Driving control circuit for an LED head

An LED printer which forms a printed image by driving an LED head comprises a smoothing circuit in the LED head driving circuit the smoothing circuit forms interpolation lines by matching the original lines of the input data signal having a vertical resolution of 200 dpi with a given pattern and produce a print data signal of 400 dpi so as to improve the resolution of the printed image. The LED head driving circuit comprises a circuit for detecting the printing condition related to the image to be printed and producing a printing condition signal comprised of a line counter for detecting whether the line to be printed is the original line or the interpolation line, a transition counter point for counting the transition points of the input data signal between black and white dots, a dot counter for detecting the print density of the input data signal and a resolution detecting portion for detecting the resolution of the input data signal. The LED head driving circuit comprises a strobe signal generator, which previously stores therein a plurality of strobe signals related to the energizing periods of respective LED elements provided in the LED head for driving the same and selectively produces the strobe signal according to the printing condition related to the image to be printed based on the input data signal. The LED head drives each LED element based on the strobe signal to form an image.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a driving control circuit for an LED head 
in LED printers, particularly those for facsimile machines. 
2. Description of the Prior Art 
A conventional LED printer for printing a given character or figure on a 
recording paper using electrophotographic technology is equipped with an 
LED head comprising a plurality of LED elements arranged in a line, and 
each of the LED elements is energized corresponding to print data so as to 
emit light. 
Dots each made by the light-emitting of each LED element form a print 
image. 
The LED head comprises a plurality of register portions and a strobe 
generation circuit provided in the control portion of the printer supplies 
a strobe signal to each register portion to thereby restrain currents in 
each LED head. 
Moreover, the LED printer comprises a smoothing circuit at the control 
portion thereof so as to form interpolation lines between original lines 
corresponding to the print data in order to enhance the resolution of 
print output. 
That is, the smoothing circuit forms interpolation lines by interpolating 
black or white dots between original dots by matching an input data signal 
of M.times.N matrix with a previously set pattern. 
For example, the smoothing circuit can perform A highly detailed printing, 
eliminating raggedness such as obliquely step-formed lines from a printed 
image by forming interpolation lines in the transferring direction of a 
recording paper so as to artificially supply print data of 200 dpi (dots 
per inch) to the LED head a pseudo print data of 400 dpi. 
When printing is performed using the pseudo print data of 400 dpi formed by 
the smoothing circuit, the strobe generation circuit varies the strobe 
width for forming dots on interpolation lines from that for forming dots 
on original lines. 
That is, the light-emitting of each LED element is reduced in intensity by 
shortening the strobe width for forming a dot on an interpolation line so 
as to form an electrostatic latent image corresponding to a small dot 
(i.e., a thin image). 
Thus, it is possible to artificially improve resolution by interpolating 
input print data and making a printed image close to the original image 
the input data by forming the dots of original lines large and the dots of 
interpolation lines small. 
However, when the smoothing circuit forms the interpolation line, white 
dots are liable to be formed in the interpolation line if black dots of 
the input data are not continuous to one another. As a result, when 
printing is performed after the interpolation lines are formed in the 
printed data, the printed image becomes thinner. 
Whereas, when the black dots of the input print data are continuous to one 
another, black dots are liable to be formed in the interpolation line. As 
a result, when printing is performed after the interpolation lines are 
formed in the printed data, the printed image becomes darker. 
Thus printer equipped with the smoothing circuit set forth above produces a 
thinner printed image from input data having a thin print image and a 
darker printed image from an input data having a dark print image so that 
the density of the printed image is varied from that of the original 
image. 
When a printer used for a facsimile, a huge amount of print data has to be 
transferred in a short time so that the amount of each print data is 
reduced, resulting in the transference of print data of low resolution. 
Such print data which vary in resolution causes different printed results 
and consequently the deteriorated quality of printed images. 
Print data of low resolution can be interpolated by the smoothing means to 
be artificially improved in resolution. However, when print data of high 
resolution which has been received is interpolated similarly, the print 
data to be supplied to the LED head becomes too much and takes too much 
time in transference, so that the LED head cannot be driven in time. 
