Recording apparatus

There is described a printer with improved synchronization, particularly in case a printer unit is separate from a control unit, by releasing clock signals from the control unit to the printer unit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a recording apparatus for image recording 
such as a laser beam printer. 
2. Related Background Art 
In a conventional apparatus for image formation on a photosensitive drum by 
light beam scanning, for example with a rotary polygonal mirror, such as a 
laser beam printer, a horizontal synchronization signal is generated from 
a detection signal indicating that the scanning light beam has reached a 
scan starting end of the photosensitive drum. For example, in a printer in 
which a light beam scans a photosensitive drum from left to right, a 
photodetector is provided close to the left-hand end of the drum, and a 
horizontal synchronization signal is generated by delaying a detection 
signal, obtained from the photodetector when the light beam passes, by a 
determined perior in such a manner that the synchronization signal 
coincides with the arrival of the light beam at the left-hand end of a 
recording sheet. Such method is satisfactory if the recording sheet, for 
receiving the image from the drum after the development of a latent image, 
is always aligned with to a left-end position regardless of the width of 
sheet, but the determined period of delay has to be altered if the 
recording sheet is supplied in such a manner that the center thereof is 
aligned with to the central axis of a scanning system, or the center of 
the photosensitive drum, for example in order to utilize better optical 
characteristics of the central portion of the scanning system. Such 
alteration can be achieved by counting pixel clock signals corresponding 
to the pixels to be recorded or other clock signals having a multiple 
frequency, from the moment the light beam passes the photodetector, and 
generating the horizontal synchronization signal at a suitable count which 
is variable according to the size of the recording sheet. 
In a conventional printer in which a printer unit is separate from a 
control unit, pixel clock signals are generated in the printer unit and 
are sent, together with the horizontal synchronization signal, to the 
control unit, which sends image signals to the printer unit in 
synchronization with the pixel clock signals, from the reception of the 
horizontal synchronization signal. In such structure, if the cable between 
the printer unit and the control unit is long, the image signals received 
by the printer unit show a considerable phase delay with respect to the 
pixel clock signals generated in the printer and often cannot be 
synchronized therewith, thus giving rise to a distortion in image or a 
loss of data. Also in the above-explained conventional printer, if the 
frequency of the pixel clock signals is changed for modifying the image 
density or for enlarging or reducing the recorded image, the 
above-mentioned count for generating the horizontal synchronization signal 
must also to be modified. 
SUMMARY OF THE INVENTION 
An object of the present invention is to eliminate the above-mentioned 
drawbacks of conventional printers. 
Another object of the present invention is to provide an improved recording 
apparatus. 
Still another object of the present invention is to provide a recording 
apparatus capable of achieving exact transfer of image data or record 
data. 
Still another object of the present invention is to provide a recording 
apparatus capable of preventing loss of record data. 
Still another object of the present invention is to provide a recording 
apparatus capable of stable image recording. 
Still another object of the present invention is to provide a recording 
apparatus capable of satisfactory image recording. 
Still another object of the present invention is to provide a recording 
apparatus allowing easy adjustment or control even in case of a 
modification in the recording density or of an enlargement or reduction of 
image size. 
The foregoing and still other objects of the present invention will become 
fully apparent from the following description to be taken in conjunction 
with the attached drawings, and from the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, the present invention will be explained in detail by embodiments 
thereof shown in the attached drawings. 
FIG. 1 schematically shows the entire structure of a recording apparatus 
constituting an embodiment of the present invention, wherein shown are a 
control unit 100 for controlling the entire apparatus; a printer unit 200 
for recording an image with a laser beam; a sheet size detector 3 for 
detecting the size of a sheet on which the image is to be recorded; an 
input unit 2 of the printer unit; a horizontal synchronization signal S1; 
pixel clock signals S3, S4; an image signal S2 supplied from a latch 11 in 
the control unit 100, to be explained later. These components will be 
explained later in more detail, in relation to FIGS. 2 to 4. 
In the printer unit 200, shown in FIG. 1, an image signal 24 from a latch 
12, provided as will be explained later in the printer input unit 2, is 
supplied through a driver 27 to an acousto-optical modulating element 
(light or optical modulator) 13 for modulating a laser beam 23 emitted 
from a laser unit 22, and the modulated laser beam scans, by means of a 
rotary polygonal mirror 21, a photosensitive drum 20 in a direction 
indicated by the arrow, while being focused thereon by a lens 25. Upon 
reaching the left-hand end of the photosensitive drum 20, the laser beam 
23 is reflected by a mirror 26 and detected by a photodetector (or optical 
detector) 5, of which detection signal (BD) S7 is supplied to the input 
unit 2. 
