Gray tone reproduction apparatus

A spirally interlacing jet drop recorder prints gray tones on a recording sheet mounted upon a rotating drum. The number of jets and the jet spacing is selected in such a manner as to facilitate time sharing of a matrix pattern memory by control circuits controlling operation of each of the jets. Matrices representing the gray levels to be produced are progressively printed from column to column by different ones of the array of jets.

BACKGROUND OF THE INVENTION 
This invention relates generally to gray tone reproduction and has 
particular application to gray tone reproduction utilizing an ink jet 
printer of the general type described in Sweet et al U.S. Pat. No. 
3,373,437. As disclosed in the Sweet et al patent, gray tones may be 
reproduced by controlling the rate at which drops are deposited upon a 
moving web. Behane et al U.S. Pat. No. 3,604,846 teaches an improved gray 
scale reproduction technique wherein drops are deposited at various 
positions within a two dimensional matrix in accordance with the gray 
level to be reproduced. Variations on the matrix approach are disclosed in 
Berry et al U.S. Pat. No. 3,977,007 and in Wong U.S. Pat. No. 4,032,978. 
This invention applies more particularly to an ink jet printer of the type 
taught by Paranjpe et al in U.S. Pat. No. 4,112,469. In printers of that 
type there is provided a print head having a row of spaced jets which are 
scanned across a recording member in the form of a sheet mounted upon a 
rotating drum. The scanning is carried out by transporting the print head 
axially along the length of the drum in such a manner as to cause spiral 
interlacing of the printed tracks produced upon the sheet by the different 
jets. As taught by Paranjpe et al, spiral interlacing may be achieved by 
providing the print head with any convenient number of jet producing 
nozzles and separating those nozzles by an appropraite integral number of 
printing track widths. An "appropriate" integral number is any number 
which has no factor other than 1 as a common factor with the number of 
nozzles. As also taught by Paranjpe, the print head should be axially 
advanced at a speed such that during one rotation of the drum, the axial 
advance is equal to the width of one printed track, multiplied by a number 
equal to the number of nozzles. Printing control is accomplished by 
simultaneously scanning a document, which is positioned upon a document 
plane, and sweeping an image of the document past a line of photocells 
arranged in correspondence with the arrangement of jet printing nozzles. 
Each photocell is connected to switch an associated jet into a catching 
position whenever the observed light level from the document is above some 
predetermined threshold. 
This invention also has application to a spirally interlacing printer of 
the type disclosed in Fox U.S. Pat. No. 4,069,486, which may include a 
plurality of rows of nozzles all spirally interlacing under control of 
data signals produced by an appropriately configured data system. Neither 
the Fox patent or Paranjpe et al teach any method for operating such 
spirally interlacing rows of jets to reproduce gray levels appearing on 
the original document. 
Other techniques for controlling an ink jet printer to reproduce gray tones 
are disclosed in Loughren U.S. Pat. No. RE27,555, Loughren U.S. Pat. No. 
3,476,874, Chen U.S. Pat. No. 3,846,800, Sagae et al U.S. Pat. No. 
3,928,718, Berry U.S. Pat. No. 4,065,773, and in Hertz et al U.S. Pat. No. 
3,416,153. These latter patents generally relate to methods for 
controlling the rate at which ink is deposited at a given location on a 
recording medium. 
SUMMARY OF THE INVENTION 
According to the present invention gray level matrix patterns are printed 
by an arrangement of marking elements which are positioned so as to extend 
progressively in the widthwise direction along a print receiving sheet 
mounted on a support member. Transport means are provided for repetitively 
transporting the recording sheet in the heightwise direction across the 
active region of the marking elements. Other transport means produce 
relative movement of the marking elements in the widthwise direction so as 
to produce interlacing of the tracks which the marking elements print on 
the recording sheet. The number of marking elements is selected in such a 
manner as to differ by 1 from the product produced when the number of 
columns in any marking matrix is multiplied by any positive, non-zero 
integer. Also, the spacing between the marking elements is equal to the 
width of one of the matrices divided by any non-zero positive integer, 
which evenly divides the number of columns in a matrix. 
