JP2001111822A - Image reader and its method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image reader that cancels a noise of a digital signal processing system superimposed on a valid part of an analog signal so as to read an image with high accuracy. SOLUTION: The image reader superimposes data of a digital system and a fixed noise of a data clock system onto a sensor output signal 33 even at acquisition of reference data and outputs a data output by canceling the fixed data at the output of image read data. In this case, a number of image read data may largely be different from the number of data at acquisition of the reference data depending on the resolution. In order to avoid mismatching between a dummy clock output timing at the acquisition of reference data and a data output timing at image reading, the number of data output terminals is selected to select and set a proper timing for each resolution to match the clock output timing at the reference data acquisition with the data and clock output timing at reading the image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to, for example, an image reading apparatus and a method which are interchangeably mounted on a recording apparatus with a recording head.

[0002]

2. Description of the Related Art Conventionally, a general ink jet recording apparatus has only a function as a recording printer equipped with a recording head.

On the other hand, there is known a recording apparatus having a scanner function by detachably attaching an image reading apparatus (scanner unit) to a carriage portion of the recording apparatus (Japanese Patent Publication No. 1-20832, Japanese Patent Publication No. Hei 2-20832). No. 21712, JP-B-2-21711, etc.).

Further, as an output format of image data, there has been used an image reading apparatus which outputs in one output format such as a serial output and an 8-bit parallel output.

[0005]

However, in a conventional image reading apparatus having only one output format, if the read data amount differs for each resolution, the processing speed, the power consumption associated with this processing speed, and the noise There was no degree of freedom for the removal effect and the like, and it was not possible to read an image with high accuracy.

Specifically, image data read during the storage time required by the sensor must be transmitted as digital data. However, it is necessary to select an optimal combination for power consumption, processing speed, noise prevention, and the like. Therefore, complicated control processing is performed, and the processing speed is reduced.

Further, it is difficult to process digital signal processing system noise superimposed on an analog signal as a sensor output, that is, digital signal processing system noise (fixed noise) in a so-called analog / digital signal mixed circuit. Could not be read.

Therefore, the present invention can read a high-quality image with high accuracy and a high S / N ratio by canceling out noise of a digital signal processing system superimposed on an effective portion of an analog signal. It is an object to provide an image reading apparatus and method.

[0009]

In order to achieve the above object, an image reading apparatus according to a first aspect of the present invention comprises a read data obtained by reading an image and a reference data obtained by reading a reference image. In an image reading apparatus that outputs a difference from data as image data, a resolution setting unit that sets a resolution at the time of reading the image, and a read data obtaining unit that obtains a number of the read data according to the set resolution Output clock generation means for generating an output clock for outputting the image data when reading the image, and a dummy clock for generating a dummy clock for the output clock when reading the reference image and obtaining the reference data Generating means for reading the image and outputting read data; timing of the output clock; Characterized by comprising a timing setting means for combining the timing of the click.

According to a second aspect of the present invention, in the image reading apparatus according to the first aspect, the timing setting means includes an output bit number changing means for changing an output bit number of the image data. The number of bits may be changed to match the timing of the output clock with the timing of the dummy clock.

According to a third aspect of the present invention, in the image reading apparatus according to the second aspect, the output bit changing means includes a serial output format 1 bit and a parallel output format 2 bit number of an integer power. It can be changed to

According to a fourth aspect of the present invention, in the image reading apparatus according to the first aspect, the timing setting means includes an output clock frequency changing means for changing a frequency of the output clock, and The frequency is changed to match the timing of the output clock with the timing of the dummy clock.

According to a fifth aspect of the present invention, in the image reading apparatus according to the first aspect, the timing setting means includes a driving frequency changing means for changing a driving frequency of a sensor for reading the image. The frequency is changed to match the timing of the output clock with the timing of the dummy clock.

According to a sixth aspect of the present invention, in the image reading apparatus according to the first aspect, the read data acquiring unit determines a portion where the timing of the output clock does not match the timing of the dummy clock. It is not acquired as read data.

An image reading apparatus according to a seventh aspect of the present invention is an image reading apparatus which is mounted on a recording apparatus so as to be exchangeable with a recording head, wherein the image reading means reads an image of a document mounted on the recording apparatus; An output unit capable of outputting the read image data in a serial output format and a plurality of parallel output formats, and an output format selection unit for selecting the output format are provided.

