GB2264024A - Image forming apparatus - Google Patents

Description

IMAGE FORMING APPARATUS Z1264024 The present invention relates to an image

forming apparatus of the type forming a halftone image by multilevel image data, e.g., a laser beam printer, laser copier or laser facsimile transceiver.

A laser beam printer using an electrophotographic system is attracting increasing attention as a high-speed and lownoise printer. Since a laser beam printer is mainly used to form characters, lines, figures and other bilevel images and usually does not process hafitone images, the structure of the printer and an image processing circuit incorporated therein are simple. Even with such a bilevel printer, it is possible to form a halftone image if use is made of a dither method or a density pattern method. However, a bilevel printer using the dither method or the density pattern method cannot form a halftone image in high resolution. In light of this, a printer capable of implementing a high resolution halftone image by use of a bilevel recording system has been recently reported. This printer drives a laser by the pulse width modulation (PWM) of an image signal and is desirably applicable to, among others, color images. There has also been reported a method which forms a halftone image by driving a laser by thepower modulation (PM) of an image signal. Specifically, in the PM scheme, the-intensity of a laser beam is changed in matching relation to a tone to determine whether or not to lower the potential distribution of a latent image in the depthwise direction, i.e., the potential on a photoconductive element. As a result, the amount of toner to be deposited on the photoconductive element is changed to provide each pixel a particular density change, i.e., a particular tone.

Image forming apparatuses using the PWM scheme are disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 192966/1990, 308473/1988, and 49781/1987.

However, all of such prior art apparatuses have some problems

1 5 left unsolved.

is, therefore, an nim it of the present invention to provide an image forming apparatus capable of forming a high quality circuit image at high speed with an inexpensive and simple arrangement.

In accordance with the present invention, an image forming apparatus for forming an image by modulating a light beam by PWM in matching relation to a tone of image data has 2-5 first timing generating means for generating a plurality of first timing signals in synchronism with timings for writing image data. The first timing signals each define a timing for starting modulating a pulse width. Second timing generating means generates a plurality of second timing signals in synchronism with the timings for writing image data. The second timing signals each define a timing for ending modulating the pulse width. control means selects one of the first and second timing signals matching the tone of image data. PWM signal generating means generates a modulating signal having the pulse widths of the first or second timing signal selected by the control means.

The Features and advantages of 1 5 the present invention will become niore appargmt from the following detailed description of an examplary embodiment when taken with the accompanying drawings in which:

FIG. I is a block diagram schematically showing an image forming apparatus embodying the present invention; FIG.2 is a block diagram schematically showing first and second timing signal generating means in detail; FIG. 3 is a timing chart representative of a relation between a first and a second timing signal and a pulse width; FIG. 4 is a timing chart similar to FIG. 3, showing a case wherein a modulation start timing is fixed; FIG. 5 is a timing chart similar to FIG. 3, showing a case wherein a modulation end timing is fixed; FIG. 6 is a timing chart similar to FIG. 3, showing a case wherein the modulation start timing is fixed to form a narrow line image; FIG. 7 is a timing chart similar to FIG. 6, showing a case wherein the modulation end timing is fixed for the same purpose; FIG. 8 shows pixels formed by PWM; FIG. 9 shows pixels of different colors color image in combination; FIG. 10 is a block diagram schematically showing part of an alternative embodiment of the present invention; FIG. 11 is a block diagram showing a specific construction of beam control means included in the alternative embodiment; FIG. 12 is a timing chart representative of the combination of PWM and PW; FIGS 13A-13D show threshold data stored in ROMs included in the beam control means; FIGS. 14A-14D show specific output patterns of digital comparators also included in the beam control means; FIG. 15 shows a specific drive level pattern for a semiconductor laser also included in the beam control means; constituting a FIGS. 16A and 16B show a relation between energy distributions of the semiconductor laser and pixels recorded thereby; - FIGS. 17A-17D show other specific drive level patterns for the semiconductor laser; FIG. 18 is a block diagram schematically showing part of a conventional image forming apparatus; FIG. 19 is a timing chart indicative of essential signals appearing in the apparatus of FIG. 18; 1 0 FIGS. 20A and 20B show triangular waves appearing in the conventional apparatus; FIG. 21 is a graph plotting the pulse width characteristic of a PWM signal relative to an input image particular to the conventional apparatus; FIG. 22 is a graph plotting an image density characteristic relative to the pulse width of the PNYM signal also particular to the conventional apparatus; and FIG. 23 is a graph plotting the pulse width of the PWM signal relative to an input image.

