THERMAL PRINTER

Abstract: A thermal printer including a plurality of storage devices for respectively storing different types of print data and a thermal head for printing on a recording medium on the basis of the print data. The thermal printer has a voltage supply for supplying a plurality of D.C. voltages to the thermal head. A control circuit selects one of the storage devices and causes the voltage supply to supply one of the D.C. voltages in accordance with the selected storage device.

Description: THERMAL PRINTER Technical Field The present invention relates to a thermal printer suitable for printing different types of data such as image and character data (text) on a recording medium. Background Art Referring now to Fig. 1 a conventional thermal printer is schematically shown. In the figure, video signals obtained by scanning photos, documents and the like are transmitted to an image input unit 1, where the signals are A/D—converted. Digital data output from the image input unit 1 are supplied to an image storage device 2. On the other hand, the data pertaining to character data such as the depicted Chinese characters are created by a text input unit 3 such as a personal computer, a word processor and the like, and are then stored in a text storage device 4 such as a text memory. The image memory 2 or the text memory 4 supplies a thermal head driver 6 with the data read therefrom through a color converter 5 for correcting the properties of an ink donor film when making a color print. A thermal head 7 is equipped with a plurality of heating elements (not illustrated); and the thermal head driver 6 is capable of causing the heating elements to produce the heat by selectively energizing a group of the heating elements in accordance with the data supplied thereto. The color dye is transferred from the ink donor film 8 to a recording medium 9 by heat produced by the heating elements. A desired image is formed on the recording medium 9. In this figure, the reference numeral 10 represents a platen drum on which the recording medium 9 is wound; 11 denotes a constant—voltage power supply for supplying a constant voltage whereby the heating elements generate heat; and 12 refers to a control circuit which selects one of the memories 2 and 4 and permits the data to be read from the selected memory, viz., the .image memory 2 or the text memory 4. Next, the operations of the conventional thermal printer will be described. The video signals output from a video signal generating device such as a-video camera and the like are A/D—converted by the image input unit 1 and are then stored in the image memory 2. The text data input from the text 3 are stored in the text memory 4. The image memory 2 or the text memory 4 is selected by the control circuit 12 depending upon the data to be printed, and the data read from the selected memory are supplied through the color converter 5 to the thermal head driver 6. Thermal head driver 6 controls the conduction time of the heating elements provided within the thermal head 7. A constant voltage is applied from the constant—voltage power supply 11 to the thermal head 7; a desired heating element is driven by the thermal head driver 6 in accordance with the data received from the color converter 5. Sinc the conduction time of the heating elements is controlled, image pixels having desired density are printed in the recording medium 9 by means o"f the thermal head 1. - Disclosure In case of printing an image and text on the same recording medium by means of the thermal printer density defects occur. It has been determined that the image printing on average involves energizing many more heating elements in a line than in text printing. The temperature of the print head will build up much more rapidly when images are to be printed. Thus with image printing heat build up is the major effect while with text printing heat radiation is the major effect. With image printing, heat build—up or accumulation effects occur, whereas with text printing heat radiation to cooler portions of a print head creates more of a problem. Consequently, when printing image and text on the same recording medium, and if the output voltage of the constant—voltage power supply 11 is set such as to correspond a voltage suitable to for the print density of an image, the print density of text will not be optimum. In contrast with this, if the Output voltage of the constant—voltage power supply 11 is set such as to correspond to a voltage suitable for text printing, the printing density of image data will be higher than the desired density. As a result, both the image and the text can not be printed on the same print with optimum densities. Disclosure of the Invention Accordingly, it is a primary object of the present invention to obviate the above—described defects and to provide a thermal printer capable of printing both text and image data on the same print with high quality densities. To accomplish the aforementioned objects, the thermal printer according to the present invention includes a plurality of devices for respectively storing different types of print data and a thermal head for printing such data on a record medium. The thermal printer also comprises voltage supply means for generating and supplying one of a plurality of D.C. voltages to the thermal head at a time, said different D.C. voltages being suitable for printing the different types of print data, respectively. Control means selects one of said storage devices to cause said selected storage device to read out the print data stored there in and supplied such data to said thermal head, and for causing said voltage supply means to generate and supply one of said different D.C. voltages to said thermal head in accordance with the type of the print data to be printed on the recording medium. Brief Description of the Drawings Fig. 1 shows a conventional thermal printer; Fig. 2 is a schematic diagram showing one embodiment of a thermal printer according to the present invention; Fig. 3 is a view showing a printing process by the thermal printer illustrated in Fig. 1; Fig. 4 is a view