JP3314523B2 - Image data converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image data converter for use in an image forming apparatus such as a laser beam printer or a digital copier, and more particularly to a method for modulating a pulse width of an image signal based on input image data. The present invention relates to an image data conversion device used for reproducing a halftone image.

[0002]

2. Description of the Related Art Conventionally, as an image forming apparatus such as a laser beam printer or a digital copying machine, for example, a laser beam is scanned and exposed on a photosensitive drum using a semiconductor laser in accordance with input image data, and the photosensitive drum is exposed. An electrostatic latent image corresponding to image data is formed on a drum, and a monochrome or color image is formed by developing the electrostatic latent image.

[0003] In such laser beam printers and digital copiers, there has been an increasing demand for higher image quality, and as one of them, half-tone (halftone) gradation reproducibility has been required. . In order to reproduce the halftone gradation in the laser beam printer or the like, instead of simply turning on and off the semiconductor laser, a digital laser is used to change the emission time of the semiconductor laser according to the gradation data. 2. Description of the Related Art An image data converter that converts grayscale data into an analog signal and generates a pulse width modulation signal having a pulse width corresponding to the grayscale data is used.

[0004] As this image data conversion device, for example, there is one configured as shown in FIG.

In FIG. 13, reference numeral 100 denotes 8-bit gradation data input from an image reading device, a computer, or the like via an I / F circuit (not shown), and 101 denotes 8-bit quantized gradation data 100. Analog signal (256
D / A converter having a function of decomposing into
2 is a first triangular waveform generator for generating a triangular waveform based on the pixel clock 103, 104 is a 1/2 frequency divider for dividing the image clock 103 into 1/2, and 105 is this 1/2 frequency divider. Second triangular waveform generators 106 and 10 for generating a half-cycle triangular waveform of the first triangular waveform generator 102 based on the pixel clock 103 frequency-divided by に よ っ て
Reference numeral 7 denotes 8-bit quantized data 108 output from the D / A converter 101 and triangular wave signals 109 and 11 output from the first and second triangular waveform generators 102 and 105.
0 and a comparator 111 for comparing these comparators 10
A selector which selects one of the pulse width modulation signals output from the signals 6 and 107 based on the select signal 112 and outputs the selected signal to the laser drive circuit.

The image data converter receives gradation data 100 having gradation information for each pixel to be printed from an image reading device or a host computer (not shown) via an I / F circuit (not shown). Then, the input gradation data 100 is converted into a pixel clock 103 by a D / A converter 101 having an 8-bit quantization resolution.
The grayscale data 100 that has been D / A-converted based on the analog signal is sent to comparators 106 and 107. Further, the triangular waveform generators 102 and 105 generate triangular wave signals 109 and 110 having the same period and a half period, respectively, in synchronization with the pixel clock 103, and these triangular wave signals 109 and 110 are also sent to the comparators 106 and 107. Sent. These comparators 106 and 107 output the triangular wave signal 10
9, 110 and gradation data 10 converted into analog signals
8 and outputs a pulse width modulation signal having a length proportional to the gradation information of the gradation data 108. This output is selected by the selector 111 and sent to the laser drive circuit as a pulse width modulation signal.

In a laser beam printer or the like using such an image data converter, a first triangular waveform which generates a triangular wave signal 109 having the same period in synchronization with the pixel clock 103 when forming a character image. Generator 10
2 is supplied to the selector 111
Is used for modulation of the semiconductor laser. When a halftone image is mainly formed, a signal that has been pulse-width modulated by a second triangular waveform generator 105 that generates a triangular wave signal 110 having a half cycle of the pixel clock 103 is selected by a selector 111. Used for modulation of the semiconductor laser.

[0008]

However, the prior art described above has the following problems. That is, in the above-described conventional image data conversion device, the triangular wave signal has the same cycle as the pixel clock 103 or 1 /
It is set to two cycles. Therefore, if the resolution of one pixel is 16 dpm (dot / mm) in a system such as a laser beam printer, the spatial frequency of the laser beam scanned and exposed by the pulse width modulation signal generated by using the image data conversion device is Inevitably, when the first triangular waveform generator 102 that generates the triangular wave signal 109 having the same cycle in synchronization with the pixel clock 103 is used,
1pm (line / mm) and the pixel clock 1
In the case where the second triangular waveform generator 105 that generates the triangular wave signal 110 having a 1/2 cycle of 03 is used, 8 lpm (li
ne / mm), but there is a problem that the structure of an image formed by a laser beam having such a spatial frequency is not always optimal in an image forming apparatus such as a laser beam printer to be designed. there were.

