Abstract:
A beam is irradiated to an information recording medium having land tracks and groove tracks alternately formed on a track formed in turn on an information recording surface thereof and pre-pits so pre-formatted as to deviate from the center of the track, a return beam of the irradiated beam is received by detectors split into two parts in a track direction to obtain a differential signal, the pre-pit is detected from this differential signal and control of recording and reproducing processings is executed by using the pre-pit detection signal so obtained as the reference.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to information recording/reproducing technologies capable of optically recording and reproducing information to and from an information recording medium (optical disk) from which the information can be read optically. More specifically, this invention relates to a method suitable for an optical disk so formatted as to correspond to high density information recording, and to an information recording/reproducing apparatus achieving the functions of this method. 
     In optical disks in general, the information is reproduced by condensing a laser beam to an information recording surface of the optical disk and detecting those reflected beams which are modulated by recording marks and pits. The recording marks are formed along a guide groove on the spiral formed on the information recording surface. As described in JP-B-4-4661, the width of a groove and the width of a portion between the grooves are arranged to be substantially equal and the depth of the groove is set to the depth at which cross-talk from adjacent tracks determined by the wavelength of the laser beam to be irradiated becomes minimal. In consequence, the recording marks can be formed similarly on the guide grooves (groove tracks) and on the portions between the grooves (land tracks), reproduction of the information can be made, and the recording density in the radial direction of the disk can be improved. 
     When the groove is formed simply and continuously from the inner circumference to the outer circumference and when the reproducing operation shifts from the continuous reproduction on the groove track to the continuous reproduction of the land track, the time involved with this shift is necessary in order to continuously reproduce the information recorded on the groove track and on the land track on the optical disk, and continuous reproduction of the information on the land and groove tracks becomes impossible. 
     Therefore, as shown in JP-A-7-141701, if a groove (single spiral) is formed in such a fashion that the land track and the groove track appear alternately and substantially in each circumference, the information recorded on the land track and on the groove track can be reproduced continuously. 
     To reproduce the recording marks formed on the land track or the groove track, the condensing position of the laser beam on the information recording surface must be scanned along the substantial center of the land track or the groove track in the radial direction of the optical disk. The condensing position of the laser beam at this time can be identified from deviation (tracking deviation amount) which is substantially proportional to the distance from the substantial center position of the land track or the groove track by using diffraction components of the laser beam by the groove. The condensing position of the laser beam is controlled (tracking control) by using this deviation amount but the polarity of the deviation reverses between the land track and the groove track. In order to carry out continuous reproduction from the land track to the groove track or vice versa, therefore, the condensing position of the laser beam must be controlled in such a fashion as to correspond to the reversion of the polarity of the deviation amount at the alternating position of the land track and the groove track. 
     Therefore, regions are set in the radial direction substantially equidistantly in the circumferential direction on the track of the optical disk and pits are formed inside these regions, as described in JP-A-6-176404. In this instance, the pits are formed so that when the information of the pits is reproduced, the alternating position (polarity reversing position) between the land track and the groove track can be identified, and the tracking control state is changed over on the basis of this information. 
     The pits are formed in advance (or “pre-formatted”) substantially equidistantly in the circumferential direction of the track. When these pre-pits are so formed as to alternately offset mutually at positions spaced apart by a quarter of the distance of the center position of each groove track adjacent to the land track in both inner and outer circumferential directions from the center position of the land track or the groove track, the signal that reproduces the bit information on the deviation signal representing the tracking deviation amount can be outputted. 
     In other words, it is possible to identify the formation position of the pre-pit (PID: Physical Identification portion) by detecting the position where the tracking deviation amount alternates to the positive and the negative in the cycle of the pit information and also to judge which of the land track and the groove track the optical spot scans at present from the positive and negative sequence of the signal output corresponding to the pits. 
