Clock signal generator having improved switching characteristics

An optical disk recorder has a clock signal generator which switches between a reproducing clock signal and a recording clock signal. The optical disk recorder records data from the end point of the recorded data after interruption of recording due to buffer under-run phenomenon. In this recording, the optical disk recorder reads out a read clock signal from the recorded data, detects the end point of the recorded data and then starts for recording from the end point based on a recording clock signal, Switching between the reproducing clock signal to the recording clock signal allows a linear change in the output frequency to thereby improve the continuity of the recorded data in the vicinity of the end point.

PREFERRED EMBODIMENTS OF THE INVENTION Now, the present invention is more specifically described with reference to accompanying drawings, wherein similar constituent elements are designated by similar reference numerals. Referring to FIG. 2 , there is shown an optical disk recorder including a clock signal generator according to a first embodiment of the present invention. The optical disk recorder includes a spindle motor 11 , an optical head 12 , a radio-frequency (RF) amplifier 13 , a servo control unit 14 , a reproducing unit 15 , a laser drive unit 16 , a recording unit 17 , a CPU interface 18 , and a recording/reproducing controller 20 . A personal computer 19 controls the optical disk recorder to perform recording/reproducing of data on an optical disk, or a CD-R, driven by the spindle motor 11 . The spindle motor 11 controls rotation of the optical disk based on the control signal supplied from the servo control unit 14 . Based on the control signal supplied from the servo control unit 14 , the optical head 12 controls irradiation by the laser, records desired data on the optical disk or supplies the read data from the optical disk to the RF amplifier 13 . The RF amplifier 13 amplifies the read data constituting a RF signal, and delivers the resultant signal to the servo control unit 14 and the reproducing unit 15 . The servo control unit 14 controls, based on read data from the RF amplifier 13 and the control signal from the recording/reproducing controller, rotation of the spindle motor 11 , focusing of the laser beam onto the signal track of the optical disk, tracking of the laser beam along the signal track of the optical disk, and sledding of the optical head 12 to move the optical head in the radial direction of the optical disk. The reproducing unit 15 performs EFM (eight to fourteen modulation) demodulation, CIRC (cross interleaved Reed-Solomon Code) decoding processing and error correction, and delivers the read data to the recording/reproducing controller 20 . The CPU interface 18 is connected to a personal computer 19 to transfer/receive data, instruction and response thereto. The recording unit 17 receives write data from the personal computer 19 via the CPU interface 18 , operates for CIRC encoding, addition of a sub-code, addition of error correcting codes, EFM modulation, etc. onto the received write data, and supplies the resultant write data to the laser drive unit 16 . The laser drive unit 16 performs drive control of the laser source in the optical head 12 based on the write data supplied from the recording unit 17 . The recording/reproducing controller 20 includes the clock signal generator 10 of the present embodiment. The recording/reproducing controller 20 controls recording processing and reproduction processing based on the instruction from the personal computer 19 and the read data from the reproducing unit 15 . The clock signal generator 10 generates a reproducing clock signal for use in reproduction of recorded data, and a recording clock signal used for use in recording of write data onto the optical disk. Referring to FIG. 3 , the clock signal generator 10 includes first and second phase comparators 21 and 22 , a selector 23 , a low-pass-filter (LPF) 24 , and a voltage controlled oscillator (VCO) 25 . The first phase comparator 21 compares the phase of the read data pulse or (read clock signal) 101 against the phase of the output clock signal 107 to generates a first difference signal representing the difference between the phases of both the clock signals 101 and 107 , and delivers the first difference signal 103 to the selector 23 . The second phase comparator 22 compares the phase of the reference clock signal 102 against the phase of the output clock signal 107 , to generate a second difference signal 104 representing the difference between the phases of both the clock signals 102 and 107 , and delivers the second difference signal 104 to the selector 23 . The selector 23 selects the first difference signal 103 during a reproducing operation, selects the second phase difference signal 104 during a recording operation, and delivers the selected difference signal to the LPF 24 . The LPF 24 smoothes the selected difference signal to output a voltage signal 106 to the VCO 25 . The voltage signal 106 has a potential which changes at a fixed time constant Tx defined by the LPF 24 . The VCO 25 oscillates based on the voltage signal 106 to generate the output clock signal 107 . Referring to FIG. 4 , the frequency of the output clock signal 107 falls from f 1 to f 2 from a reproducing operation to a recording operation in the optical disk recorder. More specifically, the selector 23 selects the first difference signal 103 before time instant t 1 , whereby the voltage signal 106 is maintained at a first potential based on the first difference signal 103 . The VCO 25 oscillates based on the voltage signal 106 to deliver the output clock signal 107 having a stable frequency f 1 . At time instant t 1 , the selector 23 selects the second difference signal 104 , whereby the selected difference signal 105 abruptly changes the waveform thereof with respect to the pulse width and the period thereof. Between the time instants t 1 and t 2 , the selector 23 selects the second difference signal 104 , whereby the voltage signal 106 linearly falls from the first potential to a second potential. The frequency of the output clock signal 107 from the VCO 25 linearly falls from the frequency f 1 to a frequency f 2 in proportion to the voltage signal 106 . After the time instant t 2 , the selector continues to select the second difference signal 104 , whereby the voltage signal 106 is maintained at the second potential based on the selected difference