METHOD FOR VARYING THE RESPONSE OF A PRINT DEVICE TO AN INPUT SIGNAL AND SYSTEM AND PRINTING APPARATUS IN WHICH THIS METHOD IS APPLIED

Abstract: Method of generating a control signal (c) for a writing device (1) which is suitable for altering the density (d) of each picture element of a surface of a substrate divided into picture elements as a function of the level of the control signal (c), being possible to choose the control signal (c) from more than two values corresponding to particular values of the density (d) and the control signal being generated on the basis of an input signal (i) whose levels have a pre-selectable relationship (c(i)), comprising two sub-relationships, with the levels of the control signal (c) which is such that levels of the density (d) to be predetermined are obtained for the levels of the input signal (i). The invention also comprises a printing system comprising a writing device (1) suitable for altering the density (d) of each picture element of a surface of a substrate divided into picture elements as a function of the level of a control signal (c) for the writing device (1), being possible for the control signal (c) to have any of more than two levels corresponding to particular values of the density (d), with conversion means (2; 2', 2'') for delivering corresponding levels of the control signal (c) on the basis of levels of an input signal (i) in accordance with a predetermined relationship (c(i)), stored in the conversion means and comprising two sub-relationships, between levels of the signals (i, c) which is such that predetermined levels of the density (d) are obtained for the levels of the input signal (i).

Description: Short title: Method for varying the response of a print device to an input signal and system and printing apparatus in which this method is applied. The invention relates to a method according to the preamble of Claim 1, and also to a system according to the preamble of Claim 9 in which this method is used. A method and a system of the abovementioned type is disclosed by the US Patent Specification 4,620,196 in which the writing device is exemplified by an ink jet device with which, as a function of the level of a digital control signal converted into a time interval, a number of droplets corresponding to the level of the control signal can be selectively deposited on every picture element (pixel) of a surface of a substrate divided into picture elements, in order to obtain a corresponding optical density. The drawback of the known method is that the density delivered by the writing device in response to the control signal is dependent on the physical operation of the writing device. If the pressure of the ink fed to a nozzle of the device is increased or reduced, the extent of the droplets formed will increase or decrease respectively, the number of droplets per unit time remaining essentially unchanged, as a result of which more or less ink respectively is deposited with a correspondingly higher or lower density respectively on the substrate than was desired or expected on the basis of the level of the control signal. In a practical embodiment, the nozzle of the ink jet device has a diameter of 10 micrometres and, to obtain a satisfactory result from the operation of the device, the maximum tolerance in the diameter is 0.1 micrometre. This demands a measuring and manufacturing accuracy which is at the limit of what is practically achievable, as a result of which the cost price of the writing device is relatively high. The abovementioned drawback is all the more serious the more writing devices, for one single colour or for different colours, a printing system or printing equipment comprises because the human eye is very sensi- tive to perceiving differences in density, in particular, in mutually adjacent groups of picture elements for the same control signal levels delivered to the different writing devices. On the basis of practical procedures it is known to use a separate, accurately adjustable ink pump for each writing device. Such pumps are, however, very expensive. The object of the invention is to eliminate the drawbacks of the known method. The object of the invention is also to provide an improved method for generating a control signal for a writing device which makes it possible to simulate a printing process, for example offset, to be chosen regardless of the operation of the writing device. These objectives are achieved for the method specified in the preamble of Claim 1.by means of the measures specified in the characterization of Claim 1. The relationship can be represented entirely or partially by a function or a table of mutually corresponding signal levels. With a suitably chosen relationshi , the result delivered by the writing device on the substrate surface will be identical to the result delivered by a standard¬ ized writing device without the agency of the relation¬ ship on the substrate surface. In manufacturing writing devices, such a relationship can be determined for each writing device and can preferably be stored in a non¬ volatile memory (ROM) and concomitantly delivered with the writing device for installation in printing equip¬ ment. The relationship may comprise two sub-relation- ships of which the first sub-relationship is chosen in a manner such that, if the second sub-relationship is linear, the same result is obtained with the writing device as with a standardized writing device without the agency of the first sub-relationship. In a practical embodiment, the second sub-relationship for simulating a printing process may be non-linear. The first sub-rela¬ tionship may in this case be advantageously stored in a non-volatile memory (ROM) associated with the writing device, while, depending on the printing process to be simulated, the second sub-relationship can be stored in a volatile memory (RAM) from a data processing system. To obtain transitions which are less well per¬ ceived by the eye between different densities of mutually adjacent groups of picture elements and to avoid the occurrence of undesirable patterns easily perceivable by the eye as a consequence of quantization faults, the method according to Claim 7 is preferably used. To determine or apply the relationship, the method according to Claim 8 is