Print head with multiplexed resistances controlling supply of current to image blocks

In an LED head operated on a time sharing basis, a multiplexer switches resistances for determining LED currents on LED arrays and assigns appropriate resistance to each array. The multiplexer is operated by block select signals of a block select circuit for time sharing operation, where the number of resistances corresponds to the number of blocks. The multiplexer capacity may also be less than the number of blocks, where the number of resistances less than the number of blocks and the same resistances correspond to more than one block. Output dispersions between arrays are adjusted by appropriate assignment of the resistances. sharing operation. Capacity of the multiplexer correspond to the number of blocks required for time. Gray scale printing is achieved by switching the current determining resistances in accordance with shading degree of image with the multiplexer.

FIELD OF THE INVENTION 
The present invention relates to print heads such as light emitting diode 
(LED) heads, thermal heads, electro luminescence (EL) heads, liquid 
crystal shutter array heads, and PLZT heads which utilize, in particular, 
a multiplexer in order to adjust the output dispersions of image arrays or 
to implement gray scale printing. 
PRIOR ART 
A print head is a device to expose a light sensitive medium or to heat ink 
ribbons by activating a large number of image formation modules with 
operating circuits. Image formation modules are usually integrated as an 
array which often works as a unit to conduct a time-sharing operation in a 
print head. In this context, an array of image formation modules is called 
as an image block. It is also possible to use two image arrays as an image 
block. 
As far as print heads are concerned, printing quality is influenced by the 
output dispersions of image formation modules. Although dispersions are 
small within an array, those of inter-array are relatively large. One 
widely known technique to adjust the output dispersions is to vary the 
time length of strobe signals sent to arrays. However, with the printers 
performing higher speed operation, it is becoming harder to adjust the 
time length of strobe signals. For example, when a line is scanned in 1.3 
msec and 2560 image formation modules are operated in 40 divisions for 
each line, the operating time for one image block is around 30 .mu.sec. To 
adjust the output dispersion of every array with the accuracy of .+-.3%, 
it is required to vary the operating time with the accuracy of 1 .mu.sec. 
In order to achieve this by strobe signals, the time length of a strobe 
signal has to be controlled with the accuracy of approximately 1 .mu.sec 
for every image array. This is extremely difficult. 
Printing ratio is defined as a ratio of image formation modules which 
generate printing output out of all image formation modules in an image 
array. When a printing ratio increases, both load on an operating circuit 
and a wiring resistance increase, resulting output decrease of image 
formation modules. Thus, it is necessary to keep the output of image 
formation modules stable even when the printing ratio varies. 
It is preferable that print heads are capable of gray scale printing. A 
technique widely known for gray scale printing is to vary strobe time for 
each image formation module. However, it requires complicated operating 
circuits. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a new technique to adjust 
the output dispersions among image arrays especially in a simple manner 
with simple circuits. 
Another object of the present invention is to prevent the fluctuation of 
output from image formation modules caused by various printing ratios. 
Still another object of the present invention is to provide a new technique 
for gray scale printing. 
In the present invention, an operating signal is adjusted to be the optimum 
value for each corresponding image block by switching current determining 
resistances to be connected to the operating circuit. Accordingly, the 
strobe time control for each image block is not necessary. A multiplexer, 
for example, can be used for switching the current determining 
resistances. Those resistances are to be provided individually for each 
image block and the multiplexer scans them one after another. Instead of 
having individual resistance for each block, however, a common resistance 
can be applied to all the normal blocks. In such a case, only for image 
arrays with large dispersions (hereinafter referred as `abnormal blocks`), 
a current determining resistance specific to each of them is to be 
provided. In any case, those abnormal blocks constitute only a small part 
of total arrays. 
According to the present invention, a multiplexer switches the size of 
operating signals of operating signal generating circuits. That is, a 
printing line is first divided into several sub-lines, and the multiplexer 
switches the size of operating signal for each of them in order to 
implement gray scale printing. For example, printing of 16 gray scales can 
be achieved by using 4 sub-lines and assigning the size of operating 
signal for each of them with binary codes. Operating signals will be 
current signals for the current-operated print heads such as LED heads and 
thermal heads. And they will be voltage signals for voltage-operated print 
heads such as liquid crystal shutter array heads.

Embodiment 1! 
