Thermal recording method
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a thermal recording method. More particularly, it relates to a thermal recording method employing a thermal recording head in which the opposing sides of the adjacent ones of a large number of heating resistor sections arrayed into a row are connected to a common lead electrode. 
     2. Description of the Prior Art 
     There has been known a thermal recording method wherein a current is caused to flow through a heating resistor section and heat thus generated in the heating resistor section is applied to a thermally responsive recording paper thereby to record information. An apparatus for such recording is disclosed in U.S. Pat. No. 3,984,844 (inventors: Kiyohiko Tanno, Yasuyuki Kojima; filed: Nov. 18, 1975; Appl. No. 633,115; patent: October 5, 1976). With the recording method described in the patent specification, a leakage current flows to a heating resistor section adjacent to a resistor section which ought to heat. Accordingly, the heating resistor section which is energized for recording after the flow of the leakage current undergoes a quantity of heat larger than that of the heating resistor section which has been energized previously. Therefore, nonuniform recording is incurred. 
     SUMMARY OF THE INVENTION 
     Object 
     An object of this invention is to provide a thermal recording method which reduces nonuniformity in the recording density. 
     Summary 
     In a thermal recording method wherein one recording block is made up of a plurality of heating resistor sections and wherein the heating resistor sections within the recording block are successively supplied with recording power through lead electrodes, this invention consists in executing a plurality of such energizing cycles for the heating resistor sections, thereby to heat the respective heating resistor sections up to a required temperature and to record dots on a thermally responsive recording paper. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a principle diagram of a recording head as well as a control circuit therefor, 
     FIGS. 2A and 2D are diagrams for explaining recording cycles, 
     FIG. 3 is a table which indicates quantities of power injection into respective heating resistor sections, 
     FIG. 4 is a table which indicates numbers of power leakage to the respective heating resistor sections, 
     FIG. 5 is a table which indicates rates of leakage power in the respective heating resistor sections, and 
     FIG. 6 is an electric diagram of a thermal recording apparatus according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a part of a recording circuit of a thermal recording head which carries out recording of 1 cycle by 4 steps within 1 block. A strip-like heating resistor 1 is divided into sections 1a, 1b, 1c, . . . by lead electrodes 2a, 2b, 2c, . . . and 3a, 3b, 3c, . . . . 
     At the first step in the case of recording full black, a recording voltage V is applied to the lead electrodes 2a and 2c and the lead electrodes 3a and 3c are earthed by a switch 4a as illustrated in FIG. 1. Then, the heating resistor sections 1a and 1e are heated by a recording current I (recording power P = V.I) and color a recording paper lying in contact with these resistor sections. In this case, also the heating resistor sections 1b, 1c, 1d, 1f and 1g undergo heating by a leakage current I/3 (leakage power P&#39; = P/9). The recording paper, however, is not colored by the injected power of P/9 owing to the coloration characteritistic thereof. 
     The recording voltage V and the earth switches 4a and 4b are selected so as to inject the recording power P into the heating resistor sections 1b and 1f at the second step, into the heating resistor sections 1c and 1g at the third step and into the heating resistor section 1d at the fourth step. Then, as in the first step, the leakage power P/9 is injected into the other heating resistor sections into which the recording power P is not injected. 
     These are put in order as to one block in FIG. 2A. One block is made up of the recording positions 1a-1d, which correspond to the symbols of the heating resistor sections respectively. 
     Supposing that power is injected into the recording positions 1a and 1d in succession in I cycle, the situation is as indicated in FIG. 3 (I cycle of steps 1-4) in the case of the full black recording. More specifically, at the recording position 1a, the leakage power P/9 is not injected before the recording power P is injected (hereinbelow, the preceding injection of the leakage power shall be termed &#34;leakage power supply&#34;). At the recording position 1b, the leakage power P/9 is injected while the recording power P is injected into the recording position 1a at the step 1, so that the number of leakage power supply is 1. Likewise, the number of leakage power supply is 2 at the recording position 1c, and it is the greatest and 3 at the recording position 1d. The numbers of leakage power supply are put in order in FIG. 4. 
     The outline of this invention will now be described with reference to FIG. 2B (the full black recording). FIG. 2B illustrates a case where the number of recording cycles is II. As regards the period of time during which the recording power is injected, letting τ denote the time in the case of one recording cycle, the time becomes (τ/2) × 2 in the case of two recording cycles. Therefore, the whole recording time does not change. 
     In the case where II recording cycles are performed, the recording sequence becomes 1a → 1b → 1c → 1d → 1a → 1b → 1c → 1d as illustrated in FIG. 2B. The recording power P and the leakage power P/9 are injected as indicated in FIG. 3 (steps 1-8). Accordingly, whereas the number of leakage power supply is zero at the recording position 1a with the prior-art method (recording cycle: I), it becomes 3 (FIG. 4). Here, let the maximum value of the numbers of leakage power supply be 1. Then, as seen from FIG. 5, the difference of the effects of the leakage power supply becomes less than in the prior-art method. Therefore, the density nonuniformity ascribable to the effect of the leakage power supply diminishes. 
