JPH05161013A - Digital recorder - Google Patents

Abstract

PURPOSE:To keep high picture quality stably at all times by preventing the degradation of the picture quality caused by the change in environment (temperature and humidity), deterioration in a density detection sensor and surface deterioration of a photosensitive body, etc., in the digital recorder of the electrophotographic system such as a laser printer. CONSTITUTION:The recorder has a feedback system to a picture signal density processing system, high density processing data and low density processing data as a test pattern given from a control circuit 30 are emitted from a laser unit 1 alternately onto the photosensing body 2 to form an electrostatic latent image. The electrostatic latent image is visualized by toner of a developer 3 and the density on the photosensitive body 2 to be visualized is detected by sensor sections 4, 5. Then a maximum value circuit 7 and mean value circuits 8, 9 are used to obtain a (maximum value)/b(mean value) (or b/a) based on the maximum value (a) and the mean value (b) of the level detected and amplified by an amplifier 6 and the control circuit 30 modulates the density of the picture data being a processing object through, e.g. an LUT (lookup table) 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital recording device, and more particularly to stabilization of image quality of an electrophotographic digital recording device such as a laser beam printer, an LED printer, a liquid crystal printer or the like.

[0002]

2. Description of the Related Art In recent years, in this kind of apparatus, input image data is subjected to gradation processing to reproduce a halftone image, and C
Various image processing methods for improving image quality such as smoothing processing for improving the image quality of characters and line drawings combined with G (character generator) have been proposed.

For example, when a halftone image is reproduced by a digital printer, generally, a host device or a printer controller performs a binarization process using a dither threshold value, and then sends two kinds of signals to the printer. .. On the printer side, black dots are attached to the printing paper based on the ON / OFF signal for each dot unit, and the greater the number of the attached black dots per unit area, the higher the density, and the smaller the number, the lower the density. The halftone was expressed by the area gradation method such as expressing the density.

Further, as a high resolution printer, it is possible to obtain a high gradation image output with a high number of lines by further dividing one dot and modulating the dot width (dot diameter) according to the density. Printers have also been proposed.

[0005]

However, in a typical laser beam printer as a printer that can print by dividing one dot as described above, the electrophotographic method is used, and in the electrophotographic method, the environment is used. Since the charging characteristic and the developing characteristic react relatively sensitively to (temperature and humidity), the halftone output from the printer is susceptible to environmental changes.

In view of the above points, an object of the present invention is to prevent deterioration of image quality due to environmental changes, deterioration of a density detection sensor, surface deterioration of a photoconductor, etc., and to always maintain stable high image quality. It is to provide a possible digital recording device.

[0007]

In order to achieve the above object, the present invention provides a developing means after an electrostatic latent image having a dot structure is formed by exposing and scanning a photoconductor with a light beam according to an image signal. In a digital recording device for visualizing an electrostatic latent image on the photoconductor, a test pattern for generating a test pattern for alternately forming high-density pattern and low-density pattern electrostatic latent images on the photoconductor. Generating means, detecting means for detecting the density of the pattern on the photoconductor after the electrostatic latent images of the high density pattern and the low density pattern are developed and visualized, and the density detected by the detecting means. Calculating means for calculating the maximum value a and the average value b of the signal and calculating the value a / b (or b / a) from the values a and b;
An image data modulating means for changing the image forming condition in the image data generating process based on the value of a / b (or b / a).

Further, as one form of the present invention, the image data modulating means sets the value of the dither matrix to the a /
It can be characterized in that it is changed based on the value of b (or b / a).

Furthermore, as another aspect of the present invention, the image data modulating means can change the dot width based on the value of a / b (or b / a).

[0010]

The present invention has a feedback system to the density processing system of the image signal, and alternately irradiates a high density processing data and a low density processing data as a test pattern on the photoconductor to form an electrostatic latent image. Then, the electrostatic latent image is formed, and the density on the visualized photoreceptor is detected, and a (maximum value) / b (average value) (from the maximum value a and the average value b of the detected level) ( Alternatively, b / a) is obtained, and the image data to be processed is modulated according to the result (a / b). In this way, since the output image density detection data is obtained as a normalized numerical value, if the detection system (light emitting diode and light receiving diode) for detecting the density is contaminated with toner,
Even if there is the background density of the photoconductor itself, this normalization process can suppress the control error of the control system to a minimum and obtain a stable image.