characteristic A PWM circuit for a conventional image forming apparatus of the type forming a halftone image by the PWM of image signals is disclosed in Japanese Patent Laid-Open No.

2 5 19296611990and will be described with reference to FIGS. 1 8 and 19. As shown, 8-bit parallel image signals ofTTL (Transistor-Transistor Logic) level a are latched by a TTL latch 101 and then converted to an emitter coupling logic (ECL) level by a level converter 102. The image signals of ECL level are transformed to an analog signal b by an ECL digital-to-analog converter (DAQ 103. The analog signal b is applied to one input of an ECL comparator 104. A clock oscillator (OSC) 106 generates a clock signal having a frequency 2f. A triangular wave generator 107 crenerates a substantially ideal triangular wave (pattern signal) having a frequency f in synchronism with the clock signal from the OSC 106. The triangular wave is fed to the other input of the ECL comparator 104. A 1/2 frequency divider 108 halves the frequency 2f of the clock signal to feed the resulting pixel clock signal having a frequency f and a duty ratio of 50 % to the TTL latch 101. In response to the pixel clock signal, the TTL latch 101 latches the parallel image signals a. Specifically, as shown in FIG. 19, the periods of the pixel clock signal and triangular signal are identical with the period of the pixels (image signals a). The ECL comparator 104 outputs a PWM signal of ECL level corresponding to a difference between the analog signal b from the DAC 103 and the triangular signal from the triangular wave generator 107. A level converter 105 converts the ECL level of the PWM signal to the TTL level. A laser driver 109 causes a laser diode to flash in matching relation to the pulse width of the PWM signal. As a result, a halftone latent image is electrostatically formed on a photoconductive element.

However, the problem with the conventional circuitry described above is that since the image forming speed depends on the frequency f, the former cannot be increased unless the latter is increased. Assuming that the frequency f is 5 MHz, the period of time assigned to a single pixel is less than 200 nsec.

Hence, it is difficult to generate an ideal triangular signal such as shown in FIG. 20A at a short period. The actual triangular signal having a short period is noticeably distorted, as shown in 0 FIG. 20B. Specifically, the accuracy of a pulse width depends on how close a reference wave is to an ideal wave. Generally, the higher the repetition frequency of the reference wave, the greater the distortion of the wave and the scattering among While higher speed and more accurate elements may be used to eliminate such distortions, they will increase the cost of the circuit. The most critical problem is that once the reference wave is distorted, no adjustment measures are available even if the circuit itself is accurate.

When an ideal pattern signal (reference signal) is used, the pulse width of the PWM signal associated with the input image signals a changes linearly, as indicated by a dashed line in FIG. 2 1. However, when use is made of a distorted pattern signal, the pulse width does not change linearly, as indicated by a solid curve in FIG. 21. Further, in an image forming apparatus 1 5 machinesbecome using an electrophotographic process, image density does not change linearly relative to the pulse width of the PWM signal, as shown in FIG. 22. Therefore, to correct this characteristic, it is necessary to provide the width of the PWM signal associated with the image signals with a particular characteristic shown in FIG. 23. In addition, the width of the PWM signal has to be changed when the electrostatic process conditions change due to aging, changes in ambient temperature, etc.

Japanese Patent Laid-Open Publication No. 4978111987 proposes to change the period of a pattern signal in matching relation to a character image or a photographic image without changing the amplitude and bias. However, when the image forming speed is increased, a pattern signal having a desired 0 1 5 waveform cannot be generated, as stated earlier.