     FIG. 2 is a schematic block diagram of an optical disk apparatus including a PID position detector. An optical disk medium  1  is rotated by a spindle motor  2  substantially at a constant linear velocity. An optical head  3  includes optical components for condensing for a laser beam on an information recording surface of the optical disk, a detector for detecting a reflected beam, a PD track head for making the condensing position variable, and so forth. 
     Automatic gain controllers  1  (AGCS)  11  and  12  control an RF (Radio Frequency) signal as the level change of the reflected beam from the optical head and an HPP (High-Frequency Push-Pull) signal corresponding to the tracking deviation amount to constant amplitude levels. The amplitude of the HPP signal changes with variance of the groove depth of the optical disk, with variance of the reflection factor of the optical disk, with variance of the laser beam quantity irradiated to the optical disk and with sensitivity variance of a detector which receives the reflected beam from the optical disk and converts it to an electric signal. Further, the waveform amplitudes vary between the land track and the groove track when the laser beam is so condensed at the PID portion as to deviate from the center position of the track or when the laser beam turns to an elliptic spot at the condensing position while deviating from the vertical axis of the optical disk. 
     For this reason, PID position detection cannot be carried out stably at the slice level of the comparator described above as a fixed level against the variation of the HPP signal amplitude, and PID detection is effected after the amplitude is controlled to a predetermined amplitude by the automatic gain controllers (AGCs). Because the signal amplitude of the RF signal changes with the recording condition, the amplitude is rendered constant by the AGC so that digitization can be carried out stably. 
     A switch  13  switches the PID portion and the data track on the optical disk by a signal switch signal  32  from the PID position detector, selects at the PID portion the output of the AGC  13  that has processed the HPP signal and selects on the data track the output of the AGC  12  that has processed the RF signal. The output of this switch  13  is inputted to a DC corrector  14  to render a DC level of each signal substantially constant. For example, envelopes at the upper and lower portions of each signal are detected in a reference signal region at the leading part of each signal and control is made so that the mean level of the upper and lower envelopes attains an arbitrary level. 
     The output of the DC corrector  14  is inputted to an automatic slice level controller (ASC)  15  and is digitized at a suitable slice level. The output of this ASC  15  is inputted to a phase-locked loop (PLL)  16  and a reproduction clock synchronized with the digitized signal is generated. The reproduction clock is inputted with the digitized signal to a reproduction signal demodulator  17  so that recording information is demodulated from the RF signal of the data track while the position information of the optical disk is demodulated from the HPP signal of the PID portion. When this HPP signal is demodulated, the information about alternation of the land track and the groove track can be obtained, too. 
     The pit information of the PID position contains the information about the position on the optical disk and the information as the reference for the reproduction processing, and the position information is formed in succession to the reference information. Generally, the signal frequency of the reference information is constant and the reproduction processing system executes control by using this reference information, so that the position information can be stably reproduced. Incidentally, the RF signal, too, contains the reference information at the leading part thereof in the same way as the PID portion, and the recording information is formed in succession to this reference information. 
     SUMMARY OF THE INVENTION 
     The PID position detector for distinguishing the PID portion from the recording region acquires the envelope waveforms of the HPP signal waveform by using a peak detector and a bottom detector from the output waveform (HPP signal) corresponding to the tracking deviation amount and the comparator compares the peak slice level with the bottom slice level to detect the PID portion. The PID portion can be detected also by suppressing the high frequency components of the HPP signal corresponding to the pit information by a high-pass filter as the waveforms inputted to the comparator. 
     No problems develop, in particular, as long as the PID position controller executes the control so that the laser beam is appropriately condensed to the center position of the track (tracking control), but when the tracking control is not effected at the time of seek from the track which is reproduced at present to another or when the tracking control gets unstable due to noises, for example, the tracking deviation amount fluctuates and the HPP signal cannot be distinguished from the signal fluctuation by the pit at the PID portion and consequently, the PID portion is erroneously detected from time to time. 
     It is therefore an object of the present invention to provide a method of controlling a signal processing system which can detect the PID portion in a satisfactory manner by a simple processing system when the tracking control is not made or when it gets unstable, and an information recording/reproducing apparatus using such a method. 
