Circuit for fetching run-length-limited coded write data

Abstract: In writing (1-7) RLL coded write data into a magnetic disk, a write data fetching circuit wherein a clock signal (VFO3F) of a frequency three times a fundamental frequency (1f) of a write clock signal (WCLK) is generated based on a signal from a servo head (12) of the magnetic disk, three clock signals (VFO1FA,VFO1FB,VFO1FC) are generated from the clock signal (VFO3F), one of the three clock signals and the write clock signal (WCLK) are synchronized, and the write data (WDAT) output in accordance with the write clock signal (WCLK) is subjected to (1-7) RLL encoding using a clock signal (FCLK) synchronized with one of the three clock signals (VFO1FA,VFO1FB,VFO1FC), whereby stable (1-7) RLL encoding is made possible without the use of a precise clock with a 5⊘ percent duty ratio.

Description: The present invention relates to a magnetic disk apparatus, more particularly to a write data fetching circuit, provided in a magnetic disk apparatus, which fetches write data for an encoding circuit of one of the run-length-limited (RLL) codes, such as a (1-7) RLL code. To increase the capacity of magnetic disk apparatuses, use is made of a. the method of increasing the number of magnetic disks, b. the method of raising the track density of the magnetic disks, c. the method of raising the bit density, etc. The method a. can be easily realized, but since the number of disks increases, the cost of the product also rises. The method b. cannot be easily realized, so considerable development costs and time are required. The method c. is advantageous in terms of product cost and development costs, so is most often used for achieving larger capacities of magnetic disk apparatuses. To raise the bit density of the magnetic disks, it is necessary to select a suitable recording code. That is, when the SN ratio, head floating height, thickness of the recording medium, magnetic characteristics, and other conditions are set, the recording code is selected so as to maximize the planar density. In this case, consideration is given to the characteristics of the magnetic recording medium, and it is demanded that the code be resistant to defects of the magnetic medium and intersymbol interference and that a high recording density can be obtained. To satisfy these demands, use is currently made of the non-return zero inversion (NRZI), 4/5 GCR, modified frequency modulation (MFM), RLL, and other coding methods. The RLL code requires the presence of a minimum (d) and maximum (k) slots in the transition between two codes and is thus called a "(d-k) RLL code". For example, the RLL code includes a (2-7) RLL code and (1-7) RLL code. The (2-7) RLL encoding method is disclosed in, for example, the specification of U.S. Patent No. 3,689,899, and the (1-7) RLL encoding method is disclosed in the specification of U.S. Patent No. 4,488,142. The frequency of the clock used in the transmission and reception of data between a magnetic disk apparatus using a (2-7) RLL code and the control apparatus of the host controller is a fundamental frequency (1f), but in the (2-7) RLL-code encoding circuit in the magnetic disk apparatus, the frequency of the clock is two times the fundamental frequency (2f). This double frequency 2f clock is prepared by a servo circuit in the magnetic disk apparatus. Below, this so prepared double frequency 2f clock will be abbreviated as VFO2F. Since the fundamental frequency 1f and the VFO2F prepared in the magnetic disk apparatus are asynchronous, when fetching data from the control circuit to the encoding circuit in the magnetic disk apparatus, it is necessary to synchronize the fundamental frequency 1f with the clock VFO1F, which consists of the VFO2F which is prepared by the magnetic disk apparatus divided in half and which has the same frequency as the fundamental frequency. In this case, it is necessary to synchronize with the clock VFO1F of the fundamental frequency no matter what the state, so the VFO1F is made a two-phase clock so that data can be fetched. The VFO1F and two-phase clock signal at this time can be easily prepared from the VFO2F in the magnetic disk apparatus. Even in a magnetic disk apparatus using the (1-7) RLL signal, the fundamental frequency for transmission and reception of data between the control circuit and encoding circuit is 1f in the case of use of a (2-7) RLL code too. However, a (1-7) RLL-code encoding circuit operates on a clock (3/2)f of a frequency 1.5 times the fundamental frequency 1f. In this case, the magnetic disk apparatus issues a clock VFO3F having a frequency 3f three times the fundamental frequency. By dividing this, a clock VFO3/2F is prepared having a frequency 3/2 times the fundamental frequency. The transmission and reception of data performed between the control circuit and encoding circuit are performed by a clock of the fundamental frequency 1f, so it is necessary to divide the VFO3F prepared in the magnetic disk apparatus into three to prepare a clock VFO1F having a fundamental frequency and to synchronize it with the fundamental frequency 1f. In the case of the (1-7) RLL signal, as explained in detail below, it is necessary to prepare a precise clock VFO1F with a duty ratio of 50% and to synchronize it with the fundamental frequency 1f. However, a clock of a digital signal with a high frequency differs from an analog signal. It is extremely difficult to divide it precisely except by a power of 2. Therefore, the resultant frequency-division circuit cannot have sufficient precision and stability. The above problems will be explained later with reference to the drawings. An embodiment of the present invention can provide a highly reliable write data fetching circuit which does not use a clock of a fundamental frequency 1f with a 50% duty ratio and which can prepare a write data fetching clock with a simple circuit construction and perform fetching of write data without error. US-A-3 643 228 discloses circuitry in accordance with the preambles of accompanying claims 1 and 8. In this circuitry, a four-times frequency clock signal is generated and from this, four clock signals successively 90° apart in phase are produced, which are used for synchronization purposes in RLL encoding. According to a first aspect of the present invention, there is provided a circuit for fetching run-length-limited coded write data upon receiving a write clock signal having a fundamental frequency, and write data, comprising:- an n-times frequency clock signal generating circuit arranged to generate a clock signal of a frequency which is n-times the fundamental frequency; a system clock signal generating circuit arranged to receive the n-times frequency clock signal and to generate n clock signals respectively differing in phase by 360/n°; a synchronization and fetching circuit arranged to perform synchronization of the write data in dependence on one of the n clock signals; and an RLL encoding circuit arranged to perform RLL encoding on the synchronized write data; characterized in that:- said n-times frequency clock signal generating circuit is a triple frequency clock signal generating circuit arranged to generate a triple frequency clock signal having a frequency three times the fundamental frequency based on a signal from a servo head of a disk apparatus; said system clock signal generating circuit is arranged to generate from the triple frequency clock signal, first to third clock signals differing from each other by 120° in phase, and a frequency divided signal formed by dividing in half the triple frequency clock signal; a fetching clock generating circuit is provided, which is arranged to receive a write gate signal indicating the start of a write operation, the write clock signal, and the first to third clock signals from the system clock generating circuit, and to generate a fetching clock signal synchronized with at least one of the first to third clock signals depending on the states thereof at a transition of the write clock signal directly after the generation of the write gate signal; said synchronization and fetching circuit is arranged to perform a first synchronization and fetching operation on the write data using the write clock signal to produce first synchronized write data, and to perform a second synchronization and fetching operation on the first synchronized write data using the fetching clock signal, to produce second synchronized write data; and said RLL encoding circuit is a RLL encoding circuit arranged to perform RLL encoding on the second synchronized write data based on the frequency divided signal from the system clock generating circuit. Further, in an embodiment of the present invention, the system clock generating circuit can further generate a clock signal formed by delaying one of the above three clock signals by within 1/6 of the fundamental frequency. According to a second aspect of the present invention, there is provided a circuit for fetching run-length-limited coded write data upon receiving a write clock signal having a fundamental frequency and write data, comprising:- an n-times frequency clock signal generating circuit arranged to generate a clock signal of a frequency which is n-times the fundamental frequency; a system clock signal generating circuit arranged to receive the n-times clock signal and to generate n clock signals respectively differing in phase by 360/n°; a synchronization circuit arranged to perform synchronization of the write data in dependence on one of the n clock signals; and an RLL encoding circuit arranged to perform RLL encoding on the write data; characterized in that:- said n-times frequency clock signal generating circuit is a triple frequency clock signal generating circuit arranged to generate a triple frequency clock signal having a frequency three times a fundamental frequency of the write clock signal based on a signal from a servo head of a disk apparatus; said system clock signal generating circuit arranged to generate from the triple frequency clock signal, first to third reference clock signals each having a frequency half that of the fundamental frequency of the write clock signal and respectively successively shifted by one period of the triple frequency clock signal, a first frequency divided clock signal formed by dividing in half the triple frequency clock signal and a second frequency divided clock signal formed by inverting the first frequency divided clock signal, and six synchronization clock signals each having a frequency half the fundamental frequency, having a 2-bit pulse width, and respectively successively shifted by one period of the triple frequency clock signal; a synchronization signal generating circuit is provided, which is arranged to receive a write gate signal indicating the start of a write operation, the first to third reference clock signals from the system clock signal generating circuit, and the six synchronization clock signals, and to generate a load signal having a frequency half the fundamental frequency and synchronized with at least one of the six synchronization clock signals in accordance with the state of the first to third reference clock signals at the time of change of the write clock signal directly after the generation of the write gate signal, and further arranged to generate an encoding clock signal having a frequency 1.5 times the fundamental frequency and synchronized with one of the first or second frequency divided clock signals in accordance with the state of the first to third reference clock signals; a data shift circuit is provided, and is arranged to receive the write data and the write clock signal, to synchronize the write data to the write clock in phase, and to output four bits of data in two 2-bit units each shifted by one bit; said RLL encoding circuit is a RLL encoding circuit arranged to receive the four bits of data from the data shift circuit and to perform RLL encoding on them to produce parallel code data; and said synchronization circuit is arranged to receive the load signal and the encoding clock signal from the synchronization signal generating circuit and to convert to a serial format the parallel code data from the RLL encoding circuit to a 3-bit unit of code data synchronized with the encoding clock signal. Embodiments of the present invention will be described below in detail with reference to the accompanying drawings, in which: Fig. 1 is an explanatory view of the constitution of a conventional (1-7) RLL code write data fetching circuit; Figs. 2a to 2d are signal waveform diagrams of a conventional (1-7) RLL code write data fetching circuit; Figs. 3a to 3i1 to 3i3 are signal waveform diagrams of when the REF1F of the conventional (1-7) RLL code write data fetching circuit has a 5⊘ percent duty ratio; Figs. 4a to 4i1 to 4i3 are signal waveform diagrams of when the REF1F of the conventional (1-7) RLL code write data fetching circuit does not have a 5⊘ percent duty ratio; Fig. 5 is a view of the basic construction of a first embodiment of the present invention; Figs. 6a to 6l are signal waveform diagrams of Fig. 5; Figs. 7a to 7k1 to 7k4 are explanatory views of the write data fetching operation of Fig. 5; Fig. 8 is a concrete circuit diagram of an embodiment of the present invention; Fig. 9 is a diagram of the system clock preparing circuit of Fig. 8; Figs. 1⊘a to 1⊘n are signal waveform diagrams of the system clock preparing circuit of Fig. 9; Fig. 11 is a view of the basic construction of a second embodiment of the present invention; Figs. 12a to 12m are signal waveform diagrams of Fig. 11; Figs. 13a to 13p are signal waveform diagrams of the write data fetching operation of Fig. 11; Fig. 14 is a concrete circuit diagram of the second embodiment of the present invention; Fig. 15 is a view of the basic construction of a third embodiment of the present invention; Figs. 16a to 16x