According to another aspect of the present invention, there is provided an image reading apparatus which is mounted on a recording apparatus so as to be exchangeable with a recording head, wherein the image reading means reads an image of a document mounted on the recording apparatus; A clock generator for generating an output clock for outputting the read image data; and a clock frequency changing unit for changing a frequency of the output clock.

An image reading apparatus according to a ninth aspect of the present invention is an image reading apparatus which is interchangeably mounted on a recording apparatus with a recording head, wherein the image reading means reads an image of a document mounted on the recording apparatus. A driving frequency changing unit for changing a driving frequency of the image reading unit.

An image reading method according to claim 10, wherein a difference between read data obtained by reading an image and reference data obtained by reading a reference image is output as image data. Setting the resolution at the time of reading, obtaining the number of the read data according to the set resolution, and generating an output clock for outputting the image data at the time of reading the image, Generating a dummy clock of the output clock when reading the reference image and obtaining the reference data; anda period of reading the image and outputting read data, the timing of the output clock and the timing of the dummy clock. And a combining step.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an image reading apparatus and method according to the present invention will be described. The image reading apparatus according to the present embodiment is applied to an ink jet printer which is mounted to be exchangeable with a recording head.

[First Embodiment] FIG. 1 is a block diagram showing the configuration of an ink jet printer. This ink jet printer has an apparatus main body 4 connected to a host computer 11 via an interface 10, a recording head 1 and a scanner unit 16 which are interchangeably connected to a head connection unit 2.

The apparatus main body 4 includes a head connection section 2, a head connection line 3, a controller 5, motor drivers 6, 7, an arithmetic and control section 8, a sensor 9, and an interface (I / F) 1.
0, CR motor 12, LF motor 14, A / D converter 2
4, a memory 25, and the like.

The scanner unit 16 includes a light source 17, a sensor 18, an amplifier 19, an A / D converter 20, an image processing I
C21 and an oscillator 26 are provided.

In the ink jet printer having such a configuration, recording data such as characters and images is transferred from the host computer 11 through the interface (I / F) 10 under the control of the host computer 11 and the arithmetic control unit 8. You. When receiving the print data, the controller 5 uses the print head 1 to print a print sheet (not shown).
Process to data to be recorded in Further, the controller 5 controls the head connection line 3 while receiving the control of the arithmetic control unit 8.
The recording data is sent to the recording head 1 via the head connection unit 2, and the recording head 1 records characters, images, and the like on recording paper.

FIG. 2 is a diagram showing a configuration of a main part of the printing mechanism. The cylindrical body 15 is a platen, which is attached to a recording paper and has an arithmetic control unit 8, a controller 5, and a motor driver 6.
Rotated by a drive mechanism (not shown) controlled by
It becomes a recording table that sends the recording paper and supports the recording paper.

The carriage 13 arranged close to the circumferential surface of the platen 15 is movably arranged along the axial direction of the platen 15. The carriage 13 mounts the recording head 1 at the time of recording and a scanner unit (image reading device) 16 described later at the time of image reading, and conveys them along the surface of a recording paper or a document for image reading. The movement of the carriage 13 is performed by a carriage drive mechanism (not shown) controlled by the arithmetic control unit 8, the controller 5, and the motor driver 7.

The sensor 9 detects whether a recording sheet or a document for reading an image is set on a paper tray (not shown), and whether the carriage 13 is at a start position. You. Recording is performed using such a printing mechanism.

Next, when the present apparatus (inkjet printer) operates as an image reading apparatus, the scanner unit 16 moves an image on a document (not shown) in the same motion as the recording head 1 during recording. A reading scan is performed.

An original is illuminated by a light source 17 in a scanner unit 16 and reflected light such as characters and images is detected by a sensor 18 having photoelectric conversion characteristics. The image signal detected by the sensor 18 is converted into an analog /
Amplify to the optimal level that can be handled by digital converter 20,
Input to the A / D converter 20. The digital data converted by the A / D converter 20 is subjected to shading correction (unevenness correction), correction such as binarization, or image processing by the image processing IC 21 and is transferred to the apparatus main body 4 as image data.