SUMMARY OF THE INVENTION 
It is the first object of the present invention to provide a driving 
control circuit of an LED head capable of forming a printed image of high 
resolution in an LED printer. 
It is the second object of the present invention to provide a driving 
control circuit for an LED head capable of printing a print image of high 
resolution at a stable print density corresponding to that of the input 
print data. 
It is the third object of the present invention to provide a driving 
control circuit of an LED head which does not deteriorate the quality of 
the printed image regardless of the different resolutions of input print 
data. 
The above objects are attained by an LED head driving control circuit of 
the present invention comprising the following components. 
An LED head equipped with a plurality of LED elements; 
a smoothing means which produces a print data signal which has been 
converted from an input data signal in accordance with a preset pattern; 
a printing condition signal output means which detects a printing condition 
related to an image to be printed according to the input data signal and 
produces a printing condition signal; and 
a strobe signal generation means which previously stores therein a 
plurality of strobe data related to the energizing period for driving the 
LED elements of the LED head, selects strobe data based on the printing 
condition signal and produces a strobe signal corresponding to the 
selected strobe data; wherein 
the LED head drives the LED elements based on the strobe signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An LED printer for printing a given character or figure on a recording 
paper using electrophotographic technology comprises an electrostatic 
charging unit, an exposure unit, a developing unit, a transfer unit and a 
fixing unit. 
The electrostatic charging unit charges a photosensitive drum with 
electricity at a given potential. 
The exposure unit performs exposure by applying the light emitted by the 
LED head thereof to the photosensitive drum to form an electrostatic 
latent image thereon. 
The electrostatic latent image on the photosensitive drum is developed by 
the developing unit so as to form a toner image thereon. 
The toner image is further transferred onto a recording paper by the 
transfer unit and is fixed thereon by the fixing unit. 
The LED head comprises a plurality of LED elements arranged in a line and 
an LED head driving control circuit controls the energizing of each LED 
element for light-emitting. 
The embodiments of the present invention will be described hereinafter with 
reference to drawings. 
In FIG. 1, an LED head driving control circuit 1 comprises an LED head 2 
which has a plurality of LED elements and four register portions 2a to 2d 
for restraining currents in each of the LED elements, a smoothing means 3 
which is composed of a first smoothing circuit 3a, a second smoothing 
circuit 3b, a third smoothing circuit 3c and a fourth smoothing circuit 
3d, a selector 4 which selectively outputs output signals from the 
smoothing means 3, a line counter 5 which produces a line signal related 
to a printing line, a transition point counter 6 which produces an image 
decision signal related to an image of the input data signal, a dot 
counter 7 which produces a print density signal related to the print 
density of the input data signal, a resolution detecting portion 8 which 
produces a resolution signal related to the resolution of the input data 
signal, a table RAM 9 which previously stores therein strobe data related 
to the energizing periods of the LED elements to be supplied to the 
register portions 2a to 2d of the LED head 2 and a strobe generation 
circuit 10 which produces a strobe signal based on the strobe data in the 
table RAM 9. Together, the RAM 9 and the strobe generation circuit 10 
define a strobe signal generation means (i.e. strobe signal generator 12). 
Each of the first to fourth smoothing circuits 3a to 3d of the smoothing 
means 3 has a smoothing pattern which corresponds to a printing condition 
different from the others. 
The first smoothing circuit 3a has a smoothing pattern of the fixed slice 
level method which converts an input data having 200 dpi vertical 
resolution into that having 400 dpi vertical resolution. 
The second smoothing circuit 3b has a smoothing pattern of the dither 
method which converts an input data having 200 dpi vertical resolution 
into that having 400 dpi vertical resolution. 
The third smoothing circuit 3c has a smoothing pattern of the fixed slice 
level method which converts an input data having 100 dpi vertical 
resolution into that having 400 dpi vertical resolution. 
The fourth smoothing circuit 3d has a smoothing pattern of the dither 
method which converts an input data having 100 dpi resolution into that 
having 400 dpi resolution. 
