Abstract:
An optical disc discriminating system comprises: a tracking error signal generating means adapted to generate a tracking error signal whose amplitude level varies with a groove depth of information tracks on an optical disc; a tracking servo means for directing a laser beam along an information track on the optical disc in accordance with the tracking error signal; a comparator adapted to compare the amplitude level of the tracking error signal supplied from the tracking error signal generating means with a reference level, with a tracking servo loop of the tracking servo means in an opened condition; a discriminating means adapted to discriminate the first and second groups of optical discs in accordance with a result from the comparator.

Description:
This is a divisional application of U.S. patent application Ser. No. 09/061,959 filed Apr. 17, 1998, now U.S. Pat. No. 6,144,625. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an optical disc discriminating system, in particular to a system for discriminating a read-only disc such as a DVD-ROM (Digital Video Disc-Read Only Memory) and a writable disc such as a DVD-R (Digital Video Disc-Recordable). 
     FIG. 7 indicates a conventional system for discriminating the types of different optical discs to determine if it is a read-only disc such as a CD-ROM (Compact Disc-Read Only Memory) or a writable disc such as a CD-R (Compact Disc-Recordable). 
     In FIG. 7, a reference character D is an optical disc to be discriminated,  101  is an optical pickup,  102  is a spindle motor for rotating the optical disc D,  103  is a motor control circuit for controlling the rotation of the spindle motor  102 ,  104  is a head amplifier,  105  is a servo circuit,  106  is an information pit detector,  107  is a CPU (Central Processing Unit). 
     The optical disc D to be discriminated is mounted on a turntable (not shown) of the spindle motor  102  which rotates in accordance with a control signal from the motor control circuit  103 . The motor control circuit  103 , in accordance with a command from the CPU  107 , compares a pulse signal (FG) having a frequency proportional to the motor rotation speed, with another pulse signal (FG) indicating a rotation speed previously set by a setting section  171  of the CPU  107 , so as to produce a frequency difference between these two pulse signals. Then, a control signal for rendering the frequency difference to become zero is fed to the spindle motor  102 . In this manner, a servo loop is formed for controlling the rotation of the optical disc D. Therefore, the optical disc D which is to be discriminated later is controllably rotated at a speed previously set by the CPU  107 . 
     On the other hand, a laser beam irradiating from the optical pickup  101  is reflected on the recording surface of the optical disc D (to be discriminated), a reflection diffraction light bearing the information from the optical disc D is received by a photodetector (not shown) of the optical pickup  101 . Then, the reflection diffraction light is converted into an electric signal to be fed to the head amplifier  104 . The head amplifier  104  processes the above electric signal so as to first produce and then amplify a RF signal and an error signal to be used in focusing control/tracking control. The amplified signals are fed to the servo circuit  105  and the information pit detector  106 . The servo circuit  105 , in view of an error signal applied hereto and in accordance with an command from the CPU  107 , produces a servo control signal for controlling a slid motor (not shown) which may effect a radial movement of the optical pickup  101 , a focus actuator (not shown) and a tracking actuator (not shown) both of which are contained in the optical pickup  101 . Further, the servo circuit  105  produces a tracking lock signal (indicating a condition determined by a tracking control operation after the tracking servo loop is closed) to the information pit detector  106 . The information pit detector  106  extracts an envelope of the RF signal, and determines whether the envelope has a higher level than a predetermined level. If the envelope has a higher level than a predetermined level, the pit detector  106  produces a signal (indicating that the optical disc D has formed thereon information pits) to the CPU  107 . 
     The types of different optical discs may be discriminated with the use of the system of FIG. 7 in a manner which will be described in detail below. 
     At first, the CPU  107  gives a command to the servo circuit  105  to drive a slide motor (not shown) so as to move the optical pickup  101  in a radial direction to a position corresponding to a TOC (Table of Contents) area on the optical disc D. Then, the CPU  107  produces a command signal to the motor control circuit  103  to cause the optical disc D to rotate in a predetermined speed. Subsequently, a laser beam is emitted from the optical pickup  101 , a focus control and a tracking control are effected by means of the servo circuit  105  such that the laser beam is converged on the information recording surface of the optical disc D and trace along information tracks thereon. After that, if the pit detector  106  has produced a signal indicating that the optical disc D has information pits formed thereon, it is determined that the optical disc D is a read-only CD or a finalized CD-R which does not contain any unrecorded portions. On the other hand, if the pit detector  106  does not produce such a signal, it is determined that the optical disc D is a CD-R which contains un-recorded portions. 