In other words, when the first triangular waveform generator 102 that generates the triangular wave signal 109 having the same period in synchronization with the pixel clock 103 is used in the image data converter, the generated pulse width modulation signal The spatial frequency of the laser beam to be scanned and exposed is 16 lpm
(Line / mm), and when reproducing a character image existing on an image to be formed, it is possible to reproduce a high-resolution and good character image, but at the same time, a photographic image simultaneously existing on the image to be formed In the case of reproducing a halftone image such as that described above, there is a problem in that the gradation characteristic is extremely changed in an area of an adjacent image, and a good reproduction of a halftone image cannot be performed. On the other hand, in the image data converter, the triangular wave signal 1 having a half cycle of the pixel clock 103 is used.
In the case where the second triangular waveform generator 105 for generating the pulse width modulation signal 10 is used, the spatial frequency of the laser beam scanned and exposed by the generated pulse width modulation signal is reduced by half to 8 lpm.
(Line / mm), and when reproducing a halftone image such as a photographic image present on an image to be formed, it appears that the gradation is continuously changed visually in an adjacent image region, Although a good halftone image can be reproduced, when reproducing a character image that is simultaneously present on the formed image, there is a problem that the resolution is reduced and a good character image cannot be reproduced. ing.

As described above, as far as the conventional image data conversion apparatus is concerned, if only one of a halftone image such as a character image and a photographic image is used, it is used for pulse width modulation of gradation data. Although it is possible to form a good image by switching the frequency of the triangular wave signal, if a halftone image such as a character image and a photographic image is mixed on the same image, both images are converted to human images. Cannot be formed in a visually satisfactory state.

In order to solve such a problem and to form both the character image and the halftone image in a state that is good for human eyes, as shown in FIG.
The frequency of the triangular wave signal 121 used when performing pulse width modulation on the gradation data
It is also conceivable to perform pulse width modulation on the grayscale data using the triangular waveform generator 120 set to an arbitrary frequency intermediate between the frequency of the same cycle as the pixel clock 103 and the frequency of the half cycle. In FIG. 13, reference numeral 122 denotes the triangular wave signal 121 and the gradation data 10 converted into an analog signal.
8 and outputs a pulse width modulation signal having a length proportional to the gradation information of the gradation data 108.

However, when an image is formed using a triangular wave signal of an arbitrary frequency in this manner, the highest frequency component of the gradation signal,
That is, the frequency component of the image becomes the highest when the black and white dots are repeated for each pixel when the resolution of one half of the pixel clock (for example, when the resolution of one pixel is 16 dots / mm), is 16 dots / mm. Half 8 dots / mm
Is the highest frequency component) and a beat (spatially generated beat on the image) generated between an arbitrary frequency of the triangular wave signal and an undesirably unsightly striped pattern or the like. A new problem occurs.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to provide an image in which a character image and a halftone image are mixed in a state which is visually satisfactory to humans. Therefore, even when a triangular wave signal of an arbitrary frequency is used, unsightly beats can be prevented from being generated on an image, and an image in which a character image and a halftone image are mixed can be visually recognized by humans. An object of the present invention is to provide an image data conversion device that can be formed in a proper state.

[0014]

Means for Solving the Problems The technical problems described above are:
The image data converter according to claim 1 of the present invention converts input grayscale data for controlling the emission of a laser beam into an analog signal, compares the analog signal with a reference signal, and responds to the input grayscale data. An image data converter for converting a pulse width modulated signal having a pulse width
The highest frequency component of the input grayscale data and the spatial frequency of the beat generated at the greatest common divisor of the frequency of the reference signal for converting the input grayscale data into a pulse width modulation signal are spatial frequencies at which human visibility is low. This is achieved by setting the frequency of a reference signal for converting the input grayscale data into a pulse width modulation signal so as to form a region.