FIG. 2 shows the relation between the printer input unit 2 and the control 
unit 100, wherein the same components as those in FIG. 1 are represented 
by the same reference numbers or symbols. There are also shown a part 1 of 
the control unit 100; a counter 4 for obtaining 3 count corresponding to 
the sheet size; an internal oscillator 6 of the printer unit for 
generating clock signals which are counted by the counter 4 for supplying 
an output signal to a horizontal synchronization signal generator 7; an 
address counter 9; an image memory 10 composed, for example, of a random 
access memory (RAM) for storing image signals which are read in 
synchronization with pixel clock signals S4 supplied from the address 
counter 9 and the pixel clock oscillator 6; and data latches 11, 12. 
FIG. 3 shows, in more detail, the relation of the oscillator 6, counter 4, 
horizontal synchronization signal generator 7, address counter 9 and image 
memory 10. FIG. 4 is a corresponding timing chart. 
In a state where the sheet size detector 3 supplies a currently selected 
sheet size to the counter 4, when the photodetector 5 detects the laser 
beam 23, the detection signal S7 is supplied as a load pulse to the 
counter 4 whereby a number corresponding to the sheet size is preset in 
said counter 4. In response, a zero-detecting NOR gate 301 releases a 
low-level output signal 302 (timing T1 in FIG. 4), whereby the counter 4 
is released from the reset state and starts a count-down operation at the 
end of the clock signal S4. After a counting operation corresponding to 
the sheet size, the output signal 302 of the zero-detecting circuit 301 is 
again shifted to a high level (timing T2 in FIG. 4) and is supplied to the 
horizontal synchronization signal generator 7 to generate a horizontal 
synchronization signal S1 of a determined pulse duration. In this 
embodiment, there is employed a simple one-shot circuit triggered by the 
start of an input signal. 
On the other hand, in the control unit 100, image signals are stored in the 
image memory 10 by an unrepresented process and can be read by address 
signals released from the address counter 9. The address counter is 
composed of a flip-flop 303, a counter 9-2 for release addresses of a 
line, and a counter 9-1 for releasing scanning line addresses. In response 
to the horizontal synchronization signal S1, the Q output 305 of the 
flip-flop 303 is shifted to the high level state (timing T2 in FIG. 4) 
whereby the counter 9-2 starts a counting operation from zero, thus 
counting the ends of the pixel clock signals S4 and designating the 
address of the image memory 10 in cooperation with the counter 9-1. On the 
other hand, said counter 9-1 performs a stepwise increment in response to 
each carry signal 304 released from the counter 9-2 (timing T3 in FIG. 4) 
and releases output signals Y0 - Ym indicating the addresses of the 
scanning lines. 
CT0 - CTn in FIG. 4 indicate the output of the address counter 9, wherein 
CT0, CT1, . . . , CTn respectively indicate the timings of reading the 
addresses 0, 1, . . . , n of the image memory 10. The latch 11 is 
activated at the start of the pixel clock signal S4 to release an output 
signal S2 as shown in FIG. 4. The signal S2 is supplied to the latch 12 of 
the printer input unit 2, which is activated at the end of the pixel clock 
signal S4 in consideration of a delay in timing. FIG. 4 also shows the 
timing of the signal 24 from said latch 12. 
The counter 9-2, which corresponds to the number of data in a line, 
releases a carry signal 304 (timing T3 in FIG. 4) after beam scanning of a 
line. The carry signal 304 is supplied, through an inverter 306, to the 
preset terminal of the flip-flop 303 to shift the output Q thereof to the 
low level state (timing T3 in FIG. 4), whereby the counter 9-2 is cleared 
and remains zero until the horizontal synchronization signal S1 is entered 
again. The counter 9-1 performs a stepwise increment by the carry signal 
304 to release the addresses of a next scanning line through the lines Y0 
- Ym. For example, if a line contains 4096 pixels at maximum, the counter 
9-2 releases outputs Q0-Q11 assuming values from 0 to FFF in hexadecimal 
number (0-4095 in decimal number), while the outputs Y0-Ym of the counter 
9-1 represent fourth and upper digits in hexadecimal number, or 13th and 
upper bits of the address of the image memory 10. 
After the scanning of a line, when the beam is detected again by the 
photodetector 5 at the start of scanning of a succeeding line, data 
corresponding to the sheet size, obtained from the sheet size detector 3 
are loaded in the counter 4 (timing T4 in FIG. 4), and a procedure similar 
to the foregoing is executed. This procedure is different from the 
foregoing in that the output of the address counter 9 starts, for example, 
from an address 1000, in hexadecimal number, and the content of the image 
memory 10 is released from the address 1000, such as CT1000, CT1001, . . . 
as shown in FIG. 4, since the output Y0 of the counter 9-1 is in a state 
"1". 