     A principle for discriminating the types of different optical discs in the above system of FIG. 7 may be explained as follows. 
     Namely, a CD-ROM or a finalized CD-R, which is a read-only optical disc, has formed in its TOC area a lot of pits representative of TOC information. Thus, when tracks are traced a RF signal will be read out which has an envelope whose level will be higher than a predetermined level, thereby producing a signal indicating that pits have been detected. On the other hand, a CD-R which contains unrecorded portions has not formed in its TOC area any pits representative of TOC information (because contents to be recorded have not yet been finally decided and thus the disc itself has not been finalized), although when tracks are traced a RF signal will be read out, an envelope of the RF signal will have a level which equals to zero, hence failing to produce any signal indicating that pits have been detected. 
     Recently, there has been developed a further advanced optical disc such as a read-only DVD-ROM (Digital Video Disc-Read Only Memory) which has a much larger information recording capacity and a higher recording density than a read-only CD. Also, there have been developed further advanced writable optical discs including a DVD-R (Digital Video Disc-Recordable), DVD-RAM (Digital Video Disc-Random Access Memory), which have an identical signal format as the read-only DVD-ROM but can optionally record movies or music a user has selected. Further, it has been suggested that a general system be designed to enable the DVD, DVD-R and DVD-RAM to be used so as to perform recording or reproducing on only one recording/reproducing device. With such a general system, it is necessary to discriminate the types of different optical discs so as to perform a desired changeover into an equivalent circuit having a characteristic corresponding to an optical disc to be used for recording or reproducing. 
     However, as shown in FIG. 8, when pit height or groove depth of an optical disc is λ/4n (λ is a wave length of a laser beam, n is an index of refraction of a substrate of an optical disc), an RF signal from the optical disc will reach a maximum level, thus a C/N (Carrier to Noise) of the RF signal is in its optimum condition. On the other hand, a push-pull signal (tracking error signal) indicating a difference between two outputs from a two-section (equally divided) photo-detector will become maximum when pit height or groove depth is λ/8n, but will become minimum when the height or the depth is λ/4n. 
     Where an optical disc is a CD-ROM, a pit height or a groove depth is designed to be λ/6n, so as to obtain a RF signal having a level equal to or higher than a predetermined value, and at the same time to surely perform a tracking control using a push-pull signal. 
     Where an optical disc is a DVD-ROM, since its recording density is seven times as high as a CD-ROM, an optical pickup, which is to be used in the above-described general system, has to be designed with an extremely high precision so as to meet the requirements prescribed in MTF (Modulation Transfer Function). In order to reduce a burden on the designing of such an optical pickup which is for use in the general system, the height of the pits formed on the DVD-ROM is designed to be λ/4n so as to obtain a RF signal having an optimum signal quality. Consequently, as a tracking control on the DVD in a comprehensive general system, it is impossible to employ a Push-pull Method, rather it is generally possible to employ a Phase Difference Method. 
     On the other hand, where an optical disc is a writable disc such as a DVD-R or a DVD-RAM, the recording of information on such an optical disc usually requires that a laser beam should have a high output power. Thus, it is preferable that a laser beam being emitted from a light source be used as efficiently as possible. Accordingly, as a tracking control on the DVD-R, it is preferable to employ a Push-pull Method which does not need to divide a laser beam, or allows a divided beam (for use in tracking control) to be much smaller than a beam for use in recording information. In contrast, the Phase Difference Method is not suitable for a DVD-R or a DVD-RAM, although the method allows a divided beam (for use in tracking control) to be smaller than a beam for recording information, because there are no pits formed on an unrecorded portions of a writable disc, making it impossible to obtain error signal. Further, in order to make a good trade-off (a balance) between the push-pull signal and the RF signal, a pit height or a groove depth is required to be approximately λ/5n so as to ensure that both the push-pull signal and RF signal reach or exceed their predetermined levels. 
     As is understood from the above description, a tracking control using Push-pull Method can not be used for a read-only optical disc such as a DVD-ROM, on the other hand, a tracking control using Phase Difference Method can not be used for a writable optical disc such as a DVD-R or a DVD-RAM. 
     Since there is no common tracking control which is useful for both a DVD-ROM and a DVD-R (or DVD-RAM), a conventional discriminating method can not be used for discriminating a DVD-ROM from a DVD-R (or DVD-RAM), because a conventional method requires a prerequisite that a tracking control be performed to render a laser beam to correctly trace tracks on an optical disc. 