are signal waveform diagrams of Fig. 15; Figs. 17a to 17l are signal waveform diagrams of the system clock preparing circuit of Fig. 15; Figs. 18a to 18h6 and Figs. 19a to 19h6 are signal waveform diagrams of the synchronization signal generating circuit of Fig. 15; Fig. 2⊘ is a detailed circuit diagram of the third embodiment of the present invention; Fig. 21 is a diagram of the system clock preparing circuit of the same embodiment; Figs. 22a to 22s are signal waveform diagrams of the system clock preparing circuit of Fig. 21; and Fig. 23 is a diagram of the synchronization circuit of the third embodiment. Before explaining the preferred embodiments of the present invention, an explanation will be made of the conventional (1-7) RLL code write data reading circuit and its write data fetching operation with reference to Figs. 1 to 4. Figure 1 is a block diagram of the constitution of a write data fetching circuit in the case of a (1-7) RLL code. Figure 2 is an overall signal waveform diagram. Figure 3 is a signal waveform diagram in the case where the duty ratio of the signal REF1F of the fundamental frequency 1f is 5⊘ percent, and Fig. 4 is a signal waveform diagram in the case where the duty ratio of REF1F is not 5⊘ percent. In Fig. 1, when a control circuit 21 is writing into a magnetic disk apparatus (not shown), the control circuit 21 issues a write clock WCLK of a fundamental frequency 1f (see Fig. 2l), a write data signal WDAT (see Fig. 2g), and a write gate signal WGT (see Fig. 2h). WDAT is issued synchronously with WCLK after a fixed time delay. WGT is issued asynchronously with WCLK. The servo circuit 22 is provided internally with a servo head 221, a preamplification circuit (PREAMP) 222, a phase-locked oscillation circuit (PLO circuit) 223, and a variable frequency oscillation circuit (VFO circuit) and issues a triple frequency clock VFO3F with a frequency 3f three times the fundamental frequency 1f (see Fig. 2a). This VFO3F is asynchronous with the WCLK of the frequency 1f. Note that the frequency VFO3/2F, 3/2 the fundamental frequency 1f, i.e., the 1/2 divided signal of VFO3F, is produced in the process of preparation of the VFO1F. The system clock preparing circuit 23 receives the VFO3F prepared by the servo circuit 22, divides this in 1/3, and prepares and outputs the VFO1F (see Fig. 2c) and shifts by one clock of the VFO3F this VFO1F to prepare and output the one-clock shifted signal VFO1FSR (see Fig. 2d). On the other hand, the frequency 1f reference signal (REF1F) preparing circuit 27 receives the VFO3F prepared by the servo circuit 22 and prepares and outputs REF1F, which has a duty ratio of 5⊘ percent and a frequency of 1f (see Fig. 2e), and its inverted signal *REF1F. As such an REF1F preparing circuit, there may be mentioned the clock-pulse Dubler (doubler) modules CDM1⊘2, CDM2⊘2, and CDM5⊘2 of JPC Co. Note that the "*" in *REF1F indicates inversion and is similarly used in other codes. Reference numeral 24 is a fetching clock (FCLK) preparing circuit which is provided internally with a D-type flip-flop (DFF) 241 and 242, a NAND circuit 243, AND circuits 244 to 246, an OR circuit 247, and an inverter 248 and issues a fetching clock RICLK for fetching write data. When DFF241 receives WGT from the control circuit 21, it is latched at the rising edge of the first subsequent WCLK and issues a synchronization gate signal (SYCGT) (see timing Ts portions of Figs. 2f, Fig. 2h, and Fig. 2i). DFF242 is latched at the rising edge of REF1F issued by the REF1F preparing circuit 24 and SYCGT, issues the write clock window signal WCLWD from the terminal Q, and issues the inverted signal *WCLWD to the terminal *Q (see timing Ts portions of Fig. 2i and Fig. 2j). Note that when the REF1F level is "⊘" at the rising edge of SYCGT, WCLWD does not rise and the WCLWD of the terminal Q remains at the "⊘" level and the *WCLWD of the terminal *Q remains at the "1" level. The NAND circuit 243 receives VFO1F from the system clock preparing circuit 23, REF1F from the REF1F preparing circuit 27, and *VFO3F from the inverter 248 and issues a fetching clock A (FCLKA) at the timing where the AND conditions of these three signals stand (see Fig. 2k). This FCLKA is issued at the center of the first half cycle (1 level) of the clocks of REF1F and has a cycle of a frequency of 1f. On the other hand, the AND circuit 244 receives the VFO1FSR from the system clock preparing circuit 23, the *REF1F from the REF1F preparing circuit 24, and the VFO3F from the servo circuit 22 and issues a fetching clock B (FCLKB) at the timing when the AND conditions of these three signals stand (see Fig. 21). This FCLKB is issued at the center of the latter half cycle (⊘ level) of the clocks of the REF1F and has a cycle of the frequency 1f. When the *WCLWD received from the DFF 242 is "1" (WCLWD is "⊘"), the AND circuit 245 supplies the FCLKA input from the NAND circuit 243 to the OR circuit 247. On the other hand, when the WCLWD received from the DFF 242 is "1", the AND circuit 246 supplies the FCLKB input from the AND circuit 244 to the OR circuit 247. Therefore, in the interval when the WCLWD is not issued from the OR circuit 247 (the interval where WGT is not input), the fetching clock FCLK is synchronized with the FCLKA. After Ts when the WCLWD is issued (after WGT is input), the FCLK is synchronized with the FCLKB (see Fig. 2m). The FCKL issued in this way is supplied to the synchronization fetching circuit 25 and the 1-7 encoding circuit 26. The synchronization fetching circuit 25 is comprised of the two DFFs 251 and 252. The WCLK issued from the control circuit 21 is input to the clock terminal CL of the first DFF 251 of the synchronization fetching circuit 25, WDAT is input to the terminal D, and WGT to the clear terminal CR. By this, WDAT is latched at the rising edges of the WCLKs and is shaped to the write data WDT of 1f width (see Fig. 2f, Fig. 2g, and Fig. 2n). The FCLK output from the FCLK preparing circuit 24 is input to the CL terminal of the next stage DFF 252, the WDT from the DFF 251 is input to the terminal D, and the WGT from the control circuit 21 is input to the terminal CR. By this, the WDT becomes "non-return to zero write data" NRZWD of a frequency 1f synchronized with the FCLK (FCLK B). The NRZWD synchronized with the FCLK prepared in this way is input along with the FCLK to the (1-7) encoding circuit 26 and converted to a (1-7) RLL code, then written into the magnetic disk apparatus (not shown). Note that in Fig. 2, when the REF1F is at the ⊘ level at the point of the rising edge of the SYCGT, the WCLWD does not rise but remains at the level of "⊘". In this case, the AND circuit 246 does not issue the FCLKB and the FCLKA output from the AND circuit 245 passes through the OR circuit 247 to become FCLK, so NDZWD is issued synchronously with the FCLK, i.e., FCLKA. Next, referring to Fig. 3, an explanation will be given of the performance of the fetching of write data synchronized with FCLK in the case of REF1F with a 5⊘ percent duty ratio. From the phases and synchronization relationships of REF1F, WGT, SYCGT, WCLWD, WDT, FCLKA, FCLKB, and NRZWD of Fig. 2, when the rising edge of the WDT is at the "1" level of REF1F, the NRZWD is issued synchronously with the FCLKB. When the rising edge of the WDT is at the "⊘" level of the REF1F, the NRZWD is issued synchronously with the FCLKA, it is understood. Figures 3a to 3e show the VFO3F, REF1F, FCLKA, FCLKB, and WDT of Figs. 2a, 2e, 2k, 2l, and 2n. VFO3F, REF1F, FCLKA, and FCLKB are in a synchronous relationship, and WDT is in an asynchronous relationship with the three signals. Which of Figs. 3f1 to 3f2, Figs. 3g1 to 3g3, Figs. 3h1 to 3h3, and Figs. 3i1 to 3i3 synchronization is performed with is shown using WDT as a reference. a. Case A: Figs. 3f1 to 3f2 At the rising edge of the WDT, when the REF1F is the "1" level, the WDT is fetched synchronously with the FCLKB. b. Case B: Figs. 3g1 to 3g3 At the rising edge of the WDT, when the REF1F is the "⊘" level, the WDT is fetched synchronously with the FCLKA. c. Case C: Figs. 3h1 to 3h3 When the rising edge of the WDT and the trailing edge of the REF1F coincide, there are cases where the REF1F will be "1" at the rising edge of the WDT and will be "⊘". In the former case, the WDT is fetched synchronously with the FCLKB and in the latter case synchronously with the FCLKA. If either of FCLKB or FCLKA is first determined, as clear from the synchronization relationship of WGT, SYCGT, WCLWD, FCLKA, FCLKB, FCLK, and WDT of Figs. 2h to 2n, there is subsequently synchronization with the same signal, so one of FCLKA or FCLKB is synchronized with and WDT fetched. d. Case D: Figs. 3i1 to 3i3 When the rising edge of the WDT and the rising edge of the REF1F coincide, in this case too, depending on whether the REF1F is at the "1" or "⊘" level, the FCLKB or FCLKA will be synchronized with. If either of FCLKB or FCLKA is first determined, there is subsequently synchronization with the same signal, so one of FCLKA or FCLKB is synchronized with and WDT fetched. As explained above, in the case of an REF1F with a 5⊘ percent duty ratio, no matter what phase relationship the WDT and REF1F are in, the WDT is fetched synchronized with FCLK. However, when the duty ratio of REF1F is not 5⊘ percent, there are cases where the WDT is not fetched synchronized with FCLK. Below, an explanation will be made of the fetching operation in this case with reference to Figs. 4a to 4i3. Figures 4a to 4e show again the VFO3F, REF1F, FCLKA, FCLKB, and WDT of Fig. 2 and Fig. 3. However, the duty ratio of the REF1F is off from 5⊘ percent. a′. Case A′: Figs. 4f1 to 4f2 At the rising edge of the WDT, when the REF1F is the "1" level, the WDT is fetched synchronously with the FCLKB. b′. Case B′: Figs. 4g1 to 4g2 At the rising edge of the WDT, when the REF1F is the "⊘" level, the WDT is fetched synchronously with the FCLKA. c′. Case C′: Figs. 4h1 to 4h3 When the rising edge of the WDT and the trailing edge of the REF1F coincide, there are cases where the REF1F will be "1" at the rising edge of the WDT and will be "⊘". In the case of "⊘", the WDT is fetched synchronously with the FCLKA, so no problem arises. However, in the case of "1", the WDT is fetched synchronously with the FCLKB. Since the rising edges of WDT and FCLKB coincide, the fetched data becomes "1" or "⊘" so is not constant, so the correct WDT cannot be fetched. d′. Case D′: Figs. 4i1 to 4i3 When the rising edge of the WDT and the rising edge of the REF1F coincide, depending on whether the REF1F is at the "1" or "⊘" level, the FCLKB or FCLKA will be synchronized with. If either of FCLKB or FCLKA is first determined, there is subsequently synchronization with the same signal, so one of FCLKA or FCLKB is synchronized with and WDT fetched. In this way, if the duty ratio of the REF1F is not 5⊘ percent, when the rising edge of the WDT and the trailing edge of the REF1F coincide, if the FCLKB is synchronized with, the WDT will not be fetched. If the duty ratio of the reference clock REF1F of the fundamental frequency 1f were not correctly 5⊘ percent, the conventional (1-7) RLL code write data fetching circuit suffered from the disadvantage of incorrect fetching of write data in some cases. In the case of digital signals, a 1/2 n frequency division can be easily and accurately performed, but a frequency division of an odd denominator is difficult and therefore it is difficult to prepare a clock REF1F of frequency 1f and a duty ratio of 5⊘ percent from the triple frequency clock VFO3F. The obtained frequency division circuit is also not satisfactory in precision and stability, so it is difficult to realize a reliable write data fetching circuit. Below, an explanation will be made of embodiments of the present invention. A. First Embodiment [Basic Constitution] An explanation will now be made of the basic constitution of a first embodiment using the present invention with reference to Fig. 5. In Fig. 5, reference numeral 11 is a control circuit which is connected to a host controller and which transmits a write clock WCLK of a fundamental frequency 1f, a write data WDAT issued synchronously with the WCLK, and a write gate WGT indicating the start of the write operation. Reference numeral 12 is a VFO3F generating circuit which generates, asynchronously with the WCLK, a clock VFO3F three times the fundamental frequency 1f. Reference numeral 13 is a synchronization fetching circuit which receives WCLK and WDAT from the control circuit 11, shapes the WDAT to the write data WDT synchronized with the WCLK, and fetches the write data synchronized with the FCLK received from the FCLK preparing circuit from the WDT. Reference numeral 14 is a (1-7) encoding circuit which converts the write data input from the synchronization fetching circuit 13 to a (1-7) RLL code. Reference numeral 15 is a system clock preparing circuit, which receives the VFO3F from the VFO3F generating circuit 12 and prepares the three clocks VFO1FA, VFO1FB, and VFKO1FC of the frequency 1f synchronized with the VFO3F. Reference numeral 16 is a fetching clock preparing circuit which receives the WCLK and WGT from the control circuit 11, receives the VFO1FA, VFO1FB, and VFO1FC from the system clock preparing circuit 15, and, in accordance with the state of the VF⊘1Fa, VF⊘1FB, and VFO1FC at the point of change of the WCLK directly after the issuance of the WGT, prepares the fetching clock FCLK synchronized with one of the same. The write data fetching operation of Fig. 5 will be explained with reference to the signal waveform diagrams of Fig. 6 and Fig. 7. The control circuit 11 issues a WCLK of the fundamental frequency 1f (see Fig. 6f), the WDAT issued synchronized with the WCLK (see Fig. 6g), and the WGT instructing the start of the write operation, which is issued asynchronously with these two signals (see Fig. 6i). On the other hand, the VFO3F generating circuit 12 