signal. Thus, the VCO 25 oscillates to output a clock signal 107 having a stable frequency f 2 . The time interval between t 1 and t 2 is determined by the time constant Tx of the LPF 24 . The time constant Tx of the LPF 24 is set a desired value for assuring the specified range of deviation allowed for recording data on the CD-R after the interruption caused by the buffer under-run phenomenon. The read clock signal 101 is obtained by reading the data recorded on the CD-R prior to the interruption. The reference clock signal 102 is obtained by multiplying an output frequency from a crystal oscillator, if the rotation of the spindle motor 11 is to be controlled at a constant linear velocity of the CD-R. On the other hand, if the spindle motor is to be controlled at a constant angular velocity of the CD-R, the reference clock 102 signal is obtained by extracting wobble signal component having a frequency of 22.05 kHz from a pre-groove signal which is output from the RF amplifier 13 , and generating a clock signal in synchrony with the wobble signal component. The frequency f 2 of the reference clock signal 102 may be higher than the frequency f 1 in the clock signal generator 10 . Now operation of the optical disk recorder for additionally recording data on a CD-R after an interruption is described. It is assumed that data is recorded on the CD-R up to the end point before interruption of recording due to the buffer under-run phenomenon. When the optical disk recorder receives an instruction for recording from the personal computer 19 , the recording/reproducing controller 20 starts for processing of additional recording. The servo control unit 14 operates for focusing control and tracking control of the optical head 12 , and for rotational control of the spindle motor 11 . The recording unit 17 receives write data from the personal computer 19 and operates for processing of the received write data. The optical disk recorder judges whether the focusing, tracking and rotational controls operate in normal conditions and whether the recording unit 17 is ready for recording, before the optical disk starts for reading the recorded data to detect the starting position for recording. The selector 23 selects the first difference signal 103 , whereby the VCO 25 generates a reproducing clock signal in synchrony with the read clock signal 101 . The optical disk recorder reproduces the data recorded on the optical disk before the interruption based on the reproducing clock signal, thereby detecting the end point of the recorded data as the starting position for the recording. Upon detection of the starting point, the selector 23 selects the second difference signal 104 , whereby the VCO 25 generates a recording clock signal in synchrony with the reference clock signal 102 . The recording unit 17 starts for recording operation based on the recording clock signal. The optical disk recorder thus records the write data on the optical disk via the laser drive unit 16 and the recording unit 17 from the starting point. During switching from the reproducing operation to the recording operation, the selected phase difference signal 105 from the selector 23 changes abruptly in the pulse waveform thereof. However, the time constant Tx of the LPF 24 suppresses the abrupt change in the potential of the voltage signal 106 , which changes linearly. Thus, the output frequency from the VCO 25 changes linearly, as illustrated in FIG. 4 , before the output clock signal synchronizes with the reference clock signal 102 . In reproduction of the data recorded by the above optical disk recorder, the optical disk reader receives a read clock signal having an excellent continuity between the vicinity of the end point and the vicinity of the starting point. Referring to FIG. 5, a clock signal generator according to a second embodiment of the present invention is similar to clock signal generator of the first embodiment, except that a pair of charge pumps 31 and 32 are provided in the present embodiment between the phase comparators 21 and 22 and the selector 23 instead of the LPF 24 shown in FIG. 3 , and each phase comparator 21 or 22 generates a pair of phase difference signals 108 and 109 or 110 and 111 . More specifically, each charge pump 31 or 32 includes a p-ch transistor Q 1 and an n-ch transistor Q 2 connected in series between the VCC source line and the ground. The transistor Q 1 charges the output line 113 or 114 , i.e., capacitor C 1 , through the selector 23 , whereas the transistor Q 2 discharges the output line 113 or 114 , i.e., capacitor C 1 , through the selector 23 . The relationship between the phase difference signals 108 and 109 or 110 and 111 from each phase comparator 31 or 32 and the rise/fall of the output signal line 113 or 114 of the each charge pump 31 or 32 is shown in Table 1. 1 TABLE 1 108, 110(Q1) L H H L 109, 111(Q2) L H L H 113, 114 Rise Fall Hold Prohibited Each phase comparator 21 or 22 judges whether the output clock signal 107 advances or lags with respect to the read clock signal 101 or reference clock signal 102 , and delivers a pair of phase difference signals 108 and 109 or 110 and 111 having H- or L-levels depending on the results of the comparison. The first charge pump 31 connected to the first phase comparator 21 has a higher current driveability compared to the second charge pump 32 connected to the second phase comparator 22 . By this configuration of the clock signal generator, the reproducing clock signal locks with the read clock signal 101 at a higher rate compared to the locking rate of the recording clock signal with respect to the reference clock signal 102 . This allows a higher shift rate in the shift of the optical disk recorder from a recording operation to a reproducing operation compared to the shift from a read operation to a recording operation, whereby continuity of the recorded data is further improved. According to the present embodiment, the charge pumps provided instead of the LPF suppress the ripple components on the voltage signal supplied to the VCO, thereby widening the frequency range of the input clock signal to the clock signal generator which can be compared against the output clock signal from the clock signal generator. Since the above embodiments are described only for examples, the present invention is not limited to the above embodiments and various modifications or alterations can be easily made therefrom by those skilled in the art without departing from the scope of the present invention.