     Moreover, in discriminating a DVD-R or a DVD-RAM having unrecorded portions, a conventional method (which discriminates a DVD-R or a DVD-RAM by confirming if there are information pits formed on the TOC area of an optical disc) fails to be used therefor because there are not information pits formed on the TOC area of a DVD-R or a DVD-RAM 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved system for discriminating the types of different optical discs even if there is no common tracking control useful for optical discs of different types, so as to solve the above-mentioned problems peculiar to the above-mentioned prior arts. 
     According to the present invention, there is provided an optical disc discriminating system adapted to discriminate a first group of optical discs each formed with information tracks having a predetermined groove depth and a second group of optical discs each formed with information tracks having a different groove depth. The discriminating system comprises: a tracking error signal generating means adapted to generate a tracking error signal whose amplitude level varies with a groove depth of information tracks on an optical disc; a tracking servo means for directing a laser beam along an information track on the optical disc in accordance with the tracking error signal; a comparator adapted to compare the amplitude level of the tracking error signal supplied from the tracking error signal generating means with a reference level, with a tracking servo loop of the tracking servo means in an opened condition; a discriminating means adapted to discriminate the first and second groups of optical discs in accordance with a result from the comparator. 
     In one aspect of the present invention, an optical disc discriminating system may be made adapted to discriminate from the second group of optical discs a specific optical disc which has information tracks wobbled with a wobble signal having a predetermined frequency and has prepits formed between adjacent tracks on the optical disc. Thus, the discriminating system further comprises: an extractor for extracting the above wobble signal and/or a prepit detector for detecting the above prepits; a discriminating means adapted to discriminate a specific optical disc from the second group of optical discs, in accordance with output signal(s) fed from the above extractor and/or a prepit detector when the tracking servo loop of the tracking servo means is in a closed condition. 
     The above objects and features of the present invention will become more understood from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a block diagram showing an optical disc discriminating system formed according to the present invention. 
     FIG. 2 is a schematic diagram indicating in detail a regenerative amplifier for use in the system of the present invention. 
     FIG. 3 is an enlarged perspective view showing the structure of a DVD-R to be discriminated in the system of the present invention. 
     FIG. 4 is an explanatory view showing the structure of a DVD-RAM to be discriminated in the system of the present invention. 
     FIG. 5 is a flowchart showing an operation of a CPU in the system of the present invention. 
     FIG. 6 is a flowchart showing another operation of a CPU in the system of the present invention. 
     FIG. 7 is a block diagram showing an optical disc discriminating system according to a prior art. 
     FIG. 8 is a graph showing a relationship between a pit depth and the level of RF signal, and a relationship between a pit depth and the level of a tracking error signal produced in a Push-pull method. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a reference character D is an optical disc to be discriminated, S is an optical disc discriminating system of the present invention for discriminating the type of the optical disc D. 
     The optical disc D to be discriminated in the discriminating system of FIG. 1 will be described below. 
     Firstly, the optical disc D may be a read-only disc such as a DVD-ROM which has a pit height or a groove depth of λ/4n with respect to a laser beam having a wave length of 635 nm. Secondly, the optical disc D may be a writable disc such as a DVD-R or a DVD-RAM which does not have a pit height or a groove depth of λ/4n with respect to a laser beam having a certain wave length. 
     The recording format of information recorded on a DVD-ROM is described in the following. 
     The information recorded on a DVD-ROM is divided into a plurality of sync-frames each serving as an information unit. Every twenty six sync-frames form one recording sector, and every sixteen recording sectors form one ECC (Error Correcting Code) block. In detail, each sync-frame has a length which is 1488 times as long as a unit length (hereinafter referred to as T) corresponding to a pit interval. Further, a synchronizing signal SY having a length of 14T which is for obtaining a synchronizm with each sync-frame, is recorded on the front portion of each sync-frame. 
     On the other hand, a writable DVD-R  38  is formed with a coloring matter layer  35 , as shown in FIG.  3 . Referring to FIG. 3, groove tracks  39  serving as information recording tracks, and land tracks  33  for leading a laser beam B along the groove tracks  39  are all formed on the disc. Further, a protection film  37  and a gold-deposited film  36  (serving as a reflecting layer) are formed on the opposite sides of the disc. 
     Formed in and along each land track  33  are a plurality of prepits  34  (separated from each other at a predetermined interval) corresponding to pre-information. 