That is, in the present invention, the output density monitor used for feedback control is obtained by measuring the amount of light reflected (or absorbed) from the surface of the photoconductor and normalizing the level of the amount of light by arithmetic processing. It is possible to prevent deterioration of the image quality due to deterioration of the sensitivity correction effect of the sensor, deterioration of the surface of the photosensitive drum, and the like, and to maintain high image quality stably.

[0012]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 shows the circuit arrangement of a digital recording apparatus (laser beam printer) according to the first embodiment of the present invention. In the figure, 1 is a laser unit having a laser diode, 2
Is a photoconductor drum, 3 is a developing device, 4 is a light emitting diode for detecting output density, 5 is a photodiode for receiving light for detecting output density, 6 is a signal amplifier, 7 is a maximum value sampling circuit, 8 is an average value circuit, Reference numeral 9 is a division circuit, 10 is a level latch circuit, and 11 is an A / D (analog / digital) conversion circuit.

Reference numeral 12 is a LUT (look-up table) for halftone processing of multi-valued image data, the output signal of which is used as a laser drive signal for the laser unit 1 described above.
Is output to. Reference numeral 13 is a latch circuit for taking in multivalued image data. Reference numeral 14 is an oscillator that generates a clock 4f having a constant period that represents the density of the printer in the main scanning direction, and is a synchronous oscillator that operates in synchronization with the host-side video clock VCLK (18). If the frequency of the clock VCLK is f, the synchronous oscillator 14 outputs 4 times the clock VCLK.
The density clock 4f having the double frequency is output. Reference numeral 15 is a main scanning counter that counts up in response to the clock 4f of the synchronous oscillator 14, and 16 is a horizontal synchronization signal HSYNC (1
It is a sub-scanning counter that counts up with the number of 9).

The LUT 12 is a table memory, which inputs multi-valued input image data and output values of the counters 15 and 16 as addresses, and dithers the multi-valued input image data by a predetermined threshold matrix to "1". The result of ",""0" is output. Reference numeral 17a is a video data input terminal for inputting multi-valued image data VIDEO (17), 18a is an input terminal for the video clock VCLK (18), 19a is an input terminal for the horizontal synchronizing signal (19), and 20a.
a is an input terminal of the vertical synchronization signal VSYNC (20), 21
Is a binary signal output from the LUT 12 which is a table memory, that is, the dithered image data VDO. Note that this dithered binary signal VDO21
Is a modulation signal for turning on / off the laser unit 1.

Next, the operation of the circuit shown in FIG. 1 will be described.

First, before starting normal printing, the LUT
A (look-up table) 12 is a control circuit (CP) such as a microprocessor, which alternately uses high-density code data and low-efficiency code data, which will be described later, as test pattern data for output density detection and feedback control.
U) 30. The LUT 12 compares the code data from the control circuit 30 with the below-mentioned threshold data programmed internally, and the comparison result is used as a laser lighting time control signal.
Supply to. The laser unit 1 irradiates the photosensitive drum 2 with a laser beam in accordance with the lighting time indicated by the signal 21, and forms a latent image on the photosensitive drum 2. The photoconductor drum 2 is previously charged by a charger. After that, the photosensitive drum 2 is rotated, and toner is attached by the developing device 3 according to the latent image, so that a visualized density pattern is formed on the surface of the drum 2.

This visualized density pattern is emitted by using the light emitting diode 4 as a light source, and the amount of reflected light is read by the photodiode 5 for receiving light. Since the output voltage of the photodiode 5 for light reception is a minute voltage of mV (millivolt) level, voltage amplification is performed by the amplifier 6. The voltage-amplified level is input to the maximum value sampling circuit 7, and the maximum value sampling circuit 7 outputs the maximum value of the level. At the same time, the output of the amplifier 6 is also input to the average value circuit 8, and the average value circuit 8 outputs the average value of the levels by the integration process.