issues, asynchronously with WCLK, the clock VFO3F of a frequency 3f three times the fundamental frequency 1f (see Fig. 2a). The system clock preparing circuit 15 prepares the three clocks VFO1FA, VFO1FB, and VFO1FC of the frequency 1f synchronized with the VFO3F (see Figs. 6c, 6d, and 6e). Note that in the process of preparation of these three signals, the VFO3/2F of the frequency 3/2f is produced (see Fig. 6b). This VFO3/2f is supplied as a (1-7) RLL-code encoding clock to the (1-7) encoding circuit. When the fetching clock preparing circuit 16 receives the WCLK and WGT from the control circuit 11 and receives the VFO1FA, VFO1FB, and VFO1FC from the system clock preparing circuit 15, in accordance with the state of the VFO1FA, VFO1FB, and VFO1FC at the point of change of the WCLK directly after the issuance of the WGT (point of rise of WCLK in case of Fig. 2), it fetches one of the same and selects it as the clock FCLK. Table 1 shows an example of the correspondence between the state of the VFO1FA, VFO1FB, and VFO1FC and the signals selected as the FCLK. Details are explained in the explanation on the detailed circuit construction. For example, in the case of Fig. 6, the state of VFO1FA, VFO1FB, and VFO1FC at the point of change (point of rise Ts) of the WCLK directly after the rise of the WGT is "1⊘1", so after the issuance of the WAT, the FCLK synchronizes with the VFO1FC (see Figs. 6i, 6f, 6c, 6d, 6e, and 6k). When the synchronization and fetching circuit 13 receives the WCLK and WDAT from the control circuit 11 and receives the FCLK from the fetching clock selection circuit 16, it synchronizes and shapes the WDAT to the write data WDT synchronized with the WCLK (see Fig. 6h), performs fetching of NRZ type write data (NRZWDT) synchronized with FCLK by the WDT, and inputs the same into the (1-7) encoding circuit 14. The (1-7) encoding circuit 14 performs (1-7) RLL encoding by the write data (NRZWDT) and VFO3/2F. Note that an explanation of the SYCGT of Fig. 6i will be given in the explanation of the detailed circuit construction. Next, an explanation will be made of a write data fetching operation in the correspondence relationships of the three fetching clocks VFO1FA, VFO1FB, and VFO1FC and WDT. Looking at the phases and the synchronization relationships of the signals of VFO1FA, VFO1FB, VFO1FC, WCLK, WDAT, WDT, WGT, and FCLK of Fig. 6, it will be understood that the clock to which FCLK is synchronized is selected from the state of the VFO1FA, VFO1FB, and VFO1FC at the point of the rise of the WDT. Below, note is taken of this point and an explanation made of the write data fetching operation at the phase relationships of the three signals and WDT with reference to Fig. 7 and Table 1. The signals of Figs. 7a to 7e show the VFO3F, VFO3/2F, VFO1FA, VFO1FB, and VFO1FC of Figs. 6a to 6e. Figures 7f1 to 7f2 to 7k1 to 7k3 show the relationship of the rising edge of the WDT and the states of the three signals and the three signals which become FCLK in each case. a. Case A: Figs. 7f1 to 7f2 If the rising edge of the WDT is made in reference to the VFO1FA, when it is in the first (1/3)f of the same, the state of the VFO1FA, VFO1FB, and VFO1FC is "1⊘1", so the FCLK is synchronized with the VFO1FC from Table 1. b. Case B: Figs. 7g1 to 7g2 When the rising edge of the WDT is in the center (1/3)f of the VFO1FA, the state of the VFO1FA, VFO1FB, and VFO1FC is "11⊘", so the FCLK is synchronized to the VFO1FA from Table 1. c. Case C: Figs. 7h1 to 7h2 When the rising edge of the WDT is in the latter (1/3)f of the VFO1FA, the state of the VFO1FA, VFO1FB, and VFO1FC is "⊘11", so the FCLK is synchronized to the VFO1FB from Table 1. d. Case D: Figs. 7i1 to 7i3 When the rising edge of the WDT coincides with the rising edge of the VFO1FA, the state of the VFO1FA, VFO1FB, and VFO1FC becomes one of the following four states: (i) ⊘11 (ii) ⊘⊘1 (iii) 1⊘1 (iv) 111 In the case of (i) and (ii), the VFO1FB becomes FCLK from Table 1. In the case of (iii) and (iv), the VFO1FC becomes FCLK from Table 1. When synchronizing to either of the same, subsequently the same signal is synchronized to, so the FCLK is synchronized to either of VFO1FB and VFO1FC. e. Case E: Figs. 7j1 to 7j3 When the rising edge of the WDT coincides with the rising edge of the VFO1FB, the state of the VFO1FA, VFO1FB, and VFO1FC becomes one of the following four states: (i) 1⊘1 (ii) 1⊘⊘ (iii) 111 (iv) 11⊘ In the case of (i) to (iii), the VFO1FC becomes FCLK from Table 1. In the case of (iv), the VFO1FA becomes FCLK from Table 1. That is, the FCLK is synchronized to either of VFO1FC or VFO1FA. f. Case F: Figs. 7k1 to 7k3 When the rising edge of the WDT coincides with the rising edge of the VFO1FC, the state of the VFO1FA, VFO1FB, and VFO1FC becomes one of the following four states: (i) ⊘1⊘ (ii) 11⊘ (iii) ⊘11 (iv) 111 In the case of (i), the VFO1FA becomes FCLK from Table 1. In the case of (ii) and (iii), the VFO1FB becomes FCLK from Table 1. In the case of (iv), the FCLK is synchronized with VFO1FC from Table 1. That is, the FCLK is synchronized with one of the VFO1FA, VFO1FB, or VFO1FC. In none of the above cases A to F other than case F (iv) does the rising edge of the FCLK coincide with the rising edge of the WDT, so it is possible to perform correct fetching of write data by the selected FCLK. In the case F (iv), the rising edge of the VFO1FC selected for the FCLK and the rising edge of the WDT coincide, so the fetching of the correct write data is not guaranteed. However, this case only rarely occurs and in practice does not pose a particular problem. In the above way, it is possible to realize a simple, stable, and highly reliable write data fetching circuit using the three phase VFO1FA, VFO1FB, and VFO1FC, which can be easily and stably prepared from the VFO3F, without use of an REF1F with a 5⊘ percent duty ratio. Note that the three VFO1FA, VFO1FB, and VFO1FC shown in Figs. 6c to 6e show one embodiment of the three fetching clock signals used in the present invention and that the present invention is not limited to the same. The same is true for Table 1. [Detailed Circuit Construction] An explanation will now be made, with reference to Fig. 6 to Fig. 1⊘, of a more detailed embodiment of the above-mentioned first aspect of the invention. Figure 8 is an explanatory view of the construction of an embodiment of the present invention, Fig. 9 is an explanatory view of a system clock preparing circuit of the same embodiment, and Fig. 1⊘ is a signal waveform diagram of the system clock preparing circuit. Overall Circuit Construction In Fig. 8, the control circuit 11, the VFO3F generating circuit 12, the synchronization and fetching circuit 13, the (1-7) encoding circuit 14, the system clock preparing circuit 15, and the fetching clock preparing circuit 16 are the same as explained in Fig. 5. In the VFO3F generating circuit 12, reference numeral 121 is a servo amplification circuit (SERVO AMP) which detects a clock signal from the magnetic disk (not shown). Reference numeral 122 is a preamplification circuit (PRE AMP) which amplifies the clock signals of the servo amplification circuit. Reference numeral 123 is a phase-locked oscillation circuit (PLO), which issues a signal of a frequency 3f. Reference numeral 124 is a variable frequency oscillation circuit (VFO) which receives the output of the PLO 123 and generates a clock VFO3F. In the synchronization and fetching circuit 13, reference numerals 131 and 132 are D-type flip-flops (DFF), CL is a clock terminal, D is a data input terminal, CR is a clear terminal, and Q is an output terminal. To the CL, D, and CR terminals of the DFF 131 are input, respectively, the WCL, WDAT, and WGT from the control circuit 11, whereby the WDT is output from the Q terminal. To the CL terminal of the DFF 132 is input the FCLK from the fetching clock preparing circuit 16. To the D terminal is input the WDT from the DFF 131. To the CR terminal is input the WGT from the control circuit 11. The NRZWDT is output from the Q terminal. In the fetching clock preparing circuit 16, reference numeral 161 is a D-type flip-flop (DFF) which receives as input at the CL terminal and D terminal the WCLK and WGT from the control circuit 11 and receives as input at the CR terminal the WGT (cleared by the inverted signal of the WGT). From the Q terminal is output the synchronization gate SYCGT. Reference numerals 132A, 132B, and 132C are DFFs, to the CL terminals of which are input the SYCGT from the DFF 161 and to the CR terminals of which are input the WGT from the control circuit 11. To the D terminal of the DFF 162A is input the VFO1FA from the system clock preparing circuit 15. At the point of rise of the SYCGT, this is latched and the latch signals X and *X are output from the Q and *Q terminals (* being a symbol of inversion, the same for all following symbols). To the D terminal of the DFF 162B is input the VFO1FB. This is latched at the point of the rise of the SYCGT, and the latch signals Y and *Y are output from the Q and the *Q terminals. To the D terminal of the DFF 162C is input the VFO1FC. This is latched at the point of the rise of the SYCGT, and the latch signals Z and *Z are output from the Q and the *Q terminals. Reference numerals 163A₁, 163A₂, 163A₃, 163B₁, 163B₂, 163C₁, 163C₂, and 163C₃ are AND circuits which perform the logical operations of the afore-mentioned Table 1. That is, the AND circuit 163A₁ receives the VFO1FA from the system clock preparing circuit 15, receives the *X, *Y, and *Z from the DFFs 162A to 162C, and, when the AND conditions of the same stand, outputs the fetching clock FCLKA synchronized with the VFO1FA. Similarly, the AND circuit 163A₂ outputs the FCLKA synchronized with the VFO1FA when the AND conditions of the VFO1FA, X, Y, and *Z stand. The AND circuit 163B₁ outputs the FCLKB synchronized with the VFO1FB when the AND conditions of the VFO1FB, *X, Y, and *Z stand. The AND circuit 163B₂ outputs the fetching clock FCLKB synchronized with the VFO1FB when the AND conditions of the VFO1FB, *X, Y, and Z stand. Similarly, the AND circuit 163B₃ outputs the fetching clock FCLKB synchronized with the VFO1FB when the AND conditions of the VFO1FB, *X, *Y, and Z stand. The AND circuit 163C₁ outputs the fetching clock FCLKC synchronized with the VFO1FC when the AND conditions of the VFO1FC, X, *Y, and Z stand. Similarly, the AND circuit 163C₂ outputs the fetching clock FCLKC synchronized with the VFO1FC when the AND conditions of the VFO1FC, X, *Y, and *Z stand. The AND circuit 163C₃ outputs the FCLKC synchronized with the VFO1FC when the AND conditions of the VFO1FC, X, Y, and Z stand. Reference numeral 164 is an OR circuit which outputs a fetching clock FCLK using OR outputs of the FCLKA, FCLKB, and FCLKC input from the AND circuits. Note that FCLKA, FCLKB, and FCLKC, as clear from the contents of the logical operations, are never simultaneously issued. System Clock Preparing Circuit Figure 9 shows an example of the system clock preparing circuit, and Fig. 1⊘ is a signal waveform diagram of the same. DFF 151 functions as a 1/2 frequency divider, to the Cl terminal of which is input the VFO3F from the VFO3F generating circuit 12 (Fig. 1⊘a). The *Q terminal and D terminal are connected and the CR terminal receives as input the clear power ready signal from the control circuit 11. By this construction, the VFO3/2F of a frequency of (3/2)f is output to the Q terminal (Fig. 1⊘b). To one of the input terminals of the NOR circuit 152A is input the *VFO3/2F from the DFF 151. To the other input terminal is input the VFO1/2F (Fig. 1⊘d) from the DFF 156, explained later. By this, the VFO-1 is output as the NOR output of the two from the NOR circuit 152A (Fig. 1⊘f). To one of the input terminals of the NOR circuit 152B is input the VFO3/2F from the DFF 151. To the other input terminal is input the *VFO1/2F (Fig. 1⊘d) from the DFF 156, explained later. By this, the VFO-2 is output as the NOR output of the two from the NOR circuit 152B (Fig. 1⊘g). To the first input terminal of the NOR circuit 153A is input the VFO-1 from the NOR circuit 152A. To the second input terminal is input the *VFO1FA (inverted signal of VFO1FA of Fig. 1⊘f) from the DFF 155, explained later. To the third input terminal is input the VFO-2 from the NOR circuit 152B. By this, the VFO-5 is output as the NOR output of the three from the NOR circuit 153A (Fig. 1⊘1). To one of the input terminals of the NOR circuit 153B is input the *VFO1FA from the DFF 155. To the other input terminal is input the *VFO1/2F (inverted signal of VFO1/2F of Fig. 1⊘d) from the DFF 156. By this, the VFO-3 is output as the NOR output of the two from the NOR circuit 153B (Fig. 1⊘i). To one of the input terminals of the NOR circuit 153C is input the VFO1/2F from the DFF 156 (Fig. 1⊘d). To the other input terminal is input the VFO1FA (Fig. 1⊘c) from the DFF 155. By this, the VFO-4 is output as the NOR output of the two from the NOR circuit 153C (Fig. 1⊘h). The OR circuit 154A outputs the VFO-6 as the OR output of the VFO-5 from the NOR circuit 153A and the *VFO1FA from the DFF 155 (Fig. 1⊘j). The OR circuit 154B outputs the VFO-7 as the OR output of the VFO-3 from the NOR circuit 153B and the VFO-4 from the NOR circuit 153C (Fig. 1⊘k). The CL terminal of the DFF 155 receives as input the VFO3F (Fig. 1⊘a). The D terminal receives as input the VFO-6 from the OR circuit 154A (Fig. 1⊘j). The CR terminal receives as input the clear power ready signal. By this, the VFO1FA is output from the Q terminal of the DFF 155 (Fig. 1⊘c). The *Q terminal outputs the *VFO1FA. On the other hand, the CL terminal of the DFF 156 receives as input the VFO3F (Fig. 1⊘a), while the D terminal receives as input the VFO-7 from the OR circuit 154B. The CR terminal receives as input the power ready signal. By this, the Q terminal of the DFF 156 outputs the VFO1/2F (Fig. 1⊘d), and the *Q terminal outputs the *VFO1/2F. Reference numeral 