     To accomplish the object described above, a method of controlling a signal processing system according to this invention comprises the steps of irradiating a laser beam to an information recording medium having a construction in which land tracks and groove tracks are alternately formed on tracks formed on an information recording surface, and pre-pits to be pre-formatted are so formed as to deviate from the track center, receiving the return beam of the irradiated beam by detectors split into two parts in the track direction to obtain differential signals, detecting the pre-pit portions from the differential signals so obtained, and controlling recording and reproducing processings on the basis of the detected pre-pit detection signal as the reference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a reproduction processing system of an information recording/reproducing apparatus according to the present invention; 
     FIG. 2 is a block diagram of a reproduction processing system of an information recording/reproducing apparatus according to the prior art; 
     FIG. 3 is a block diagram of an example of a PID position detector according to the present invention; 
     FIG. 4 is a signal waveform diagram of each portion of the example of the PID position detector according to the present invention shown in FIG. 3; 
     FIG. 5 is a block diagram of another example of the PID position detector according to the present invention; 
     FIG. 6 is a signal waveform diagram at each portion of the example of the PID position detector according to the present invention shown in FIG. 5; 
     FIG. 7 is a block diagram of still another example of the PID position detector according to the present invention; 
     FIG. 8 is a signal waveform diagram of each portion of the example of the PID position detector according to the present invention shown in FIG. 7; 
     FIG. 9 is a block diagram of still another example of the PID position detector according to the present invention; 
     FIG. 10 is a signal waveform diagram of each portion of the example of the PID position detector shown in FIG. 9; 
     FIG. 11 is a block diagram of an example of a drive controller according to the present invention; 
     FIG. 12 is a block diagram of an example of a signal width protector according to the present invention; 
     FIG. 13 is a block diagram of an example of a signal cycle measurer according to the present invention; 
     FIG. 14 is a block diagram of an example of a signal cycle protector according to the present invention; 
     FIG. 15 is a block diagram of another example of the drive controller according to the present invention; and 
     FIG. 16 is a timing chart of another example the driving controller according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic block diagram of an embodiment of the present invention, wherein the explanation of the same portion as the one shown in FIG. 2 is omitted. The afore-mentioned RF signal from the optical head and the HPP signal are controlled to the predetermined amplitude levels by the automatic gain controllers (AGCs)  11  and  12  described already, respectively. The drive controller  18  improves response of control at the leading part of each signal by the AGC signal  31  in such a manner as to rapidly attain a predetermined amplitude and to thereafter lower the response so as not to follow a quick amplitude change but to execute stable control. 
     The switch  13  described already changes over the PID track and the data track on the optical disk by the signal switch signal  32  from the drive controller  18 , selects on the PID track the output of the AGC  12  obtained by processing the HPP signal and selects on the data track the output of the AGC  11  obtained by processing the RF signal. 
     The output of the switch  13  is inputted to the DC corrector  14  described above, which makes the substantial DC level of each signal constant. This operation is executed under the control of the DC correction control signal  33  from the drive controller  18  which turns on or off the DC correction operation. 
     The output of the DC corrector  14  is inputted to the afore-mentioned automatic slice level controller (ASC)  15  and is digitized at a suitable slice level. This ASC  15  is executed under control of the ASC control signal  34  from the drive controller  18  which turns on or off the ASC operation. 
     The output of the ASC  15  is inputted to the PLL  16  and a reproduction clock synchronized with the digitized signal is generated. This reproduction clock is inputted with the digitized signal to the reproduction signal modulator  17  so that the recording information is demodulated from the RF signal of the data track while the position information of the optical disk is demodulated from the HPP signal of the PID track. When this HPP signal is demodulated, the information about alternation of the land track and the groove track can be acquired, too. 
     The PLL  16  is so controlled by the PLL control signal  35  from the drive controller  18  as to switch the signal to be synchronized to the reproduction signal or to the reference signal. The drive controller  18  obtains the PID information signal  36  from the reproduction demodulator  17 . 