According to a second aspect of the present invention, there is provided an image data converter for converting input gradation data for controlling emission of a laser beam into an analog signal, and converting the analog signal to one of a plurality of reference signals. An image data converter for converting a pulse width modulation signal having a pulse width corresponding to the input gradation data into a pulse width modulation signal, wherein the highest frequency component of the input gradation data and the input gradation data are converted into a pulse width modulation signal. The frequency of all the reference signals is set so that the spatial frequency of the beat generated at the greatest common divisor with each frequency of the plurality of reference signals for conversion is a spatial frequency region where human visibility is small, This is achieved by switching and using the reference signal according to the characteristics of the input gradation data.

According to a third aspect of the present invention, there is provided the image data conversion apparatus according to the second aspect, wherein the D / A for converting the input gradation data into an analog signal is provided. This is achieved by providing a plurality of filter means for filtering the output signal of the converter with optimum characteristics in conjunction with the frequencies of the plurality of reference signals.

Here, the image data conversion apparatus according to the present invention is applied to, for example, a laser beam printer, but is of course applicable to other image forming apparatuses such as an electrophotographic digital copying machine. .

The resolution of the input gradation data is, for example, 16 dpm (dot / mm), but is not limited thereto. The resolution of the input gradation data is 24 dpm (dot / dot). / Mm), it is needless to say that the same can be applied even if it is higher or lower than the above numerical value.

Further, in the present invention, "the highest frequency component of the input grayscale data" means the resolution of the input grayscale data, that is, the half frequency of the pixel clock. That is, when the resolution of the input gradation data is, for example, 16 dpm (dot / mm), the maximum spatial frequency of the 16 dpm image is reduced to 1/2 dp by repeating black and white dots alternately. (Do
t / mm). Therefore, in this case, 8 dp
m (dot / mm) is the “highest frequency component of the input gradation data”.

In the present invention, "the greatest common divisor" means that when there are two numbers a and b (a and b are not necessarily integers), m Is a divisor, a / m and b / m must both be integers.

Now, when a is 8, the “greatest common divisor” is
The value of b is 12, 32/3, 16/3, 10, 48 /
5, 28/3, 64/7, etc., and the greatest common divisor m is 4, 8/3, 8/3, 2, 8 /
5, 8/6 and 8/7. In this case, a / m is 2, 3, 3, 4, 5, 6, 7 respectively, and b / m is
3, 4, 2, 5, 6, 7, and 8, the above relationship is established among a, b, and m. Further, in the present invention,
The spatial frequency region where human visibility is small is, for example, 1
A spatial frequency of 4 lines or more per mm corresponds to this. However, by performing appropriate signal processing, there are cases where it is possible to reduce human visual sensitivity to beats even if it is less than 4 lines per mm. Therefore, in this case, the spatial frequency range may be less than 4 lines per mm.

[0022]

In the image data converter according to the first and second aspects of the present invention, basically, the highest frequency component of the input gradation data and the input gradation data are converted into a pulse width modulation signal. Of the reference signal for converting the input grayscale data into a pulse width modulation signal so that the spatial frequency of the beat generated at the greatest common divisor of the frequency of the reference signal becomes a spatial frequency region where human visibility is small. It is configured to set a frequency. Therefore, in the image data conversion device, for example, when gradation data having a resolution of 16 dpm is input, the highest frequency component of the gradation data is 8 lpm (line / mm). If this gradation data is compared with a 12 lpm (line / mm) reference signal and the gradation data is pulse width modulated, the resolution will be higher than when using a 8 lpm (line / mm) signal as the reference signal. A copy image that satisfies both the design and the character can be obtained. Further, the spatial frequency of the beat generated at the highest common divisor of the highest frequency component (8 lpm) of the gradation data and the reference signal (12 lpm) is 4 lpm, which does not become an unsightly beat even in terms of human visibility.

According to a third aspect of the present invention, there is provided an image data conversion apparatus according to the second aspect, wherein a D / A for converting input gradation data into an analog signal is provided. It is configured to include a plurality of filter means for filtering the output signal of the converter with optimum characteristics in conjunction with the frequencies of the plurality of reference signals. For this reason, in the image data conversion device, for example, when grayscale data having a resolution of 16 dpm is input, if three types of reference signals, 16 lpm, 12 lpm, and 8 lpm, are prepared, only for characters, for pictograms, It can be used for pictures and for different modes. On the other hand, the data after the gradation data is converted into an analog signal by the D / A converter has a step-like waveform as a characteristic of the D / A converter, and contains unnecessary high-frequency components. A low-pass filter as a plurality of filter means for removing these unnecessary high-frequency components is:
By providing one having a cutoff frequency corresponding to the three types of reference signals and switching the low-pass filter according to the mode switching, it is possible to maintain a good relationship between the gradation data and the reference signal in any mode.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on the illustrated embodiment.