In this data transfer, the data is sent to the host computer 11 through a path reverse to the flow of the recording data at the time of recording. That is, image data is sent from the image processing IC 21 to the host computer 11 through the head connection unit 2, the head connection line 3, the controller 5, and the interface 10. At this time, the controller 5
Converts the image data received from the image processing IC 21 into a format that can be easily sent by the interface 10 or a format that can be easily handled by the host computer 11, and transfers the data under the control of the arithmetic control unit 8.

When reading an image, the printing mechanism performs almost the same operation as during recording. That is, the original for image reading is attached to the platen 15, and the platen 15 is rotated by a driving mechanism (not shown) to feed the original, and serves as an original table for supporting the original, similarly to the case of recording. Then, the carriage 13 on which the scanner unit 16 is mounted conveys the scanner unit 16 along the surface of the image reading document and reads the image. The movement of the carriage 13 is performed by a carriage drive mechanism (not shown) as in the case of recording. The sensor 9 operates in the same manner as the operation described above. As described above, also during the image reading operation, the printing mechanism performs the same movement as during the recording operation.

FIG. 3 is a timing chart showing changes in signals of various parts during an image reading operation. At the time of image reading, the LNST signal 30 supplied from the apparatus main body 4 and setting the reading cycle and the accumulation time serves as a temporal reference signal for the operation of each block.

The LNST signal 30 is generated by the arithmetic and control unit 8 and the controller 5 in the apparatus main body 4 and input to the image processing IC 21 through the head connection unit 2.
The image processing IC 21 has an oscillator 26, and all logical movements in the scanner unit 16 operate in synchronization with the clock of the oscillator 26. That is, the image processing IC2
The LNST signal 30 input to 1 is synchronized with the clock of the oscillator 26, and the image processing IC 21 generates the signal φROG31 for driving the sensor 18.

At the same time, the image processing IC 21 generates a clock φCLK 32 for driving the sensor 18 and outputs a signal φ
It is sent to the sensor 18 together with the ROG 31. Clock φ
CLK32 and the signal φROG31 are synchronized, and the signal φROG31
The sensor 18 reads a white image, for example, in the cycle of the ROG 31, and outputs a sensor output signal 33 thereof. The S0 portion of the sensor output signal 33 in the figure is a signal of information read by the sensor 18 during the accumulation time T0. The sensor output signal 33 is a signal for outputting information for one pixel in one clock of the clock φCLK32. That is, the sensor 18
Is a sensor for 256 pixels, and a clock φCLK32
Is a 1 MHz clock, 256 pixels × (1/1)
1 MHz) = 256 μsec, sensor output signal 3
3 will be output. Similarly, clock φCLK
When 32 is 2 MHz, 256 pixels × (1/2 MH)
z) = 128 μsec, the sensor output signal 33 is output.

A plurality of clock frequencies of φCLK 32 can be selected by instructions such as register setting from the arithmetic control unit 8 to the image processing IC 21. FIG. 4 is a block diagram showing a partial configuration of the image processing IC 21. The image processing IC 21 includes a DTCK clock generation block and a frequency selection block 51, an output terminal selection block 52,
A register block 53, a sensor drive clock generation block, a frequency selection block 54, and the like are provided.
The clock frequency of φCLK 32 is selected by instructing the sensor drive clock generation block and the frequency selection block 54 to set a register value.

The sensor output signal 33 is an analog signal having a level on the order of several mV to several + mV. The sensor output signal 33 is amplified by the amplifier 19 to a level suitable for converting an analog signal into a digital signal.
After being converted into digital data by the / D converter 20,
It is input to the image processing IC 21. The image processing IC 21 performs image data processing such as non-uniformity correction and outputs image data.

At this time, as the output format from the image processing IC 21, the 1-bit output as a serial output format, and the 2-bit output, 4-bit output, and 8-bit output as the parallel output format can be performed.
Are provided for eight terminals.

In the selection of the output format, the number of output terminals is set to the image processing IC 21 according to an instruction such as register setting through the arithmetic control unit 8 or the controller 5. As described above, by appropriately selecting a plurality of output terminals, the output time of the data output 1 signal 34 can be selected according to the number of output terminals. In other words, the data output 1 signal 34 is a 1-bit output, and if the data output 2 signal 35 which is a 2-bit output is selected, the data output is completed in half the time of the data output 1 signal 34. . Similarly, when the 4-bit output is selected, the data output takes only 1/4 time, and when the 8-bit output is selected, the data output takes only 1/8 time.

