Circuit for Controlling the Printing Position of a Video Printer and Method Therefor

The present invention relates to a method for controlling the printing position of a video printer, whereby the initial position of printing paper is precisely controlled to prevent misplacement of the printing paper and thus deterioration of the picture quality.

Generally, the video printer employs a ribbon having three dyestuffs of yellow (Y), magenta (M) and cyan (C) which are sequentially sublimated by the heat of a thermal head and deposited on a single printing paper which is to circulate three times so as to print a video signal thereon with full colors. In this case, the amount of the generated heat by the thermal head is precisely controlled so as to regulate the amount of each of the dyestuffs subliminated, thereby freely making intermediate colors (color gradation).

Such conventional video printer as is shown in FIG. 1A includes a thermal head 100, a color ribbon 102, and a paten drum 106. The color ribbon 102 includes three colors of Y, M, C, which are bounded by a black strip with adjacent three colors, as is shown in FIG. 1B.

A printing paper 104 in close contact with the platen drum 106 runs between the platen drum 106 and the color ribbon 102 beneath the thermal head 100 at the same speed as the platen drum 106. The thermal head 100 heat generates according to the input video signals to sublimate each of the dyestuffs on the color ribbon 102, so that the sublimated dyestuffs are deposited on the printing paper 104, thereby reproducing the original colors of the input video signals. In order to fully reproduce the colors of the input video signal, whenever the printing paper 104 passes the theraml head 100, the position of the printing paper relative to the thermal head must always be the same, so that the dyestuffs of Y, M, C are sequentially deposited on the same spot with respect to each picture element. Here, the platen drum 106 circulates three times (one i.e., time for each of the three components Y, M, C) in order to complete one picture sheet.

Therefore, it is very important to precisely adjust the initial printing position of the thermal head 100, and if the initial printing position is misplaced, the printing spot with respect to each picture element is misplaced so as to separate the three color elements from each other, thus causing deterioration of the picture quality and forming a strip of M or C color along both sides of the printed picture as is shown in FIG. 3.

In such a conventional video printer, a circuit for sensing and controlling the position of the printing paper is constructed as shown in FIG. 2. The signal that is produced by photo-coupler 108 to sense the entrance of the printing paper 104 is amplified by amplifier 112, and is applied to microcomputer 114 to control the speed of the platen drum 106 according to the signal. For example, in order to reduce the printing time of the video signal, the platen drum is rotated at the normal speed when printing, while rotating at two times the normal speed when not printing.

If the time that is taken from the moment of the photo-coupler sensing the printing paper for the moment of the signal processing of amplifier 112 and microcomputer 114 to the normal speed of the motor 120 is different for each of the Y, M, C components, or the rotation of the motor 120 is affected by the backlash of gear 122, etc., the initial printing position is misplaced.

Thus, the speed control error of the motor 120 results when the signal sensed by the photo-coupler 108 is applied to the microcomputer 114, such that there occur differences of the sensing response time according to the moment when the program counter of the microcomputer 114 makes the sensing determination (present counting).

Consequently, if the initial printing position is misplaced, the printing position of the Y, M, C comonents is misplaced on the printing paper by a gap "a" or "b" as shown in FIG. 3, so that the picture quality is deteriorated throughout.

It is therefore, an object of the present invention to provide an improved circuit and method for controlling the position of printing on print media.

It is another object to provide a circuit and method for precisely controlling the initial position of the printing paper by equalizing for each revolution of the platen drum the time that is taken from the moment of sensing the printing paper until the speed of the platen drum reaches the normal speed under the control of a microcomputer.

According to an aspect of the present invention, a video printer for sequentially printing the video signals of a video processing system on a paper in full colors includes a sensor for sensing printing paper; an amplifier for amplifying the output of the sensor; a microcomputer for generating a control signal to temporarily stop or to drive at either the normal speed or two times the normal speed, a motor according to the receiving of the output of the amplifier; a control circuit for controlling the motor to conform to the initialization of the position of multi-color printing according to the control signal of the microcomputer and the sensing signal of the sensor; and a driver for driving the motor according to the control signal of the microcomputer.

The present invention will now be described more specifically with reference to the drawings attached only by way of example.

For a better understanding of the invention and to show how the smae may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which:

FIG. 1A & 1B are cross-sectional views for illustrating the essential part of a conventional apparatus for printing a video signal;

FIG. 2 is a block diagram of a conventional circuit for controlling the printing position of a video signal;

FIG. 3 is an example of a picture printed by a conventional video printer;

FIG. 4 is a block diagram for illustrating a circuit for controlling the printing position of a video printer according to the present invention; and

FIG. 5 is a detailed diagram for illustrating a circuit for controlling the printing position of a video printer according to one embodiment of the present invention.