FIG. 2 shows an embodiment of a laser beam printer to which the image data converter according to the present invention can be applied.

In FIG. 2, reference numeral 21 denotes a photosensitive drum as an image bearing member.
Is driven to rotate at a predetermined rotational speed along a direction of an arrow by a driving unit (not shown). Around the photosensitive drum 21, a scorotron charger 22 for uniformly charging the surface of the photosensitive drum 21, a laser optical system 23, and developers of yellow, magenta, cyan, and black, respectively, were accommodated. Four color developing devices 24a, 24b, 24c, 24d and a transfer charger 2
5 and an intermediate transfer belt 26 for transferring and holding the toner images of the respective colors sequentially formed on the photosensitive drum 21 in a superimposed state are sequentially arranged along the rotation direction of the photosensitive drum 21. Around the intermediate transfer belt 26, the toner images of a predetermined number of colors superimposed on each other on the intermediate transfer belt 26 are conveyed to the position facing the intermediate transfer belt 26 at a predetermined timing. Transfer paper 2
Transfer roll 2 for batch transfer onto 0
7, a conveyance belt 28 that conveys a recording sheet 20 as a recording medium on which the toner image of a predetermined number of colors has been transferred to a fixing device 29 described below, and a recording sheet 20 conveyed by the conveyance belt 28. Fixing device 2 for fixing toner image
9 are arranged.

The scorotron charger 22 for uniformly charging the surface of the photoreceptor drum 21 has two or more discharge wires so that stable and uniform charging can be performed even when images of the second and subsequent colors are formed. It is desirable to use the scorotron charger used.

Further, the color developing units 24a, 24b,
As the components 24c and 24d, for example, a two-component developing device having a known configuration can be used. Subsequent photosensitive drum 21
In order to avoid contact with the toner image formed thereon, a two-component developer supported on a developing roll is arranged such that the ears of the two-component developer face the surface of the photosensitive drum 21 via a gap. Further, each of the color developing units 24a,
As a developing bias applied to 4b, 24c and 24d, a DC voltage (DC + AC voltage) on which an AC voltage is superimposed so as not to disturb the toner image formed on the photosensitive drum 21 and to obtain a desired high image quality. ) Is applied to adjust, for example, the AC component of the developing bias.

FIG. 3 is a schematic diagram showing a laser optical system used in the laser beam printer shown in FIG.

The laser optical system 23 includes a dual semiconductor laser array 31 driven by a laser drive circuit described later, a collimator lens 32, a polygon mirror 33, and an fθ lens 34.
The semiconductor laser array 31 is turned on / off by being modulated by a laser drive circuit described later, and emits a plurality of laser beams 35 modulated by a so-called pulse width modulation method. This semiconductor laser array 3
A plurality of laser beams 35 emitted from 1 are applied to the surface of the polygon mirror 33 via the collimator lens 32 and the polygon mirror 3 rotating at a high speed.
3 is reflected and deflected by the surface of the photosensitive drum 21 via the fθ lens 34 in the main scanning direction (photosensitive drum 21).
(Scan direction).

In the laser beam printer constructed as described above, after the surface of the photosensitive drum 21 is uniformly charged to a predetermined potential by the scorotron charger 22, the surface of the photosensitive drum 21 An image corresponding to the first color image information is scanned and exposed by the system 23 to form an electrostatic latent image. The first-color electrostatic latent image formed on the photosensitive drum 21 is developed by a first-color developing device, for example, a yellow color developing device 24a, to become a toner image. The first color yellow toner image is electrostatically transferred onto the intermediate transfer belt 26 by the charging of the transfer charger 25. Thereafter, the photoreceptor drum 21 on which the toner image of the first color is formed is subjected to the charging, exposure, and development steps again for the predetermined number of colors in the same manner as described above without going through the transfer step and the cleaning step. Photoconductor drum 21
The four toner images of yellow, magenta, cyan, and black sequentially formed thereon are sequentially transferred onto the intermediate transfer belt 26 while being superimposed on each other. Then, the four color toner images formed on the intermediate transfer belt 26 are collectively transferred onto the recording paper 20 by the transfer roll 27 which comes into contact with the intermediate transfer belt 26 via the recording paper 20 at a predetermined timing. After the transfer, the recording paper 20 is conveyed to a fixing device 29 by a conveyance belt 28. The fixing device 29 fixes the four color toner images on the recording paper 20 in a molten and mixed state, discharges the recording paper 20 out of the apparatus, and ends the color image forming process.