Maximum value a obtained by the circuits 7 and 8
And the average value b are input to the division circuit 9 and a / b (or b
/ A) division processing is performed. Generally, this division circuit 9
In the case of the analog voltage system as described above, is composed of a subtraction circuit using an operational amplifier (operational amplifier) and a logarithmic amplifier. The result of division by the division circuit 9 has the same value even when the absolute value of the level changes as shown in FIG. 2, that is, even when the detection sensitivity changes due to the ambient temperature of the apparatus, dirt on the sensors 4, 5, and the like. You can see that From this fact, similarly, with respect to the background density of the photosensitive drum 2, it is possible to exclude the influence of the background density from the detection elements by performing the above-described normalization processing.

The data normalized by the division circuit 9 is temporarily stored in the level latch circuit 10 for A / D conversion. After that, with this data, the A / D conversion circuit 1
Converted to digital value by 1 (for example, 4-bit data)
This digital value is sent as a feedback signal to the LUT 12 through the control circuit 30.

The LUT 12 performs dither processing in consideration of this feedback signal to successively generate a binary video signal from a multi-valued image signal in real time. next,
The operation of the LUT 12 will be described in detail.

The 6-bit multi-valued image data VIDEO (17) sent from a host device (not shown) such as a reader or a host computer also has a constant peripheral video clock VCLK (18) sent from the host device. ), It is latched in the latch circuit 13. Latch circuit 13
The multi-valued image data 17 latched by is input as an address signal of the LUT 12, which is a table memory.

The video clock 18 also includes a synchronous oscillator 14
, And the output of the synchronous oscillator 14 counts up the main scanning counter 15. The main scanning counter 15 represents its count value by parallel binary 4 bits, and this 4-bit data is the LUT 12 which is a table memory.
The multi-valued image data 17 is input to the upper side of the input bit. The main scanning counter 15 has a horizontal synchronizing signal HSY.
It is reset by the NC (19), and operates as a pointer address that subdivides the pixel area (1 dot area of 1200 dpi).

The sub-scanning counter 16 is counted up by the horizontal synchronizing signal HSYNC (19), and the count value is represented by parallel binary 4 bits. The 4-bit data is the count value of the counter 15 of the LUT 12 which is a table memory. Input to the upper bits of the bits input by. The sub-scanning counter 16 has a vertical synchronization signal VS.
Reset at the top of the image area by YNC (20),
It operates as a pointer in the sub-scanning direction.

Next, a halftone processing method for printing with a printer in which the main scanning density is higher than the transfer rate of the input image data (provisionally, 300 dpi) will be described. 3 to 7 are views for explaining the processing contents of the LUT 12, which is a table memory that executes dither processing.

As described above, in the memory address of the LUT 12, as shown in FIG. 3, the upper 2 bits are the count value of the sub scanning counter 16, the lower 4 bits are the count value of the main scanning counter 15, and the lower 6 bits. Is multi-valued image data VIDEO. FIG. 4 shows a dither matrix in this embodiment, and each numerical value in the figure represents a threshold value for binarization. Normally, these threshold values are sequentially accessed by the count values of the sub-scanning counter 16 and the main scanning counter 15 and compared with the input multi-valued image data. In this example, as shown in FIG. The data of "1" and "0" that has been dithered by the dither matrix is stored in advance in the LUT 12 of the table memory.

Then, the count values of the sub-scanning counter 16 and the main scanning counter 15 and multi-valued data are directly input as addresses to the LUT 12 of the table memory, and the LUT 12 of the table memory binarizes "1" and "0". Only the data is retrieved. In the following description, for simplification of description, the sub-scanning counter 16 and the main-scanning counter 15 sequentially access each threshold value. Then, the sub-scanning counter 16 outputs the horizontal synchronizing signal HSY.
Each time NC is input, it operates as a pointer address in the row direction of the dither threshold matrix shown in FIG. 4, and the lower main scanning counter 15 receives the density clock 4f generated from the synchronous oscillator 14 as an input and every clock. Operates as a column-direction address pointer.