Further in accordance with the practice of this invention each of the 
marking elements is provided with a density code generator for generating 
gray scale codes representative of gray levels to be printed and a shift 
register for storing bit strings representing columns of dots to be 
printed. The bit strings comprising all possible dot columns are stored in 
a common memory for writing out to each of the shift registers on a time 
sharing basis. A rotation counter cooperates with the density code 
generators to generate addresses for those bit strings which are to be 
written out to the shift registers. 
In preferred embodiment the recording sheet is mounted on a rotating drum, 
and the marking elements are drop streams generated by a jet print head. 
Also in preferred embodiment the density code generators comprise sensing 
elements which are arranged in correspondence with the arrangement of the 
printing streams and which observe an original image which is scanned 
therepast. 
It is therefore an object of the invention to provide apparatus for 
interlaced printing of matrix patterns representing the gray levels in an 
image to be recorded. 
It is another object of the invention to provide gray scale printing 
capability for a spirally interlacing jet drop recorder. 
Other objects and advantages of the invention will be apparent from the 
following description, the accompanying drawings and the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As generally illustrated in FIG. 1 a copier operating in accordance with 
this invention may comprise a document illumination station 10, a printing 
system 12, and a paper transport system 13. In order to produce one or 
more copies an original document 14 is placed flat on a planar support 
glass 15. A suitable control switch activates a pair of lamps 16 to 
illuminate document 14 for imaging by a scanning lens 17. 
The image scanned by lens 17 is reflected from the surface of a rotating 
mirror 18 into a focusing lens 19 for imaging upon a photodiode array 20 
positioned at the image plane of focusing lens 19. A line of individual 
photodiodes in the array 20 are spaced to observe spaced points along a 
line such as line 42. The rotation of mirror 18 causes the line of 
observed points to move in a direction as indicated by the arrow 43. For 
one embodiment as hereinafter described, there may be eighty-six 
individual photodiodes spaced to observe eighty-six points along the line 
42. A typical spacing between photodiodes may be about 0.025 inches, 
center-to-center. 
Scanning system 11 is supported by a table 21 driven by a synchronous drive 
motor 22 under control of a control unit 24. A jet drop print head 26 is 
transported axially along a worm 27 in synchronism with the movement of 
table 21. During this axial movement, print head 26 directs a series of 
jets 65 toward a recording sheet 29 mounted on a rotating drum 30. As 
taught in the above mentioned Paranjpe et al patent, the spacing of jets 
65 corresponds to the spacing of the photodiodes in the array 20. The jets 
65 print a series of tracks 66, as best illustrated in FIG. 2. Printing 
drum 30 is driven by a drive motor 31 under control of control unit 24 for 
movement in synchronism with the movement of mirror 18 and its drive motor 
25. The recording sheet 29 is transferred between drum 30 and transport 
system 13 by transfer elements not illustrated in detail. 
Print head 26 generally comprises a fluid supply manifold 46, an orifice 
plate 44, a charge ring plate 48, deflection electrodes 49, and a catcher 
50, all as illustrated in FIG. 3. The manifold 46 contains a supply of ink 
52, which flows under pressure through orifices 45 to form the streams 65. 
Streams 65 comprise a series of uniformly sized and regularly spaced 
drops, which are generated under the control of a stimulator, as 
schematically illustrated at 51. 
Charge plate 48 includes a series of electrodes 53, each of which is 
connected to a charging control line 67. As taught in the above mentioned 
Paranjpe et al patent and in prior art cited therein, the stream 65 can be 
controlled to deposit either upon the sheet 29 or into catcher 50 by 
applying control signals to the line 67. 
FIG. 4 illustrates the control circuitry which generates printing control 
signals for the charge control line 67. Under control of that control 
circuitry, all of the lines 67 cause the jets 65 to print in a cooperative 
fashion such that different gray scales appearing on the document 29 are 
reproduced in matrix fashion. A series of matrix patterns, A through Z, 
for reproducing twenty-six gray levels ranging from white to black are 
illustrated in FIG. 5. Control unit 24 includes a memory 68 containing 
pattern information corresponding to the twenty-six patterns of FIG. 5. 