     Further, on the DVD-R  38  shown in FIG. 3, the groove tracks  39  are formed in a wobbled manner in accordance with a frequency (140 KHz) corresponding to the rotating speed of the DVD-R. 
     The recording format of pre-information recorded on a DVD-R  38  is described in the following. 
     The pre-information recorded on the DVD-R are in fact recorded on a plurality of sync-frames each serving as an information unit. Each cync-frame has recorded thereon a synchronizing signal SY. A land track  33  has formed therein one prepit  34  (adjacent to the synchronizing signal SY) serving to indicate a synchronizing signal in pre-information. Further, the land track  33  has formed therein two or one prepit  34  (adjacent to the front half of a sync-frame, other than the above synchronizing signal SY) serving to indicate address data of pre-information to be recorded on the land track  33 . In this embodiment, prepits  34  are formed to have pre-information recorded on only even-numbered cync-frames or only on odd-numbered cync-frames in one recording sector. Therefore, within one recording selector, every two cync-frames (having a periodic interval of about 115 μs) have at least one prepit serving to indicate a cynchronizing signal. 
     When information (such as picture data other than pre-information) is to be recorded on the DVD-R  38 , a wobbling frequency of the groove track  39  is extracted so as to obtain a clock signal (for controlling information recording) synchronized with the rotation of DVD-R  38 . At the same time, pre-information is obtained by detecting prepits  34 , a laser beam B is then adjusted to have the most appropriate output power in accordance with the pre-information. Meanwhile, address data indicating various positions on the DVD-R  38  is obtained, so that the information (such as picture data) is recorded on certain position on the DVD-R  38  in accordance with the address data. However, the depth of the groove tracks  39  is not λ/4n, but λ/5n so as to ensure that both the push-pull signal and RF signal reach or exceed their predetermined levels. In fact, the depth of groove tracks of DVD-R is shallower than the pit height of a DVD-ROM. 
     On the other hand, if a writable optical disc is a DVD-RAM (DVD-Random Access Memory), such as a repetitive writable/erasable optical disc made of a phase-changeable optical memory material shown in FIG.  4 . Referring to FIG. 4, groove tracks and land tracks are alternatively arranged on the disc. On the front portion of each recording sector, there is formed an information recording area (called “head”) for recording pre-information such as address information. Such pre-information are formed in terms of prepits along straight lines (dotted lines in FIG.  4 ). Similar to a DVD-R, the depth of groove tracks of DVD-RAM is shallower than the pit depth of a DVD-ROM. 
     Further, the groove tracks are formed in a wobbled manner in accordance with a frequency (140 KHz) corresponding to the rotation speed of the DVD-RAM, except on the “head” portion of each recording sector. 
     The optical disc discriminating system S according to the present invention will be described in detail below with reference to FIG.  1 . 
     Referring to FIG. 1, the system S includes an optical pickup  1 , a spindle motor  2 , a motor control circuit  3 , a switch  4 , an equalizer  5 , an amplifier  6 , a regenerative amplifier  7 , a decoder  8 , a peak detector  10 , an A/D converter  11 , a band pass filter (BPF)  12 , a prepit signal detector  13 , a prepit signal decoder  14 , a wobble signal extractor  15 , a comparator  16 , a MMV (Monomulti-vibrator)  17 , and a MVV  18 . 
     FIG. 2 is a schematic diagram indicating in detail the regenerative amplifier  7 . Referring to FIG. 2, the regenerative amplifier  7  comprises a push-pull signal generator  7   a , a phase difference signal generator  7   b , and a switch  7   c.    
     As shown in FIG. 2, the push-pull signal generator  7   a  is adapted to produce a push-pull error signal S PP using a Push-pull Method in accordance with detected signals S DT fed from a 4-divided photodetector  1   a  provided on the optical pickup  1 . The phase difference signal generator  7   b  is adapted to produce a phase difference error signal S PH using a Phase Difference Method in accordance with a RF signal S P which is an overall signal of the detected signals S DT fed from the 4-divided photodetector, also directly in accordance with the detected signals S DT. 
     Although not shown in FIG. 1, the optical pickup  1  includes a laser diode, a beam spliter, an objective lens, a photodetector  1   a . The optical pickup  1 , in response to a drive signal, irradiates a laser beam B on to the information recording surface of the optical disc D. A reflected light is detected by the photodetector  1   a  which produces signals S DT to be supplied to the regenerative amplifier  7 . 