     When the PID information signal  36  from the reproduction demodulator  17  is detected, the drive controller  18  generates various drive control signals  31 ,  32 ,  33 ,  34  and  35  with the PID information as the reference. When the area on the optical disk at which a desired information is recorded is sought or when the PID information cannot be obtained stably and continuously at the start of the operation of the optical disk apparatus, the drive controller  18  generates various drive control signals  31 ,  32 ,  33 ,  34  and  35  on the basis of the PID detection signals IP1 and IP2 from the PID position detector  19 . These IP1 and IP2 are the detection signals that correspond to the land track and the groove track, respectively. 
     Incidentally, when the PID information signal  36  is detected, too, various control signals  31 ,  32 ,  33 ,  34  and  35  may be generated on the basis of the PID detection signals IP1 and IP2 of the present invention. 
     These various drive control signals  31 ,  32 ,  33 ,  34  and  35  make it possible to stably reproduce the information of the HPP signal and the RF signal by controlling each signal processing system for a predetermined period from the information (VFO) area as the reference of the leading part of each of the HPP signal and the RF signal. 
     FIG. 3 shows an example of the PID position detector  19  according to the present invention. FIG. 4 shows the signal waveform at each portion in this example. Positive and negative pit information appear in the HPP signal. The waveform also changes to positive and negative at the portion where a tracking deviation amount is great. The HPP signal is inputted to the peak detector  41  and to the bottom detector  42  to obtain a peak detection waveform  51  and a bottom detection waveform  52 , respectively. These waveforms are compared with the slice levels V 1  and V 2  by comparators  44  and  45 , respectively, to obtain a positive portion detection signal  53  and a negative portion detection signal  54 . In this case, the positive portion detection signal  53  and the negative portion detection signal  54  occur at the PID portion to attain “1”. At the portion having the large deviation amount, too, these positive portion detection signal  53  and negative portion detection signal  54  occur to attain “1”. 
     Therefore, a signal detection signal  55  representative of the existence of the RF (Radio Frequency) component such as the pit information is obtained by the signal level detector  43 . The signal detection signal  55  attains “1” at portions at which the signal having the RF component exists, such as at the pit portion, and attains “0” at portions where the signal having the RF signal does not exist. For this reason, the signal detection signal  55  turns to “0” at the portions where the tracking deviation amount is great. Assuming that the logical ORs carried out between this signal detection signal  55  and the positive portion detection signal  53  and between the signal detection signal  55  and the negative portion detection signal  54  are IP1 and IP2, IP1 and IP2 become the signals having “1” at only the pit portions of the PID portion. The signal level detector  43  detects the existence/absence of the signal component from the change of the envelope waveform by extracting the RF component, for example. 
     FIG. 5 shows another example of the PID position detector  19  according to the present invention, and FIG. 6 shows the signal waveform at each part in this example. The HPP signal is inputted to a low-pass filter (LPF)  48  to obtain a signal component  56  as a low frequency component signal of the HPP signal, and is compared with a slice level V 3 , V 4  by each comparator  44 ,  45  to obtain each of the positive portion detection signal  53  and the negative portion detection signal  54 . In this case, the positive portion detection signal  53  and the negative portion detection signal  54  occur at the IPD portion and attain “1” and at the portion where the tracking deviation amount is great, too, these positive portion detection signal  53  and negative portion detection signal  54  occur and attain “1”. Therefore, assuming that the logical ORs carried out between the signal detection signal  55  that attains “1” at the pit portions and the positive portion detection signal  53  and between the signal detection signal  55  and the negative portion detection portion  54  by the signal level detector  43  are IP1 and IP2, these IP1 and IP2 become the signals which are “1” at only the pit portions of the PID portion. 
