate from the image sensor. 
Other objects, together with the forgoing are attained in the exercise of 
the invention in the following description and resulting in the embodiment 
illustrated in the accompanying drawings.

PREFERRED EMBODIMENT--DESCRIPTION 
Referring now to the drawings, in which like numerals refer to like parts 
throughout the several views. FIG. 1 shows a systemic diagram of a 
configuration in which the present invention may be practiced. Referenced 
by 100 is a process or scanner that coverts a paper-based scanning object 
102 to a corresponding image 104. The paper-based scanning object 102 may 
be a piece of paper containing black-andwhite or colorful printed 
information such as text, graphics, tables and etc. The image 104 
comprises a plurality of pixels, each pixel represented by a numerical 
value representing the intensity of the light reflectance falling on a 
sensor in the scanner 100 from a corresponding dot in the scanning object 
102. For example, the paper-based scanning object 102 is a 8.5 inch by 11 
inch paper, the resultant image 104 has a size of 850 by 1100 pixels and 
is in 8-bit format, that means each inch square of the scanning object 102 
is represented by 100 by 100 pixels. If all the pixels in the inch square 
are 255, the corresponding inch square in the scanning object 102 is white 
and oppositely if all the pixels in the inch square are 0, the 
corresponding inch square in the scanning object 102 is dark. It can be 
understood that any pixels having a value between 0 and 255, i.e. the gray 
scale, represent the variations of contents in the scanning object 102. 
When the scanner 100 is capable of reproducing colors, the image 104 
comprises 3 individual gray scale images, each generally representing red, 
green and blue intensity. In other words, each dot in the scanning object 
102 is represented by a 3-intensity-value matrix, such as [23, 45, 129]. 
The scanner 100 comprises a sensing module 106, a post-signal processing 
circuitry 108 and a working memory 110. The present invention is 
preferably embodied in the sensing module 106 therefore other processes or 
hardware in the scanner 100 are not to be described in detail to avoid 
unnecessarily obscuring aspects of the present invention. 
Referring now to FIG. 2, there is shown a cross section view of a typical 
sensing module. A color light source 114 provide 3 different 
illuminations, e.g. red, green, and blue lights, to the scanning object 
over the cover glass 112. The scanning object, not shown in the figure, 
may be a sheet of paper placed face down on the cover glass 112 such that 
the scanning side is illuminated by the light source 114. The cover glass 
112 is transparent and provides a focus means for the paper to be properly 
scanned. When the light source 114 emits light onto the paper as indicated 
by 116, the light reflected from the paper through the cover glass 112 is 
directed at the optical lens 118 which is generally an array of one-to-one 
erect graded index micro (cylindrical or rod) lens. It should be 
understood that the present invention is independent of the optical lens 
and the light source. The use of the particular light source and the lens 
array in this configuration facilitate the description of the present 
invention and impose no limitation thereof. Under the optical lens 118, 
there is an image sensor 120 comprising an array of photodetectors made of 
CMOS or CCD sensors. The array can be configured as one-dimensional array 
or two-dimensional array, often referred to linear sensor or area sensor 
respectively. It should be noted that the following description is based 
on the linear sensor, those skilled in the art will appreciate that the 
principles of the present invention can be equally applied to the 
two-dimensional array as well. The optical lens 118 collects the reflected 
light onto the photodetectors that convert the reflected light to 
electronic signals proportionally representing the intensity of the 
reflected light. The electronic signals are then transferred to the data 
bus 122 that is coupled to the memory device 110 through the connector 
124. 
For the paper over the cover glass 112 to be completely scanned, the paper 
and the image sensor 120 has to move against each other. In the flatbed 
scanners, the paper is held still while the image sensor is driven to move 
along the paper at a fixed speed. In the sheet-fed scanners, the image 
sensor 120 is held still and the paper is rolled along the image sensor at 
a fixed speed. In both cases, the motion is performed by a moving 
mechanism, not shown in the figure, that determines the scanning 
resolution. In other words, the moving speed is conformed to the image 
vertical resolution in the resultant image and hence synchronized by a 
sensor clock signal that may be generated from an oscillator. 
When a line of the paper is being scanned, the paper on the cover glass 112 
is kept still. After one line is scanned, the paper is advanced one scan 
line by the moving mechanism. The moving distance depends on the vertical 
resolution. When a color image is generated, the light source 114 first 
emits a red light. This red light is directed at the paper and the 
reflected light is focused onto the image sensor 122 by the optical lens 
118. The image sensor 122 integrates the reflected light and generates a 
sequence of pixel signals, each representing a pixel value. The pixels are 
then sequentially readout, one at a time, to the data bus 122 and the 
connector 124 to the memory device, such as the memory 10 in FIG. 1. The 
readout process will be described in more detail in the following. After 
the scanning process for the red light is finished, the same process is 
repeated respectively for the green light and blue light. 