showing one example of power supply which can be used in the thermal printer of Fig. 2.; Fig. 5 is a " block diagram of the principal portion of another thermal printer according to the present invention; and Fig. 6 is a view showing a print produced by the thermal printer of Fig. 5. Modes of Carrying Out the Invention The Fig. 2 schematically shows a thermal printer according to the present invention; and the same components as those in Fig. 1 are marked with the same reference numerals. Differences between the embodiment of Fig. 2 and the conventional structure of Fig. 1 are that a variable voltage power supply 13 is provided instead of the prior art constant—voltage power supply, and that a control signal is transmitted from the control circuit 12 to the variable voltage power supply 13. The variable voltage power supply 13 comprises a voltage supply unit 14 for supplying a voltage to the heating elements of the thermal head 7 and a voltage setting unit 15 for changing the output voltage of the voltage supply unit 14 in response to a control signal from the control circuit 12. The operation of this embodiment will now be explained. As in the case in Fig. 1, the control circuit 12 permits the data to be read from the image memory 2 or the text memory 4 in conformity with the types of data to be printed on the recording medium 9. At this time, control circuit 12 sends to the voltage setting unit 15, the control signal indicating which of the image data or the text data is to be read out. Upon a receipt of the control signal from the control circuit 12, the voltage setting unit 15 sets the output voltage of the voltage supply unit 14. More specifically, when the control signal indicates that image data are to be read from the image memory 2, the voltage setting unit 15 sets the output voltage of the voltage supply unit 14 to a voltage V. suitable for the image print density. When the control signal indicates that text data are to be read from the text memory 4, the voltage setting unit 15 sets the output voltage of the voltage supply unit 14 to a voltage V„π suitable for text print density. As described in. reference to the conventional printer of Fig. 1, the heat build—up or accumulation effects act on the heating elements of the thermal head 7 in the case of the image printing, whereas the heat radiation effects work on the heating elements in the case of the text printing. Therefore, the voltage V u π suitable for the text printing generally should be higher that the voltage V " τ suitable for the image printing. With this arrangement, it is feasible to print both high quality image and text on the same recording medium each with its own desired density and to further maintain the desired print density of image and the text throughout the recording medium. Referring to Fig. 3, there is schematically shown a concept of the above—described printing according to the present invention. First, an image is printed on the recording medium 9 by means of the thermal head 7, and then characters are likewise printed thereon. In this case, the output voltage of the voltage supply unit is kept at V during the image printing but is kept at V during the text or π character printing. The variable voltage power supply 13 may be constructed by a wide variety of types of circuits. Fig. 4 shows one example of such a circuit. The control signal output from the control circuit 12 is input to the base 17 of a switching transistor 16. The emitter of the switching transistor 16 is grounded through a resistor 18, and its collector is connected to a junction point J of two resistors 19 and 20 which are connected in series between an output terminal OUT of a rectifier circuit R and " ground. The junction point J is connected to a switching pulse generator and a drive circuit 21 generating first and second pulse trains for turning on and off a switching transistor S. The switching transistor S is connected to a DC power supply B and to the primary winding of a transformer T. The secondary winding of the transformer T is connected to the rectifier circuit R. The operation of the circuit illustrated in Fig. 4 now will be explained. The switching transistor 16 stays in the OFF—state while the control circuit 12 supplies the base 17 of the switching transistor 16 with the control signal which indicates the readout of the image data from the image memory 2. At this time, the switching transistor S is turned on and off by the first pulse train having pulses of a predetermined short width supplied from the switching pulse generator and drive circuit 21. Pulses are correspondingly generated in the secondary winding of the transformer T. These generated pulses are rectified by the rectifier circuit R, thereby obtaining the desired voltage V at the output terminal "OUT". Thereafter, the switching transistor 16 is changed to the ON—state during the period in which the signal from the control circuit 12 indicates that the text data are. to be read from ' the text memory and to the transistor base 17. The resistor 18 is connected in parallel to the resistor 20. This varies the voltage at the junction point J obtained by dividing the voltage V. at the output terminal OUT. As a result, the switching pulse generator and driver circuit 21 detect this variation, and generates the second pulse trains having pulses of a longer pulse width. Owing to this step, the conduction period of the switching transistor S is extended, and the output voltage of the rectifier circuit R becomes Vπ u which is higher than the previous voltage V . As can be understood from the description so far, the switching transistor 16, the resistors 18," 19, and 20, the switching pulse generator, and drive circuit 21, the DC power supply B, the switching transistor S and the transformer T from the voltage setting unit 15; and the rectifier circuit R constitutes the voltage supplying unit 14. In the above—described first embodiment, the period in which the image is printed is offset in time from the period in which the text is printed. The present invention can, however, be applied to a case where the image and the