     FIG. 7 shows another example of the PID position detector  19  according to the present invention, and FIG. 8 shows the signal waveform at each portion in this example. The HPP signal is the differential signals obtained by receiving the return beam of the beam irradiated to the optical disk by the detector disposed eccentrically with respect to the track center and split into two parts in the track direction. The output signals of these two split detectors, that is, a tracking deviation (+) and tracking deviation (−), are inputted to the signal level detectors  49  and  50  to obtain the signal detection signals  56  and  57 , respectively. The signal level detectors  49  and  50  are substantially the same as the level detector  43  described above and attain “1” at the pit portions of the PID portion at which signals having a relatively high frequency component exist and at the data portions at which the recording information exists. Since the tracking deviation (+) and the tracking deviation (−) are the common mode signals having substantially the same amplitude on the data track, the amplitude is extremely small in the HPP signal and the signal level detector  43  attains “0”. 
     Assuming hereby that the logical ORs carried out by the signal level detectors  49 ,  50  and the signal level detector  43  and IP1 and IP2, respectively, these IP1 and IP2 are the signals which attain “1” at only the pit portions of the PID portion. 
     FIG. 9 shows still another example of the PID position detector  19  according to the present invention, and FIG. 10 shows the signal waveform at each portion in this example. The HPP signal is inputted to the low-pass filter (LPF)  48  and a low frequency component signal  56  of the HPP signal is obtained. A peak clamper  61  clamps the upper part of the waveform at a potential Va while a bottom clamper  62  clamps the lower part of the waveform, thereby yielding upper and lower portion clamp waveforms  63  and  64 , respectively. These upper and lower portion clamp waveforms  63  and  64  are compared by the comparators  44  and  45  at slice levels obtained by adding addition potentials Vc and Vd to the clamp potentials Va and Vb by adders  65  and  66  to obtain a positive portion detection signal  53  and a negative portion detection signal  54 , respectively. 
     In this case, the positive portion detection signal  53  and the negative portion detection signal  54  occur at the PID portion and attain “1”, and at the portions where the tracking deviation amount is great, too, these positive and negative portion detection signals  53  and  54  develop and attain “1”. Therefore, assuming that the logical ORs carried out by the signal level detector  43  between the signal detection signal  55 , which attains “1” at the pit portion, and the positive and negative portion detection signals  53  and  54 , respectively, are IP1 and IP2, these IP1 and IP2 are the signals which attain “1” at only the pit portions of the PID portion. 
     Incidentally, when the fluctuation of the tracking deviation amount at portions other than the PID portion has a relatively low frequency, the positive portion detection signal  53  and the negative portion detection signal  54  for suppressing the fluctuation by the peak clamper  61  and the bottom clamper  62  do not occur. Therefore, masking by the signal level detection signal  55  need not be effected. 
     In the examples described above, if the track comprising the groove track and the land track other than the PID portion of the optical disk is so formed as to slightly wobble in the track direction, IP1 and IP2 can be detected at least stably by using the HPP signal having a substantially constant amplitude by the AGC  12  by setting the slice levels V 3  and V 4  in the following way. 
     In the optical disks having the wobbles formed thereon, the tracking deviation amount changes due to the wobbles at portions other than the PID portion of the HPP signal. Generally, the tracking deviation amount resulting from the wobbles is sufficiently smaller than the deviation amount due to the pits of the PID portion lest it affects tracking control. Therefore, the slice levels V 3  and V 4  need be set to the levels which do not respond to the fluctuation of the tracking deviation amount resulting from the wobbles. 
     The slice levels can be set, for example, by gradually increasing the slice levels V 3  and V 4  from the center level to both positive and negative, changing the levels until the wobble pattern does not occur in IP1 and IP2 and setting the slice levels V 3  and V 4  to somewhat higher levels than the levels at which the wobble pattern does not occur, in consideration of the fluctuation of the tracking deviation amount due to the wobbles. 
     When the optical disk is divided in the recording unit (sector) and the PID portion exists in each sector, each of the control signals  31  to  35  from the drive controller  18  of the present invention is outputted for each of the positive and negative pit portions of each sector and the PID portion, and each signal processing system is controlled. 