     The motor control circuit  3  is adapted to control the rotation of the optical disc D by rendering the spindle motor  2  to rotate in a speed corresponding to an FG signal fed from CPU  9 . 
     Referring again to FIG. 2, the push-pull signal generator  7   a  of the regenerative amplifier  7  comprises adders  19 ,  20  and a subtractor  21 . The adder  19  is adapted to add together two output signals from sections B 2  and B 3  of the photodetector  1   a  to produce a sum signal, the adder  20  is adapted to add together two output signals from sections B 1  and B 4  of photodetector  1   a  to produce another sum signal. The sutractor  21  is adapted to produce a difference signal between the two sum signals from the adders  19 ,  20 . In fact, the difference signal is a push-pull error S PP signal which is then supplied to the peak detector  8 , the band pass filter  12 , the prepit signal detector  13 , and the switch  7   c . However, such a push-pull error signal S PP is also a tracking error signal whose signal level changes with the groove depth of an optical disc, as shown in FIG.  8 . 
     Referring further to FIG. 2, the phase difference signal generator  7   b  has adders  22 ,  23 ,  24 , a subtractor  25 . The adder  22  is adapted to add together two output signals from sections B 1  and B 3  of the photodetector  1   a  to produce a sum signal, the adder  23  is adapted to add together two output signals from sections B 2  and B 4  of photodetector  1   a  to produce another sum signal. The adder  24  is adapted to add together the two sum signals from the adders  22  and  23 , so as to produce a RF signal S P which is then fed to the decoder  8  (FIG.  1 ), a rising pulse generator  26 , a decay pulse generator  27 . The rising pulse generator  26  is adapted to produce a pulse signal in synchronism with a positive amplitude period of RF signal S P fed from the adder  24 , so as to supply the pulse signal as a gate pulse to the gate circuit  28 . The decaying pulse generator  27  is adapted to produce a pulse signal in synchronism with a negative amplitude period of RF signal S P fed from the adder  24 , so as to supply the pulse signal as a gate pulse to the gate circuit  29 . 
     Meanwhile, the sutractor  25  is adapted to produce a difference signal between the two sum signals from the adders  22 ,  23 . This difference signal is fed to gate circuits  28  and  29  as a signal (to be sampled), respectively. The gate circuit  28 , during a period when a gate pulse is supplied from the rising pulse generator  26 , will sample the above difference signal, and supply a sampled value to a hold circuit  30 . Similarly, the gate circuit  29 , during a period when a gate pulse is supplied from the decaying pulse generator  26 , will sample the above difference signal, and supply a sampled value to a hold circuit  31 . The output from the hold circuit  30  and the output from the hold circuit  31  are all supplied to a subtractor  32  so that a difference signal is produced. Such a difference signal is used as a phase difference error signal S PH (which is also a tracking error signal in a Phase Difference Method) to be fed to the switch  7   c.    
     The switch  7   c , in response to a changeover signal from the CPU  9 , supply either a push-pull error signal S PP or a phase difference error signal S PH to the switch  4  (FIG.  1 ). 
     The switch  4  is adapted to open or close a tracking servo loop for the disc discriminating system. In detail, the switch  4 , which operates in accordance with a command form the CPU  9  for opening or closing the tracking servo loop, will supply a tracking error signal fed from the switch  7  to the equalizer  5 . 
     The equalizer  5  is adapted to perform a predetermined wave equalization so that either of the above error signals will be processed to have a desired frequency characteristic and be fed to the amplifier  6 . The amplifier  6  is adapted to amplify the error signal to a predetermined desired level and supply the amplified signal to the optical pickup  1 . In fact, the amplified signal serves as a drive signal for driving a tracking actuator (not shown) provided in the optical pickup  1 . 
     On the other hand, the decoder  8  is adapted to perform predetermined demodulation and deinterleave on the RF signal S P fed from the regenerative amplifier  7  so as to decode the RF signal S P, and supply a demodulated signal S DM to the CPU  9 . 
     The peak detector  10  is adapted to detect a peak level of the push-pull error signal S PP and to hold the peak level so as to supply the peak level to the A/D converter  11 . In the A/D converter  11 , the peak level is converted into a digital value which is then fed to the CPU  9 . The CPU  9 , in response to the digital value and in accordance with a predetermined procedure, will determine the type of an optical disc. 
     Meanwhile, the band pass filter  12  is adapted to remove noise components contained in the push-pull error signal fed from the regenerative amplifier  7 , so as to produce a composite signal S PC which is then supplied to the prepit signal detector  13  and the wobble signal extractor  15 . 
