Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-10-28
2002-10-29
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S250000
Reexamination Certificate
active
06473110
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image recording apparatus which records and reproduces an image by performing a scan on a photoreceptor with a light beam which is modulated based on an image signal. More specifically, the present invention relates to an image recording apparatus which simultaneously records a plurality of line images by scanning and exposing the photoreceptor with a plurality of laser beams.
2. Description of the Background Art
In the market of image recording apparatuses such as a laser printer having a laser recording apparatus or a digital copying machine, the conventional machines which have been introduced into the market range from the low-cost and low-speed machines to the high-cost and high-speed machines. In the market of image recording apparatuses operating at high recording speed, all high-level functions which are not only high-speed output but high image quality are required relative to all specifications.
Regarding the laser recording apparatus, the high-speed output could be achieved by increasing the number of revolutions of a photoreceptor and enhancing the scanning speed of a laser scanning device so as to record an image on the photoreceptor rotating at high speed.
In order to enhance the scanning speed of the laser scanning device, a rotational polygon mirror should be rotated at high speed for scanning with the laser beam in a predetermined direction (main scanning direction).
However, if the method of enhancing the speed of rotational polygon mirror is employed, a motor for the rotational polygon mirror inevitably increases in size, so that a problem of radiating heat from the motor arises. At present, the size of the entire apparatus and the space where the apparatus is to be installed should be considered, and the problems such as the noise due to enhanced speed and increase in weight of the apparatus have arisen. Therefore, the extent of enlargement of the motor is limited.
In the high speed laser recording apparatuses of today, the enhancement of the speed is achieved by not only speeding up the rotation of the rotational polygon mirror but simultaneously recording a plurality of line images by one scanning operation with a plurality of laser beams.
However, when a plurality of laser beams are used to simultaneously record a plurality of line images, the points at which a scan is started on the plurality of line images on the photoreceptor could deviate from each other (jitter) in the main scanning direction. This is due to displacement of positions where a plurality of laser sources are arranged in parallel with each other.
A method is now known for recording and reproducing an image with high resolution by inclining a plurality of laser sources and arranging them in parallel with each other, so as to decrease the pitch of the line images in the sub-scanning direction and accordingly achieve continuous gradations. In this case, the plurality of inclined laser sources start scan at points deviated from each other in the main-scanning direction.
When the positions where the exposure and scan are started by the laser beams are thus slightly displaced on the photoreceptor (the scan starting points are not aligned in the main-scanning direction), it is impossible to achieve fine gradations of an image which is recorded and reproduced.
A method is accordingly devised for aligning the scan starting points in the sub-scanning direction on the photoreceptor, with which each laser beam is detected before starting the exposure and scan of the photoreceptor, and after a predetermined time set for each laser source has passed (after counting reference clock a predetermined number of times), each laser source is driven (turned on) with modulation based on image data.
In order to implement this method, conventionally a plurality of image clock signals having different phases are generated in advance, and an image clock signal which satisfies a predetermined condition is determined when each laser beam is detected before starting exposure and scan of the photoreceptor. The laser source is then driven with modulation by the determined image clock signal.
The structure mentioned above is disclosed in U.S. Pat. No. 4,393,387 and Japanese Patent Laying-Open Nos. 60-153259 and 9-1859.
U.S. Pat. No. 4,393,387 discloses a recording apparatus capable of correctly matching image writing starting positions with a simple structure by inclining a plurality of laser beams and synchronizing a plurality of clock signals that correspond to respective beams based on the result of detection by a beam detector.
Specifically, a plurality of laser beams are inclined, and one position signal is generated based on the result of detection by the detector which detects a particular beam which is turned on. Based on the position signal, a plurality of clock signals having the same period and different phases are produced. The plurality of clock signals are applied to a plurality of memories which store recording signals, and recording signals corresponding to respective beams are read out.
Japanese Patent Laying-Open No. 60-153259 discloses a synchronizing device of a laser printer capable of decreasing the jitter by providing a clock delay unit which successively delays reference clocks with a constant phase and selecting one of thus produced clocks as an image clock which maintains a constant relation with an output of a photodetector.
Specifically, when laser beam reaches the photodetector, a photo-detection output pulse HS is output. The photo-detection output pulse HS is supplied to a latch circuit, and a combination of a plurality of clocks T
0
and T
1
is supplied to a data input of a data selector. After clocks T
0
and T
1
are respectively delayed by ¼ period to generate pulses T
2
and T
3
, any one of pulses T
0
to T
3
is selected. Pulse data supplied to select inputs SA and SB of the data selector are synchronized by the photo-detection output pulse HS. Therefore, the pulse data selected by the data selector and output from an output terminal Y is substantially synchronous with pulse HS. The output is accordingly used as an image clock so as to implement image recording with reduced jitter.
Japanese Patent Laying-Open No. 9-1859 discloses an image forming apparatus capable of synchronizing a synchronizing signal with an image clock precisely without using a high-frequency reference clock and without using a number of clocks having different phases, by utilizing an analog periodic signal.
Specifically, a synchronizing signal indicating a starting timing of image formation is generated for each line in the main-scanning direction. According to the timing at which the synchronizing signal is generated, the voltage value of the analog periodic signal with periodically changing voltage value is sampled and held. Thus, the timing at which the synchronizing signal is generated relative to a reference clock signal is converted to the voltage value to be stored. Comparison between the analog periodic signal and the stored voltage value allows a pixel clock to be produced with a phase corresponding to the timing at which the synchronizing signal is generated.
According to the methods disclosed in U.S. Pat. No. 4,393,387 and Japanese Patent Laying-Open No. 60-153259, however, a plurality of image clock signals having different phases respectively should be generated in advance. In order to minimize the displacement in the recording starting position in the main-scanning direction, the phases of the image clock signals should be displaced finely from each other so as to generate a large number of image clock signals. As a result, the number of delay circuits for displacing the phases increases, leading to a complicated circuit structure.
Consequently, the clock frequency of a plurality of image clock signals increases, making it difficult to implement the circuit, or may cause problems such as high-frequency noise and increase in cost due to employment of a high-speed element.
Further, the delay circuits
Birch & Stewart Kolasch & Birch, LLP
Pham Hai
Sharp Kabushiki Kaisha
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