157 is a shift transistor, which successively shifts the VFO1FA input from the DFF 155 by the VFO3F and outputs the VFO1FB and VFO1FC (Figs. 1⊘l, 1⊘m, 1⊘n). The VFO1FA of Fig. 1⊘l shows again the VFO1FA of Fig. 1⊘c. In the above way, the VFO3/2F, VFO1FA, VFO1FB, and VFO1FC are output from the system clock preparing circuit 15. Explanation of Operation The operation of the embodiment of Fig. 8 will now be explained with reference to the signal waveform diagram of Fig. 6. The control circuit 11 transmits the WCLK, the WDAT generated in synchronization with the WCLK, and the WGT generated asynchronously with these two signals (Figs. 6f, 6g, 6i). On the other hand, the VFO3F generating circuit 12 generates the VFO3F asynchronously with the WCLK and inputs it to the system clock preparing circuit 15 (Fig. 6a). The system clock preparing circuit 15, when receiving the VFO3F, as explained in the afore-mentioned Fig. 5 and Fig. 6, outputs the VFO3/2F, VFO1FA, VFO1FB, and VFO1FC (Figs. 6b, 6c, 6d, and 6e). The DFF 161 of the fetching clock preparing circuit 16, when receiving the WCLK and WGT from the control circuit 11, issues the SYCGT at the point of the rise Ts of the WCLK directly after the rising edge of the WGT (Figs. 6f, 6i, 6j). The DFF 162A receives the VFO1FA from the system clock preparing circuit 15 at the D terminal, receives the SYCGT from the DFF 161 at the CL terminal, latches the state of the VFO1FA at the point of the rise Ts of the SYCGT, outputs the latching signal X to the Q terminal, and outputs the latching signal *X to the *Q terminal. In the case of Fig. 6, X = 1 and *X = ⊘. The DFF 162B latches the state of the VFO1FB received from the system clock preparing circuit 15 at the point of the rise Ts of the SYCGT received from the DFF 161, outputs the latching signal Y to the Q terminal, and outputs the latching signal *Y to the *Q terminal. In the case of Fig. 6, Y = ⊘ and *Y = 1. The DFF 162C, in the same way, latches the state of the VFO1FC received from the system clock preparing circuit 15 at the point Ts of the rise of the SYCGT received from the DFF 161, outputs the latching signal Z to the Q terminal, and outputs the latching signal *Z to the *Q terminal. In the case of Fig. 6, Z = 1 and *Z = ⊘. The AND circuit 163A₁ receives the VFO1FA from the system clock preparing circuit 15, receives the latching signals *X, *Y, and *Z from the DFFs 162A to 162C, and, when the AND conditions of the same stand, outputs the FCLKA synchronized with the VFO1FA. Similarly, the AND circuit 163A₂, when the AND conditions of the VFO1FA, X, Y, and *Z stand, outputs the FCLKA synchronized with the VFO1FA. In the case of Fig. 6, X = 1, *X = ⊘, Y = ⊘, *Y = 1, Z = 1, *Z = ⊘, so the FCLKA is not output. Similarly, the AND circuit 163B₁, when the AND conditions of the VFO1FA, *X, Y, and *Z stand, outputs the FCLKB synchronized with the VFO1FB. The AND circuit 163B₂, when the AND conditions of the signal VFO1FB, *X, Y, and Z stand, and the AND circuit 163B₃, when the AND conditions of the VFO1FB, *X, *Y, and Z stand, respectively output the FCLKB synchronized with the VFO1FB. In the case of Fig. 6, the FCLKB is not output. Similarly, the AND circuit 163C₁, when the AND conditions of the signal VFO1FC, *X, Y, and Z stand, the AND circuit 163C₂, when the AND conditions of the VFO1FC, X, *Y, and Z stand, and the AND circuit 163C₃, when the AND conditions of the VFO1FC, X, Y, and Z stand, respectively output the FCLKC synchronized with the VFO1FC. In the case of Fig. 6, X = 1, Y = ⊘, and Z = 1, so the AND conditions of the AND circuit 163C₁ stand and the FCLKC is output. The OR circuit 164 outputs the FCLKC input from the AND circuit 163C₁ as the FCLK and supplies the same to the CL terminal of the DFF 132. The afore-mentioned Table 1 shows to which of the three clock signals the FCLK is synchronized with in accordance with the various states of the VFO1FA, VFO1FB, and VFO1FC latched at the point of the rise of the SYCGT. On the other hand, the DFF 131 of the synchronization and fetching circuit 13, when receiving the WCLK, DAT, and WGT from the control circuit 11, issues the WGT, then synchronizes and shapes the WDAT by the WCLK and outputs the resultant DAT to the Q terminal (Figs. 6f, 6g, 6h, and 6i). The DFF 132 synchronizes and shapes the DAT received from the DFF 131 by the FCLK received from the fetching clock preparing circuit 16 and outputs the NRZ type write data NRZWDT (Figs. 6h, 6k, and 6l). The (1-7) encoding circuit 14 receives the write data NRZWDT from the DFF 132, receives the VFO3/2F from the system clock preparing circuit 15, and prepares the (1-7) RLL code. By performing the fetching operation of the write data from the WDT by the FCLK prepared in this way, it is possible to perform a correct write data fetching operation, as explained with reference to Fig. 7 previously. Above, an explanation was made of an embodiment of th present invention, but the present invention is not limited to this embodiment. For example, for the three clock signals, use may be made of three clock signals different from the waveforms shown in Fig. 6. Effects of the First Embodiment As explained above, according to the first embodiment of the present invention, the following effects are obtained: (1) The write data fetching clock is prepared by three stable, precise phase signals, so it is possible to fetch write data with less error than by the past method. (2) Instead of a 5⊘ percent duty ratio REF1F, use is made of three easy, stable, and precise clocks, so the system clock preparing circuit and the overall write data fetching circuit are simplified and the reliability is improved. B. Second Embodiment The write data fetching circuit of the first embodiment does not require a clock VFO1F (REF1F) of a 5⊘ percent duty ratio, so made possible fetching of write data of (1-7) RLL encoding with a simple circuit construction and no errors. However, as mentioned above, in the case F when the state of the three clocks is "111", the rising edge of the VFO1FC which becomes the FCLK and the rising edge of the RDT coincide, so the write data becomes "1" or "⊘" and is unstable, making it impossible to guarantee the fetching of the correct write data. This case occurs only very rarely, but to completely eliminate errors during fetching of write data, it is desirable to fetch the correct data even in such very rare cases. The second embodiment has as its object the provision of a highly reliable (1-7) RLL coded write data fetching circuit which prepares the write data fetching clock FCLF with a simple circuit construction without using a clock REF1F of a 5⊘ percent duty ratio and a fundamental frequency 1f and which completely eliminates the errors occurring when fetching write data. [Basic Constitution of Second Embodiment The basic constitution of the second embodiment, as shown in Fig. 11, is the same as that in Fig. 5, which showed the basic constitution of the first embodiment. The difference is that one of the three signals sent from the system clock preparing circuit 15a to the fetching clock preparing circuit 16 is DLVF⊘1FC. Below, the embodiment will be explained based on the points of difference with the first embodiment. Reference numeral 15a is a system clock preparing circuit which receives the VFO3F from the VFO3F generating circuit 12 and which prepares three clocks of the frequency 1f synchronized with the VFO, one of the clocks among the same being delayed within the range of 1 clock (1/6F) from the correct phase (VFO1FA, VFO1FB, DLVFO1FC: VFOIFC). Reference numeral 16 is a fetching clock preparing circuit which receives the WCLK and WGT from the control circuit 11, receives the three clocks (VFO1FA, VFO1FB, and DLVFO1FC) from the system clock preparing circuit 15, and prepares a fetching clock FCLK synchronized with one of the same in accordance with the state of the three clocks (VFO1FA, VFO1FB, and DLVFO1FC) at the point of the change of the WCLK directly after the generation of the WGT. Note that the invention is not limited to the one VFO1FC being given a delay in the three clocks. The VFO1FA or the VFO1FB may also be delayed. The operation of the write data fetching circuit of Fig. 11 will be explained with reference to the signal waveform diagrams of Fig. 12 and Fig. 13. The control circuit 11 transmits the WCLK of the fundamental frequency 1f (Fig. 12g), the WDTA generated synchronously with the WCLK (Fig. 12h), and the WGT which indicates the start of the write operation and is generated asynchronously with these two signals (Fig. 12j). On the other hand, the VFO3F generating circuit 12 generates the VFO3F of a frequency three times the fundamental frequency 1f asynchronously with the WCLK (Fig. 12a). The system clock preparing circuit 15a prepares the three phase clocks of the frequency 1f synchronized with the VFO3F, one of the phase clocks being delayed in the range of one clock ((1/6)f) from the correct phase. In the case of Fig. 12, there is shown the case where the VFO1FC among the correct three phase clocks VFO1FA, VFO1FB, and VFO1FC is delayed by one clock's worth (shown as DLVFO1FC) (Figs. 12c, 12d, 12e, and 12f). Note that in the process of preparation of the three clock signals, the VFO3/2F of a 3/2 frequency is produced (Fig. 12b), so the VFO3/2F is supplied to the (1-7) encoding circuit as the synchronization clock for the (1-7) RLL-code encoding. The fetching clock preparing circuit 16, when receiving the WCLK and WGT from the control circuit 11 and receiving the VFO1FA, VFO1FB, and the DLVFO1FC from the system clock preparing circuit 15a, selects as the fetching clock FCLK one of the same (in Fig. 12, in accordance with the state of the VFO1FA, VFO1FB, and DLVFO1FC) at the point of the rise Ts of the WCLK directly after the generation of the WGT (Figs. 6f, 6i, 6j). Table 2 shows an example of the correspondence relationship between the state of the VFO1FA, VFO1FB, and DLVFO1FC and the FCLK synchronized clock signals. Note that the asterisk mark indicates cases which actually never occur. Further, the meanings of X, Y, and Z are as mentioned later. VFO1FA (X) VFO1FB (Y) DLVFO1FC (Z) FCLK sync'ed. clock ⊘ ⊘ ⊘ ⊘ VFO1FA 1 ⊘ ⊘ 1 VFO1FB 2 ⊘ 1 ⊘ VFO1FB 3 ⊘ 1 1 VFO1FB *4 1 ⊘ ⊘ DLVFO1FC 5 1 ⊘ 1 DLVFO1FC 6 1 1 ⊘ VFO1FA 7 1 1 1 DLVFO1FC For example, in the case of Fig. 12, the state of the VFO1FA, VFO1FB, and DLVFO1FC at the point of change (point of rise Ts) of the WCLK directly after the rising edge of the WGT is "1⊘1", so after the generation of the WGT, the FCLK synchronizes with the DLVFO1FC (Figs. 12c, 12d, 12f, 12g, 12j, and 12l). The synchronization and fetching circuit 13, when it receives the WCLK and the WDAT from the control circuit 11 and receives the FCLK from the fetching clock preparing circuit 16, synchronizes and shapes the write data WDT synchronizing the WDAT to the WCLK (Fig. 12m). By this WDT, the fetching of the NRZ type write data (NRZWDT) synchronized with the FCLK is performed and the result input to the (1-7) encoding circuit. The (1-7) encoding circuit 14 performs (1-7) RLL encoding by the write data (NRZWDT) and the VFO3/2F. Next, an explanation will be given of the write data fetching operation at the various correspondence relationships of the three clocks VFO1FA, VFO1FB, and VFO1FC and WDT. Looking at the phases and the synchronization relationships of the signals of VFO1FA, VFO1FB, DLVFO1FC, WCLK, WDAT, WDT, WGT, and FCLK of Fig. 12, it will be understood that the clock to which FCLK is synchronized is selected from the state of the VFO1FA, VFO1FB, and DLVFO1FC at the point of the rise of the WDT. Below, note is taken of this point and an explanation made of the write data fetching operation at the phase relationships of the three clocks and WDT with reference to Fig. 13 and Table 2. Figures 13a to 13f show the VFO3F, VFO3/2F, VFO1FA, VFO1FB, and VFO1FC of Figs. 12a to 12f. Figures 13g to 13p show the relationship of the rising edge of the WDT and the states of the three clocks and the three clocks to which the FCLK becomes synchronized in each case. For example, in Fig. 13g (case A), if the rising edge of the WDT coincides with the rising edge of the VFO1FA, the state of the VFO1FA, VFO1FB, and DLVFO1FC becomes one of the following: (i) ⊘11 (ii) ⊘⊘1 (iii) 111 (iv) 1⊘1 Referring to the above-mentioned Table 2, in the case of (i) and (ii), the FCLK synchronizes with the VFO1FB. In the case of (iii) and (iv), the FCLK synchronizes with the DLVFO1FC. Similarly below, for Figs. 13g to 13p (case A to J), the types of states occurring in the three clocks at the point of the rising edge of the WDT and the clocks to which the FCLK synchronizes at that time are summarized and shown in Table 3. Note that A and B in the "FCLK synchronized VFO1F clock" column mean VFO1FA and VFO1FB and that (DL)C means the DLVFO1FC. The meanings of X, Y, and Z are explained later. Phase relation of WDT rising edge and 3 clocks VFO1FA (X) VFO1FB (Y) DLVFO1FC (Z) FCLK sync VFO1F clock (A) Coincides with rising edge VFO1FA ⊘ 1 1 B ⊘ ⊘ 1 B 1 1 1 (DL)C 1 ⊘ 1 (DL)C (B) Between rising edges VFO1FA and VFO1FB 1 ⊘ 1 (DL)C (C) Coincides with rising edge VFO1FB 1 ⊘ 1 (DL)C 1 1 1 (DL)C (D) Between rising edge VFO1FB and trailing ing edge DLVFO1FC 1 1 1 (DL)C (E) Coincides with trailing edge DLVFO1FC 1 1 1 (DL)C 1 1 ⊘ A (F) Between trailing edge DLVFO1FC and trailing edge VFO1FA 1 1 ⊘ A (G) Coincides with trailing edge VFO1FA 1 1 ⊘ A ⊘ 1 ⊘ B (H) Between trailing edge VFO1FA and rising edge DLVFO1FC ⊘ 1 ⊘ B (I) Coincides with rising edge DLVFO1FC ⊘ 1 ⊘ B ⊘ 1 1 B (J) Between rising edges DLVFO1FC and VFO1FA ⊘ 1 1 B As shown in Table 3, no matter what the case A to J, the rising edges of the FCLKs do not coincide with the rising edge of the WDT, so it is possible to fetch the correct write data with no error by the selected FCLK. Note that the delayed clock is not limited to the VFO1FC, as mentioned earlier. In this way, it