preferably used. The invention also relates to a printing system according to one of Claims 9 to 16 inclusive in which the method according to the invention is used. The invention also relates to printing equipment in which such a printing system is installed, so that- load is removed from a data processing system and/or data transmission system and such systems only have to gene¬ rate and deliver the input signal which is independent of the operation of the printing equipment. The invention is explained by reference to the drawings. In the drawings: Figure 1 shows a block diagram of a first embodi¬ ment of a printing system according to the invention in which the method according to the invention is used. Figure 2 shows a diagram explaining the operation of the system of Figure 1. Figures 3, 4 and 5 show block diagrams of second,, third and fourth embodiments of a printing system accord¬ ing to the invention in which the method according to the invention is used. The printing system shown in Figure 1 comprises a writing device 1 and a conversion circuit 2. The conversion circuit 2 receives an input signal i from a data processing system and/or data transmission system 3 and, in response thereto, delivers a control signal c having a level corresponding to the level of the received input signal i to the writing device 1. The writing device 1 may be any sort with which a change in the density d of any picture element of a surface of a substrate divided into picture elements can be obtained as a function of the control signal c, with a choice of more than two density levels. The writing device may be, for example, a light-emitting element such as a laser or light-emitting diode or a light interrupt¬ ing element such as a liquid crystal display (LCD) fitted between a light source and a substrate with which, as a function of the level of the control signal delivered to the element, a photosensitive layer of the substrate can be exposed in order to alter the optical density thereof. The writing device may also be an ink jet device for spraying ink droplets selectively onto the substrate surface or a heating element for selectively heating heat-sensitive dyestuffs deposited on a carrier between the writing device and the substrate or deposited on the substrate. The density may be, for example, an optical density or a density of an electrical charge provided on a suitable substrate, for example a semiconductor sub- strate. With the electrically charged substrate as starting point, it is possible to obtain a corresponding imprin . Figure 2 shows a diagram having four quadrants with an origin 0 in the centre and with the functions d(i), d(c) and c(i) respectively in three quadrants, i corresponding to the levels of the control signal, d to the optical density obtained by means of the writing device 1 on the substrate and c to the levels of the control signal. The function d(c) thus indicates the intensity d which is obtained by means of the writing device 1 on a particular substrate if the writing device 1 is controlled by the control signal c. The function . d(i) gives the desired density d in response to the input signal i received by the conversion circuit 2. For the desired density d to be obtained, the input signal i received by the conversion circuit 2 has to be converted into the control signal c in accordance with the function c(i). For example, for an input signal i having level i l r the corresponding desired density d having level ά x is obtained by means of a control signal c having level c x and this leads to a point i x of the function c(i). Simi¬ larly, for an input signal i having level i 2 , the corresponding desired density d 2 is obtained by means of a control signal c having level c 2 , and this leads to a point f 2 of the function c(i), and so on for the other levels of the input signal and the control signal c. The relationship.thereby obtained between levels of the input signal i and the control signal c may be entirely or partially represented by a function (formula) which, for the relationship shown as an example in Figure 2, may be the following: c(i) - k x i - k 2 sin(k 3 i) in which k 17 k 2 and k 3 are constant. The relationship may, however, also be represen¬ ted entirely or partially by a table of mutually corresponding levels of the input signal i and the control signal c, and in this case the conversion circuit 2 may comprise a simple memory in which the table is stored. If the printing system 1, 2 always has to operate in the same manner, the table may advantageously be stored in a non-volatile memory (ROM) . In that case, the relationship can be determined and stored for each writing device 1 to be manufactured in a manner such that the printing system 1, 2 obtained with said writing device 1 delivers the same result as a standardized writing device without the agency of the relationship. The conversion circuit 2 in that case forms a specific conversion circuit 2 associated with the non-standardized writing device 1. If it is desired to simulate a printing process to be chosen, it is preferable to split the conversion circuit 2 into two parts 2' and 2 " , as shown in Figure 3. In this case, the part 2" preferably functions as the specific conversion circuit mentioned above, which, together with the writing device 1, should be interpreted as a printing system which delivers the same result as a standardized writing device. The part 2' may be used for storing any relationship depending on the printing process, for example offset, to be simulated, it being possible for said relationship to be determined inde¬ pendently of the operation of the writing device by the data system 3 and to be used unaltered in any similar printing system 1, 2', 2". Figures 4 and 5 show embodiments of the type shown in Figure 1, comprising a memory 4, a buffer register 5 at the input of the memory 4 and a number of adders 6 to 8 inclusive between the data system 3 and the buffer register 5, each of the adders 6 to 8 inclusive receiving a different signal value from the memory 4, the values intended for the adders 7 and 8 being delayed by means of delay circuits 9, 10 by one or two transfer cycles, respectively, of the buffer register 5. The embodiment according to Figures 4 and 5 is used if the number of