Clock independent pulse width modulation

Details Clock independent pulse width modulation Clock independent pulse width modulation

2000-03-24

2002-04-02

Pham, Hai (Department: 2861)

Incremental printing of symbolic information

Light or beam marking apparatus or processes

Scan of light

C348S252000

Reexamination Certificate

active

06366307

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains generally to image transfer technology. More particularly, this invention relates to clock independent pulse width modulation that enables generation of laser printer data at a desired frequency regardless of the operating frequency of the pulse width modulator.
BACKGROUND OF THE INVENTION
When rendering images using laser printer technology, a latent image is created on a surface of an insulating, photo-conducting material usually in the form of a rotating drum by selectively exposing areas on the surface of the drum to light. In operation, laser printers print pages by applying black toner onto selected small regions of fixed size referred to as pixels. By placing toner in only a portion of a pixel region, it is possible to create the effect of shades of gray. One presently recognized technique for placing toner in only a portion of a pixel region uses pulse width modulation (PWM). However, there exists a need to improve presently available prior art techniques as described below.
Pursuant to laser printer technology, a latent image is created on a surface of an insulating, photo-conducting material by selectively exposing areas of the surface to light. For the case of a laser printer, the surface is in the form of a rotating drum. A difference in electrostatic charge density is created between areas on the surface depending on the degree to which such areas are exposed to light. A visible image is then developed on the drum using one or more types of electrostatic toner. For the case of black and white printing, a single, black toner is used. For the case of color printing, multiple different color toners are used. Each toner is selectively attracted onto the photoconductive surface of the drum either exposed or unexposed to light, depending on the relative electrostatic charges on the photoconductive surface, characteristics of the development toner, and the type of toner used. Depending on the particular implementation, the photoconductive surface may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
The developed image is then transferred from the drum surface onto a sheet of paper. More particularly, a transfer roller is imparted with an electrostatic charge that is opposite to that of the toner. The transfer roller is rotated in proximity with the photoconductive surface of the drum. The transfer roller pulls the toner from the photoconductive surface, transferring the toner onto a charged sheet of paper. The transferred toner maintains the pattern of the image that was developed on the photoconductive surface.
In operation, a laser printer scans a laser beam horizontally across the photosensitive, electrically charged drum. By modulating the laser beam via a pulse width modulator (PWM), resulting variations in charge will impart proportionate amounts of toner being deposited onto a sheet of paper.
More particularly, laser printers print pages of information onto individual sheets of paper by applying a particular toner, such as black toner, to selected small regions of fixed size referred to as pixels. By placing toner in only a portion of a pixel region, it is possible to create the effect of shades of gray. One technique for placing toner in only a portion of a pixel region uses a pulse width modulation (PWM) technique.
Laser printers are distinguished from other types of printers by their ability to place precise amounts of toner into very small regions of a page at relatively high speed. As a result, laser printers generate image quality that is far greater than most other types of printers. However, laser printers operate by scanning a photoconductive drum upon which a rendered image is held. This results in an intrinsic quantization of the image in the vertical direction of the page. Additionally, there exist limitations in circuitry that is used to modulate the horizontal scanning. These limitations result in quantization of the image so that a single cell, or pixel, is effectively formed. If pixels are made small enough, the quantization effects can be made imperceptibly small to the human eye. However, there are practical limits. First, the vertical quantization is limited by the scan rate and the speed with which the photoconductive drum is rotated. Secondly, horizontal quantization is limited by the ability to transfer data in serial form to the scanning laser. The horizontal quantization limits the number of transitions that modulate the scanning laser, thereby limiting the density of horizontal dots that are placed onto a printed page.
In an effort to increase the resolution capability of laser printers, various techniques have been used to increase the number of horizontal dot components of a laser video signal generated by a laser. Irrespective of the technique used to generate horizontal dot components, the laser needs to be phase locked to a single signal edge that is referred to as a beam detect. The beam detect provides a reference signal that indicates when the scanning laser begins to sweep across the photosensitive drum.
In operation, a pixel clock is phase locked to the beam detect signal. One technique uses a clock generator as described in U.S. Pat. Nos. 5,438,353 and 5,760,816 listing Applicant as the inventor, and describing such clock generators. Such U.S. Pat. Nos. 5,438,353 and 5,760,816 are herein incorporated by reference.
According to the above-referenced clock generator prior art technique, a clock generator uses a chain of delayed clocks in order to generate phase locked, variable phase video output signals. More particularly, the clock generator comprises a variation of a pulse width modulator (PWM). For example, U.S. Pat. No. 5,438,353 discloses a variable phase version clock generator. Additionally, U.S. Pat. No. 5,760,816 discloses a reduced clock domain version clock generator which is referred to as a phase adjusted pulse width modulator (PWM). Such U.S. Pat. Nos. 5,438,353 and 5,760,816 are herein incorporated by reference as indicating details presently understood in the art.
Improvement has been made to the clock generator technique described in the above-referenced patents. More particularly, it is desirable to generate horizontal dots within a single pixel clock time, thereby increasing the effective resolution of a printed page. Such a technique generates sub-pixels within a pixel dot clock cycle. This technique can be accomplished using a phase adjusted, or phase locked, pulse width modulator. See U.S. patent application Ser. No. 09/534,747, entitled “A METHOD AND DEVICE FOR TIME SHIFTING TRANSITIONS IN AN IMAGING DEVICE”, filed Mar. 24, 2000, naming the inventors as Robert D. Morrison and Eugene A. Roylance, which application describes one technique using phase adjusted transition placement. Such U.S. patent application Ser. No. 09/534,747 is herein incorporated by reference. Such U.S. Patent Application provided a substantial improvement over previous prior art techniques disclosed in U.S. Pat. Nos. 5,438,353 and 5,760,816 wherein a clock generator of such previous prior art techniques introduces error in dot placement when using the clock generator on pixel boundaries. A standard phase locked loop generally cannot be used in such an environment because it is not possible to generate an error signal that would correct the pixel dot clock. More particularly, there is only one beam detect edge that the pixel dot clock can lock phase to, and that phase is required to be rigidly maintained for the length of a scanned line.
However, there is still a problem which results from implementing the above-referenced clock generators because the clock frequency that specifies the width of video pixels is usually different than the clock that is used to run the formatter processor. The clock used to run the formatter processor formats user data into a raster image that can modulate the laser. As a result, three problems occur. First, there are significant delays induced by the handshaked transfer of data f

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