The clock DTC at the data output
Also for K36, a plurality of clock frequencies can be selected. That is, the clock DTCK36 is
Although the number of clocks output according to the number of clocks corresponding to the data amount of the data output is a clock that can be output at a frequency of, for example, 2 MHz, when the clock DTCK 36 for the data output 1 signal 34 is 4 MHz, the clock DTCK 37 The data transfer becomes possible in half the time as shown in FIG.

The selection and setting of the clock frequency are also
In the same manner as the selection and setting of the data output terminal, the operation is performed on the DTCK clock generation block and the frequency selection block 51 of the image processing IC 21 by an instruction such as register setting through the arithmetic control unit 8 or the controller 5.

Next, the data output during the operation of acquiring the reference data (read data of the reference image) from the white correction plate or the like will be described. For example, when a reference image such as a white correction plate is read at the time of acquiring reference data, the acquired reference data is directly stored in a reference data memory (not shown) in the image processing IC 21. In other words, when reading the reference data, the data output and the data output clock DTCK are not required for the output from the data output terminal or the like. Therefore, when it is desired to read the acquired reference data, the reference data is read from the reference data memory.

However, as described above, the sensor output signal 33 is an analog signal having only an output voltage of several mV to several tens mV, and is easily affected by disturbance.
In particular, spike-like noise generated by the data output and the data output clock DTCK affects the power supply system and the ground system as noise, and is superimposed on the output waveform of the sensor output signal 33. It has also been found that it is quite difficult to remove the noise itself.

Therefore, in order to eliminate the influence of such noise, the data output clock DTCK is output as a dummy clock in a pseudo manner when the reference data is obtained, as in the case of normal image reading. in this way,
As in the image reading, almost the same noise is superimposed on the sensor output signal 33 in the reference data reading, thereby canceling (removing) spike noise (fixed noise) due to the data output clock DTCK. .

In other words, the normal image reading data expresses the difference between the level of the sensor output 33 at the time of the reference data and the level of the sensor output 33 at the time of reading the image (the error in the image processing IC 21). Therefore, the image reading data is output in a state where at least the fixed noise caused by the data output clock DTCK is removed.

The generation of the dummy clock is performed by the arithmetic control unit 8.
Are set in the register block 53 in the image processing IC 21, and a dummy clock is output from the DTCK clock generation block and the frequency selection block 51.

As described above, the configuration is such that the number of data output bits can be selected, that the data clock frequency can be selected, that the sensor drive frequency can be selected, and that the dummy clock is generated when the reference data is obtained. A description will be given of a method of removing digital data and fixed noise of the data clock system which are superimposed on the output signal from the sensor 18 because of the possible configuration.

The method of removing the fixed noise has been described to some extent in the description of the configuration capable of generating a dummy clock when acquiring the reference data, but will be described again. FIG.
5 is a timing chart showing a change in a signal of each unit at the time of data output.

With respect to the output time Ts of the sensor output signal 33, a time Tsd for outputting at the same timing within the image reading data output time Td occurs as a signal output timing. Digital data and data clock fixed noise are superimposed on the sensor output signal 33 within the time Tsd.

Here, also at the time of acquiring the reference data, digital data and data clock fixed noise are superimposed on the sensor output signal 33, and the data output obtained by canceling the fixed noise at the time of image reading data output is output. You do it. However, when acquiring the reference data, the reference data is acquired over all the pixels (256 pixels) of the sensor 18, but when reading the image, the number of image read data differs for each reading resolution. For example, in this embodiment,
Since the configuration of the sensor 18 and the optical system has a basic resolution of 720 dpi, 360 dpi, 180 dpi, 90d
To obtain a reduced resolution such as pi, this is realized by performing an averaging process between pixel data in the image processing IC 21 from the read data of 720 dpi.

At this time, the number of image data of each resolution in one reading cycle is, for example, assuming that multi-valued data is 8 bits, and when 720 dpi, 256 pixels × 8 bits = 2048
For data at 360 dpi, 256 pixels × (１ ／) × 8 bits = 1024 data For 180 dpi, 256 pixels × (１ ／) × 8 bits = 512 data For 90 dpi, 256 pixels × (１ ／) × 8 bits = 256 data. Depending on the resolution, the number of data at the time of acquiring the reference data may greatly differ, and the output timing of the dummy clock at the time of acquiring the reference data and the data output timing at the time of image reading may not match (see FIG. 5).