FIG. 1 shows a circuit structure of the embodiment. 2 in the figure is a 
control circuit which receives image signals (print data), clock pulses 
and reset signals from a printer, more specifically, for example, from a 
CPU of a printer and controls the LED head. 4 is, for example, a 64 bit 
shift register and 6 is, for example, a 64 bit latch circuit of 64th. A1 
through A64 are 64 AND gates. 8 is a constant current power supply which 
consists of 64 circuits B1 through B64 with, for example, the constant 
current of 5 mA. Shift register 4, latch 6, gates A1 to A64 and power 
supply 8 together constitute a driving circuit for driving the LED head, 
one block at a time. 
Current outputs of the power supply 8 vary according to the external 
current determining resistances. In the embodiment, 40 resistances from R1 
to R40 are employed. Mirror constant current circuits, for example, can be 
used in the power supply 8 and each circuit of B1 through B64 is set to 
output the same amount of current as that of the current determining 
resistance. The power supply 8 can be replaced by a constant voltage power 
supply. 10 is a multiplexer. 40 FET switches are applied in the 
multiplexer 10 to select one of the 40 resistances R1 to R40. The selected 
resistance is used as the current determining resistance for the power 
supply 8. 
12 is bus lines which consist of 64 lines of 12-1 through 12-64. L1 through 
L40 are 40 LED arrays in each one of which 64 LEDs are integrated. In the 
embodiment, one LED array is used as an image block. However, two arrays 
may also be used as an image block. T1 through T40 are switching 
transistors. 14 is a block select circuit. 
FIG. 2 shows a variation of the multiplexer. 20 in the figure is a new 
multiplexer. 22 is, for example, a resistance ladder circuit of 6 bits 
accuracy. 24 is a memory such as EPROM which stores the adjustment data of 
LED current of each LED array L1 to L40 individually with, for example, 6 
bits accuracy. Then the memory reads out the adjustment data unique to 
each image block (one of the 40 LED arrays L1 to L40) which is selected by 
a signal of the block select circuit 14. The signal from block select 
circuit 14 controls the operation of multiplexer 20. According to the 
adjustment data, the multiplexer 20 controls resistance outputs of the 
resistance ladder circuit 22 with 6 bits accuracy. As a result, current 
outputs of the power supply 8 are controlled with 6 bits accuracy 
(.+-.1.5%) and LED currents to the LED arrays L1 through L40 are 
controlled with .+-.1.5% accuracy. 
A variation shown in FIG. 2 is identical to the embodiment shown in FIG. 1 
except the multiplexer 20. The difference between the variation in FIG. 2 
and the embodiment in FIG. 1 is that the resistance ladder circuit 22 and 
the memory 24 for adjustment data are required in the case of the 
variation, while in the embodiment, the multiplexer 10 alone is capable of 
scanning 40 resistances from R1 to R40 one after another with image block 
select signals of the block select circuit 14. Furthermore, the 
multiplexer 20 in FIG. 2 is more complex than the one in FIG. 1. The 
reason is that the multiplexer 20 requires at least 6 switches to activate 
the 6 bits resistance ladder circuit 22. These 6 switches are controlled 
by 40 data retrieved from the EPROM 24. 
Now, the operation of the embodiment will be described with reference to 
FIG. 1. Clock pulses CLK and print data DATA are transferred serially from 
the printer with the frequency of, for example, 2 MHz. The control circuit 
2 transfers them to the shift register 4 of 64th. On the receipt of clock 
pulses CLK, the shift register 4 recognizes them as shift clocks and 
shifts the data to store them in a specific address. When all the 64 data 
are registered in the shift register 4, they are transferred to the latch 
circuit 6 and wait there for, for example, two clocks. Subsequently, 
strobe signals of the width of 62 clocks are sent to the AND gates A1 
through A64. Then, LED current is sent to the bus lines 12 from the 
constant current circuit B1 through B64. 
The block select circuit 14 consists of, for example, a 64 bit counter, a 
64 bit decoder, AND gates and a shift register of 1+40th. The top bit of 
the shift register is a special bit for bit-set. A reset signal is used 
for setting a data bit in the top bit. Every time the counter counts 64 
clocks, it generates a shift clock and shifts a data bit of the shift 
register one by one. On the completion of printing a line, the data bit is 
returned to the second bit which corresponds to the array L1. As a result, 
bit-set of the data is completed in the way that data for each array 
respectively correspond to the transistor T1 to T40 to be operated. 