Referring to FIG. 4, sensor 110 senses the printing paper 104 passing between platen drum 106 and thermal head 100, providing the output thereof to amplifier 112 which amplifies and delivers it to microcomputer 114 and control circuit 116. The microcomputer 114 supplies a control signal to the control circuit 116 that controls the power supply to motor driver 118, so as to properly control the speed of the motor 120, thereby initializing the printing position of a video signal of the Y, M, C components on the printing paper.

Referring to FIG. 5, photo-coupler 108 includes light-emitting diode and photo-transistor respectively having current-limiting resistors 124 and 126 connected to the ground. The output of the photo-transistor is delivered through its emitter to interrupt terminal T.sub.1 of the microcomputer 114 and first buffer 128, while the signals of the output terminals O.sub.1, O.sub.2, O.sub.3 of the microcomputer 114 are applied to the selecting terminals S.sub.1, S.sub.2, S.sub.3 of the motor driver 118 (preferably, it may be a single chip designated as M54548L). The first and second outputs Q.sub.1, Q.sub.2 of the motor driver 118 are connected to the motor 120. The power supply terminal VP of the motor driver 118 is commonly connected to the collectors of first and second transistors 140, 142 whose emitters are commonly connected to the power supply Vcc. The base of the first transistor 140 is connected through a resistor 134 to first buffer 128, while the base of the second transistor 142 is connected through a resistor 132 to second buffer 130. The input of the second buffer 130 is connected to the second output O.sub.2 of the microcomputer 114.

In the drawing, if the photo-transistor of the photo-coupler 108 does not sense the printing paper, the photo-transistor is turned off, cutting off the power supply applied to the photo-coupler 108. Hence, the base of the first transistor 140 has the voltage level of low state, thereby causing the first transistor 140 to conduct. At this time, the second transistor 142 is turned off by the second output signal generated according to the sensing signal from the microcomputer 114, which at the same time outputs a control signal representing two times the normal speed of the motor. Conversely, if the photo-coupler 108 senses the printing power, the first and second transistors 140 and 142 are simultaneously turned off so as to temporarily stop the motor 120. Thereafter, if the sensing signal applied to the microcomputer 114 causes the first to third output terminals to generate the signals for rotating the motor 120 at the normal speed, the second transistor 142 simultaneously conducts to rotate the motor 120 at the normal speed, thus performing the printing process of a video signal. The revolution speed of the motor 120 according to the states of the selecting terminals S.sub.1 -S.sub.3 of the motor driver is shown in the following Table 1.

As described above, at the moment when the photo-coupler senses the printing paper, the motor is temporarily stopped, and at the moment when the program executed by the microcomputer responds to the sensing, the speed change of the motor and heat generation by the thermal head occur, so that the initial printing position is precisely controlled during the revolutions of printing for the three Y, M, C components, thus improving the printing quality of a video printer according to the present invention.

Motor driver 118 (a single chip, M54548L) produces the motor driving outputs selectively for the "normal speed" or for the "two times the normal speed", responsively to the control signals applied to the selection terminals S.sub.1, S.sub.2, S.sub.3 thereof. Therefore, as described in Table 1, in order to rotate the motor at normal speed, the logic states of the control signal applied to the selection terminals S.sub.1, S.sub.2, S.sub.3, must be respectively Low, Low and High (L-L-H); the "two times the normal speed" rotation, the logic states must be respectively Low, High and Low (L-H-L).

When photo-coupler 108 can not sense the entry of printing paper, the system rotates motor 120 at two times the normal speed so as to save printing time. That is, if the entry of a printing paper is not sensed, the transistor of photo-coupler 108 is turned OFF, and thus transistor 140 is turned ON because, at this moment, the output of buffer 128 is at a logic low state. At the same moment, microcomputer 114 generates the logic states L-H-L through the respective output terminals O.sub.1 -O.sub.2 -O.sub.3 in response to the logic low signal of the interrupt terminal I.sub.1. At the base of transistor 142, a logic high voltage is applied, and thus transistor 142 is turned off. Therefore, in this case, motor driver 118 can be provided with power through transistor 140.

Meanwhile, if the photo-coupler 108 senses the entry of the printing paper, the base of transistor 140 and the interrupt terminal I.sub.1 of microcomputer 114 are provided with power supply Vcc by way of the transistor of photo-coupler 108. However, since it takes a while for the microcomputer 114 to program, the previous logic states L-H-L are held for a predetermined interval. Therefore, at this moment, the two transistors 140, 142 are simultaneously turned off so as to cut of the power supply provided to motor driver 118. As s result, the motor stops running.

Therefore, if microcomputer 114 terminates counting the program and changes the logic states to L-L-H for the normal speed driving, transistor 142 is turned on because the output state O.sub.2 is at logic low level. Accordingly, motor driver 118 can be provided with power supply Vcc by way of transistor 142.

It may be seen therefore, that transistors 140 and 142 for controlling the power supply provided to motor driver 118 and the rotation speed of the motor is dependent upon the control signal applied to the selection terminals S.sub.1, S.sub.2, S.sub.3.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that modifications in detail may be made without departing from the spirit and scope of the invention.