After the transfer process is completed, the surface of the photoreceptor drum 21 is erased by a static eliminator (not shown) to remove the residual toner charge and to remove the residual toner and the like by a cleaning device having a blade. The residual charge is further erased by the lamp, and is prepared for the next color image forming step.

In this embodiment, an apparatus for forming a full-color image using the intermediate transfer belt 26 has been described as a laser beam printer. However, the present invention is not limited to this. Forming a full-color image by sequentially transferring the toner images of the respective colors onto recording paper or an intermediate transfer medium held on an intermediate transfer drum, or a plurality of photosensitive drums, A so-called tandem-type color image forming apparatus that forms a full-color image by sequentially transferring the plurality of toner images formed in (1) to recording paper sequentially conveyed to the transfer positions of the respective photosensitive drums, etc. The good thing is, of course.

FIG. 1 is a block diagram showing an image data conversion apparatus according to an embodiment of the present invention used in a laser beam printer configured as described above.

In the image data converter according to this embodiment, the highest frequency component of the input grayscale data and the maximum frequency of any one of the reference signals for converting the input grayscale data into a pulse width modulation signal. Any one of a plurality of reference signals for converting the input grayscale data into a pulse width modulation signal so that the spatial frequency of a beat generated as a common divisor is in a spatial frequency region where human visibility is small.
One frequency is set.

In FIG. 1, reference numeral 40 denotes gradation data having a resolution of 16 dpm (dot / mm) at 8 bits input from an image reading device or a computer through an I / F circuit (not shown), and reference numeral 41 denotes this gradation data. A D / A converter having a function of decomposing the data 40 into an 8-bit quantized analog signal (256 gradations); 42, a reference frequency oscillator for outputting a 54 MHz reference signal (F ₀ ) 43;
Is a frequency synchronizing circuit for synchronizing the reference signal (F ₀ ) 43 output from the reference frequency oscillator and the SOS signal (scanning reference signal) 45 output from the laser optical system, and 46 is output from the reference frequency oscillator 42 54M
The 1/3 frequency divider 48 divides the 1 Hz reference signal (F ₀ ) 43 into a 1/3 18 MHz reference signal 47.
18 MHz reference signal 4 divided by frequency divider 46
7, a first triangular wave generator 50 for generating a triangular wave signal 49 of 16 lpm (line / mm),
A 1/2 frequency divider for further dividing the 18 MHz reference signal 47 divided by the 3 frequency divider 46 into a 1/2 9 MHz reference signal 51, and a frequency divider 52 is divided by the 1/2 frequency divider 50. 8 lpm based on the 9 MHz reference signal 51
(Line / mm) second triangular wave generator for generating a triangular wave signal 53, the reference signal (F ₀₎ of 54MHz outputted from the reference frequency oscillator 42 is 54 43 to 1 1/4 in
A 1/4 frequency divider that divides into a 3.5 MHz reference signal 55,
56 is divided by the ４ frequency divider 54.
12 lpm (lin) based on a 5 MHz reference signal 55
e / mm), a third triangular wave generator 58 for generating a triangular wave signal 57, and the first to third triangular wave generators 48,
Any of the triangular wave signals 49 output from 52 and 56,
A selector for selecting 53 and 57 based on a resolution switching signal 59; a low-pass filter 60 for removing high-frequency components from an 8-bit quantized analog signal (256 gradations) 61 output from the D / A converter 41 ,
Reference numeral 62 denotes 8-bit analog quantized data 61 output from the low-pass filter 60 and a triangular wave signal 4 output from the first to third triangular wave generators 48, 52, and 56.
9, 53, and 57 are voltage comparators that output a pulse width modulation signal 63 having a pulse width corresponding to the grayscale data by comparing the data with the pulse width modulation signal.