Signals of the counter are decoded by the decoder. The AND gates activate 
the signals of the data bits of the shift register during the 1st clock 
and the 62nd clock out of 64 clocks and the transistors T1 to T40 are 
operated. 
The LED arrays are operated by LED currents from the power supply 8, and 
also by the operating signals of the corresponding transistors among T1 
through T40 sent from the block select circuit 14. Operating frequency of 
1 line is, for example, 1.3 msec and time allocation for one image block 
(one LED array) is, for example, 32 .mu.sec in which LED time is 31 
.mu.sec. 
There are output dispersions within the range of, for example, 
approximately .+-.25% in LED arrays L1 to L40. In this context, an 
abnormal LED array is defined as an LED array having about 25% more or 
less output than the median. To secure high quality printing, it is 
necessary to hold the output dispersion of each array less than .+-.5%, or 
more preferably, less than .+-.3%. The influence of dispersions is 
remarkable in printing picture images and dispersions between adjoining 
arrays are also quite noticeable. However, supposing that LED arrays L1 to 
L40 are selected so that the dispersions can be eliminated, defect rate of 
LED arrays increases, and thus the cost of LED head increases. 
In the embodiment, LED outputs of the LED arrays L1 through L40 are 
measured at the occasion of assembly of LED head so that the average 
output of each array can be calculated. Thin plate resistances, for 
example, can be used for the resistances R1 through R40. In order to 
adjust the output dispersions of LED arrays L1 through L40, resistance 
values are trimmed by means such as a laser. As current output of the 
constant current power supply 8 decreases in inverse proportion to the 
value of current determining resistance, the resistance value is to be 
trimmed in the way that the resistances of smaller value are to be applied 
to the LED arrays of smaller outputs and the resistances of larger value 
are to be applied to the LED arrays of larger outputs. 
When the above mentioned LED head is operated, signals of the block select 
circuit 14 are sent to the multiplexer 10 and make it turn on the internal 
40 switches one by one. The resistances R1 to R40 are used as the current 
determining resistances of the power supply 8. As a result, the resistance 
R1 becomes the current determining resistance for the LED array L1, the 
resistance R2 for the LED array L2 . . . , the resistance R40 for the LED 
array L40. Thus, even the parallel allocation of 40 switches alone can be 
used as the multiplexer 10. In such a case, signals from the block select 
circuit 14 scan these 40 switches in due order. 
FIG. 3 is a control flow illustrating the control upon the power supply 8 
by the multiplexer 10. In response to the signals of block select circuit 
14, transistors T1 through T40 are operated one after another. AT the same 
time, the multiplexer 10 is also operated and it starts to scan the 
current determining resistances from R1 to R40 in order. 
FIG. 4 shows the principle of adjustment in the output dispersion of LED 
arrays L1 through L40. The larger the output of a LED array is, the larger 
the value of resistance to be applied becomes and the smaller the output 
of a LED array is, the smaller the value of resistance to be applied 
becomes. The outputs of the constant current circuits B1 through B64 are 
not determined individually, but are determined in common according to the 
value of the current determining resistance which is externally connected 
to the power supply 8. The reason is that the current on the current 
determining resistance is to be a basic current which is common to all the 
constant current circuits B1 through B64. As shown in FIG. 4, resistances 
of smaller value are applied to the arrays of smaller outputs in order to 
increase the LED current. On the contrary, resistances of larger value are 
applied to the arrays of larger outputs in order to decrease LED current. 
Accordingly, the output dispersions of LED arrays L1 through L40 are 
adjusted. In the case that the structure of the power supply 8 is changed 
and an individual current determining resistance is employed for each LED, 
the output dispersion of each LED can be adjusted by mounting a 
multiplexer 10 for each LED. 
The following is the comparison between the embodiment and the prior arts 
which adjust the dispersions of LED arrays L1 through L40 according to the 
width of strobe signals. In order to hold the dispersion less than .+-.3% 
by means of prior arts, the width of strobe signals has to be adjusted 
with the accuracy of about 1 .mu.sec (30 .mu.sec.times.0.03) per every LED 
array L1 to L40. It also implies that time length of a strobe signal has 
to be changed with the accuracy of about 2 clocks for each one of the 
arrays. This is quite difficult in terms of the circuit structure. Control 
of the strobe time requires a memory with the adjustment data of every 
array and a decoder to modify the time length of strobe signals. Data in 
the memory change the operating conditions of the decoder. Because it is 
necessary to control the time length with about 2 clocks accuracy, the 
memory and decoder have to be capable of high speed performance. On the 
other hand, the embodiment shown in FIG. 1, in contrast with the prior 
arts, requires only the multiplexer 10 which consists of 40 switches and 
40 trimmed resistances from R1 to R40. Neither a memory with adjustment 
data nor high speed decoder is necessary. 