Memory 68 is addressed by rotation codes from a rotation counter 81 and by 
gray scale codes from a series of density code generators 35. Each jet has 
its own density code generator 35, and all jets share memory 68. 
In order to enable such memory sharing, the jets 65 are spaced for 
simultaneous printing of corresponding columns from a field of matrices 
comprising the copy being printed. For example, a printed sheet may 
comprise a two dimensional field of matrices printed with different ones 
of the twenty-six different matrix patterns illustrated in FIG. 5. Each 
such matrix pattern is printed by a sequence of different jets, each 
printing columnar sets of dot patterns collectively comprising the overall 
matrix patterns. For 5.times.5 matrix patterns as illustrated in FIG. 5, 
the columnar dot patterns are printed within five-cell columns, which 
extend in the vertical or heightwise direction and which are positioned 
side-by-side in the widthwise direction. As illustrated in FIG. 5, the 
matrix columns have a width p and are designated from left to right by the 
designations a through e. 
The rotation of drum 30 causes the jets to scan in the heightwise 
direction, while the translation of head 26 along worm 27 causes printing 
to progress in the widthwise direction, with sequential printing of the 
columns a through e. For printing the illustrated 5.times.5 matrices, the 
jets 65 have a center-to-center spacing of 5p. Thus if one jet is printing 
column a from one of the illustrated matrix patterns, all jets will be 
printing column a from that or another matrix pattern. Upon printing, 
these columnar patterns take on the form of the tracks 66 of FIG. 2. 
The general spacing requirement of this invention requires that the jets, 
or other marking elements, be spaced in the widthwise direction at a 
spacing equal to the width of a marking matrix divided by any non-zero 
positive integer which evenly divides the number of columns comprising a 
matrix. Expressed in equation form: 
EQU s=p(m/I.sub.1) 
where m is the number of columns in a matrix pattern, p is the width of one 
column, s is the distance between the marking elements, and I.sub.1 is the 
dividing integer. 
From a practical design point of view, m and p are established by the 
nature of the graphics being reproduced, so the jet spacing s is the 
dependent variable. In the above case where m=5, there is no integer other 
than 1 which evenly divides m. Thus for the case of the example, s must 
have a value of 5p. If m had a value of 15 (i.e. 15.times.15 matrices), 
then the spacing of the marking elements could be any of 5p, 10p, or 15p 
corresponding respectively to I.sub.1 values of 3, 2 or 1. 
Further in order to print matrix patterns in accordance with this 
invention, the number of marking elements is selected in accordance with 
the equation: 
EQU n=I.sub.2 m.+-.1 
where n is the number of marking elements and I.sub.2 is any positive 
non-zero integer. For the embodiment herein described, I.sub.2 has a value 
of 17, so that there may be either 84 or 86 jets. Tabulated data are 
hereinafter set forth for both cases. 
It may be demonstrated that the above limitations upon jet spacing and the 
number of jets are but a special case of the general teaching of Paranjpe 
et al in U.S. Pat. No. 4,112,469. The patent discloses that spiral 
interlacing of a row of jets may be achieved if the jets are spaced apart 
some number of track widths having no factor other than 1 as a common 
factor with the number of jets. The patent also discloses a further 
requirement that the advance speed of the print head must be so related to 
the drum rotation speed that during one rotation of the drum, the head 
moves a distance equal to the number of jets multiplied by the width of a 
printed track. This invention follows that teaching, so that in one 
special case, as hereinafter described, print head 26 advances along the 
worm 27 a distance 86p during one rotation of drum 30. In the other 
described special case the head 26 advances a distance 84p during the same 
period of time. 
It will be appreciated that the selection of the number 17 as a value for 
I.sub.2 is entirely arbitrary. The factor I.sub.2 could just as easily 
have a value of 1, 2 or 3, which means that a five-column matrix may be 
printed by a row of 4, 6, 9, 11, 14 or 16 jets. As the number of jets 
increases the cost of the system also increases, but the time for 
producing a printed copy decreases. 