     It is further possible to detect the polarity judgement positions of the land and groove tracks in accordance with the sequence of the occurrence of IP1 and IP2 of the PID position detector by using IP1 and IP2. 
     Though the explanation has thus been given on the control of the reproduction signal processing system, the timing of the start of recording by the in-recording data portion may be generated from IP1 and IP2. 
     FIG. 11 shows an example of the drive controller  18  shown in FIG.  1 . Initially, the circuit construction will be described. 
     This driving controller comprises a signal width protector  101  which effects sampling by an internal clock by inputting IP1 and IP2, employs an input signal value only when the number of times of continuous detection of “0” or “1” exceeds a predetermined value and holds a previous value at other times, a signal interval measurer  102  which measures the signal interval by using the IP1a and IP2a signals that are inputted, as the input signals, a signal cycle protector  103  which selectively outputs only the IP1a signal  131  and IP2a signal  132 , that occur periodically, by using the IP1 detection window signal  133  and the IP2 detection window signal  134  outputted by this signal interval measurer  102 , and a controller  104  which inputs the IP1b and IP2b signals  135  and  136 , whose cycle is so protected, and generates various control signals  31  to  35 . 
     Next, the operation will be explained. 
     The IP1 and IP2 signals which are normally inputted from the PID detector  19  are sometimes outputted due to irregular signals (noise, off-tracking, track jump, etc.) and these IP1 and IP2 signals generated in other portions than the IPD portion must be removed. In the PID portion, too, there is the possibility that various drive control signals  31  to  35  are erroneously outputted due to the irregular signals such as the noise. 
     To cope with the occurrence of the IP1 and IP2 signals due to the irregular signals, protection is effected by dividing the irregular signals into an irregular signal having a short signal width and an irregular signal having a long signal width and in this way, the erroneous output of various drive control signals  31 ,  32 ,  33 ,  34  and  35  due to various irregular signals can be suppressed. 
     First, as to the irregular signals having a short signal width, the signal width of each of IP1 and IP2 is detected and change detection is not made for the signal change below a predetermined level. This processing is executed by the signal width protector  101 . 
     Next, as to the irregular signals having a long signal width, the occurrence cycle of IP1 and IP2 is measured and IP1 and IP2 occurring outside the cycle are rejected. In this way, the irregular signals of this kind can be suppressed. 
     More concretely, the signal intervals of the IP1a signal  131  and the IP2a signal  132  outputted from the signal width protector  101  are measured, and the IP1 detection window signal  133  and the IP2 detection window signal  134  are outputted at the point when the signal interval is confirmed. Incidentally, the IP1 detection window signal  133  and the IP2 detection window signal  134  are kept always valid at the point when signal interval is not confirmed. 
     Incidentally, the signal interval measurer  102  has also the function of outputting the IP1 detection window signal  133  and the IP2 detection window signal  134  from an PID information signal  36  in place of the IP1a signal  131  and the IP2a signal when the PID information signal  36  inputted from the reproduction signal modulator  17  is detected, and can more stably control against the irregular signals by appropriately switching the functions in accordance with the reproduction condition. 
     The signal interval protector  103  outputs only the IP1a signal  131  generated during the period in which the IP1 detection window signal  133  is varied, as the IP1b signal  135  having the protected signal interval. This also holds true of IP2 and the IP2b signal  136  having the protected signal interval is outputted. 
     Since the IP1b signal  135  and the IP2b signal  136  are generated from only the IP1 and IP2 signals generated in the PID portion, various drive control signals  31 ,  32 ,  33 ,  34  and  35  can be generated by the controller  104  without being affected by the irregular signals by using these IP1b and IP2b signals as the references. 
     As described above, even when various irregular signals are contained in IP1 and IP2 inputted from the PID detector  19 , various drive control signals  31 ,  32 ,  33 ,  34  and  35  outputted from the drive controller  18  can generate the control signals not affected by the irregular signals. 