is possible to realize a simple, stable, and error-free, highly reliable write data fetching circuit by using three clocks (VFO1FA, VFO1FB, and DLVFO1FC) which can be easily prepared from the VFO3F without use of a REF1F of a 5⊘ percent duty ratio. [Detailed Constitution of the Second Embodiment] Details of the second embodiment will be explained now with reference to Fig. 12 to Fig. 14. Figure 14 is a detailed circuit diagram of the second embodiment of the present invention. The signal waveform diagrams of Figs. 12 and 13 may be also used as the signal waveform diagrams for the circuit of the embodiment of Fig. 14. Detailed Circuit Construction In Fig. 14, the control circuit 11, the VFO3F generating circuit 12, the synchronization amd fetching circuit 13, the (1-7) encoding circuit 14, and the fetching clock preparing circuit 16 are the same as explained with reference to the first embodiment. In the system clock preparing circuit 15a, reference numeral 151 is a three-clock generating circuit which has the same construction as the system clock preparing circuit 25 (Fig. 9) in the write data fetching circuit of the first embodiment and issues the *VFO32F, VFO1FA, VFO1FB, and VFO1FC (Figs. 12b, 12c, 12d, and 12e). Reference numeral 158 is a DFF to whose D terminal is input the VFO1F1C from the three-clock generating circuit 151, to whose CL terminal is input the *VFO3F through the inverter 159, and to whose CR terminal is input the reset signal from the three-clock generating circuit 151. By this construction, the delayed clock DLVFO1FC delayed from the VFO1FC by one clock (f/6) is prepared. In the fetching clock preparing circuit 16, reference numeral 161 is a D-type flip-flop (DFF), to whose CL terminal and D terminal are input the WCLK and WGT from the control circuit 11 and to whose CR terminal is input the WGT (cleared by the inverted signal of the WGT). The synchronization gate SYCGT is output from the Q terminal. Reference numerals 162A, 162B, and 162C are DFFs, to whose CL terminals are input the SYCGT from the DFF 161 and to whose CR terminals are input the WGT from the control circuit 11. The D terminal of the DFF 162A receives as input the VFO1FA from the system clock preparing circuit 15. This is latched at the point of the rise of the SYCGT, and the latching signals X and *X are output from the Q and *Q terminals. The D terminal of the DFF 162B receives as input the VFO1FB. This is latched at the point of the rise of the SYCGT, and the latching signals Y and *Y are output from the Q and *Q terminals. The D terminal of the DFF 162C receives as input the VFO1FC. This is latched at the point of the rise of the SYCGT, and the latch signals Z and *Z are output from the Q and *Q terminals. Reference numerals 163A₁, 163A₂, 163B₁, 163B₂, 163B₃, 163C₁, 163C₂, and 163C₃ are AND circuits which perform the logical operations of the afore-mentioned Table 2. That is, the AND circuit 163A₁ receives the VFO1FA from the system clock preparing circuit 15, receives the *X, *Y, and *Z from the DFF 162A to 162C, and, when the AND conditions of the same stand, outputs the fetching clock FCLKA synchronized with the VFO1FA. Similarly, the AND circuit 163A₂ outputs the FCLKA synchronized with the VFO1FA when the AND conditions of the VFO1FA, X, Y, and *Z stand. The AND circuit 163B₁ outputs the fetching clock FCLKB synchronized with the VFO1FB when the AND conditions of the VFO1FB, *X, Y, and *Z stand. Similarly, the AND circuit 163B₂ outputs the fetching clock FCLKB synchronized with the VFO1FB when the AND conditions of the VFO1FB, *X, Y, and Z stand. The AND circuit 163B₃ outputs the FCLKB synchronized with the VFO1FB when the AND conditions of the VFO1FB, *X, *Y, and Z stand. The AND circuit 163C₁ outputs the fetching clock FCLKC synchronized with the VFO1FC when the AND conditions of the VFO1FC, X, *Y, and Z stand. Similarly, the AND circuit 163C₂ outputs the fetching clock FCLKC synchronized with the VFO1FC when the AND conditions of the VFO1FC, X, *Y, and *Z stand. The AND circuit 163C₃ outputs the FCLKC synchronized with the VFO1FC when the AND conditions of the VFO1FC, X, Y, and Z stand. Reference numeral 164 is an AND circuit which outputs a fetching clock FCLK using OR outputs of the FCLKA, FCLKB, and FCLKC input from the AND circuits. Note that FCLKA, FCLKB, and FCLKC, as clear from the contents of the logical operations, are never simultaneously issued. Note also that the AND conditions X, *Y, and *Z of the AND circuit 163C₂ never stand, but by providing this AND circuit 163C₂, there is the advantage of common use of the circuit with the fetching clock preparing circuit 25 of the write data fetching circuit of the first embodiment. Explanation of Operation The operation of the circuit of Fig. 14 will now be explained with reference to the signal waveform diagram of Fig. 12. The system clock preparing circuit 15a, when receiving the VFO3F, outputs the VFO3/2F, VFO1FA, VFO1FB, and DLVFO1FC (Figs. 12b, 12c, 12d, and 12f). The DFF 161 of the fetching clock preparing circuit 16, when receiving the WCLK and WGT from the control circuit 11, issues the SYCGT at the point of rise Ts of the WCLK directly after the rising edge of the WGT (Figs. 12g, 12j, and 12k). The DFF 162A receives the VFO1FA from the system clock preparing circuit 15a at the D terminal, receives the SYCGT from the DFF 161 at the CL terminal, latches the state of the VFO1FA at the point of rise Ts of the SYCGT, outputs the latching signal X to the Q terminal, and outputs the latching signal *X to the *Q terminal. In the case of Fig. 12, X = 1 and *Y = ⊘. The DFF 162B similarly latches the state of the VFO1FB received from the system clock preparing circuit 15a at the point of rise Ts of the SYCGT received from the DFF 161, outputs the latching signal Y to the Q terminal, and outputs the latching signal *Y to the *Q terminal. In the case of Fig. 12, Y = ⊘ and *X = 1. The DFF 162C, in the same way, latches the state of the DLVFO1FC received from the system clock preparing circuit 15a at the point Ts of rise of the SYCGT received from the DFF 161, outputs the latching signal Z to the Q terminal, and outputs the latching signal *Z to the *Q terminal. In the case of Fig. 12, Z = 1 and *Z = ⊘. The AND circuit 163A₁ receives the VFO1FA from the system clock preparing circuit 15a, receives the latching signals *X, *Y, and *Z from the DFF 162A to 162C, and, when the AND conditions of the same stand, outputs the FCLKA synchronized with the VFO1FA. Similarly, the AND circuit 163A₂, when the AND conditions of the VFO1FA, X, Y, and *Z stand, outputs the FCLKA synchronized with the VFO1FA. In the case of Fig. 12, X = 1, *X = ⊘, Y = 1, Z = 1, *Z = ⊘, so the FCLKA is not output. Similarly, the AND circuit 163B₁, when the AND conditions of the VFO1FA, *X, Y, and *Z stand, the AND circuit 163B₂, when the AND conditions of the signal VFO1FB, *X, Y, and Z stand, and the AND circuit 163B₃, when the AND conditions of the VFO1FB, *X, *Y, and Z stand, respectively output the FCLKB synchronized with the VFO1FB. In the case of Fig. 12, the FCLKB is not output. Similarly, the AND circuit 163C₁, when the AND conditions of the signal VFO1FC, X, *Y, and Z stand, the AND circuit 163C₂, when the AND conditions of the VFO1FC, X, *Y, and *Z stand, and the AND circuit 163C₃, when the AND conditions of the VFO1FC, X, Y, and Z stand, respectively output the FCLKC synchronized with the VFO1FC. In the case of Fig. 12, X = 1, Y = ⊘, and Z = 1, so the AND conditions of the AND circuit 163C₁ stand and the FCLKC is output. The OR circuit 164 outputs the FCLKC input from the AND circuit 163C₁ as the FCLK and supplies the same to the CL terminal of the DFF 132. The afore-mentioned Table 2 and Table 3 show to which of the three clock signals the FCLK is synchronized in accordance with the various tates of the VFO1FA, VFO1FB, and VFO1FC latched at the point of rise of the SYCGT. On the other hand, the DFF 131 of the synchronization and fetching circuit 13, when receiving the WCLK, DAT, and WGT from the control circuit 11, issues the WGT, then synchronizes and shapes the WDAT by the WCLK and outputs the resultant DAT to the Q terminal (Figs. 12g, 12h, 12i, and 12j). The DFF 132 synchronizes and shapes the DAT received from the DFF 131 by the FCLK received from the fetching clock preparing circuit 16 and outputs the NRZ type write data NRZWDT (Figs. 12i, 12l, and 12m). The (1-7) encoding circuit 14 receives the write data NRZWDT from the DFF 132, receives the VFO3/2F from the system clock preparing circuit 15a, and prepares the (1-7) RLL code. By performing the fetching operation of the write data from the WDT by the FCLK prepared in this way, it is possible to perform a correct write data fetching operation, as explained with reference to Fig. 13 previously. Above, an explanation was made of a second embodiment of the present invention, but the present invention is not limited to this embodiment. For example, the clock given the delay may be VFO1FA or VFO1FB instead of VFO1FC. Further, it is possible to set the amount of delay given to anywhere within the range of the clock ((1/6)f). Effects of the Second Embodiment As explained above, according to the second embodiment of the present invention, the following effects are obtained: (1) The write data fetching clock is prepared by three clocks of a fundamental frequency 1f, one clock of which is delayed within the range of one clock (f/6) from the correct phase, so it is possible to fetch write data with completely no error. (2) Instead of a 5⊘ percent duty ratio VFO1F, use is made of three easy, stable, and precise clocks, so the system clock preparing circuit and the overall write data fetching circuit are simplified and the reliability is improved. C. Third Embodiment As explained above, the conventional (1-7) RLL coded write data fetching circuit suffered from the problem that it sometimes could not fetch the correct write data unless the REF1F had a duty ratio of exactly 5⊘ percent. In the case of digital signals, it is difficult to prepare an REF1F with a duty ratio of 5⊘ percent from the VFO3F and the resultant frequency division circuit is unsatisfactory in precision and stability, so realization of a reliable write data fetching circuit is difficult. The third embodiment of the present invention, like the first embodiment but using a method different from the first embodiment, has as its object performing without error a write data fetching operation by a simple circuit construction and the provision of a highly reliable write data fetching circuit. [Basic Constitution of Third Embodiment] The reason why the conventional write data fetching circuit required an REF1F with a duty ratio of 5⊘ percent was that, in the conventional write data fetching circuit, the handling of data between the control circuit and the encoding circuit was performed in units of the fundamental frequency 1f, i.e., 1 bit. However, in a (1-7) RLL code, 3-bit encoded data is prepared based on the current 2-bit data and the next 2-bit data. That is, the write data is fetched in the (1-7) encoding circuit in 2-bit units, whereby normally (1-7) RLL encoding is possible. In this case, as explained below, an REF1F with a duty ratio of 5⊘ percent is not required. The third embodiment of the present invention was made based on this idea. Figure 15 shows the basic constitution of the third embodiment of the present invention by a block diagram. In Fig. 15, reference numeral 11 is a control circuit which transmits a write clock WCLK of a fundamental frequency 1f, a write data WDAT by which data is generated at 1 bit/1f synchronized with the WCLK, and a write gate WGT which indicates the start of the write operation. Reference numeral 12 is a data shift circuit which makes the WDAT input from the control circuit phase-synchronized with the WCLK and generates four bits of data X₁, X₂, Y₁, and Y₂ in units of 2 bits and shifted one bit each. Reference numeral 13 is a (1-7) encoding circuit which performs (1⊘7) RLL coding processing based on the X₁, X₂, Y₁, and Y₂ input from the data shift circuit 12 and outputs the three-bit unit parallel code data Z₁, Z₂, and Z₃. Reference numeral 14 is a VFO3F generating circuit which generates asynchronously with the WCLK the clock VFO3F of a frequency three times the fundamental frequency 1F. Reference numeral 15 is a system clock preparing circuit which receives the VFO3F from the VFO3F generating circuit and issues the three reference clocks VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC of the frequency 1/2(f) shifted by one clock each and synchronized with VFO3F, the clock VFO3/2F and its inverted clock *VFO3/2F (where * is an inversion symbol, same below for other symbols) divided from VFO3F and with a frequency of (3/2)f, the six synchronization clocks VFO1/2A1, VFO1/2FB⊘, VFO1/2FC4, VFO1/2FA6, VFO1/2FB7, and VFO1/2C3 shifted one clock each and synchronized with the VFO3F and having a frequency of (1/2)f and a width of 2 bits. Reference numeral 16 is a synchronization signal generating circuit which receives the WGT from the control circuit 11, receives the three reference clocks from the system clock preparing circuit 15, the VFO3/2F, the *VFO3F, and the six synchronization clocks, and outputs a load signal LOAD of a frequency of (1/2)f synchronized with one of the six synchronization clocks in accordance with the state of the three reference clocks at the point of change of the WCLK directly after the generation of the WGT and an encoding clock ENCOD 3/2F of a frequency of (3/2)f synchronized with one of the VFO3/2F and *VFO3/2F in accordance with the state of the three reference clocks. Reference numeral 17 is a synchronization circuit which receives the LOAD signal and the ENCOD3/2F from the synchronization signal generating circuit 16 and converts to a serial format the parallel code data Z₁, Z₂, and Z₃ input from the (1-7) encoding circuit 13 to a 3-bit unit code data CODE DATA synchronized with the ENCOD3/2F. The operation of Fig. 15 will be explained with reference to the operational waveform diagrams of Figs. 16 to 19. Figure 16 is an overall signal waveform diagram, Fig. 17 is a signal waveform diagram of the system clock preparing circuit, and Figs. 18 and 19 are signal waveform diagrams of the synchronization signal generating circuit. The control circuit 11 transmits the clock WCLK with the fundamental frequency 1f, the write data WDAT synchronized with the WCLK and by which data is generated at 1 bit/1f, and the WGT generated asynchronously with the two signals and indicating the start of the write operation (Figs. 16f, 16g, and 16h). After the generation of the WGT, the data shift circuit 12 synchronizes the phases of the WDAT and WCLK input from the control circuit 11 and generates the four pieces of data X₁, X₂, Y₁, and Y₂ in 2-bit units shifted one bit each (one clock's worth) (Fig. 16f, 16g, 16h, and 16l to 16p). The (1-7) encoding circuit 13 performs the known (1-7) RLL encoding processing based on the X₁, X₂, Y₁, and Y₂ input from the data shift circuit 12 and outputs the 3-bit unit parallel code data Z₁, Z₂, and Z₃. When the WDAT is all "⊘", "X₁, X₂" and "Y₁, Y₂" are both "⊘⊘". In that case, the "Z₁, Z₂, Z₃" is coded as "⊘⊘1" (Fig. 16g, 16l to 16s). On the other hand, the VFO3F generating circuit 14 generates asynchronously with WCLK the clock VFO3F of a frequency three times the fundamental frequency 1f (Figs. 16a and 16f). The system clock preparing circuit 15 receives the VFO3F from the VFO3F generating circuit 14 and generates the three reference clocks VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC of the frequency (1/2)f shifted by one clock each and synchronized with VFO3F (Figs. 16a, 16c, 16d, 16e, and 17a, 17d, 17e, and 17f), the clock VFO3/2F and its inverted clock *VFO3/2F divided from VFO3F and with a frequency of (3/2)f (Figs. 16 and 17b, however, *VFO3/2F not shown), and the six synchronization clocks VFO1/2A1, VFO1/2FB⊘, VFO1/2FC4, VFO1/2FA6, VFO1/2FB7, and VFO1/2C3 shifted one clock each and synchronized with the VFO3F and having a frequency of (1/2)f and a width of 2 bits (Figs. 17a, 17g to 17l). The synchronization signal generating circuit 16 receives the WGT from the control circuit 11, receives the three reference clocks from the system clock preparing circuit 15, the VFO(3/2)F, the *VFO3F, and the six synchronization clocks, and outputs the LOAD signal of a frequency of (1/2)f synchronized with one of the six synchronization clocks in accordance with the state of the three reference clocks at the point of change of the WCLK directly after the generation of the WGT and the ENCOD3/2F of a frequency of (3/2)f synchronized with one of the VFO3/2F and *VFO3/2F in accordance with the state of the three reference clocks. Note that the synchronization gate SYCGT shown in Fig. 16i is a gate signal generated inside the synchronization signal generating circuit at the point of change Ts of the WCLK directly after the generation of the WGT. Table 4 shows an example of the correspondence relationship between the state of the three reference clocks VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC at the point of time Ts of generation of the SYCGT and the six synchronization clocks to which the LOAD signal is synchronized. VFO1/2FRA VFO1/2FRB VFO1/2FRC LOAD signal sync. VFO1/2FR ⊘ ⊘ ⊘ ⊘ VFO1/2FB⊘ 1 ⊘ ⊘ 1 VFO1/2FA1 3 ⊘ 1 1 VFO1/2FC3 4 1 ⊘ ⊘ VFO1/2FC4 6 1 1 ⊘ VFO1/2FA6 7 1 1 1 VFO1/2FB7 As clear from the comparison of Fig. 17 and Table 4, seen from the reference point of time of the state of the three reference clocks covered, the next rising synchronization clock is synchronized to. For example, in the case of Fig. 16, the state of the reference clocks VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC at the point of change (point of rise Ts) of the WCLK directly after the rising edge of the WGT is "⊘⊘1", so a LOAD signal synchronized with the VFO1/2FRA1 is generated after the generation of the WGT (Figs. 16c, 16d, 16e, 16f, 16h, and 16k). Further, when the state of the reference clocks VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC at the point of change of the WCLK directly after the rising edge of the WGT, i.e., the point of generation (point of rise Ts) of the SYCGT, is "⊘⊘⊘", "11⊘", "⊘11", that is, "⊘,6,3", the ENCOD3/2F synchronizes with the VFO3/2F and when "⊘⊘1", "1⊘⊘", or "111", that is, "1,4,7", the ENCOD3/2F synchronizes with the *VFO3/2F. The synchronization circuit 17 receives the LOAD signal and the ENCOD3/2F from the synchronization signal generating circuit 16 and converts to a serial format the parallel code data Z₁, Z₂, and Z₃ input from the (1-7) encoding circuit 13 to 3-bit unit CODE DATA synchronized with the ENCOD3/2F. Note that the content of the waveforms and circuits in Fig. 16 and Fig. 17 not explained will be discussed later. By using the LOAD signal and the ENCOD3/2F prepared in this way, as explained in reference to Figs. 18 and 19 next, it is possible to reliably synchronize the coding data output by the (1-7) encoding circuit at each of the states of the three clocks at the generation of the SYCGT. Looking at the phases and the synchronization relationships of the signals of VFO1/2FRA, VFO1/2FRB, VFO1/2FRC, WCLK, WDAT, SYCGT, LOAD, and ENCOD3/2F, it will be understood that which of the six synchronization clocks synchronize(s) the LOAD signal and ENCOD3/2F is determined from the state of the VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC at the point of the rise of the SYCGT. Below, note is taken of this point and an explanation made of the synchronization and generating operation of the LOAD signal and the ENCOD3/2F at the phase relationships of the three reference clocks and the SYCGT with reference to Figs. 18 and 19 and Table 4. Figures 18b to 18e and Figs. 19b to 19e show the VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC shown in Figs. 16b to 16e. Figures 18f1 to 18f6 to Figs. 18h1 to 18h6 (cases 1 to 3) and Figs. 19f1 to 19f6 to Figs. 19h1 to 19h6 (cases 4 to 6) show the SYCGT generated synchronized with the WCLK directly after the rising edge of the WGT at the zones (19 to (6) in the 2 bits (2f) and the six synchronization clocks for synchronizing the LOAD signal and ENCOD3/2F and VFO3/2F (*VFO3/2F) from the relation of the states of the three reference clocks at the point of the rise of the SYCGT. For example, in case 1, the state of VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC at the point of rise of the SYCGT (zone (1)) is "⊘⊘1". Referring to Table 4, in the case 1, the LOAD signal synchronizes with the synchronization clock VFO1/2FA1 and the ENCOD3/2F synchronizes with the *VFO3/2F. Table 5 summarizes and shows the types of states caused by the three reference clocks at the point of rise of the SYCGT (zone (1) to (6)) for the cases 1 to 6 and the six synchronization clocks to which the LOAD signal synchronizes and the VFO3/2F (*VFO3/2F) to which the ENCOD3/2F synchronizes. Case Corres. bet. SYCGT rising edge and ref. clock VFO1/2 FRA VFO1/2 FRB VFO1/2 FRC LOAD signal sync'ed sync. clock ENCOD3/2F sync'ed VFO3/2F 1 Zone (1) ⊘ ⊘ 1 VFO1/2FA1 *VFO3/2F 2 Zone (5) 1 1 1 VFO1/2FB7 *VFO3/2F 3 Zone (2) ⊘ ⊘ ⊘ VFO1/2FB⊘ VFO3/2F 4 Zone (3) 1 ⊘ ⊘ VFO1/2FC4 *VFO3/2F 5 Zone (6) ⊘ 1 1 VFO1/2FC3 VFO3/2F 6 Zone (4) 1 1 ⊘ VFO1/2FA6 VFO3/2F As shown in Table 5, even if the point of the rise of the SYCGT is in the zone (1) or (2), the LOAD signal and the ENCOD3/2F are unilaterally synchronized. Further, even if the point of the rise of the SYCGT occurs at the boundary of zones, the LOAD signal and ENCOD3/2F are unilaterally synchronized. For example, if the point of the rise of the SYCGT occurs at the boundary of the zones (1) and (2), the state of the VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC becomes either the "⊘⊘1" or "⊘⊘⊘". The former case is the same as case 1, where the LOAD signal synchronizes with the VFO1/2FA1 and the ENCOD3/2F synchronizes with the *VFO3/2F. The latter case is the same as the case 3, where the LOAD signal synchronizes with the VFO1/2FB⊘ and the ENCOD3/2F synchronizes with the VFO1/21F. The same applies when the SYCGT is generated at the boundary of other zones and synchronization is effected by one of the cases of the two zones. In either case, the synchronized LOAD signal and ENCOD3/2F subsequently maintain that synchronization state. In this way, by fetching directly the WDAT in 2-bit units into the (1-7) encoding circuit, it is possible to perform a fetching operation where an REF1F with a 5⊘ percent duty ratio is not used. Further, it is possible to synchronize the encoding data to the ENCOD3/2 by the LOAD signal and ENCOD3/2F prepared using the three reference clocks and six synchronization clocks, which can be easily and stably prepared from the VFO3F. By this, it is possible to perform without error a write data fetching operation by a simpler circuit construction in the past and to realize a highly reliable write data fetching circuit. [Detailed Constitution of Third Embodiment] Details of the third embodiment of the present invention will be explained with reference to Figs. 16 to 23. Figure 2⊘ is a circuit diagram of the third embodiment of the present invention, Fig. 21 is a diagram of the system clock preparing circuit of the embodiment, Fig. 22 is a signal waveform diagram of the system clock preparing circuit, and Fig. 23 is an explanatory view of the synchronization circuit of the embodiment. The operational waveform diagrams of Fig. 16 to Fig. 19 are also used for the explanation of the operation of the circuit of the embodiment of Fig. 2⊘. Detailed Circuit of Third Embodiment In Fig. 2⊘, the control circuit 11, the data shift circuit 12, the (1-7) encoding circuit 13, the VFO3F generating circuit 14, the system clock preparing circuit 15, the synchronization signal generating circuit 16, and the synchronization circuit 17 are as explained with reference to Fig. 15. In the VFO3F generating circuit, reference numeral 141 is a servo amplification circuit (SERVO AMP) which detects a clock signal from a magnetic disk (not shown). Reference numeral 142 is a preamplification circuit (PRE AMP) which amplifies the clock signal of the servo amplification circuit 141. Reference numeral 143 is a phase locked oscillation circuit (PLO) which receives the clock of the PRE AMP 122 and generates the signal of the frequency 3f. Reference numeral 144 is a variable frequency oscillation circuit which receives the output of the PLO143 and generates the clock VFO3F. In the synchronization signal generating circuit 16, reference numeral 161 is a DFF which receives as input at its clock (CL), data (D), and clear (CR) terminals the WCLK and WGT from the control circuit 11 and outputs the SYCGT from its Q terminal. The clear operation is performed by the inverted signal (trailing edge) of the WGT. Reference numerals 162A, 162B, and 162C are three stages of DFFs which receive as inputs at their CL terminals the SYCGT from the DFF 161 and receive as inputs at their CR terminals the WGT. The D terminal of the DFF 162 receives as input the VFO1/2FRA from the system clock preparing circuit 15 and the output terminals Q and *Q output the value of the VFO1/2FRA latched at the point of the rising edge of the SYCGT and its inverted value. The D terminal of the DFF 162B receives as input the VFO1/2FRAB and the output terminals Q and *Q output the value of the VFO1/2FRB latched at the point of the rising edge of the SYCGT and its inverted value. The D terminal of the DFF 162C receives as input the VFO1/2FRC and its output terminals Q and *Q output the value of the VFO1/2FRC latched at the point of the rising edge of the SYCGT and its inverted value. Reference numeral 163 is a phase selection circuit which generates one of the signals SELECT ⊘, 1, 3, 4, 6, and 7 for selecting the six synchronization clocks in accordance with the values of the DFF 162A to 162C, i.e., the states "⊘⊘⊘", "⊘⊘1", "⊘11", "1⊘⊘", "11⊘", and "111" (that is, "⊘", "1", "3", "4", "6", and "7") of the VFO1/2FRA to VFO1/2FRC at the point of rise of the SYCGT. The SELECT ⊘ selects the VFO1/2FB⊘, SELECT 1 selects the VFO1/2FA1, SELECT 3 selects the VFO1/2FC3, SELECT 4 selects the VFO1/2FC4, SELECT 6 selects the VFO1/2FA6, and SELECT 7 selects the VFO1/2FB7. Reference numeral 164 is a clock selection circuit which receives the six synchronization clocks VFO1/2FA1 to VFO1/2FC3 from the system clock preparing circuit 15, receives the SELECT ⊘ to 7 from the phase selection circuit 163 (only input of SELECT ⊘ is shown), and outputs the input synchronization clock indicating the SELECT (⊘ to 7) as the LOAD signal. Reference numeral 165 is an encoding clock phase detection circuit which receives the VFO3/2F and *VFO3/2F from the system clock preparing circuit 15, receives the SELECT ⊘, 6, and 3 and the SELECT 1, 4, and 7 from the phase selection circuit 163, and, when one of the SELECT ⊘, 6, and 3 is input, outputs the VFO3/2F as the ENCOD3/2 and, when one of the SELECT 1, 4, and 7 is input, outputs the *VFO3/2F as the ENCOD 3/2F. In the synchronization circuit 17, reference numeral 171 is a LOAD circuit which receives from the (1-7) encoding circuit the parallel coding data Z₁, Z₂, and Z₃, receives from the clock selection circuit 164 the LOAD signal, latches the parallel coding data Z₁, Z₂, and Z₃ at the rising edge of the LOAD signals, and outputs the latching signals FF₁, FF₂, and FF₃. Reference numeral 172 is an ENCODE DATA circuit which receives the latching signals FF₁, FF₂, and FF₃ from the LOAD circuit 171, receives the LOAD signal from the clock selection circuit 16, receives the ENCOD3/2F from the encoding clock phase detection circuit 165, and converts the latching signals FF₁, FF₂, and FF₃ input from the LOAD circuit 171 to the serial CODE