Both speed and resolution can easily be improved in the LED head of the 
embodiment. However, it is not the case in the prior arts which conduct 
the adjustment by strobe time. Because in the prior arts, for example, if 
the direction of vertical scan is divided into 4 lines in order to achieve 
higher resolution, it is necessary to control the strobe time only in 0.25 
.mu.sec (1 .mu.sec.times.1/4). On the contrary, as the embodiment employes 
the multiplexers 10 and 20 to switch resistances for the purpose of 
adjustment of the output dispersion of LED arrays L1 through L40, the 
strobe time does not affect to the adjustment operation, no matter how the 
strobe time is shortened. 
Embodiment 2! 
FIG. 5 shows the second embodiment. 26 in the figure is a waveform shaping 
circuit. A comparator, for example, can be applied to the circuit 26 to 
detect that the switching transistor is on and send signals. 28 is an OR 
gate. 30 is a counter. 32 is a decoder. 34 is a new multiplexer. Reset 
signals from the printer reset the counter 30. The counter 30 and the 
decoder 32 can be replaced by a shift register. In such a case, output 
bits of the shift register are to be shifted one after another in response 
to the signals of OR gate 28. 
The number of abnormal blocks in an LED head is usually from 1 to 3. It is 
assumed that the outputs of LED arrays L2 and L38 in FIG. 5 exceed the 
designated range and thus are recognized as being abnormal. All other LED 
arrays are normal with their outputs within the limit. Then the waveform 
shaping circuit 26 detects that the corresponding switching transistor 
such as T1 is on for the top LED array L1, for the abnormal LED array L2, 
for the next LED array L3, for another abnormal LED array L38 and for the 
next LED array L39. Any other normal LED arrays are not to be connected to 
the waveform shaping circuit 26. 
When the LED head is reset, the waveform shaping circuit 26 detects that 
the top LED array L1 is on according to collector-emitter voltage of the 
switching transistor T1. And the initial value of the counter 30 is set to 
one via the OR gate 28. The decoder 32 decodes the value and the 
multiplexer 34 operates the current determining resistance R1 which 
corresponds to normal LED array L1. These processes are synchronized with 
the initial stage of the operation of LED array L1. When the abnormal LED 
array L2 is operated, the counter 30 is incremented by 1 in response to 
the signal of waveform shaping circuit 26 and the decoder 32 decodes the 
value of the counter 30, then the next current determining resistance R2 
is operated. As for the normal LED array L3 through L37, the counter 30 is 
incremented by 1 only at the operation of LED array L3 via the waveform 
shaping circuit 26 and also via the OR gate 28. A common current 
determining resistance R3 is applied to L3 through L37. When the second 
abnormal LED array L38 is operated, the counter 30 is incremented by 1 and 
the current determining resistance R4 is applied. As for the normal LED 
arrays L39 and L40, the counter 30 is incremented by 1 in response to the 
signal of the operation of LED array L39 and the last current determining 
resistance R5 is applied to both of them. 
When the number of the abnormal LED arrays is n, the number of necessary 
current determining resistances is approximately 2n+1. As the number of 
abnormal LED arrays is unknown until the inspection of the LED head, it is 
preferable to prepare larger capacity for each component such as the 
waveform shaping circuit 26, the counter 30, the decoder 32 and the 
multiplexer 34 (in the embodiment, all those components are able to handle 
up to 4 abnormal LED arrays, while the number of abnormal arrays are 
usually from 1 to 3). The excess capacity of, for example, the multiplexer 
34 may be used for the connection with the current determining resistance 
for normal LED arrays. Similarly, the waveform shaping circuit 26 can be 
individually connected to the switching transistors T1 through T40 by 
using, for example, jumper lines after the inspection of outputs of LED 
head. This technique can also be applied in the embodiments shown in FIG. 
6 and FIG. 7. 