It can be appreciated that the pixel readout time could be considerably 
lengthy if the image sensor has a larger number of photodetectors, hence 
the larger number of pixels in one scanning line. To fully understand the 
principles of the present invention, FIG. 3 shows the internal functional 
diagram of the sensing module. According to one embodiment of the present 
invention, the light source 114 comprises three light emitting diodes 
(LED), each being a green 132, a red 134 and a blue 136 diode, 
respectively and controlled individually and successively by an "ON" 
signal at respective connectors 138, 140, and 142. The green 132, red 134 
and blue 136 diode is turned on when the "ON" signal, often an appropriate 
voltage, is applied to the respective connectors 138, 140, and 142, 
wherein three intensity images, representing the red, green and blue 
components in the scanning object are so generated. For a monochrome 
scanning, only one of the LED diodes, preferably the green one, is turned 
on such that only one intensity image is generated. The rod lens array 118 
collects the reflected light from the scanning object and focus it onto 
the image sensor 120 underneath. The image sensor 120 comprises, for 
example, N photodetectors. Each of the photodetectors collects light cast 
thereon during each integration process and generates a pixel signal. Upon 
the completion of the integration process, the pixel signals, each 
respectively generated by one of the photodetectors, are sequentially 
readout to the video bus 139 as a scanning signal via the readout switch 
array 140. The switch array 140 comprises the same number of the readout 
switches as the number of the photodetectors in the image array 120. It is 
understood to those skilled in the art that each of the readout switches 
may be implemented by a diode that becomes on or "passing through" when a 
proper voltage is applied across. As shown in the figure, the scanning 
signal is coupled to a gain & offset control circuit 142. The scanning 
signal is processed, including amplified and offset, in the gain & offset 
control circuit 142 with respect to a desired adjustment. 
It can be readily appreciated that the switch array 140 may be replaced by 
a shift register when the image sensor is CCD. The shift register 
comprises the same number of memory cells as the number of the 
photodetectors 120. Upon the completion of the integration process, the 
pixel signals are serially shifted out, one pixel signal at one clock 
cycle from one memory cell to another, subsequently to produce a scanning 
signal in the video bus 139. 
FIG. 4 shows one embodiment of the present invention at printed circuit 
board (PCB) level and should be understood in conjunction with FIG. 3. 
Referenced by 162, 163, 164 and 165 are four readout passages, each 
comprising a local video bus 150, 152, 154 or 156 and a switch array 151, 
153, 155 or 157. Each of the four readout passages is responsible for 
reading out the pixel signals in one portion of the image sensor. In other 
words, the image sensor is virtually, preferably equally, divided into 
four groups, or virtual groups, each of which is coupled to one of the 
readout passages and hence the video buses 150, 152, 154 or 156 are 
referred to as the local buses. The four local video buses 150, 152, 154 
and 156 concurrently generate four respective segmented scanning signals 
that are to be combined to produce an interleaved scanning signal at the 
regular video bus, such as the one referenced by 139 in FIG. 3. 
The four readout passages 162, 163, 164 and 165 may also be implemented 
using four shift registers in the case of CCD. To avoid unnecessarily 
obscuring aspects of the present invention, the following description of 
the figure is based on the case of CMOS using the local video buses and 
the switch arrays, those skilled in the art will understand that the 
description below is equally applied to the shift registers in CCD case. 
Each of the four readout passages 162, 163, 164 and 165 can be respectively 
represented by FIG. 3 and the operation thereof has been described. Each 
of the local video buses 150, 152, 154 and 156 is coupled to a respective 
switch array that preferably comprises an equal number of readout switches 
therein. For example, there are M readout passages, each of the M readout 
passages comprising K readout switches so that K times M is equal to the 
total number of the photodetectors in the image sensor. To be more 
specific, if the image array comprises 2700 photodetectors and four 
readout passages are used, each of the four switch arrays will have 675 
readout switches, therefore each of the photodetectors is coupled exactly 
to one readout switch. It should be understood by those skilled in the art 
that the image sensor virtually divided into four groups, hence four 
switch arrays and four local buses herein, is not an implied limitation of 
the present invention, rather they are used in a specific embodiment 
illustrated herein to describe the present invention. 
When the image sensor starts the readout phase, all the charges in the 
photodetectors are concurrently readout to the four local buses via the 
readout switches and each of the local video buses generates respectively 
a segmented scanning signal OA, OB, OC or OD. The four segmented scanning 
signals OA, OB, OC and OD, representing four consecutive segments of a 
scanning signal, are connected to a multiplexer 158 that is used to 
sequentially multiplex the four segmented scanning signals OA, OB, OC and 
OD and produce an interleaved signal Vo that goes to the regular video bus 
and coupled to the gain & offset control circuit, not shown in this 
figure. 
Prior to describing how the four segmented scanning signals OA, OB, OC and 
OD are multiplexed to produce the interleaved signal Vo, it is necessary 
to describe four control signals, each derived from a sensor clock signal, 
to control the four local video buses 150, 152, 154 and 156. FIG. 5 shows 
a set of signal waves. Referenced by 170 is the sensor clock signal having 
a clock cycle T. It is understood to those skilled in the art that the 
sensor clock signal 170 may be drew from an oscillator circuit in a 
scanner. Control signals, CLKA 172, CLKB 174, CLKC 176 and CLKD 178 are 
derived from the sensor clock signal 170 and each is orderly delayed by a 
clock cycle T as indicated by the downward edges 182, 184, 186 and 188. It 
is further indicated in the figure that each clock cycle in the controls 
signals are four times of that in the sensor clock signal 170. The timing 
circuit 166 in FIG. 4 receives the sensor clock signal 170 and produces 
the control signals, CLKA 172, CLKB 174, CLKC 176 and CLKD 178. It is 
understood to those skilled in the art that many commercially available 
counters can be used to implement the timing circuit 166 and preferably 
implemented in an application specific integrated circuit (ASIC). 