     If the tracking servo is in its closed condition and an optical disc D to be discriminated is a DVD-R, the composite signal S PC will be an overlapped signal in which a pulse signal caused due to prepits formed in the land tracks is overlapped on a wobbling signal from groove tracks). On the other hand, if an optical disc to be discriminated is a DVD-RAM, a pulse signal caused due to prepits and in synchronism with the head portion (having prepits) of each recording sector, and a wobble signal from the groove tracks, will be intermittently generated alternatively and periodically. 
     Further, the prepit signal detector  13  includes a comparator adapted to compare the composite signal S PC with a predetermined reference value, capable of extracting from the composite signal S PC a pulse signal generated due to prepits formed on a DVD-R or a DVD-RAM, producing a prepit detection signal S PD (which is a two-value signal only indicating a high or low value) to be fed to the CPU  9  and the prepit signal decoder  14 . 
     The prepit signal decoder  14  is adapted to demodulate the prepit detection signal S PD so as to produce a pre-information decoded signal S PJ (indicating pre-information recorded on a writable optical disc) to the CPU  9 . 
     On the other hand, the wobble signal extractor  15 , also acting as a limiter, is adapted to remove from the composite signal S PC the pulse signal and noise signal mixed with the wobble signal, so as to produce a extracted wobble signal S WB which is then supplied to the comparator  16 . 
     The comparator  16  is adapted to compare the extracted wobble signal S WB with a predetermined threshold value (a reference value) so as to produce a two-value wobble signal S NWB in synchronism with the period of the extracted wobble signal S WB. The wobble signal S NWB is then fed to MMV  17 . 
     The MMV  17  is adapted to produce a pulse signal S MP 1  which will reach a high level H in synchronism with a rising edge of the two-value wobble signal S NWB. The pulse signal S MP 1  is then fed to the CPU  9  and the MMV  18 . Here, the pulse signal S MP 1  has a longer time period than that of a wobble signal period (about 7.2 μs) of DVD-R or a DVD-RAM. Further, the MMV  17  is so constructed that during a period a pulse signal S MP 1  is occurring, once the rising edge of a next two-value wobble signal is inputted hereto, a further pule signal S MP 1  having a predetermined time period will be generated from that time point. Therefore, when groove tracks are continuously formed over entire circumference of an optical disc, a pulse signal S MP 1  may be maintained constantly at a high level. 
     The MMV  18  is adapted to produce a pulse signal S MP 2  which will reach a high level in synchronism with a rising edge of the signal S MP 1 . The pulse signal S MP 2  is then fed to the CPU  9 . Here, the pulse signal S MP 2  has a longer time period than that of one recording sector period (about 1.48 ms) of DVD-RAM. Further, the MMV  18  is so constructed that during a period a pulse signal S MP 2  is occurring, a pulse signal S MP 1  will not be accepted even if it is supplied hereto. Therefore, with a pulse signal S MP 2 , there will occurs a decaying edge after a time period (set to be longer than one recording sector period) has passed. 
     The CPU  9 , in accordance with a demodulated signal S DM, is adapted to supply through an interface circuit (not shown) reproduction signals corresponding to digital signals recorded on an optical disc, and to control the optical disc discriminating system S as a whole. 
     As understood from the above description, the CPU  9  is adapted to discriminate the type of an optical disc through its operations involving the use of the prepit detected signal S PD, the pre-information decoded signal S PJ, the pulse signal S MP 1 , the pulse signal S MP 2 , and the peak level of the push-pull signal fed from the A/D converter  11 . 
     An operation of the optical disc discriminating system S according to the present invention, will be described in the following with reference to FIG.  5 . 
     FIG. 5 is a block diagram showing a flowchart of an operation of the CPU  9  for discriminating the type of an optical disc. However, it is assumed that at this moment a focus control on the laser beam irradiating from the optical pickup  1  has been performed, whilst a tracking control has not been performed yet with the switch  4  in its opened position. 
     Referring to FIG. 5, at step S 1 , the CPU  9  operates to move the optical pickup  1  to a predetermined position on an optical disc D (for example, a position adjacent to an inmost circumference of a recording area). At step S 2 , the CPU  9  operates to supply an FG data (corresponding to the above position on the optical disc D) to the rotation control circuit  3  so as to start the rotation of the spindle motor  2 . 
     At step S 3 , a changeover signal is supplied from the CPU  9  to the switch  7   c  to render the switch  7   c  to be connected to the push-pull signal generator  7   a.    