     FIG. 15 shows still another example. When the signal interval measurer  102  outputs the IP1 detection window signal  133  and the IP2 detection window signal  134  from the PID information signal  36 , the signal interval measurer  102  first generates signals  137  and  138  corresponding to the IP1b signal  135  and the IP2b signal  136 , which are then inputted to the controller  104 , so that the control signals highly resistant to the irregular signals can be generated by switching the IP1b and IP2b signals  135 ,  136  and the signals  137 ,  138  depending on the reproduction condition. 
     FIG. 12 shows an example of the signal width protector  101  shown in FIG. 11 that pertains to IP1. Incidentally, the same circuit construction can be employed for IP2, as well. 
     First, the circuit construction will be explained. 
     The circuit comprises n-notation counters  140  and  141  which are counted up when an EN input is “1” at the rise of CK and are cleared when a CLR input is “1” at the rise of CK (all the Q outputs are set to “0”), a comparator  142  having a comparison output  152  which outputs “1” only when a D1 input is equal to n−1 at the rise of CK and a comparison output  153  which outputs “1” only when a D0 input is equal to n−1 at the rise of CK, and an RS-FF  143  which sets the Q output (IP1a signal)  131  to “1” when “1” enters its S input, sets its Q output (IP1a signal)  131  to “0” when “1” enters its R input, and latches a previous value at other times. 
     Next, the circuit operation will be explained. 
     When IP1 is “1”, the n-notation counter  140  is counted up whenever CK rises and at this time, the n-notation counter  141  remains cleared. When counted up to n−1, the n-notation counter  140  returns to 0 and thereafter repeats once again its count-up operation. 
     When IP1 is “0”, the n-notation counter  140  remains cleared, contrary to the above, and the n-notation counter  141  so operates as to repeat its count-up operation. 
     The comparator sets its comparison output  152  to “1” when the n-notation counter  140  coincides with n−1. Similarly, the comparator sets its comparison output  153  to “1” when the n-notation counter  141  coincides with n−1. 
     Therefore, when IP1 changes to “1” and “1” continues for a period corresponding to (n−1)×(internal clock), “1” enters the S input of the RS-FF  143  and the IP1a signal  131  changes to “1”. Similarly, when IP1 changes to “1”, the IP1a signal  131  changes to “0”. 
     Incidentally, when the IP1 again changes to “1” after IP1 changes from “1” to “0” but before the n-notation counter  141  is counted up to n−1, “1” is not outputted from the comparison output  153 . In consequence, the IP1a signal remains “1” without changing to “0”. 
     This also holds true of the case when IP1 changes from “0” to “1” and returns to “0” before count-up of n−1, on the contrary. 
     As a result of the operations described above, the IP1 signal change below n−1 as calculated by the internal clock is not reflected on the IP1a signal and the detection of IP1 for the signal width below a predetermined value can be protected. This also holds true of IP2. 
     The value n and the frequency of the internal clock may be selected suitably within the range which can sufficiently remove the noise, etc, of IP1 and IP2. 
     FIG. 13 shows an example of the signal interval measurer  102  relating to IP1. Incidentally, the same circuit construction can be employed for IP2, too. 
     First, the circuit construction will be explained. 
     This circuit comprises a rising edge detector  160  for detecting the rising edge of the IP1a signal  131 , a PID information judgement circuit  165  for executing detection judgement of the PID information from the PID information signal  36 , a selector  161  for switching and selecting a PID start detection signal A 170  outputted from the rising edge detector  160  and a PID start detection signal B 175  outputted from a PID information judgement circuit  165 , a signal interval counter-and-decoder  162  including a signal interval counter, for clearing the count value when the PID start detection signal C 171  outputted from the selector  161  is inputted during the effective period of the PID detection window C 133  and keep counting always at other times and a decoder for generating a necessary decoding output signal, a detection counter  164  for executing counting when the PID start detection signal A 170  is detected during the effective period of the detection window signal A 172  outputted from the signal interval counter-and-decoder  162 , and a detection window controller  163  for outputting and controlling the IP1 detection window signal  133  by each input signal  173 ,  174  and  176 . 