Embodiment 3! 
In the embodiment shown in FIG. 6, the number of current determining 
resistances is further decreased. R1 in the figure is a current 
determining resistance for the normal LED arrays, while R2 and R3 are 
resistances for the abnormal LED arrays L2 and L38. 36 is an inverter. 38 
is a one-shot multivibrator. 40 is a multiplexer with a shift register. 42 
is a switch circuit which consists of two FETs. When the normal LED arrays 
such as L1 are operated, S terminal of the switch circuit 42 turns on by a 
signal of the inverter 36 (normal signal) via the one-shot multivibrator 
38. Then, via S terminal, the current determining resistance R1 for the 
normal LED arrays is connected to the power supply 8. When the first 
abnormal LED array L2 is operated, R terminal of the switch circuit 42 
turns on in response to a signal of the OR gate 28 (abnormal signal) via 
one-shot multivibrator 38. As a result, the current determining resistance 
R2 of the multiplexer 40 is connected to the power supply 8. As for the 
second abnormal LED array L38, an output bit of the shift register in the 
multiplexer 40 is shifted 1 bit by a signal of the one-shot multivibrator 
38 and the current determining resistance R3 is connected. The switch 
circuit 42, also in this case, connects the power supply 8 to the 
multiplexer 40. 
To sum up the above mentioned, the normal LED arrays are operated by one 
common current determining resistance R1, while the abnormal LED arrays L2 
and L38 are operated by the resistances R2 and R3 respectively. Thus, the 
number of current determining resistances to be required equals to the 
number of abnormal LED arrays plus one. In this embodiment, the 
combination of switch circuit 42 and multiplexer 40 is substantially 
considered as one multiplexer. 
Embodiment 4! 
FIG. 7 shows the forth embodiment. 44 in the figure is a presettable 
counter. 46 is a dip switch which determines set values of the presettable 
counter 44. In this embodiment, the abnormal LED arrays are detected at 
the inspection of the LED head and the dip switch 46 accordingly 
determines the set values of the presettable counter 44. The presettable 
counter 44 is to be reset by reset signals and is incremented by 1 by a 
block select signal of the block select circuit 14. When the value of the 
counter 44 is identical to the predetermined set value, the OR gate 28 
generates a signal. In the embodiment, signals are generated for the 
abnormal LED arrays L2 and L38. Every time an abnormal LED array is 
selected, the current determining resistance connected to the multiplexer 
40 is switched in response to the signal of the one-shop multivibrator 38. 
When normal LED arrays are operated, the switch circuit 42 connects the 
current determining resistance R1 with the power supply 8. This embodiment 
can be implemented without using jumper lines. 
Embodiment 5! 
FIGS. 8 and 9 illustrate the embodiment which detects printing ratio per 
image block and varies the current determining resistances by a 
multiplexer in order to adjust the output dispersion. In the following 
description of the embodiments, the above mentioned symbols represent the 
same objects. As long as there are no additional explanations nor special 
remarks, they can be implemented according to the above mentioned 
embodiments. 
82 in FIG. 8 is a comparator. An AD converter, for example, can be applied. 
80 is a multiplexer. R0 through R4 are current determining resistances. 
The LED current from the power supply 8 is determined by the value of one 
of the current determining resistances of R0 to R4 selected by the 
multiplexer 80. The LED current iS controlled by switching the resistances 
R0 to R4 according to the printing ratio. 
FIG. 9 shows the operation of the embodiment. The control circuit 2 sends 
strobe signals with a constant width to AND gates A1 through A64. The 
comparator 82 detects the voltage on the voltage detecting resistance R 
in, for example, five stages and controls the multiplexer 80. Then a 
current determining resistance is selected among five stages of R0 through 
R4 in, for example, the latter 1/3 of strobe signal time. The current 
output of the power supply 8 is also varied. That is, the multiplexer 80 
controls the current output of the power supply 8 in the latter 1/3 of 
strobe signal time in order to prevent the fluctuation of exposure energy 
of an LED caused by various printing ratio. 
Embodiment 6! 
FIGS. 10 through 13 show the sixth embodiment. 100 in the figures is a 
driving signal generating circuit which receives, for example, clock 
pulses of 8 MHz and reset signals from the printer. The shift register 4 
is, for example 64 bits and serially receives print data at 8 MHz. 102 is 
a new multiplexer which controls the current output of the power supply 8. 