The smoothing means 3 performs data conversion by referring to the input 
data signal DATA of M.times.N matrix supplied from a central unit and 
converting the same according to a preset pattern. 
For example, 3.times.2 matrix data (data of two original lines each 
containing 3 dots) is converted into 3.times.3 matrix data or 3.times.5 
matrix data by interpolating black or white dots therebetween according to 
a given pattern. 
That is, when the smoothing means 3 forms an interpolating line from two 
original lines, it produces 3.times.3 matrix data, while when it forms 
three interpolating lines, it produces 3.times.5 matrix data. 
FIG. 2 is a view for explaining the algorithm of the smoothing process in 
the smoothing means 3, wherein 3.times.3 matrix data is formed from 
3.times.2 matrix data. 
In FIG. 2, denoted at A.about.C and D.about.F are dots of the original 
lines of 3.times.2 matrix in the input data, while denoted at P is a dot 
in an interpolation line added by the smoothing process. 
Each dot is composed of logical "1" (black dot) or logical "0" (white dot), 
and dot P is determined by the following formula. 
EQU P=A.times.E.times.F+C.times.D.times.E+B(D+E+F) 
The smoothing means 3 forms a print data signal of pseudo 400 dpi by 
forming three interpolating lines when the input data signal has a 
resolution of 100 dpi and forms a print data signal of pseudo 400 dpi by 
forming an interpolating line when the input data signal has a resolution 
of 200 dpi. 
In this way, it is possible to print a highly detailed print image 
eliminating raggedness in the oblique stepped lines of printed character 
etc. by way of the smoothing means 3 which artificially improves 
resolution by adding the data signal of interpolation lines to the input 
data signal. 
The line counter 5 receives a latch signal LATCH (FIG. 4(e)) for changing 
lines from a central unit and decides whether the line to be printed by 
the LED head 2 is an original line or an interpolation line added by the 
smoothing means 3 based on the latch signal LATCH. 
Thereafter the line counter 5 supplies a line signal indicating whether it 
is an original line or an interpolation line to the selector 4 and the 
table RAM 9. 
For example, when the smoothing means 3 forms three interpolation lines in 
the input data signal DATA, the line signal produced by the line counter 5 
is composed of four kinds of signals of an original line, a first 
interpolation line, a second interpolation line and a third interpolation 
line. 
When the smoothing means 3 forms an interpolation line, the line signal is 
composed, two kinds of signals of the original line and the first 
interpolation line. 
The transition point counter 6 counts the changes from a black dot to a 
white dot and those from a white dot to a black dot (the number of 
transitions between black and white dots) on reception of the input data 
signal DATA. 
FIG. 3 shows the circuit diagram of the transition counter 6. 
In FIG. 3, denoted at 61 is a flip-flop circuit, 62 is an exclusive OR 
(XOR) circuit and 63 is a counter. 
The filp-flop circuit 61 receives a data signal DATA and produces a data 
signal preceding the same (the data signal of the preceding dot) DATA' in 
synchronism with a clock signal CLOCK supplied by the central unit. 
The exclusive OR circuit 62 receives the data signal DATA and the preceding 
data signal DATA' produced by the flip-flop circuit 61. 
The exclusive OR circuit 62 produces a logical "1" when the data signal 
DATA is different from the preceding data signal DATA', i.e. the input 
data signal transits from black to white or from white to black. 
The data produced by the exclusive OR circuit 62 is supplied to the counter 
63 to be counted thereby. 
The counter 63 a produces logical "1" when the counted number exceeds a 
preset number (e.g. "512"). 
When the number counted by the counter 63 exceeds the preset number, it 
means that there are many dot transition points, i.e., the input data 
contains many portions where dots are not continuous to one another 
(uncontinuous portions). 
When there are many uncontinuous portions of dots, the input data signal is 
generally an image data best suited for the dither method. 
The counter 63 produces logical a "0" when the counted number does not 
exceed the preset number. 
When there are few uncontinuous portions, the input data signal is 
generally an image data suited for the fixed slice level method. 