text concurrently exist on the same line. This case will now be explained with reference to Figs. 5 and 6. The second embodiment shown in Fig. 5 differs from the first embodiment of Fig. 2 in that the control signals transmitted from the control circuit 12 are supplied not only to the variable voltage power supply 13 but also to the thermal head driver 6. The thermal head 7 is capable of enabling the heating elements a group at a time. As illustrated in Fig. 6, where both text and an image are printed on one line, the heating elements can be classified into the first group for printing the text and the second group for printing the image. In other words, it is possible to set,a group of heating elements of the thermal head in accordance with a width occupied by such a group of the heating elements on the recording medium come under the image data or text data. Desired printing densities are produced by enabling each group of the heating elements on a time—division basis and by causing the variable voltage power supply 13 to supply the thus enabled heating elements with a voltage suitable for printing. The control circuit 12 performs a function to supply the control signals to the variable voltage power supplies 13 and the thermal head driver 6. For example, when printing a line X—X illustrated in Fig. 6, the control signals indicating that the text is to be printed are first supplied to the variable voltage power supply 13 and then thermal head driver 6. During printing the text, the variable voltage power supply 13 outputs the voltage vVπ. suitable for printing the text and the thermal head driver 6 outputs an enable signal EN., to enable only the first group of the heating elements used for the text printing. At the time point when variable voltage power supply 13 outputs the voltage V τ suitable for the image printing, the thermal head driver 6 outputs another enable signal EN_ to enable the second group of the heating elements used for the image printing. In the second embodiment of Fig. 5, it is thus feasible to produce the same effects as those for the first embodiment. The description of the first and second embodiments has heretofore been focused on the case in which only image and the text are to be printed with a desired densities. The present invention can, however, be applied to other cases wherein printing is effected on the basis of three or more different types of data such as image, text, and graphics. The arrangement of these cases may be such that memories are provided for each t pe of data, and that the variable voltage power supply is capable of outputting voltages suitable for printing each type of data. As understood from the first and second embodiments, the voltage suitable for the printing density of the data is supplied to the heating elements of the thermal head, depending on the type of the data to be printed, so it is possible to print all kinds of data on the same recording medium with desired densities. This will remarkably enhance the print—quality. The thermal printer according to the present invention is suitable for printing both an image like a portrait in the identification card and alphabets or Chinese characters showing the personal information. Industrial Applicability and Advantages The invention is particularly suitable for use in printing images leaving text and continuous tone images.

Claims: Claims : 1. In a thermal printer including a plurality of storage devices for respectively storing different types of print data and a thermal head for printing such print data on a recording medium, characterized by: a. voltage supply means for generating and supplying one of a plurality of D.C. voltages to said thermal head at a time, said different D.C. voltages being suitable for printing the different types of print data, respectively; and b. control means for selecting one of said storage devices to cause said selected storage device to read out the print data stored therein and supplied such data to said thermal head, and for causing said voltage supply means to generate and supply one of said different D.C. voltage to said thermal head in accordance with the type of the print data to be printed on the second medium. 2. A thermal printer for printing an image and text on a recording medium, comprising: a. first and second storage devices for respectively storing image and text data to be printed on the recording medium; b. a control circuit for selecting one of said storage devices in accordance with the data to be printed on the recording medium and for permitting the data to be read from said selected storage device; c. a thermal head having heating elements and responsive to a D.C. voltage and the data read from said selected storage device for printing an image on the recording medium by selectively driving said heating elements in accordance with the data and the D.C. voltage; and d. a variable voltage power supply for supplying first and second D.C. voltage to said heating elements with the first voltage being proportional to a print density suitable to print image data and the second voltage being proportional to a print density suitable to print text data, said control circuit being adapted to cause said power supply to respectively produce said first and second voltages, whereby said thermal printer produces image and text data at desired densities on a recording medium. 3. The thermal printer as set forth in claim 1 wherein said variable voltage power supply includes: a. a pulse generating circuit responsive to said control circuit to selectively generate first and second pulse trains having short and longer pulse widths respectively; and b. means responsive to said selected pulse train to produce said first and second D.C. voltages. 4. The thermal printer as set forth in claim 1 wherein said heating elements which can be enabled a group at a time, and wherein a group of said heating elements is enabled in response to said control circuit.

Inventors: YUMINO MASAMICHI, YUMINO, MASAMICHI
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1988
Main classification: H04N1
IPC codes: B41J2, H04N1