In order to match this timing, as a first method, the number of data output terminals is selected, and the 2-bit output of the parallel output format is compared with the time when the 1-bit output of the serial output format is used. , The data output time is reduced to 1/2, the 4-bit output is set to reduce the data output time to 1/4, or the 8-bit output is set to reduce the data output time to 1/8. An appropriate timing combining the clock output timing at the time of acquiring the reference data and the data and clock output timing at the time of image reading is selected and set for each resolution. FIG.
Is the timing indicating the clock output timing when acquiring the reference data and the data and clock output timing when reading the image when selecting and setting an appropriate timing for each resolution by selecting the number of data output terminals. It is a chart. As described above, when the image is read, the data amount differs depending on the resolution. Therefore, the output timing can be adjusted by selecting and setting the number of output bits.

As a second method, the frequency of the data clock is selected for each resolution and the timing is adjusted. For example, at 720 dpi, when the clock frequency is 8 MHz, at 360 dpi, when the clock frequency is 4 MHz, at 180 dpi, when the clock frequency is 2 MHz, at 90 dpi, the clock frequency is 1 MHz, the time Tsd becomes the same timing, and the reference data acquisition becomes the same. An appropriate timing combining the clock output timing at the time and the clock output timing at the time of image reading is selected and set for each resolution. FIG. 7 shows the clock output timing at the time of acquiring reference data, the data and clock output timing at the time of image reading, when the appropriate timing is selected and set for each resolution by selecting the frequency of the data clock for each resolution. FIG.

As described above, when reading an image, the data amount differs for each resolution.
The output timing can be adjusted by selecting and setting the frequency K). At this time, since the clock of the reference data at the time of acquiring the reference data is also output as a dummy clock, a data output clock of a frequency selected for each resolution is output at the time of image reading.

As a third method, the timing is adjusted by changing the driving frequency of the sensor 18 for each resolution. For example, at 360 dpi, the sensor driving frequency is 1 MHz, and at 180 dpi, the sensor driving frequency is selected and set to 2 MHz, and as shown in FIG.
The time Ts360 and the time Td360 of 180 dpi are longer, but the time Ts180 and the time Td180 of 180 dpi are longer.
In both cases, the time Tsd has the same timing,
The sensor driving frequency is selected and set for each resolution so that the clock output timing at the time of acquiring the reference data and the clock output timing at the time of image reading are combined so as to be appropriate timing. FIG. 8 shows a clock output timing at the time of acquiring reference data and a clock at the time of image reading when selecting and setting a sensor drive frequency for each resolution so that an appropriate timing is obtained by changing the drive frequency of the sensor 18 for each resolution. 6 is a timing chart showing output timing.

As described above, the first, second, and third methods are used to select and set an appropriate combination for each resolution in consideration of the power consumption, the noise removal rate, the radiation noise generation state, and the like. Accurate and high-quality image reading can be performed.

[Second Embodiment] In the second embodiment,
This shows a case where noise of the digital signal processing system cannot be canceled. In other words, when the number of data output terminals is restricted due to the space and cost of the device and only one bit output is provided, and the means shown in the first embodiment cannot be taken, FIG. As shown in the figure, the timing of the time Tsd with respect to the sensor output signal 33 cannot be completely obtained. FIG. 9 is a timing chart showing a change in the signal of each unit when the timing of the time Tsd with respect to the sensor output signal 33 cannot be completely obtained.

In the figure, in the period Tnsd, only the noise of the reference data clock becomes the noise of the digital signal processing system, and there is a portion where the clock noise at the time of reading the image and the dummy clock at the time of obtaining the reference data cannot be offset. Would be.

However, the period T during which this noise cannot be canceled out
The nsd image read data is not used as output data, that is, is processed as invalid data, and only the portion where noise is canceled is used as final image data, so that the noise is canceled and the S / N ratio is high. High image reading can be realized. The processing of such invalid data is processed by the arithmetic and control unit 8 and the controller 5 in the apparatus main body 4.

[0058]

According to the present invention, high-precision, high-S / N ratio, high-quality images can be read by canceling out noise in a digital signal processing system superimposed on an effective portion of an analog signal. Can be. In addition, energy saving, radiation noise reduction, and the like can be achieved.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an inkjet printer.

FIG. 2 is a diagram illustrating a configuration of a main part of a printing mechanism.