In the foregoing description, the counter 4 need not be composed of a 
down-counter but may be composed of an up-counter or a combination of an 
up-counter and a comparator. Also, the image memory 10 and the address 
counter 9 in the control unit 1 are shown simply for the explanation of 
the interface, and can be composed of any other component capable of 
releasing digital pixel signals in synchronization with horizontal 
synchronization signals and pixel clock signals, such as a television 
camera or a CCD sensor. Also, in the foregoing explanation it is assumed 
that the latch 11 performs latching operation at the start of the pixel 
clock signals S3, S4 while the latch 12 performs latching operation at the 
end of said pixel clock signals, but both latches 11, 12 may perform the 
latching operation at the start or end of the pixel clock signals S3, S4. 
In the following, there will be explained a second embodiment of the 
present invention, wherein components equivalent to those in the first 
embodiment will be represented by the same reference characters. 
FIG. 5 schematically illustrates the entire structure of a recording 
apparatus constituting a second embodiment of the present invention, 
wherein shown are a control unit 100' for controlling the entire 
apparatus; a printer unit 200' for image recording with a laser beam; a 
sheet size detector 3 for detecting the size of a recording sheet 
employed; an input unit 2' of said printer unit; a horizontal 
synchronization signal S1'; a pixel clock signal S3'; and an image signal 
S2' supplied from a latch 11', to be explained later, of the control unit 
100'; which will be explained in more detail in relation to FIGS. 6 to 9. 
In the printer unit 200' shown in FIG. 5, an image signal 24' from a latch 
12' provided, as will be explained later, in the printer input unit 2' is 
supplied through a driver 27 to an acousto-optical modulating element 
(light modulator) 13 for modulating a laser beam 23 emitted from a laser 
unit 22, and the modulated laser beam scans, by means of a rotary 
polygonal mirror 21, a photosensitive drum 20 in a direction indicated by 
the arrow, while being focused thereon by a lens 25. Upon reaching the 
left-hand end of the photosensitive drum 20, the laser beam 23 is 
reflected by a mirror 26 and detected by a photodetector 5, of which 
detection signal (BD) S7 is supplied to the input unit 2'. 
FIG. 6 shows the relation between the printer input unit 2' and the control 
unit 100', wherein same components as those in FIG. 1 are represented by 
same numbers or symbols. There are also shown a part 1' of the control 
unit 100'; a counter 4' for obtaining a count corresponding to the sheet 
size; an internal oscillator 6' of the printer unit for generating clock 
signals which are counted by the counter 4' for supplying an output signal 
to a horizontal synchronization signal generator 7'; a pixel clock 
oscillator 8' of the control unit; an address counter 9'; an image memory 
10 composed, for example, of a random access memory (RAM) for storing 
image signals which are read in synchronization with pixel clock signals 
S3' supplied from the pixel clock oscillator 8' and the address counter 
9'; and data latches 11', 12'. 
FIG. 7 shows, in more detail, the relation of the oscillator 6', counter 
4', horizontal synchronization signal generator 7', address counter 9' and 
image memory 10', and FIGS. 8 and 9 are corresponding timing charts. In a 
state where the sheet size detector 3 supplies a currently selected sheet 
size to the counter 4', if the photodetector 5 detects the laser beam 23, 
the detection signal S7 is supplied as a load pulse to the counter 4' 
whereby a number corresponding to the sheet size is preset in said counter 
4'. In response a zero-detecting NOR gate 301' releases a low-level output 
signal 302' (timing T1 in FIG. 9), whereby the counter 4' is released from 
the reset state and starts a count-down operation at the end of the clock 
signal S4' (timing T2 in FIG. 9). After a counting operation corresponding 
to the sheet size, the output signal 302' of the zero-detecting circuit 
301' is again shifted to a high level (timing T3 in FIG. 9) and is 
supplied to the horizontal synchronization signal generator 7' to generate 
a horizontal synchronization signal S1' of a determined pulse duration. In 
this embodiment, there is employed a simple one-shot circuit triggered by 
the start of an input signal. 
On the other hand, in the control unit 100', image signals are stored in 
the image memory 10' by an unrepresented process and can be read by 
address signals released from the address counter 9'. The address counter 
9' is composed of a flip-flop 303', a counter 9-2' for releasing addresses 
of a line, and a counter 9-1' for releasing scanning line addresses. In 
response to the horizontal synchronization signal S1', the output Q 305 of 
the flip-flop 303' is shifted to the high level state (timing T2 in FIG. 
9) whereby the counter 9-2' starts a counting operation from zero, thus 
counting the ends of the pixel clock signals S3' and designating the 
address of the image memory 10 in cooperation with the counter 9-1'. On 
the other hand, said counter 9-1' performs a stepwise increment in 
response to each carry signal 304' released from the counter 9-2' (timing 
T3 in FIG. 4) and releases output signals Y0 - Ym indicating the addresses 
of the scanning line. 