Accordingly, the transition counter 6 decides whether the input data signal 
DATA is an image data for the dither method or that for the fixed slice 
level method and supplies an image decision signal composed of one of the 
two kinds of signals to the selector 4 and the table RAM 9. 
The dot counter 7 receives the data signal DATA to count black dots in the 
data of a line to thereby determine the print density. 
That is, the dot counter 7 classifies the density of the input data signal 
DATA into four print densities, high, medium high, medium low and low 
according the ratio of black dots in the whole dots of a line. 
Then the dot counter 7 supplies one of four print density signals 
corresponding to the four of print densities, i.e. high, medium high, 
medium low and low, to the table RAM 9. 
The resolution detecting portion 8 detects the resolution of the input data 
signal. 
For example when a printer is used in a facsimile machine, the data signal 
to be printed is transferred together with a control code representing the 
resolution thereof from the supply side. 
When standard resolution (e.g. 100 dpi) is selected on the supply side the 
printer used in a facsimile machine receives a DCS signal indicating the 
resolution and when high resolution (e.g. 200 dpi) is selected on the 
supply side it receives an NSF signal indicating the resolution. 
The resolution detecting portion 8 decides the resolution of the input data 
signal on the basis of the input control code (DCS signal or NSF signal) 
and supplies a resolution signal indicating either of 100 dpi and 200 dpi 
to the selector 4 and the table RAM 9. 
The selector 4 receives a line signal from the line counter 5, an image 
decision signal from the transition counter 6 and a resolution signal from 
the resolution detecting portion 8. 
The selector 4 selects one of the smoothing print data signals from the 
first to fourth smoothing circuits 3a to 3d according to the three 
received signals set forth above and supplies the selected smoothing print 
data signal to the LED head 2. 
The table RAM 9 previously stores therein a plurality of strobe data 
related to the energizing periods of LED elements. 
The table RAM 9 receives the line signal from the line counter 5, the image 
decision signal from the transition counter 6, the print density signal 
from the dot counter 7 and the resolution signal from the resolution 
detecting portion 8 strobe data selects particular from the stored strobe 
data on the basis of the received signals and supplies a signal 
corresponding to the selected strobe data signal to the strobe generation 
circuit 10. 
FIG. 4 exemplifies a strobe data signal produced by the table RAM 9. 
FIG. 4 (a) shows a strobe data signal when the line signal is the original 
line, the image decision signal is the systematic dither, the print 
density signal is low and the resolution signal is 200 dpi. 
FIG. 4 (b) shows a strobe data signal when the line signal indicates the 
interpolation line, the image decision signal indicates the dither method, 
the print density signal is medium high and the resolution signal is 200 
dpi. FIG. 4 (c) shows a strobe data signal when the line signal indicates 
the original line, the image decision signal indicates the fixed slice 
level method, the print density signal is high and the resolution signal 
is 100 dpi and FIG. 4 (d) shows a strobe data signal when the line signal 
indicates the interpolation line, the image decision signal indicates the 
fixed slice level method, the print density signal is low and the 
resolution signal is 100 dpi. 
In FIGS. 4 (a) to 4 (d), denoted at S1 is the strobe width of the strobe 
signal STB0, S2 is that of the strobe signal STB1, S3 is that of the 
strobe signal STB2 and S4 is that of the strobe signal STB3. 
As evident from these figures, when the dither method is selected the 
strobe width P2 of the interpolation line is shorter than the strobe width 
P1 of the original line and the difference therebetween is large. 
Whereas when the fixed slice level method is selected, the difference 
between the strobe width P2 of the interpolation line and the strobe width 
P1 of the original line is small. 
The table RAM 9 selects the optimum strobe data meeting the conditions of 
the line signal, image decision signal, print density signal and 
resolution signal respectively. 
That is, the table RAM 9 selects the long strobe width when the line signal 
indicates the original line, while the short strobe width when the line 
signal indicates the interpolation line. 