First, when a vertical synchronization signal VSYNC (20) is sent from a host device (not shown) when receiving image data, the sub-scanning counter 16 is reset, and FIG.
The "00H" line shown in is addressed.

Next, the horizontal scanning signal HSYNC (19) resets the main scanning counter 15 to "00H".
Address columns. At this time, the accessed threshold value is "34H" (see FIG. 4). Therefore,
When the value of the input multi-valued image data is "34H", the lower 6 bits of the memory address are "34H" and the upper 6 bits of the pointer address are all "0". Therefore, in order to set "1" (determined as a black dot) to the multi-valued image data VIDEO having a value of "34H" or more with "34H" in the 0th row and 0th column as the threshold value, as shown in FIG. At memory address "0034H"
After that, it is achieved by setting the output data up to the address "00FFH" to "1".

Next, the density clock 4f of the synchronous oscillator 14
When one pulse is input to the main scanning counter 15, the main scanning counter 15 counts up, and the threshold value “24” is set in the “00” row and “01” column.
Since it is "H" (see FIG. 4), when the value of the input image data is "24H", the memory address is address "0124H". Therefore, as shown in FIG.
By setting the output data from the address "0124" to the address "01FFH" to "1", all the black dots are output when the input image data is "24H" or more.

Similarly, in the sub-scanning direction, the row switching operation is performed by the horizontal synchronizing signal HSYNC, and the LUT 12
The table changes. Although the dither threshold matrix of FIG. 4 can be arbitrarily created by the above processing method, specifically, the optimum dither matrix may be selected and switched according to the printer characteristics indicated by the feedback signal.

FIG. 6 shows the timing of the density clock 4f and the image density data VIDEO (17) sent by the host device. Now, assuming that the value of the image data (17) is "1FH", and the state of dots when expressing the density of 50% by 1FH / 3FH is shown in FIG. As shown in FIG. 7, 4 rows and 16 columns, that is, 64 gradations are represented by 4 rows and 4 columns of image data, and at the same time as reproducing high-resolution and high-gradation halftones, as described above, The output level of the amplifier 6 changes because the light-emitting diode 4 for density detection and the photodiode 5 for light reception are contaminated with toner and the like due to changes in the sensitivity characteristics of the photoconductor drum due to changes in conditions and usage. Even in that case, since the feedback process is performed to adjust the dither value according to the ratio of the specified high density level and the low density level (that is, the normalization of the measurement value) after the detection, stable output of high image quality is possible. Is possible.

Although the density data is fed back to the LUT 12 in the embodiment of FIG. 1, a table RAM for performing γ correction before the halftone processing (generally, data is directly intervened in the incoming data). The same effect can be obtained by changing or switching the contents of (converting) according to the density data.

(Other Embodiments) FIG. 8 shows the configuration of the second embodiment of the present invention. In FIG. 8, 50 is the incoming VDO
It is a selector for selecting under control of the control circuit 30 whether to send a signal to the laser side or to send a low-density pattern input from the control circuit 30 for density measurement to the laser side. Reference numeral 51 is a selector using a plurality of delay gates 52 and having a plurality of delay inputs.
The delay amount can be freely switched by the signal from.
53 is an AND circuit. 54 is a constant current circuit,
The switching drive current of the laser unit 1 is determined via the switching circuit 55. The same parts as those of the circuit of FIG. 1 used in the first embodiment of the present invention described above are designated by the same reference numerals and the description thereof will be omitted.

First, the control circuit 30 is switched to select the signal of the control circuit for the density measurement from the control circuit 30, and the pattern signal is sent from the control circuit 30. This pattern signal (binary signal) has a current value determined by the constant current circuit 54 via the AND circuit 53, and drives the laser unit 1 via the switching circuit 55 by this current value. As a result, an electrostatic latent image is formed on the photoconductor drum 2 as described in the first embodiment, and then the electrostatic latent image is visualized by toner adhesion by the developing device 3, and then the reflection type sensor unit. By the (light emitting diode 4 and the light receiving photodiode 5), a voltage signal is obtained as the amount of reflected light according to the toner image on the photosensitive drum 2. Subsequent processing is the same as that described in the first embodiment, and will be omitted.

