Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-10-12
2002-11-05
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S247000
Reexamination Certificate
active
06476844
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for modulation of a laser diode, and, more particularly, to a method and apparatus for minimizing electromagnetic interference resulting from modulation of a laser diode.
2. Description of the Related Art
Monochrome and color electrophotographic printers using laser scanning unit imaging require accurate, high-bandwidth modulation of the laser diode to produce high resolution halftone and color images. The time per pixel is determined by process speed, addressability, and laser scan efficiency. Higher speed and higher addressability printers possess shorter pixel times and higher video frequency content. Sub-pixel modulation used for half-tone or multi-level imaging at each pixel results in an even higher bandwidth requirement. Here, modulation is binary, i.e., the laser is switched between an ON state and an OFF or standby state.
The video path from the raster-image-processor data source to a properly designed laser driver typically limits the quality of image reproduction when tradeoffs are made to prevent undesirable electromagnetic radio-frequency emissions and interference. These tradeoffs typically involve adding inductors and capacitors to reduce electromagnetic interference at the expense of limiting or distorting video signals. Driver and receiver rise and fall times, turn-on and turn-off delays, and drive current capability also limit bandwidth and signal fidelity. Cable length, impedance, termination impedance, and losses further limit bandwidth and signal fidelity.
Lexmark printers using laser-scanning-unit printheads are now available with process speeds as high as 34 pages (8.5″×11″) per minute at 600 dots per inch (dpi) addressability and 17 pages per minute at 1200 dpi. Time per pixel for these printers is approximately 33.5 nanoseconds at 600 dpi and 18.6 nanoseconds at 1200 dpi. Each 600-dpi pixel is subdivided into eight 4.23 nanosecond slices, and each 1200 dpi pixel is subdivided into four 4.65 nanosecond slices. Each imaged pixel is ON for typically 25.4 nanoseconds (six of eight slices) at 600 dpi and 9.3 nanoseconds (two of four slices) at 1200 dpi. For consistent reproduction of half-tone images and fine-feature graphics, a laser diode
10
(
FIG. 1
) must faithfully reproduce the binary video signal generated at a raster image processor
12
. This involves faithful transmission of the video signal over a cable
14
from a driver module
16
of raster image processor
12
to a receiver module
18
remotely located on a laser driver
20
of a laser scanning unit
22
. This may be a distance d of 4 inches to 30 inches or more.
It is known to transmit a single-ended (not differential) signal over a twisted pair or shielded coaxial cable
24
(FIG.
2
). The binary video signal at driver
16
is attenuated to a fraction of a TTL signal swing (from typically 3.4 V peak-to-peak to 1.8 V peak-to-peak) and level-shifted to be centered around the threshold voltage at receiver module
18
. This approach minimizes signal swing and consequently radiated electromagnetic signal amplitude. However, this approach also introduces sensitivity to receiver, threshold variation, which can result in unexpected changes in laser ON time.
Another method of transmitting data that retains signal fidelity and minimizes electromagnetic interference is the use of a paired differential driver
26
(
FIG. 3
) and receiver
28
. Here, electromagnetic interference is minimized to the extent that equal and opposite polarity signals are present in video cable
30
, leading to cancellation of radiated radio frequency energy. Signal amplitude can be reduced for lower electromagnetic emissions by using a differential receiver
28
with less sensitivity to receiver threshold variation as compared to the single-ended receiver
18
. Frequency content in leach of two signal wires
32
,
34
is that of the high-frequency binary data. Commercially available differential driver-receiver pairs
26
,
28
designed for high speed data tend to produce large switching currents. Using these drivers, differences in driver switching times and differences in line impedance have resulted in more electromagnetic interference than expected.
Serial-to-parallel conversion of data prior to transmission, and parallel-to-serial conversion at laser driver
20
, as shown in
FIG. 4
, offers another way of keeping data frequency low over a video cable
36
interconnecting a raster image processor
38
and a laser scanning unit
40
. However, to properly recreate the serial binary sub-pixel modulated data, both a data clock and a high-frequency gating signal must be carried in lines
42
and
44
of cable
36
. This may create a worse electromagnetic compatibility problem than transmitting the serial data, since the gating signal has the same sub-pixel frequency content as the serial data for every pixel time. Another problem is that additional wires add to cost.
What is needed in the art is a simple and low cost method of passing a high bandwidth video modulation signal from a raster image processor to a laser print head driver with reduced electromagnetic interference.
SUMMARY OF THE INVENTION
Thee present invention provides a two-wire on-off encode and exclusive-OR decode of binary video data to pass a high bandwidth modulated video signal from source to receiver with reduced electromagnetic interference.
The invention comprises, in one form thereof, a method of driving a laser device in a electrophotographic machine. A binary image data signal having a first switching rate is provided. A plurality of binary encoded data signals are transmitted over a transmission medium. Each of the encoded data signals is dependent upon the image data signal and has a respective second switching rate lower than the first switching rate. The image data signal is recreated by using the encoded data signals. The laser device is driven with the recreated image data signal.
The invention comprises, in another form thereof, an apparatus for driving a laser device in an electrophotographic machine. The apparatus includes a raster image processor having an image data source outputting a binary image data signal with a first switching rate. A data encoder receives the binary image data signal and transmits a plurality of binary encoded data signals over a transmission medium. Each of the encoded data signals is dependent upon the image data signal and has a respective second switching rate lower than the first switching rate. A laser scanning unit includes a data decoder receiving the binary encoded data signals over the transmission medium and recreating the image data signal by using the encoded data signals. A laser driver drives the laser device with the recreated image data signal.
An advantage of the present invention is that electromagnetic interference is reduced at low cost.
Another advantage is that signal quality is sufficient to reproduce the sub-pixel modulation required for halftone reproduction in a 20 pages per minute, 1200 dpi addressable electrophotographic printer using laser scanning unit imaging.
REFERENCES:
patent: 4928289 (1990-05-01), Dingeman
patent: 4991920 (1991-02-01), Peczalski
patent: 5283658 (1994-02-01), Hayashi et al.
patent: 5363208 (1994-11-01), Ogino et al.
patent: 5379126 (1995-01-01), Seto et al.
patent: 5387985 (1995-02-01), Loce et al.
patent: 5392061 (1995-02-01), Vondran, Jr.
patent: 5436644 (1995-07-01), Motoi et al.
patent: 5479175 (1995-12-01), Cianciosi et al.
patent: 5521625 (1996-05-01), Morrison
patent: 5535343 (1996-07-01), Verseput
patent: 5579090 (1996-11-01), Sasanuma et al.
patent: 5581292 (1996-12-01), Cianciosi et al.
patent: 5633669 (1997-05-01), Hada et al.
patent: 5638463 (1997-06-01), Ohshita
patent: 5652660 (1997-07-01), Seto et al.
patent: 5659399 (1997-08-01), Lin et al.
patent: 5712962 (1998-01-01), D'Entremont
patent: 5742323 (1998-04-01), Fukushima
patent: 5751470 (1998-05-01), Damon
patent: 5793406 (1998-08-01), Trask et al.
patent: 585465
Lexmark International Inc.
Pham Hai
Taylor & Aust P.C.
LandOfFree
Method and apparatus for electrophotographic laser video... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for electrophotographic laser video..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for electrophotographic laser video... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2975122