Thus, it is possible to practically print an image which is close in shape 
to the original image of the input data by forming the dots on the 
original line large (dark) and the dots on the interpolation line small 
(thin). 
The table RAM 9 selects a larger strobe width in the original line and a 
smaller strobe width in the interpolation line by increasing the 
difference between the strobe widths of the original line and the 
interpolation line when the image decision signal indicates the dither 
method is selected (i.e. there are many portions where the dots are not 
continuous to one another). The table RAM 9 decreases the difference 
between the strobe widths of the original line and the interpolation line 
when the image decision signal indicates the fixed slice level method is 
selected (i.e. there are many portions where the dots are continuous). 
As a result, when an input data signal indicating the dither method is 
selected wherein black dots are not continuous to one another is subjected 
to the smoothing process, a printed image is not so influenced by the 
white dots in the interpolation line although the dots formed in the 
interpolation line are liable to be white. 
On the other hand, when the input data signal indicates the fixed slice 
level method is selected, the printed image is influenced by the smoothing 
effect since the dots of the original line and those of the interpolation 
line are printed with respective strobe widths which are not so different 
from each other. 
The table RAM 9 selects a short strobe width when the print density signal 
is high (i.e. the ratio of black dots in a line is high) while a long 
strobe width when the print density signal is low. 
The table RAM 9 selects a short strobe width for the interpolation line 
when the resolution signal corresponding to the vertical resolution of the 
input data signal indicates 200 dpi and selects a short strobe width for 
the first and third interpolation lines and a long strobe width for the 
second interpolation line of the three interpolation lines when the 
vertical resolution signal indicates 100 dpi. 
The strobe generation circuit 10 operates in synchronism with the system 
clock SYSCLOCK from the central unit and produces strobe signals STB0 to 
STB3 having a strobe width corresponding to the strobe data from the table 
RAM 9. 
FIG. 5 is a circuit diagram of the strobe generation circuit. 
In FIG. 5, denoted at 101 is an inverter circuit, 102 is an AND (2AND) 
circuit, 103 is a counter, 104 is an inverter circuit and 105 is a phase 
switching portion. 
The counter 103 receives the strobe data signal from the table RAM 9 and 
set a starting number according to the strobe data, i.e. the strobe width 
of the strobe data signal as illustrated in FIG. 4. 
The counter 103 counts up in synchronism with the system clock SYSCLOCK 
starting at the starting number and produces a carry signal when it counts 
up to a give number. 
The phase switching portion 105 is composed of shift registers which 
successively switch the phases of the strobe signals STB0 to STB3 
according to the carry signal. 
That is, the counting time of the counter 103 up to the given number is 
varied according to the starting number which is set in accordance with 
the strobe width of the strobe data signal, to thereby vary the strobing 
period for driving the LED elements of the LED head 2. 
The latch signal LATCH from the inverter circuit 101 and the carry signal 
from the inverter circuit 104 are supplied to the AND circuit 102. 
The counter 103 sets a starting number corresponding to the strobe data 
signal based on the load signal from the AND circuit 102. 
The LED head 2 comprises LED elements arranged in a line (not shown) 
wherein each LED element is energized according to the print data from the 
selector 4 so as to emit light. 
The LED head 2 comprises four register portions 2a to 2d. The strobe 
signals STB0 to STB3 which are successively produced by the strobe 
generation circuit 10 are supplied to the register portions 2a to 2d 
respectively. 
Each of the register portions 2a to 2d of the LED head 2 is composed of 
shift registers and latch circuits (not shown). 
The print data produced from the selector 4 is successively supplied to the 
shift registers in synchronism with the clock signal CLOCK, and when the 
latch circuit receives a latch signal LATCH, the print data stored in the 
shift registers are latched by the latch circuits. 
The LED elements corresponding to the latched print data is energized to 
emit light. 
Since the LED elements are energized according to the strobe signals STB0 
to STB3 from the strobe generation circuit 10, a photosensitive drum (not 
shown), is irradiated during the time which corresponds to the strobe 
width of the strobe signal so as to form an electrostatic latent image of 
a dot having a given size thereon.