FIG. 3 is a timing chart illustrating a change in a signal of each unit during an image reading operation.

FIG. 4 is a block diagram showing a partial configuration of an image processing IC 21.

FIG. 5 is a timing chart showing a change in a signal of each section at the time of data output.

FIG. 6 shows a clock output timing at the time of acquiring reference data, a data and clock output timing at the time of image reading, when selecting and setting an appropriate timing for each resolution by selecting the number of data output terminals. FIG.

FIG. 7 shows a clock output timing at the time of acquiring reference data and a data and clock output timing at the time of image reading when an appropriate timing is selected and set for each resolution by selecting a frequency of a data clock for each resolution. FIG.

FIG. 8 shows a clock output timing at the time of reference data acquisition and a clock output timing at the time of image reading when selecting and setting a sensor driving frequency for each resolution so that an appropriate timing is obtained by changing a driving frequency of the sensor 18 for each resolution. 6 is a timing chart showing clock output timing.

FIG. 9 is a timing chart showing changes in signals of respective units when the timing of time Tsd with respect to the sensor output signal 33 cannot be completely obtained.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Recording head 2 Head connection part 4 Main body 5 Controller 8 Operation control part 16 Scanner unit 18 Sensor 21 Image processing IC

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Tadashi Yamamoto 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term in Canon Inc. (reference) 5B047 BB02 CA06 CB17 5C072 AA01 EA04 FB08 5C077 LL02 MM02 PQ05 SS01

Claims (10)

[Claims] An image reading apparatus that outputs a difference between read data obtained by reading an image and reference data obtained by reading a reference image as image data, wherein a resolution for setting a resolution at the time of reading the image is provided. Setting means, read data obtaining means for obtaining the read data of a number corresponding to the set resolution, output clock generating means for generating an output clock for outputting the image data when reading the image, A dummy clock generating means for generating a dummy clock of the output clock when reading the reference image and obtaining the reference data; anda period for reading the image and outputting read data, a timing of the output clock and a timing of the dummy clock. Image characterized by comprising timing setting means for adjusting the timing Winding device. 2. The timing setting means comprises output bit number changing means for changing the number of output bits of the image data, and changing the number of output bits to change the timing of the output clock and the timing of the dummy clock. The image reading device according to claim 1, wherein the image reading device is combined with the image reading device. 3. The image reading apparatus according to claim 2, wherein said output bit changing means is capable of changing the number of bits to one power of a serial output format and the number of bits to the power of 2 in a parallel output format. 4. The timing setting means comprises output clock frequency changing means for changing the frequency of the output clock, and changing the frequency of the output clock to match the timing of the output clock with the timing of the dummy clock. The image reading device according to claim 1, wherein: 5. The timing setting means includes a driving frequency changing means for changing a driving frequency of a sensor for reading the image, and changing the driving frequency to match the timing of the output clock with the timing of the dummy clock. The image reading device according to claim 1, wherein: 6. The image reading apparatus according to claim 1, wherein the read data acquisition unit does not acquire a portion where the timing of the output clock does not match the timing of the dummy clock as the read data. 7. An image reading apparatus which is mounted on a recording apparatus so as to be exchangeable with a recording head, comprising: image reading means for reading an image of a document mounted on the recording apparatus; and serially outputting the read image data. An image reading apparatus comprising: an output unit capable of outputting in a format and a plurality of parallel output formats; and an output format selection unit that selects the output format. 8. An image reading device which is exchangeably mounted on a recording device with a recording head, wherein the image reading device reads an image of a document mounted on the recording device, and outputs the read image data. An image reading apparatus, comprising: a clock generating means for generating an output clock for the same; and a clock frequency changing means for changing a frequency of the output clock. 9. An image reading device which is mounted on a recording device so as to be exchangeable with a recording head, wherein the image reading device reads an image of a document mounted on the recording device, and a driving frequency of the image reading device is changed. An image reading apparatus comprising: 10. An image reading method for outputting a difference between read data obtained by reading an image and reference data obtained by reading a reference image as image data, wherein a resolution at the time of reading the image is set. Acquiring a number of the read data according to the set resolution; generating an output clock for outputting the image data when reading the image; reading the reference image and reading the reference data Generating a dummy clock of the output clock when acquiring the data, and a step of matching the timing of the output clock with the timing of the dummy clock during a period of reading the image and outputting read data. Image reading method.