     At step S 4 , an output signal from the A/D converter  11  is fed to the CPU  9  to check the level of a push-pull signal S PP. 
     At step S 5 , the CPU  9  determines whether a digital value indicating an amplitude level of a push-pull signal fed from the A/D converter  11  is higher than a predetermined value (reference value A). If the digital value is not higher than the reference value A, the optical disc is supposed to be a read-only DVD-ROM, thus the program transfers to step S 6 . Namely, if an optical disc is a DVD-ROM, since its pit height (or groove depth) is λ/4n, the amplitude level of the push-pull error signal will be almost zero regardless of a positional relationship between a laser beam spot and an information track on the disc. On the other hand, if an optical disc is a DVD-R or a DVD-RAM, since its pit height (or groove depth) is larger or smaller than λ/4n, a push-pull error signal will have a certain amplitude level corresponding to a positional relationship between a laser beam spot and an information track. 
     An step S 6 , the CPU  9  supplies a changeover signal to render the switch  7  to connect the phase difference controller  7   b , so that a phase difference error signal S PH is fed to the switch  4 . 
     At step S 7 , the CPU  9  supplies a switch closing signal to the switch  4  to close the switch  4 , so as to start a tracking control on the laser beam B. At step S 8 , the CPU  9  gets an output signal (demodulation signal S MD) from the decoder  8  so as to read out a control code recorded on a predetermined area (such as a TOC area of a DVD-ROM) of an optical disc. 
     At step S 9 , it is determined whether the control code read out at step S 8  is an inherent predetermined code of a DVD-ROM. If it is an inherent predetermined code of a DVD-ROM, the program transfers to step  10  to determine that the optical disc is a read-only disc DVD-ROM. 
     Afterwards, the CPU  9  operates to indicate that the optical disc is a read-only disc DVD-ROM, and to perform a changeover to a predetermined circuit suitable for the read-only disc DVD-ROM. 
     On the other hand, at step S 9 , if a control code read out at step S 8  is not an inherent predetermined code of a DVD-ROM, the program transfers to step S 17  to determine that the optical disc is neither a DVD-ROM nor a DVD-R or DVD-RAM, so as to stop all the possible operations after that. 
     Moreover, at step S 5 , if a digital value indicating an amplitude level of a push-pull error signal fed from the A/D converter  11  is higher than a predetermined value (reference value A), the program transfers to step S 11 . At step S 11 , a switch closing signal is supplied to the switch  4  to render the tracking servo loop to be in a closed condition. Then, the program transfers to step S 12 , the CPU  9  supplies an EN signal to each of the MMV  17  and the MMV  18  so as to render them in an operational condition. 
     At step S 13  a discrimination flag N is set to be zero, and at step S 14  a timer is started. 
     In step S 15 , a rising edge of a pulse signal S MP 2  fed from the MMV  18  is detected. 
     At step  15 , if a rising edge of a pulse signal S MP 2  fed from the MMV  18  can not be detected, the program transfers to step S 16  at which it is determined whether a predetermined time from the start of the timer has passed or not. If the predetermined time has not passed yet, the program goes back to step S 15 . At step S 15 , a rising edge of a pulse signal S MP 2  fed from the MMV  18  is again detected. Similarly, if a rising edge of a pulse signal S MP 2  fed from the MMV  18  can not be detected, the program transfers to step S 16  again. Such a process is repeated until it is determined that the above predetermined time from the start of the timer has passed. 
     When it is determined at step S 16  that the above predetermined time from the start of the timer has passed, the program transfers to step S 17 , to determine that the optical disc is neither a DVD-ROM nor a DVD-R or DVD-RAM, so as to stop all the possible operations after that. 
     At step S 15 , if a rising edge of a pulse signal S MP 2  fed from the MMV  18  has been detected, the program transfers to step  18  to detect whether there has occurred a decaying edge in a pulse signal S MP 1  fed from the MMV  17 . 
     If there has occurred a decaying edge in a pulse signal S MP 1  fed from the MMV  17 , the program transfers to step S 19 . At step S 19 , the discrimination flag N is changed from zero to 1, and the program transfers to step  20 . 
     On the other hand, at step S 18  if a decaying edge in a pulse signal S MP 1  can not be detected, the program transfers directly to step S 20 . 