4 current determining resistances from R1 to R4 are connected to the 
multiplexer 102. By switching the 4 resistances R1 to R4 with the 
multiplexer 102, the current output of the power supply 8 varies in the 
range, for example, from 1 mA to 8 mA. 14 is the aforementioned block 
select circuit. A clock pulse of 8 MHz is divided into 2 MHz signals by a 
divide circuit 104 and the circuit 14 selects LED arrays to be operated 
per a line. 
FIG. 11 shows the details of generating circuit 100. 106 in the figure is, 
for example, a 64 counter. 108 is a decoder. 110 is a 4 bit counter which 
produces multiplexer control signals, 112 is a flip-flop. Both 114 and 116 
are NAND gates. 
Now, the operation of the embodiment will be described with reference to 
FIGS. 10 to 13. First of all, data input from the printer into the LED 
head will be explained. In order to achieve the printing of 16 gray 
scales, print data of 4 bits word long are required for each LED. In the 
printer, among the print data of 4 bits word long, data of the least 
significant bit (LSB), for example, are first transferred to the head for 
one array, in the next place, data of the bit weighted 2 are transferred, 
then data of the bit weighted 4 are transferred. At the end, data of the 
most significant bit (MSB) weighted 8 are transferred. In the embodiment, 
the print data of 4 bits word long are divided into 4 sub-lines for the 
sake of 16 gray scale printing. On sub-line 1, print data of LSB are 
operated by the LED current of the power supply 8 of, for example, 1 mA 
each. On sub-line 2, data of the next order are operated by the LED 
current of, for example, 2 mA each. Accordingly on sub-line 3, data of the 
third order are operated by the LED current of 4 mA each and on sub-line 
4, data of MSB are operated by the LED current of 8 mA each. 
FIG. 12 shows an operating waveform of the embodiment. In the printer, the 
gray scale print data of 4 bits are sliced per a bit. The LSB is the first 
to be input into the LED head, and the MSB is the last to be input into 
the head. The print data sent to the head per each bit are individually 
registered by the shift register 4. In the control circuit 100, the 
counter 106 counts the number of clock pulses and when it counts 64 
clocks, the flip-flop 112 is set and the 4 bit counter 110 is incremented 
by 1. The decoder 108 decodes the output of the counter 106 and for every 
input of 64 data, the NAND gate 116 is operated and the print data in the 
shift register 4 are latched to the latch circuit 6. Right after the 
latching, a strobe signal is generated in the NAND gate 114 by a signal of 
the decoder 108. 
The sub-line 1 is printed by using the LSB of the print data. The sub-line 
4 is by the MSB. In other words, print data of 4 bits word long are 
divided into 4 sub-lines and printed separately according to the weight of 
each bit in the embodiment. The condition of the multiplexer 102 varies 
every 64 clock counts of the control circuit 100. Resistance R1 is applied 
for the sub-line 1, R2 for the sub-line 2, R3 for the sub-line 3 and R4 
for the sub-line 4 of the MSB. With the change of the resistances, the LED 
currents of the constant current circuits B1 to B64 also vary. For 
example, 1 mA for the sub-line 1, 2 mA for the sub-line 2, 4 mA for the 
sub-line 3 and 8 mA for the sub-line 4. As a result, the relative value of 
the accumulation of LED currents on the LED arrays L1 to L40 varies in 16 
stages from 0 to 15, and thus the printing of 16 gray scales is achieved. 
The divide circuit 104 divides a clock pulse into 4 segments and controls 
the block select circuit 14 which, then, switches the LED arrays L1 to L40 
one by one every time the printing of 4 sub-lines 1 though 4 is completed. 
In stead of sequentially printing 4 sub-lines for each LED array, it is 
also possible to print by each sub-line for the 40 LED arrays L1 to L40 
allocated in the direction of horizontal scan. That is, only after the 
sub-line 1 is completed, the sub-line 2 is printed and so on. However, it 
is not preferable because this method requires a large amount of memory in 
the printer. In order to scan the 40 LED arrays L1 to L40 serially along 
the direction of sub-lines, the printer has to be equipped with a memory 
of about 10K bits (2560.times.4). Each bit of the memory is required to be 
sliced and read out. On the contrary, the embodiment shown in FIG. 10 
requires relatively small amount of memory for the printer. 