DATA synchronized with the ENCOD3/2F. System Clock Preparing Circuit Figure 21 shows an example of the system clock preparing circuit, and Figs. 17 and 22 are signal waveform diagrams of the same. The DFF 151 functions as a 1/2 frequency divider and receives as input at its Cl terminal the VFO3F from the VFO3F generating circuit 12 (Fig. 22a). The *Q terminal and D terminal are connected and the CR terminal receives as input the clear power ready signal from the control circuit 11. By this construction, the VFO3/2F of a frequency of (3/2)f is output to the Q terminal (Fig. 22b) and the *VFO3/2F is output to the *Q terminal. To one of the input terminals of the NOR circuit 152A is input the *VFO3/2F from the DFF 151. To the other input terminal is input the VFO1/2FRA (Fig. 22d) from the DFF 156A, explained later. By this, the VFO-1 is output as the NOR output of the two from the NOR circuit 152A (Fig. 22g). To one of the input terminals of the NOR circuit 152B is input the VFO3/2F from the DFF 151. To the other input terminal is input the *VFO1/2FRA from the DFF 156A, explained later. By this, the VFO-2 is output as the NOR output of the two from the NOR circuit 152B (Fig. 22h). To the first input terminal of the NOR circuit 153A is input the VFO-1 from the NOR circuit 152A. To the second input terminal is input the *VFO1F (inverted signal of VFO1F of Fig. 22c) from the DFF 155, explained later. To the third input terminal is input the VFO-2 from the NOR circuit 152B. By this, the VFO-5 is output as the NOR output of the three from the NOR circuit 153A (Fig. 22i). To one of the input terminals of the NOR circuit 153B is input the *VFO1F from the DFF 155. To the other input terminal is input the *VFO1/2FRA (inverted signal of VFO1/2FRA of Fig. 22d) from the DFF 156A. By this, the VFO-3 is output as the NOR output of the two from the NOR circuit 153B (Fig. 22k). To one of the input terminals of the NOR circuit 153C is input the VFO1/2FRA from the DFF 156A (Fig. 22d). To the other input terminal is input the VFO1F (Fig. 22c) from the DFF 155. By this, the VFO-4 is output as the NOR output of the two from the NOR circuit 153C (Fig. 221). The OR circuit 154A outputs the VFO-6 as the OR output of the VFO-5 from the NOR circuit 153A and the *VFO1F from the DFF 155 (Fig. 22j). The OR circuit 154B outputs the VFO-7 as the OR output of the VFO-3 from the NOR circuit 153B and the VFO-4 from the NOR circuit 153C (Fig. 22m). The CL terminal of the DFF 155 receives as input the VFO3F (Fig. 22a). The D terminal receives as input the VFO-6 from the OR circuit 154A (Fig. 22j). The CR terminal receives as input the clear power ready signal. By this, the VFO1F is output from the Q terminal of the DFF 155 (Fig. 22c). The *Q terminal outputs the *VFO1FA. On the other hand, the CL terminal of the DFF 156A receives as input the VFO3F (Fig. 22a), while the D terminal receives as input the VFO-7 from the OR circuit 154B. The CR terminal receives as input the power ready signal. By this, the Q terminal of the DFF 156A outputs the VFO1/2FRA (Fig. 22d and Fig. 17d), and the *Q terminal outputs the *VFO1/2FRA. Reference numeral 157A is a shift register, which successively shifts the VFO1/2FRA input from the DFF 156A by the VFO3F and outputs the VFO1/2FRB and VFO1/2FRC (Figs. 17a, 17d, 17e, 17f). Next, to one of the input terminals of the NOR circuit 153D is input the *VFO3/2F from the DFF 151. To the other input terminal is input the *VFO1/2FA1 (Fig. 16e) from the DFF 156B, explained later. The VFO-8 is output as the NOR output (Fig. 22n). To one of the input terminals of the OR circuit 154D is input the VFO-4 from the NOR circuit 153C. To the other input terminal is input the VFO-8 from the NOR circuit 153D. The VFO-9 is output as the OR output (Fig. 22p). To the CL terminal of the DFF 156B is input the VFO3F. The D terminal receives as input the VFO-9 from the OR circuit 154D. The CR terminal receives as input the power ready signal. By this, the VFO1/2FA1 is output from the Q terminal of the DFF 156B, and the *VFO1/2FA1 is output from the *Q terminal (Figs. 22a, 22e, 22p, and 17g). Reference numeral 157B is a shift register, which successively shifts the VFO1/2FA1 input from the DFF 156B by the VFO3F and outputs the VFO1/2FB⊘ and VFO1/2FC4 (Figs. 17a, 17g, 17h, and 17i). To one of the input terminals of the NOR circuit 152E is input the *VFO1/2FRA from the DFF 156A. To the other input terminal is input the VFO1F from the DFF 155. The VFO-1⊘ is output as the NOR output of the same (Fig. 22q). Further, one of the input terminals of the NOR circuit 153F receives as input the VFO1/2F from the DFF 151. The other input terminal receives as input the *VFO1/2FA6 (Fig. 17j) from the DFF 156C, explained later. The VFO-11 is output as the NOR output (Fig. 22r). One of the input terminals of the OR circuit 154C receives as input the VFO-1⊘ from the NOR circuit 153E. The other input terminal receives as input the VFO-11 from the NOR circuit 153F. The VFO-12 is output as the OR output (Fig. 22s). The CL terminal of the DFF 156 receives as input the VFO3F. The DF terminal receives as input the VFO-12 from the OR circuit 154C. The CR terminal receives as input the power ready signal. By this, the VFO1/2FA6 is output from the Q terminal of the DFF 156C, and the *VFO1/2FA6 is output from the *Q terminal (Fig. 22f and 17j). Reference numeral 157C is a shift register, which successively shifts the VFO1/2FA6 input from the DFF 156C by the VFO3F and outputs the VFO1/2FB7 and VFO1/2FC3 (Figs. 17j, 17k, and 17l). In this way, from the system clock preparing circuit 15 can be prepared the three reference clocks VFO1/2FRA to VFOFRC, VFO3/2F, *VFO3/2F, and the six cycle clocks VFO1/2FA1 to VFO1/2FC3. Synchronization Circuit Figure 23 shows an example of the synchronization circuit. The LOAD circuit 171 is comprised of a parallel circuit of the three DFFs 173A to 173C. The CL terminals receive as input the LOAD signal from the clock selection circuit 164 and the CR terminal receives as input the clear power ready circuit. The D terminals of the DFF 173A, 173B, and 173C receive as input the parallel code data Z₁, Z₂, and Z₃ from the (1-7) encoding circuit 13. By this, the Z₁, Z₂, and Z₃ are latched at the rising edge of the LOAD signals and the latching outputs FF₁, FF₂, and FF₃ are output to the Q terminals of the DFF 173A, 173B, and 173C. In the ENCODE DATA circuit 172, reference numerals 174A to 174E are AND circuits, 175A and 175B are OR circuits, 176A to 176C are DFFs, and 178 is an inverter. One of the input terminals of the AND circuits 174A, 174C, and 174E receive as inputs the LOAD signal from the clock selection circuit 164. The other terminal receives as input the FF₁, FF₂, and FF₃ from the DFF 173A to 173C. One of the input terminals of the AND circuit 174B and 174D receive as input the *LOAD signal through the inverter 178. The other input terminal of the AND circuit 174B receives as input the output of the Q terminal of the DFF 176B. The other input terminal of the AND circuit 174D receives as input the output of the Q terminal of the DFF. The CL terminals of the DFF 176A to 176C receive as input the ENCOD3 /2F, while the CR terminals receive as input the clear power ready signal. The D terminal of the DFF 176A is supplied with the OR output of the AND circuit 174A and 174B from the OR circuit 175A. The D terminal of the DFF 176B is supplied with the OR output of the AND circuit 174C and 174D from the OR circuit 175B. The D terminal of the DFF 176C is supplied with the output of the AND circuit 174E. By this constitution, when the LOAD signal turns on (1 level), the AND circuits 174B and 174D are shut by the *LOAD signal, and the FF₁, FF₂, and FF₃ are latched to the DFF 173A, 173B, and 173C, respectively, through the AND circuit 174A, 174C, and 174E. When the LOAD signal is off (⊘ level), the AND circuits 174A, 174C, and 174E are closed and, during the serial conversion, the FF₁ to FF₃ of the DFF 173A to 173C are prevented from being input to the DFF 176A to 176C. On the other hand, the AND circuits 174B and 174D turn on and, together with the DFF 176A to 176C, constitute a shift transistor. The FF₁ to FF₃ latched to the DFF 176A to 176C, i.e., the Z₁ to Z₃, are output serially synchronously with the ENCOD3/2F and converted to serial CODA DATA (Fig. 16h, 16q to 16x). As mentioned above, processing is performed to latch the parallel code data from the (1-7) encoding circuit by the LOAD signal and to convert this to the 3-bit unit serial CODE DATA synchronized with the ENCOD3/2F. Explanation of Operation of Third Embodiment The operation of the third embodiment will now be explained with reference to the signal waveform diagrams of Fig. 16 to Fig. 19. The control circuit 11 transmits the WCLK having the fundamental frequency 1f, the WDAT by which data is generated by 1 bit/1f in synchronization with the WCLK, and the WGT generated asynchronously with these two signals which indicates the start of the write operation (Figs. 16f, 16g, 16h). The data shift circuit 12 synchronizes in phase the WDAT input from the control circuit 11 to the WCLK after the generation of the WGT and issues the four pieces of data X₁, X₂, Y₁, and Y₂ of 2-bit units shifted 1 bit each (one clock) (Fig. 16f, 16g, 16h, 16l to 16p). The 1-7 encoding circuit 13 performs the known (1-7) RLL encoding processing based on the X₁, X₂, Y₁, and Y₂ input from the data shift circuit 12 and outputs the 3-bit unit parallel code data Z₁, Z₂, and Z₃. When the WDAT is all "⊘", the "X₁, X₂" and the "Y₁, Y₂" are both "⊘⊘". The "Z₁, Z₂, and Z₃" in that case is encoded to "⊘⊘1" (Fig. 16g, 16l to 16s). On the other hand, the VFO3F generating circuit 14 generates, asynchronously with the WCLK, the clock VFO3F with a frequency three times the fundamental frequency 1f (Figs. 16a and 16f). The system clock preparing circuit 15 receives the VFO3F from the VFO3F generating circuit 14, as explained in the above-mentioned Figs. 21 and 22), generates the three VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC of the frequency (1/2)f synchronized with the VFO3F and shifted by one clock each (Figs. 16a, 16c to 16e and Fig. 17a, 17d to 17f), the clock VFO3/2F formed by dividing the VFO3F and with a frequency of (3/2)f and its inverted clock *VFO3/2F (Fig. 16e and Fig. 17b, however, *VFO3/2F not shown), and the six synchronization clocks VFO1/2FA1, VFO1/2FB⊘, VFO1/2FC4, VFO1/2FA6, VFO1/2FB7, and VFO1/2FC3 (Figs. 17a, 17g to 17l) synchronized with the VFO3F shifted one clock each and with a frequency of (1/2)f and a width of 2 clocks (2 bits). The DFF 161 of the synchronization signal generating circuit 16 receives the WCLK and the WGT from the control circuit 11, generates the SYCGT at the point of rise of the WCLK directly after the generation of the WGT, and inputs the same to the CL terminals of the DFF 162A to 162C (Figs. 16f, 16h, and 16i). The three parallel DFF 162A, 162B, and 162C, when receiving the three reference clocks VFO1/2FRA, VFO1/2FRB, and VFO1/2FRC from the system clock preparing circuit 15 at their D terminals, latch the state of the reference clocks at the point of rise Ts of the SYCGT input from the DFF 161 and input the latched values to the phase selection circuit 163. In the case of Fig. 16, the state of the DFF 162A, 162B, and 162C at the point of rise Ts of the SYCGT is "⊘⊘1". The phase selection circuit 163 generates the signal SELECT (⊘ to 7) for selecting the six synchronization clocks in accordance with the state of the DFF 162A to 162C. In the case of Fig. 2, the state of the DFF 162A to 162C is "⊘⊘1", so the SELECT 1 for selecting the six synchronization clock VF⊘1/2FA1 is output and supplied to the clock selection circuit 164 and the encoding clock phase detection circuit 165. The clock selection circuit 164, when receiving the six synchronization clocks VFO1/2FA1 to VFO1/2FC3 from the system clock preparing circuit 15, selects the VFO1/2FA1 in accordance with the SELECT 1 as the LOAD signal and inputs it into the LOAD circuit 17. On the other hand, the encoding clock phase detection circuit 165, when receiving the VFO3/2F and *VFO3/2F from the system clock preparing circuit 15, selects the *VFO3/2F as the ENCOD3/2F in accordance with the SELECT 1 input from the phase selection circuit 163 and supplies it to the ENCODE DATA circuit 172. In this way, no matter at what phase of the VFO3F the point of rise of the SYCGT occurs, the LOAD signal and ENCOD3/2F are unilaterally prepared, as explained in reference to Table 4, Table 5, and Figs. 18 and 19. Further, the parallel code data Z₁ to Z₃ are fetched from the (1-7) encoding circuit 13 by the LOAD circuit 171 receiving the LOAD signal. Using the ENCOD DATA circuit 174 receiving the ENCOD3/2F and LOAD signal, the parallel code data Z₁ to Z₃ is converted to a serial format to prepare the CODE DATA, as mentioned above. Above, an explanation was made of an embodiment of the present invention, but the present invention is not limited to the constitution of this embodiment and various modifications are possible. Effects of Third Embodiment As explained above, according to the third embodiment of the present invention, the following effects can be obtained: (1) By fetching the WDAT direct into the (1-7) encoding circuit in 2-bit units, it is possible to fetch data without use of a REF1F with a 5⊘ percent duty ratio. (2) Encode data can be synchronized with the ENCOD3/2F by the LOAD signal and ENCOD3/2F prepared using the three reference clocks and the six synchronization clocks, which can be easily and stably prepared from the VFO3F. (3) By the above (1) and (2), write data can be fetched without error by a circuit construction simpler than the past and it is possible to realize a highly reliable write data fetching circuit. Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in this specification except as defined in the appended claims.