Therefore, the density value obtained as a result is input to the control circuit 30 as the output of the A / D conversion circuit 11, and the selector 51 and the constant current circuit 5 are supplied from the control circuit 30.
An instruction signal as a modulation signal of the pulse width and the laser light amount according to the density value is sent to 4. As can be seen from the above configuration, the normalized density data obtained by the control circuit 30 indicates and selects one of the delay elements input to the selector 51, and the output of the selector 51 is the AND gate 5
Since the signal is input to one end of 3, a modulation signal having a pulse width is obtained at the output side of the AND gate 53. Further, since the constant current set value for driving the laser unit 1 is switched according to the density data, it is possible to select and realize any desired characteristic.

The present invention is not limited to laser beam printers, but can be applied to other electrophotographic digital recording devices such as LED printers and liquid crystal printers.

[0038]

As described above, according to the present invention,
Since the density data is normalized to perform feedback control of the image signal, a high-quality image output can be output stably regardless of environmental changes, and a sensor that detects the density (reflected light amount) There is an effect that it is possible to make maintenance-free by minimizing the influence of dirt and the like.

[Brief description of drawings]

FIG. 1 is a block diagram showing a circuit configuration of a density feedback processing system of a digital recording apparatus (laser beam printer) according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating the principle of density data normalization processing according to the present invention.

FIG. 3 is a diagram showing an example of a memory address format of a LUT 12 of FIG.

FIG. 4 is a diagram showing an example of a dither matrix executed by the LUT 12 of FIG.

5 is a diagram showing an example of data contents corresponding to FIG. 4 in the table memory of the LUT 12 of FIG.

FIG. 6 is a diagram illustrating density clock 4f and image density data (V
3 is a timing chart showing the timing with (IDEO).

FIG. 7 shows that the value of image data is “1FH” and 1FH /
It is a figure which shows the mode of a dot when expressing the density of 50% by 3FH.

FIG. 8 is a block diagram showing a circuit configuration of a digital recording apparatus according to a second embodiment of the present invention.

[Explanation of symbols]

 1 laser unit 2 photoconductor drum 3 developing device 4 light emitting diode 5 light receiving photodiode 6 signal amplifier 7 maximum value sampling circuit 8 average value circuit 9 division circuit 10 level latch circuit 11 A / D conversion circuit 12 LUT (look-up table) ) 13 latch circuit 14 synchronous oscillator 15 main scanning counter 16 sub-scanning counter 50, 51 selector 52 delay gate 53 AND circuit 54 constant current circuit 55 switching circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical display location G03G 15/00 303 15/04 116 9122-2H

Claims (3)

[Claims] 1. A digital image forming apparatus that develops an electrostatic latent image on the photosensitive member by developing means after exposing and scanning the photosensitive member with a light beam in accordance with an image signal to form an electrostatic latent image having a dot structure. In the recording device, a test pattern generating means for generating a test pattern for alternately forming an electrostatic latent image of a high density pattern and a low density pattern on the photoconductor, and an electrostatic charge of the high density pattern and the low density pattern. After the latent image is developed and visualized, detection means for detecting the density of the pattern on the photoconductor, and maximum value a and average value b of the density signal detected by the detection means are obtained. From the value of b, a / b (or b /
It is characterized by comprising an arithmetic means for arithmetically operating the value of a) and an image data modulation means for changing the image forming condition in the image data generation processing based on the value of a / b (or b / a). Digital recording device. 2. The digital recording apparatus according to claim 1, wherein the image data modulation means changes the value of the dither matrix based on the value of the a / b (or b / a). 3. The digital recording apparatus according to claim 1, wherein the image data modulation means changes the dot width based on the value of the a / b (or b / a).