     At step S 20 , it is determined whether there has occurred a decaying edge in a pulse signal S MP 2  fed from the MMV  18 . If a decaying edge in a pulse signal S MP 2  can not be detected, the program goes back to step  18 . In fact, the step  18  and the step S 20  are repeated until it is determined that there has occurred a decaying edge in a pulse signal S MP 2  fed from the MMV  18 . 
     At step S 20 , if it is determined that there has occurred a decaying edge in a pulse signal S MP 2  fed from the MMV  18 , the program transfers to step S 21  to confirm whether a discrimination flag N is 1 or not. If the discrimination flag N is 1, the program transfers to step S  22  to determine that the optical disc D is a DVD-RAM. On the other hand, if at step S 21  the discrimination flag N is zero, the program transfers to step S 23  so as to determine that the optical disc is a DVD-R. 
     Another operation of the optical disc discriminating system S, according to the present invention, will be described in the following with reference to FIG.  6 . 
     FIG. 6 is a block diagram showing a flowchart of another operation of the CPU  9  for discriminating the type of an optical disc. 
     Referring to FIG. 6, at step S 1 , the CPU  9  operates to move the optical pickup  1  to a predetermined position on an optical disc D (for example, a position adjacent to an inmost circumference of a recording area). At step S 2 , the CPU  9  operates to supply an FG data (corresponding to the above position on the optical disc D) to the rotation control circuit  3  so as to start the rotation of the spindle motor  2 . 
     At step S 3 , a changeover signal is supplied from the CPU  9  to the switch  7   c  to render the switch  7   c  to be connected to the push-pull signal generator  7   a.    
     At step S 4 , an output signal from the A/D converter  11  is fed to the CPU  9  to check the level of a push-pull signal S PP. 
     At step S 5 , the CPU  9  determines whether a digital value indicating an amplitude level of a push-pull signal fed from the A/D converter  11  is higher than a predetermined value (reference value A). If the digital value is higher than the reference value A, the program transfers to step S 11 . At step S 11 , a switch closing signal is supplied to the switch  4  to render the tracking servo loop to be in a closed condition. In this way, the tracking servo loop is formed so as to perform a tracking control in accordance with the push-pull error signal supplied from the regenerative amplifier  7 . 
     Then, the program transfers to step S 24  at which a timer is started. 
     Afterwards, at step S 25 , it is determined whether a prepit detected signal S PD has been produced from the prepit signal detector  13  (indicating whether prepit has been detected from an optical disc which is to be discriminated). If prepit can not be detected from an optical disc, the program transfers to step S 26 . At step S 26 , it is determined whether a predetermined time from the start of the timer has passed or not. If it is determined at step S 16  that the above predetermined time from the start of the timer has passed, the program transfers to step S 17 , to determine that the optical disc is neither a DVD-ROM nor a DVD-R or DVD-RAM, so as to stop all the possible operations after that. 
     On the other hand, if the predetermined time has not passed yet, the program goes back to step S 25  at which it is determined whether prepit can be detected from an optical disc which is to be discriminated). 
     At step S 25 , if prepit has been detected, the program transfers to step S 27 . At step S 27 , it is necessary to wait until a time period has passed which is sufficient for reading minimum prepit signal to decode the prepit signal so as to obtain pre-information, by virtue of the prepit signal decoder  14 . 
     At step S 28 , it is determined whether an error has been detected when performing decoding process in the prepit signal decoder  14 . If an error can not be detected, the program transfers to step S 23  in which the CPU  9  determines that the optical disc D is a DVD-R. 
     On the other hand, at step S 28  if it is determined that an error was detected when performing decoding process in the prepit signal decoder  14 , the program transfers to step  29  at which the CPU  9  determines whether the detected error can be corrected in accordance with the prepit signal format of a DVD-R. If it is determined that the detected error can be corrected in accordance with the prepit signal format of a DVD-R, it may be determined that the optical disc is a DVD-R. 
     On the other hand, at step S 29  if it is determined that the detected error can not be corrected in accordance with the prepit signal format of a DVD-R, the program transfers to step S 22  at which it may be determined that the optical disc is a DVD-RAM. 
     As may be understood from the above description, with the use of the optical disc discriminating system according to the present invention, an optical disc may be discriminated using a fact that a read-only disc and a writable disc have different groove depths and using the amplitude level of a push-pull error signal obtained when the tracking servo loop is in its opened condition. Therefore, it is possible to correctly perform disc discrimination even if optical discs to be discriminated are treated in different tracking error signal generating methods. 
     While the presently preferred embodiments of the this invention have been shown and described above, it is to be understood that these disclosures are for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.