Tables I and II present summaries of matrix printing operations for 
printing heads producing eighty-six and eighty-four jets respectively. The 
tables present printing information for a series of printing columns 
number from left to right across the axially extending dimension of print 
sheet 29. For each column there is presented the identification number of 
the stream which effects printing thereof, the rotation number for the 
rotation of drum 30 during which printing is accomplished, the 
identification number of the matrix encompassing that printing column (the 
matrices being numbered from left to right across the entire field of 
printing) and the designation of the matrix column which is being printed. 
In each case printing is commenced with the right hand stream printing the 
first or left hand printing column. At this time all other streams are 
positioned to the left of the print sheet 29 and are in a catching 
position. As a matter of convention, drum 30 is deemed to be rotating 
through rotation number 0 at the time when printing column number 1 is 
being printed. 
Looking now at Table I it will be seen that stream number 86 prints matrix 
column a during rotation 0. This means that at all times during rotation 
0, stream number 86 will print dot patterns from matrix column a of one of 
the twenty-six illustrated patterns A-Z of FIG. 5. During the time period 
between rotation 0 and rotation 1 print head 26 moves from left to right a 
distance equal to the width of eighty-six printing columns. Thus during 
rotation number 1, stream number 86 prints printing column number 87. 
During the course of this rotation stream 86 will print matrix column b 
from selected ones of the twenty-six matrix patterns, the printing being 
performed in the eighteenth matrix as numbered from left to right. 
Similarly stream 86 prints matrix columns c, d, and e on successive drum 
rotations. 
In like manner stream number 69 prints a "b" matrix column within printing 
column number 2 during drum rotation number 1 and then on successive drum 
rotations prints matrix columns c, d, e, and a. The printing from all 
streams will be seen to interlace, and each stream moves progressively 
through the matrix column sequence from left to right with each rotation 
of drum 30. Moreover, as indicated by Table I all streams print common 
matrix columns during the same rotation of drum 30. Thus during rotation 
number 2, all streams then within the printing area of recording sheet 29 
are printing matrix column c, as may be verified by looking at the 
tabulated data for printing column numbers 3, 8, 13, 18, 23, 88, and 173. 
This feature of common matrix column printing and progessive printing of 
matrix columns greatly simplifies the data handling system, as will be 
described in detail below. 
Table I as described above, represents the type of progressive printing 
which occurs for the case where the number of jets is one greater than an 
number obtained by multiplying the number of matrix columns by a non-zero 
positive integer. For the case where the number of jets is one less than a 
number obtained by multiplying the number of matrix columns by a non-zero 
positive integer, reference may be made to Table II, which relates to the 
printing of 5-column matrices by an arrangement of 84 jets. For this 
arrangement, matrix column printing progresses from right to left, while 
print head 26 is physically transported from left to right. Thus during 
rotation number 1 all streams print "e" columns from various ones of the 
matrix patterns. On rotation number 2 all streams print "d" columns. This 
is followed by printing of "c", "b", and "a" columns on successive 
rotation. As shown in the table, printing of the five columns of the 
matrices occupying printing column Nos. 1-5 is accomplished by stream 
numbers 84, 17, 34, 51, and 68 during rotation numbers 0, 4, 3, 2, and 1 
respectively. Thus it requires a total of five rotations of drum 30 to 
complete the printing of any given matrix on print sheet 29, but during 
this period of time the data for printing the matrices is processed in a 
relatively simple and convenient manner. 
FIG. 4 illustrates data handling circuitry for conducting a printing 
process as described above with reference to Tables I and II. In general 
the data handling circuitry comprises memory 68, together with its input 
and output gating and a series of density code generators 35. Each density 
code generator 35 includes a photodiode 69, which for this example is one 
of 86 (or 84) photodiodes comprising the photosensor array 20. Photodiode 
69 observes points along a series of lines on document 14; a new line of 
image information being observed with each rotation of mirror 18. The 
output signal from photodiode 69 varies in accordance with the amount of 
light being observed, which corresponds to the point to point variation in 
the gray level of the original document. 