Another conventional image forming apparatus i S disclosed in Japanese Patent Laid-Open Publication No. 30847311988 and elaborated to change the start points (or end points) of pulses of a continuous PWM signal depending on the 2 0 kind of an image, e.g., a text image or a photographic or similar graphic image. This, however, needs a plurality of different kinds of reference waves and, therefore, scales up the circuit.

Referring to FIG. 1, an image forming apparatus embodying the present invention will be described. As shown, the apparatus has first and second timing signal generating means 1 and 2, respectively. The timing signal generating means 1 and 2 generate respectively a first and a second timingsignal synchronous- to a pixel-by-pixel write timing signal in response to a control signal fed from image data control means 5. The two timing signals are applied to PWM signal generating means 3. The control signal from the control means 5 matches the tonality of the input image data, i.e., tone data including a pulse signal start command.

As shown in FIG. 2, the timing signal generating means 1 and 2 have delaying means 10 for delaying respectively the write timing signal by a plurality of time units tl, t2, t3 and so on and a plurality of time units t'l, t2, t3 and so on, and data selecting means 11 for selecting one of the timing signals delayed by the delaying means 10 in response to a control signal from the ima.e data control means 5. As a result, as shown in 0 FIG. 3, the first and second timing signals are produced each being delayed by predetermined periods of time from the write timing signal in association with the image data (tones). As also shown in FIG. 3, the PWM signal generating means 3 delivers to beam control means 4 a pulse signal in the form of pulses which go high in synchronism with the first timing signal and go low in synchronism with the second timing signal ( t'l - tl, t '2 - t2, t'3 - t3 and so forth). The beam control means 4 produces a light beam having been modulated by such a pulse signal to thereby form an electrostatic latent image matching the image data on a photoconductive element, not shown. The latent image is developed by a toner. Pulse width correcting means 6 and image forming condition detecting means 7 will be described specifically later.

One of the first and second timing signals may be fixed, a s follows. When an image is to be formed by the PWM system, the density, resolution and quality of the image noticeably changes depending on the time when the modulation begins in the period of one pixel and on the pulse width. Regarding a graphic image whose density varies over the entire area, it is preferable that the output image has constant periodicity. In this case, as shown in FIG. 4 or 5, if the data selecting means 11 selects the first timing signal such that tl, t2, t3 and so forth are equal or selects the second timing signal such that t'l, t2, t'3 and so 1 5 forth are equal, the timing for starting or ending the modulation will remain constant.

Further, one of the first and second timing signals may be fixed in matching relation to the kind of an image, as follows.

Fixing one of the two timing signals renders the density of the output image uniform and is, therefore, desirable for - graphic images, as stated above. However, should such an implementation be applied to a document mainly constituting of characters, small characters would not be fully filled up and, in the worst case, would cause conspicuous stripes to appear therein, as shown in FIG. 6. Regarding dense andbold -1 1characters, the stripes are not conspicuous since the pulse width increases in proportion to the density. However, since the boundaries between the characters and the paper or background are read as the average density of the characters, stripes conspicuously appear around the characters, as also shown in FIG. 6.

The stripes described above can be eliminated if the modulation start or end timing is fixed at the higher density side, i.e., if the modulating pulses concentrate at the center of a character, as shown in FIG. 7. Specifically, FIG. 7 indicates a condition wherein the second timing signal having the pulse width of a pixel I and the first timing signal having the pulse 1 5 width of the next pixel II are fixed.

As shown in FIG. 8, when an image is outputted in multiple levels by the PWM system, a single pixel is constituted by a single frame; a density is represented by hatching. In FIG. 8, it is assumed that all the pixels have the same density, and that the modulation start and end timings are identical throughout the pixels. In this case, therefore, the pixels extend inthe subscanning direction (in which the image carrier moves) perpendicular to the main scanning direction (in which the beam moves).