Claims: A circuit (Fig. 5) for fetching (1-7) run-length-limited (RLL) coded write data upon receiving a write clock signal (WCLK) having a fundamental frequency (1f), and write data (WDAT), comprising:- an n-times frequency clock signal generating circuit (12) arranged to generate a clock signal of a frequency which is n-times the fundamental frequency; a system clock signal generating circuit (15) arranged to receive the n-times frequency clock signal and to generate n clock signals respectively differing in phase by 360/n°; a synchronization and fetching circuit (13) arranged to perform synchronization of the write data in dependence on one of the n clock signals; and an RLL encoding circuit arranged to perform RLL encoding on the synchronized write data; characterized in that:- said n-times frequency clock signal generating circuit (12) is a triple frequency clock signal generating circuit arranged to generate a triple frequency clock signal (VFO3F) having a frequency (3f) three times the fundamental frequency (1f) based on a signal from a servo head (221) of a disk apparatus; said system clock signal generating circuit (15) is arranged to generate from the triple frequency clock signal (VFO3F), first to third clock signals (VFO1FA, VFO1FB, and VFO1FC) differing from each other by 120° in phase, and a frequency divided signal (VFO3/2F) formed by dividing in half the triple frequency clock signal (VFO3F); a fetching clock generating circuit (16) is provided, which is arranged to receive a write gate signal (WGT) indicating the start of a write operation, the write clock signal (WCLK), and the first to third clock signals (VFO1FA, VFO1FB, and VFO1FC) from the system clock generating circuit (15), and to generate a fetching clock signal (FCLK) synchronized with at least one of the first to third clock signals (VFO1FA, VFO1FB, and VFO1FC) depending on the states thereof at a transition of the write clock signal (WCLK) directly after the generation of the write gate signal (WGT); said synchronization and fetching circuit (13) is arranged to perform a first synchronization and fetching operation on the write data (WDAT) using the write clock signal (WCLK) to produce first synchronized write data, and to perform a second synchronization and fetching operation on the first synchronized write data using the fetching clock signal (FCLK), to produce second synchronized write data; and said RLL encoding circuit is a (1-7) RLL encoding circuit arranged to perform (1-7) RLL encoding on the second synchronized write data based on the frequency divided signal (VFO3/2F) from the system clock generating circuit (15). A circuit according to claim 1, wherein the triple frequency clock signal generating circuit (12) has a phase synchronization circuit (123) arranged to synchronize the phase of the signal from the servo head and to output the triple frequency clock signal (VFO3F). A circuit according to claim 2, wherein the system clock circuit (15) is comprised of:- first to fifth NOR gates (152A, 152B, 153A, 153B, 153C), first and second OR gates (154A, 154B), first to third D-type flip-flops (151, 155, 156), and a shift register (157); wherein the triple frequency clock signal (VFO3F) is connected to the clock terminal of the first flip-flop (151), the negative output terminal of the first flip-flop (151) is connected to the delayed input terminal of the first flip-flop (151), and the frequency divided signal (VFO3/2F) is output from the positive output terminal of the first flip-flop (151); the triple frequency clock signal (VFO3F) is connected to the clock terminals of the second and third flip-flops (155, 156) and to the shift register (157), the inputs of the first NOR gate (152A) are connected to the negative output terminal of the first flip-flop (151) and the positive output terminal of the third flip-flop (156), the inputs of the second NOR gate (152B) are connected to the positive terminal of the first flip-flop (151) and the negative terminal of the third flip-flop (156), the inputs of the third NOR gate (153A) are connected to the output terminals of the first and second NOR gates and the negative output terminal of the second flip-flop (155), the inputs of the fourth NOR gate (153B) are connected to the negative output terminals of the second and third flip-flops (155, 156), the inputs of the fifth NOR gate (153C) are connected to the positive output terminals of the second and third flip-flops (155, 156), the inputs of the first OR gate (154A) are connected to the output terminal of the third NOR gate and the negative output terminal of the second flip-flop (155), the inputs of the second OR gate (154B) are connected to the output terminals of the fourth and fifth NOR gates (153B, 153C), the output of the first OR gate (154A) is connected to the delayed input terminal of the second flip-flop (155), the output of the second OR gate (154B) is connected to the delayed input terminal of the third flip-flop (156), the first clock signal (VFO1FA) is output from the positive output terminal of the second flip-flop (155) and is also applied to the shift register (157), whereby said shift register is operable to generate the second and third clock signals (VFO1FB, VFO1FC). A circuit according to claim 2, wherein, when the first to third clock signals (VFO1FA, VFO1FB, and VFO1FC) are in the output level states listed in the following table, the fetching clock generating circuit (16) is arranged to generate the fetching clock signal (FCLK) based on one of the first to third clock signals as follows:- VFO1FA VFO1FB VFO1FC FCLK 0 0 0 VFO1FA 0 0 1 VFO1FB 0 1 0 VFO1FB 0 1 1 VFO1FB 1 0 0 VFO1FC 1 0 1 VFO1FC 1 1 0 VFO1FA 1 1 1 VFO1FC A circuit according to claim 4, wherein the synchronization and fetching circuit (13) has:- a fourth D-type flip-flop (131) whose clock terminal is connected to receive the write clock signal (WCLK), whose delayed input terminal is connected to receive the write data (WDAT), and arranged to latch the write data in accordance with the write clock signal; and a fifth D-type flip-flop (132) whose delayed input terminal is connected to the positive output terminal of the fourth D-type flip-flop (131), whose clock terminal is connected to receive the fetching clock signal (FCLK), and arranged to latch the output from the fourth D-type flip-flop (131) in accordance with the fetching clock (FCLK). A circuit according to claim 1, wherein the system clock generating circuit (15a) is arranged to generate a delayed clock signal (DLVFO1FX) formed by delaying one of the first to third clock signals (VFO1FA, VFO1FB, and VFO1FC) by within 1/6 of the period corresponding to the fundamental frequency (1f). A circuit according to claim 6, wherein the system clock circuit (15a) comprises:- first to fifth NOR gates (152A, 152B, 153A, 153B, 153C), first and second OR gates (154A, 154B), first to third D-type flip-flops (151, 155, 156), a shift register (157), and a sixth D-type flip-flop (158); wherein the triple frequency clock signal (VFO3F) is connected to the clock terminal of the first flip-flop (151), the negative output terminal of the first flip-flop is connected to the delayed input terminal of the first flip-flop, and wherein in use the first frequency divided signal (VFO3/2F) is output from the positive output terminal of the first flip-flop; the triple frequency clock signal (VFO3F) is connected to the clock terminals of the second and third flip-flops (155, 156) and to the shift register (157), the inputs of the first NOR gate (152A) are connected to the negative output terminal of the first flip-flop (151) and the positive output terminal of the third flip-flop (156), the inputs of the second NOR gate (152B) are connected to the positive output terminal of the first flip-flop (151) and the negative output terminal of the third flip-flop (156), the inputs of the third NOR gate (153A) are connected to the output terminals of the first and second NOR gates (152A, 152B) and the negative output terminal of the second flip-flop (155), the inputs of the fourth NOR gate (153B) are connected to the negative output terminals of the second and third flip-flops (155, 156), the inputs of the fifth NOR gate (153C) are connected to the positive output terminals of the second and third flip-flops (155, 156), the inputs of the first OR gate (154A) are connected to the output terminal of the third NOR gate (153A) and the negative output terminal of the second flip-flop (155), the inputs of the second OR gate (154B) are connected to the output terminals of the fourth and fifth NOR gates (153B, 153C), the output of the first OR gate (154A) is connected to the delayed input terminal of the second flip-flop (155), and the output of the second OR gate (154B) is connected to the delayed input terminal of the third flip-flop (156), whereby in use the first clock signal (VFO1FA) is output from the positive output terminal of the second flip-flop (155) and is also applied to the shift register (157), and the second and third clock signals (VFO1FB, VFO1FC) are output from the shift register (157), one of the first to third clock signals (VFO1FA, VFO1FB, and VFO1FC) being applied to the delayed input terminal of the sixth flip-flop (158), and an inverted signal of the triple frequency clock signal (VFO3F) being applied to the clock terminal of the sixth flip-flop (158), whereby said delayed clock signal (DLVFO1FX) is output from the positive output terminal of the sixth flip-flop (158). A circuit (Fig. 15) for fetching (1-7) run-length-limited (RLL) coded write data upon receiving a write clock signal (WCLK) having a fundamental frequency (1f) and write data (WDAT), comprising:- an n-times frequency clock signal generating circuit (14) arranged to generate a clock signal of a frequency which is n-times the fundamental frequency (1f); a system clock signal generating circuit (15) arranged to receive the n-times clock signal and to generate n clock signals respectively differing in phase by 360/n°; a synchronization circuit (17) arranged to perform synchronization of the write data in dependence on one of the n clock signals; and an RLL encoding circuit (13) arranged to perform RLL encoding on the write data; characterized in that:- said n-times frequency clock signal generating circuit (14) is a triple frequency clock signal generating circuit arranged to generate a triple frequency clock signal (VFO3F) having a frequency (3f) three times a fundamental frequency (1f) of the write clock signal (WCLK) based on a signal from a servo head (221) of a disk apparatus; said system clock signal generating circuit (15) is arranged to generate from the triple frequency clock signal (VFO3F), first to third reference clock signals (VFO1/2FRA, VFO1/2FRB, VFO1/2FRC) each having a frequency (f/2) half that of the fundamental frequency of the write clock signal (WCLK) and respectively successively shifted by one period of the triple frequency clock signal (VFO3F), a first frequency divided clock signal (VFO3/2F) formed by dividing in half the triple frequency clock signal (VFO3F) and a second frequency divided clock signal (*VFO3/2F) formed by inverting the first frequency divided clock signal, and six synchronization clock signals (VFO1/2FA1, VFO1/2FB0, VFO1/2FC4, VFO1/3FA6, VFO1/2FB7, and VFO1/2FC3) each having a frequency half the fundamental frequency, having a 2-bit pulse width, and respectively successively shifted by one period of the triple frequency clock signal (VFO3F); a synchronization signal generating circuit (16) is provided, which is arranged to receive a write gate signal (WGT) indicating the start of a write operation, the first to third reference clock signals (VFO1/2FRA, VFO1/2FRB, VFO1/2FRC) from the system clock signal generating circuit (15), and the six synchronization clock signals, and to generate a load signal (LOAD) having a frequency (f/2) half the fundamental frequency and synchronized with at least one of the six synchronization clock signals in accordance with the state of the first to third reference clock signals at the time of change of the write clock signal (WCLK) directly after the generation of the write gate signal (WGT), and further arranged to generate an encoding clock signal (ENCODE3/2F) having a frequency 1.5 times the fundamental frequency and synchronized with one of the first or second frequency divided clock signals (VFO3/2F, *VFO3/2F) in accordance with the state of the first to third reference clock signals; a data shift circuit (12) is provided, and is arranged to receive the write data (WDAT) and the write clock signal (WCLK), to synchronize the write data to the write clock in phase, and to output four bits of data (X₁, X₂, Y₁, Y₂) in two 2-bit units each shifted by one bit; said RLL encoding circuit is a (1-7) RLL encoding circuit arranged to receive the four bits of data from the data shift circuit (12) and to perform (1-7) RLL encoding on them to produce parallel code data (Z₁, Z₂, and Z₃); and said synchronization circuit (17) is arranged to receive the load signal (LOAD) and the encoding clock signal (ENCODE3/2F) from the synchronization signal generating circuit (16) and to convert to a serial format the parallel code data (Z₁, Z₂, and Z₃) from the (1-7) RLL encoding circuit to a 3-bit unit of code data (CODE DATA) synchronized with the encoding clock signal.

Inventors: MASE HIROYUKI, NAGAKURA FUMIAKI, NISHIMURA MASAHIRO, MASE, HIROYUKI, NAGAKURA, FUMIAKI, NISHIMURA, MASAHIRO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1988
Main classification: H03M5
IPC codes: H03M5