Referenced by 190, 192, 194 and 196 are the respective segmented scanning 
signals from the four local buses 150, 152, 154 and 156 in FIG. 4. The 
four switch arrays 151, 153, 155 and 157, each respectively in response to 
the downward edges in the respective control signals, CLKA 172, CLKB 174, 
CLKC 176 and CLKD 178, one such edge in each of the control signals being 
indicated by 182, 184, 186 and 188, turn on one readout switch therein to 
readout a pixel signal from a respective photodetector to the respective 
local buses 150, 152, 154 and 156, resulting in four respective segmented 
scanning signals 190, 192, 194 and 196. The pixel signal readout time, 
i.e. the time it takes to produce a segmented scanning signal, is the time 
it takes to successively turn on all the readout switches in a switch 
array regardless of the number of the switch arrays. 
Returning to FIG. 4, the timing circuit 170 generates the four control 
signal, CLKA 172, CLKB 174, CLKC 176 and CLKD 178, each respectively and 
independently controlling the operations of the four switch arrays. In 
addition, the timing circuit 170 passes the sensor clock signal 170 to 
control the operation of the multiplexer 158. For every rising edge in the 
sensor clock signal 170, the multiplexer samples one of the four arrived 
scanning signals OA, OB, OC and OD. For example, the scanning signals OA, 
OB, OC and OD are successively sampled at 183, 185, 187 and 189 in FIG. 5 
at the rate of T. It can be now appreciated that the readout rate of the 
pixel signals from the four local video buses remains the same as each 
cycle T and still produces one pixel signal, yet the capacitance of the 
inherent capacitor resulting from each switch array has been significantly 
reduced, which makes it possible to apply a higher system clock to further 
increase the pixel readout rate from the image sensor. Generally, if there 
are N number of photodetectors in the image sensor and M switch arrays, 
each of the M switch arrays equally comprising K readout switches and 
K=N/M, are used to accommodate the pixel signals from the photodetectors, 
the readout rate can be potentially improved by M times if a higher sensor 
clock signal with a cycle of T/M is applied, without requiring a 
high-speed image sensor. In other words, with the present invention, a 
regular image sensor can now be used to accommodate the higher sensor 
clock signal. 
It should be appreciated by those skilled in the art that the readout 
passages 162, 163, 164 and 165 may be implemented using four shift 
registers. Each of the registers comprises the same number of memory cells 
as the photodetectors in each of the virtual groups of the image sensor. 
Instead being readout through an array of readout switches, the pixel 
signals are dumped to the memory cells and then readout by shifting, one 
pixel signal at a time, to the data bus to produce the segmented scanning 
signal OA, OB, OC and OD, respectively. 
The output VO from the multiplexer is, as described above, an interleaved 
scanning signal. Referring now to FIG. 6, there is illustrated an example 
of the process from four readout passages to produce the interleaved 
signal. There are four readout passages 202, 204, 206 and 208, each having 
five readout switches therein, respectively. The twenty pixels are labeled 
consecutively and from corresponding photodetectors in the image sensor. 
The consecutively numbered twenty pixels represent four segmented scanning 
signals from the four readout passages 202, 204, 206 and 208. When the 
four readout passages 202, 204, 206 and 208, controlled by their own clock 
signals, such as the CLKA, CLKB, CLKC, CLKD in FIG. 5, readouts 
respectively one pixel at one system clock cycle, as indicated by 183, 
185, 187 and 189 caused by the corresponding downward edges 182, 184, 186 
and 188 respectively in FIG. 5, the multiplexer 210 samples the four 
inputs sequentially, e.g. from OA to OD, and produces the interleaved 
signal 212. As indicated by the numbered pixels in the figure, the first, 
second, third and forth pixel are from the first pixel of the first, 
second, third and forth shift register, hence the interleaved scanning 
signal. Generally the interleaved scanning signal is difficult to 
visualize, an ordering process 214 may be used to reorder the pixels to 
produce a normal scanning signal 216 reflecting exactly what is captured 
in the photodetectors. It is understood to those skilled in the art that 
the reordering process can be implemented in the post-signal processing 
108 or memory addressing in the memory 110 in FIG. 1. 
The present invention has been described in sufficient detail with a 
certain degree of particularity. It is understood to those skilled in the 
art that the present disclosure of embodiments has been made by way of 
example only and that numerous changes in the arrangement and combination 
of parts as well as steps may be resorted s without departing from the 
spirit and scope of the invention as claimed. Accordingly, the scope of 
the present invention is defined by the appended claims rather than the 
forgoing description of one embodiment.