However, it is a common practice with a color image forming apparatus to form a color image by superposing a yellow, magenta and cyan toner images and, if necessary, a black toner image. Usually, these toner images are not formed at the same time, but they are formed and superposed one after another. This is apt to bring the toner images out of register, resulting in irregular colors. To eliminate this problem, as shown in FIG. 9, the illustrative embodiment changes the modulation start timing (i.e. modulation end timing) while maintaining the pulse width constant. This is equivalent to providing each color with a screen angle as in the plate making art and, therefore, successful in preventing irregularities in 1 0 color and moire from occurring due to misregistration.

Regarding an image forming apparatus using an electrophotographic process, the image quality, particularly the linearity of density which is the gamma characteristic of an image, slightly differs from one apparatus to another due to irregularities in the sensitivity characteristic of a photoconducive element, gamma characteristic of development, and shape and power of a light beam, as shown in FIG. 22. In addition, the image quality changes due to aging, ambient conditions, and so forth. To eliminate the scattering among the apparatuses and to reduce changes in image quality against aging and varying environment, it is necessary to maintain adequate process conditions and to select and set the pulse width of the PWM signal for each image data adequately, as shown in FIG. 23. To meet this requirement, the embodiment sets a correction value matching an irregularity particular to the apparatus via the pulse width correcting means 6, as shown in FIG. 1. This allows an adequate pulse width to be selected and set to- thereby compensate for the scattering among the apparatuses. Specifically, the pulse width can be finely adjusted if an adjustable period of time At is added to the generation timing of, for example, the first timing signal.

To prevent the image quality from being effected by aging :0 and environment, as shown in FIG. 1, the image forming c) condition detecting means 7 detects an image forming condition corresponding to a process condition at regular or irregular intervals and automatically or in response to a command from the outside. Specifically, the detecting means 7 delivers a signal representative of the instantaneous image forming 0 condition on the photoconductive element to the pulse width correcting means 6. In response, the correcting means 6 calculates a correction amount of the pulse for each image data and feeds it to the image data control means 5. Then, the control means 5 determines a first and a second timing signal matching the correction amount, thereby controlling the data selecting means, FIG. 2.

The image forming condition may be detected in terms of at least one of the charge potential of the photoconductive element and the surface potential of the element undergone exposure, amount of toner deposition, image density, etc. if desired, such image forming conditions may becombined although the control method will be complicated. Further, temperature and humidity may be detected, or the number of times that an image is formed may be memorized to detect various kinds of image forming conditions.

The embodiment, therefore, enhances the accuracy, reproducibility and stability of pulse width, compared to the conventional apparatus using a single reference wave. Since the embodiment is capable of freely controlling the pulse width and the position for generating a pulse in response to the write timing signal, it allows the image quality to be designed with greater freedom. Further, the embodiment can freelycorrect changes in image forming conditions due to aging and varying environment by manipulating the pulse width. In addition, such correction can be done without effecting the parameters of an image processing circuit which will precede the embodiment.

Referring to FIGS. 10-17D, an alternative embodiment of the present invention will be described which modulates both of pulse width and power. As shown in FIG. 10, the embodiment is essentially similar to the previous embodiment except for beam control means 4'. As shown in FIG. 11, the beam control means 4' generates a light beam on the basis of a level matching a pulse signal SG1 from the PWM signal generating means 3 and 0 tone data S 1. The beam control means 4' is generally made up of a tone processing circuit 250, and a laser driver 260 for modulating a semiconductor laser 43 by combining PWM and PM, as shown in FIG. 12. In FIG. 12 the levels of the 1 a s e r modulating signal indicated by dashed lines and the levels of the same indicated by solid lines are respectively representative of PWM and the combination of WPM and PM.