     In case where the number of recording cycles is n, the rate of leakage power at the recording position 1a at which the number of leakage power supply is the minimum becomes (1 - 1/n) when the maximum numbers of leakage power supply is denoted by 1. Accordingly, in order to lessen the difference of the effects of the leakage power supply among the recording positions, the number of recording cycles n may be increased. However, when n is too large, the thermal efficiency is degraded. 
     When n = 4 or so, the thermal efficiency is not problematic, and the difference of the effects of the leakage power supply can be confined to about 25%. Therefore, the density nonuniformity poses almost no problem. 
     A concrete electric circuit of this invention will now be described with reference to FIG. 6. 
     In FIG. 6, numeral 1 designates a heating resistor, which is divided into sections 1a, 1b, 1c, . . . by lead electrodes 2a, 2b, 2c, . . . and 3a, 3b, 3c, . . . . 4a and 4b indicate group selector switches which are alternately closed. The switch 4a earths the lead electrodes 3a, 3c, 3e, . . . (odd-numbered group), while the switch 4b earths the lead electrodes 3b, 3d, 3f, . . . (even-numbered group). Numberals 5 and 6 represent groups of reverse current-blocking diodes. Information signal-impressing switches 7a, 7b, 7c, . . control the bestowal of outputs of a buffer memory 8 on the lead electrodes 2a, 2b, 2c, . . . . Shown at 9 is a line memory, which stores information for one scanning line. By the function of a transfer circuit 10, the line memory 9 bestows the information on the buffer memory 8 so that the information corresponding to the heating resistor sections 1a, 1b, 1c, . . . may appear at output terminals 8a, 8b, 8c, . . . of the buffer memory 8. Numeral 11 designates a synchronism control circuit, which controls the group selector switches 4a and 4b, the information signal-impressing switches 7a, 7b, 7c, . . . , the line memory 9 and the transfer circuit 10. 
     The operation of the above construction will be explained as to a case where n cycles each consisting of four steps are performed. 
     (1) Among the information for one scanning line stored in the line memory 9, the information corresponding to the heating resistor sections 1a, 1b, 1e, 1f, 1i, 1j . . . (odd-numbered group of lead electrodes 3a, 3c, 3e, . . .) are transferred to the buffer memory 8 by the transfer circuit 10 so as to be provided at the output terminals 8a, 8b, 8c, . . . thereof. 
     (2) The group selector switch 4a is closed. 
     (3) The information signal-impressing switches 7a, 7c, 7e, . . . are closed for a time of τ/n (= τ 0 ), to cause currents to flow to the heating resistor sections 1a, 1e, 1i, . . . (first step in the first cycle). 
     (4) Subsequently, the information signal-impressing switches 7b, 7d, 7f, . . . are closed for the time of τ 0 , to cause currents to flow to the heating resistor sections 1b, 1f, 1j, . . . (second step in the first cycle). 
     (5) The buffer memory 8 is cleared, and the contents of the line memory 9 are transferred so that the information corresponding to the heating resistor sections 1c, 1d, 1g, 1h, 1k, 1l, . . . (even-numbered group of lead electrodes 3b, 3d, 3f, . . . )may appear at the output terminals 8b, 8c, 8d, . . . of the buffer memory 8. 
     (6) The group selector switch 4b is closed. 
     (7) The information signal-impressing switches 7b, 7d, 7f, . . . are closed for the time of τ 0 , to cause currents to flow to the heating resistor sections 1c, 1g, 1k, . . . (third step in the first cycle). 
     (8) The information signal-impressing switches 7c, 7e, 7g, . . . are closed for the time of τ 0 , to cause currents to flow to the heating resistor sections 1d, 1h, 1l, . . . (fourth step in the first cycle). 
     The above series of operations are repeated n cycles, to cause the current to flow to each heating resistor section for a time of τ 0  × n = τ as the whole. Thus, a recording paper is heated up to the coloring temperature thereof. 
     After completion of the operations of the n cycles, the synchronism control circuit 11 refreshes the contents of the line memory 9 into information for the next scanning line and again makes similar recursive current controls of n cycles each consisting of four steps. 
     In the foregoing embodiment, the case where one scanning line forms one block has been referred to. However, even in case where it is divided into a plurality of blocks, the 4-step n-cycle recording can be performed by executing current controls similar to the above every block. 
     As set forth above, according to this invention, a plurality of recording cycles are executed within one block, so that the difference of the effects of the leakage power supply dependent upon the recording positions can be lessened and that a thermosensitive recording free from the density nonuniformity can be attained.