     Next, the circuit operation will be explained. 
     First, the circuit operation when the PID information signal  19  from the reproduction signal demodulator  17  under the initial state will be explained. 
     The selector  161  selects the PID start detection signal A 170  outputted from the rising edge detector  160  and outputs it as the PID start detection signal C 171 . 
     In the signal interval counter-and-decoder  162 , on the other hand, the IP1 detection window signal  133  is always effective under the initial state (refer to the following explanation of the operation of the detection window controller  163 ). When the PID start detection signal C 171  becomes effective, the signal interval counter of the signal interval counter-and-decoder  162  is cleared and counting starts in synchronism with the PID start detection signal C 171 . Next, the count value near the timing at which the PID start detection signal C 171  is inputted is decoded and the detection window signal A 172  and the detection window signal B 173  are outputted. 
     When the PID start detection signal C 171  is inputted to the detection counter  164  while the detection window signal A 172  is effective, the detection count value  174  is counted up. However, this count-up operation is stopped when the count value reaches a predetermined count value. When the PID start detection signal C 171  is not inputted to the detection counter  164  while the detection window signal A 172  is effective, the detection count value  174  is cleared. 
     When the detection count value  174  exceeds a predetermined value, the detection window controller  163  outputs the detection window signal B 173  as the IP1 detection signal  133 . Incidentally, when the detection count value  174  is below the predetermined value, the detection window controller  163  outputs the IP1 detection window signal  133  so that it is always effective. Therefore, the IP1 detection window signal  133  is always effective under the initial state. 
     The operations described above make it possible to check the signal interval by the rising edge detection of IP1 from the initial state and to output the IP1 detection window signal  131  for periodically protecting the IP1 a signal  131  after the signal interval is confirmed. This also holds true of IP2. 
     FIG.  16 (A) is a timing chart from the initial state till the start of the signal interval protection by the IP1 detection window signal  133  and FIG.  16 (B) shows the timings during the signal interval protection. 
     Referring to FIG.  16 (B), it can be appreciated that even when any irregular signal occurs in the IP1a signal  131 , the irregular signal does not affect the IP1b signal  135 . 
     Next, the circuit operation when the PID information signal from the reproduction signal demodulator  17  becomes effective will be explained. 
     When the PID information judgement circuit  165  confirms effectiveness of the PID information signal  36 , the PID start detection signal B 175  is outputted on the basis of the PID information signal and at the same time, the PID information judgement signal  176  becomes effective. 
     The selector  171  selects the PID start detection signal B 175 , which is inputted as the PID start detection signal C 171  to the signal interval counter-and-decoder  162 . As a result, the detection window signal B 173  is generated on the basis of the PID information signal. The detection window controller  163  does not employ the detection count value  174  for the judgement when the PID information judgement signal  176  is effective, but outputs always the detection window signal B 173  as the PID detection window signal  133 . 
     As described above, the operation can be switched to the generation of the IP1 and IP2 detection windows by using the PID information signal  36  contained in the reproduction signal. 
     Incidentally, when the PID information judgement signal  176  becomes invalid due to the change of the reproduction state, etc, the operation is returned to the generation of the PID detection window signal  133  by the original PID start detection signal A 170 . 
     FIG. 14 shows an example of the circuit portion in FIG. 11 relating to the signal cycle protector  103 . 
     The logical ORs  180  and  181  may be carried out between the IP1a signal  131  and IP2a signal  132  outputted from the signal width protector  101  and the IP1 detection window signal  133  and the IP2 detection window  130  outputted from the signal interval measurer  102 , respectively. 
     As described above, the present invention can satisfactorily detect the PID portion even when tracking control is not effected or when tracking control is unstable, and can control the signal processing system by using this detection signal. Therefore, the present invention can accomplish an information recording/reproducing apparatus capable of stably recording and reproducing the information.