In the laser beam printer to which the image data conversion device according to this embodiment is applied in the above-described configuration, an image in which a character image and a halftone image are mixed in a visually satisfactory state can be obtained as follows. It can be formed. That is, in the above laser beam printer,
When an image is formed, a pulse width modulated image signal to be applied to a semiconductor laser is generated using an image data converter as follows.

In this image data converter, as shown in FIG.
The z reference signal (F ₀ ) 43 is synchronized with the SOS signal (scanning reference signal) 45 output from the laser optical system by the frequency synchronization circuit 44. Next, the 54 MHz reference signal (F ₀ ) 43 synchronized with the SOS signal (scanning reference signal) 45 by the frequency synchronization circuit 44 is 1
The 18 MHz pixel clock (16 dp)
m of the gray scale data clock) 47 and the first triangular wave generator 48 generates 16 lpm based on the 18 MHz reference signal 47 divided by the １ ／ frequency divider 46.
A (line / mm) triangular wave signal 49 is generated. On the other hand, the pixel clock 47 that has been frequency-divided to 18 MHz by the １ ／ frequency divider 46 is further frequency-divided by the 分 frequency divider 50 to a 9 MHz clock 51. 9MHz divided by 1/2 frequency divider 50
8 lpm (line / mm) based on clock 51
Is generated.

Further, 54 synchronized with the SOS signal (scanning reference signal) 45 by the frequency synchronization circuit 44
The MHz reference signal (F ₀ ) 43 is frequency-divided into a 13.5 MHz clock 55 by a ５４ frequency divider 54, and the third triangular wave generator 56 divides the frequency by the ４ frequency divider 54. 12l based on the obtained 13.5 MHz reference signal 55
A triangular wave signal 57 of pm (line / mm) is generated.

Therefore, in this embodiment, the gradation data 40
Frequency component (8 lpm) and the third triangular wave generator 5
The greatest common divisor of the frequency (12 lpm) of the triangular wave signal generated in 6 is 4 lpm.

In this embodiment, the 8-bit gradation data 40 input from an image reading device, a host computer or the like via an I / F circuit (not shown) converts the 54 MHz reference signal (F ₀ ) 43 into one. It is transmitted in synchronization with an 18 MHz 16 dpm grayscale data clock 47 divided by a ３ frequency divider 46. The gradation data 40 is converted into an analog signal 61 by a D / A converter 41 having an 8-bit quantization resolution, and the gradation data 40 converted into the analog signal 61 passes only low-frequency components. The signal is input to the voltage comparator 62 via the low-pass filter 60.

In the first triangular wave generator 48, 1 /
18 MHz 16 dp divided by 3 divider 46
The 16 lpm triangular wave signal 49 is further divided by the second triangular wave generator 52 into 1 /
9 MHz clock 5 divided by 2 divider 50
1, a triangular wave signal 53 of 8 lpm is generated. Here, the triangular wave signal of 12 lpm (line / mm) refers to a triangular wave signal for forming an image of 12 lpm. Further, in the third triangular wave generator 56, 1 /
A 12 lpm triangular wave signal 57 is generated based on the 13.5 MHz clock 55 divided by the 4 frequency divider 54. These first to third triangular wave generators 48, 5
The three types of triangular wave signals 49, 53 generated in 2, 56,
57, one triangular wave signal is a resolution switching signal 59
, And is input to the voltage comparator 62. The voltage comparator 62 outputs a pulse width modulation signal 63 having a pulse width corresponding to the gradation of the gradation data 40.

Incidentally, the above 16 lpm, 8 lpm, 1 lpm
The three types of triangular wave signals 49, 53, 57 of 2 lpm are, for example, triangular wave signal 53 of 8 lpm for a pattern composed of a halftone image such as a photograph, triangular wave signal 49 of 16 lpm for characters, and triangular wave signal 57 of 12 lpm for a photograph. By selectively using these triangular wave signals 49, 53, and 57 for picture / character mixing in which a halftone image and a character image are mixed, it is possible to obtain high-quality print / copy according to the type of image. .