levels chosen for the input signal i is larger than the number of levels with which the control signal c can effectively control the writing device 1. The number of locations of the memory 4 is in this case equal to the number of chosen levels of the input signal i and the maxima and minima of the input signal i and of the control signal c then correspond to _each other. The locations of the memory 4 contain the level of the control signal c which is proportionally nearest the address thereof and also a difference value which is fed back to the adders 6 to 8 inclusive. If the difference value is 0, as is the case when the system is switched on, the next address at the output of the buffer register 5 corresponds to the level, delivered by the data system 3, of the input signal i. The memory 4 delivers in response thereto the nearest corresponding level of the control signal c and a dif¬ ference value which is equal to the difference between the address of the memory 4 corresponding to the level of the input signal i and an address value corresponding to the control signal without difference value. If, for example, the input signal i has 256 levels 0, 1, 2, ...., 255 and the control signal has 16 levels 0, 1, 2, , 15 corresponding to the levels 0, 15, 31 .... 255 of the input signal i, after the system has been switched on and if a linear relationship is chosen by way of example, the memory 4 will deliver for the level 24 of the input signal i the level 2 of the control signal c with a difference value of 31 - 24 = 7 corresponding to the input signal i, which value is added to the next level of the input signal i by the adder 6. The embodiment according to Figure 5 is expanded with respect to the embodiment of Figure 4 in that the difference value is composed of three sections which are each delivered to the respective adders 6 to 8 inclusive with suitable delays. Of course, this type of weighting of the difference value can be expanded by using more adders and more delay circuits. It is pointed out that the contents of the locations of the memory 4 can be chosen in a manner such that the section containing the circuits 4 to 6 inclusive or 4 to 10 inclusive, respectively, has the same response as the part 2" of Figure 3 and that an additional con¬ version circuit such as the part 2' of Figure 3 may be provided between the data system 3 and the adder 6 or 8 respectively. „_The conversion circuits such as 2 may be provided entirely or partially (for example 2") remotely from printing equipment ~ -(not_shown) comprising the writing device 1. Preferably, however, the conversion circuit is entirely provided in the printing equipment, so that the data system 3, which is in general a computer intended for general applications, and the transmission paths thereof to the printing equipment are relieved of load and the printing equipment can be used at any point in time and at any position regardless of the operation and the data file of the data system 3, to deliver the same result as printing equipment having a corresponding number of standardized writing devices. It is pointed out that, because the relationship c(i) can be introduced into the conversion circuit 2 in any known manner, with serial or parallel transmission, for the sake of clarity, not all the conductors which are necessary to transmit the relationship and the signals i and c are shown in the figures. It is further pointed out that a relationship can be concomitantly delivered to a writing device, which relationship compensates for mutual differences between different writing devices. The relationship can be concomitantly provided in the form of a chip which is incorporated in the writing device and which is read out by the computer which controls the printing system after the writing device has been plugged in. Alternatively, the relationship can be concomitantly delivered with the writing device stored on, for example, a floppy, and in this case the floppy data should also be read in when the writing device is plugged in. - Claims -

Claims: C A I M S 1. Method of generating a control signal (c) for a writing device (1) which is suitable for altering the density (d) of each picture element of a surface of a substrate divided into picture elements as a function of the level of the control signal (c), it being possible to choose the control signal (c) from more than two values corresponding to particular values of the density (d) , characterized in that the control signal (c) is generated on the basis of an input signal (i) whose levels have a pre-selectable relationship (c(i)) with the levels of the control signal (c) which is such that levels of the density (d) to be predetermined are obtained for the levels of the input signal (i). 2. Method according to Claim 1, characterized in that the relationship (c(i)) is chosen as a function of the operation of the.writing device (1) in a manner such that the writing device (1) delivers, for particular levels of the input signal (i), the same values of the density (d) on the substrate as a standardized writing device would deliver for said levels of the input signal (i) without the agency of the relationship (c(i)). 3. Method according to Claim 1, characterized in that the relationship (c(i)) comprises two sub-relation¬ ships, the x-axis values of the first sub-relationship corresponding to the levels of the input signal (i), the y-axis values of the first sub-relationship corresponding to the x-axis values of the second sub-relationship and the y-axis values of the second sub-relationship cor¬ responding to the levels of the control signal (c) and the first sub-relationship being chosen as a function of the operation of the writing device (1) in a manner such " that, if the other sub-relationship is a linear sub- relationship, the writing device (1) delivers, for particular levels of the input signal (i), the same values of the density (d) on the substrate as a stan¬ dardized writing device would deliver without the agency of the relationship (c(i)) for said levels of the input signal (i) . 4. Method according to Claim 1, characterized in that the relationship (c(i)) is represented at least partly by a function. 