The output from photodiode 69 is amplified by amplifier 70 and supplied to 
Miller integrator 71, the output voltage of which has a slope equal to the 
level of the input voltage. The output from Miller integrator 26 is 
applied through gate 72 to an A to D convertor 73. Gating of signals 
through gate 72 is controlled by strobe pulses on line 74. 
Strobe pulses on line 74 have a dwell time equal to the printing time for 
printing five cell positions comprising one of the matrix columns a--e 
illustrated in FIG. 5. The strobe signal is generated by flip-flop 75, 
which is set and reset by a modulo 5 counter 76. Counter 76 in turn counts 
individual cell pulses from a clock 77. Clock 77, counter 76 and flip-flop 
75 serve all printing channels, while density code generator 35 is 
dedicated to a single channel. 
The strobe pulses generated on line 74 are differentiated by a 
differentiating network 85, and the leading and trailing edge spikes are 
squared off to produce leading edge and trailing edge pules. The leading 
edge pulses appear at the output of amplifier 78 and are used to reset the 
Miller integrator. The trailing edge pulses appear at the output of 
amplifier 79 to gate the integrated value of the observed light level 
through gate 72 and into the A to D convertor 73. 
The A to D converter 73 generates a five bit code representing one of the 
twenty-six matrix patterns of FIG. 5. This five bit code is supplied to a 
channel register 80, together with a three bit code from rotation counter 
81. Rotation counter 81 is advanced one count for each rotation of drum 
30, and the resulting output code represents one of the five matrix 
columns a-e. For the eight-six jet arrangement as herein described, the 
count proceeds in a forward direction for successive indication of columns 
a-e. For the alternative eighty-four jet arrangement, the rotation count 
proceeds in a reverse direction for indication of matrix columns e-a. The 
output of rotation counter 81 is reset after every fifth count. 
As a result of the operation of rotation counter 81 and A to D converter 
73, channel register 80 is loaded with codes representing the 130 columns 
comprising the twenty-six matrix patterns of FIG. 5. Bit strings 
corresponding to these 130 columns are stored in pattern cell memory 68 at 
addresses corresponding to the codes loaded into channel register 80. 
Addressing of pattern cell memory 68 is accomplished by a gating network 
82, which operates under control of memory controller 84. Memory 
controller 84 generates counting signals on a series of lines as generally 
illustrated at 86 for sequential gating of addresses from 86 (or 84) 
channel registers 80 into pattern cell memory 68. The signals on the lines 
86 are READ control signals which enable sharing of the pattern cell 
memory by all of the recording channels. 
Output writing of bit strings from pattern cell memory is also under 
control of memory controller 84, output writing being controlled by WRITE 
control signals on a set of lines, generally illustrated at 87, which are 
applied to an output gating network 83. Each of the printing control lines 
67 has an associated shift register 81 into which printing control bit 
strings are shifted. Memory controller 84 controls the READ/WRITE sequence 
such that each shift register 81 receives bit strings only from memory 
locations designated by its associated density code generator 35 
(operating in cooperation with rotation counter 81). 
Tables III and IV illustrate some of the bit strings which may be loaded 
into the shift registers 81. For example, if a density code generator 35 
generates a density code corresponding to matrix pattern B of FIG. 5, then 
the associated shift register 81 will be loaded with one of the five codes 
listed in Tabel III. The five codes correspond to matrix columnS A-e, as 
selected by the output code from rotation counter 81. If the output code 
from density code generator 35 remains constant, then the same bit string 
will be read out repeatedly during one rotation of drum 30. Rotation 
counter 81 will cause the other strings tabulated in Table III to be read 
out on consecutive rotations of the drum, and the associated jet 65 will 
print corresponding patterns. Similarly the jet will carry out printing 
under control of the codes tabulated in Table IV when its density code 
generator 35 generates an address portion specifying matrix pattern D. 