FIG. 13 shows a data structure of the embodiment. 4 bits word long is 
enough to represent the print data of 16 gray scales. The LSB data a4 are 
operated on the sub-line 1 by LED current of 1 mA, data a3, next in the 
order, are operated on the sub-line 2 by LED current of 2 mA, the next 
data a2 are operated on the sub-line 3 by LED current of 4 mA and the MSB 
data al are operated on the sub-line 4 by LED current of 8 mA. 
Accumulation of LED currents on each sub-line makes 16 currents from 0 mA 
to 15 mA with 1 mA interval. As a result, 16 gray scales are achieved. 
FIGS. 14 and 15 show a variation which further divides a sub-line into 
three micro lines. For each micro line, a strobe signal of different width 
is applied in order to achieve higher resolution. In this variation, the 
LED head processes the bit slice of print data. 
Modified part of the circuit is indicated in FIG. 14. All the other parts 
are identical to those of FIG. 10. 120 in FIG. 14 is a new control circuit 
which divides a sub-line into three micro lines and generates a strobe 
signal of appropriate width for each micro line. The width of a strobe 
signal is, for example, relative value 1 for the micro line 1, 2 for the 
micro line 2, and 4 for the micro line 3. A shift register of 192.times.3 
bits, for example, can be employed for the shift register 122 which 
receives print data from the printer in 3 bits parallel by the frequency 
of 8 MHz. 
FIG. 15 shows the operation of the variation. The printer simultaneously 
inputs the print data for the 3 micro lines in 3 bits parallel. Input is 
implemented in due order from the sub-line 1 of the LSB, then the sub-line 
2, the sub-line 3, and the sub-line 4 of the MSB. The control circuit 120 
generates a latch signal for each micro line. On the micro line 1, data of 
1st to 64th addresses of the LSBs in the shift register 122 are latched to 
the latch circuit 6. On the micro line 2, data of 65th to 128th addresses 
of the middle bits in the register 122 are latched to the circuit 6. On 
the micro line 3, data of 129th to 192nd addresses of the MSBs in the 
register 122 are latched to the circuit 6. 
The width of a strobe signal for each micro line is, for example, width of 
8 clocks for the micro line 1, width of 16 clocks for the micro line 2, 
and width of 32 clocks for the micro line 3. As the number of clocks for a 
sub-line is 64, the remaining 8 clocks are to be used for switching 
signals. The multiplexer 102 is operated in the same manner as in the 
embodiment shown in FIG. 10. On the sub-line 1, the multiplexer 102 
controls the constant current circuits B1 to B64 end the constant current 
of 1 mA is output for every micro line. Accordingly, 2 mA current is 
output for the sub-line 2, 4 mA current for the sub-line 3, and 8 mA 
current for the sub-line 4. 
As each micro line has different print data, printing of 7 gray scales of 
1, 2, 3, 4, 5, 6 and 7 can be achieved by changing the width of a strobe 
time in a sub-line. By combining it with the printing of 15 gray scales 
implemented in 4 sub-lines 1 to 4, printing of 105 gray scales can be 
accomplished. 
FIG. 16 illustrates the operating characteristics of the power supply 8. In 
the figure, the abscissa indicates voltage outputs and the ordinate 
indicates current outputs of each constant current circuit B1 to B64. The 
current determining resistance is varied from 100 K.OMEGA. to 280 K.OMEGA. 
in the figure, and respective change of the current output is illustrated. 
As is demonstrated in the figure, the outputs of the constant current 
circuits B1 through B64 vary in accordance with the value of the current 
determining resistances R1 through R4. By switching the resistances with 
the multiplexer 102, it becomes possible to change the current outputs in 
the power supply 8. Moreover, the multiplexer 102 is capable of switching 
the current determining resistances in high speed and in simple manner. 
These advantages can be well reflected in the ongoing speedup of printers. 
Embodiment 7! 
FIG. 17 illustrates the embodiment which realizes the printing of 256 gray 
scales. 130 in the figure is a new control circuit. 132 is a shift 
register for input buffer from the printer. The shift register 132 may be 
provided, if necessary. 134 is a shift register of 64 bits. 136 is a latch 
circuit of 64 bits. A101 through A164 are AND gates for the less 
significant 4 bits. 138 is a constant current power supply. 102 is the 
aforementioned multiplexer which determines the current outputs for the 
more significant 4 bits by switching the current determining resistances 
R1 to R4. 142 is a new multiplexer which determines the current outputs 
for the less significant 4 bits by switching the current determining 
resistances R5 to R8. 144 is a data bus which combines the current outputs 
from both the power supplies 8 and 138 and provides them to the LED arrays 
L1 through L40. 146 is a new block select circuit. 