The tone processing circuit 250 has ROMs (Read Only Memories) 251, 252, 253 and 254 and digital comparators 255, 256, 257 and 258. The ROMs 251, 252, 253 and 254 store respectively threshold data of laser power shown in FIGS. 13A, 13B, 13C and 13D. The image signal S1 having eight bits is 1 0 commonly applied to the inputs A of the digital comparators 255-258. Threshold data SDa, SDb, SDc and SDd read out of the ROMS 251-254 are respectively applied to the other inputs B of the comparators 255-258. In the illustrative embodiment, the address terminal of each of the ROMS 251-254 has four bits.

Lower two bits SSx of a scanning address in the main scanning direction are applied to two of the four bits of each address terminal while lower two bits SSy of a scanning address in the subscanning direction are applied to the other two bits.

Therefore, the ROMs 251-254 each outputs data corresponding 2 0 to the instantaneous scanning position.

As shown in GIGS.

stores sixteen (= 4 x 4) hexadecimal numbers in the each matrix in the main 2 5 designated by the bits SSx 13A-13D5 the ROMs 251-254 each different 8-bit data (represented by figures). The addresses x and y of and subscanning directions are and SSy, respectively. Specifically, each 4 x 4 assigns a particular threshold value to each pixel position in a small area covering a plurality of pixels.

Generally, _such a matrix is used to execute dither processing.

Further, all the matrices shown in the figures are different with respect to the data. Specifically, at corresponding pixel positions, the threshold of the ROM 252 is greater than that o f the ROM 251 by four, the threshold of the ROM 253 is greater than that of the ROM 252 by four, and the threshold of the ROM 254 is greater than that of the ROM 253 by four.

Therefore, at each scanning position, the digital comparators 255-258 compare the image signal S1 with the respective threshold values SDa-SDd. When the image signal S 1 is greater than the threshold values SDa-SDd (A > B), the comparators 255-258 produce respectively high level signals Soa, Sob, Soc and Sod, as shown in FIGS. 14A-14D by way of example. In FIGS. 14A-14D, all the pixels of the image signal S1 are assumed to have a density level 59; ONEs and ZEROs indicate black and white, respectively.

NAND gates 262, 263, 264 and 264 are included in the laser driver 260 and gate respectively the output signals Soa-Sod of the comparators 255-258 on the basis of the pulse signal SG1 having the previously mentioned width. The output 0 signals of the NAND gates 262-265 are applied to the bases of PNP transistors Q1, Q2, Q3 and Q4, respectively. A constant voltage power source 261 applies a constant voltage to the - 1 7- emitters of the transistors Ql-Q4 via associated resistors Ra, Rb, Rc and Rd. Also, the power source 261 is connected to the collectors of the transistors Ql-Q4 via a resistor Ro and connected to the semiconductor laser 43. In this configuration, as the output signals Soa-Sod of the comparators 255-258 go high, the associated transistors Ql-Q4 turn on to cause their emitter currents to flow to the collectors. The emitter currents are added to the corresponding output signals Soa-Sod of the comparators 255-258. Assuming that the output signals Soa-Sod shown in FIGS. 14A-14D appear, the resulting sums shown in FIG. 15 are applied to the laser 43 to modulate the output power of the laser 43.

In the specific circuit shown in FIG. 11, the constant voltage from the constant voltage power source 261 is applied 1 5 to the laser 43 as a bias voltage Vo. Assume that the emitter resistances Ra-Rd have the same value r, that the ON resistance of the transistors Ql-Q4, the voltage between the terminals of the laser 43, and the bias current to the laser 43 are neglected, and that the number of transistors Q1-Q4 turned on is n. Then, a current I to be fed to the laser 43 is expressed as:

I=n- Vo / r It follows that the output power of the laser 43 is modulated in five consecutive steps ("0" to W) on the basis of the 4-bit signals Soa-Sod from the tone processing circuit 250. Since 1 -1 8- the laser power is not linear relative to the drive. current I, it is caused to become linear by the bias resistor Ro.