That is, when the image data converter uses a triangular wave signal 53 of 8 lpm, the spatial frequency of the laser beam scanned and exposed by the generated pulse width modulation signal 63 is 8 lpm (line / m).
m), and when a halftone image such as a photographic image existing on the formed image is reproduced, the gradation characteristic appears to be continuously changed visually in an adjacent image region, and a good halftone image It can be reproduced and is suitable for pictures composed of halftone images such as photographs. When the 16 lpm triangular wave signal 49 is used, the spatial frequency of the laser beam scanned and exposed by the generated pulse width modulation signal 63 becomes 16 lpm (line / mm), and characters existing on the formed image are displayed. When an image is reproduced, a high-resolution and good character image can be reproduced, which is suitable for characters.

Further, when the image data converter uses a 12 lpm triangular wave signal 57, the spatial frequency of the laser beam scanned and exposed by the generated pulse width modulation signal 63 is 12 lpm (line / line).
mm), and when a halftone image such as a photographic image existing on the formed image is reproduced, the gradation in the adjacent image region is slightly inferior to the case where a triangular wave signal of 8 lpm is used. It looks like it changes continuously continuously, and a halftone image can be reproduced well. In addition, even when a character image existing on an image to be formed is reproduced, although it is slightly inferior to the case of using a 16 lpm triangular wave signal 49, a high-resolution and good character image can be reproduced. Any of the halftone images can be formed in a state that is visually satisfactory to humans, and is suitable for mixed use of pictures / characters in which halftone images such as photographs and character images are mixed.

However, in the above-mentioned image data converter, 1
When the 2 lpm triangular wave signal 57 is used, when the resolution of one pixel of the laser beam printer is 16 lpm, the maximum frequency (8 lpm) of the gradation signal and the reference frequency (12 lpm) are used.
pm), the highest frequency (8
1pm) and a beat of 4 lpm which is the greatest common divisor of the reference frequency (12 lpm) occurs on the image. However, as is clear from the graph showing the relationship between the relative contrast sensitivity indicating the human visibility and the spatial frequency shown in FIG. 4, the value indicating the relative contrast sensitivity of the human is 1.42, which is as large as 1.41 ( (The larger the numerical value, the lower the human visibility is.) Even when an image including a beat of 4 lpm is viewed with the human eyes, the beat of 4 lpm is not conspicuous and does not lower the image quality. Therefore, in order to form an image in which a character image and a halftone image are mixed together in a visually satisfactory state by using a triangular wave signal of 12 lpm, unsightly beats can be prevented from being generated on the image. Therefore, it is possible to form an image in which a character image and a halftone image are mixed in a state visually satisfactory to humans.

In FIG. 4, the horizontal axis is spatial frequency, which is cpd (cycle / deg). Therefore, at the distance of clear vision (250 mm), 4 lpm (line / mm)
Becomes 17.5 cpd (cycle / deg), and the sensitivity at this time is reduced to about 1/5 compared to the peak and to about 1/3 compared to 2 lpm.

5 and 6, the resolution of one pixel is 16
In the case of dpm, dot patterns are shown when using a 12 lpm triangular wave signal and when using a 10 lpm triangular wave signal. As can be seen from FIGS. 5 and 6, when an image signal having a period of 8 lpm is subjected to pulse width modulation and a triangular wave signal of 12 lpm is used, 4 lp is used.
While a repeating pattern with a period of m occurs, 1
When a triangular wave signal of 0 lpm is used, a repetition pattern having a period of 2 lpm is formed, and the repetition of 2 lpm becomes a beat. FIG. 4 is a graph showing the relationship between the relative contrast sensitivity and the spatial frequency, which represents the human visual sensitivity. As is clear from FIG. 4, the value indicating the relative contrast sensitivity of a human is as small as 0.5 (the smaller the numerical value, the higher the visibility of a human), and a visually unsightly beat is generated on the image.

As described above, in the case of a laser beam printer in which the resolution of one pixel is 16 dpm, the determination of the combination of the highest frequency (8 lpm) of the gradation signal and the reference frequency signal is shown in FIG. FIG. 8 shows the determination of the combination of the highest frequency (8 lpm) of the gradation signal and the reference frequency signal when the resolution is 24 dpm.

In FIG. 7, a mark about a beat cycle of about 2.5 is indicated by a triangle. this is,. If used as it is, the beat becomes noticeable for the reason described above, but the beat can be scattered and made inconspicuous by combining the phase shift technique of the triangular wave signal. This technique is shown in FIG. FIG. 10 shows an example of the dot pattern at this time.