The operation of the embodiment will be described with reference to FIG. 
17. The printer (not shown in the figure) transfers gray scale data and 
print data which indicate whether each dot should be printed or not. A 
line is printed in 4 divided sub-lines and the degree of gray scale is 
allocated to each one of them. At the same time, for each sub-line, the 
current outputs from both the power supply 8 (for the more significant 
data) and the power supply 138 (for the less significant data) are 
combined. The current outputs of the power supplies 8 and 138 are switched 
by the multiplexers 102 and 142 respectively for every sub-line. Vertical 
scan control signals from the printer are used for the switching. As a 
result, two outputs, one of which is the output corresponding to the 
degree of gray scale designated by the multiplexer 102 and the other is 
the one designated by the multiplexer 142, are combined in each sub-line. 
The print data are assigned per each sub-line for both the power supplies 
8 and 138 separately. Thus, requisite data for each sub-line are the gray 
scale data to be sent to the multiplexers 102 and 142 and 128 (64.times.2) 
print data. 
In the embodiment, the printing of 256 gray scales of 8 bits data is 
implemented. Supposing that the data are (a1, a2, a3, a4, a5, a6, a7, a8) 
with a1 in the MSB side and a8 in the LSB side, the more significant 4 
bits (a1, a2, a3, a4) are processed by the power supply 8 and the less 
significant 4 bits (a5, a6, a7, a8) are processed by the power supply 138. 
Then, for example, on the sub-line 1, data (a1, a5) are processed and the 
multiplexer 102 selects the current output for data al and the multiplexer 
142 selects the current output for data a5. Likewise, data (a2, a6) are 
processed on the sub-line 2, data (a3, a7) are processed on the sub-line 
3, and data (a4, a8) are processed on the sub-line 4. As the print data 
are assigned for each sub-line per each dot for both the power supplies 8 
and 138 separately, the printing of 256 gray scales of 8 bits, in total, 
can be realized. 
This embodiment can be achieved by adding the multiplexer 142, the shift 
register 134, the latch circuit 136 and the constant current power supply 
138 to the embodiment of 16 gray scales shown in FIGS. 10 through 13. The 
multiplexer 102 processes 16 gray scales of the more significant 4 bits, 
and the multiplexer 142 processes the less significant 4 bits in order to 
accomplish the printing of 256 gray scales. For this purpose, the current 
outputs from both the power supplies 8 and 138 are combined. 
Embodiment 9! 
FIG. 18 shows the ninth embodiment. 150 in the figure is a new control 
circuit. 152 is a new multiplexer of 3 channels. In this embodiment, the 
LED head shown in FIG. 1 and that of FIG. 10 are integrated so that the 
output dispersions of LED arrays L1 through L40 can be adjusted and at the 
same time the printing of 16 gray scales can be accomplished. The control 
circuit 150 controls the LED head in order to divide a printing line into 
4 sub-lines. The multiplexer 10 is applied for the sub-line of MSB and 
adjusts the output dispersions by switching the current determining 
resistances R01 to R040 for each LED array L1 to L40. The multiplexer 152 
is applied for the less significant 3 sub-lines and implements the 
printing of 3 bits gray scales by switching the current determining 
resistances R1 to R3 for each sub-line. The degree of gray scale in total 
is 16 gray scales of 4 bits (3 bits+1 bit). As is mentioned above, the 
adjustment of output dispersions is conducted only for 1 bit of the MSB. 
It should be understood that the present invention is not limited to the 
specific embodiments described in this specification. For instance, 
although the embodiments are described using LED heads as examples, 
thermal heads can also be applied to the embodiments. In such a case, LED 
arrays L1 to L40 are replaced by the arrays of thermal modules. In the 
case of liquid crystal shutter array heads, LED arrays L1 to L40 are 
replaced by liquid crystal shutter arrays and the constant current power 
supply 8 is also replaced by a constant voltage power supply for the 
constant voltage operation. In the embodiments, the time sharing operation 
is employed. However, static operation may also be employed.