FIG. -16A shows energy distributions of the laser beam at consecutive drive levels L1, L2, L3 and L4 while FIG. 16B shows pixel sizes AR1, AR2, AR3 and AR4 defined by the laser beam at the drive levels Ll-L4, respectively. In FIG. 16B, the rectanaular area ARP is representative of a theoretical recording area to which a single pixel is allocated. As far as the electrophotographic system is concerned, the toner density of a single dot is highest at the center of the pixel and sequentially decreases toward the periphery.

FIGS. 17A-17D pertain to the density level Ll of the input image S1 and indicate respectively the numbers of transistors Ql-Q4 to turn on at each pixel position, i.e., density levels and 1 5 the mean densities Lp of the 4 x 4 image area when the density level L is 5D, 61, 65 and 69. As these figures indicate, a change in the density level Ll directly translates into a change in the mean density Lp. Therefore, by PWM and PM, it is possible to generate more different kinds of beam energy than in the case with PWM only. This, coupled with the fact that the position forming a dot can be freely defined, allows high quality images to be formed at high speed by an inexpensive and simple circuit arrangement.

In the circuitry shown in FIG. 11, only seventeen tones are available with PM only since dither processing is executed on the basis of a matrix of 4 x 4 pixels. By contrast, in FIGS.

17A-17D, sixty-five tones (= 256/4+1) can be rendered in total since four - different conversions of the input data of 256 consecutive steps influence the density.

If desired, the emitter resistors Ra-Rd may each be provided with a particular resistance, and even the 4 x 4 matrix may be replaced with another.

In summary, it will be seen that the present invention provides an image forming apparatus capable of forming high 1 0 quality images with no regard to the waveform of a reference wave which has heretofore been an issue. Processing can be executed by an inexpensive and simple circuit arrangement.

Since a graphic image whose density varies over the entire area is outputted with certain periodicity, not only such an image 1 5 but also a text image achieve high quality. An effect equivalent to providing a screen angle to each color in the plate making art is achievable to eliminate irregular colors and moire ascribable to the dislocation of pixels. A high quality image can be formed by inexpensive and simple circuitry with no regard to the scattering among machines, aging, or varying environment.

Moreover, as PWM and PM are combined to modulate a light beam, more different kinds of beam energy than in the case with PWM only can be formed. In addition, since the position for forming a dot can be freely defined, high quality image is achievable with inexpensive and simple circuitry.

1 1

Claims (8)

1. An image forming apparatus for forming -'an image by modulating a light beam by pulse width modulation (PWM) in matching relation to a tone of image data, said apparatus comprising:

first timing generating means for generating a plurality of first timing signals in synchronism with timings for writing image data, said plurality of first timing signals each defining a timing for starting modulating a pulse width; second timing generating means for generating a plurality of second timing signals in synchronism with the timings for writing image data, said plurality of second timing signals each defining a timing for ending modulating the pulse width; control means for selecting one of said first and second timing signals matching a tone of image data; and is PWM signal generating means for generating a modulating signal having the pulse widths of said first or second timing signal selected by said control means.

2. An apparatus as claimed in claim 1, wherein said control means constantly fixes and selects one of said first and second timing signals with no regard to the tone of image data while selecting the other timing signal on the basis of the tone of image data.

3. An apparatus as claimed in claim 1 or 2, wherein said control means fixes one of said first and second timing signals matching the kind of image data.

4. An apparatus as claimed in claim 1, 2 or 3, wherein said controls means selects, when a color image is to be formed, said first timing signal color by color to change the timing for starting modulating the pulse width.

5. An apparatus as claimed in claim 1, 2, 3 or 4 wherein said control means selects one of said first and second timing signals matching the tone of image data and an irregularity particular to said apparatus.

6. An apparatus as claimed in any one of claims 1, to 5 wherein said control means selects one of said first and second timing signals matching the tone of image data and an electrophotographic image forming condition.

7. An apparatus as claimed in any one of claims 1 to 6 further comprising power modulating signal generating means for generating a modulating signal which modulates an intensity of the light beam at a level matching the tone of image data.

8. An image forming apparatus constructed and arranged to operate substantially as herein before described with reference to, and as illustrated in, the accompanying drawings.