FIG. 11 shows a dot pattern in the case of 10 lpm in combination with the phase shift technique of the triangular wave signal. In this case, even if the phase of the triangular wave signal is shifted, another beat is generated between the original beats of 2 lpm, which is equivalent to 4 lpm, and the beats are still inconspicuous. At 2 lpm or less, it is difficult to reduce the beat even if a phase shift is added.

Embodiment 2 FIG. 12 shows a second embodiment of the present invention. The same parts as those of the above embodiment are denoted by the same reference numerals. The D / A converter 41 is combined with one low-pass filter 60 to remove only the 16 dpm grayscale data clock component mixed with the output 61 of the D / A converter 41 and its harmonic components. In Example 2, three low-pass filters 60a, 60b, and 60c are provided, the cutoff frequencies are set to 8, 4, and 6 lpm, and the triangular wave used is 16
8 lpm for lpm, 4 lpm, 12 lp for 8 lpm
If m, the operation of the selector 58b is linked to the selector 58a so that the output of the low-pass filter having a cutoff frequency of 6 lpm is input to the comparator. By doing so, no matter which of the triangular waves 49, 53, 57 is used, the components of the respective triangular waves 49, 53, 57 that are equal to or higher than the Nyquist frequency are suppressed, so that unnecessary aliasing distortion of the image can be reduced.

The other configuration and operation are the same as those of the first embodiment, and the description thereof will be omitted.

[0054]

According to the present invention, a triangular wave signal of an arbitrary frequency is formed in order to form an image in which a character image and a halftone image are mixed in a good state visually by a human. An image data conversion device that can prevent unsightly beats from appearing on an image even when used, and can form an image in which a character image and a halftone image are mixed in a human-visually good state. Can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of image data conversion according to the present invention.

FIG. 2 is a configuration diagram showing a laser beam printer.

FIG. 3 is a configuration diagram showing a laser optical system.

FIG. 4 is a graph showing a relationship between a spatial frequency and a relative contrast sensitivity.

FIG. 5 is a timing chart showing a signal processing state;

FIG. 6 is a timing chart showing a signal processing state.

FIG. 7 is a chart showing a result of determining a triangular wave reference signal frequency.

FIG. 8 is a chart showing a result of determining a triangular wave reference signal frequency.

FIG. 9 is a timing chart showing a signal processing state.

FIG. 10 is a schematic diagram showing a formed image.

FIG. 11 is a schematic diagram showing a formed image.

FIG. 12 is a block diagram showing image data conversion according to another embodiment of the present invention.

FIG. 13 is a block diagram showing conventional image data conversion.

[Explanation of symbols]

40 gradation data, 41 D / A converter, 42 reference frequency oscillator, 43 reference signal (F ₀ ), 48 first triangular wave generator, 49 triangular wave signal, 52 second triangular wave generator, 53 triangular wave signal, 56 3 triangular wave generator, 57 triangular wave signal.

Claims (3)

(57) [Claims] 1. An apparatus for converting input gradation data for controlling emission of a laser beam into an analog signal, comparing the analog signal with a reference signal, and converting the analog signal into a pulse width modulation signal having a pulse width corresponding to the input gradation data. The spatial frequency of a beat generated at the greatest common divisor of the frequency of a reference signal for converting the input grayscale data into a pulse width modulation signal and the highest frequency component of the input grayscale data is human. An image data conversion apparatus, wherein a frequency of a reference signal for converting the input grayscale data into a pulse width modulation signal is set so as to be in a spatial frequency region where the visibility is small. 2. The method according to claim 1, wherein the input gradation data for controlling the emission of the laser beam is converted into an analog signal, and the analog signal is compared with any one of a plurality of reference signals to have a pulse width corresponding to the input gradation data. In an image data converter for converting a pulse width modulation signal, the greatest common difference between the highest frequency component of the input gradation data and the respective frequencies of a plurality of reference signals for converting the input gradation data into a pulse width modulation signal. The frequencies of all the reference signals are set so that the spatial frequency of the beat generated by the number becomes a spatial frequency region where human visibility is small, and the reference signals are switched and used according to the characteristics of the input gradation data. An image data conversion device, characterized in that: 3. An image data converter according to claim 2, wherein the output signal of the D / A converter for converting the input gradation data into an analog signal is linked to the frequency of a plurality of reference signals. An image data conversion device comprising a plurality of filter means for filtering with optimum characteristics.