CT0 - CTn in FIG. 8 indicate the output of the address counter 9', wherein 
CT0, CT1, . . . , CTn respectively indicate the timings of reading the 
addresses 0, 1, . . . , n of the image memory 10. The latch 11' performs 
latching operation at the start of the pixel clock signal S3' to release 
an output signal S2' as shown in FIG. 8, wherein (n) indicates the data of 
an address n in the memory. The signal S2' is supplied to the latch 12' 
which performs latching operation at the end of the pixel clock signal S3' 
in consideration of a delay in timing. FIG. 8 also shows the timing of the 
signal 24' from the latch 12'. 
The counter 9-2', which corresponds to the number of data in a line, 
releases a carry signal 304' (timing T3 in FIG. 8) after beam scanning of 
a line. The carry signal 304' is supplied, through an inverter 306', to 
the preset terminal of the flip-flop 303' to shift the output Q thereof to 
the low level state (timing T3 in FIG. 8), whereby the counter 9-2 is 
cleared and remains zero until the horizontal synchronization signal S1' 
is entered again. The counter 9-1' performs a stepwise increment by the 
carry signal 304' to release the addresses of a next scanning line through 
the lines Y0 - Ym. For example, if a line contains 4096 pixels at maximum, 
the counter 9-2' releases outputs Q0-Q11 assuming values from 0 to FFF in 
hexadecimal number (0-4095 in decimal number), while the outputs Y0 - Ym 
of the counter 9-1' represent fourth and upper digits in hexadecimal 
number, or 13th and upper bits of the address of the image memory 10'. 
After the scanning of a line, when the beam is detected again by the 
photodetector 5 at the start of scanning of a succeeding line, data 
corresponding to the sheet size, obtained from the sheet size detector 3 
are loaded in the counter 4' (timing T4 in FIG. 8), and a procedure 
similar to the foregoing is executed. This procedure is different from the 
foregoing in that the output of the address counter 9' starts, for 
example, from an address 1000, in hexadecimal number, and the content of 
the image memory 10' is released from the address 1000, in the order of 
CT1000, CT1001, . . . as shown in FIG. 8, since the output Y0 of the 
counter 9-1' is in a state "1". 
In the foregoing description, the counter 4' need not be composed of a 
down-counter but may be composed of an up-counter or a combination of an 
upcounter and a comparator. Also the image memory 10' and the address 
counter 9' in the control unit 1' are shown simply for the explanation of 
the interface and can be composed of any other component capable of 
releasing digital pixel signals in synchronization with horizontal 
synchronization signals and pixel clock signals, such as a television 
camera or a CCD sensor. Also, in the foregoing explanation it is assumed 
that the latch 11 performs a latching operation at the start of the pixel 
clock signal S3' while the latch 12' performs a latching operation at the 
end of said pixel clock signal, but both latches 11, 12 may perform a 
latching operation at the start or end of the pixel clock signal S3'. 
Also in the foregoing description, a second clock generator 8' is provided 
in the control unit independently from the clock signal of the printer 
unit, but it is also possible to adopt a high clock frequency in the 
printer unit and to utilize a frequency obtained by dividing said high 
frequency, in place for said second clock generator. A change in clock 
frequency, for example for changing the image density, is attained in this 
case by a change in the ratio of said frequency division. 
In the above-explained structures, the timing of synchronization signal is 
not affected at all by a change in the frequency of pixel clock signals 
for the purpose of a change in the image signal density of an enlargement 
or reduction of the image. 
Though the foregoing embodiments are limited to recording apparatus in 
which image signals are supplied from the control unit to the printer 
unit, the present invention is not limited to such embodiments but is 
applicable also to a recording apparatus in which code signals are sent 
from the control unit to the printer unit. In such case, in response to a 
received code signal, the printer unit makes an access to a character 
generator to form a determined pattern such as a character or a graphic 
pattern. 
Also, in the foregoing embodiments, the control unit 100 or 100' may be a 
reader which generates image signals by reading an original documents, 
places on a support table, with an image sensor such as CCD, or a host 
computer in which pixel data or code signals, collectively called 
recording data, to be supplied to the printer unit, are stored in a memory 
medium such as a magnetic tape. Also, the control unit 100 or 100' can be 
an electronic file. 
Also, in the foregoing embodiments, the printer unit may also be composed, 
instead of a laser beam printer, of another printer such as an ink jet 
printer, a thermal printer or a wire dot printer. 
Furthermore, the present invention is not limited to the foregoing 
embodiments but is subject to various modifications within the scope and 
spirit of the appended claims.