It will be understood that memory controller 84 generates eighty-six READ 
signals for gating network 82 and eighty-six WRITE signals for gating 
network 83 during the time that a five-bit string is being shifted out of 
shift register 81, this being necessary to enable time sharing of pattern 
cell memory 68 by eighty-six different recording channels. It will also be 
appreciated that the recording control signals shifted out of the shift 
registers 81 may control ON/OFF printing of pen recorders or other marking 
elements not specifically described herein. Furthermore the density 
control generators 35 may be any type of code generator capable of 
generating codes representing a series of different dot matrix patterns 
and may service marking elements which are arranged in a plurality of rows 
spaced around drum 30. 
While the forms of apparatus herein described constitute preferred 
embodiments of the invention, it is to be understood that the invention is 
not limited to these precise forms of apparatus, and that changes may be 
made thereto without departing from the scope of the invention. 
TABLE I 
______________________________________ 
Printing Stream Rotation Matrix Matrix 
Column No. No. No. No. Column 
______________________________________ 
1 86 0 1 a 
2 69 1 1 b 
3 52 2 1 c 
4 35 3 1 d 
5 18 4 1 e 
6 1 5 2 a 
7 70 1 2 b 
8 53 2 2 c 
9 36 3 2 d 
10 19 4 2 e 
11 2 5 3 a 
12 71 1 3 b 
13 54 2 3 c 
14 37 3 3 d 
15 20 4 3 e 
16 3 5 4 a 
17 72 1 4 b 
18 55 2 4 c 
19 38 3 4 d 
20 21 4 4 e 
21 4 5 5 a 
22 73 1 5 b 
23 56 2 5 c 
24 39 3 5 d 
25 22 4 5 e 
-- -- -- -- -- 
-- -- -- -- -- 
87 86 1 18 b 
88 69 2 18 c 
89 52 3 18 d 
-- -- -- -- -- 
-- -- -- -- -- 
173 86 2 35 c 
174 69 3 35 d 
175 52 4 35 e 
-- -- -- -- -- 
-- -- -- -- -- 
259 86 3 52 d 
260 69 4 52 e 
261 52 5 53 a 
-- -- -- -- -- 
-- -- -- -- -- 
345 86 4 69 e 
-- -- -- -- -- 
-- -- -- -- -- 
______________________________________ 
TABLE II 
______________________________________ 
Printing Stream Rotation Matrix Matrix 
Column No. No. No. No. Column 
______________________________________ 
1 84 0 1 a 
2 17 4 1 b 
3 34 3 1 c 
4 51 2 1 d 
5 68 1 1 e 
6 1 5 2 a 
7 18 4 2 b 
8 35 3 2 c 
9 52 2 2 d 
10 69 1 2 e 
11 2 5 3 a 
12 19 3 3 b 
-- -- -- -- -- 
-- -- -- -- -- 
83 50 3 17 c 
84 67 2 17 d 
85 84 1 17 e 
86 17 5 18 a 
87 34 4 18 b 
88 51 3 18 c 
-- -- -- -- -- 
-- -- -- -- -- 
167 50 4 34 b 
168 67 3 34 c 
169 84 2 34 d 
170 17 6 34 e 
171 34 5 35 a 
172 51 4 35 b 
-- -- -- -- -- 
-- -- -- -- -- 
253 84 3 51 c 
-- -- -- -- -- 
-- -- -- -- -- 
337 84 4 68 b 
______________________________________ 
TABLE III 
______________________________________ 
MATRIX B 
Column Code 
______________________________________ 
a 0 0 0 0 0 
b 0 0 0 0 0 
c 0 0 1 0 0 
d 0 0 0 0 0 
e 0 0 0 0 O 
______________________________________ 
TABLE IV 
______________________________________ 
MATRIX D 
Column Code 
______________________________________ 
a 0 0 0 0 0 
b 0 0 1 0 0 
c 0 0 1 1 0 
d 0 0 1 0 0 
e 0 0 0 0 0 
______________________________________