5. Method according to Claim 1, characterized in that the relationship (c(i)) is represented at least partly by a table of levels of the control signal (c) corresponding to different levels of the input signal (i)- 6. Method according to Claim 1, characterized in that at least a section of the relationship (c(i)) can be repeatedly altered. 7. Method according to Claim 1, in which the picture elements are scanned according to a predetermined path, characterized in that the chosen number of levels of the input signal (i) is greater than the number of levels of the control signal (c), the respective maxima and minima of the signals corresponding to each other, in that the level of the control signal (c) which is nearest is chosen for a value of the control signal corresponding to a received level of the input signal (i) according to the relationship (c(i)), and in that the difference between the received level of the input signal (i) and the level of the input signal (i) corresponding to the chosen level of the control signal (c) is added to the next levels of the input signal (i) to be received according to a predetermined distribution and delay algorithm prior to choosing subsequent levels of the control signal (c). 8. Method according to Claim 1, characterized in that, for a number of levels of the input signal (i) a " number of small areas with corresponding values of the density (d) are formed on the substrate, in that the density of each small area is measured, and in that the relationship (c(i)) is used to obtain desired densities for said levels of the input signal (i) in accordance with the measured values of the density. 9. Printing system comprising a writing device (1) suitable for altering the density (d) of each picture element of a surface of a substrate divided into picture elements as a function of the level of a control signal - li ¬ fe) for the writing device (1), it being possible for the control signal (c) to have any of more than two levels corresponding to particular values of the density (d), characterized by conversion means (2; 2', 2") for deli- vering corresponding levels of the control signal (c) on the basis of levels of an input signal (i) in accordance with a predetermined relationship (c(i)), stored in the conversion means, between levels of the signals (i, c) which is such that predetermined levels of the density (d) are obtained for the levels of the input signal (i). 10. System according to Claim 9, characterized in that the relationship (c(i)) is chosen as a function of the operation of the writing device (1) in a manner such that the writing device (1) delivers, for certain levels of the input signal (i), the same values of the density (d) on the substrate as a standardized writing device would deliver without the agency of the relationship (c(i)) for said levels of the input signal (i). 11. System according to Claim 9, characterized in that the relationship (c(i)) comprises two sub-relation¬ ships, the x-axis values of the first sub-relationship corresponding to the levels of the input signal (i), the y-axis values of the first sub-relationship corresponding to the x-axis values of the second sub-relationship and the y-axis values of the second sub-relationship corresponding to the levels of the control signal (c), and the first sub-relationship being chosen as a function of the operation of the writing device (1) in a manner such that, if the other sub-relationship is a linear sub- relationship, the writing device (1) delivers, for certain levels of the input signal (i), the same values of the density (d) on the substrate as a standardized writing device would deliver without the agency of the relationship (c(i)) for said levels of the input signal (i). 12. System according to Claim 9, characterized in that the conversion means (2; 2', 2") comprise a memory which is suitable for storing a function representing at least partly the relationship (c(i)). 13. System according to Claim 9, characterized in that the conversion means (2; 2', 2") comprise a memory which is suitable for storing a table, representing at least partly the relationship (c(i)), of levels of the control signal (c) corresponding to different levels of the input signal (i) corresponding to addresses of the memory. 14. System according to Claim 9, characterized in that the conversion means (2; 2 ' , 2") comprise a non- volatile- memory for storing at least a section of the relationship (c(i) ) 15. System according to Claim 9, in which the picture elements are scanned according to a predetermined path, characterized in that the number of levels of the input signal (i) is greater than the number of levels of the control signal (c), the respective maxima and minima of the signals corresponding to each other, in that the conversion means (2; 2' ,2") choose the nearest level of the control signal (c) for a value of the control signal corresponding to a received level of the- input signal (i) according to the relationship (c(i)), and in that the conversion means (4 to 6 inclusive; 4 to 10 inclusive) comprise difference determining means, adding means (6; 6 to 8 inclusive) and delay means (5; 5, 9, 10) which are suitable for delivering the difference between the received level of the input signal (i) and the level of the input signal (i) corresponding to the chosen level of the control signal (c) and for adding delayed sections of the difference to subsequent levels of the input signal (i) to be received according to a predetermined distribution and delay algorithm prior to choosing subsequent levels of the control signal (c) . 16. System according to Claim 9, characterized by a density meter which is suitable for measuring the density (d) of each of a number of small areas of the substrate coloured by means of suitably chosen levels of the control signal (c), in that the density meter is coupled to the conversion means (2; 2', 2") and in that the conversion means are suitable for applying the relationship (c(i)) as a function of the measured density levels and of levels of the density (d) assigned before¬ hand to levels of the input signal (i). 17. Printing equipment comprising a system according to one of Claims 9 to 16 inclusive.

Inventors: DIEDEREN JACOBUS HENRICUS, DIEDEREN, JACOBUS, HENRICUS
Applicants: STOCK X CEL B V, STOCK X-CEL B.V.
Application year: 1989
Main classification: H04N1
IPC codes: H04N1